Ideal coolant temperature for fuel economy? 2

[dicer]

Any new discoverys on this topic?
Years past adiabatic was the buzz word for diesel economy improvement, what about SI engines?
Also induction temperatures? Ideal for power is cool induction, what about for economy?

 

[Lou Scannon]

Tough questions. Are you talking theoretical, or practical?
For practical, just reverse engineer a state of the art application.
For theoretical, read up on your thermo, choose a heat engine cycle, and do the analysis.

One of the issues encountered with SI engines is that beyond a point, optimization for efficiency reduces power density, due to knock limited BMEP. We're talking about compression ratio and coolant temnperature here. Of course if compression ratio and coolant temperature can be dynamically adjusted with load, then less compromise is necessary.

I'm not as well versed in diesel engines, but I believe one of the limitations encountered with so-called adiabatic engines is the most of the heat not extracted by the absent cooling system is not able to be extracted in the expansion stroke, and instead is lost to the exhaust. The reasons for this I don't recall exactly. I believe it is at least partially due to the need for a rapid process (high power density).

 

[tbuelna]

dicer,

Your question is quite broad.

Most coolant circuit temperatures are established by the coolant properties (ie. boiling point) or by the safe thermal structural limits of the head and block materials. But keeping internal engine surfaces that are exposed to combustion gas at as high a temperature as practical minimizes heat transfer to that surface, which is good for efficiency.

The adiabatic engine sounded great in theory, but in practice it had serious shortcomings. Engine parts like pistons and rings that have sliding contacts need lubrication, and engine lubricants have temperature limits. So the surfaces the lubricant films contact must be kept below a certain temperature, which means they must be actively cooled.

Diesel engines don't have detonation limits, so high intake temperatures aren't a big problem. However, most current turbodiesel engines use lots of charge air cooling to minimize NOx emissions.

Water/glycol is actually a very good coolant, since it is cheap, safe and has good specific heat value. Maybe someone will develop a fluid that is both safe and cheap, but with better specific heat properties than water/glycol.

I'm sure other posters can add much more.

Regards.
Terry

 

[patprimmer]

Keeping the maximum heat in the chamber is best for thermal efficiency, however a piston seized in a bore is really bad for mechanical efficiency and a piston with a big hole in it lets a lot of heat out.

Like everything in engine design, it is about the optimum balance of the various compromises.

Regards
Pat
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[sbullet86]

To pretty much sum up what everyone is saying:

The only reason for real world engines to have a max temp is to keep the material from melting. Theoretical otto/diesel/dual cycles will have higher thermal efficiency with higher compression ratios and tempuratures.

 

[SMOKEY44211]

Air cooled SI engines run head temps around 450 deg F. I've run liquid cooled engine at 300 deg. F using evans coolant and sythetic oil and intake air temps around 450. 11:1 expansion ratio on 87 octane fuel. Gapless rings, ceramic coating of piston head and exhaust valve, along with retarding the cam 10 deg. oil control strategy and aerodynamic shaping of crankshaft counter wieghts yeilds about 30% fuel economy improvement 48% boost in power. About 5% of those numbers can be attributed to raising coolant temps. -----Phil

 

[evelrod]

Phil, I'm a great Smoky Yunick fan and being so made me try one of his experiments some 20 years ago. I converted an engine to a propylene glycol coolant that operated at 300f. Difficult in keeping the oil temp down to manageable temps, but I could hold around 270f in the sump. Even more difficulty in keeping the fuel mix cool, including the use of a 'cool can' ( in a road race car this is silly). I tried and failed. I'm not Smoky. I found the effort largely a waste of time and money as the end results were little if any gain in power and, by the time I fixed all the other problems the effort caused, fuel efficiency was a non starter.

Not saying it can be taken out of the realm of theory, just that I could not manage to accomplish it.

Rod

 

[LionelHutz]

...yeilds about 30% fuel economy improvement 48% boost in power. About 5% of those numbers can be attributed to raising coolant temps.

It's fun to dream and call conspiracy between the auto manufacturers and big oil but be serious, you know how to increase a typical engine's power by 43% while also increasing the fuel economy by 25% (numbers are less 5% and not using the higher engine operating temperature) and yet these "secrets" remain untapped even with the millions being spent on engine R&D?

