Supercharging

an increase in the quantity of the fresh charge of a combustible mixture that is supplied to an internal-combustion engine by raising the intake pressure. Supercharging is usually used to increase power by 20–45 percent without increasing the weight and size of the engine, and also to compensate for loss of power under high-altitude conditions. Supercharging with “quality control” can be used to “purify” the process—that is, to reduce the toxicity and smokiness of the exhaust gases.

A supercharger unit uses a compressor, a turbocompressor, or a combination of the two. The most common type has a turbo-compressor driven by energy from the exhaust gases. Supercharger assemblies are used on almost all forms of vehicle dieseis (marine, locomotive, and tractor engines); in carburetor engines supercharging is limited by the onset of detonation. Among the main drawbacks of a supercharger assembly are increased mechanical and thermal stresses on the engine (caused by the higher pressure and temperature of the gases), reduced economy, and increased complexity of design.

The “built-in” superchargers include the dynamic type (formerly called the inertia, resonance, or acoustic type), in which the effect is obtained by means of vibratory phenomena in pipes; the ram type, which is used in piston aircraft engines at altitudes above the rated altitude and speeds above 500 km/hr; and the refrigeration superchargers, which achieve the effect by evaporating into the air supply a fuel or other combustible fluid with a low boiling point and a high heat of vaporization.

Dynamic supercharging, which can raise the admission coefficient ƞ_v, to 0.92–0.96 over a wide range of engine speeds with minor changes in the manifold design, is becoming increasingly common in vehicle internal-combustion engines. An increase in ƞ_v, by supercharging makes it possible to boost a diesel engine’s power when the fuel feed per cycle is increased or to improve economy while maintaining power (with the same fuel feed per cycle). Dynamic supercharging increases the life of components in the cylinder-piston group because of lower temperatures when operating on lean mixtures.