FIRST PRINCIPLES: A PRIMER ON TURBOCHARGER ENERGY USE

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"Energy can be neither created nor destroyed. It can only change forms." -- First Law of Thermodynamics (abridged)

Many Car Crafters first learn about this through the Bernoulli Principle, as applied to carburetion. Speeding up air through a narrower passage (a carburetor's venturi) lowers the pressure of the air stream and allows outside air pressure (through the fuel bowl vents) to force fuel through the metering system into the venturi air stream. In other words, Pressure energy is briefly exchanged for velocity energy.

Changing energy into different forms is also at the core of how turbocharging works. Pressure, velocity and temperature of the gas passing through the compressor and turbine are interrelated and change predictably at different points of the turbocharging process.

For example, the compressor impeller increases the velocity of the intake air by pumping it through its blades at a high r.p.m. This air velocity energy is then tranformed into stable flow and higher pressure through the diffuser section of the compressor housing. Although some energy is converted into heat, the bulk of the energy input has transformed from rotational energy into increased air pressure.

Remember that the difference between the outside air pressure and the post-compressor air pressure is called the pressure ratio.

More energy is, of course, added to the system through the combustion process. Thus, on the turbine side, the hot, pressurized and pulsing exhaust gas is accelerated through a volute in the turbine housing(think: funnel bent around a circle) to a nozzle. The exhaust's expansion from the nozzle through the turbine blades to the low-pressure exhaust rotates the turbine, converting velocity energy and sometimes pulse energy into the rotational force that powers the compressor.