Diesel Engines

The combustion processes in the petrol engine and the diesel engine differ in the following significant features: in the petrol engine the petrol-and-air mixture is drawn into the cylinder, compressed (compression ratio ranging from 4:1 to 10:1), and ignited by a spark.

In the diesel engine, on the other hand, air alone is drawn into the cylinder and is compressed to a much higher ratio (14:1 to 25: 1) than in the petrol engine. As a result of this high compression the air is heated to a temperature of 700°-900° C.

Only then is a certain quantity of diesel fuel injected into the cylinder. Because of the prevailing high temperature, the fuel ignites spontaneously. However, combustion does not take place immediately when the fuel particles enter the combustion chamber, but after an interval of about 0.0001 sec. This is because the fuel droplets first have to mix intimately with the air in the combustion chamber and must then be heated up and vaporised before they can burn.

The time that elapses between injection and ignition is called the ignition lag. On injection of the fuel, it is broken up by the nozzle into smaller and larger droplets, according to a certain pattern.

The smaller droplets occur more particularly in the edge zone of the injected fuel spray and are the first to ignite. Next, the larger droplets in the interior of the spray are ignited. Fuel injection continues after the first flame has formed (main combustion).

If some of the diesel fuel is incompletely burned in this combustion process, or if it accumulates and then burns suddenly and violently at the next main combustion, the engine is said to be ‘knocking’.

In the petrol engine, on the other hand, the petrol-and-air mixture first ignites in the vicinity of the spark plug. The heat given off by the burning fuel particles causes the adjacent particles to ignite, so that a flame front, starting from the sparking plug, spreads through the combustion chamber. As a result of thermal radiation and rise in pressure, but also in consequence of the presence of ‘hot pockets’ in the combustion chamber, a fresh ignition and flame front formation may occur in the still unburned mixture not yet reached by the initial flame front.

Thus, in certain circumstances, the entire unburned mixture may undergo sudden and violent combustion, causing ‘knocking’ of the petrol engine.