FOUR-STROKE PETROL ENGINE OPERATING PRINCIPLE

As with the a variety of repeating cycles of events in nature, so does the motor vehicle petrol engine need to operate on a continuously repeating cycle known as the four-stroke principle. It would seem to be normally accepted that the first internal combustion engine to operate successfully on the four-stroke cycle was created in 1876 by Nicolaus August Otto (1829–91).

ENGINE OPERATING PRINCIPLE

Nicolaus August Otto (1829–91)

This self-taught German engineer was to become one of the wonderful brilliant researchers of his time and a partner in the firm of Deutz close to Cologne, which for many years was the largest manufacturer of internal combustion engines in the world.

Although the Otto engine ran on gas, which was then regarded as a suitable and reliable fuel to use, it nevertheless incorporated the necessary ideas that led to the development in 1889 of the first victorious liquid-fuelled motor vehicle engine.

ENGINE OPERATING PRINCIPLE

Gottlieb Daimler (1834– 1900)

This was the twin-cylinder Daimler engine, patented and constructed by the German automotive pioneer Gottlieb Daimler (1834– 1900) who, like Otto, had been connected with the Deutz firm. The Daimler engine was subsequently adopted by numerous other car manufacturers and, in most respects, it can be regarded as the true forerunner of the recent four-stroke petrol engine (Figure 1).

petrol engine

Fig.1 Cross-section of a modern four-stroke petrol engine (Jaguar)

In this type of engine the following sequence of events is continuously repeated all the time it is running (Figure 2):

1. The induction stroke, during which the combustible charge of air and fuel is taken into the combustion chamber and cylinder, as a result of the partial vacuum or depression created by the retreating piston.

2. The compression stroke, which serves to raise both the pressure and temperature of the combustible charge as it is compressed into the lesser volume of the combustion chamber by the advancing piston.

3. The power stroke, immediately preceding which the combustible charge is ignited by the sparking plug and during which the gases expand and perform useful work on the retreating piston.

4. The exhaust stroke, during which the products of combustion are purged from the cylinder and combustion chamber by the advancing piston, and discharged into the exhaust system.

petrol engine

Fig.2 four-stroke petrol engine cycle

It thus follows that one entire cycle of operations occupies two complete revolutions of the engine crankshaft. Since energy is necessarily required to carry out the initial induction and compression strokes of the engine piston before firing occurs, an electrical starter motor is used for beginning cranking of the engine. Once the engine is running the energy required for performing subsequent induction, compression and exhaust strokes is derived from the crankshaft and flywheel system, by virtue of its kinetic energy of rotation. Kinetic energy is a term used to communicate the energy possessed by a body due to its mass and motion. The theory of an engine flywheel is therefore to act as a storage reservoir for rotational kinetic energy, so that it absorbs energy upon being speeded up, and sends it when slowed down.

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In the four-stroke cycle, the engine inlet and exhaust valves perform the functions of admitting the combustible charge before its compression, and releasing the burnt gases after their expansion. The opening and closing of the inlet and exhaust valves are not, in real practice, timed to coincide exactly with the beginning and ending of the induction and exhaust strokes; nor is the spark timed to occur exactly at the beginning of the power stroke. At a later stage the reasons for these departures in valve and ignition timing from the fundamental four-stroke operating cycle will be made clear.


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