Rudolf Diesel developed the idea for the diesel engine and obtained the German patent for it in 1892. His goal was
to create an engine with high efficiency. Gasoline engines had been invented in 1876 and, especially at that time,
were not very efficient.
The main differences between the gasoline engine and the diesel engine are:
A gasoline engine intake a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel
engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air
lights the fuel spontaneously.
A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high
as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.
Gasoline engines generally use either carburetor, in which the air and fuel is mixed long before the air enters the cylinder,
or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use
direct fuel injection -- the diesel fuel is injected directly into the cylinder.
Note that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel
directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a
The injector on a diesel engine is its most complex component and has been the subject of a great deal of
experimentation -- in any particular engine it may be located in a variety of places. The injector has to be able to
withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist
circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special
induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise
improve the ignition and combustion process.
One big difference between a diesel engine and a gas engine is in the injection process. Most car engines use
port injection or a carburetor rather than direct injection. In a car engine, therefore, all of the fuel is loaded into the
cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression
ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking.
A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the
more power is generated.
Some diesel engines contain a glow plug of some sort. When a diesel engine is cold, the compression process may
not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire that helps
ignite the fuel when the engine is cold so that the engine can start.
All functions in a modern engine are controlled by the ECM communicating with an elaborate set of
sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position
(i.e., T.D.C.). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and
retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder
is compressed more, creating more heat, which aids in starting.