1. Diesel Locomotive Technology
Contents
The Diesel Locomotive - The Diesel Engine - Diesel Engine Types - Size Does Count - To V or
not to V - Tractive Effort, Pull and Power - Starting - Governor - Fuel Injection - Fuel Control -
Engine Control Development - Power Control - Cooling - Lubrication - Transmission - Parts of
a Diesel-Electric Locomotive - Mechanical Transmission - Hydraulic Transmission - Wheel Slip
- DMUs - More Information (Links).
The Diesel Locomotive
The modern diesel locomotive is a self contained version of the electric locomotive. Like the
electric locomotive, it has electric drive, in the form of traction motors driving the axles and
controlled with electronic controls. It also has many of the same auxiliary systems for cooling,
lighting, heating, braking and hotel power (if required) for the train. It can operate over the same
routes (usually) and can be operated by the same drivers. It differs principally in that it carries
its own generating station around with it, instead of being connected to a remote generating
station through overhead wires or a third rail. The generating station consists of a large diesel
engine coupled to an alternator producing the necessary electricity. A fuel tank is also essential.
It is interesting to note that the modern diesel locomotive produces about 35% of the power of a
electric locomotive of similar weight.
The UK Class 47 is typical of the general New SD90MAC 6,000 hp heavy freight US
purpose diesel-electric locomotives introduced diesel-electric locomotives with AC drive first
in the 1960s. built in 1998
Click on an image for the full size view.
Parts of a Diesel-Electric Locomotive
The following diagram shows the main parts of a US-built diesel-electric locomotive. Click on
the part name for a description.
Diesel Engine
This is the main power source for the locomotive. It comprises a large cylinder block, with the
cylinders arranged in a straight line or in a V (see more here). The engine rotates the drive shaft
at up to 1,000 rpm and this drives the various items needed to power the locomotive. As the

2. transmission is electric, the engine is used as the power source for the electricity generator or
alternator, as it is called nowadays.
Main Alternator
The diesel engine drives the main alternator which provides the power to move the train. The
alternator generates AC electricity which is used to provide power for the traction motors
mounted on the trucks (bogies). In older locomotives, the alternator was a DC machine, called a
generator. It produced direct current which was used to provide power for DC traction motors.
Many of these machines are still in regular use. The next development was the replacement of
the generator by the alternator but still using DC traction motors. The AC output is rectified to
give the DC required for the motors. For more details on AC and DC traction, see the Electronic
Power Page on this site.
Auxiliary Alternator
Locomotives used to operate passenger trains are equipped with an auxiliary alternator. This
provides AC power for lighting, heating, air conditioning, dining facilities etc. on the train. The
output is transmitted along the train through an auxiliary power line. In the US, it is known as
"head end power" or "hotel power". In the UK, air conditioned passenger coaches get what is
called electric train supply (ETS) from the auxiliary alternator.
Motor Blower
The diesel engine also drives a motor blower. As its name suggests, the motor blower provides
air which is blown over the traction motors to keep them cool during periods of heavy work.
The blower is mounted inside the locomotive body but the motors are on the trucks, so the
blower output is connected to each of the motors through flexible ducting. The blower output
also cools the alternators. Some designs have separate blowers for the group of motors on each
truck and others for the alternators. Whatever the arrangement, a modern locomotive has a
complex air management system which monitors the temperature of the various rotating
machines in the locomotive and adjusts the flow of air accordingly.
Air Intakes
The air for cooling the locomotive's motors is drawn in from outside the locomotive. It has to be
filtered to remove dust and other impurities and its flow regulated by temperature, both inside
and outside the locomotive. The air management system has to take account of the wide range of
temperatures from the possible +40°C of summer to the possible -40°C of winter.
Rectifiers/Inverters
The output from the main alternator is AC but it can be used in a locomotive with either DC or
AC traction motors. DC motors were the traditional type used for many years but, in the last 10
years, AC motors have become standard for new locomotives. They are cheaper to build and
cost less to maintain and, with electronic management can be very finely controlled. To see

3. more on the difference between DC and AC traction technology try the Electronic Power Page
on this site.
To convert the AC output from the main alternator to DC, rectifiers are required. If the motors
are DC, the output from the rectifiers is used directly. If the motors are AC, the DC output from
the rectifiers is converted to 3-phase AC for the traction motors.
In the US, there are some variations in how the inverters are configured. GM EMD relies on one
inverter per truck, while GE uses one inverter per axle - both systems have their merits. EMD's
system links the axles within each truck in parallel, ensuring wheel slip control is maximised
among the axles equally. Parallel control also means even wheel wear even between axles.
However, if one inverter (i.e. one truck) fails then the unit is only able to produce 50 per cent of
its tractive effort. One inverter per axle is more complicated, but the GE view is that individual
axle control can provide the best tractive effort. If an inverter fails, the tractive effort for that
axle is lost, but full tractive effort is still available through the other five inverters. By
controlling each axle individually, keeping wheel diameters closely matched for optimum
performance is no longer necessary. This paragraph sourced from e-mail by unknown
correspondent 3 November 1997.
Electronic Controls
Almost every part of the modern locomotive's equipment has some form of electronic control.
These are usually collected in a control cubicle near the cab for easy access. The controls will
usually include a maintenance management system of some sort which can be used to download
data to a portable or hand-held computer.
Control Stand
This is the principal man-machine interface, known as a control desk in the UK or control stand
in the US. The common US type of stand is positioned at an angle on the left side of the driving
position and, it is said, is much preferred by drivers to the modern desk type of control layout
usual in Europe and now being offered on some locomotives in the US.
Batteries
Just like an automobile, the diesel engine needs a battery to start it and to provide electrical
power for lights and controls when the engine is switched off and the alternator is not running.
Cab
Most US diesel locomotives have only one cab but the practice in Europe is two cabs. US
freight locos are also designed with narrow engine compartments and walkways along either
side. This gives a reasonable forward view if the locomotive is working "hood forwards". US
passenger locos, on the other hand have full width bodies and more streamlined ends but still
usually with one cab. In Europe, it is difficult to tell the difference between a freight and
passenger locomotive because the designs are almost all wide bodied and their use is often
mixed.

4. Traction Motor
Since the diesel-electric locomotive uses electric transmission, traction motors are provided on
the axles to give the final drive. These motors were traditionally DC but the development of
modern power and control electronics has led to the introduction of 3-phase AC motors. For a
description of how this technology works, go to the Electronic Power Page on this site. There
are between four and six motors on most diesel-electric locomotives. A modern AC motor with
air blowing can provide up to 1,000 hp.
Pinion/Gear
The traction motor drives the axle through a reduction gear of a range between 3 to 1 (freight)
and 4 to 1 (passenger).
Fuel Tank
A diesel locomotive has to carry its own fuel around with it and there has to be enough for a
reasonable length of trip. The fuel tank is normally under the loco frame and will have a
capacity of say 1,000 imperial gallons (UK Class 59, 3,000 hp) or 5,000 US gallons in a General
Electric AC4400CW 4,400 hp locomotive. The new AC6000s have 5,500 gallon tanks. In
addition to fuel, the locomotive will carry around, typically about 300 US gallons of cooling
water and 250 gallons of lubricating oil for the diesel engine.
Air reservoirs are also required for the train braking and some other systems on the locomotive.
These are often mounted next to the fuel tank under the floor of the locomotive.
Air Compressor
The air compressor is required to provide a constant supply of compressed air for the locomotive
and train brakes. In the US, it is standard practice to drive the compressor off the diesel engine
drive shaft. In the UK, the compressor is usually electrically driven and can therefore be
mounted anywhere. The Class 60 compressor is under the frame, whereas the Class 37 has the
compressors in the nose.
Drive Shaft
The main output from the diesel engine is transmitted by the drive shaft to the alternators at one
end and the radiator fans and compressor at the other end.
Gear Box
The radiator and its cooling fan is often located in the roof of the locomotive. Drive to the fan is
therefore through a gearbox to change the direction of the drive upwards.
Radiator and Radiator Fan

5. The radiator works the same way as in an automobile. Water is distributed around the engine
block to keep the temperature within the most efficient range for the engine. The water is cooled
by passing it through a radiator blown by a fan driven by the diesel engine. See Cooling for
more information.
Turbo Charging
The amount of power obtained from a cylinder in a diesel engine depends on how much fuel can
be burnt in it. The amount of fuel which can be burnt depends on the amount of air available in
the cylinder. So, if you can get more air into the cylinder, more fuel will be burnt and you will
get more power out of your ignition. Turbo charging is used to increase the amount of air
pushed into each cylinder. The turbocharger is driven by exhaust gas from the engine. This gas
drives a fan which, in turn, drives a small compressor which pushes the additional air into the
cylinder. Turbocharging gives a 50% increase in engine power.
The main advantage of the turbocharger is that it gives more power with no increase in fuel costs
because it uses exhaust gas as drive power. It does need additional maintenance, however, so
there are some type of lower power locomotives which are built without it.
Sand Box
Locomotives always carry sand to assist adhesion in bad rail conditions. Sand is not often
provided on multiple unit trains because the adhesion requirements are lower and there are
normally more driven axles.
Truck Frame
This is the part (called the bogie in the UK) carrying the wheels and traction motors of the
locomotive. More information is available at the Bogie Parts Page or the Wheels and Bogies
Page on this site.
Wheel
The best page for information on wheels is the Wheels and Bogies Page on this site.
Mechanical Transmission
A diesel-mechanical locomotive is the simplest type of diesel locomotive. As the name suggests,
a mechanical transmission on a diesel locomotive consists a direct mechanical link between the
diesel engine and the wheels. In the example below, the diesel engine is in the 350-500 hp range
and the transmission is similar to that of an automobile with a four speed gearbox. Most of the
parts are similar to the diesel-electric locomotive but there are some variations in design
mentioned below.

