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  Power and Efficiency

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   The power developed by an engine depends mainly on three factors:
  1. The amount of air which can be passed through the combustion chamber per minute - Volumetric Efficiency
  2. The completeness with which the air can be mixed with fuel and burnt – Thermal Efficiency
  3. How well the chemical energy of the burning fuel can be converted into mechanical work at the crankshaft - Mechanical Efficiency.

Volumetric Efficiency
Volumetric efficiency is a measurement of how well the gas is passed through the engine. It requires large and unobstructed ports offering the minimum obstruction to the flow of gases – both air and exhaust. This is why engines are gas flowed for additional power; fettling the cylinder heads to match the shape of the manifolds, and radiusing unnecessary metal inside the ports. Merely bolting on a larger exhaust often has disappointing results – more air has to get in before a better exhaust can have an effect.

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 highest possible compression ratio which can be used without knocking (detonation) and the minimum loss of heat to the combustion walls are the two main requirements for thermal efficiency. For both reasons the combustion chamber needs to be as compact as possible, because a compact chamber loses less heat through its smaller surface area. The compact chamber has a further benefit - the flame has less distance to travel and therefore burns quicker, reducing the risk of knocking and burning the mixture more thoroughly which helps to reduce emissions.

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.

To produce more powerful engines at higher compression ratios and yet with less emissions manufacturers have paid careful attention to the design of turbulence inside the chamber. Using offset inlet ports produces a ‘swirl’ in the gas which helps to spread the flame more quickly (induction turbulence) and then using the movement of the piston towards a part of the head can produce more turbulence – sometimes called a squish action – and this violent disturbance is also most beneficial. Extensive research has also identified ways in which the compression turbulence can be prolonged into ripples of micro-turbulence right up to the point of ignition.

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
The designs of the mechanical parts of the engine all have a bearing on how well the result of the burning of the fuel equates to turning the crankshaft. The loss of some efficiency due to the increased effort of turning multiple camshafts against more valve springs is offset by the inherent advantages of the overhead cam designs.




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