Tech file: Adiabatic efficiency

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"Adiabatic efficiency" is a phrase that rarely gets thrown around when advertising for turbocharger or supercharger kits, yet it's an incredibly important factor in choosing a compressor. The term "adiabatic" refers to a process where no heat is gained or lost. Adiabatic engines are already used in industrial applications by way of ceramic engine blocks (and sometimes internal parts) since the ceramic make-up neither emits nor absorbs heat.

In a perfect world, all turbos and blowers would work at 100% efficiency. This just simply can't happen since as any gas is compressed it's temperature raises as it's molecules rub together to create friction, thus creating heat. In the world of compressors the roots-style blower ranks at the bottom of adiabatic efficiency at about 50%. This is largely due to the fact that it does not directly compress air but rather compresses air that has already been compressed when entering the intake tract. Most other forms (twin screw blowers, turbos, centrifugal blowers) usually fall in around 60%-80% efficiency...the best centrifugal blowers maxing out around 80% (although, since their operation is rpm-based, it's a trade off to lower rpm range boost/performance). Obviously, in terms of adiabatic efficiency, a higher percentage is better since it means less heat is gained which, in turn, means that the charge is more dense meaning more power and less chance for detonation.

With turbos more than any other form of forced induction, the adiabatic efficiency is paramount to it's performance gains since it is starting out with hot exhaust gases to begin with. Yes, the after effects of air charge temperature raises can be fought with intercoolers and water/methanol injection, but it's important to start off with an efficient turbo to begin with to make even better the effects of such products. Efficiency levels of a turbo should be listed on it's compressor map which will be posted shortly in another article. It's important when selecting/ordering a turbo to look at it's compressor map (any reputable manufacturer should make them available) and check out the potential it has for the best possible performance gains. These days turbos shouldn't fall under around 65% (at the worst), but it's best to be on the safe side. You don't want that $3,000 investment giving you all kinds of hassle, do you?