Both the turbocharger and supercharger are a kind of air pump, which force the air into the cylinder to increase the mass of the intake air. The conventional engine draws air in, using the vacuum generated when the piston goes down. The pressure inside the intake manifold becomes higher because the turbocharger or supercharger force the intake air into the cylinders at a pressure (boost pressure) higher than atmospheric pressure. Thus, the mass of the intake air into the cylinder is increased.
1. Charging efficiency
The engine’s ability to draw air in is called charging efficiency. The charging efficiency of a conventional engine is about 65-85% due to the resistance of the intake system and the exhaust gas remaining in the exhaust system. However the charging efficiency of an engine equipped with a turbocharger or supercharger can be more than 100%.
2. Compression ratio
The gasoline engine has a greater tendency of knocking as the compression pressure is increased. Since air is compressed to a pressure greater than the atmosphere pressure and is fed to the cylinders of a turbocharged or supercharged engine, the compression pressure becomes higher than that of the conventional engine and knocking occurs more easily. Therefore the compression ratio of a turbocharged or supercharged engine is set lower than that of a conventional engine to prevent knocking.
Mass of air actually taken in
Charging efficiency(%) = —————————————– X 100
Mass of air in cylinder under standard conditions
*Standard conditionsStandard atmosphere pressure at 2C(68 F)