6. Fluid Coupling
In a diesel-mechanical transmission, the main drive shaft is coupled to the engine by a fluid
coupling. This is a hydraulic clutch, consisting of a case filled with oil, a rotating disc with
curved blades driven by the engine and another connected to the road wheels. As the engine
turns the fan, the oil is driven by one disc towards the other. This turns under the force of the oil
and thus turns the drive shaft. Of course, the start up is gradual until the fan speed is almost
matched by the blades. The whole system acts like an automatic clutch to allow a graduated start
for the locomotive.
Gearbox
This does the same job as that on an automobile. It varies the gear ratio between the engine and
the road wheels so that the appropriate level of power can be applied to the wheels. Gear change
is manual. There is no need for a separate clutch because the functions of a clutch are already
provided in the fluid coupling.
Final Drive
The diesel-mechanical locomotive uses a final drive similar to that of a steam engine. The
wheels are coupled to each other to provide more adhesion. The output from the 4-speed
gearbox is coupled to a final drive and reversing gearbox which is provided with a transverse
drive shaft and balance weights. This is connected to the driving wheels by connecting rods.

7. Hydraulic Transmission
Hydraulic transmission works on the same principal as the fluid coupling but it allows a wider
range of "slip" between the engine and wheels. It is known as a "torque converter". When the
train speed has increased sufficiently to match the engine speed, the fluid is drained out of the
torque converter so that the engine is virtually coupled directly to the locomotive wheels. It is
virtually direct because the coupling is usually a fluid coupling, to give some "slip". Higher
speed locomotives use two or three torque converters in a sequence similar to gear changing in a
mechanical transmission and some have used a combination of torque converters and gears.
Some designs of diesel-hydraulic locomotives had two diesel engines and two transmission
systems, one for each bogie. The design was poplar in Germany (the V200 series of
locomotives, for example) in the 1950s and was imported into parts of the UK in the 1960s.
However, it did not work well in heavy or express locomotive designs and has largely been
replaced by diesel-electric transmission.
Wheel Slip
Wheels slip is the bane of the driver trying to get a train away smoothly. The tenuous contact
between steel wheel and steel rail is one of the weakest parts of the railway system.
Traditionally, the only cure has been a combination of the skill of the driver and the selective use
of sand to improve the adhesion. Today, modern electronic control has produced a very effective
answer to this age old problem. The system is called creep control.
Extensive research into wheel slip showed that, even after a wheelset starts to slip, there is still a
considerable amount of useable adhesion available for traction. The adhesion is available up to a
peak, when it will rapidly fall away to an uncontrolled spin. Monitoring the early stages of slip
can be used to adjust the power being applied to the wheels so that the adhesion is kept within
the limits of the "creep" towards the peak level before the uncontrolled spin sets in.
The slip is measured by detecting the locomotive speed by Doppler radar (instead of the usual
method using the rotating wheels) and comparing it to the motor current to see if the wheel
rotation matches the ground speed. If there is a disparity between the two, the motor current is
adjusted to keep the slip within the "creep" range and keep the tractive effort at the maximum
level possible under the creep conditions.
Diesel Multiple Units (DMUs)
The diesel engines used in DMUs work on exactly the same principles as those used in
locomotives, except that the transmission is normally mechanical with some form of gear change
system. DMU engines are smaller and several are used on a train, depending on the
configuration. The diesel engine is often mounted under the car floor and on its side because of
the restricted space available. Vibration being transmitted into the passenger saloon has always
been a problem but some of the newer designs are very good in this respect.
There are some diesel-electric DMUs around and these normally have a separate engine
compartment containing the engine and the generator or alternator.

8. The Diesel Engine
The diesel engine was first patented by Dr Rudolf Diesel (1858-1913) in Germany in 1892 and
he actually got a successful engine working by 1897. By 1913, when he died, his engine was in
use on locomotives and he had set up a facility with Sulzer in Switzerland to manufacture them.
His death was mysterious in that he simply disappeared from a ship taking him to London.
The diesel engine is a compression-ignition engine, as opposed to the petrol (or gasoline) engine,
which is a spark-ignition engine. The spark ignition engine uses an electrical spark from a
"spark plug" to ignite the fuel in the engine's cylinders, whereas the fuel in the diesel engine's
cylinders is ignited by the heat caused by air being suddenly compressed in the cylinder. At this
stage, the air gets compressed into an area 1/25th of its original volume. This would be
expressed as a compression ratio of 25 to 1. A compression ratio of 16 to 1 will give an air
pressure of 500 lbs/in² (35.5 bar) and will increase the air temperature to over 800°F (427°C).
The advantage of the diesel engine over the petrol engine is that it has a higher thermal capacity
(it gets more work out of the fuel), the fuel is cheaper because it is less refined than petrol and it
can do heavy work under extended periods of overload. It can however, in a high speed form, be
sensitive to maintenance and noisy, which is why it is still not popular for passenger
automobiles.
Diesel Engine Types
There are two types of diesel engine, the two-stroke engine and the four-stroke engine. As the
names suggest, they differ in the number of movements of the piston required to complete each
cycle of operation. The simplest is the two-stroke engine. It has no valves. The exhaust from
the combustion and the air for the new stroke is drawn in through openings in the cylinder wall
as the piston reaches the bottom of the downstroke. Compression and combustion occurs on the
upstroke. As one might guess, there are twice as many revolutions for the two-stroke engine as
for equivalent power in a four-stroke engine.
The four-stroke engine works as follows: Downstroke 1 - air intake, upstroke 1 - compression,
downstroke 2 - power, upstroke 2 - exhaust. Valves are required for air intake and exhaust,
usually two for each. In this respect it is more similar to the modern petrol engine than the 2-
stroke design.
In the UK, both types of diesel engine were used but the 4-stroke became the standard. The UK
Class 55 "Deltic" (not now in regular main line service) unusually had a two-stroke engine. In
the US, the General Electric (GE) built locomotives have 4-stroke engines whereas General
Motors (GM) always used 2-stroke engines until the introduction of their SD90MAC 6000 hp "H
series" engine, which is a 4-stroke design.
The reason for using one type or the other is really a question of preference. However, it can be
said that the 2-stroke design is simpler than the 4-stroke but the 4-stroke engine is more fuel
efficient.
Size Does Count

9. Basically, the more power you need, the bigger the engine has to be. Early diesel engines were
less than 100 horse power (hp) but today the US is building 6000 hp locomotives. For a UK
locomotive of 3,300 hp (Class 58), each cylinder will produce about 200 hp, and a modern
engine can double this if the engine is turbocharged.
The maximum rotational speed of the engine when producing full power will be about 1000 rpm
(revolutions per minute) and the engine will idle at about 400 rpm. These relatively low speeds
mean that the engine design is heavy, as opposed to a high speed, lightweight engine. However,
the UK HST (High Speed Train, developed in the 1970s) engine has a speed of 1,500 rpm and
this is regarded as high speed in the railway diesel engine category. The slow, heavy engine
used in railway locomotives will give low maintenance requirements and an extended life.
There is a limit to the size of the engine which can be accommodated within the railway loading
gauge, so the power of a single locomotive is limited. Where additional power is required, it has
become usual to add locomotives. In the US, where freight trains run into tens of thousands of
tons weight, four locomotives at the head of a train are common and several additional ones in
the middle or at the end are not unusual.
To V or not to V
Diesel engines can be designed with the cylinders "in-line", "double banked" or in a "V". The
double banked engine has two rows of cylinders in line. Most diesel locomotives now have V
form engines. This means that the cylinders are split into two sets, with half forming one side of
the V. A V8 engine has 4 cylinders set at an angle forming one side of the V with the other set
of four forming the other side. The crankshaft, providing the drive, is at the base of the V. The
V12 was a popular design used in the UK. In the US, V16 is usual for freight locomotives and
there are some designs with V20 engines.
Engines used for DMU (diesel multiple unit) trains in the UK are often mounted under the floor
of the passenger cars. This restricts the design to in-line engines, which have to be mounted on
their side to fit in the restricted space.
An unusual engine design was the UK 3,300 hp Class 55 locomotive, which had the cylinders
arranged in three sets of opposed Vs in an triangle, in the form of an upturned delta, hence the
name "Deltic".
Tractive Effort, Pull and Power
Before going too much further, we need to understand the definitions of tractive effort, drawbar
pull and power. The definition of tractive effort (TE) is simply the force exerted at the wheel rim
of the locomotive and is usually expressed in pounds (lbs) or kilo Newtons (kN). By the time the
tractive effort is transmitted to the coupling between the locomotive and the train, the drawbar
pull, as it is called will have reduced because of the friction of the mechanical parts of the drive
and some wind resistance.
Power is expressed as horsepower (hp) or kilo Watts (kW) and is actually a rate of doing work.
A unit of horsepower is defined as the work involved by a horse lifting 33,000 lbs one foot in

10. one minute. In the metric system it is calculated as the power (Watts) needed when one Newton
of force is moved one metre in one second. The formula is P = (F*d)/t where P is power, F is
force, d is distance and t is time. One horsepower equals 746 Watts.
The relationship between power and drawbar pull is that a low speed and a high drawbar pull can
produce the same power as high speed and low drawbar pull. If you need to increase higher
tractive effort and high speed, you need to increase the power. To get the variations needed by a
locomotive to operate on the railway, you need to have a suitable means of transmission between
the diesel engine and the wheels.
One thing worth remembering is that the power produced by the diesel engine is not all available
for traction. In a 2,580 hp diesel electric locomotive, some 450 hp is lost to on-board equipment
like blowers, radiator fans, air compressors and "hotel power" for the train.
Starting
A diesel engine is started (like an automobile) by turning over the crankshaft until the cylinders
"fire" or begin combustion. The starting can be done electrically or pneumatically. Pneumatic
starting was used for some engines. Compressed air was pumped into the cylinders of the engine
until it gained sufficient speed to allow ignition, then fuel was applied to fire the engine. The
compressed air was supplied by a small auxiliary engine or by high pressure air cylinders carried
by the locomotive.
Electric starting is now standard. It works the same way as for an automobile, with batteries
providing the power to turn a starter motor which turns over the main engine. In older
locomotives fitted with DC generators instead of AC alternators, the generator was used as a
starter motor by applying battery power to it.
Governor
Once a diesel engine is running, the engine speed is monitored and
controlled through a governor. The governor ensures that the engine speed stays high enough to
idle at the right speed and that the engine speed will not rise too high when full power is
demanded. The governor is a simple mechanical device which first appeared on steam engines.
It operates on a diesel engine as shown in the diagram below.
The governor consists of a rotating shaft, which is driven by the diesel engine. A pair of
flyweights are linked to the shaft and they rotate as it rotates. The centrifugal force caused by

