Factors that influence ignition delay in diesel engine (Compression Ignition or ‘CI’ engine) are
- Compression ratio
- Inlet air temperature
- Coolant temperature
- Jacket water temperature
- Fuel temperature
- Intake pressure
- Air-fuel ratio and
- Engine size
1. Compression ratio: With the increase in compression ratio reduces ignition lag, a higher pressure increases density resulting in closer contact of the molecules which reduce the time of action when fuel is injected.
2. Inlet air temperature: With the increase in inlet temperature increases the air temperature after compression and hence decreases the ignition delay.
3. Coolant temperature: Increase in engine speed increases cylinder air temperature and thus reduces ignition lag. The increase in engine speed increases turbulence and this reduces the ignition lag.
4. Jacket water temperature: With the increase in jacket water temperature also increases compressed air temperature and hence delay period is reduced.
5. Fuel temperature: Increase in fuel temperature would reduce both physical and chemical delay period.
6. Intake pressure (supercharging): Increase in intake pressure or supercharging reduces the auto-ignition temperature and hence reduces delay period. Since the compression pressure will increase with intake pressure, the peak pressure will be higher. Also, the power output will be more air and hence more fuel can be injected per stroke.
7. Air-fuel ratio (load): With the increase in air-fuel ratio (leaner mixture) the combustion temperatures are lowered and cylinder wall temperatures are reduced and hence the delay period increases, with an increase in load, the air-fuel ratio decreases, operating temperature increases and hence, delay period decreases.
8. Engine size: The engine size has little effect on the delay period in milliseconds. As large engines operate at low revolutions per minute (rpm) because of inertia stress limitations, the delay period in terms of crank angle is smaller and hence less fuel enters the cylinder during the period. Thus combustion in large slow speed Compression Ignition engines is smooth.
In below table, we have mentioned how the increase in each variable effects the ignition delay period and the reason for the cause of the delay.
|Increase in Variable|
Effect on Delay Period
|Cetane number of fuel||Reduces||Reduces the self-ignition temperature|
|Injection pressure||Reduces||Reduces physical delay due to greater surface volume ratio.|
|Injection timing advance||Reduces||Reduced pressure and temperature when the ignition begins|
|Compression ratio||Reduces||Increase air temperature and pressure and reduces autoignition temperature|
|Intake temperature||Reduces||Increases air temperature|
|Jacket water temperature||Reduces||Increases wall and hence air temperature|
|Fuel temperature||Reduces||Increases chemical reaction due to better vaporization|
|Intake pressure (supercharging)||Reduces||Increases density and reduces auto ignition|
|Engine Speed||Reduces in the order of milliseconds||Reduces loss of heat|