READING TURBO COMPRESSOR MAPS, PART IV: EFFICIENCY ISLANDS

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

"Efficiency is doing things right." -- Peter Drucker

Anyone who has mistakenly put their hand on the shop compressor discharge line will always remember that compressing air results in heat. And NASCAR TV viewers often hear about "starting on low tire pressures" to avoid "pressure build-up."

Both of these are examples of the relationship between air volume, air pressure and air temperature.

Ideal gas law states that:

(air pressure x air volume)/air temperature = remains constant (Miller 19-20) Thus, increases in air pressure of a fixed quantiy of air result in increases in temperature.

When any compressor takes a "gulp" of air and compresses it into a smaller space, heat is necessarily produced. Often, the temp increase is explained as the result of increased friction between air moleules rubbing together when jammed into a smaller space.

Engineers and scientists describe the ideal temperature rise from air compression as "adibatic" -- neither gaining or losing any heat (beyond what the Ideal Gas Law predicts, that is).

However, no air compressor is 100% efficient. Internal air movement, impeller friction, pumping losses, and other inefficiencies add extra heat to the compressed air. This inefficiency is represented on turbo compressor maps and is critical to determining the actual density of the compressed charge.

Looking at the old T66 turbo map, the lines in the middle of the map show zones or "islands" of efficiency. The percentages shown are the calculated efficiency of the compressor, based on measurement of the discharge temperature of the compressed air.

Thus, when someone reports that a particular compressor is operating in the 75% efficient zone at a particular pressure ratio, they're saying that the compressor is heating the air 25% MORE than the Ideal Gas Law adibatic temperature rise formula predicts.

By way of comparison, when a traditional Roots blower is operating in a 50% efficiency island, then it is heating the air 50% more than the ideal temp rise formula predicts.

Extra heat in the compressor discharge air indicates two things. First, the extra heaing means that the dischared air is less dense than under ideal conditions. Second, the extra heat shows that not all of the "work" applied to compressing the air actually resulted in air compression -- some of the energy was "lost" to heating the compressed air. (engineers and scientists refer to this as "isentropic efficiency.")

However, for Car Crafters, the more important things are to determine how much density has been lost and how to recover as much of it as possible.