11. the rotation causes the weights to be thrown outwards as the speed of the shaft rises. If the speed
falls the weights move inwards.
The flyweights are linked to a collar fitted around the shaft by a pair of arms. As the weights
move out, so the collar rises on the shaft. If the weights move inwards, the collar moves down
the shaft. The movement of the collar is used to operate the fuel rack lever controlling the
amount of fuel supplied to the engine by the injectors.
Fuel Injection
Ignition is a diesel engine is achieved by compressing air inside a cylinder until it gets very hot
(say 400°C, almost 800°F) and then injecting a fine spray of fuel oil to cause a miniature
explosion. The explosion forces down the piston in the cylinder and this turns the crankshaft.
To get the fine spray needed for successful ignition the fuel has to be pumped into the cylinder at
high pressure. The fuel pump is operated by a cam driven off the engine. The fuel is pumped
into an injector, which gives the fine spray of fuel required in the cylinder for combustion.
Fuel Control
In an automobile engine, the power is controlled by the amount of
fuel/air mixture applied to the cylinder. The mixture is mixed outside the cylinder and then
applied by a throttle valve. In a diesel engine the amount of air applied to the cylinder is
constant so power is regulated by varying the fuel input. The fine spray of fuel injected into each
cylinder has to be regulated to achieve the amount of power required. Regulation is achieved by
varying the fuel sent by the fuel pumps to the injectors. The control arrangement is shown in the
diagram left.
The amount of fuel being applied to the cylinders is varied by altering the effective delivery rate
of the piston in the injector pumps. Each injector has its own pump, operated by an engine-
driven cam, and the pumps are aligned in a row so that they can all be adjusted together. The
adjustment is done by a toothed rack (called the "fuel rack") acting on a toothed section of the
pump mechanism. As the fuel rack moves, so the toothed section of the pump rotates and
provides a drive to move the pump piston round inside the pump. Moving the piston round,
alters the size of the channel available inside the pump for fuel to pass through to the injector
delivery pipe.
The fuel rack can be moved either by the driver operating the power controller in the cab or by
the governor. If the driver asks for more power, the control rod moves the fuel rack to set the

12. pump pistons to allow more fuel to the injectors. The engine will increase power and the
governor will monitor engine speed to ensure it does not go above the predetermined limit. The
limits are fixed by springs (not shown) limiting the weight movement.
Engine Control Development
So far we have seen a simple example of diesel engine control but the systems used by most
locomotives in service today are more sophisticated. To begin with, the drivers control was
combined with the governor and hydraulic control was introduced. One type of governor uses
oil to control the fuel racks hydraulically and another uses the fuel oil pumped by a gear pump
driven by the engine. Some governors are also linked to the turbo charging system to ensure that
fuel does not increase before enough turbocharged air is available. In the most modern systems,
the governor is electronic and is part of a complete engine management system.
Power Control
The diesel engine in a diesel-electric locomotive provides the drive for the main alternator
which, in turn, provides the power required for the traction motors. We can see from this
therefore, that the power required from the diesel engine is related to the power required by the
motors. So, if we want more power from the motors, we must get more current from the
alternator so the engine needs to run faster to generate it. Therefore, to get the optimum
performance from the locomotive, we must link the control of the diesel engine to the power
demands being made on the alternator.
In the days of generators, a complex electro-mechanical system was developed to achieve the
feedback required to regulate engine speed according to generator demand. The core of the
system was a load regulator, basically a variable resistor which was used to very the excitation of
the generator so that its output matched engine speed. The control sequence (simplified) was as
follows:
1. Driver moves the power controller to the full power position
2. An air operated piston actuated by the controller moves a lever, which closes a switch to
supply a low voltage to the load regulator motor.
3. The load regulator motor moves the variable resistor to increase the main generator field
strength and therefore its output.
4. The load on the engine increases so its speed falls and the governor detects the reduced speed.
5. The governor weights drop and cause the fuel rack servo system to actuate.
6. The fuel rack moves to increase the fuel supplied to the injectors and therefore the power
from the engine.
7. The lever (mentioned in 2 above) is used to reduce the pressure of the governor spring.
8. When the engine has responded to the new control and governor settings, it and the generator
will be producing more power.
On locomotives with an alternator, the load regulation is done electronically. Engine speed is
measured like modern speedometers, by counting the frequency of the gear teeth driven by the
engine, in this case, the starter motor gearwheel. Electrical control of the fuel injection is
another improvement now adopted for modern engines. Overheating can be controlled by

13. electronic monitoring of coolant temperature and regulating the engine power accordingly. Oil
pressure can be monitored and used to regulate the engine power in a similar way.
Cooling
Like an automobile engine, the diesel engine needs to work at an optimum temperature for best
efficiency. When it starts, it is too cold and, when working, it must not be allowed to get too
hot. To keep the temperature stable, a cooling system is provided. This consists of a water-
based coolant circulating around the engine block, the coolant being kept cool by passing it
through a radiator.
The coolant is pumped round the cylinder block and the radiator by an electrically or belt driven
pump. The temperature is monitored by a thermostat and this regulates the speed of the (electric
or hydraulic) radiator fan motor to adjust the cooling rate. When starting the coolant isn't
circulated at all. After all, you want the temperature to rise as fast as possible when starting on a
cold morning and this will not happen if you a blowing cold air into your radiator. Some
radiators are provided with shutters to help regulate the temperature in cold conditions.
If the fan is driven by a belt or mechanical link, it is driven through a fluid coupling to ensure
that no damage is caused by sudden changes in engine speed. The fan works the same way as in
an automobile, the air blown by the fan being used to cool the water in the radiator. Some
engines have fans with an electrically or hydrostatically driven motor. An hydraulic motor uses
oil under pressure which has to be contained in a special reservoir and pumped to the motor. It
has the advantage of providing an in-built fluid coupling.
A problem with engine cooling is cold weather. Water freezes at 0°C or 32°F and frozen cooling
water will quickly split a pipe or engine block due to the expansion of the water as it freezes.
Some systems are "self draining" when the engine is stopped and most in Europe are designed to
use a mixture of anti-freeze, with Gycol and some form of rust inhibitor. In the US, engines do
not normally contain anti-freeze, although the new GM EMD "H" engines are designed to use it.
Problems with leaks and seals and the expense of putting a 100 gallons (378.5 litres) of coolant
into a 3,000 hp engine, means that engines in the US have traditionally operated without it. In
cold weather, the engine is left running or the locomotive is kept warm by putting it into a heated
building or by plugging in a shore supply. Another reason for keeping diesel engines running is
that the constant heating and cooling caused by shutdowns and restarts, causes stresses in the
block and pipes and tends to produce leaks.
Lubrication
Like an automobile engine, a diesel engine needs lubrication. In an arrangement similar to the
engine cooling system, lubricating oil is distributed around the engine to the cylinders,
crankshaft and other moving parts. There is a reservoir of oil, usually carried in the sump, which
has to be kept topped up, and a pump to keep the oil circulating evenly around the engine. The
oil gets heated by its passage around the engine and has to be kept cool, so it is passed through a
radiator during its journey. The radiator is sometimes designed as a heat exchanger, where the
oil passes through pipes encased in a water tank which is connected to the engine cooling
system.

14. The oil has to be filtered to remove impurities and it has to be monitored for low pressure. If oil
pressure falls to a level which could cause the engine to seize up, a "low oil pressure switch" will
shut down the engine. There is also a high pressure relief valve, to drain off excess oil back to
the sump.
Transmissions
Like an automobile, a diesel locomotive cannot start itself directly from a stand. It will not
develop maximum power at idling speed, so it needs some form of transmission system to
multiply torque when starting. It will also be necessary to vary the power applied according to
the train weight or the line gradient. There are three methods of doing this: mechanical,
hydraulic or electric. Most diesel locomotives use electric transmission and are called "diesel-
electric" locomotives. Mechanical and hydraulic transmissions are still used but are more
common on multiple unit trains or lighter locomotives.
Diesel-Electric Types
Diesel-electric locomotives come in three varieties, according to the period in which they were
designed. These three are:
DC - DC (DC generator supplying DC traction motors);
AC - DC (AC alternator output rectified to supply DC motors) and
AC - DC - AC (AC alternator output rectified to DC and then inverted to 3-phase AC for the
traction motors).
The DC - DC type has a generator supplying the DC traction motors through a resistance control
system, the AC - DC type has an alternator producing AC current which is rectified to DC and
then supplied to the DC traction motors and, finally, the most modern has the AC alternator
output being rectified to DC and then converted to AC (3-phase) so that it can power the 3-phase
AC traction motors. Although this last system might seem the most complex, the gains from
using AC motors far outweigh the apparent complexity of the system. In reality, most of the
equipment uses solid state power electronics with microprocessor-based controls. For more
details on AC and DC traction, see the Electronic Power Page on this site.
In the US, traction alternators (AC) were introduced with the 3000 hp single diesel engine
locomotives, the first being the Alco C630. The SD40, SD45 and GP40 also had traction
alternators only. On the GP38, SD38, GP39, and SD39s, traction generators (DC) were standard,
and traction alternators were optional, until the dash-2 era, when they became standard. It was a
similar story at General Electric.
There is one traction alternator (or generator) per diesel engine in a locomotive (standard North
American practice anyway). The Alco C628 was the last locomotive to lead the horsepower race
with a DC traction alternator.
face="Times New Roman">Below is a diagram showing the main parts of a common US-built
diesel-electric locomotive. I have used the US example because of the large number of countries

