View Single Post
Old 01-27-2010, 01:56 PM   #1 (permalink)
EcoModding Lurker
Join Date: Jan 2010
Location: Montana
Posts: 33
Thanks: 0
Thanked 26 Times in 10 Posts
thermal thoughts

I drive a '97 Chevy full size half ton 2 wheel drive pickup extended cab, 4.3 V6, 4L60E automatic, 3.07:1 gear ratio. I have had a scangage on it since I bought it 2.5 years ago at just over 100K miles. It turns in decent mileage if I drive it decently, and it has loads of power and with the Vortec engine at 210 HP, it will pull a trailer with as much as 10K lbs weight reasonably well provided you keep engine RPM up. I have modified the 4L60E with a dash mounted switch that sets up the torque converter lock, and a circuit that fools the computer so it doesn't generate constant error codes as a result. I have also disabled the PWM valve in the valve body that modulates pressures to allow clutch slippage for smoothness. I average about 22mpg on extended trips. Here in Montana, mileage is highly variable due to terrain and other conditions.

Temperature seems to have a significant effect on mileage. In warm weather I typically do better than in cold.... but that isn't news.

I had a huge brainstorm the other day on efficiency.

It is well known that higher compression results in higher efficiency....nobody with any experience would challenge that statement. A look at high compression reveals that the result of high compression is higher temperatures prior to combustion.... the thermodynamics of it. High compression unfortunately results in pre-ignition at full throttle at low speeds. In fact this is how diesel engines fire.
Gas engines throttle by cutting off air.... necessary to maintain a proper air fuel ratio. The result is that at low throttle settings, the compression is far lower, and thus the actual efficiency is lower than at high throttle settings if you look at fuel burned per HP produced.... rather than MPG. Volumetric efficiency is the only valid measure of efficiency for an engine.
OK.... so how do we combat this issue..... we combat it by using higher compression ratio.... which means that we have to use higher octane fuel, which means that we shoot ourselves in the foot because the high octane fuel will be LESS, NOT MORE efficient at low throttle settings.
Thus in a perfect world we would have either a variable chamber size so the actual pressure and temp prior to ignition was constant....... or barring that, we would have variable octane fuel. Neither of these is practical in the real world.... though the latter is doable.
So here is the brainstorm..... the solution I have never heard anybody even suggest. The solution requires a conclusion before hand... that it is the temperature prior to combustion, NOT the pressure that makes the difference. In reality it is probably some of both.
The obvious solution is HOT AIR....... Mention hot air, and everybody throws up their hands and says "you can't do lose power"... or some such mindless reaction.
The truth is that if we are producing 50 HP at full throttle at 2500 RPM for example.... with cold air.... and we only need 20 HP, we need to REDUCE power by throttling. We can reduce power with hot air... and carry more throttle than we otherwise would because the hot air is less dense. We also increase the temp at the top of the compression stroke proportionately. This both reduces throttle suction drag, and improves conditions for efficient combustion in the chamber.
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???


  Reply With Quote