Technical paper on the subject of Direct Fuel Injection 2-stroke engine for range extender application

1. The Small Gasoline DI 2-Stroke Engine: an Keynote Paper
Presented in
Adapted Range Extender for Electric Vehicles ? SIAT-2011
Pierre Duret
Present Position:
Director of the Center for “Engines & Utilization of Hydrocarbons” at the IFP School
Educational Background:
1981: Graduated from the French Engineer School “Ecole Centrale de Paris”
Job Profile & Experience:
1982 – 1987: Research Engineer at IFP responsible of the study and development of two-
stroke engines with direct fuel injection
1987 – 1996: IFP “Two-Stroke Engines” Projects Leader, responsible of a research and
development group working on several projects of design and development of low emissions
high fuel economy two-stroke engines and gasoline controlled auto-ignition engines for
world-wide customers.
1996 - 2001: Assistant Director “Engines & Energy” at IFP
2001- 2003: Deputy Director of IFP “Engines & Energy” Technology Business Unit
Since September 2003, Director of the Center for “Engines and Utilization of
Hydrocarbons” at the IFP School
In parallel, since May 2005, Chairman of the “Powertrain” Committee of the French
Society of Automotive Engineers
R&D Recent Involvement
Expert for French Public authorities and for the European Commission in internal
combustion engines
Co-ordinator of several EU Projects, Network of Excellence and International Consortium
Projects driven by IFP
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Publications & Events
More than 30 families of granted patents and more than 50 international publications on
engines and powertrains for automotive and other applications
Organiser and chairman of several International Congresses on Powertrains
Six “Best paper” Awards including one at the SIAT’99
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The Small Gasoline DI 2-Stroke Engine: an Adapted Range
Extender for Electric Vehicles ?
Pierre Duret
IFP School, Rueil-Malmaison, France
ABSTRACT vehicle range (compared to the pure electric vehicle range)
The main purpose of this paper is to discuss the possibility with only a few litres of gasoline.
of using a small gasoline direct injected twostroke engine as
a range extender for electric vehicles. INTRODUCTION
In the first part of the paper, the most recently available In early 90’s, high fuel economy on a 500 kg concept car
results from DI two-stroke engines produced outside with a 2-cylinder 500 cc DI 2-stroke of 24 kW has already
automotive as well as the performances achieved in the past been demonstrated by the author [1-3] asshown in the
of some advanced DI two-stroke automotive concepts will be Fig.1. Nevertheless, this project was not further developed,
reviewed and compared with the required specifications for a in particular because gasoline DI (direct fuel injection)
range extender application. From this technical constructive technology was not mature at this period where in addition
review, it then becomes clearly possible to point out the emissions regulations were less severe than today’s and
advantages and limitations in considering the use of such future standards.
engine technology as a range extender of electric vehicles.
Then a detailed simulation study of a small electric
automotive vehicle equipped with a range extender is
undertaken and their results are presented. These calculations
are done for several vehicle specifications (especially in
terms of maximum performance when the vehicle operates
in range extender mode). Compared to its two-cylinder
four-stroke counterpart, it is expected that a DI two-stroke
would have a smaller displacement, size and weight, a lower
cost (significantly lower if a single-cylinder configuration
is chosen), much better NVH characteristics (if a two-
cylinder is chosen), easier and less expensive maintenance
and significantly higher fuel economy. In addition the lower
Figure 1. Fuel Economy Achieved in the Early 90’s with
maximum in-cylinder pressure of the two-stroke would make
a 24 kW DI 2-Stroke in a 500kg Concept Car
it particularly adapted to be combined with a starter generator
in the range extender application. From the simulation, it is
possible to understand that the main issue that would have It is particularly interesting to see that the first Ultra Low
to be carefully considered is probably the control of NOx Cost Car (ULCC) introduced in the Indian market weights
emissions to avoid the use of a costly DeNOx aftertreatment. 600 kg and is equipped with a 2-cylinder 623 cc 4-stroke
of almost the same power output (25 kW). And this vehicle
Finally the detailed results of the simulation show however has been homologated with a fuel economy 23,5 km/l. Even
that in the case of the range extender application, such if has been probably not obtained under the same conditions
target can be achievable provided that the engine operation (same driving cycle), it seems to be not as good as what
can be maintained in the ultra low NOx Controlled Auto- was obtained in the past with the DI 2-stroke.
