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Automobile Mechanical and Electrical Systems Second Edition Edited by Tom Denton

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PREFACE:

Before a vehicle can be constructed a prototype has to be submitted for type approval. When awarded this means the vehicle has passed very stringent tests and that it meets all current safety requirements.

 Different countries have different systems, which means some modifi cations to a car may be necessary if it is imported or exported. The European Union (EU) has published many directives that each member country must incorporate into its own legislation. This has helped to standardize many aspects.

 In the UK the Road Vehicles (Construction & Use) Regulations 1986 is the act that ensures certain standards are met. If you become involved in modifying a vehicle, e.g. for import, you may need to refer to the details of this act. Other countries have similar legislation.

The air above the Earth’s surface is like a fluid that exerts a pressure on all points around it because of the Earth’s gravitational force pulling it down. 

This creates a pressure known as atmospheric pressure and is 101.325 kPa, 760 mmHg, 29.92 inches Hg, 14.696 psi or 1013.25 millibars (let’s stick with approximately 1 bar or 15 psi).

A naturally aspirated engine (one that does not use forced induction by a supercharger or turbocharger) relies on atmospheric pressure to charge the cylinder with gas (air or air/fuel mixture) ready for the combustion process.

 As the piston moves down the cylinder (from TDC to BDC), the volume increases and this causes the pressure in the cylinder to reduce, becoming lower than atmospheric pressure.

 This creates a pressure difference between the inside and outside of the cylinder, and as a result the atmospheric pressure (the higher pressure) forces gases into the cylinder (where there is lower pressure) until the pressure is balanced. Note that any restriction to the flow of gas will reduce the effectiveness of the cylinder charging process.

After the combustion chamber has been charged with gas (air or fuel/air) during the induction stroke, the cylinder inlet and exhaust valves are both closed and seal the combustion chamber. 

The piston begins to rise in the cylinder, thus reducing the volume of the cylinder space and hence increasing the pressure of the trapped gas charge in the cylinder before combustion. The opening and closing of the valves is executed in sequence via the engine valve gear, synchronized with the four-stroke cycle and piston position.

During compression of the fuel/air mixture in a petrol engine, heat energy and kinetic energy (due to gas movement) are imparted into the mixture owing to the reducing volume and rising pressure. This creates a significant temperature increase and the magnitude of this increase depends upon the speed of the compression process and the amount of heat rejected to the surroundings (via the cylinder combustion space, walls, head, etc.). 

The temperature rise elevates to a point just below the self-ignition temperature of the fuel/air charge, which will combust at or above the flash point when ignited via an external source (i.e. the spark plug). 

Note that if the temperature of the mixture was too high, spontaneous self-ignition could occur and this would be a limiting factor for the maximum compression ratio in a petrol engine.

 After compression of the inlet charge, combustion of the fuel creates heat and pressure energy, which is imparted on the piston to generate mechanical work. In a petrol engine, this process is initiated by the high-voltage arc at the spark plug electrodes in the cylinder.

The quality of petrol (gasoline) is measured by a parameter called the octane rating, which gives an indication of the fuel’s resistance to engine ‘knock’ or uncontrolled, spontaneous combustion, which causes engine damage.

 Fuels with a higher octane rating burn more slowly and in a more controlled manner, and hence have a greater resistance to knock. The octane rating of the fuel determines the limit of ignition advance for a given engine speed and load condition.

 Therefore, it is particularly important to operate the engine on the correct fuel, to prevent damage to the engine due to knocking.


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