The name diesel comes from the inventor of the diesel engine, R. Diesel. There are both four- and two-stroke-cycle diesel engines. Most automotive diesels are four-stroke engines. The intake stroke on the diesel engine draws only air into the cylinder. The air is then compressed during the compression stroke. At near maximum compression, finely atomized diesel fuel (a gas oil having a high flashpoint) is sprayed into the hot air, initiating auto ignition of the mixture. During the subsequent power stroke, the expanding hot mixture works on the piston, then burnt gases are purged during the exhaust stroke.
Since diesel engines do not use a spark plug, they are also referred to as compression ignition (CI) engines. In the case of petrol engines, too high a temperature in the combustion chamber ignites the petrol spontaneously.
When this occurs, the plug cannot control the moment of ignition. This unwanted phenomenon is often referred to as ‘knocking’.
The diesel is an injection engine. A petrol engine normally needs a throttle valve to control airflow into the cylinder, but a diesel engine does not. Instead, the diesel uses a fuel injection pump and an injector nozzle sprays fuel right into the combustion chamber at high pressure. The amount of fuel injected into the cylinder controls the engine power and speed. There are two methods3 by which fuel is injected into a combustion chamber, direct or indirect injection.
With direct injection engines (DI) the fuel is injected directly into the cylinder and initial combustion takes place within the bowl that is machined into the piston head itself. With indirect injection engines (IDI) the fuel is injected and initial combustion takes place in a small pre-combustion chamber formed in the cylinder head. The burning gases then expand into the cylinder where combustion continues. Pistons for IDI engines usually have shallow depressions in their heads to assist the combustion process. Although an IDI engine has
some advantages, it cannot match the efficiency of a DI engine, which is why most new automotive diesel engines entering production are DI designs.
Turbo charged engines are mainly used because diesels can generate only a low power output without turbocharging. Turbocharging with an intercooler is used in large engines. Diesel engines produce lean combustion, having an air-fuel ratio of about 15:1 up to 100:1. The diesel’s leaner fuel mixture generates higher fuel economy compared to that of a petrol engine. The peak cylinder pressure can be in excess of 15 MPa. The HC and CO contents in the exhaust gas are lower compared to those of petrol engines, but the particulate soot and NOx emissions cause environmental problems. In comparison with petrol engines, the components in a diesel engine are exposed to significantly more arduous operating conditions. Up until the 1980s, the noise, exhaust smoke and poor performance of diesel engines made them less attractive.
However, recently improved diesel engines with high torque now offer a more attractive alternative to petrol engines. A Stirling engine is another type of engine that uses a piston-cylinder construction. There are, however, other engines, such as the rotary and gas turbine engines, that do not use the piston-cylinder mechanism.
The science and technology of materials in automotive engines
Hiroshi Yamagata
Woodhead Publishing and Maney Publishing
on behalf of
The Institute of Materials, Minerals & Mining
CRC Press
Boca Raton Boston New York Washington, DC
WOODHEAD PUBLISHING LIMITED
Cambridge England
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