Misinformation about effect of temp on MPG?

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>

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

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.

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

dclark said:

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.

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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.

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

Colder air increases aerodynamic drag on the car.

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.

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.

DFWDieselJet said:

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

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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.

Don't forget the effects of winterized fuel.

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.

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.

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

air temp

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

rodejetta said:

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

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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.

Almagne said:

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.

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That's my stance. There's a happy medium there with extremities on either side that will hurt MPG and performance.

In my city the temperatures can range from -40 to +35 celcius from summer to winter. My fuel economy stays pretty much the same all the way from +35 to -15 once it goes below -15 or so I loose 5-10 MPG. I always get lower economy when it gets that cold. Looking at my fuel economy spreadsheet the lowest economy ive ever had was durring 2 weeks in jan-feb when it was -25 to -30. Over the past week its been around -20 here and i can already see a difference again.

However, these results may be skewed a bit because I stay in 4th instead of going into fifth to warm the engine up a bit faster. How much this will affect the results im not very sure.

DFWDieselJet said:

The conventional wisdom is "warmer air equals better mileage for SI engines under certain conditions."

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Fixed it. Diesels are CI btw...

earthworm said:

The reason for the high MPG from Diesel is its greater off load efficiency and ~12-14% greater energy content per gallon..

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I'm not sure where this "conventional wisdom", as it was called, was sourced. Well documented fact here that high air temps lead to high fuel temps which cause the ECU to back off timing and/or boost. A/C use not even considered, my best mileage has always been acheived in moderate to cool ambient temps (lots of temp variation where I live-highs in 90s this summer, -6F this AM and getting colder). I think most of the COLD temp diesel inefficiency is way overstated. I will still average almost 50mpg on my commute in temps down to 0F, so that is only 3-5mpg down from my "above freezing" average and some of that is due to the blended fuel/BTU issue. As the temp goes down well past -0F of course it continues to drop off. It's not the hit gassers take at those temps IMO. Cold air is denser, carrying more oxygen. That is why 2 cycle engines (mostly snowmobile related up here) run "lean" at cold temp and have to be re-jetted or run "rich" as a cautionary measure most of the time to avoid piston meltdown. All things being equal (which they are not) cooler should be more effecient, but only to a point where frictional loads and cold air not being compression ignition friendly take over. Turbos and the high degree of electronics involved in the TDIs make for a lot of variables and when you're like me and have no idea as to how ECU "mapping" and all the numerous sensors work together you just tend to focus on the end results, great mileage and how to maybe make it better.
Drivebywire has suggested covering the intercooler in cold weather conditions to help raise the thermal temps inside TDI engines. Most likely an excellant idea, the cooler certainly isn't needed in sub-zero temps, and I plan to give it a try-soon.

xs650 said:

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.

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correct.


There is actually legislation working its way thru California capitol hill, wanting all oil and gas stations to compensate for temperature differences of the gasoline/diesel when it is sold to drivers .
At warmer temps in summer, each gallon will have slightly less energy, since the fuel molecules are slightly more expanded and occupy more volume....thus your one gallon of gas is slightly less dense and has less BTU> YOu get less fuel for your dollar in summer.

In winter, the opposite is true....with cooler temps, the fuel molecules are denser and closer together....and thus, each gallon of fuel has MORE energy and BTUs..... YOu get more fuel for your dollar in Winter.

Therefore, one should get relatively better mileage on slightly cooler temps during fillups . IN winter when you fill up with cooler fuel, YOur tank will have slightly more BTUs in 10 gallons than say, if you filled up the same 10 gallons in summer.

The oil companies argue that while this is true, the effects are reversed in winter....that consumers get MORE fuel and BTU per dollar , and that would even out/cancel out any loss incurred during the summer. Plus all the extra costs in standardizing fuel temps and fuel pumps....



COlder air works the same way, after the vehicle is warmed up. COOLER air has more densely packed air molecules.. more oxygen, and thus give slightly more power and boost . The intercooler also works nicely to cool down the air more ....so better thermal efficiency and more MPG during winter months and with colder fuel and colder air .

However, these effects are relatively minor. Cold air is denser and increases aerodynamic coefficients, meaning it is harder for our cars to push thru the air at freeway speeds. Other main factors include loss of air pressure in tires, weight of unnecessary items in vehicle, and of course, driving habits...

But of course knowing oil companies if that legislation passes, you're not going to get a discount for warmer fuel in the summer. You're going to get charged more for cold fuel.

"3.599 that's only at 60F sir, you're actually getting more BTUs so it's 3.659"

Haven't the mythbusters done this one yet? The ground temp in MOST climates in the US is rather stable. I'd be surprised if you saw a wide temp between 100 F and 30 F in fuel temps.

highender said:

correct.

