Last update: 08/03/2005 |
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The power developed by an engine depends mainly
on three factors:
Volumetric Efficiency But this is only the beginning. The volumetric efficiency is also affected by the valve timings and these need to be carefully selected and maintained. A hotter cam is often the next stage in improving performance, changing the valve timing, opening rate and depth from the ‘average’ values selected by the manufacturer may make more mixture available and therefore increase power. Lastly, the temperature of the fuel/air mixture needs to be as low as possible when the inlet valve closes. This is because a hot gas is much less dense than a cold one and because of thermal expansion a hot cylinder will hold a smaller mass of mixture. This explains why there are often intercoolers built into the air inlet of turbocharged engines. The first charge of air will be hot and therefore less dense after it has made a pass through the turbo compressor and passing it through a radiator cooled by the airstream increases its density before it enters the engine. Thermal Efficiency The design of the cylinder head can reduce the risk of detonation (a premature and unplanned explosion in a part of the mixture) and while until recently fuels would contain additives to suppress knocking, these have largely been phased out so that manufacturers have increasingly turned to a very compact and efficient design of compression chamber – the Hemispherical and Pent Roof designs. These designs have high volumetric and thermal efficiency and also, for the large part, reduce emissions to acceptable levels. Turbulence Further design complexities have produced areas within the chamber where the mixture varies – richer nearer the spark plug and leaner elsewhere. The effect of all of these design complexities and the addition of another set of inlet and exhaust valves makes it possible to run engines on as little as 22:1 air/fuel, which would have been considered impossible only a generation ago. These latest engines are now running at such high pressures that NOx emissions are possible when running at full power. For this reason, many new designs have Exhaust Gas Recirculation in which exhaust gases are mixed with the fuel air to ‘quench’ combustion at the most critical time. Mechanical Efficiency
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