Thermodynamics
Thermodynamics plays a large part in the system of a turbocharger. Gas, pressure,
temperature, and volume are all constantly changing, and their relationships
can be seen through thermodynamic properties. The process that starts the
turbocharger is the flow of exhaust gasses coming from the engine. The turbocharger
is placed inline of the exhaust between the engine, and basically the open
air. When the exhaust is released from the engine it is at a very high temperature
and pressure. It travels to the exhaust turbine in the turbocharger, where
it quickly wants to get to an area of less energy, which is on the other side
of the turbine. The turbine does not spin in most part due to the kinetic
energy of the gas pushing against it as one may think, such as a pinwheel
in the wind, instead it is the effect of the expanding gas. The open side
which is effectively at atmospheric temperature and at a relatively low temperature
compared to the exhausted gas, causes the gas to quickly expand and cool,
starting the turbine to spin, and creating work. This work can then be used
to spin the compressor turbine. In the opposite way as the exhaust, the compressor
turbine brings in fresh air, and by spinning at a very high speed compresses
the air. We know that pressure and heat are proportional to one another, so
as the gas is compressed its temperature also rises, by a large amount actually.
This is a downside however because hotter air is less dense than cooler air,
and therefore less air molecules per unit of volume will enter the engine
than would air at outside temperature. To solve this an intercooler is placed
inline before the engine. This is a large metal container similar to a radiator.
Outside air is able to pass by it cooling it, and effectively cooling the
compressed gas inside. Now a large amount of air is able to reach the engine,
thereby increasing the amount of gas fed to the engine, which increases power.
Although similar in appearance, the compressor and exhaust turbines are not of identical design. Remember that the fresh air half consists only of air. The exhaust side adds combusted gasoline to the fresh air, creating a much larger volume. Therefore the turbines must be designed to operate under much different conditions.
Mathematically the power increase due to the turbocharger can
be found if we assume the process to be adiabatic. This is where no heat enters
or leaves the system, which is almost true. Therefore the change in energy
of the gas equals the work done by it, and in that way the power increase
can be found. However in reality there will always be some losses due to friction,
and this can only be used as an estimator.