15. which use them. There are obviously many variations in layout and European practice differs in
many ways and we will note some of these in passing.
More Information
This page is just a brief description of the main points of interest concerning diesel locomotives.
There aren't too many technical sites around but the following links give some useful
information:
Diesel Locomotive Systems - A good description of the operation of the equipment of the
modern UK diesel-electric Class 60 locomotive. It written in simple terms and gives the reader a
basic understanding of the technology.
US Diesel Loco Operating Manuals - Copies of some of the older US diesel locomotive manuals
issued to staff. Contains some very interesting details.
Diesel-Electric and Electric Locomotives - by Steve Sconfienza, PhD.D. - >Includes some
technical background on the development of diesel and electric traction in the US, an illustration
of the PRR catenary system and some electrical formulae related to different traction systems.
Diesel-Electric Locomotive Operation - A general list of US diesel locomotive types, designs
and statistics with a summary of their development. A useful introduction to the US diesel loco
scene.
Sources:
The Railroad, What it is, What it Does by John H Armstrong, 1993, Simmons Boardman Books
Inc.; BR Diesel Traction Manual for Enginemen, British Transport Commission, 1962; BR
Equipment, David Gibbons, Ian Allan, 1986 and 1990; Modern Railways; International
Railway Journal; Railway Gazette International; Mass Transit; Trains Magazine.

16. What a Modern Locomotive Is -- The Short Version
This is the really simple version. Modern locomotives have electric motors
connected to the drive axles. The electric motors receive electric power either from
an on-board power source (e.g., a diesel motor) or from a central power source via
a distribution system (e.g., a thrid rail). The link between the electric motor and the
source of the electricity is called the transmission. The electrical power lines that
criss-cross our towns and cities are called electric "transmission" lines; the link
from a diesel motor to the electric motors on a locomotive's axles is called the
transmission. That's it!
[ back to page index ]
Why it looks the way it does
Why do modern freight locomotives look the way they do -- a cab at one end,
lots of bulky equipment at the other? Why do Amtrak, LIRR, and other passenger
locomotives that have been recently designed without regard to any freight
predecessors have one cab with, at most, a hostler's position at the other end?
The issue of cabs on locomotives has a number of "histories" that have
converged to produce the style seen today. First, many early U.S. diesel
locomotives did have two cabs, such as Baldwins built for Jersey Central (while
the same locomotives for other roads had only one cab), as did other diesel and
electrics such as various boxcabs and the GG1, and of course the AEM7s of today
have cabs at each end. What gnaws at ones mind, though, is really about the big
freight locomotives (like the SD80MACs of Conrail). So . . .
When the big frieght roads first dieselized, there were questions about MUs
and crews. The railroads did not want to put a crew in each cab of an MUed set, so
that brought forth such oxymorons as referring to the evolutionarilly critical A-B-
B-A FTs from General Motors as "a locomotive." Calling it one locomotive (one
with a cab at each end!) meant it needed only one crew (and note that the B-units
had no cab, or if any just a hostler's position). As these evolved into F3s, F7s and
F9s, and A-units unpaired and mixed with other units, the ubiquity of single-cab
units was assured. Roads taking E-unit derivatives (i.e., double-engine units, even
those from other builders) such as the Jersey Central Baldwins (DR-6-4-20) did
sometimes take two cabs when it was clear that the unit would only be operating as

17. a single unit; alternatively, some cab units came semi-permanently coupled back-
to-back with a draw-bar instead of a coupler (e.g., PRR's DR-12-8-1500/2),
essentially a single unit with a cab at each end (today, similar issues are resovled in
passenger operations through push-pull, with a cab at the other end of the train).
The other key development was of the non-hood units, the "road switchers."
General Motors again had the critical development with the GP-7. The unit was
logically divided into two hoods, with a somewhat centered cab for ease of bi-
directional movement. Under one hood -- the longer one -- was the motive
equipment, while under the other -- the shorter one -- was usually a steam
generator for passenger service. Local builder ALCO had similar locomotives with
the RS-1s, RS-2s, and RS-3s, leading ultimately into its Century line. Passenger
operations into the 1970s (e.g., the Long Island) took road switchers long-hood
forward to protect the engine crew from the steam generator in case of an accident,
particularly a grade crossing accident where a vehicle could have flipped over the
anti-climber (where the anti-climber would have been had the units been so
equipped) and crushed the steam generator into the cab (shades of an exploding
steam locomotive!). Much earlier, many roads had begun turning their road
switchers around and running them short hood forward for the better visability
(who needed a long hood like a steam engines' boiler, anyway?) -- except for The
Norfolk and Western [N&W] and The Southern [SOU], which seemed to delight in
taking the most incrediably long hood and designating it front, with the cab so
configured; and the Erie Lackawanna, which apparently had some SD-45s
configured for either direction. Long-hood forward and bi-directional units
sometimes had two control stands, one on each side of the cab facing "forward,"
but sometimes had just a single stand sitting parallel to the cab side for use with the
engineer facing in either direction. As the time came when passenger equipment no
longer needed steam, the short hood was cut and shortened, and that gave the
configuration of the road switcher of today (note that during the overlap when
short hoods were being cut but some passenger steam-generators were still needed
in operation, GM's SDP-35, SDP-40, and SDP-45 had the steam generators at the
coupler-end of the long hood).
At this point, a second cab could have been added, but only a hand-full of
units have ever been built that way (some electrics on mining roads come to mind).
For the most part, the big roads run with two or more units anyway, so having east-
west pairs is not difficult (turning facilities such as wyes and loop tracks seem to
be plentiful). The smaller roads simply don't want the expense. And, of course, it
seems, N&W and SOU successor Norfolk Southern would just as soon run them
long-hood anyway!

18. [ Motors Graphic ][ back to page index ]
Baisc DC Motor Concepts
The traditional electric motor on a diesel-electric or electric locomotive is a
DC motor. Internally, DC motors have two main components: the stator is the
stationary outside part of a motor. The armature is the inner part which rotates. To
get the armature to rotate, electric motors require two sets of windings, the field
winding (on the stator) to develop the magnetic field within which the armature
will turn, and the armature winding (which in many DC motors can make
transition between series and shunt winding). In series, current passes through both
the field and armature windings "in series", that is, one after the other, while in
shunt the current is divided and passes either through the field or armature
windings. DC motors with series windings develop high starting torque, while
motors in shunt develop high speed. (A third category, the compound, combines
both series and shunt windings simultaneously, with mixed properties: c.f. a motor
that can transition between series and shunt, but not use both at the same time; a
fourth category is the permanent-magnet motor, which not surprisingly uses a
permanent magnet for the field, and is only used in relatively low-power
applications.)
DC motors turn because an electrical field rotates. The field rotates because an
electrical current passing into the armature changes polarity, with the armature
tugged forward with each change. (N.B., it is the field in the armature that is
changing, not the field in the stator.) In order to accomplish this change in polarity,
the windings in the armature are connected to the outside world by means of a
commutator, a conductive sheaf that allows for the current in the windings to have
a change in polarity by breaking and making the connection. The electricity flows
into the commutator through conductive brushes (usually carbon). These are
sources of friction, heat, and general wear in the DC motor.
Series-wound motors are also called universal motors (see below), universal in
the sense that they will run equally well using either AC or DC: simultaneously
reversing the polarity of both the stator and the rotor cancel out, thus the motor will
always rotate in the same direction regardless of the voltage polarity. Sometimes
called "AC motors" instead of "DC series-wound motors" or "universal motors,"
these are not the motors to which one refers when referring to AC traction motors.

19. Note: In the motor wiring diagrams, the DC motors do not have separately
excited windings. While some first generation diesel-electrics had an auxiliary
generator to provide current to separately excited windings, which required manual
switching by the engineer, that design is now obsolete. Contemporary U.S. road
locomotives do not have separately excited windings; however, there has been
other work in this direction, based in computer-control systems, that has included
wheel-slip detection and control and wheel-creep systems, in which the windings
are under separate control (see below).
[ back to page index ]
Transmissions
When speaking of a vehicle such as a railroad locomotive, transmission is the
process by which power is transmitted from one location and used in another.
Often this implies some type of changing process, for example the manner in the
rotational force of a crankshaft is converted to electrical force in a generator.
In a modern diesel-electric locomotive, this is a multi-staged process that goes
from fuel oil to turning wheels. In the days of steam the intermediate process was,
well, it was steam. Coal or oil was burned in a firebox. The heat generated heated
water, turning it into steam (and continued to heat the steam), which in turn drove a
piston in a back-and-forth motion, which -- through the drive gear -- turned the
wheels. In diesel-electric locomotives, the fuel is burned in cylinders, driving
pistons in a back-and-forth motion, which -- through the crank shaft -- turns an
electricity generating device (a generator or alternator or both), which provides
electricity for electric motors that are connected to the axles of the trucks, which
turn the wheels.
FIGURE ONE: BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE
diesel motor

20. generator
electric motors
[ Transition Graphic ][ back to page index ]
Transition
Transition is the process by which the transmission of a diesel-electric
locomoitve is brought from series wiring to parallel wiring. When in series, all
current in the locomotive pass through all motors: this produces maximum low-
speed force in the motors, i.e., maximum starting torque. When in parallel, current
is divided among the motors: this produces maximum high-end efficency, i.e.,
highest motor speed. This is just as with the wiring internal to DC motors, where
having the motor wound in series develops high starting torque, while placing the
motor in parallel will develop high speed. Electrically, as current increases through
the motors in a circuit with a given total current and voltage, the voltage drop
across each motor will decrease: parallel circuits apply the total voltage to each
load (i.e., in this case, motor), while series circuits apply the total current to each
load.
Not all locomotives can make transition -- yard locomotives are often wired
only for series. The motors on a diesel-electric road locomotives are often capable
of making multiple transitions, with both trucks and motors on a truck capable of
being switched into series or parallel wiring.
See the motor diagram for an example of a DC motor capable of transition and
the transition diagram for examples of two- and three-axle transition schemes. (A
note on how the construction of the motors is depicted is above). These diagrams
are quite general: some locomotives would put like axles on different trucks in
series (first axle with first axle, second with second), or have other arrangements.