Ignition (CAI) combustion range. Beside the achievement
of the Euro 6 NOx target, remarkably low level of average Beside this new interest for ultra low cost passenger car,
CO2 emissions can be achieved with impressively increased there is also a recent trend in powertrain development
towards progressive electrification. Among the variouslevel
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of powertrain electrification, the Electric Vehicle (EV) is Taking into account these two considerations (results achieved
the extreme one. Even if a lot of car manufacturers became in the 90’s with a DI 2-stroke with a non mature technology
more and more involved in this direction during the last few & recent availability of well proven DI 2- stroke technology
months, it is generally considered that, due to its drawbacks outside automotive) it seems natural to wonder if for the ultra
mainly linked to the electric energy storage system (batteries low cost car as well as for the range extender application, a
are very expensive, very heavy and need a lot of space for small DI 2-stroke engine could be a well adapted engine in
their packaging in the vehicle), the purely electric vehicle will place of the more conventional 4-stroke engine technology
be limited for some specific applications. This could change widely used in automotive applications. The Ultra Low Cost
in case of drastic progresses from the batteries in terms Car (ULCC) application of the DI 2-stroke engine has already
of cost and energy storage. But during the transition, the been studied in details in a previous paper [5]. In this new
solution to increase the chance of acceptance of EV by the paper the discussion will be specially focused on the range
public in a large scale could be to keep a limited pure EV extender application.
range (with therefore minimum battery cost) corresponding
to most of the urban usages and to equip the vehicle with THE PRINCIPLE ADVANTAGES OF
a lightweight range extender. Such range extender would THE 2-STROKE CYCLE
allow to exceptionally multiply by several times the pure
EV range without sacrifying the global CO2 emissions. And The 2-stroke engine is well known for its main followingv
again as for the Nano example, it is interesting to remind specific advantages resulting from the principle of the 2-
ourselves that Citröen presented in 1998 at the Paris Auto stroke cycle [1]:
Show an electric vehicle (based on a Saxo Citröen model) 1. Low friction losses: this is particularly true with pump
and equipped with a small direct injected gasoline 2-stroke crankcase configuration (roller bearings for crankshaft,
engine as range extender. This innovative vehicle (vehicle rod and piston pin; no oil ring retainer; no valves train
mass 1050 kg; max speed 120 km/h) was presented with to drive, one driving cycle every revolution), no oil
a pure EV range of 80 km and an extended range up to pump to drive especially during cold start;
340 km. The auxiliary power unit used was a prototype
DI 2-stroke engine technology, 2 cylinder opposite 200 cc, 2. Low pumping losses : the pumping work decreases
delivering a power of 6,5 kW and directly coupled with a in absolute value (almost constant in relative value as
starter generator. The auxiliary power unit (thermal engine shown by Figure 3b) when the load decreases. It is the
+ starter generator) was remarkably packaged with overall contrary in a SI 4-stroke (Fig. 3a)
dimensions of vol. 30x30x25 cm & a mass of 20 kg.
3. Double combustion cycle frequency when compared to
With such small size, it was possible to implement this a 4-stroke engine
auxiliary unit under the rear seat of the Saxo car. A schematic
view of the whole powertrain of the car is presented in The advantages 1 and 2 result in significantly higher effective
Fig. 2. power for the same indicated power, especially at part
load as shown by the Fig. 3a and 3b. This should give a
potentially higher 2-stroke fuel economy than SI 4-stroke.
The advantage 3 results in higher specific torque and power
output but nevertheless lower than 2 times the power of
an equivalent 4-stroke because all the expansion stroke is
not useful for producing power (exhaust port opens during
the last part of the expansion stroke). As a consequence,
the size and weight of a 2-stroke can be much smaller. It
Figure 2. The Citroën Saxo Dynavolt: an Electric
also allows to have drastically better 2-stroke NVH (noise,
Vehicle Concept Presented in 1998 with a Small DI
vibration and harshness) characteristics as we will also see
2-Stroke Engine as Range Extender [4]
later in this paper.
However in a classical carburetted 2-stroke engine, the
But this very interesting project was not further investigated potential fuel economy advantages 1 and 2 are unfortunately
after the 1998 Paris Auto Show for two main reasons: firstly, masked by the main 2-stroke drawbacks:
it was not the right period for electric vehicles (too much in
advance !) and again 2-stroke gasoline DI technology was 1. The short-circuiting of fuel directly to the atmosphere
not yet mature ! (above 50 % of maximum engine load) solved by DI
(Direct fuel Injection)
In parallel, during the last decade the DI 2-stroke technology
has been further developed outside automotive and successfully 2. The poor combustion or misfiring (below 50% of
applied in production for marine outboards and 2-3 wheelers maximum engine load) solved by combined CAI
engines. (Controlled Auto-Ignition) & DI
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Figure 3a. Distribution of Indicative Power in Figure 4. Negative Effect of Losses from Mixture
Effective Power, Friction Losses and Pumping Losses Shortcircuiting and of Losses from Irregular Combustion
Versus Engine Load in a 4-Stroke Engine on Specific Fuel Consumption of a Carburetted 2-Stroke
Engine [6]
2. Its combination with CAI combustion (Controlled Auto
Ignition) for NOx emissions control and improved
combustion stability [3,11,12] with the AR (Activated
Radicals) combustion as an example of production
available technology [6,13,14,15].
