COMBUSTION PERIODS OF FUEL

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There are 4 main phases to the combustion of fuel.

1.         Injection delay. This is the small delay between closing of the spill ports/valve and the opening of the fuel injector. This delay is affected by the rate of pressure rise within the pumping element (influenced by wear in the pump), the pressure in the injector line (influenced by the delivery valve), very slight compression of the fuel and expansion of the HP pipe.

2.         Ignition delay. This is the delay between the start of injection and the start of combustion. In order that a fuel can burn, it must be transformed into a vapour and mixed with the correct ratio of air. In order to do this the fuel must break up into small droplets (10-20mm), and heat up to its self ignition temperature once in the cylinder (cylinder air temp 550-700^oC), therefore dictated by physical and chemical processes.

3.         Pre-mixed combustion. This phase of combustion commences immediately after ignition. The fuel which combusts in this phase is:- all the fuel which has been injected, and the fuel which is still being injected. Thus the severity of this uncontrolled combustion is dictated by the length of ignition delay, and the quantity of fuel injected during this delay. Large pre-mixed combustion produces increased max cylinder pressure and rate of pressure rise. This large pressure rise produces diesel `knock', which increases engine noise, and shock loading to bearings, piston rings, combustion cylinder, etc.

4.         Diffusion combustion. This phase of combustion commences when the rate of fuel combusting is the same as that being injected. The length of this period is dictated by the type of fuel burnt (HFO takes longer to evaporate or prepare), and the rate at which fuel is injected (fast rate produces small fuel droplets and shorter time to burn). Depends on how rapidly the oxygen and fuel can be mixed, as the initial supply of oxygen near the fuel droplets will have been used during the pre-mixed combustion phase.

If this period of combustion is too long, exhaust soot particles and temperature will increase, and useful power output will fall.