Efficiency increase with a warmer engine? (195F vs 165F)

[Zyzzyx]

Had some work done on the car this week (t-belt, water pump, t-stat, and other stuff). With the change of the thermostat came a change in the operating temperature of the car.

Looking back I should have realized that the car was running cool, especially the last couple weeks since I got the ScanGauge and had a digital temp reading. Most likely the t-stat was opening way too early, or even staying open slightly. The cars 'normal' operating temp was ~165F, and would cool down when in DFCO, sometimes quite noticeably (down to 120F on one long downhill in 5th gear). Thus it was still flowing coolant through the radiator and cooling it even when then there was no extra heat load.


With the new thermostat I'm now seeing the factory gauge "where it should be", and the digital gauge is telling me ~195F (its a 188F t-stat, tested and it does open right at that). Also the engine temp is steady now on a long deceleration with fuel cutoff.



So what I'm wondering, is there some efficiency gained now that the engine is running 30F warmer than it used to? I'd say the oil is slightly thinner at the higher temps, but that's questionable with the viscosity modifiers. What about the cylinders, any chance of better combustion with the warmer engine block?

[NeilBlanchard]

Absolutely! You may have not been getting the fuel injection controller out of the enriched mode, and more of the fuel will be vaporized.

[pgfpro]

Here's a coolant temperature correction map off my Haltech.

As you can see when the engines coolant is cooler there is more of a increase based in % added to the pulse width of the injectors.

Factory ecu's will even have more of a correction all the way up to normal operating temperatures.

[MechEngVT]

Your brake thermal efficiency increases with increased engine operating temperature. The theoretical Otto cycle efficiency is always higher than actual thermal efficiency due to the adiabatic assumption in the theory not applying in the real world. The hotter your engine runs, the less heat of combustion is lost to the block walls, and the closer you approximate the adiabatic (zero heat transfer) ideal.

[busypaws]

"In theory there is no difference between theory and practice. In practice there is."
Don't remember where I got the quote but your last post, I think, said the same thing.

[Thymeclock]

Originally Posted by NeilBlanchard

Absolutely! You may have not been getting the fuel injection controller out of the enriched mode, and more of the fuel will be vaporized.

Agreed. I'll bet your old thermo wasn't opening fully, hence the reduced operating temp. Thermos are a PITA in that they can fail partially and be problematic in operating only marginally. You probably didn't have much heat from the heater in the interior either, if you noticed.

[gone-ot]

...would co$t more, but an electronic thermostat (controlled by ECU) would get the job done much better! (servo-mechanical flow control or electric pump control)

...it could hold the flow totally off until it's actually needed, and then only pump or pass just enough coolant to keep thermal equilibrium.

[MechEngVT]

Originally Posted by Old Tele man

...would co$t more, but an electronic thermostat (controlled by ECU) would get the job done much better! (servo-mechanical flow control or electric pump control)

...it could hold the flow totally off until it's actually needed, and then only pump or pass just enough coolant to keep thermal equilibrium.

It's been a long time, but I remember pan-testing a thermostat on the stove. I seriously doubt you would be able to detect any cooling performance increase from a servo-actuated thermostat as I recall the spring thermostat nearly jumped out of the pan when it opened up, and they continuously adjust pretty well if temperature drops too low. They are fast-opening on warmup and maintain low enough flow to stay at their design temperature pretty well. An old, tried-and-true design not likely to be replaced until our vehicles no longer need liquid cooling systems.

[gone-ot]

...agreed, not a co$t-effective replacement today, but when electric water pumps eventually become more common, under ECU control, it would certainly be more energy efficient to modulate total flow and use energy pumping coolant around only when you really have to.

...instead of "pumping" all the time and then apply "throttling" on top of that.

[Christ]

If the electric water pump is ECU controlled, why would you need a t-stat at all?

I mean, the ECU could just shut off the pump and monitor temps until the pump needed to move the hot coolant.