THE KEY SUCCESS FEATURES OF
THE DI 2-STROKE ENGINE FOR
RANGE EXTENDERS OF ELECTRIC
VEHICLES
After this introduction, the section of this paper will be
organized in four main sub sections discussing the four main
Figure 3b. Distribution of Indicative Power in Effective issues that can be considered are key success features of
Power, Friction Losses and Pumping Losses Versus the DI 2-stroke engine as a powertrain for range extenders:
Engine Load in a 2-Stroke Engine Simple, lightweight and compact: DI + exhaust throttling CAI
- NVH issues and low production cost: single-cylinder +
These two different sources of unburned fuel and therefore balancing shaft or 2-cylinder without balancing shaft
of poor efficiency are clearly illustrated in the Fig. 4 as a - Easy maintenance and high fuel economy for low
function of engine load. operating cost: 2-stroke principle advantages and
downsizing
The technologies to solve these two drawbacks already exist
and have been successfully introduced in production several - DeNOx free emissions control: oxidation catalyst
years ago outside of automotive: with fast cold start lighting and CAI combustion for
aftertreatment free NOx emissions control
1. The gasoline direct fuel injection for HC emissions
control and best fuel economy with several examples
of production available technologies: The 2-Stroke Engine: A Simple, Compact
and Lighweight Powertrain for Range
- Air assisted direct fuel injection on marine outboard
engines, autorickshaw, 2-wheelers [7] Extender
- IAPAC compressed air assisted fuel injection on This is a well-known advantage of the conventional 2-stroke
marine outboard engines [8,9] engine versus 4-stroke. The following Fig. 5 showing the
compact range extender DI two-stroke engine arrangement
- Direct liquid fuel injection on marine outboard under the rear seat of the Saxo Dynavolt clearly illustrate
engines [10] this advantage.
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The reduction of the cylinder unit displacement is nevertheless
limited towards low values by the increase of losses and the
decrease of efficiency. On the other side, the reduction of
the number of cylinders is limited by turbo charging and
NVH issues.
A 2-cylinder 4-stroke presents only one combustion cycle
every engine revolution. It needs a balancing shaft to improve
its NVH characteristics. For cost reduction, the best would
be to use a single-cylinder engine, but if the 4-stroke cycle
is still used, it would mean only one combustion cycle every
two engine revolutions and therefore unacceptable NVH
behaviour for automotive application.
To use the 2-stroke cycle would double the combustion
frequency and then provide an interesting solution to NVH
issues at minimum production cost. A 2-cylinder opposite
2-stroke engine configuration (as shown by the Citroën
Saxo example) would be the best solution in terms of NVH
issues for a range extender application. A single-cylinder
2-stroke engine is even possible for minimum cost with
NVH performance equivalent at least to a 2-cylinder 4-stroke
engine and even better as shown in a previous paper [5].
Low Production Cost Engine Characteristics
The 2-stroke engine is also particularly interesting in terms
Figure 5. The Minimum Size of the 200 cc DI 2-Stroke of production cost. Its lighter weight means less materials
Range Extender Engine Installed under the Rear Seat of and therefore less raw materials cost. It is a simpler engine
the Saxo Dynavolt [4] with much less components:
the complete 4-stroke valve train system is deleted, in
In the previous section, we already explained that the addition if a 2-cylinder 2-stroke is used, there is no need
2-stroke configuration adapted for such range extender of balancing shaft
application should include DI technology combined with CAI
There is also a way of significant further production cost
combustion. As shown by several previous studies, the most
saving (without sacrifying the NVH behaviour as explained
cost effective solution for implementing the CAI combustion
previously) if a single-cylinder 2-stroke is used in place of
in a gasoline two-stroke engine is to use an exhaust throttling
a 2-cylinder 4-stroke:
device [2,16]. This device allows to control the exhaust back
pressure and consequently the upstream internal scavenging
and stratification process between the fresh charge and the there is still no valve train (but a balancing shaft becomes
residual gases. To use an AR (Activated Radical) exhaust necessary as in 2-cylinder 4-stroke) the number of moving
valve [6,13] or a transfer throttling valve [17,18,19] could parts (pistons, rings, rods,...) is reduced (divided by two)
be slightly more efficient solutions but at a rather significant the number of fixed parts (fuel supply and injectors, ignition
incremental cost not justified for a range extender application. system,....) is also similarly reduced some parts become
simpler: intake and exhaust manifold, crankshaft,...
