A supercharger is an air compressor that helps an engine make more power by pushing extra air into the cylinders. More air means the engine can burn more fuel each cycle, which increases the force on the pistons. This is called forced induction because the intake air is driven in under pressure instead of entering only by atmospheric pressure.
Superchargers matter because they can increase horsepower without making the engine much larger.
Understanding Automotive Technology: How a Supercharger Works
There are two common supercharger designs. A positive displacement unit traps a measured amount of air in rotating lobes or screws, then carries it from the inlet to the outlet. It can build useful boost at low engine speed, so the driver feels strong pulling force soon after pressing the accelerator.
A centrifugal unit uses a fast spinning impeller, much like a compact air pump. Its boost rises more strongly as engine speed increases.
This gives a different driving feel, with more effect at higher revolutions. The pulley size matters because it sets how fast the compressor turns compared with the crankshaft.
Compressing air creates heat. Hotter air is less dense, so each cylinder receives fewer oxygen molecules than it would with cool air at the same pressure. Heat also increases the chance of knocking.
Knock is uncontrolled combustion that can damage pistons, rings, or bearings. Many supercharged engines use an intercooler between the compressor outlet and the intake ports.
The intercooler transfers heat from the compressed air to outside air or engine coolant. Cooler intake air improves density and gives the engine computer more room to use safe ignition timing.
A supercharger does not create power for free. The crankshaft must turn the belt or gears, and that takes torque. At light throttle, many systems use a bypass valve.
This valve lets air circulate around the compressor instead of forcing it into the engine. The engine then wastes less energy pumping air when extra power is not needed.
When the driver opens the throttle, the bypass closes and boost can arrive quickly. This quick response is a major difference from many turbochargers, which are driven by exhaust flow and may need time to spin up.
Fuel delivery and engine control must match the extra airflow. Fuel injectors need enough capacity, and the fuel pump must maintain pressure when the engine is under load. The engine computer uses sensors for air pressure, air temperature, throttle position, and knock.
It adjusts fuel amount and ignition timing to protect the engine. Students can connect this to everyday driving when a vehicle climbs a hill, tows a trailer, or accelerates onto a highway.
When learning this system, pay attention to the full chain of energy. The crankshaft drives the compressor, the compressor changes the air conditions, the cylinders burn a matched fuel charge, and some of the added output is used to keep the compressor turning.
Key Facts
- Power increases when more air and fuel burn in each cylinder per cycle.
- A supercharger is driven by the engine, usually through a belt, chain, or gears connected to the crankshaft.
- Boost pressure is the pressure above atmospheric pressure in the intake manifold.
- Absolute intake pressure = atmospheric pressure + boost pressure.
- Engine air flow roughly increases in proportion to absolute intake pressure if temperature and efficiency are similar.
- Power used by the supercharger comes from the engine, so net power gain = added engine power minus compressor drive power.
Vocabulary
- Supercharger
- A mechanical compressor that forces extra air into an engine intake so the engine can burn more fuel and produce more power.
- Boost pressure
- The extra pressure above normal atmospheric pressure that a supercharger creates in the intake system.
- Intake manifold
- The set of passages that distributes incoming air, or air and fuel mixture, to the engine cylinders.
- Crankshaft
- The rotating engine shaft that converts piston motion into rotational motion and can also drive a supercharger.
- Intercooler
- A heat exchanger that cools compressed intake air to make it denser and reduce engine knock risk.
Common Mistakes to Avoid
- Assuming a supercharger makes free power is wrong because the compressor takes mechanical power from the crankshaft to spin.
- Confusing a supercharger with a turbocharger is wrong because a supercharger is mechanically driven by the engine, while a turbocharger is driven by exhaust gas energy.
- Ignoring air temperature is wrong because compressing air heats it, and hot air is less dense than cool air at the same pressure.
- Adding boost without adding fuel is wrong because the engine needs the correct air fuel ratio to burn safely and make power.
Practice Questions
- 1 An engine has an intake pressure of 14.7 psi at atmospheric pressure. A supercharger adds 7.0 psi of boost. What is the absolute intake pressure in psi?
- 2 A naturally aspirated engine produces 200 hp. If supercharging increases the engine's gross power by 70 hp but the supercharger takes 15 hp to drive, what is the net horsepower?
- 3 Explain why an intercooler can help a supercharged engine make more power even though the supercharger has already compressed the air.