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Turbochargers Blow Strength Into Small Car Engines

Fredrik Westin
Department of Machine Design
KTH (Royal Institute of Technology, Sweden)
June 2002

To increase the power of a car engine, a designer can choose to use a turbocharger. If the turbocharger is used on a small engine, this small engine can do the same work as a large one, but weigh less and consume less fuel. The turbocharger uses otherwise wasted energy from exhaust gases and returns it to the engine. However, because too little is known about turbochargers, they are less efficient on the engine than they could be. Increased understanding of how turbochargers work is necessary in order to utilize them in building less polluting car engines.

The most important issues for today's car manufacturers, and therefore for mechanical engineers, concern the environment. With a turbocharger, or turbo, the engine can be reduced in size and weight, but still produce the power required. With a lighter engine, the rest of the car can also be made lighter, and therefore the mass reduction of the car will be larger than just the mass reduction of the engine. A lighter car will require less fuel, making it less polluting. The turbo can be used on any kind of combustion engine regardless of what fuel type it uses, and with a better fuel than gasoline the pollution reduction will be even larger.

The turbo can be described as two fans sitting on a common shaft. The engine's exhaust gases drive one of the fans, causing the shaft to rotate. The fan at the other end of the shaft is connected to the intake pipe of the engine, where the engine takes in the fresh air needed for combustion. When the shaft rotates, this intake fan blows more air into the engine than what the engine normally would have ingested. With more air, the engine can burn more fuel and produce more power. Therefore, exhaust gases that would otherwise be wasted along with their energy, are used to drive the turbo. But of course there is a catch; when an engine is designed to use a turbo, other parts of the engine must be changed, and these unwanted changes lead to a decrease in efficiency. With a better understanding of the turbo, this unwanted decrease in efficiency would not be necessary.

The normal procedure for an engineer or researcher to take is to make measurements in the real world and try to describe them after evaluating the data. One of the main difficulties in finding out how the turbo really works is that there are no measurement techniques good enough to measure everything we would like to know. With the help of computer simulations, one can get data that otherwise would be unknown. But the problem is that since it is still not completely known how the turbo works, one cannot make perfect computer simulations.

An example of data that is very difficult to measure is exhaust gas flow and temperature at the inlet of the turbo. In the engine, these values fluctuate heavily and the maximum values are extreme, up to 500 m/s and 1300 deg C, or like the speed of a fighter airplane and a temperature almost high enough to melt steel. Measurement equipment, therefore, either melts down or is blown away. The simulations, however, have no problem in calculating this data, so what I do is take measurements on the engine with the best possible equipment, and then conduct computer simulations to try to reproduce the measurement results. However, since I cannot measure everything I want, and the simulations are not completely perfect, neither method gives the complete picture. But, if I combine data from both methods then the data together can give new information that could not be acquired from using just one method. This new information can then be used to make better computer simulations that will aid in the development of better engines and turbochargers.

My reason for conducting this research is that with a better understanding of how the turbo works, one can develop engines that consume less fuel than the engines of today, but which are still as strong or stronger. And lower fuel consumption will of course lead to decreased pollution.