The DI 2-Stroke Engine: A Low Production
For all these reasons, the 2-stroke engine technology can
Cost Powertrain with Significant NVH be considered as probably the cheapest to produce while
Advantages in parallel giving the best NVH characteristics. What can
have a negative impact on the cost of a DI 2- stroke are
NVH Issues and Low Production Cost mainly the direct injection system and the possible need for
For a range extender application, a small size small an expensive specific DeNOx aftertreatment. Concerning the
displacement engine is required for both compactness cost of DI 2-stroke technology, the progress done during the
and lightweight (as described before) and also for best last few years and its various applications outside automotive
efficiency. The reduction of the overall engine displacement show that it can be probably considered as slightly higher
can be achieved by two different ways: the reduction of the but almost similar to the cost of 4-stroke port fuel injection
cylinder unit displacement and the reduction of the number technology. Concerning the NOx emissions control, we
of cylinders. will also see in a following section that there are some
possibilities to achieve it without specific aftertreatment.
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This is a key issue to keep the 2-stroke inherently low reduces HC emissions to a level almost similar (or slightly
production cost. higher) than 4-stroke, NOx emissions are significantly
lower due again to the principle of the 2-stroke cycle (one
The DI 2-Stroke Engine: Easy Maintenance combustion every cycle with half the 4-stroke IMEP) and of
and High Fuel Economy For Low Operating the inherent internal EGR dilution.
Cost There are some possibilities for further reduction to ultra
low level at low load thanks to the CAI combustion, Raw
Easy and Lower Maintenance Cost emissions of CO are generally significantly lower (lean burn
operation at part load)
for the customer The following 2-stroke engine specific
features have to be considered by the customer as providing A significant amount of scavenging air is directly short-
easier maintenance at a lower cost: circuited and lost in the exhaust which means that there is
always an excess of O2 in the exhaust.
the 2-stroke mechanics is the simplest one and therefore some
limited maintenance operations can in some cases be directly This has two main consequences:
done by the user himself, as it is done for example in India, - The exhaust conditions are highly favourable for
there is no requirement of oil change as in a 4-stroke engine. providing high efficient HC and CO conversion by
The oil tank can be easily refilled by the user himself on a an oxidation catalyst
regular basis as it would be recommended by the manufacturer - To maintain a minimum cost, a conventional 3-way
This is something which has to be positively considered for catalyst aftertreatment cannot be the solution for NOx
a low cost automotive range extender application. reduction and therefore the raw emissions of NOx have
High fuel economy for low operating cost Several examples to be maintained very low in order to avoid complex
of DI 2-stroke engines in production outside automotive show DeNOx aftertreatment in oxidizing conditions
the 2-stroke versus 4-stroke higher fuel economy thanks to
the principle advantages of the 2-stroke cycle. To illustrate it, If we look now again to some examples of DI two-stroke
five different examples of applications have been described engines, we can start first with the liquid direct fuel injected
in a previous paper [5]. : 2-stroke outboard [10]. What is remarkable with this engine
is that it is the first (and only one) outboard engine that
50 cc 3,5 kW single-cylinder scooter application in Europe received the Clean Air Excellence Award of the US EPA
144 cc 6,6 KW single-cylinder 3-wheeler application in India ! Its raw emissions performances were compared with
250 cc 20 kW single-cylinder DI AR 2-stroke (compared to other technologies Including Fuel Injected (EFI) 4-stroke
400 cc 4-stroke) for large scooter application technology. Almost the same HC + NOx emissions were
obtained with significantly better CO emissions.
680 cc 37 kW 2-cylinder marine outboard application
1230 cc 52 kW 3-cylinder DI CAI 2-stroke automotive
CAI Combustion for Aftertreatment Free NOx
prototype compared to 1360 cc 4-stroke Emissions Control
This paper clearly show the benefits in terms of fuel economy The emissions specifications for future vehicles will require to
of the DI 2-stroke versus the 4-stroke engine. And this benefit meet a level similar to Euro 6: with high efficient oxidation
is increasing when the engine size is decreasing (due to the catalyst (close coupled metallic substrate) and fast lighting
incremental effect of the lower friction losses). control strategy for HC and CO emissions control and with
aftertreatment free NOx emissions control The strategy used
Nox Aftertreatment Free Emissions Control: for this purpose is also already described in details in a
the Main Issue Of DI 2-Stroke for Range recent paper related to the Ultra Low Cost Car application
[5]. NOx emissions can be controlled by using the ultra low
Extender NOx CAI (Controlled Auto-Ignition) combustion.
