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 that....you 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???

Howard

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 :eek: ). With controlled use of throttle at low rpms the theory is interesting enough to investigate though. :confused:

Hello and welcome to EM Howard. :)


Lotus is developing a variable compression ratio engine. See here:
http://ecomodder.com/forum/showthrea...ase-11392.html

Also, I believe Saab was working on one a while back. Haven't heard anything on it recently.

Also also, I believe Volvo has a electronically controlled hot/cold air mixing valve on some of their engines. I don't know Volvo stuff well at all, but I've heard someone here mention that they have them. Hopefully someone else can elaborate.

Interesting idea and we here have promoted warm air intakes for quite some time here. A lot of us don't mind loosing power, so the mixing valve isn't even necessary.

If you want to negate the power loss, a simple valve in the intake tract will open the intake back to cold air, and within a split second, the ECM will react to the cold air, compensating with more fuel, creating more power.

Gee, that was hard. :rolleyes:

I stand firm that any engineering thought that crosses anyone's mind should be filtered through normal, every day people first, before proceeding to think. Engineers have a tendency to work things out on paper, in 2 dimensions (metaphorically). Once you throw a square into the 3d world, it becomes a cube, and everything is changed. All of a sudden, your formulae just don't work anymore, and you're stuck back at square (cube?) 1.

That said, the idea is a sound one, and there are engine builders working on engines with such things as dynamic compression in mind. Have been since the 70's, when 15:1 static was do-able in a gasoline ICE. Somewhere along the lines, people forgot what's already been done, and now subscribe to some nonsense which says that it can't be done, with no premise for the thought.

Regarding the "less potential for expansion", this is just a ridiculous sentiment, when you consider that the percent change in temperature between 60 degrees to combustion temp and 140 degrees to combustion temp is very small.

Without looking it up, imagine for a second that cylinder combustion temps are approx 1,000degF.

The difference between 60degF and 1,000degF is 940degF. The difference between 140degF and 1,000degF is 860degF, or a total change of 80degF between the two (not counting changes in combustion temps due to the hotter air).

80degF is 8% of 1,000degF... so what noticeable effect is 8% expansion going to have?

Of course, if you count differences in pressure because of lessened combustion gasses, etc... the number increases some. It's still not going to make a huge difference.

that is very thought out. after doubting all manufacturing with compression levels set too low, ports too small, heat rise too much, and on and on and on...

the babble stops after increasing everything. Strain the mechanicals is also equaling efficient.

I have yet to exceed compression to need avaition fuel extremes. Anyone can swap an oe gasket to copper for example, even tiny engines dramatically effected by this change, and stay at 87 octane.

and lastly, variable valves suck. solid states will win a looong time to come. Get it right the first time, this variable stuff does not neeed to happen.

you the throttle, the road. Oh my god. that got confusing...:rolleyes:

I came across this article a couple of days ago while I was researching vapor carburetors. The basic idea is that in order for fuel to vaporize completely in the combustion chamber it needs to be pre-heated. These guys built a heat exchanger and ran their fuel through it before it got to the intake manifold. Then, in order to minimize condensation of the fuel, they used another heat exchanger to heat the air mixing with the vaporized fuel. The article explains the process and several different experiments they conducted. In the end, they used a three valve butterfly system, two cold air and one hot air, intake system. Read the article and share your thoughts. The logic is sound as far as I can tell.

Fuel Vaporization Through Pre-Heating: Anthony P. O'Donnell Experiements

It's not going to change anything when fuel "droplets" are microns in dimension anyway. For all intents and purposes, modern fuel delivery systems atomize the fuel with nearly 99% effectiveness.

"Injector" is sort of a misnomer in this case - they should be referred to as "Vaporizers", since that is more directly related to their job.

Is it still insignificant if it is actually 8% ;)

Good catch


Even 8% of expansion isn't going to be noticed, especially considering that a portion of the expansion of gasses happens during and after the exhaust stroke.