There are many ways to make an internal combustion engine work more efficiently, they include cylinder heads that use dual overhead camshafts, variable length intake manifolds, turbochargers, and superchargers. This paper will discuss the last of these devices listed, the supercharger. All good supercharging systems also integrate an intercooler to increase efficiency, so lets take a look at the two units together and discuss the physics of what they do and how they work. A supercharger is actually a very simple device; it is nothing more than an air pump. In the past decade superchargers have become a somewhat popular and cost effective way to increase an engines volumetric efficiency without substantial losses to fuel economy. The specific type of supercharger that will be examined is the Eaton roots type. The roots type supercharger consists of two helical shaped rotors that accelerate opposite of one another in a metal housing.
The rotors are belt driven off of the engines crankshaft and turn at velocities of up to five times that of the engine. Once the air is compressed by the supercharger, it then passes into the intercooler. The intercooler acts as cooling system that reduces the temperature of the compressed air. It is extremely important that a supercharging system has an intercooler because of the high levels of friction that are created and experienced by the process of compressing the air. From the intercooler the compressed air then enters the engine and is mixed with the appropriate amount of fuel. Now that we have established how a roots type supercharger with intercooler works, lets take a look at the basic physics of what is happening. Obviously when the supercharger is at rest, not turning, there is an extremely large amount of potential energy.
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As previously stated the supercharger is belt driven off of the engines crankshaft, so therefore it requires the input of ordered energy to work. By using a system of pulleys, it is able to transfer the mechanical energy from the engine required to turn it. There are two pulleys used, one and the crankshaft and one on the supercharger. The two pulleys are different sizes; this creates a mechanical advantage. The supercharger pulley may be as much as five times smaller than the crankshaft pulley, allowing the supercharger to spin much faster than the engine. Once the engine is started the supercharger then begins to produce kinetic energy by turning as well.
The supercharger then begins to displace air and actually perform work. The two helical shaped rotors inside the housing turn in opposite directions of each other, they are of equal size but have exactly opposite shapes that intertwine almost like gears. The two rotors have parallel axes of rotation and accelerate opposite of each other. There is enough space provided by the shape of the rotors and the way they intertwine to move molecules of air through the system. The idea behind the supercharger is that it needs to move the air from a region where it naturally exists and force it into a region that it normally wouldnt; just like any other pump would do. Because of the fact that the rotors are constantly turning and air is always flowing through the machine, it creates pressure by pushing more molecules into a system of a constant volume. With increased pressure also comes an increased temperature, because the air molecules are being forced from one region into another against their natural flow.
The increase in temperature is due to many factors, obviously moving the air against its will is going to increase kinetic energy and therefore heat, but also from the friction of the superchargers internal components and passageways. The Eaton Company coats the rotors with Teflon in an attempt to reduce the friction of the two rotors spinning against each other. Pressurizing air creates an incredibly large amount of heat. The intercooler is then put in line after the supercharger in order to reduce the temperature of the pressurized air. An intercooler is really nothing more than a simple radiator. It is a system of tubes with cooling ducts and fins that allows air to pass by and cool it off. The process by which the intercooler releases heat is convection. The air passing by the intercooler transfers heat from the intercooler to the surrounding air which has a lower temperature. This effectively reduces the temperature of the compressed air inside the intercooler and creates an even more dense flow of air into the engine, thereby increasing the engines volumetric efficiency and most importantly allows more fuel (energy source) to be added to the mixture.
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The supercharger and intercooler systems complement each other greatly when added to even the most basic of engines. They are very simple machines that make a dramatic improvement in the performance of an engine. If you have never driven a vehicle with a supercharger, you are missing out on an engineering marvel. Now that you know the physics of how they work, its time to test drive a car with one and witness the difference it makes.
