Increasing engine operating temp to increase MPG
 

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.

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.

increasing efficiency
 

[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?

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.

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.

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.

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;)

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.

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.

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.

I have seen a small improvement from doing a partial grill block. This has helped my engine reach operating temperature quicker in the cold weather and it runs slightly warmer.

I agree with a hotter thermostat being more efficient. Here is someone else's experience:

https://www.jeepforum.com/forum/f11/...5-ats-1551782/
I have also read experiences where people have resolved their bad gas mileage by replacing their 165 degree thermostat with an OE 195 thermostat.

increasing efficiency
 

Nice find, Taylor! Actual hands on experience! So, if the LJ averaged 16mpg, going up by 1-2MPG translates into 6-12% improvement in MPG, switching to a 205F thermostat!
Thanks for the data!

This has nothing to do with a hotter thermostat. It means that the old thermostat was not fully closing, so the engine warmed up slowly. If a thermostat is fully closed, the engine will warm up at the same rate regardless of the thermostat opening temperature.

Running hotter does typically increase efficiency, 20% seems a bit high to put squarely on operating temperature change. Perhaps hotter under hood temps also caused a warm air intake effect.

The reason mfg's don't go much hotter than 200 is thermal expansion and oil degradation. The metal parts push on eachother more at the same time the oil life is being reduced. Perfect recipe for destroying an engine for 80% of vehicle owners who dont maintain ther vehicle properly.

My Insight uses double spark plugs, has EGR and a thermostatic valve that will open partially at 81 degrees Celsius (177.8F), and fully at 92.

Usually the temperatures will stay in the low (1)80's even with my partial grill block. But on longer trips the intake temp will gradually build and at some point that reflects in the coolant temp reaching to the 90s.
All is fine up to 94 degrees (200 F, indeed). Above that MPG suffers noticeably. I then set the blower to max and the cabin temp up; coolant temp drops quickly and good MPG returns.

So it depends.
You need to monitor what's going on, and don't expect miracles when you already have EGR.
I don't see anything more than a small gain (2-3%) and taking it too far does hurt. But a different car may react differently.

Could be, I don't remember if the old mk 2 Jetta heater core was on a separate line from the thermostat.

The big question, is:
Have you had any experience raising the coolant temp to increase MPG? If so, this is the place to share that experience.

He refers to the thermostat that closes off the radiator loop when the engine is cold, not that of the heater core.

If the radiator stat is bad it will even when the engine is not yet warm allow coolant to the radiator which feeds back cold coolant to the engine, preventing it from heating up quickly.
This is indeed very bad for economy. I had this happen in my '85 Civic. I had no economy readout back then, but it made itself felt at the pump.

You do kinda want to keep the internal parts temperatures below 250 because at that temp the oil starts to break down and lose lubrication factors.

Typically at the rod to piston junction, but I don't have current technology information and have no way to measure that. Historically rod bearings fail from heat/lube failure.

Additionally, since there seems to be a dearth of information, some experimentation needs to be performed because typically most ICE favor different operational temperature efficiency.

I am 100% in favor of grill block, test, modify, test rinse, repeat until trends show you went too far or oil temps get to be about 225f, or the smoke point of whatever oil you use. Smoke point should be available on the mfr website.

My 05 TJ has a 210* thermostat stock. I don't like running so close to boiling point. Remember this is an ancient engine. It's ridiculously fast to warm up considering there's 200+lbs of iron in it! Lots of excess heat.

All 'new' cars I've had run 190 degrees and in normal driving the thermostat doesn't actually open fully since their normal operating temp is 176 degrees. Its been this way on my last three Euros so it's not just coincidence. Fans kick on at 200 and it seems the only way for this to happen is with a really aggressive grill block.

Could also be that this is done for NOx control. Two of them have 20k mile oil change intervals so if lower temps reduce oil deg then it could be done for that reason too.

But it seems to me new cars run cool.

Honda put a hotter thermostat in the first gen Insight than any other car of their lineup at the time, which tells me there is SOME efficiency to be gained. Although the claimed in this thread doesn't seem realistic to me. Seat of the pants driving my Insight and watching the temps tells me it's probably <5%.

On some cars (like mine) the head gasket blows faster and more readily above 200F water temps.

Above is best demonstration about hot spots. Leads me to believe the head is stress relieving itself to something other than flat.

A 203 degree stat improved my MPG by about 0.5 MPG.

0.5 MPG in what vehicle, and what was the stat before, and what % improvement does that represent, and how did you control for other variables in measuring the improvement?

Anecdotes are nearly worthless just by the nature of them being anecdotes, but they are made even more useless when the details are omitted.

I had a disc burst and impinge on the spinal nerve at L5-S1 that left me in excruciating pain. Desperate to do anything to relieve the pain, I visited a chiropractor. He told me the bad news is that my back is messed up (duh), but the good news is I should see a great reduction in pain if I saw him 42 times in the course of 2 months. Rather than do that, I did nothing and my inflammation/pain subsided in 2 months. Had I made the 42 visits, I'd likely attribute my excellent results to the therapy when in fact my body recovered naturally.

My point is that correlation does not equal causation, especially when practically no variables are controlled for that could potentially confound the conclusion.

Jeez dude! I did that mod nearly 18 years ago.

If memory serves it went from 19.3 to 19.8 MPG. It was my first ecomod.

I think the old stat was 190 degrees. The old school bus motors are designed to run fairly hot.


I don't think there are any hotter stats available for that engine.

I strive for good MPGs as much as reasonable, but I won't run the water or oil any hotter than the manufacturer thinks it should go. The longevity of an engine trumps eking another MPG or two out of a gallon of gas.

Since I live in the South, I'm always watching for excessive oil and water temperatures during summer months and look for ways to keep them in check. In fact, I even put louvers in my Mazda3's hood to manage excessive engine bay temperatures which kill batteries and other things which can be degraded by excessive temperatures.

IOW, fuel economy is important to me, but it doesn't trump everything.

Since this is on the topic of thermostats, what do we think of high flow 'stats?

https://www.stewartcomponents.com/in...ormation_id=11

I think the perfect combination would be a high temp high flow stat, but they don't offer that for my vehicle. The thermostat only regulates the minimum coolant temperature, so having a high flow high temp stat would help keep the engine coolant in a more narrow range. I'm sure it would be particularly helpful in hot climates.