This section deals with the emissions of a DI Two-stroke We can conclude from this second main section of this paper
engine and about their control. that a small DI 2-troke engine presents some specific features
– simple, compact and lightweight, low production cost with
Oxidation Catalyst for HC and CO Emissions
significant NVH advantages, easy maintenance and high fuel
Control economy for low operating cost, NOx aftertreatment free
DI 2-stroke engines present different emissions profiles than emissions control – that make it particularly well adapted
4-stroke engines. : as a powertrain for ultra low cost passenger car application
or as a range extender for electric vehicle.
HC emissions are generally higher (intake and exhaust open
simultaneously in the 2-stroke cycle) but DI drastically
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THE DI 2-STROKE ENGINE EV The engine displacement is not fixed and will be determined
(as it will be described in the next subsection) according
RANGE EXTENDER APPLICATION to the thermal engine power required to meet the target of
In this third main section of the paper, we will study more in maximum vehicle speed achievable in range extender mode
details the DI 2-stroke range extender application for electric only. To fix the engine displacement, we considered a specific
vehicles. For this purpose we undertook a simulation study. power of 42 kW/l which is easily achievable in a small DI
The conditions of this simulation will be first introduced and 2-stroke engine at a rather moderate maximum engine speed
then the results will be presented and discussed. (4500 rpm) in order to minimize engine noise.
The combustion system of the DI 2-stroke engine is chosen
Conditions of the Simulation Study to avoid stratified charge direct injection generally responsible
Vehicle Specifications of higher NOx emissions. Indeed since the control of NOx
emissions without after treatment is probably the most
We chose for this study an urban type of EV (electric important key issue, we prefer to chose the ultra low NOx
vehicle). Its specifications are summarized in the Table 1 CAI (Controlled Auto-Ignition) mode at part load and to keep
here below. homogeneous charge when the engine load increases. For a
low cost small two-stroke engine, the simplest solution to get
Table 1. Electric Vehicle Specifications the CAI combustion mode will be to use an exhaust throttling
control valve, the position of the valve being controlled by
the engine management system as a function of the engine
load (intake throttle position sensor) and the engine speed.
Regarding the exhaust conditions, due to the inherent 2-stroke
scavenging process, there will always be an excess of short-
circuited air in the exhaust. A closed
loop 3-way catalyst cannot therefore be used. This is the
reason why raw emissions of NOx have to be sufficiently
It can be seen in the Table that a small urban vehicle was low to meet the legislation without complex and costly
chosen. We chose a vehicle mass of 580 kg (similar to the DeNOx aftertreatment system. We also consider that an
Tata Nano used in the ULCC study [5]) with two possible oxidation catalyst has to be used for the control of CO and
pure electric range. A load of 75 kg corresponding to the HC emissions. The excess of short-circuited air in the exhaust
driver was added. In the case of a 60 km EV range, the gases allows the oxidation to be extremely efficient. In DI
additional battery mass used in the simulation is 51 kg for 2-stroke engine applications, a metallic type of oxidation
such small and light vehicle. It is increased to 104 kg for catalyst is preferred in order to obtain catalyst lighting at
the 120 km electric range. low exhaust temperature.
During all the following simulations we also considered that Finally for all the simulations, we used engine efficiency
the vehicle was equipped with advanced low friction tyres (BSFC and CO2 emissions) as well as raw emissions of
and that there is a permanent electric power consumption of NOx coming directly for the extensive IFP DI 2-stroke
150 W (power required by the auxiliaries). engine data base build during the last 25 years of experience
[1,2,3,17,18].
Specifications of the thermal engine used as range extender
Regarding the thermal engine used as range extender, its Efficiency of the Starter Generator, of the Electric
specifications are described in the following Table 2. Motor and of the Battery
The main simplified assumptions used in the simulation
Table 2. Specifications of the DI 2-Stroke Engine used
regarding the efficiency of the energy conversion components
as Thermal Engine Range Extender
are summarized in the Fig. 6.
As shown by the figure, we assume an efficiency of 0,9
for the starter generator to produce electric power from the
thermal engine power. The efficiency of the electric motor
is also assumed to 0,9 in both directions, to produce power
to the wheels or reversely to recover energy during braking.
Concerning the battery, we also use a simplified average
efficiency of 0,8 for the storage of electric energy (coming
either from the generator coupled with the thermal engine
or from the electric motor during braking energy recovery)
and for its redelivery from the battery to the electric motor.
