How does a Diesel Engine work ?


An internal combustion engine transforms the chemical energy in fuel to mechanical rotational energy. Diesel engines, which have been serving mankind for over a century, are the most versatile and economical IC engines.

A detailed webpage version of the video is given below.

The Basic Working

To release the chemical energy in diesel effectively, an atomized form of the fuel is made to contact with high temperature and high pressure air. The chemical energy release (Combustion) is shown in the Fig.1. In diesel engines, this energy is effectively transferred as mechanical rotational energy.

So the operation of a diesel engine is all about producing high temperature and high pressure air continuously. We will see how this is achieved in this article.

Slider-Crank Mechanism & The basic Assembly

Piston, connecting rod, crank and cylinder form a mechanism called slider-crank mechanism. Here the linear motion of the piston is transformed to a rotary motion at the crank.

During the motion of the piston, the top most point it can reach is called Top dead centre (TDC) and the bottom most position the piston can reach is called as Bottom dead centre (BDC).
In an IC engine, this mechanism is properly supported in an engine block. Cylinder head, valves and fuel injector are fitted above the engine block.

The working

When the piston moves downwards, inlet valves open and fresh air from outside is sucked in, or, in other words, the engine breathes. This stroke is called as suction stroke.

During the return stroke, inlet and exhaust valves are closed and the air inside the cylinder gets compressed. During the compression stroke, the piston does work on the air. So the temperature and pressure of the air will rise to a level which is higher than the self ignition value of the diesel.

An atomized form of diesel is injected into this compressed air. The fuel gets evaporated and undergoes an uncontrolled spontaneous explosion. As a result, the pressure and temperature rise to high level values.

The high energy fluid pushes the piston downwards. The hot air does work on the piston and energy in the fluid is converted to the mechanical energy of the piston. This is the only stroke where the piston absorbs power from the fluid.

Due to inertia of the system, the piston moves upwards again. This time the exhaust valves open and the exhaust is rejected. Again the suction stroke happens.

This cycle, which has a total 4 strokes, is repeated over and over for continuous power production.

Use of Bowl in diesel engine piston

You may have noticed that a bowl is provided on top of the diesel engine piston. During the compression stroke this bowl helps produce air that is rapidly swirling. Thus the injected fuel gets mixed with the air effectively.

The Mechanical Design Aspects

The mechanical design of IC engines, particularly that of diesel engines, is a challenging and interesting task.
Since the combustion process in diesel engines is never uniform and smooth, they are prone to more vibration and noise compared to petrol engines. Thus diesel engines require a rugged structural design.
Out of the four strokes, it is only during the power stroke that a tremendous amount of force is exerted on the piston. So a single cylinder engine will always have high force non uniformity . Similarly the output power will also have a fluctuating nature. The variation of force and output power with the piston movement is plotted in Fig. 10.

 

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