Page 107 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 91

The more effi ciently the engine cylinders can fi ll with gas, the more air, or fuel/
air, is available for the combustion process and this improves overall engine
effi ciency ( Fig. 2.18 ). The process of getting gases into and out of the engine is
known as ‘aspiration’ or ‘engine breathing’.
Combustion in the engine cylinder takes place because of a chemical reaction
Key fact
between the carbon and hydrogen in the fuel and the oxygen in the air. This
reaction releases energy from the fuel in the form of heat that generates pressure Combustion in the engine cylinder
in the cylinder to force movement of the piston. To achieve effi cient combustion, takes place because of a chemical
reaction between the carbon and
the quality of the fuel/air mixture is important; that is, how evenly mixed the fuel
hydrogen in the fuel and the oxygen
droplets are in the induced air. Movement of the air as it enters the cylinder is
in the air.
important for this process and the requirements are different for petrol and diesel
engines. The required air movements for each engine type are created by careful
design of the components that form the inlet tract and combustion chamber.
The inlet valve opens and closes according to piston position and controls
the incoming gas charge into the engine. It generally remains open for a small
period after the piston has reached BDC (i.e. beyond the end of the inlet stroke).
This allows the energy of the moving gas column in the inlet tract to assist in
the cylinder charging process, which helps to increase volumetric (and engine)
effi ciency.
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.
It is important that the closed cylinder is sealed properly to maintain the
appropriate pressures in the cylinder during the working cycle. Any losses in
pressure would signifi cantly reduce the effi ciency of the engine. To seal the
piston and bore, piston rings are fi tted into radial groves near the top of the

piston and provide a gas-tight seal between the moving piston and the cylinder
bore. When the cylinder volume is reduced during the compression stroke,

Figure 2.18 This very effi cient engine produces 98 g of CO 2 /km. (Source: GM Media)