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Obviously more precise and more optimized efficiency data cc, it means that the preferred configuration (for minimum
could be used compared to what is used in this study. cost) will be to use a single cylinder engine if a REX vehicle
Nevertheless, it is important to point out that the main top speed of no more than 110 km/h is targeted, which will
purpose of this study is too really show the potential of DI be the most probable case for such type of urban vehicle.
2-stroke engine (especially to demonstrate the capability to Above such targeted RE vehicle top speed of 110 km/h a
meet NOx emissions legislation without DeNOx and to show two cylinder engine would probably be necessary.
the low CO2 emissions and range extension potentials). It is
not at this stage to predict an actual project of range extender.
Figure 6. Schematic View of the Thermal Engine and
Starter Generator Package, of the Electric Motor, of the Figure 7. DI 2-Stroke Engine Displacement Versus
Battery and of the Energy Management System Including Targeted Vehicle Top Speed in Range Extender (RE)
Corresponding Efficiencies Mode
Dimensioning of the DI 2-Stroke Thermal Instantaneous Power Required to Drive
Engine the Vehicle and Distribution of the
Corresponding Energy Fluxes
The power required for the thermal engine range extender
(REX) depends on the target for the maximum vehicle speed Instantaneous Power Required to Drive the Vehicle
achievable in range extender mode only (which means with on the NEDC Cycle
battery almost empty or with a charge below a minimum
acceptable level). We made calculations of the power required The simulation model used is based on Excel. It calculates
to drive the vehicle for different choices of top speed from the instantaneous power required by steps of 0,5 second (as
60 to 120 km/h. These calculations were done with a road mentioned before, this instantaneous power include the 150
slope of 3% in order to give some margin in the use of the W permanent electric power consumption). The Fig. 8 shows
vehicle. From such calculations and taking into account the an example of calculation for a vehicle with a top speed of
different efficiencies described in the previous subsection, 80 km/h in range extender mode. According to the previous
it is then possible to calculate the corresponding engine figure, such vehicle is then equipped with a thermal engine
power. From this engine power, and considering a DI of 273 cc with a maximum power of 11,5 kW.
2-stroke specific power of 42 kW/l, we can then obtain the
engine displacement necessary versus the vehicle top speed
targeted. Such results are reported in the following Fig. 7.
Two curves can be seen, each one corresponding to two
different EV range.
The dotted line is slightly above the full line because with
120 km EV range the vehicle is slightly heavier (+ 53 kg of
battery) which explains the slightly higher engine isplacement
required. Nevertheless, as it can be seen in the figure, the
differences between the two curves are very low.
From such figure, it can be seen that if a vehicle top speed
of 60 km/h is targeted in REX mode only, a DI 2- stroke
engine of about 170 cc is sufficient while an engine of
600 cc is necessary for a vehicle targeted top speed of 120 Figure 8. Instantaneous Power Required to Drive the
km/h. If we consider that the largest unit displacement used Vehicle Versus Time During the NEDC Driving Cycle
in small DI 2-stroke engine is generally no more than 500 (with 80 km/h Maximum Vehicle Speed)
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Two curves are presented on this figure: the vehicle speed
(scale on the left side of the figure) versus time and the
corresponding instantaneous power required to drive the
vehicle (scale on the right side of the figure) along the
NEDC cycle with maximum speed limited to 80 km/h. It can
be seen that such instantaneous power oscillate a lot being
maximum during the accelerations, being very low during
the vehicle stabilized speed (only the remaining 150 W
when the vehicle is stopped) and becoming negative during
deceleration and braking.
Distribution of the Energy Dluxes During NEDC
Driving Operation
Figure 10. Relative State of Charge of the Battery
In this example the calculated average power required by During the NEDC Cycle in RE Mode
the vehicle along all NEDC cycle from the generator is 1,94 (with 80 km/h maximum vehicle speed)
kW, which means 2,15 kW delivered by the thermal engine
(with 0,9 efficiency of the generator).
This figure shows that the state of charge of the battery
This average electric power supplied by the engine/generator globally increases during the urban part of the driving cycle
package is plotted on the Fig. 9 (full line with constant (even if some limited decrease can be observed during each
value). This figure presents also the instantaneous power acceleration) and then decreases significantly during the
required by the vehicle (dotted line) and the power supplied stronger accelerations of the extra urban part of the cycle.
by the battery. In this figure, the instantaneous power of the At the end, the battery state of charge is even slightly higher
battery is negative when the battery supplies electric power to than at the beginning because of the energy recovery during
the electric motor and is positive when the battery is loaded the last deceleration and braking.
by electricity coming either from the electric motor (during
braking) or from the generator (when it supplies an excess Final Results: NOx Emissions in Rex Mode,
of electric power).
