Lesson 2/Learning Event 2
(The term "volumetric" efficiency is slightly misleading because engine efficiency is actually a ratio of
weights and not "volumes." However, when this term is used in connection with pumps and compressors,
the ratio is of volumes, and this term has become applied to engine efficiency also.)
Properties of Gases and Their Relation to Supercharging
A review of mass, volume, and density, as applied to gases, is necessary to explain the principle of
All matter can be classified into solids and fluids. Fluids are, in turn, subdivided into liquids and gases. The
weight of any substance depends upon its density, and the density of solids and liquids may be considered to
be essentially constant. This is not the case with gases, for the density of gases is proportional to the
pressure applied. In other words, when the pressure upon a gas is increased, its density will also increase.
Therefore, two identical volumes of gas under different pressures will not weigh the same. For example, at
sea level, 13 cubic feet of air weighs one pound, but at high altitudes where the atmospheric pressure is
lower, the same 13 cubic feet of air weighs a good deal less.
Mass, weight, and volume. To an engineer, mass has a different meaning than weight, but in general, the
two may be considered the same. "Volume" refers to the space occupied by matter, and "weight" refers to
the force which a certain volume of matter would exert on a scale. The weight of any substance is directly
proportional to its density and volume.
Boyle's law. The volume of a gas can be reduced by applying pressure. This is known as compression.
Boyle's law states: The volume of an ideal gas is inversely proportional to the pressure upon it, if
temperature is maintained constant. (There is no "ideal" gas, but air conforms closely and is considered
A supercharger is essentially a pump whose main function is to compress the air or air-fuel mixture to
increase the density of the charge. A supercharger increases the power output in two ways.
- It increases the density of the air-fuel mixture delivered to the cylinders of the engine.
- It increases the maximum compression pressure.