The important difference between the petrol and the diesel engine (Figure 1) is that the previous relies on spark ignition (SI) and the latter on compression ignition (CI). More specially, the combustion process in the diesel engine is initiated by natural ignition of the fuel when it is injected into a extremely compressed charge of air, which has reached about 800°C. Diesel engine combustion also tends to happen at constant pressure rather than at constant volume as in a petrol engine. This means that in the diesel engine the combustion pressure continues to increase steadily as the piston retreats and the cylinder volume increases, whereas in the petrol engine the combustion process is so quick that there is very little movement of the piston while it occur and hence very little boost in cylinder volume.
During its early development this type of engine was described variously as compression ignition, Diesel, oil and heavy oil, but it has long because come to be known generically as the diesel engine (with a small ‘d’). This acknowledges the major personal input made to its development by Dr Rudolph Diesel (1858–1913), who was born in Paris of German parents.
He become a student of mechanics and later on entered the well-known engineering works of Sulzer Brothers in Winterthur, Switzerland. It was in the early 1890s that he developed his theories on what we now know as the diesel engine principle and consequently took out a variety of patents, including a British one granted in 1892. A few years later his theoretical effort was embodied in a working engine of practical form built by the famous firm of MAN at Augsburg.
Fig.1 Cross-section of a four-stroke turbocharged direct-injection diesel engine.
In fairness, though, it must be added that Diesel’s concept of sparkless ignition was really predated by the pioneering work of an English engineer, Herbert Ackroyd Stuart (1864–1927). In 1890 he patented an engine operating on a related principle, but which required a vaporizer surface at the ending of the cylinder. For starting the engine, the vaporizer required the application of external heat. Hence, the first true compression ignition engine is usually attributed to Rudolph Diesel.
In the four-stroke diesel cycle the following series of events is continuously repeated all the time the engine is running refer (Figure 2):
Fig.2 The four-stroke diesel engine cycle: (a) induction (b) compression (c) power (d) exhaust
1] The induction stroke, during which air only is taken into the combustion chamber and cylinder, as a result of the partial vacuum or depression produced by the retreating piston.
2] The compression stroke, in which the advancing piston compresses the air into the very small volume of the combustion chamber and increases its temperature high enough to make sure self-ignition of the fuel charge. This demands compression pressures significantly in excess of those employed in the petrol engine.
3] The power stroke, right away preceding which the fuel charge is injected into the combustion chamber and mixes with the extremely hot air, and during which the gases of combustion expand and perform useful work on the retreating piston.
4] The exhaust stroke, during which the products of combustion are purged from the cylinder and combustion chamber by the advancing piston and discharged into the exhaust system. As in the case of petrol engine, the timing for the opening and closing of the inlet and exhaust valves and that for injecting the fuel, departs from the basic four-stroke operating cycle.