Two-stroke and four-stroke engines have different valve timing strategies. Combustion in engines occurs in distinct phases - ignition lag, flame propagation, and after burning in SI engines, and ignition delay, premixed combustion, controlled combustion, and after burning in CI engines. Factors like fuel type, engine speed, load, and air-fuel ratio affect the timing and progression of combustion.

1. VALVE TIMING AND COMBUSTION
PHASING IN IC ENGINES
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2. Two Stroke IC Engines
 Compression stroke
 Power stroke
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3. Two Stroke SI Engines
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• Intake port is uncovered at the end of power stroke before
BC and is blocked again at the start of compression stroke

4. Two Stroke SI Engines
Compression stroke
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5. Two Stroke SI Engines
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TDC BDC
Piston gets to
TDC

6. Two Stroke SI Engines
TDC BDC
At the end of power stroke exhaust port is uncovered and
exhaust starts leaving the chamber while intake is close
Still closed

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2-Stroke SI Cycle Valve timing
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8. 2-stroke Diesel Cycle
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9. 9
2-Stroke Diesel

10. Valve Timing in 4-Stroke
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12. Effects On Valve Timing
Intake valve opening
Early – Dilution of intake with exhaust
Intake valve closing
Late – Reduces cylinder pressure
Early – Increases cylinder pressure
Exhaust valve opening (BBDC , Blowdown)
Late – Pumping losses
Early – Power reduction
Exhaust valve closing
Late(ATDC) – Reduces vacuum
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13. 17
Valve Positions
Overlap
•Both valves are open
•End of exhaust & start of intake
•Low pressure in exhaust port
Ram Effect
• Exhaust valve remain open after BC to utilize air
momentum to raise volumetric efficiency
• More efficient at high rpm
Blow down
•Exhaust valve opens before BDC
•To help evacuate cylinder before piston reverses
•Reduce exhaust stroke work losses
•At High rpm blow down angle is increased

14. Combustion in IC Engine
 A rapid chemical combination/reaction of HCs in fuel
with air (oxygen) resulting in liberation of energy
 Requirements:
Air fuel mixture formation
Some means to initiate reaction
Stabilization and propagation of flame in combustion
chamber
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15. Combustion Phasing in SI Engine
Three phases of combustion in SI engines
 Ignition lag stage
 Flame propagation stage
 After burning stage
Richard’s Theory:
 Growth and development of self propagating nucleus
flame(ignition lag)
 Spread of flame trough combustion chamber
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16.  Ignition lag (A-B)
 Flame propagation stage(B-C)
 After burning
(C-D)
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17. Ignition lag (1st Stage)
 Ignition lag is the time between spark initiation and
the point of significant pressure rise due to
combustion
 This interval consist on duration during which the
molecules are heated to self ignite and become able
to self propagate a flame nucleus
 This is very small interval (0.00015-0.0002 seconds)
corresponds to 35 degrees and its advance
increases with the speed
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18. A chemical process depending on
 Nature of fuel
 Temperature and pressure specially around spark
 Proportions of exhaust gas
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19. Flame propagation (2nd stage)
 As mixture is premixed so laminar flame produces
 As flame is formed it is self sustained and able to
propagate in whole chamber
 Heat produced by burning should be greater than
heat loss to surrounding
 Initially the heat loss/consumed is larger so slow
propagation and mass burning rate is low
 This is compensated by 25-35 advance angle so
peak pressure occur at about 15 degree ATDC
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20. After burning (3rd stage)
 Combustion does not stop at max. P
 The combustion process after max. P is after burning
 Rich mixture results in after burning
 Detonation(abnormal combustion, rise of T & P )
 Noise
Mechanical damage
Increase exhaust heat transfer
Pre-ignition(before sparking in next cycle)
Decrease in power out put
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21. Factors affecting the flame
propagation
 A/F ratio(max. flame speed at 10% rich)
 Compression ratio(Inc. flame speed and after
burning stage as mass/surface decrease)
 Load on engine
(high load = P inc. = speed inc.) (lower load and higher throttle = P
decreases & mixture is diluted by residual gases = ignition lag, advance
mechanism is needed. Rich mixture is provided = prolonged After
Burning and results in discharge of large amount of CO exhaust gases)
 Turbulence and engine speed
 Other factors(Engine Speed, )
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22.  A/F ratio
 Engine Speed
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23. Combustion Phasing in CI Engine
Four phases of combustion
 Ignition delay period
 Premixed combustion
 Controlled combustion
 After-burning
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25. Ignition Delay(1st Phase)
 The time/crank angle covered between the start of
injection and the start of ignition
 Some physical and chemical process takes place during
this interval are
 Physical
(atomization, vaporization and mixing of fuel with hot air)
 Good atomization require high fuel P, smaller injector
nozzle diameter, optimum fuel viscosity
 Vaporization of fuel depends on droplet diameter,
velocity, volatility, air temperature and pressure
 Lighter fuel => shorter ignition delay
 Heavier fuel => Longer Ignition delay
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26.  Chemical reaction (multiple reactions occur on
different sections of droplets/vapors as cracking)
 Start of slow chemical reactions initiate chemical
delay
 Chemical delay is longer than physical delay and is a
very strong function of temperature
 Determines the nature of the preceding phases of
combustion
 Injection of more fuel during this phase means more
violent second phase leading to detonation causing
shock waves
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27. Factors Affecting Delay Period
 Compression ratio(DP decrease as CR increase)
 Crank angle (other than optimum result in enhanced
delay)
 Load(higher load results in more fuel so temperature of
chamber rise so reduced delay for next cycle)
 Intake Air T & P(higher values result in lesser delay)
 Fuel Atomization(DP decreases with fine atomization)
 Cetane number (DP decrease with higher CN)
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29. Pre-mixed Combustion(2nd Phase)
 Combustion of the fuel which has mixed with the air to within the
flammability limits (air at high-temperature and high-pressure) during
the ignition delay period occurs rapidly in a few crank angles
 It is main Ignition event but is uncontrolled combustion
 Characterized by sudden increase in temperature and pressure
Rate of pressure rise is affected by
 The delay in the atomization of the fuel; it depends on the design of
the fuel injection system.
 Extent to which fuel is evaporated during the delay period
 The extent of distribution and mixing of the fuel throughout the
cylinder, longer delay periods, and high engine speed will increase
the mixing.
 The quantity of fuel injected during the delay period
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31. Controlled Combustion(3rd Phase)
Controlled phase of combustion depends upon
 The rate of fuel injection
 Type and intensity of air motion (swirl, turbulent )
 Piston motion
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32. After Burning (4th Phase)
As majority of energy is extracted from fuel but still there is
fuel left to burn resulting in exothermic reaction
Factors responsible for after burning are
 Slow burning
 High viscosity
 High density
 High carbon content of the fuel
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33. Excessive after burning can result in
 Higher exhaust temperature
 Increased heat transfer losses
 Reduced thermal efficiency
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34. Ignition Delay and Cetane Number
The quantity of fuel burned during each of the phases is
mainly a function of the Cetane number of the fuel
 CN=%hexadecane+0.15(%HMN)
 Iso-Cetane(HMN) 15, Cetane (n-Hexadecane) 100
 It affects the chemical ignition delay, which in turn
dominates the performance and emissions
characteristics
 For low CN delay is long and most of fuel is injected
before auto ignition, this may result in diesel knock
 Higher Cetane number reduces delay periods so small
fraction of fuel burned during the premixed phase
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