Misinformation about effect of temp on MPG?

DFWDieselJet · November 24th, 2007, 23:31

Ok, question for you gurus...

In my brief esperience with my TDI, I have observed something which seems to counter popular wisdom here on this board.

The conventional wisdom is "warmer air equals better mileage."

I've observed the opposite...as the temps have fallen (from a high of about 95F to 35F), my MPGs have climbed from the low 40s to around 50, under otherwise identical driving conditions (highway, 73-75mph).

And there is one big "problem" in my mind with the warmer air=better MPG idea. Why do we have intercoolers? TDI's never generate enough boost for the temp to REALLY get up there, and detonation is not a problem as in gas engines, and gas engines of similar boost levels have been sold without intercoolers...so why do we have them? Germans are pretty good engineers, and accountants too...they wouldn't needlessly put an intercooler in these cars if it did not have a positive effect on HP or MPG.

And if warmer air helped MPG, why isn't bypassing the intercooler a common hypermiler-mod? In fact, it's the opposite...keeping it clean and efficient is routine maintenance for the economy-minded.

Sure sure, there is the air density argument...but I'm really having a hard time wrapping my head around the idea that it's a significant difference. And at city-driving speeds, where wind resistance isn't a huge factor, any difference would be futher marginalized.

And as for the colder temps = less rolling resistance, intuitively, I think that would be the exact opposite since colder temps=harder tires and road surface, and harder tires usually means better MPG.

Ok, so fuel is reformulated in colder temps for gelling reasons, but that's not directly a temperature effect...it's indirect. I suspect this is the main..if not ONLY...culprit here.

<flame suit ON>

40X40 · November 24th, 2007, 23:46

You're kinda going off in all directions there... but I can say that my car likes temps around 60F with small excursions in either direction having little effect. Once you get to 30F or 85F bad things happen to the average MPG though...
Don't underestimate the VW engineers... they gave us one of the most fuel efficient cars ever seen in mass production.

JMO.

Bill

dclark · November 25th, 2007, 00:36

Ever start up a TDi when it is around 0 degrees? it isn't pretty. The car sounds rattly and until it warms up, it is loud.
Until it is warmed up, a tdi can get some pretty poor mpg. I read somewhere that Bosch figured it could get as low 16mpg when it is real cold, during the time it takes to warm up.
Also, all the fluids, fro the oil in the engine, fluid in the tranny, even the grease in the wheels bearings, get thick and it takes more energy to move.So, if you are in a cold area, this all gets factored into why winter mpg drops and that's not even touching the issue of winter fuel.

DPM · November 25th, 2007, 04:13

Yup. And re. intercoolers, at 1.1bar, turbo outlet temps are 100c or worse...

DFWDieselJet · November 25th, 2007, 09:37

Quote:

Originally Posted by dclark

Ever start up a TDi when it is around 0 degrees? it isn't pretty. The car sounds rattly and until it warms up, it is loud.
Until it is warmed up, a tdi can get some pretty poor mpg. I read somewhere that Bosch figured it could get as low 16mpg when it is real cold, during the time it takes to warm up.
Also, all the fluids, fro the oil in the engine, fluid in the tranny, even the grease in the wheels bearings, get thick and it takes more energy to move.So, if you are in a cold area, this all gets factored into why winter mpg drops and that's not even touching the issue of winter fuel.

Ok, that makes sense. MPG is not optimum until the car is up to operating temp, and in extreme cold temps that can easily take enough time to have an impact on overall MPG.

But once the engine IS at operating temps, I still think colder air = better MPG/efficiency/power/etc.

Part of my reasoning for starting this thread is I've noticed (from my scangauge) that as temps have dropped, my IATs have dropped to just below 60deg F. This has corresponded with my increased MPG, and it's the only factor I can attribute to the increased economy. I believe there is a pretty direct relationship here, and it's easy to test. Just accelerating briskly will bring the IATs up into the 70-80 degree range. I then return to "cruise" and see a corresponding drop in MPG. As the intercooler cools off and the IATs fall back to the 60 deg range, the MPG climbs back up. This got me thinking about water injection and more efficient intercoolers. So i started searching old threads, where I ran across a few posts making the hotter air=better MPG claim.

xs650 · November 25th, 2007, 10:39

Cooler air into the engine is generally good for thermal efficiency.

