How a Turbo Works (or: How Turbos Work)

Turbochargers harness some of an engine's wasted energy to compress the air going into that engine - thereby increasing the output of the engine. Perhaps you can sense something cyclic about the previous sentence. That's because a turbo charger is by its very nature, cyclic. Use of a turbocharger can increase the output of an engine by 30 percent or more without a significant increase in the weight of the vehicle.

The Turbocharger Turbine
At the end of combustion, as the piston is traveling back up the cylinder, the exhaust valve(s) opens and hot air is forced out of the cylinder. This exhaust is wasted in a naturally aspirated engine, simply being released into the atmosphere. A turbocharger, however, recycles this energy. In a turbocharged setup, the exhaust gases are forced through the turbine of the turbocharger, this causes the turbine wheel to spin.

The Turbocharger Compressor
The turbine wheel is directly attached to another wheel in the compressor of the turbo. On this side of the turbo, often referred to as the compressor or the "cold" side, the spinning compressor wheel compresses the intake air. This pressurized air is then forced into the engine. Since pressurized air has more oxygen per unit volume (PV = nRT), there is more opportunity for combustion. This additional oxygen can either accompanied by additional fuel for increased power or in some diesel applications, the advantage is in the excess oxygen which allows for cleaner and more complete burning of the fuel, resulting in reduced emissions and increased efficiency.

Compression Generates Heat
When air is compressed so that more air can fit in the same volume, its temperature is increased. This is compounded by the fact that the turbo is not 100% efficient and generates heat when compressing the air. Since the air is hotter, it is less dense, which means less oxygen is in the cylinder than if the air were cooler. This explains why a 14.7 psi turbo gauge pressure will not double the power of an engine. The heat also explains why an engine under boost is more prone to knocking (predetonation or auto-detonation). In order to cool the charged air, an intercooler or water/alcohol injection is often used.

Turbocharger Selection
Understanding how turbochargers work isn't much help if you can't pick one that is properly matched to your engine. Selecting a properly sized turbocharger is a relatively laborious and challenging process when done the traditional way - by hand. It involves calculating the air requirements of the engine when under boost, accounting for many variables, and plotting these requirements on what are known as compressor maps, which tell how efficiently each turbo will be at providing that amount of air.