How does a Diesel Engine work ?

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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.

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Fig.1 When an atomised form of diesel is made to contact with high temperature and pressure air it leads to the release of chemical 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.

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Fig.2 Using Slider-Crank mechanism linear motion of the piston is transformed into 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.

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Fig.3 The engine block support the Slider-crank mechanism; Cylinder head is 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.

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Fig.4 During the suction stroke fresh air which is rich in Oxygen content is sucked in

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.
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Fig.5 During compression stroke, the piston does work on the air; so both its pressure and temperature rises

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.

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Fig.6 Atomized form of fuel is injected into the compressed air

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.

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Fig.7 During the power stroke piston absorbs power from the high energy gas

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.

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Fig.8 Here exhaust valve is open and the exhaust is rejected

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.

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Fig.9 The bowl above the diesel engine piston helps to create a rapidly swirling air

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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Fig.10 The variation of piston force and output power of a single cylinder engine

As a result of high force non uniformity a single cylinder engine will never have a smooth operation; rather it would have a noisy and vibrating operation.

Smooth operation with more number of cylinders

But with more number of cylinders one can overcome these problems. Consider this 4 cylinder engine. Here 4 different strokes occur at a time.

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Fig.11 In a 4 cylinder engine all the 4 strokes occur at a time

So the power stroke is always present in the engine. The total force and total power in 4 cylinder engine is shown in the Fig. 12 & Fig.13 respectively. It is clear that a four cylinder engine will have better force and power uniformity. In short, the more cylinders an engine has the smoother it will operate.
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Fig.12 Total force in a 4 cylinder engine

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Fig.13 Total power in a 4 cylinder engine

A four cylinder engine generally operates on the following firing order.It operates on the firing order of 1-3-4-2. Such a firing order makes sure that the combustion force is balanced along the length of the engine also.
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Fig.14 Generally an IC engine operates on the firing of 1-3-4-2

A heavy flywheel which acts like a power reservoir further helps in smoothing out non uniformity of power. When the engine supplies extra power the flywheel absorbs the power. During the low power operating regimes, the flywheel releases the power to the engine. Thus the power output will have a better uniform nature as shown in the Fig.15.

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Fig.15 The flywheel helps to smooth out the output power

Use of the Counter weights

A huge unbalanced force arises in the form of dynamic unbalance due to the excessive mass at the connecting rod side. Such unbalanced force is balanced by providing counterweights on the crank side.

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Fig.16 Counter weights are provided on the crank side to balance the dynamic force of the connecting rods

Controlling the valve operation

Opening and closing of valves are accurately controlled by a pair of cam shafts. Cam shafts derive motion from the engine.

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Fig.17 Camshafts help to operate the intake and exhaust valve quite accurately

During a complete engine cycle, the crankshaft rotates twice, but the exhaust/intake valve open only once. This means that the camshaft need to be activated only once in an engine cycle. Thus the cam shafts need to rotate at the half the speed of crank shaft. This speed reduction is achieved by using a double sized wheel at Camshaft side than in the the crank side.

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