6. • Each stroke takes 180° of
crankshaft rotation to complete
• All cylinders fire in 720° of
crankshaft rotation
• 720 divided by number of
cylinders = firing interval
• Odd fire V-6 engine (90° block
with 120° rod journals)
Four stroke cycle
(Petrol)
6
11. 11
Engine Cooling System
Engine heat is transferred
through walls of the
combustion chambers and
through the walls of cylinders
12. Piston Dwell Time
Piston travel is at a minimum
TDC and BDC
Crank moves horizontally
Piston velocity
Maximum when rod is 90° to crank
Acceleration
Maximum 30° earlier
Best VE is obtained by synchronizing valve
opening with piston speeds
12
13. Other Valve Position
Overlap
• Both valves are open
• End of exhaust & start of intake
• Low pressure in exhaust port
Blow down
• Exhaust valve opens before BDC
• To help evacuate cylinder before piston
reverses
• Pumping losses at end of exhaust
stroke
13
14. Valve Mechanism
• Intake valve opening
BTDC
Low pressure in cylinder Intake valve closing
• Intake valve closing
ABDC
Cylinder pressure is effected by timing
• Exhaust valve opening
BBDC
Residual pressure helps blow down
• Exhaust valve closing
ATDC
Low pressure in exhaust port draws air in
mmpmm 14
15. Effects On Valve Timing
Intake valve opening
Late – Reduced VE
Early – Dilution of intake with exhaust
Intake valve closing
Late – Reduces cylinder pressure
Early – Increases cylinder pressure
Exhaust valve opening
Late – Pumping losses
Early – Power reduction
Exhaust valve closing
Late – Reduces vacuum
Early – Reduces VE
15
16. COMBUSTION
Spark ignition
Maximum cylinder pressure 15° ATDC
Tumble and swirl
Motion reduces misfires
Excess motion inhibits flow
AFR 14.7:1 at part throttle, 12.5:1 under load
Compression ignition
18:1 direct injection
23:1 pre-chambers for better starting
Compression heats to 800-1200 °F
16
17. Valve Mechanism
OHV (overhead valve)
Pushrod configuration
Many reciprocating parts
Higher valve spring pressure required
Compact engine size compared to OHC
17
18. OHC (overhead cam)
Fewer reciprocating parts
Reduced valve spring pressure required
Higher RPM capability
Cylinder head assemblies are taller
18
Valve Mechanism
21. Hydraulic lifters
To maintain zero lash
Quieter No periodic
adjustment
Anti-scuff additives
are required in oils
Valve lash compensators
21
22. Hydraulic lifter operation
Valve closed
• Oil flows through lifter
bore & past check
valve
• Plunger return spring
maintains zero lash
22
23. Valve open
• Check valve seats
and limits the slippage
• Now operates as a
solid lifter
Hydraulic lifter operation
23
24. Hydraulic lifter operation
Return to valve closed
• New oil enters the lifter
body
• This oil replaces oil
that has leaked
between plunger and
body (predetermined
leakage)
24
31. Oil pan baffles
• To keep oil in sump during braking,
accelerating, and cornering
31
Lubricating System
32. Oil pan windage tray
• To prevent oil aeration in the sump
32
Lubricating System
33. Oil pumps
• Driven by distributors, gear
on camshaft, or crankshaft
33
Lubricating System
34. Oil pumps with pressure relief valves
• Gear type pump
• Rotor type pump
34
Lubricating System
35. Full flow oil filtering system
• Oil pump output flows
through filter first
• Bypass circuit for restricted
filters will allow oil to
flow to engine
35
Lubricating System
36. Diesel Engine
Advantages
• Higher engine torque
• Better fuel economy
• Long engine life
Disadvantages
• Engine noise
• Exhaust smell
• Hard start
36
