VOLUMETRIC EFFICIENCY

Press, Volume CI, Issue 29909, 24 August 1962, Page 9

VOLUMETRIC EFFICIENCY

V T OLUMETRIC efficiency is ’ a technical term for a vital factor in engine performance. It is, simply, concerned with the efficiency of the engine's “breathing.” When the piston in a fourstroke engine, travels downwards on the induction or “breathing in” stroke it draws a mixture of air and fuel, metered by the carburettor, into the cylinder. This mixture is then compressed and flred.

If the piston travelled slowly on the induction stroke, a full fuel mixture could be drawn into the cylinder. In practice, however, the air-fuel mixture has to travel through a number of bends and small openings in the manifolds and carburettor before reaching the cylinder. This is not the only difficulty: because the engine is hot, the mixture is heated, and this causes it to expand. These factors reduce the effective amount of the mixture which is drawn into the

cylinder, and the cylinder is not completely filled. The higher the speed, the less mixture is drawn into the cylinder. The gas has difficulty travelling through the carburettor and manifolds, and there is friction between the gas and the rough sides and edges of the manifolds and ports. This is why, in high performance engines, the ports and manifolds are usually polished to improve the gas flow.

The volumetric efficiency of an engine is the ratio between the amount of fuel mixture that is actually drawn into the cylinder and the amount of mixture that could be drawn in under perfect conditions.

At high speeds volumetric efficiency of many engines falls to as much as 50 per cent., and the cylinders are only half-filled. Polishing and porting and the addition of extra carburettors on high performance engines is intended to im-

prove the volumetric efficiency and therefore the breathing at speed. If the engine breathes better at high speed it will produce more power. There is a limit to the speed at which an engine can run, not only because of mechanical and metallurgical problems but also because the engine “runs out of breath” as volumetric efficiency falls. Similarly, when an internal combustion engines is run at high altitudes where there is less barometric pressure to push the gas into the cylinder, it loses volumetric efficiency.

As well as by polishing manifolds and ports and thereby reducing gas friction, volumetric efficiency can be improved by increasing the size of intake valves, altering the time at which the valves open, and adding extra carburettors. Yet another method is to fit a supercharger, which forces additional air into the cylinders. A supercharger is, simply, just an air pump.