Automotive Electronics and Embedded System: Diesel Engine

A diesel engine is an internal combustion engine. Diesel engine ignition system, such as the diesel engine and Homogeneous charge compression ignition engines, rely solely on heat and pressure created by the engine in its compression process for the engine. The engine works on the principle of diesel cycle. It uses the heat of compression to initiate ignition to burn the fuel that has been injected into the combustion chamber. The compression that occurs is usually more than three times higher than petrol engine. Diesel engines will take air only, and shorty before peak compression, a small quantity of diesel fuel is sprayed into the cylinder via a fuel injector that allows the fuel to instantly ignite. The compression air produced force the piston down, generating power. The engine operating cycle is repeated every four strokes (upward or downward movement) of the piston, this being known as four stroke cycle as shown in figure1.7 (a).

Figure 1.7(a) Four Stroke CI Engine(Diesel Engine)

Four stroke CI engine:-

Conventional CI engine operates using four “strokes”, with either an up or down movement of each piston. These strokes are named: Intake, Compression, Combustion and exhaust. This cycle is same as that of the four stroke petrol engine
It consist of 4 strokes, one cycle operation is completed in 4 stroke of the piston that is one cycle is completed in every 2 revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft rotation and hence a cycle consists of 720°, of crankshaft rotation.
Essential part of four stroke diesel engine as shown in figure1.7(b).

Figure1.7(b) Essential part of 4 stroke Diesel engine

Intake Stroke:-

Diesel engines typically use direct injection which injects fuel directly into the combustion chamber during the compression stroke. The stroke on a diesel engine only draws air into the cylinder.In suction stroke piston starts at Top Dead Center (TDC) of the cylinder and moves to the Bottom Dead Center (BDC). Outlet valve will be closed and inlet valve will be open to allowing the mixer of fuel & air into the cylinder. Figure1.7(c) shows the intake stroke of the diesel engine

Figure1.7(c) Intake stroke

Compression stroke:-

In diesel engine fuel is injected under high pressure towards the top of the compression stroke. In compression stroke, once piston reaches BDC & moves back TDC, inlet valve will be closed. As the piston moves towards TDC.It compresses air inside the cylinder & compression takes place. Hence it is called compression stroke. Figure1.7 (d) shows the compression stroke of the diesel engine.

Figure1.7(d) Compression stroke

Combustion stroke:-

Compression ignition engines such as the diesel engine do not use the spark plugs to ignite the fuel-air mixture.When the piston reaches the top of the compression stroke the temperature and pressure in the combustion chamber is sufficient to ignite the mixture.In expansion stroke, both the valves are closed. When piston reaches top of its stroke, the fuel is sprinkled by the Fuel Injector andthe fuel mixture is ignited due to high temperature & pressure generated inside the cylinder & push down the piston to BDC.Hence it is known as combustion or expansion stroke. . Figure1.7 (e) shows the combustion stroke of the diesel engine

Figure1.7(e) Combustion stroke

Exhaust stroke:-

Exhaust gases are pushed out of the cylinder by the upward motion of the piston following the ignition stroke. During this stroke, the outlet valve is open and inlet valve is closed.The piston moves from bottom to top and during this motion piston pushes the exhaust gases out of the cylinder at constant pressure.Typically exhaust gases contain oxygen, nitrogen, water vapor, carbon dioxide, carbon monoxide, hydrogen, nitrous oxides, particulates and unburned hydrocarbons.

Figure1.7 (f) shows the Exhaust stroke. At the one movement the inlet valve open and the new cycle starts.

Figure1.7(f) Exhaust stroke

Two stroke CI Engine:-

Two stroke engine is widely used where small power required for motor cycle like auto rickshaw, scooter. This type of engine is compact in size, easy for manufacturing and simple in operation. In two strokes engine there is no inlet or exhaust valve as in four

stroke engines.Internal combustion engine which completes a power cycle in only one crankshaft revolution and with two strokes, or up and down movements, of the piston in comparison to a "four-stroke engine",which uses four stroke. Two stroke CI engine as shown in figure1.7 (g).

Figure 1.7(g) Two Stroke CI Engine(Diesel Engine)

Intake/Compression:-

When piston moves from BDC to TDC, it first closes the transfer port and then exhausts port.The charge of air-fuel mixture in the chamber is pressurized and creates high temperature.At the same time a vacuum is developing in the crankcase, so the fresh charge is being drawn into the intake port.. As the piston moves towards TDC, the diesel is added to the pressurized and high temperature gas which in turn ignites the whole mixture inside the cylinder and forces the piston to move downwards. Figure1.7 (h) shows the Intake/Compression stroke in two stroke diesel engine.

Figure 1.7(h) Compression Stroke

Combustion/Exhaust:- In this stroke both the ports still closed condition , the pressure of the expanding gases forces the piston towards BDC.The pressure in the crankcase is already rising. Later in down stroke exhaust port will be open & force out the burnt gases.Very shortly after that the Inlet / Transfer port will also open to intake the fresh charge of fuel mixture and engine is ready to start of the cycle.Figure1.7 (i) shows the Combustion/Exhaust stroke in two stroke diesel engine.