Superchargers are components used to increase the power output of vehicles under heavy load and high speed conditions. They serve this function by pressurizing air before it enters the engine, thereby delivering a greater amount of air and fuel for combustion.
Superchargers are usually comprised of two fan-like components, each with two or three rounded lobes. One rotates clockwise and the other anti-clockwise. This movement causes their lobes to mesh, and their positioning leaves only a very small space between the meshing lobes. The rotation of the fans pulls air in from an inlet port in the bottom of the supercharger and through the small space created by the meshing lobes. After this compression takes place, the air is forced out of the top of the supercharger towards the intake manifold, where it is mixed with fuel and delivered to the engine's cylinders.
The supercharger is driven by a belt connected to the engine's crankshaft. Since the crankshaft rotates whenever the engine is operating, the supercharger is always working. While boost, or increased intake pressure, is not essential at lower engine speeds, the operation of the supercharger increases engine torque, or twisting force, at low speeds. This is an important factor in vehicle control. As engine speed increases, so does the spinning of the lobes within the supercharger, increasing the amount and pressure of air that is fed to the engine.
The speed of moving parts and small clearance within the supercharger create heat that is transferred to the air passing through. This necessitates the addition of an intercooler into the air's pathway. An intercooler is basically a small radiator positioned between the supercharger and the intake manifold. Cooling the air that enters the combustion chamber both creates a better charge and prevents heat-related ignition malfunctions such as detonation or pre-ignition.
There are times when the supercharger's services are not required, such as during engine idle and very low speeds. Since the component is driven off the crankshaft, it cannot simply be shut off. A device called a butterfly valve is used to divert the supercharger's pressurized air back into its own inlet when boost is not needed.
Butterfly valves are positioned between the intercooler and intake manifold. The opening and closing of butterfly valves are generally determined by a vacuum-sensitive diaphragm. Mid-range vacuum levels hold the diaphragm open slightly to prevent excess boost from harming the engine. High levels of vacuum, which indicate very low or idle engine speed, force the diaphragm open completely. This opens the butterfly valve, exposing a pathway leading back to the supercharger's inlet.
Superchargers serve the same function as turbochargers but operate differently. Their advantage over turbos is that they offer greater low-speed torque and do not require a pressure control device such as the turbo's wastegate valve, since supercharger speed is controlled by engine speed. Yet the supercharger uses engine power to operate since it is mechanically driven; this compromises its efficiency.
Like turbochargers, superchargers are costly and increase a vehicle's fuel consumption.