Average CO2 Emissions and Electric Vehicle
Extended Range
Relation between Thermal Engine Operating Load
and NOx Emissions
From the Fig. 7 we have seen that the thermal engine
displacement can be defined. Then from the calculations of
the cycle and the example given in Fig. 9, we can get the
thermal engine average power out put necessary to perform
the NEDC cycle in range extender mode. The Fig. 9 gives
a thermal engine average power of 2,15 kW (before the
generator) for a maximum vehicle speed of 80 km/h in
Figure 9. Instantaneous Power Distribution between the REX mode. For such given power, we made emissions and
Vehicle, the Battery and the Thermal Engine (after the efficiency/CO2 calculations for three different engine speeds:
Generator) Versus Time During the NEDC Cycle in RE 4000, 2500 and 1500 rpm. The new Fig. 11 shows that
Mode (with 80 km/h maximum vehicle speed) for a maximum speed of 80 km/h (which means a 273cc
11,5 kW DI 2- stroke engine) such average power can be
Battery State of Charge During the NEDC Cycle obtained with a BMEP of 1,02 bar @ 4000 rpm, of 1,63 bar
@2500 rpm and 2,73 bar@1500 rpm. This figure presents
in REX Mode the other engine BMEP versus the maximum vehicle speed
It is important to point out that to perform all the simulations in REX mode (which is directly correlated with the engine
in range extender mode, the main assumption is that when displacement as shown in Fig. 7 and repeated in the right
the NEDC cycle is operated in range extender mode, the axis of this figure).
thermal engine power is chosen in order to be neutral in In the next Fig. 12, we plotted the calculated BMEP of the
terms of battery state of charge & discharge. This is clearly Fig. 11 versus engine speed for the four limited vehicle top
shown by the Fig. 10 (which still corresponds to the same speed of 60, 80, 100 and 120 km/h (which correspond to
example of 80 km/h limited vehicle top speed in REX mode). a respective thermal engine displacement of 171, 273, 415
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and 606 cc). We also add in this figure the typical ultra low • Condition A: the on board electric energy storage is
NOx CAI combustion range. This CAI range is what can be fully charged.
expected with the combination of an exhaust throttling valve
with DI which is the simplest and cheapest way of getting • condition B: the on board electric energy storage is at
CAI in a DI 2-stroke. The definition of this range is based its minimum state of charge. To reach this condition,
on the IFP DI 2-stroke database [5]. the vehicle is run at 50 km/h until the thermal engine
start and the vehicle is stopped.
From this figure, it can be seen that engine speed of 2500
rpm and 4000 rpm are fully inside the ultra low NOx CAI The measurement of emissions then start after a maceration
combustion range. On the contrary, the lower 1500 rpm is period. From our understanding of the legislation, it seems
outside the range and we will see in the next subsection that, that the pollutant emissions limits will have to be met in
as we could expect, NOx emissions will be much higher at both conditions. In the example of our study, our simulated
this engine speed. vehicles have a pure EV range of either 60 or 120 km/h.
It means that there are both able to perform the condition
A of the NEDC cycle in pure EV, which means without
any pollutant emissions. For the condition B, the vehicle
has then to meet the Euro 6 legislation in REX mode.
Regarding HC and CO, there are generally low in a DI
2-stroke engine and easily converted by an oxidation catalyst
as already discussed before and demonstrated in several
papers [5,7,18,19]. They have therefore not been estimated
in this study considering that the main key issue will be the
NOx without aftertreatment. The NOx emissions have been
estimated based on the data available in the IFP DI 2-stroke
engine data base. The results are reported in the following
Fig. 13 and 14.
Regarding the calculation of the CO2, the legislation proposes
Figure 11. DI 2-Stroke Engine BMEP for 3 Different a method to calculate an average weighted value depending
Speeds Versus Limited Vehicle top Speed in RE Mode on the EV range.
The formula used in such method is:
M = (De x M1 + Dav x M2) / (De + Dav)
in which:
M = average weighted mass emissions of CO2 in g/km
M1 = mass emissions of CO2 in g/km in condition A
M2 = mass emissions of CO2 in g/km in condition B
De = range of the vehicle in pure electric mode (measured
according to the Annexe 9 of [22])
Dav = 25 km (assumed average distance between two battery
charges)
In our study, we can consider that with an EV range of 60
or 120 km, our simulated vehicle can perform the NEDC
Figure 12. DI 2-Stroke Engine Pperating BMEP in RE condition A in pure EV mode. This means that M1 = 0 and
Mode During NEDC Driving Cycle for 4 Engine the formula becomes:
Displacements (= 4 vehicle top speeds in REX mode)
M = (25 x M2) / ( 25 + 60) = (25 x M2) / 85 in g/km for
the simulated vehicle with a 60 km pure EV range and
NOx Emissions Results in REX Mode and Average
M = (25 x M2) / (25 + 120) = (25 x M2) / 145 in g/km for
CO2 Emissions the simulated vehicle with a 120 km pure EV range.
