Q I have read that diesel smoke emission is increased
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if injection timing is retarded. A fellow operator has mentioned that the cause of excessive smoking in a particular case was traced to an injection timing that was too far advanced. While I can appreciate that incomplete combustion results from retarding the injection and that this increases the amount of black smoke produced, I am at a loss to understand how advancing injection can give the same result. Could you give an explanation?
ADespite intensive research into the causes of diesel smoke over the past 20 years, experts have admitted that they still have a lot to learn about the effects of "quenching" on the production of smoke. If combustion takes place when the piston is near top-dead-centre the combustion space has a relatively small surface area, which reduces the heat transferred from the burning gases to the walls of the chamber. With retarded injection, the gases are still burning after a considerable area of the cylinder wall has been uncovered and heat transfer is increased. This tends to quench the gases and inhibit combustion, and is assumed to be one of the main causes of black smoke, notably when the engine is operating under light load.
If injection is advanced, combustion may be completed well before top-dead-centre and the extra cylinder wall area that is exposed during combustion can produce a quenching effect. Combustion in a precombustion chamber is more consistent than is the case with a direct-injection unit, partly because the surface area of the chamber is the same at all times. Quenching of the type described cannot occur unless injection is so far retarded that the gases are still burning when they are discharged into the cylinder. Although a pre-combustion chamber normally gives more efficient burning than is possible with an open chamber with direct injection, the overall efficiency provided by the latter is generally the more favourable because of the lower pumping losses involved.