Thermal Efficiency-does engine heat mean fuel milage?

[AudiVWguy]

Hi Everybody,
I was wondering if anyone had experimented with increasing engine temp will cause an increase in fuel milage. Right now I have a digital temp gauge with the sensor mounted on the head where the coolant exits to the radiator. The thermostat is an 87c so at highway speeds (65-70mph) the outlet temp is 190 to 197 degrees F. EGT varies from 580 to 680 degrees F.
Has anybody experimented with this? How much increase have you tried? Besides increasing the thermostat I will have to increase the fan switch also.
Looking forward to your comments.
-JB

[jimfoo]

Yes, in theory the hotter it runs, the more efficient it will be. In reality, you probably wouldn't notice anything I would think as you can't really increase the temp all that much more. Plus increased temp is bad for the engine oil. Yes, if you put a hotter thermostat in, you should increase the temp the fan comes on at. My car ran way hotter on hills than I wanted, up to 250 F, and I don't think I got any better mileage because of it.

[Vanagoner]

It seems that efficiency is the work achieved from the difference in temperature between the hot and cold sides of the heat cycle.

So with any heat cycle engine, you can increase the hot side of the cycle (to mechanical limits), or reduce the cold side of the cycle.  The thing I dislike most about turbo's (besides lag) is how it traps pressurized heat against the head, and reduces cold side efficiency.
 
p.s. I'm not an engineer, so correct me!  

[jimfoo]

Sounds about right to me. Also, the hotter an engine is, the more it radiates, increasing underhood temps, which heats the intake air reducing efficiency.

[Vanagoner]

So I'll venture to say that an n/a with a pulse tuned high flow exhaust (and maybe air or water mist injection into the exhaust) and high combustion temps (with a darned good oil cooler) might be inherently more efficient than a turbo.  Not more powerful, just more efficient.  Your thoughts?

[jimfoo]

Probably.

[Turbinepowered]

Recycling exhaust heat into the intake increases efficiency, as per the concept of regeneration that is used with stationary power plants.

Hot, moist air will net you better efficiency than cool, dry air. Note that the latter is better for power, the former for efficiency.

If you used a non-intercooled, non-boost compensated turbodiesel that also utilized intake air preheating you would see an increase in fuel economy. You would also see some very high temperatures inside the engine, so the oil cooler becomes quite vital.

[Vanagoner]

You're saying that a cold intake doesn't help efficiency-  So.. it is the temperature difference between the combustion moment and the exhaust moment which counts?  There has to be a maximized temp drop somewhere- right?

- I was wondering why VW went with an N/A diesel on their 300 mpg "1 liter car"-  maybe this is why.

On the question of the radiator fan temp setting, I'd leave it alone if I increased the thermostat temperature.  It would give a more immediate reserve of colder coolant to mix in, less chance of overheating the head.

[Turbinepowered]

Reciprocating piston heat engines utilize heat to create pressure, and the pressure does the work. The more heat you can load up the cylinder with, the more pressure you can generate and the more power you get for it. If you can use the heat that you've already "spent" and feed it back into the air going in, you need less heat from your fuel for the same pressure.

[edit] Sorry, cold medicines messing with my head.

The turbocharger and the hot-air induction are two separate incremental increases of efficiency. The turbocharger is easier to explain, as it simply acts to utilize exhausted heat (and the expansion of the gases so heated) to create precompression, which then augments the static compression in the cylinder. As compression goes up, so does efficiency. A turbocharger blowing ~7psi into an engine with 22:1 compression makes the engine effectively operate at 33:1 compression. Multiplying the pressure ratios, 1.5 (7psi turbocharger compression) by 22 (Static piston compression) yields 33 (total system compression).

You've increased this compression without directly demanding more work from your engine, as you would with a straight increase in static compression.

The hot air intake just serves to increase the available heat to do work in your cylinder, thus ramping up that temperature difference between hot (in cylinder) and cold (atmospheric).

[clbanman]

Some similar ideas were explored in Smokey Yunick's adiabatic engine.
http://www.eng-tips.com/viewthread.cfm?qid=78116&page=1

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4862859.PN.&OS=PN/4862859&RS=PN/4862859

[jimfoo]

So the greater final temp of the air outweighs the loss in air mass due to the higher initial temp is what you are saying?

[Turbinepowered]

So the greater final temp of the air outweighs the loss in air mass due to the higher initial temp is what you are saying?

If you're referring to efficiency, thermal efficiency, yes. If you're seeking power, then no, the hot air intake is a terrible idea and you want the air as cool as possible.

This is easier to do in a diesel, where you're running massively lean anyway, you don't have to worry so much about overfueling past stoich due to decreased air density.

[clbanman]

If you followed Smokey's whole schematic, he heated the fuel, not the air.  I'm working off my memory of the original Hot Rod article I read, but he claimed he was heating the fuel to just short of vaporizing it, had a modified "turbo" that he called a homogenizer that had a sort of screen instead of the full vane.  The idea was supposed to be to break the fuel droplets into a uniform small size which was supposed to prevent detonation despite the lean mixture.  All related much more to gasoline than diesel.

[Benjamin]

Intrested, but guys who run a bad thermostat dont have worser feul-mileage.

in the other case, some race engines run lower water-oil temperaturer and if i remember correct they do that to have more power.

Greetz, Benjamin

[upchuck]

I don't have any technical knowledge to add here, but after blowing HGs twice (not on the same VW) in the last couple years from what I perceived as minor overheating incidents,  I'd sure worry about warping your head and blowing your HG if you went any hotter than the stock VW setup.  I seem to recall reading that VWs with their stock thermostat already run on the "hot side."