21. [ Catenary Graphic ][ back to page index ]
Electric Locomotives
In straight electric locomotives or M.U. cars (multiple-unit electric), the on-
board diesel engine and generator and/or alternator is replaced by Niagara Falls or
Indian Point or some other such central power generating source. The power is
transmitted to the railroad and delivered to the trains. Electric transmission lines
are generally high-voltage AC. This is because of the greater efficiency (i.e., less
loss) of AC during transmission over certain distances, and the likewise greater
efficiency of high-voltage transmission over low-voltage transmission (see below
and also the Formulas and Concepts page under Electric Power Transmission for
more extensive notes on AC, DC, and long distance electric transmission). In rail
applications (as in most others), the most efficient transmission of electricity from
generating stations to the tracks requires transmission lines of high-voltage AC
with substations to convert this to line voltage for equipment. The power is
delivered to the individual locomotives or cars generally either by a track level
third rail (there are some fourth rail systems, too) or through overhead wires
known as catenary). While there are some exceptions, AC systems usually use
catenary, while DC systems usually use third rail.
FIGURE TWO: BLOCK DIAGRAM OF A ELECTRIC LOCOMOTIVE
central generating facility
distribution system
on-board electrical equipment

22. (e.g., transformer, rectifier, motor-generator, inverter, etc.)
electric motors
Notes on Electric Power Transmission and Distribution Systems
Long-Distance Transmission, Substations, and Local Distribution
Two principal elements of an electric system are the transmission of the
electric power from the source of the electricity (i.e., a generating plant) to the
local use area and, ultimately, distributing that power to the consumer (e.g., an
electric locomotive or a home). Thus, these parts of the system may be divided into
the transmission system and the distribution system, with transmission convenying
the power long distances (at high voltages), and the distribution system delivering
the power locally (at low voltages).
Long-Distance Transmission
As a general rule, in the United States, DC cannot be transmitted as
economically as AC in transmission systems; railroads follow the practice of AC
transmission systems, with high voltage AC stepped-down to distribution voltages
at substations.
Extensive notes on long distance electric transmission, including some
formulas, are on the Formulas and Concepts page under Electric Power
Transmission.
Local Distribution
Local distribution is almost universally accomplished by third rail or overhead
wires. The highest third rail voltage in use in the U.S. today is the 1,000 volt
system on San Francisco's BART system. The highest, historically, is reputed to
have been an interurban that ran 2400 volts (historically, only for a brief period): it
did not really work very well, as arcing and leakage were such critical issues that
the system was conveted to a lower voltage.

23. As for AC, it is generally delivered at higher volatages than third rail through
overhead wires. As for AC third rail, no such systems exist (at least that I know of,
certainly not in the U.S.). This may not be practicable: AC's advantage comes from
high voltage transmissions that can be readilly stepped up or down, with
conversion to DC in the distribution system providing easy control of motors
without including on-board rectifiers (but more on that following). (Now, with AC
traction in common usage, there may be a rationale for that to change, but
distribution is still basically low-voltage DC or higher voltage AC.)
Distribution on the North East Corridor
The NEC uses three different combinations:
• D.C. to New York City: 11kV 25Hz
• N.Y.C. to New Haven: 11kV 60Hz
• New Haven to Boston: 25kV 60Hz
Understandably, Amtrak's engineers (the slide-rule type) want everything at
25kV 60Hz, and that is the standard for new track, such as the latest
electrificationm, from New Haven to Boston. Sixty hertz has the advantage of
being compatible with the commercial grid (25Hz requires frequency converters,
which run [reportedly] $40,000,000 each), and 25kV is not an unusual voltage, so
equipment is available.
Higher voltages are also more efficient to transmit than lower: in fact, when
the voltage is doubled the amperage is halved for the same power level ( P = I * V:
increasing voltage results in a linear decrease in current at the same power level).
Since transmission losses are a function of amperage only (dissipated power = I2R,
where R is the constant line resistance [or impedance, in the case of AC]), 25kV
power can be transmitted over twice as far as 11kV power at the same loss levels.
One should note, however, that this does not solve the problem of drawing down
the current in a section by multiple trains running within it: high train density will
still require short segments (remember that this concerns only local distribution via
the catenary and its local feeders, not long-distance transmission from
powerplants).
Unfortunately, upgrading track from 11kV to 25kV is expensive, because it is
necessary to rebuild catenary to accomodate the higher arc distance at the higher
voltage, (e.g., better insulation and insulators).

24. Phase
While it is theoretically possible to run an entire rail line, hundreds of miles
long, on a single, synchronized AC phase, in practice it is not practical. Instead,
lines are generally broken into ten to twenty mile segments, each one running on a
different phase (usually 120 degrees apart). The boundry of each segment is called
a "phase break." On former Pennsy track, these were marked by a phase-break
signal, which looked like a typical position-light signal with all positions in the
entire circle lit. To prevent arcing between the sections, an insulating section of
catenary is run across the phase break. By segmenting the catenary into sections, it
is a simple matter -- with respect to stringing the catenary -- to have not only
different phases but also different frequencies, 25 or 60 hertz, or voltatges, 11kV or
25kV, on the two sides of a phase break.
Unlike former equipment, which had to stop and change internal settings,
Acela trains are capable of changing both frequency and voltage while at speed.
This is done as part of the normal "approaching phase break" message that is sent
to trains via ACSES, which includes the frequency and voltage that will be on the
other side of the break (typically no change). When the pantograph hits the
insulator between phases, the train temporarily cuts the input power, reconfigures
the leads to the windings on the primary of the main transformer, and reconnects
the input power -- all in less than a second. To the equipment downstream from
the transformer, all that is visible is an AC voltage that drops out briefly every five
to 15 minutes, as the locomotive hits the various phases. The blip is short enough
there should be no noticeable traction-motor stutter or hotel-power disruption.
FIGURE THREE: REGIONAL EXAMPLES OF MAINLINE ELECTRIC OPERATIONS
• Northeast Corridor
o 25kv, 60Hz AC via Catenary
The PRR originally built this as 11kv, 25hz and supplied its own
electricity, as this system was not compatible with commercial 60Hz
systems (complete change-over not yet completed: see above).
• Long Island Rail Road
o 600v DC via 3rd rail
• Delaware, Lackawanna, and Western (metro New Jersey lines)

25. o 3000v DC via Catenary
• Montreal Suburban
o 2500v DC via Catenary
• Other Systems
o there are also some 50kv, 60hz catenary systems
o some DC operators generated their own 25Hz AC for distribution to
substations
Substations
Substations sit between transmission and distribution systems. They are fairly
straightforward: a transformer steps down the AC voltage, then, if using a DC
distribution system, a rectifier converts the AC to DC. With their transformers to
step-down the high voltage transmission voltages to distribution levels, they are
located periodically throughout the system. For the railroad, these are much like
the ubiquitious substations with their tranformers located throughout suburban
neighborhoods.
In electric locomotive (or MU) applications, the use of DC (as on the Metro
North [ex New York Central] Hudson and Harlem lines and on the LIRR) requires
closely spaced substations to convert AC to DC, stations more closely spaced than
might be required for a similar AC distribution system. This is because of the
higher line losses at the lower distribution voltages (this is explained elsewhere
under Electric Power Transmission).
Rectification
Because of long-established AC motor issues of low starting torque and of
power control (more on that follows), the traction motors themselves have (up until
recently) been DC. This has meant that at some point in the process AC has had to
be coverted ("rectified") to DC, either at the substation or in the locomotive.
AC, which reverses direction 60 times a second (the U.S. standard), generally
resembles a sine wave in the distribution systems. A simple rectifier is an electrical
check valve: flow is only permitted in one direction, while retaining the
characteristic sine curve (one-half of the curve, just the "positive" half, let's say).
This is referred to as half-wave rectification. A more sophisticated approach is to
allow the negative alternations to pass also, but in the same direction as the

26. positive alternations (i.e., no direction change in the current). The AC thus
becomes a pulsating DC, with all pulsations of the sine wave in one direction from
zero. This is referred to as full-wave rectification.
FIGURE FOUR: RECTIFICATION
Substation-Based Rectification:
The rectifiers in modern substations are solid state, sillicon diode based. They
are efficient (the voltage drop is a fraction of a volt through the rectifier) and

27. reliable. Earlier systems, such as used on the Long Island Rail Road, used mercury
arc rectifiers, only slightly less efficient, but requiring much support (these were
known as "ingitron" systems). They were quite large, housed in large structures,
and required much cooling. Other systems, such as used by the New York City
Transit Authority, used rotary converter based substations - - very large, very
maintenance intensive.
Substations are almost always fed with three phase AC, and the three phases
overlap coming out of the rectifier, so the DC pulsates only slightly (filtering can
remove the pulsations altogether: see figure seven below for an example). From
here, the DC is fed to the third rail (or catenary) by way of breakers, current
sensors, switchgear, and whatever else.
Locomotive-Based Rectification:
Prior to the advent of a solid-state technology for converting high-power AC
to DC, massive locomotives were often the only solution to this issue when using
all AC systems: an AC motor turned a DC generator, which in turn supplied DC to
the motors on the axles. A rectifier that takes the form of an AC motor turning a
generator is called, not surprisingly, a "motor-generator."
Some railroads, such as the Pennsylvania and New Haven, had MUs with
ignitrons on them. Both of these roads also had ingitron based locomotives (known
as "rectifiers," locomotives such as the EP-5, E-44, and E-33, but not the PRR's
GG1: one of the most massive locomotives, it was actually an all-AC unit, with
power control through tranformer taps).
In the mid-1960s, high-power solid-state rectifiers became feasible, and
smaller, lighter weight electric locomotives -- and AC transmissions on diesel-
electric locomotives -- became available. In the straight-electric market, the last
GM (GMD [Canada]) motor-generator unit was the SW1200MG (2300v, 60hz),
produced from 1963 to 1971 (1971 being well after the move to solid-state
rectification, but the unit had gone into production in 1963 and was maintained for
an existing customer).
[ Catenary Graphic ][ back to page index ]

