Tuesday, April 29, 2008

The Basics of Internal Combustion in the MINI Cooper

Let’s start at the beginning. Your MINI Cooper is powered by an internal combustion engine. Aside from some electric cars, nearly all cars on the road have IC engines. All this means is that the power is produced by an explosion—combustion— that happens inside—internal to—the engine.

In contrast, in an electric car the combustion happened somewhere else, at a power plant or in the sun. The resulting energy was sent over the electric power grid to be stored in the car’s battery and then used by the car’s motor to produce power.

With a hybrid car, the power still comes from internal combustion within the car’s engine. However, what makes the car a hybrid is that it also has an electric motor that can be used as both to produce power and to generate power. Normally the car is driven by the IC engine, and any excess power is used to make the electric motor generate electricity, which can be stored and used by itself, or in conjunction with the IC engine. But we digress.

In an internal combustion engine, the power is produced by the interaction of three forces. Air is pulled into the engine, is mixed with gasoline, and then the mixture is compressed by the cylinder and ignited by a spark to produce an explosion which pushes the cylinder down, turning the crank, and producing power. Air, fuel, and spark. Once the explosion occurs, the only remaining task is to get the resulting smoke out of the engine as quickly and efficiently as possible, so a fourth factor, the exhaust, enters the equation.

In modern automobiles, to provide the fine-tuning needed to maintain performance while meeting emission regulations, the air/fuel/spark equation is controlled by an engine control unit (an ECU)—a computer which controls basic engine operations like fuel mixture and spark timing— linked electronically to the throttle pedal and to several sensors that measure engine performance.

The S Stands for Supercharger

In the MINI Cooper S, the engineers added another component, the Mini Cooper Supercharger, which is only used on high-performance cars. It is used on Jaguars and Bentleys, for example, but on few cars as inexpensive as the MINI. The presence of the supercharger is one of the few major differences between the MINI Cooper and the MINI Cooper S

The supercharger is worth a few words on its own. As we mentioned, in order for the fuel to burn, we need air. If we want more powerful combustion, then we need more air.

This principle first became an issue back in the days when all airplanes used IC engines and the designers wanted their craft to fly higher. However, the higher the airplanes flew, the thinner the air became. With less air, there was less power produced by the engine. So engineers came up with the idea of using a little component with spinning blades, powered off the engine, to compress the air coming into the engine. With more air being forced into the engine, more power could be produced. They called it “supercharging” the engine.

It wasn’t long before automobile designers were using the same invention on the ground to make race engines run faster without having to get bigger. Remember the “Blower Bentleys” that were raced at LeMans in the early 1930s? You probably don’t, unless you’re an auto history buff. They were probably the earliest well-known application of a supercharger in a racing car. But if you want to impress your car buff friends, just tell them you have a “blown” MINI and refer to your supercharger as the “blower.”

It’s that same principle we find in the MINI Cooper S today. A small turbine between the air intake and the engine is driven off the main driveshaft by a pulley and belt to compress air coming into the engine. More air means that more fuel can be added, and more power will be produced.

Incidentally, engines in some other makes of cars address the same problem of compressing the intake air by using turbochargers. The difference between a supercharger and a turbocharger is how the little vanes in the turbine are powered. In a supercharger, the power comes directly off the driveshaft, connected to the supercharger pulley by the main engine belt. In a turbocharger, there are two sets of vanes, connected by a shaft. Exhaust gas coming out of the engine spins one set of vanes, which in turn push the other vanes that push air into the engine.

The problem with a turbocharger is that you’ve got to wait for the engine to build up some exhaust pressure before the turbo kicks in—what the gearheads call “turbo lag”—which means that the added power isn’t immediately available. With a supercharged engine, the supercharger spins faster as the engine gains speed, so the added power is always on tap and ready for use.

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