 

[IRstuff]

I don't necessarily see the assertions as contradictory, given that we don't know what was actually given up. Most systems are a compromise of competing technical requirements, which are solved by the engineers solution to the problem.

When the first i286 PCs came out in the late 80's, overclockers came up with an ingenious approach to nearly doubling the processing speed on the processors, by upping the supply voltages by 75%. However, they compromised the technology used in the processor, resulting in complete processor failure within 2 months of higher voltage operation.

Likewise, we don't know how much reliability was compromised by changing the operational parameters of an engine. Yes, it might have had better performance and efficiency, but for how long? Higher operating temperatures invariably lead to earlier failures, per Arrhenius relationship.

TTFN

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[IRstuff]

Just for giggles, assuming a 0.5 eV activation energy, running an engine 50°F hotter might reduce MTBF by about 60%

TTFN

FAQ731-376
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[gruntguru]

Lionel. Smokey didn't mention what he started with. I don't think it would be impossible to improve the mpg of a 60s car by 30%.

Engineering is the art of creating things you need, from things you can get.

 

[SMOKEY44211]

The particular engine example was was a 2.2 GM 4cyl. TPI fuel injected. They have a history of head gasket failures. They were found in Caviliers and S/10 pickups from mid 90's through 2004. Just a 2 valve pushod engine. They make good guinea pigs, cheap and plentiful. I have no doubt that at the OEM R+D facilities they have been able to achieve those #s or greater. Which begs the question "why don't they offer these fuel efficient engines"? I believe the answer is a combination of cost, potential liabilities, (warranty and legal) emission and inviromental considerations. Last but not least the recipe of fuel available. Let's take the example of raising the coolant temp. to 300 deg. If we stay with water/ethelyne glycol coolant and raise the pressure to keep it liquid what happens when the heater core fails and puts 200 deg+ liquid on the legs of passenger or driver? Not good. OK lets substitute some nasty synthetic coolant that won't boil at atmospheric pressure. Collision ruptures radiator. Stuff leaks onto road bed and nearby soil. Who pays for the cleanup? Also consider the product costs about $35 a gal. You need to save a lot of fuel to justify the higher costs. I don't even want to get into the discusion of ethanol in gasoline. Read the report Archer- Daniels- McNeal. Lots of politicians with thier fingers in the cookie jar on that one. Not enough public outrage to reverse that decision.------Phil

 

[dicer]

Lets make it a bit easier, what induction temp would be ideal?
A simple hot or cold.

 

[patprimmer]

I can see one advantage in a less volatile fuel being used and heated to give it sufficient evaporation in the chamber. If all other factors are equal (which they won't be) the lower rate of evaporation at ambient means less fuel will be wasted due to evaporation into the atmosphere.

Within reasonable limits, hotter charge gives better economy but colder charge gives more power.

Regards
Pat
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[SMOKEY44211]

Optimum air inlet temp.for reasons of fuel economy is SI gasoline fueled engine is 450 deg. F at full throttle normally aspirated. ------Phil

 

[patprimmer]

And that is independent of particular fuel blend on the day, air duct size, method of injecting fuel, where fuel is injected, air speed in the ducts and manifolds, compression ratio, rpm and cam timing?

Regards
Pat
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[IRstuff]

There was a blurb on Engineering.com about hot/cold split engines:

http://www.engineering.com/Videos/T...t-And-Cold-Sided-Engines-More-Efficient-.aspx

http://www.tourengine.com/technology.html

Perhaps, that answers the question as to how to locally optimize the efficiency of the IC engine

TTFN

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[patprimmer]

I would think also a very hot charge would have lower density and without thinking it through, that seems like it would reduce pumping losses. In fact, once you look past the smoke and mirrors, that might be its biggest area of gain re fuel efficiency and high charge temperature.

Regards
Pat
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[gruntguru]

I assume you mean part throttle pumping losses will reduce due to the larger throttle opening required for a given power output. Makes sense.

Engineering is the art of creating things you need, from things you can get.

 

[Lou Scannon]

EGR also has this benefit, and other benefits.