28. The conversion to AC/DC transmission
An alternator is generally smaller and simpler than a generator of like capacity.
This is because generators, like DC motors, are equipped with a commutator and
carbon brushes, which are what reverses the electrical current as the armature
turns, preventing the current from alternating, keeping the current direct. This
simpler, lighter structure means that a diesel-electric locomotive using an alternator
instead of a generator should be more economical. With the advent of economical
and compact solid-state rectifiers, which could be routinely installed on
locomotives (see above), the greater efficiency of the alternator could finally be
realized in rail applications, and AC/DC transmission became a reality. In the mid-
1960s, all three major manufacturers begin offing AC/DC transmission units, Alco
and GM in 1965 and GE in 1966.
FIGURE FIVE: BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE WITH AN
AC/DC TRANSMISSION
diesel motor
alternator
rectifier
electric motors

29. The block diagram in figure five illustrates the AC/DC transmission. A diesel
motor turns an alternator; the AC produced by the alternator is rectified to DC for
the locomitive's DC traction motors.
GM's first applications of AC transmission were in the GP40 and SD40 of
1965 and 1966, respectively. General Electric's first AC transmission were the
U28B/U28C offerings of 1966 (earlier production of these models was straight
DC). Alco offered the top-end of its Century line with AC/DC transmissions, the
C430/C630, in 1966/1965 respectively.
[ back to page index ]
The move away from DC traction motors
One of the most important advances in locomotive technology in recent years
is the AC traction motors. AC motors have been around for many years (the
kitchen clock that plugs into a recessed electrical socket directly behind it is an
example of one). However, AC motors were never able to match the starting torque
of the DC and are notoriously difficult to control in varrying load and speed
implementations. Unfortunately, while DC motors provide high starting torque
they also have critical limitations (as was noted above). These limitations have
long made it desirable that a substitute be found.
Like generators, DC motors are equipped with a commutator and carbon
brushes, which are subjected to very high current loads. (In a generator, these are
what reverses the electrical current as the armature turns, preventing the current
from alternating -- keeping the current direct; in the motor, the commutator and
brushes reverse the current, creating the moving magnetic force that rotates the
armature.) A DC motor that would have high current loads while not in motion or
while moving slowly would receive major damage or burn-out if such a high
current were to be applied for too long a period of time. At low speeds, the high
amperage damage would occur within minutes. Because of this, until recently, all
DC locomotives all have minimum continuous speeds (for example, SD40 & 45 at
11 to 12 MPH, SD50 at 10 MPH, GP40 at 12 MPH, some swithcers and regeared
road units, such as some CSX GP38s at 7 MPH).

30. Power in a DC circuit is simply equal to the voltage times the current. This is
expressed as power (in watts, "P") = voltage (in volts, "E" [for Electromotive
force) times current (in amps, "I"), or
P=E*I, and
Power (in horsepower) = watts * 0.00134102
(going the other way, watts = horsepower * 745.6999)
In DC motors, the power relationship is simple: at a constant voltage Ohms
law requires more current to produce more power (watts = voltage * current). This
means that in DC high current levels will be needed to produce high power,
lacking a good way to vary voltage on the fly. This becomes expensive, having
necessitated heavy conductors throughout the system to carry the high current;
further, the high current produce a great deal of heat, further limiting DC traction
motors.
For example, to compute the current flow in a 1000 horsepower switcher with
DC traction motors at 600 volts,
• 1000 horsepower = 745,699.872 watts
• 745,699.872 watts / 600 volts = 1242.83312 amps.
Using today's high-horsepower DC units, e.g. a 4400 horsepower, 6 axle unit,
where each of the six motors contribute 733 horsepower to the total unit
horsepower, one can get up to 5500 amps per motor in a 600 volts system.
(Remember that in DC motors that current goes across the commutator and
brushes.) Specifically,
• 4400 horsepower / 6 motors = 733 horsepower/motor
• 733 horsepower/motor = 546,598 watts/motor
When operating in parallel, with a 600 volt drop across each motor,
• 546,598 watts/motor / 600 volts = 910.99667 amps/motor
When operating in series, with a 100 volt drop across each of six motors,
• 546,598 watts/motor / 100 volts = 5465.98 amps/motor

31. Note that when operating in parallel, 911 amps * 6 motors is 5466 amperes
total in the system.
These modern units -- like their AC brethren -- use computer control to
reduce (hopefully to eliminate) wheel slip, but even so they can still slip (and stall),
and even with arc suppressors and damping material around the brushes, flashovers
and destroyed brushes still occur, caused by low speed, wheel slip, rough track,
etc., all of which contribute to the woes of a DC traction motor.
Power-wize, contemporary DC traction motor size is getting very close to the
practical limits. This is based on such elements as magnetic saturation and the
current capacity of the electrical conductors used to build them, coupled with the
physical limits of the structure (it would be necessary to use physically larger
motors to forestall magnetic saturation: see note below).
In the 1960s, the Southern Pacific and the Rio Grande both acquired diesel-
hydraulic locomotives. In the hydraulic transmission, a driveshaft connects the
power-plant to the axles, just as in an automobile. In 1961, both roads acquired
German-built Krauss-Maffei locomotives, twin-engined 3450 hp, c-c units with a
cowl carbody. In 1963, the SP took an additional 15 units with a road-switcher
carbody. In 1964 SP acquired the Rio-Grande units. ALCO also made a forray into
the diesel-hydralluc experiments, the DH-643, a double-engine, 4300 hp, c-c unit:
three units were built, all going to SP in 1964 after testing on the New York
Central. In 1970 SP retired its German units, while the ALCOs were scrapped in
1973. The world still had two decades to wait for a better locomotive transmission.
Note: Magnetic saturation is a rather abstract concept that may best be thought
of as the limited ability of an object to be magnetized. In the case of a motor the
object is usually a piece of iron wound with wires conducting an electrical current.
With an applied voltage to the wound wires, a current is caused to flow, and that
current flow causes a magnetic field to be created. With more applied voltage,
more current and more magnetic field in proportion to the applied voltage. At some
point, the iron becomes saturated, increasing the current does not create more
magnetic field, and the linear relationship is broken: increasing voltage no longer
causes a linear increase in current but instead creates a geometric increase in
current -- that is, lots and lots of current, creating lots and lots of heat, burning out
the motor. For more on magnetic saturation, including some formulas, see our
Formulas and Concepts page under AC Motor Facts.

32. [ back to page index ]
The move to AC traction motors
It has long been known that AC motors can be more economical than DC
motors, just as with their near cousins, alternators and generators. Like alternators,
AC motors are not equipped with wear-prone commutators and brushes,
eliminating these sources of limitations of the low speed-high throttle position. AC
motors would allow locomotives to (1) have more pulling power, (2) avoid stall
burns in the traction motors, and (3) have correspondingly lower maintenance
requirements.
An early example of AC in a railroad application is the GG1 (designed in
1934), which utilized 12 six-pole motors, 400 volts AC at 25 Hz. Each motor was
rated at 385 hp, with the 12 motors mounted in pairs over each of the six driving
axles (see our GG1 page for details of the GG1 electricals). In June of 1989, GM
began the modern AC traction motor era with its demonstrator, the F69PH-AC, an
AC traction version of the F59, followed in 1991 by the SD60MAC. GM delivered
its first production unit to Burlington Northern in 1993. GE delivered its first AC-
traction unit to CSX in June of 1994.
As a brief technical aside to provide some background and standardize
terminology, series-wound DC motors (i.e., motors with commutators and brushes
where the field winding and the rotor winding are connected in series) are also
called universal motors, universal in the sense that they will run equally well using
either AC or DC: simultaneously reversing the polarity of both the stator and the
rotor cancel out, thus the motor will always rotate the same direction regardless of
the voltage polarity. So a universal motor is in a sense a type of an AC motor in as
much as it will operate on AC. The term "universal motor" differentiates it from
the more generally thought of AC motor, the AC induction motor, which lacks
commutators and brushes. Unfortunately for universal motors, the fact that they do
not lack commutators and brushes means that they do lack all of the advantages of
what are more typically thought of as AC motors -- the induction motors -- which
is the very lack of commutators and brushes! So to say the universal motor "will
run equally well using either AC or DC" may be a slight misphrasing: perhaps one
should say, "it will run equally badly!" Therefore, the universal motor does not
have a role to play in modern electric traction (although universal motors were
used in early AC applications in locomotives); rather it is the induction motor that

33. is the "AC motor" to which one refers when speaking of AC traction motors today.
That means no brushes to maintain, no flashovers, no commutator to get damaged,
no armatures to rewind, and less potential for damage at high power/low rpm
situations.
AC locomotives are more expensive due to the control problems inherent in
the AC design. An AC motor's speed is traditionally dependant on its design, but it
may be controlled by varying the frequency of the input voltage. Being able to vary
frequency has been a significant issue in the development of AC motors in high-
horsepower traction applications. To deal with the power control problem, both
EMD and GE use an AC to DC to AC conversion, control taking place in the DC
phase. In an AC traction motor application, the diesel engine drives an alternator,
crating AC. This AC is rectified (i.e., converted to DC) and power control takes
place in this stage. This is the same place that power control would take place in a
conventional AC/DC transmission. At this point, the DC (called the DC link) goes
through a solid-state "inverter," which converts the DC back to AC. This AC then
powers the motors.
FIGURE SIX, BLOCK DIAGRAM OF A DIESEL-ELECTRIC LOCOMOTIVE WITH AC
TRACTION MOTORS
diesel motor
alternator
rectifier
== DC LINK ==

