The Evolution Of Internal Combustion: Thermal Energy Management

Greg & carve, Thank you for clarifying what was a "common misconception" on my part... what clued me in was the comment about two-stroke expansion chambers. I found this longer explanation, which pulls both of your points together: http://www.turboclub.com/turbotech/TurboFun2.htm That said, the total percentage of heat energy recaptured by a turbocharger must be relatively small, and partially offset by the back pressure and frictional heat it generates. Is anyone prepared to argue that a 200-horsepower turbo-four puts out significantly less heat than a 200-horsepower normally aspirated V6?

Thanks for the reply, Don. If the turbo extracted power from the exhaust and added it to the crankshaft, then yes: they'd put less heat out the exhaust for a given power output. However, turbochargers are almost exclusivly used to compress intake air, which increases power instead of efficiency. The same heat in a smaller engine means higher temperature, which is then cooled by expanding through the turbo. Heat & even temperature output at the tailpipe of a 200 hp turbo 4 is therefore likely similar to a 200 hp V6 when both engines are making their rated 200 hp. The advantage of the turbo engine is that it's smaller & lighter, and it has reduced pumping losses while cruising. The advantage of the normally aspirated engine is that it has quicker, more predictible throttle response and is simpler. If the turbo was geared to provide power to the crankshaft directly rather than compressing intake air, exhaust temp would indeed be cooler. The effect of this added expansion would be similar in principal to an atkinson cycle engine, which expands the charge to a greater volume than the amount drawn in and compressed. The efficiency of heat engines is largely proportional to the temperature difference between the hot side (combustion fire) and cold side (ambient air). As the air in the engine is expanded, it cools off. Power output falls as the hot air continues to expand, and you need a much larger volume (bigger engine) to extract the leftover marginal heat. You'll often refer to this as "quality" of thermal energy. A small volume of high temperature air has a higher "quality" than a big volume of lower temperature air, even if the total thermal energy is the same. This is why automotive engines tend to be less thermally efficient than stationary engines and powerplants- they'd have to be enormous to keep extracting heat from the cooling combustion products, which requires a bigger, heavier car, which requires more power, and so on. Since you have to use more energy to move that enormous mass it really doesn't save you much fuel. This is also why you'll often see race cars belching fire out the exhaust- they're just using that first little bit of high-quality thermal energy to maxamize power output at the expense of fuel efficiency. Turbines are a relatively light and simple way to extract some of that marginal heat. Using them to compress intake air is a way to actually IMPROVE your power to weight ratio, so that's why they're used almost exclusively for that purpose. Still, they don't actually generate boost that often and it'd be nice to extract power from that marginal heat all the time. A turbo alternator would probably be the way to go (especially in a hybrid) since it wouldn't add any additional weight or complexity & would remove parasitic drag from the engine. The hard part though would be de-coupling the compressor from the turbine when you didn't want to make boost. Perhaps the best way is to have the turbo ONLY power the alternatore, and then have an electric supercharger.