Colder air increases aerodynamic drag on the car.

Almagne · November 25th, 2007, 18:11

I'm no expert at cars, but I always thought of it this way.

The colder air is more dense, so you get more molecules of air into the combustion chamber at a specific boost pressure than at a higher temperature. In essence cooler air acts like a slightly higher boost - without an actual increase in boost pressure. Unfortunately like all things engineering, there is a point where the colder intake temperature is no longer an advantage, but extracts a performance penalty. As the air gets colder, some of the thermal energy of the combustion will be expended to increase the temperature of the incoming air. The result is a slight decrease in performance, which is manifested as lower MPG.

I'm not sure if this actually the case, so if there are any experts, please correct me.

Powder Hound · November 25th, 2007, 18:43

In general, there is very nearly a 1:1 correspondence between the heat taken out of the intake air by the intercooler, and the lowering of the exhaust temperature. How it translates is that an intercooler allows lower thermodynamic stress on the engine.

You get an intercooler when the engineers win over the accountants in the head office. Sometimes accountants sign on for preventative engineering when they look at warranty costs.

When I have been driving up a canyon in the sinter to go skiing, I'm really glad I have an intercooler there, even though the outside air is very cold. Lots of work being done on cold air by the turbocharger, and when the air is thin, the turbo can heat it up a lot as it works even harder.

My mileage plummets here in Phoenix in the summer, and it isn't completely because of running the a/c. Excessive heat causes its own difficulties.

Likewise, my mileage isn't the best in the winter, either. The best time is during moderate weather in the spring and fall when I don't have to run a/c or heat.

darkscout · November 25th, 2007, 19:08

Quote:

Originally Posted by DFWDieselJet

The conventional wisdom is "warmer air equals better mileage."

Can you cite one example of "conventional wisdom"? To me conventional wisdom is cold air is denser. Dense air has more O2. More O2 makes things more efficient.

Souzafone · November 25th, 2007, 19:52

Don't forget the effects of winterized fuel.

MayorDJQ · November 26th, 2007, 04:59

As with the A/C running during 85*+ weather, extra load (rear defog, heater motor, longer GP time, longer crank time, etc.) in cold weathe means more drag from the alternator, which will take some of the MPGs away, but probably not too much.

kcfoxie · November 26th, 2007, 07:11

Its only dropped to about 33F here, and my TDI is "rattly" sounding in the mornings. I have noticed (though all my "good upgrades" have happened since it started getting cold, too) that my economy is better now in the winter.

rodejetta · November 26th, 2007, 07:50

The one thing you could say for the better fuel economy in warm weather is that the fuel expands with increasing temperature, and because the fuel is injected by volume, you would do better. That said, the fuel is heated up in the pump, so any gain is marginal.
I am with you on the better economy with colder weather, but when you get winterised fuel, the gains go down the drain, because it then has more kerosine in it and thus less energy.

Hans

spdracr0 · November 26th, 2007, 08:36

FWIW, and keep in mind I am no engineer by any stretch but in my experience racing, we can dyno a car straight off with only letting teh motor get up to temp, then we canlet it cool down but this time we will let it run in gear on teh dyno to get up to temp. When we let the car run on the dyno heating all the fluids there is always a significant increase in hp. Take it at face value but that has been my experience..and this was done in 100 degree south carolina weather

xs650 · November 26th, 2007, 09:06

Quote:

Originally Posted by rodejetta

The one thing you could say for the better fuel economy in warm weather is that the fuel expands with increasing temperature, and because the fuel is injected by volume, you would do better. ....
Hans

It doesn't work that way.

If the fuel is denser, it has more energy per volume so we use less of it (by volume) to do the same amount of work.