|Hotter air in equals mo’ hotta air out! Takes more power too. Translation? Laggy turbo.|
Notice that the hotter the air going into the compressor, the more power is required to compress it. Also, the increase in the air temperature is greater. Of course, this results in the final temperature being even hotter. I think comparing air inlet temps of 77F and 122F is reasonable; 77F being ambient air temp and 122F being the temp if you ingest air from the engine bay. So by sucking up the hotter engine bay air instead of cooler air from the front of the car, the compressor power required increases by 4.3hp, or about 8%. The difference in the temperature coming out of the compressor is a toasty 60.1F making the intercooler work that much harder.
Of course, there’s no such thing as a perfect compressor, so let’s see what happens with a 60% efficient compressor; this is often where the tuner industry operates as they try to squeeze as much power as possible out of turbos. Looking again at 77F and 122F inlet temps, the power and temperature differences are now 7.1hp and 70.2F!
|Crappy compressor efficiency means supa dupa hotta air! You could boil an egg on it. Mo’ laggy too.|
So now we know how crappy compressor efficiency and sucking in hot air increase the power required of a compressor to move and squeeze air. But where does that power come from? A supercharger gets it off the crankshaft of the engine and a turbocharger uses a turbine wheel in the exhaust. If you go by old school nomenclature, what we call a turbocharger was referred to as a turbo supercharger. So basically a special type of supercharger with a turbine wheel to get work out of the exhaust. Anyways, we like turbos because they use otherwise wasted energy. Going back to the very first chart, a perfect compressor doing 100lbs/min at a pressure ratio of 4 would need almost 150hp. Given the option of taking that from the engine crankshaft or the exhaust energy, we’ll take the exhaust energy. That’s the reason why pretty much every diesel engine and the majority of gasoline engines use turbos instead of superchargers. So does the turbo engine make 150hp more than the supercharged engine? The answer is no because the turbine wheel and turbine housing create back pressure in the engine reducing its volumetric efficiency. But a turbo engine will still make significantly more power than a supercharged engine given the turbo is properly sized.
Sizing of the turbine wheel is important so as to get the maximum efficiency from it. Our worst case scenario of the 2.0L engine, 60% efficient compressor, 50lbs/min mass flow rate, 2.75 PR, and 122F air inlet temperature requires 92.4 hp of shaft power from the turbine to drive the compressor wheel. A 100% efficient turbine wheel would need to get 92.4 hp worth of energy out of the exhaust. However, like compressor wheels, there’s no such thing as a 100% efficient turbine wheel.
|Throw in some moderate turbine efficiency and you end up needing a lot of exhaust power to spin a compressor.|
Plugging in a value of 60% for turbine efficiency, we can see how much energy needs to be pulled from the exhaust based on the temperature of the air going into the turbo. The difference in power required going from 77F and 122F is a lag inducing 11.9 hp, or about 8%.
So what have we learned? Maximizing compressor and turbine efficiencies reduces the exhaust energy required to get a turbo going. Sucking in colder air instead of hot air also makes compressors happy. The final conclusion to all of this is that math is cool, stay in school!