A turbocharger, as fitted to a petrol or diesel engine, is part of a forced induction system, not to be confused with a supercharger. It compresses the air flowing into the engine and the advantage of this is that it lets the engine squeeze more air into a cylinder, and more air means that more fuel can be added, giving more power from the fuel/air explosion in each cylinder. A turbocharged engine produces more power overall than the same non turbo’d engine. This will significantly improve the power-to-weight ratio for the engine.
In order to achieve this power boost, the turbocharger uses the exhaust flow from the engine to spin the turbine section of the turbocharger, which in turn spins the connected air compressor. The turbine in the turbocharger spins at speeds of up to 150,000 rotations per minute (rpm) -- that's about 30 times faster than most car engines can go. And since it is driven by the exhaust gasses, the temperatures in the turbine are also very high.
The typical boost provided by a turbocharger is 6 to 8 pounds per square inch (psi). Since normal atmospheric pressure is 14.7 psi at sea level, that is equivalent to providing about 50 percent more air into the cylinders. You might expect to get 50 percent more power, but it's not totally efficient, so you will probably get a 30 to 40 percent improvement instead. One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having turbine blades rotating in the exhaust flow increases the restriction in the exhaust. This result of this is that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts some power from the cylinders that are firing at the same time.
The turbocharger is bolted to the exhaust manifold of the engine. The exhaust from the cylinders spins the turbine . The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurises the air going into the pistons.
In order to handle speeds of up to 150,000 rpm, the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.
One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the accelerator. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you first step on the pedal and then the car surges ahead when the turbo gets moving.
One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier. One way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger does provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.
Most automotive turbochargers have a wastegate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The wastegate is a valve that allows the exhaust gasses to bypass the turbine blades. The wastegate senses the boost pressure and If the pressure gets too high, it is an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust gas from around the turbine blades. This allows the blades to slow down.
When air is compressed, it heats up and when air heats up, it expands. So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. In order to increase the power of the engine, the goal is to get more air molecules into the cylinder, not necessarily more air pressure.
An intercooler is an additional component that looks something like a radiator, except air passes through the inside as well as the outside of it. The intake air passes through sealed passageways inside the cooler, while cooler air from outside is blown across fins by the engine cooling fan.
The intercooler increases the power of the engine by cooling the pressurized air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.