Even though Carnot showed long ago that all engines are doomed to be inefficient due to the law of ever-increasing entropy, engineers have found ways of extracting maximum efficiency from engines through intelligent designs and modifications. One such smart modification is the introduction of turbochargers.
What are They?
A turbocharger is a turbine-driven air compression mechanism that is used to force pressurized (high density) air into engines to increase their power output. Denser air ingested into engines lets more fuel burn during every compression stroke, which increases the power output of engines through every engine cycle. To put it in simpler words, the engine gets more air to breathe, in order to drive the combustion, when a turbocharger is fitted in.
The turbine that drives the turbocharger compressor is operated by the feedback of exhaust gases from the engine. So the entire mechanism is essentially self-propelled by the car engine itself. The turbine that drives the compressor rotates at a speed of about 150,000 rotations per minute. It provides a boost of 6 to 8 eight pounds per square inch (psi), in terms of pressure exerted by air ingested into the engine. As the outer atmospheric pressure is 14.7 psi on an average, with every engine cycle, 50% more air is pumped in. These contraptions have been known to boost engine power by as much as 30% to 40%. They are fitted in petrol and diesel engines.
These types of engine-boosting mechanisms are called 'forced-induction systems'. Turbochargers are usually fitted in high-performance sports cars. These car parts are developed using light metals so that they do not increase the overall weight of the car significantly, while boosting its engine power. They are also used in airplane and motorcycle engines to boost power output.
Design and Working
Turbochargers are directly attached to the exhaust manifold of cars. The exhaust gases from the engine pass through a pipe and drive the turbine wheel of the compressor before getting discharged. The turbine wheel drives the compressor wheel fan attached to it, that is connected to the intake manifold. This fan acts like a centrifugal pump, drawing air into the intake manifold and compressing air in the process. This pressurized air is fed to the engine to boost its power output. Since the turbine rotates at very high speeds, fluid bearings are used to support it. Normal bearings wouldn't stand the high friction caused by the rotating shaft. Fluid bearings maintain a constant level of oil around the shaft that lubricates and cools it.
As you can see, they are driven by a feedback mechanism, powered by the exhaust gases of the engine. This feedback does have an effect on the working of the engine, as the exhaust gases need to be pushed out more forcefully than before by the engine. Also, the turbocharger takes some time to build enough compressive power and boost the engine power. So the car may experience a 'turbo lag' initially. However, this effect can be reduced by using lighter metals and ceramics in the construction, which reduces the overall inertia. An alternative way is to reduce the size of the device.
There is another danger of the turbochargers going into overdrive at high engine speeds. To prevent this, most of them have a 'waste gate' that lets the exhaust gases bypass the turbocharger when it goes into overdrive. Some cars are fitted with two of them, one small and one big, that act at different engine speeds. The efficiency of car engines gets reduced at high altitudes as the air gets more rarefied. Therefore, at high altitudes it comes in handy by pumping in pressurized air and maintaining engine efficiency.
As a comparison would reveal, turbochargers are a type of superchargers, developed to boost the efficiency of engines and their power output. Development of such engine power-boosting technologies has enabled engineers to extract the maximum power from engines within the limit on engine efficiency, set by Mr. Carnot.