[Nels]

Originally Posted by owly

The mechanism would be a heat exchanger on the exhaust that would generate hot air. A butterfly would select hot or cold air.... or a percentage of each as part of the throttle mechanism. Thus you would first start pulling greater and greater proportions of hot air as you let off throttle, and as you reached the limits of that, the throttle would operate normally.... choking off air. It's a system that would utilize waste heat.
Hot air has less expansion potential... you might say..... true enough... but you have MORE of it, so the net result is probably equal. or thereabouts.

....................... Has anybody built anything along these lines???

Sounds similar to what all the automakers were doing in the mid 70's to early 80's with their heat riser valves feeding a portion of the heated air to the intake while the engine was cold to smooth out cold weather (and cold engine) operation. That was with carbureted engines though. More options are now available with computer controlled fuel injection. There are a couple folks over on the Saturn site (saturnalia.com) experimenting with warm air intakes to increase fuel mileage (at the expense of engine power). Some success in that regard. I'm not so sure it is an increase in efficiency though. In effect they are fooling the computer into leaning out the fuel/air ratio. There is a point where this can go too far, especially if more power is needed (up to and including melting holes in the tops of pistons ). With controlled use of throttle at low rpms the theory is interesting enough to investigate though.