INJECTION VS IGNITION LAG

Ignition lag

• It is a time delay between start of injection and star of ignition. 
• Factor affecting this delay are a rise in scavenge air or cooling water temperature, retarded fuel injection timing,ignition quality, the cetane number of fuel low load, load speed.

The duration of this period is set as a definite period of time, irrespective as to how fast the engine turns, and that period depends upon the chemical structure of the fuel. Basically, the lag period depends upon the number upon the number of molecular bonds which must be broken in order to release atoms of hydrogen and carbon from the fuel molecule. The longer and more complex the molecular chain, the greater will the amount of heat energy required to release the atoms and the longer will be the amount of heat energy required to release the atoms and the longer will be the ignition lag period. Because modern residual fuels result from complex blends of crude oil of many different types, they are complex structures and the ignition quality may be very variable between nominally the same grade of fuel. 

Formerly the cetane number was used to define ignition quality but cetane is a single element fuel and relating this to the complex nature of residual fuels is not realistic. The general term ignition quality is now used.
Ignition lag is the preparation period of the fuel within the cylinder for spontaneous ignition and beginning of combustion. The physical and chemical processes occurring during this period are characterised by weak ABSORPTION and liberation of heat. Thus there is little if any deviation from the compression curve. The length of the lag period depends on the fuels ignition quality and nothing else. The higher the ignition quality, the shorter will be the lag period, and the lower the ignition quality, the longer the lag period.

INJECTION LAG

• It is the time delay between the closing of spill port /valve of the fuel pump and the opening of the fuel valve. 
• It depends on the pressure rise of the fuel pump and the pressure in the injection line there is a delay due to compression of the fuel and expansion of the pipework.

Although liquids are often classed as being incompressible, they can be compressed to some extent at the pressures involved. Pipework will expand at these pressures and a certain amount of oil must be delivered in order to take account of these factors.

 Pump timing can be adjusted to take account of this because the amount remains the same at all engine speeds. When oil pressure reaches a high enough value the injector needle will lift and injection commences