Diesel engines " Compression ignition engine

The modern diesel engine came about as the result of the internal combustion principles first proposed by Sadi Carnot . Dr. Rudolf Diesel applied Sadi Carnot's principles into a patented cycle or method of combustion that has become known as The Diesel cycle .

Carnot's patented engine operated when the
heat generated during the Compression of the
air fuel charge caused ignition of the mixture,
which then expanded at a constant pressure
during the full power stroke of the engine.

A diesel engine is similar to the gasoline engine
used in most cars . Both engines are internal
combustion engines , meaning they burn the
fuel-air mixture within the cylinders . Both are
Reciprocating engines ,being driven by pistons
moving laterally in two directions.

The majority of their parts are similar.Although
a diesel engine and gasoline engine operate
with similar components,a diesel engine,when
compared to a gasoline engine of equal horsepower, is heavier due to stronger, heavier materials used to withstand the greater dynamic forces from the higher combustion pressures present in the diesel engine.

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.

Compression Ratio and Clearance Volume

Clearance volume is the volume remaining in the cylinder when The piston is at TDC . Because of the irregular Shapes of the combustion chamber ( volume in the head ) the clearance volume is calculated empirically by filling the chamber with a measured amount of fluid while the piston is at TDC.This volume is then added to the displacement volume in the cylinder to obtain the cylinders total volume.

An engine's compression ratio is determined by taking the volume of the cylinder with piston at TDC
" Top Dead Center " and dividing the volume of the cylinder when the piston is at BDC " Bottom Dead Center ".

   where
                   Vc   : is Clearance volume.

                   VS : is Displacment or stroke volume.

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.

Another difference between a gasoline engine and a diesel engine :

The engine speed is controlled manner :

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.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 rps.
Diesel engines require a speed limiter, commonly called the governor, to control the amount of fuel
being injected into the engine.

Ignition system

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 stroke. When fuel
is injected, it vaporizes and ignites due to the heat created by the compression of the air in the cylinder.