34. inverter
electric motors
FIGURE SEVEN, VOLTAGE/FREQUENCY IN A DIESEL-ELECTRIC LOCOMOTIVE WITH
AC TRACTION MOTORS: OUTPUT OF ALTERNATOR TO OUTPUT OF INVERTER

35. Control takes place in the stages around the DC Link. The inverter converts the
DC back to AC, with the conversion frequency and voltage specifically controlled
(this is what then determines the motor's speed). However, this is not simply an
inverter, for in modern applicatons of AC motors, with a reliance made on varying
the voltage and frequency of the AC to control power more than on simply the
brute force approach of the application of current, the inverter must do more than
simply convert DC to AC. Complex electronic circuitry in the form of on-board
computers now is used to control the inverter. (This has eliminated the need for

36. that classic of the diesel age, the ammeter, in the cab of the AC-motored
locomotive, which has been replaced by a tractive effort display.)
The inverter stage is actually a group of inverters, depending on manufacturer
either one for each truck (GM) or one for each motor (GE). Each individual
inverter consists of six "gated turn-on (GTO) devices," high-power thyristors (that
is, "silicon-controlled rectifiers"), three each for the positive and the negative
phases of the AC wave in positive/negative pairs. Each positive/negative pair
alternate turning-on, chopping the DC into a square wave AC. Each of the three
positive/negative pairs turn-on 120 degrees out of phase from each other (turning-
on at 0 degrees, 120 degrees, and 240 degrees), producing three-phase AC. While
the phase remains constant, the frequency -- how many cycles per second this is
repeated -- is varried. Also able to be controlled at this stage is the voltage, how
positive and negative the AC becomes. Thus, the frequency and the voltage of the
AC arriving at the AC motor is fully controlable, providing the speed control for
the locomotive.
Since the frequency and voltage are closely controlled by onboard computer
systems, motors cannot run away as they would on a DC locomotive, and the AC
motor will not be subject to damaging wheel slip. The use of AC traction motors,
coupled with computer controlled wheel creep systems, has allowed AC units to
achieve much higher adhesion levels than similar DC units, up to 45% adhesion,
versus the 20% range on other units. This has permitted two-for-three and one-for-
two replacement of units, with resulting economies in size and maintenance
expenses that offset the added initial investment in the purchase of AC units.
(Note, however, that there are other issues with such power reductions: for
example, a two-for-one reduction on a tradtionally two locomotive run means one
locomotive, and if that one locomotive develops problems enroute [not entirely
unheard of] there is no backup.)
Computer control technology has also been applied to DC traction motors,
including wheel-slip detection and a wheel-creep systems allowing for brief [we're
talking fractions of seconds here] applications of power to facilitate very low speed
operations. While, this does not fully eliminate problems with high current flow at
low speeds in DC motors, these DC wheel-creep systems and wheel-slip detection
systems provide dramatically increased adhesion in DC units as well as in AC units
and have eliminated many of the operational issues with DC traction: CSX, for
example, does not place a minimum continuous speed on its DC-traction GM
SD60s and SD70s and GE Dash 8 and Dash 9 locomotives, the same as for all of
its AC locomotives [see also note above].)

37. While the AC traction motor is less complex and has proven itself dependable
in long term railroad use (the PRR GG1 used 12 385 hp AC motors ), the purchase
of new units with AC traction motors is an expensive undertaking, representing an
investment in new technology with maintenance and operational issues not
previously encountered, and the new generation of 1000 hp AC traction motors in
railroad use represents a new and untested technology, with some railroads still
very reluctant to make the transition. (An interesting W3 site on AC motors is
http://www.drivesys.com/asdis.html.)
[ back to page index ]
Expanded AC Motor Principals
The Short Version
This isn't expected to make sense, so don't worry. When an AC motor is at rest
and an AC voltage is first applied to it, the difference between the aramature speed
and the rotating field is 100%. Under these conditions, a high current will flow at
the moment the aramature starts to turn. At the moment of starting, the torque is at
0% of the full load torque, but as the speed increases the torque likewise increases.
This is in part because, at low speeds, the motor reactance is high, and the current
and voltage are very much out phase. This contributes to the low power factor. In
an AC motor, maximum power will be generated when the voltage and current are
closest to being in phase, so it can be seen that when the voltage and current are
out of phase the motor will not be very efficient.
The Long Version
(See the Formulas and Concepts page for more detail on AC motor operation and on
these formulas).
Power in a DC circuit versus Power in an AC circuit.
Power in a DC circuit
As noted above, Power in a DC circuit is simply equal to the voltage times the
current. This is expressed as power (in watts, "P") = voltage (in volts, "E" [for
Electromotive force) times current (in amps, "I"), or

38. P=E*I
For example, ten volts times ten amps equals 100 watts. This relationship can
be used in reverse to analyze a circuit. A 40 watt bulb on a 12 volt DC circuit must
be drawing 3.333 amps. Further, since Ohm's law states that voltage = current tims
resistance (E=I*R), it may be seen that the load here is 3.6 ohms. This is all simple
and straightforward because this is a DC circuit.
Power in an AC circuit
AC Motors
One of the miricals of the AC motor is that in the AC induction motor, one of
the of the two principal components (these two components in the AC motor are
the stator and the rotor), the rotor has no visable electrical contacts to the outside
world. Instead, it has an electrical field induced into it by the electrical field of the
stator -- no commutator, no brushes! (Acutally, some induction motors have
brushes and slip rings, but these are used for connecting control and starting
equipment to the windings). The induction of the electrical field into the rotor
happens because of the characteristic pulsing flow of current in AC. However, this
has other affects as well.
• Reactance
In an AC circuit, things are different, because in addition to there being
a pure resistive load in the circuit there is also reactance in the circuit.
Reactance is the unique effect that is displayed in opposition to AC current
flow. There are two types of reactance, inductive reactance (that is, a coil),
the tendency of the circuit to absorb and store an electrical potential, and
capacitive reactance (that is, a capacitor), the tendency of the circuit to
absorb and store current. AC circuits can always be quantified in terms of
these three forces: resistance, inductive reactance, and capacative reactance.
The total oppostion to the AC current flow is called impedance, and it is the
vectored sum of the circuit resistance plus the total reactance, inductive and
capacitive. Since inductive and capacitive reactance are forces of opposite
direction, they counter each other, thus,
1. Inductive Reactance, Xl, = 2PiFL
2. Capacitive Reactance, Xc, = 1/(2PiFC)
where
• Xl = Inductive Reactance in Ohms,

39. • Xc = Capacitive Reactance in Ohms,
and
• F = the frequency of the applied AC in Hertz (cycles per
second),
• L = the inductence of the circuit in Henries, and
• C = the capacitence of the circuit in Farads
3. Impedance, Z, = (R2 * X2)1/2, where X2 = (Xl - Xc)2
where
• Z = Impedance in Ohms,
• R = Resistance in Ohms, and
• X = Reactance in Ohms
AC induction motors are primarilly inductive circuits, so effectively
their impedence may be expressed by the formula
Z = (R2 * Xl2)1/2
• Power Factor
In an AC circuit, voltage times current does not equal power; it equals
the effective value of voltage and current, which is measured in
"voltamperes" (VA). Correcting voltamperes for "power factor" produces
the useful or actual power in the circuit, which is measured in watts. So,
P = VA * pf
and the value of pf is determined by how much the voltage and current
are out of phase. An incandescent light bulb has a power factor of anywhere
from 0.95 to 0.99; AC motors may have power factors ranging from .6 to .9;
in all of these situations, the current is lagging the voltage -- inductive
circuits.
• Phase Angle
A purely reactive circuit has a phase angle between the current and
voltage of 90 degrees, which results in a power factor of 0.0. The
relationship between phase angle and power factor is that power factor
equals the cosine of the phase angle. Therefore, power equals the cosine of
the phase angle times the voltamperes. In the above example, the cosine of
90 degrees = 0.0. So, at rest, with 90 degree phase angle (purely reactive

40. circuit -- the resistance of the motor's windings is minimal), the useful
power of the motor is . . . 0 watts! There are starting strategies, for example,
any substantive resistance in the circuit will reduce the phase angle below 90
degrees, thus increasing power factor above 0 and allowing some work to
get done. More typically, capacitor-based systems can reduce the phase
angle and can be used to start the motor.
The situation that has developed is that the power developed in an AC
motor is related to the magnitude of the voltage, the current, and the internal
resistance of the motor (i.e., the simple resistance of the wires), and the
frequency of the AC applied to the motor, because the frequency will change
the phase angle. (This concept is expanded upon on the Formulas and
Concepts page under Power Facts/AC Motor Facts.)
• Speed Control in AC Motors
Since an AC motor's speed is based on the frequency of the AC, a
change in frequency directly results in a change in speed; however, the
change in frequency also changes the reactance of the circuit (because a
reduction in the frequency causes a linear reduction in inductive reactance).
This in turn changes the impedance of the circuit, the oppositon to the flow
of AC current. Speed control may be accomplished in these motors by
utilizing solid-state, micro-processor control devices that vary the frequency
and voltage of the AC applied to the motor.
If it were intended to slow an AC motor, the frequency of the applied
AC would be reduced: as frequency decreases, circuit reactance decreases,
and therefore impedance also decreases. Given a constant voltage, current
would increase, potentially to the point where the motor would be damaged.
Therefore, a decrease in the frequency must be accompanied by a decrease
in voltage sufficient to stabilize the current.
As one last reminder, there is much more detailed information on our
Formulas and Concepts page under Power Facts/AC Motor Facts.
GASP!