We studied the emissions legislation that will be applied Based on these two formulas, the average CO2 emissions of
for plug in hybrid and for EV with range extender. The the 6O km EV range vehicle are plotted in Figure 13 and the
legislation [22] considers two conditions of operation of the average CO2 emissions of the 120 km EV range vehicle are
vehicle: plotted in Fig. 17, both together with the NOx emissions in
REX mode only. All the results are presented for a limited
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12. Symposium on International Automotive Technology 2011
vehicle top speed from 60 to 120 km/h in REX mode. Therefore to use the lowest engine speed allowing to be
in CAI combustion mode, which means around 2500 rpm
It is interesting to see that both figures 13 and 14 show that
as shown by this study, would provide the best trade off in
when the DI 2-stroke engine runs at low engine speed such
terms on average CO2 versus NOx emissions. This conclusion
as 1500 rpm, its raw emissions of NOX are too high to meet
is valid for each vehicle EV range and for each vehicle
the Euro 6 limit. On the contrary, with higher engine speeds
limited maximum speed in REX mode. This confirms that
such as 2500 rpm and 4000 rpm, the Euro 6 limit can be
there is a great flexibility in the choice of the displacement
met without NOx after treatment and with some margin.
of the DI 2-stroke engine and therefore of the maximum
This is well correlated with the Fig. 12 and confirms that
vehicle speed in REX mode. We can also see that the lowest
the DI 2-stroke range extender must preferably run in CAI
CO2 (about 15 g/km with 60 km EV range and less than 10
combustion at part load to take benefit of the ultra low NOx
g/km with 120 km EV range) are obtained when the vehicle
emissions of this combustion mode.
top speed is limited to 60 km/h. With a more reasonable
Regarding the average CO 2 emissions, the situation is 80 km/h limited vehicle speed, the CO2 emissions remain
opposite. The lowest engine speed gives the best CO 2 nevertheless quite low with less than 20 g/km with 60 km
emissions. EV range and just above 10 g/km with 120 km EV range.
DI 2-Stroke: an Efficient Solution to Extend the
EV Range
Finally to conclude this study, we calculated the extension
of the EV range if the vehicle is equipped with a fuel tank
of 10 litres of gasoline. The last Fig. 15 shows the results
for the 60 km EV range vehicle versus the thermal engine
displacement / limited vehicle top speed in REX mode.
Figure 13. NOx Emissions (G/Km) in REX Mode Only
and Corresponding Average CO2 Emissions for 3
Different Thermal Engine Operating Speeds Versus
Limited Maximum Vehicle Speed in REX Mode with 60
Km Range in Pure EV Mode
Figure 15. Extension of the Pure EV Range with Various
DI 2-stroke range extender displacement / with various
limited vehicle top speed Again it can be seen that with
the smallest thermal engine displacement (the lowest top
speed in REX mode) the vehicle range can be impressively
increased (multiplied by 9) with only 10 litres of fuel. With
a less limited vehicle speed in RE mode such as 80 km/h,
the pure EV range of 60 km can even be extended up to
more than 400 km with such low amount of fuel.
CONCLUSION
The main purpose of this paper is to review in details
Figure 14. NOx Emissions (G/Km) in REX Mode Only the most recently available results from DI two-stroke
and Corresponding Average CO2 Emissions for 3 engines recently produced outside automotive as well as the
Different Thermal Engine Operating Speeds Versus performances achieved in the past of some advanced DI two-
Limited Maximum Vehicle Speed in REX Mode with stroke automotive concepts, and to compare them with the
120 Km Range in Pure EV Mode required specifications for an ultra low cost car application as
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13. Symposium on International Automotive Technology 2011
well as for a range extender application. From the technical stroke powertrain. Finally, India, with its great expertise in
constructive review presented in the first two main sections high efficiency small engines as shown by the 2-stroke DI
of the paper, it then becomes clearly possible to point out the commercialized in autorickshaw [21], could take a leading
advantages and limitations in considering the use of such position in achieving such challenge.
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ACKNOWLEDGMENTS
The author would like to particularly thank Thierry Colliou
of the IFP Energies Nouvelles for the most recent engines
data he provided and for the very useful calculations he
made and that were used in this study and Yoichi Ishibashi
of Honda R&D for his precious advices and support, and
for some materials and results used in this paper.
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