Diesel Engine Fundamentals
The greater combustion pressure is the result of the higher compression ratio used by diesel
engines. The compression ratio is a measure of how much the engine compresses the gasses in
the engine's cylinder.
In a gasoline engine the compression ratio (which controls the
compression temperature) is limited by the air-fuel mixture entering the cylinders. The lower
ignition temperature of gasoline will cause it to ignite (burn) at a compression ratio of less than
10:1. The average car has a 7:1 compression ratio. In a diesel engine, compression ratios
ranging from 14:1 to as high as 24:1 are commonly used. The higher compression ratios are
possible because only air is compressed, and then the fuel is injected. This is one of the factors
that allows the diesel engine to be so efficient. Compression ratio will be discussed in greater
detail later in this module.
Another difference between a gasoline engine and a diesel engine is the manner in which engine
speed is controlled. In any engine, speed (or power) is a direct function of the amount of fuel
burned in the cylinders. Gasoline engines are self-speed-limiting, due to the method the engine
uses to control the amount of air entering the engine. Engine speed is indirectly controlled by
the butterfly valve in the carburetor. The butterfly valve in a carburetor limits the amount of
air entering the engine. In a carburetor, the rate of air flow dictates the amount of gasoline that
will be mixed with the air. Limiting the amount of air entering the engine limits the amount of
fuel entering the engine, and, therefore, limits the speed of the engine. By limiting the amount
of air entering the engine, adding more fuel does not increase engine speed beyond the point
where the fuel burns 100% of the available air (oxygen).
Diesel engines are not self-speed-limiting because the air (oxygen) entering the engine is always
the maximum amount. Therefore, the engine speed is limited solely by the amount of fuel
injected into the engine cylinders. Therefore, the engine always has sufficient oxygen to burn and
the engine will attempt to accelerate to meet the new fuel injection rate. Because of this, a
manual fuel control is not possible because these engines, in an unloaded condition, can
accelerate at a rate of more than 2000 revolutions per second. Diesel engines require a speed
limiter, commonly called the governor, to control the amount of fuel being injected into the
Unlike a gasoline engine, a diesel engine does not require an ignition system because in a diesel
engine the fuel is injected into the cylinder as the piston comes to the top of its compression
When fuel is injected, it vaporizes and ignites due to the heat created by the
compression of the air in the cylinder.
Major Components of a Diesel Engine
To understand how a diesel engine operates, an understanding of the major components and how
they work together is necessary. Figure 2 is an example of a medium-sized, four-stroke,
supercharged, diesel engine with inlet ports and exhaust valves. Figure 3 provides a cross
section of a similarly sized V-type diesel engine.