Increasing engine operating temp to increase MPG

[squirrl22]

It is well known that if you increase the operating temperature of an Internal combustion engine, you increase it's thermal efficiency, thereby increasing it's MPG and HP. This was demonstrated by "Smokey" Eunick with his Adiabatic Engine https://schou.dk/hvce/
I used this concept to get the most MPG from my old 1986 VW Jetta diesel (RIP- Deer strike killed it) by using a custom thermostat- would open at 230F, and Evans waterless coolant.
I have never seen this discussed on any thread, so I thought I would bring it up.
Personally, I was getting 43-46MPG on a long highway commute to and from work before my temp mod, and afterwords, I was averaging between 50-55MPG.

[redpoint5]

I'm not buying a 25% engine efficiency improvement by simply running hotter; not even close.

Sure, there's efficiency to gain by running hotter, but we're talking single digit improvements, if that.

I'm sure someone is skilled enough to show the theoretical efficiency gains by going hotter.

[squirrl22]

[QUOTE=redpoint5;594358]I'm not buying a 25% engine efficiency improvement by simply running hotter; not even close.

Skepticism is good, but denial of someone's real life experience, with zero facts and zero experience to back you up, is simply being closed minded and willfully ignorant.
Calculating a 25% increase in engine efficiency is simply bad math.
43mpg to 50mpg is a 16.27% increase in mpg. 46mpg to 55mpg is 19.56%.

Going from a 160F thermostat/running temperature to a 230F thermostat/running temperature is a thermal increase of 43.75% .

Did you read thru all the info on the link to Smoky's Adiabatic Engine?

[redpoint5]

Yeah, I misread 40 MPG instead of 43. Probably my mind was trying to keep the math simple.

160F to 230F isn't a 44% increase in temperature. It doesn't work that way because F doesn't start at 0, but instead -460. That's more like 11% hotter. I don't have the time to figure out the theoretical maximum efficiency gain by going hotter, but it's obviously not that drastic or you'd see auto manufacturers figuring out how to do just that.

I'm not saying it isn't worthwhile to run a hotter thermostat, I'm simply disagreeing with the notion that the MPG gains are primarily attributable to running hotter. I need to see how the variables were controlled so that the tests stand the least chance of having confounding factors.

I could say I used turn signals and got 30 MPG, and then stopped using turn signals and got 40 MPG. It's meaningless unless I can show how the variables were controlled.

[squirrl22]

I've thought about it over the years, Obviously some of the gains were from greater heat retention when not running, faster warm ups. I remember being thrilled that I could now get hot air blowing from the heater less than 1 mile from home, instead of 5-6 miles.

I disagree that automotive engineers would be looking at this, I have known too many in my time. Offhand I can think of a dozen examples of automotive engineers ignoring existing engineering solutions.

Still, I urge read thru all the info out there on Adiabatic Engine efficiency. I makes sense, if you think about retaining more engine heat - The more heat you can retain, the higher your efficiency.
Smoky demonstrated conclusively the advantages.

I would love to find someone else who has experimented with this. Mine was a diesel IDI engine, Smoky was successful w gasoline engines.

[ksa8907]

The equations of thermodynamic efficiency do not concern the operating temperature. The idea is to extract the heat energy from the combustion process. So, it's the difference in thermal energy between peak combustion temp and the temperature as the piston reaches bdc.

For reciprocating engines, it is difficult to improve due to the nature of the very simple and short process.

[squirrl22]

The variables were all pretty well controlled- same commute for 5+years, All known maintenance variables like tire pressure well monitored and maintained, speed rarely varied- 60mph w cruise control, variables like wind, traffic, outside temp, etc. would all even themselves out over 5 years, fuel always from the same Mobil station.

Self cancelling turn signals

[squirrl22]

You can throw out there all kinds of theory about how you think something should work, but show me your real life experiences, actual experiments and personal results. Real experience trumps theory 100% of the time.

[redpoint5]

The laws of thermodynamics are not a theory, which is why they are called laws.

I think the idea with running hotter is that less heat from combustion is absorbed by the engine since the temperature differential between combusted hot gas and engine is reduced. I dunno, I'm no expert in this area.

Still, a 20% increase in power for a given amount of fuel doesn't make sense. It doesn't pass the "reasonable" test.

[squirrl22]

There a huge difference between the laws of thermodynamics, and the theory of how those laws work inside an engine. While discussion of the theory of how those laws work inside an engine is an interesting exercise in hypotheticals, what I am hoping for is some actual feedback from people with real experience.

I realize that most thermostats only vary in range from 160F to 195F, but even people who have tried something a simple as just going from a 160F to a 195F thermostat, who have taken the time to monitor their before and after numbers would be appreciated.