hopefully, this ends up as a very long thread, filled with good information, or at least theories that have merit...

so, what is thermal efficiency(specifically, the thermal efficiency of an otto cycle engine, since by a large margin those dominate the US market)?

it's how much of the energy contained in the fuel is actually used to power the engine, rather than getting "lost" as exhaust or cooling. it's commonly quoted that roughly 1/3 of the energy contained in gasoline is actually used in a useful way(driving the piston down its bore, which turns the crank, which powers the transmission, which turns the axle, which turns the wheels, with each step losing efficiency along the way), the other 2/3 is lost to the exhaust system and to the cooling system.

now, 33% isn't that great... you could be making 3 times the torque or achieving 3 times the fuel efficiency, depending on how far your foot is in the throttle, obviously.



so, now that we know what we're working up against, what can be done to convert the energy lost in the cooling system and exhaust system and use it to drive the engine?



friction = bad, no exceptions. friction takes useful energy to make more heat, which ends up in the cooling system.

heat IS energy. pressure IS energy. anything that can be done to keep those two things inside the combustion chamber and not the surrounding metal will keep energy out of the cooling system. when the spent air/fuel charge comes out through the exhaust port is another place for it to sink into the surrounding metal. the exhaust manifolds and downpipe as well. you could actually help cold-start emissions quite a bit if you kept all of the heat coming out of the engine intact until it hits the catalytic converter.



other suggestions???

one way of extracting energy from the exhaust is obviously a turbocharger, though with increased airflow/airmass caused by boosting, don't expect to see any MPG benefit. if you're looking for more power, it's a great way to go about it, since at least the energy left in the exhaust will be doing something.

there was some brief discussion before about coating internal parts with substances that would reject heat... seems like a good place to cut cooling system losses.

it seems like the cooling system is the low-hanging fruit... extracting work out of the exhaust(besides a turbo) seems to be difficult. using the exhaust to heat up the coolant to normal operating temps faster is actually getting used on some common cars now(prius is one IIRC).

neat fact: a hotter engine absorbs less usable energy into the cooling system due to the temperature differences. a 1500*F flame hitting a 70*F piece of metal gets more of it's heat sapped away than the same flame hitting a 210*F piece of metal. this is why some rather "radical" engines(NASCAR) run really hot thermostats/coolant temps.

another neat fact: aluminum conducts heat a LOT better than iron... thus, an aluminum head will transfer more heat out of the combustion chamber than an iron-head engine of the same design would. because of this "feature", you'll generally run more advance since the engine will be less likely to detonate/pre-ignite. one way to help minimize heat loss regardless of head material would be to minimize surface area.





MOAR THOUGHTS!?!???