A supercharger pressurizes air intake so that it reaches above atmospheric pressure. This is a better way to boost a car engine's power than to build a heavier, more expensive engine. In a regular four-stroke engine, the piston moves down, creating a vacuum, and then air is sucked into the chamber. The air is combined with fuel, and the spark plug ignites the mixture to create combustion. The resulting explosion drives the piston down, and the reciprocating motion gets transferred to the wheels. If you want a more powerful explosion, though, you need more fuel. More fuel requires more oxygen. A supercharger, spinning more rapidly than the car's engine, compresses air to above atmospheric pressure, and does not create a vacuum. So more air is forced into the engine. With extra air you can add extra fuel, resulting in around 46 percent more horsepower. They don't call it a "super" charger for nothing.
Interestingly, the supercharger is the lifeblood of Ralph Miller's patented Miller cycle engine. The engine, a design harkening back to the 1940s, is not completely unlike the traditional four-stroke engine, though there are two big differences: the supercharger design, and the fact that the engine's intake valve stays open during a portion of the compression stroke, compressing against the pressure of the supercharger instead of the walls of the cylinder. The engine achieves about a 15 percent increase in efficiency using this approach. Mazda has been using the Miller-cycle engine in some of its cars in recent years.
If the term "supercharger" has you recalling the term "turbocharger," you might be wondering about the pros and cons of each approach. A turbocharger is more efficient, because its power source, the exhaust stream, is energy that would otherwise have been wasted. But a turbocharged engine also provides less initial boost due to backpressure in the exhaust system; the boost in a turbocharged engine kicks in at higher rpm. As far as superchargers go, their main advantage is ease of installation, and the primary disadvantage of a supercharged car engine is its hefty price tag.
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