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I agree on that, but the point was that IMO the fuelpump injects a specific volume into the cylinder, and because in that fuel is the molecules are farther apart in summer, you use somewhat less fuel. But, as said, the pump heats up the fuel in both winter and summer, so the difference is probably minimal.
My NA was most efficient in cooler temperatures, but before the fuel is winterised. For the TDI I do not know yet.

rodejetta said:

I agree on that, but the point was that IMO the fuelpump injects a specific volume into the cylinder, and because in that fuel is the molecules are farther apart in summer, you use somewhat less fuel. But, as said, the pump heats up the fuel in both winter and summer, so the difference is probably minimal.
My NA was most efficient in cooler temperatures, but before the fuel is winterised. For the TDI I do not know yet.

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The amount of fuel injected each time is so minute the difference at any temp would be infintismal (sp). The amount of thermal expansion of diesel fuel is rather minimal, that's why we dont need the extra room in the fuel tanks and can vent/fill right to the top. After running the engine for a while fuel temps heat up rather consistantly from recirculating through the pump and warm engine compartment.
Yes, your TDI should follow the same seasonal mpg trends as the older non-turbo vehicle, just better and a lot more fun.

rodejetta said:

I agree on that, but the point was that IMO the fuelpump injects a specific volume into the cylinder.

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That would only be true if someone or something were telling the injector to inject that same volume of fuel under both conditions. How would you do that in a car if you wanted to? (Without adding an additional control system to the car)

The injector injects the amount of fuel into the cylinder that the driver or cruise control has requested. The driver or cruise control request fuel to obtain a desired level of speed or acceleration. Neither one has knowledge of the amount of fuel being injected, only speed and acceleration.

The driver and cruise control both request fuel based on desired speed and/or acceleration so they are actually requesting a mass of fuel, not a volume whether they realize it or not.

The difference in fueling by mass or volume is small as long as we are sticking to DF2, but this is a hair splitting exercise.

And, to add some more fuzz to the equation...If you look it up, I believe you will find the differences in density of DF2 from different sources is far greater that the differences in density due to fuel tank temperatures.

In the last 12 months:

Lowest 3-tank average 41.8 mpg,US [5.63 L/100km] Feb 17 (Cold snap!)
Highest 3-tank average 50.1 mpg,US [4.69 L/100km] June 1 (Before A/C)

12-month average 46.6 mpg, US [5.05 L/100km]

I track _every_ tank but only look at three-tank averages.

when I chart my mpg for the year or lifetime, it is a single cycle (similar to a sine wave but not so clean) that has a peak at around May/June and drops from there to the bottom in February.

Pretty clear in my book that economy is lower in the winter for ..... whatever reason. I know there are many reasons for this

MK3, do your local fuel suppliers mix in some lighter fuel such a DF1 for winter fuel?

lowest 3 tank ave. 2/1-3/2/07 45.83mpg
high 3 tank ave. 5/17-6/8/07 53.35mpg
9/18-10/17/07 about same as high average
12 month average 52.297 mpg
Very similar seasonal trend as poster above from WI reports (as would be expected)

The hotter the air going into the engine the more thermodynamically efficient it will be and the less maximum power it will make.

This was recently discussed here - more links in post #21

Normally I'd respond to a previous post, but they are all over the place. The OP started this out mainly discussing air temps, but the conversation has gone in all directions.

Air temps: In hot weather the intercooler becomes less efficient, power is reduced, and we probably push down a little further on the accelerator to compensate. But at a constant cruising speed this effect may be negligible. Cold dense air is better than hot less-dense air for power and probably mileage.

Fuel temps: We know that the ECM retards the timing when fuel is hot, and this lowers fuel economy.

And if we get hotter fuel at the pump, we're actually losing about 1% by weight and BTUs. Different issue, but relevant.

Engine efficiency during warmup in cold weather: We know that TDIs use more fuel during the warmup period. This has little effect on those with longer commutes, when most of the miles are driven after the engine reaches normal operating temperature. It can have drastic effects on those who make short trips, when the engine has a chance to cool down in between.

mk3 said:

when I chart my mpg for the year or lifetime, it is a single cycle (similar to a sine wave but not so clean) that has a peak at around May/June and drops from there to the bottom in February.

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If you show me a chart like this, I can tell you whether it's from a Snow Belt car or a Sun Belt car. My peak mileage is in January, worst mileage is in August.

Turbos and mileage

As a newbie to this club, hopefully I can add some insight to turbos and mileage. All turbo have a design criteria to meet the application need. Turbos are not really meant for high density application of ambient air pressures. As for the 1.9L application, it is more than likely designed for a mid operating range of driving for the standard conditions. A standard day is based on 59 degrees farenhiet at 14.7 # ambient pressure. High denisty air on very cold days could result in an inefficient turbo. If you had a cold day at altitude at 5000 ft. it might not be a problem as the ambient air pressure is less than that at sea level. These are all factors that must be considered when setting up turbos. This is why there are different wheel designs and vane settings.