41. Kilo Newtons, kilo Watts, kilometres per Hour
So just what do terms used to describe the performance of locomotives and multiple units like Maximum
Tractive Effort, Power At Rail, and Continuous Power mean? Here is a guide to such things showing how
they influence journey times and speeds.
Some School Physics Revision
A few basic physical relationships link the various factors that influence the acceleration and speed of an
object, in this case a train! The following notes explain those relationships.
The application of a force to a mass will cause it to accelerate as governed by one of Newton's laws of
motion. The relationship is that the force necessary is the product of the mass and the acceleration rate.
i.e. Force = Mass x Acceleration (1)
Here it is useful to point out that, in strict scientific terms, weight is the force acting on a mass resulting
from the influence of the acceleration due to gravity (which is constant for all objects).
The energy consumed in moving an object over a distance is the product of the force required and the
distance.
i.e. Energy = Force x Distance
Now, power is the rate of energy usage
i.e. Power = Energy/Time
And speed is the rate of travelling a distance
i.e. Speed = Distance/Time
These relationships may therefore be combined
so Power = Force x Speed (2)
This introduction provides two relationships that will reappear later on.
Units of Measurement
All physical quantities have some unit of measurement assigned to them in order to support these
relationships numerically. The standard system of units across the world is the Systeme International (SI),
from which many units are known colloquially as "metric". Within this system, the quantification of units is
based on 10s, 100s etc, with the main divide points every 1000 (e.g. millimetres, metres and kilometres).
Before this system was introduced, various other units were used, often referred to as "imperial", where
the links between sub-units were not so mathematically straightforward (e.g. inches, yards, miles).

42. The rest of this article will use SI units for all but miles, but the following section explains the units for
each of the quantities already introduced, and shows their conversion to imperial units which may well be
more familiar to many readers.
Quantity SI Unit SI Unit Imperial Unit Imperial Unit ConversionSI Imperial
Symbol Unit
Name Symbol Name Unit
(approx.)
Force Newton N Pound force lbf 1N 0.22 lb f
Mass Kilogram kg Pound lb 1kg 2.2 lb
Distance Metre m Yard yd 1m 1.09 yd
Distance Kilometre km Mile mile 1 km 0.62 mile
Time Second s
Speed Metres per m/s Miles per hour mph 1 m/s 2.2 mph
second
Speed Kilometres km/h Miles per hour mph 1 km/h 0.62 mph
per hour
Acceleration Metres per m/s/s
second per orm/s2
second
Energy Joule J
Power Watt W
With the SI unit system, a largely standard means of sub-dividing the units using a prefix is employed so
as to keep the figures quoted sensible. These are broken down in intervals of 1000, although some
intermediate intervals occur. The following table lists the commonly used prefixes. Note that the one
exception to these is the base unit of mass being the kilogram, with a thousandth of a kilogram being a
gram and a thousand kilograms being a tonne!
Prefix Symbol Interval
milli m 1/1000

43. centi c 1/100
deci d 1/10
1
kilo k 1000
mega M 1 000 000
Anyway, now we get to the trains at last……..
Getting Going
Tractive Effort
Tractive Effort (TE) is the name for the force applied to the rail by the wheel of the train to cause
movement. The size of that force is determined by the characteristic of the power equipment installed on
the train, and how the driver uses it.
By necessity, this tractive effort is not constant throughout the speed range, and most traction units have
a characteristic that looks something like Fig 1.
Fig 1:

44. In the example characteristic shown, the TE is constant up to 20 mph, therefore in this speed range, from
relationship (1) above, the acceleration will be constant. As a result of this, speed will build up uniformly
with time as shown in Fig 2. This is the region of Maximum Tractive Effort.
Fig 2:
Above this speed, the TE falls, and in consequence the acceleration will start to fall and speed will not
build up so quickly. The plot of speed with time, now starts to curve as shown in Fig 3.

45. Fig 3:
Power
Relationship (2) above says that power is the product of force and speed. Now, if the force, or TE were to
remain constant with increasing speed, the power requirement would continue to rise throughout the
speed range. Practically, this is not possible as the necessary equipment becomes unfeasibly big and
costly, so, when the maximum power capability (or rating) of the equipment is reached, the TE must start
to be reduced as speed increases to compensate. This occurs at the "knee" point at 20mph on the above
TE-speed curve (Fig 1).
So, in the example given, the maximum TE of the unit is 100kN, and hence the maximum power may be
calculated as follows:
Speed in m/s from above table = 20/2.2 = 9.1 m/s
Power = Force x Speed
= 100kN x 9.1 m/s
= 910kW
Fig 4:

46. As this is the power needed to actually move the train it is strictly referred to as the Maximum Power at
Rail.
In reality, the total power drawn from the supply (whether overhead wire, third rail, or fuel tank) will be
greater than 910kW, due to the need for additional auxiliary loads (for lighting, heating, cooling etc) and
due to losses in the conversion process, as nothing is 100% efficient.
Further, it is highly unlikely that the equipment is capable of running at this power level continuously, and
indeed for many types of service, it would offer little advantage relative to the associated cost. Again, for
reasons of rating the characteristic of the equipment will not follow the curve of maximum power to top
speed, as indicated by the dip from 70mph onwards in Figs 1 & 4. Consequently a continuous power
rating will often also be quoted.
This continuous power rating may be derived from a number of factors based around the equipment
characteristic and will including assumptions of proportion of time at a lower tractive effort demand
(driver's controller) or coasting.
Train Resistance
So that's how a train is controlled to get it moving, but in practice there are a number of other forces which
act to make life difficult.
Friction is always present where motion is concerned, and indeed, there is a certain minimum amount
which must be overcome before any movement can take place (often known as stiction!).
Air resistance, or drag, is another important factor which becomes increasingly significant with speed.
Pointed noses help reduce this.

47. These factors are accounted for mathematically using results found by measurement and experience, as
theoretical calculation would be far too complex.
Generally train resistance is expressed as:
2
R = a + bv + cv where v = speed
The factors a, b and c characterise the particular train, with a being the stiction referred to above, b arises
from other mechanical considerations, and c is due to the air resistance.
The train resistance typically looks something like that shown in Fig 5.
Fig 5:
There are further factors to take into account which depend on the route. The main one of these is
gradient, which brings in the effect of gravity.
If the train was travelling vertically upwards (i.e. it thought it was the space shuttle at take off), it would
incur the full effect of gravity. As explained earlier, the acceleration due to gravity is constant.
Mathematically, it is known as g (as in the term g forces in also the best quality intellectual films!) and is
2
9.81 m/s .
For example, for a 150 tonne (150 x 1000 kg) train, the gravitational force acting on it is:
Force = Mass x Acceleration
= 150 x 1000 x 9.81
= 1 471 500 N

48. = 1 471.5 kN
This is the weight of the train.
Now, even the Lickey incline isn't that steep, so the gravitational resistance practically encountered isn't
nearly so great. While it's not completely accurate, for the gradients encountered by trains, it suffices to
divide the weight by the gradient to obtain the value for this resistance.
So, for example if the above train were climbing a 1 in 200 gradient, the resistance due to gravity would
be:
1 471.5/200
= 7.3575 kN
This resistance is constant irrespective of speed and thus simply adds to the train resistance. When the
train is going downhill, this figure is subtracted from the train resistance - i.e. it assists the train.
The effect of gradient is seen in Fig 6.
Fig 6:
Now, how do these forces look compared to the Tractive Effort developed by the train
Fig 7:

49. As long as the train produces Tractive Effort greater than the overall train resistance, then it will
accelerate. The point at which the two curves cross is when it will cease to accelerate and is known as
the balancing speed and is the maximum speed attainable on that particular track. In the example here it
is 95 mph on the level, but 75 mph on a 1 in 100 gradient.
The force available to accelerate the train is the difference between the Tractive Effort and the train
resistance. Thus it will be realised that an earlier statement about constant acceleration, when the TE is
constant, is not strictly correct. In practice the acceleration will reduce as the resistance increases with
speed. Additionally it will be noted that train resistance becomes increasingly significant as speed
increases.
The following curve shows the actual build up in speed allowing for train resistance (Actual Characteristic)
compared with the theoretical build up in speed seen earlier in Fig 3 (Ideal Characteristic):
Fig 8:

50. Gear Ratio
In all the above discussions, gear ratio has not been mentioned. A gearbox links the traction motor shaft
to the train axle in order to step down the rotational speed since motors run much faster than axles! As
power = force x speed, and assuming that there are no losses in the gearbox, as the rotational speed at
the axle is reduced, the torque at the axle is increased. Consequently, re-gearing is often used as a
means of obtaining a revised traction characteristic to suit alternative service patterns without other
significant change to the traction equipment.
Wheel Diameter
Before finishing, it is also worth noting that this performance will not be maintained throughout the life of
the train, since, as the wheels wear down, the tractive effort characteristic will change! A change in the
wheel diameter is effectively a change of gear ratio, and consequently as the wheels get smaller the
starting TE will increase. However, as this also means that the axle speed becomes higher for any given
train speed, the TE at higher speeds will fall off more rapidly. When train performance is being predicted,
it is normal to assume the average half-worn wheel diameter.
Fig 9 illustrates the effect of wheel diameter on the TE characteristic.
Fig 9:

51. With all this information, it is therefore possible to calculate the performance of a train over a given route.
Example Route Performance Calculation - Appleby to Settle
To provide an example of such a calculation illustrating the various influences, a train with the above TE
characteristic (based on the average wheel diameter) is shown running over the Appleby to Settle section
of the Settle & Carlisle route (Fig 10), with stops at each of the intermediate stations. In this example, the
line speed limit has been falsely set to 85mph between Kirkby Stephen and Garsdale so as to illustrate
the effect of gradient on speed (see expanded profile Fig 11).
In the following two diagrams, the train speed is indicated by the bold red line, with line speed restrictions
indicated by the pink line. The gradient profile is illustrated by the green line, and is not to any scale.
Fig 10: