Temprature Vs. Fuel Economy Question....

I've seen a few people say that their fuel mileage is "getting ready to go downhill" with the coming cold weather... I am wondering why this is?? I thought the colder the air, the more dense. And the more dense the air, the more ya can cram in the cylinders and the "Bigger the Bang"... Isn't this the same for diesels?? I know I got better economy in my SVX in the winter time... Hmmm, can someone edumicate me??

With colder weather the engine takes longer to get to operating temp. A TDI uses more fuel when it's cold, so fuel efficiency drops as the outside temp gets colder.

The effect of the cooler more dense air is probably negligable compared to what the turbo provides all the time.

The gas stations add a winterizer to the diesel which has a detrimental effect on fuel economy.

Although there is also one other small factor: a cold engine does not get good fuel mileage and when it's cold outside obviously it takes slightly longer to warm the engine. But it's really due to the additives that they have to put in the fuel.

WOW... I never would have guessed that simple temprature drop would cause the TDI engine to suffer so much!! That is realy interesting. I wonder if my Rabbit's engine (1.6ltr Diesel) will have the similar fate this winter... I am getting great fuel mileage at the moment. It was in the 50's the last time that I drove down from Charlotte, NC to Beaufort, SC... I got 47 mpg... That was the best that I had ever gotten before!! Before that I had only gotten 43mpg... Still really good for a 23 yearold car that burns oil... Hope to get some posts here about winter experiences soon ... Also, see my roadtrip post in the "Roadtrips" forum... I am leaving for PA in a few hours (from SC)... Taking the GF's Jetta TDI... We'll have to see what kinda mileage we get

On a second note Thumper: You may not have to deal with this since you live in SC. I'm not really sure if they'll have to winterize the fuel much in our balmy weather down here in the SOUTH!!

Hahahaha!!! Yea, we don't get quite as hard of a winter as you NORTHERN guys get ... Still, I don't have much fat on my body so the slightest temprature change, I FEEL!!! ... So out winters down here seem like hell to me!!!

On a second note Thumper: You may not have to deal with this since you live in SC. I'm not really sure if they'll have to winterize the fuel much in our balmy weather down here in the SOUTH!!

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__. I think that *amount* of kerosene added to "winterize" fuel varies but I've seen noticeable winter effects on my mileage (south-eastern NC). My mileage typically falls by about 2-3 MPG's. I've also heard that the winterization schedule varies by location; i.e., the northern locations get heavier doses of "winter mixture" earlier in the season than we do but I can't confirm that with real data.

__. Anyway, bottom line is that we see some winterization effect in the Carolina's but it may not be as strong as in the North and may not last as long. (And don't forget that there are other effects of temperature besides fuel.)
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With colder weather the engine takes longer to get to operating temp. A TDI uses more fuel when it's cold, so fuel efficiency drops as the outside temp gets colder.

The effect of the cooler more dense air is probably negligable compared to what the turbo provides all the time.

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I think this is overplayed. I live near Boston (we had 10 days of below zero nights last January) and have driven in -20 in Vermont. If you only drive short distances it's true. But if you typically drive far enough to get the car up to temperature (my commute is 32 miles each way) you're not going to see much of a difference. And right now (40s at night, 50s and 60s during the day) is when the conditions are best for a diesel. That sounds like a lot of winter days in SC. I don't see a significant change, maybe a mpg or two. You might even see better mileage because of the heat you have in the summer and not using A/C.

I think this is overplayed. I live near Boston (we had 10 days of below zero nights last January) and have driven in -20 in Vermont. If you only drive short distances it's true. But if you typically drive far enough to get the car up to temperature (my commute is 32 miles each way) you're not going to see much of a difference. And right now (40s at night, 50s and 60s during the day) is when the conditions are best for a diesel. That sounds like a lot of winter days in SC. I don't see a significant change, maybe a mpg or two. You might even see better mileage because of the heat you have in the summer and not using A/C.

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Yeah, I agree, down here in the south we may actually get better fuel mileage in the winter, depending on the schedule of winterization. I don't use the a/c much because I'm a stick in the mud and don't like to pay for it. But for anybody who does, you'll probably get a little better mileage in the spring and fall and maybe equal in the wintertime.

It has been my experience that best mileage is between 40 and 70*F, below 40*F it takes longer for the car to warm up so my average MPG is lower. above 70* is the A/C hit.
50-60* with 100% humidity, is great for mileage & power. Tis why I like to drive my TDI at night. (53MPG doing ~85+ down the highway)
Allan

I get 47-48 mpg in the summer here in Phoenix and 52-53 in the winter.

2003 Jetta TDI 5 speed

A few factors come into play. I think in general almost all fuel locations have some form of winterized fuel. Truckers go through all areas of the country, and trips from south to north are not uncommon. So, what happens if someone fills up in Florida and drives to New York State? There will be some level of winterization. So, this takes some form of toll on the economy.

Next, is of course the fact that diesels aren't very efficient when cold, as they work off of compression, not spark ignition. Air + fuel + heat = ignition (including the compression of the pistons) If the heat isn't at the designed level, you have a slower burn or less complete burn.

Finally, it all depends on how you drive. In the summer in Toronto I get about 49-50 MPGs as I rarely use the A/C. In the winter I get about 47-48 MPG. It isn't a big drop..maybe 50 - 100 km/tank (like 30-55 miles per tank difference). You might also hear issues as some change to winter tires, which obviously don't get the mileage as summer tires will.

I think that *amount* of kerosene added to "winterize" fuel varies

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Our’s varies throughout the winter season. Here is the posting on top of the pump this am. Not sure how much is added for each drop in temp. Perhaps I will ask @ next fill up.

Interesting stuff; makes sense that the longer you drive, the less the impact on mileage--once the engine gets warm, it goes back to running near the same mpg (except for the losses due to winterization). But if that is true, then why did my last car (gasser) go from 37 to 32mpg last winter during a cold snap? I have a 51 mile drive to work, almost all highway.

I hope I don't have an mpg drop like that this year! Or at least not continously...

I think that *amount* of kerosene added to "winterize" fuel varies

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Our’s varies throughout the winter season. Here is the posting on top of the pump this am. Not sure how much is added for each drop in temp. Perhaps I will ask @ next fill up.

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we get some 40 below here in crispy vt
brr.

.

Strange story:
Temp should be high, but with the turbo they use intercooler. Why is there not existing a intercooler bypass when temp is so critical. I can understand that TDI builders don't stay wake at night for this problem, but with the new emmision regulation in Europe this should be a nearly free of charge solution. But I think it's more complex than that.
BTW the momental fuel consumtion indication on the display doesn't change in cold (-30°) or in hot (30°) wheater 0,7-0,9L/Hr, denpending of the electrical loads.

Interesting stuff; makes sense that the longer you drive, the less the impact on mileage--once the engine gets warm, it goes back to running near the same mpg (except for the losses due to winterization). But if that is true, then why did my last car (gasser) go from 37 to 32mpg last winter during a cold snap? I have a 51 mile drive to work, almost all highway.

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In the cold a TDI gets like 25 mpg for the first 10 miles or so, then it starts climbing, so if your engine is warm you will just get the winterization losses due to lower cetane fuel (why people use additives to boost the Cetane level in the winter)
The high level of air to fuel ratio makes it take longer for the car to warm up, because it burns so little fuel at idle it will take for ever to warm up.

So if you use a TDI heater during cold weather your losses will be minimized because the car will already be warm.

My worst mileage was 39 last winter, during a spell of 10-20*F weather.
But then my commute is only ~20 miles.

Strange story:
Temp should be high, but with the turbo they use intercooler. Why is there not existing a intercooler bypass when temp is so critical. I can understand that TDI builders don't stay wake at night for this problem, but with the new emmision regulation in Europe this should be a nearly free of charge solution. But I think it's more complex than that.
BTW the momental fuel consumtion indication on the display doesn't change in cold (-30°) or in hot (30°) wheater 0,7-0,9L/Hr, denpending of the electrical loads.

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The temperature of the engine should be high, which is somewhat unrelated to the temperature of the incoming air. Besides, an intercooler can't cool air any further than ambient temperature. Therefore having an intercooler bypass during startup would do little for helping to warm the engine.

I can’t comment on my TDI yet, but have noticed I get better mileage with my gas vehicles in the winter than the summer. In Texas, that is…

Yep, same here. Our '04 Jetta mileage dropped 3-4 mpg during the middle of the summer. During September/October it has come back up again. Of course, the worst I got was about 41mpg and the best was about 45mpg. Either way, I'm not complaining. Sure beats my '91 Chevy Silverado! - Alf

This is my first winter with the TDI. But I'll pass on a tip from the Honda Insight's.

The Insight and TDI motor share the fact that they produce very little heat. Its common that Insight owners put cardboard, rubber mat, tape off etc etc the air flow from the radiator. In the winter time the Insight could see a large improvement in MPG with just the cardboard block.

I'll try it on my TDI once it gets cooler in GA.

WOW... I never would have guessed that simple temprature drop would cause the TDI engine to suffer so much!! That is realy interesting. I wonder if my Rabbit's engine (1.6ltr Diesel) will have the similar fate this winter... I am getting great fuel mileage at the moment. It was in the 50's the last time that I drove down from Charlotte, NC to Beaufort, SC... I got 47 mpg... That was the best that I had ever gotten before!! Before that I had only gotten 43mpg... Still really good for a 23 yearold car that burns oil... Hope to get some posts here about winter experiences soon

... Also, see my roadtrip post in the "Roadtrips" forum... I am leaving for PA in a few hours (from SC)... Taking the GF's Jetta TDI... We'll have to see what kinda mileage we get

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Don't you just love that 1.6 "naturally asperated" VW Diesel?

The difference is that the TDI engine's fuel system is regulated by the computer. The computer tells the injector pump to inject more fuel to maintain the set speed or rpm until the engine reaches optium operating temperature. Example: even in July at cold idle, the TDI engine will use more fuel than when its at warm idle. The idle rpm is 903 on mine engine. The warm idle fuel consumption is 2.2 milligrams per revolution. When the engine is cold the fuel consumption is much higher (3.1 to 4.0 depending on outside temps).

With your 1.6 NA engine the idle RPM will increase as the engine begins to reach operating temps. The cooler air will improve its fuel economy.

I have driven over 1,500,000 miles in the old style VW diesel Rabbits and Jettas..!

Indeed my Seat Alhambra TDI (MPV) is using a computer to inject diesel. It's thanks to the computer there is an actual fuels consumption indicator . This indication is not the real consumption of the car but an indication of what amount of diesel there should be injected as programmed in the look-up table of the processor system. This number in that table is choosen because a lot of other parameters are involved and one of them is the temp. another is the air density.
So on the display there is an indication of what is explained in the previous posting.
Now what I see. In summer I have in idle, direct after starting the engine, an consumption of 0.7 L/h (because car is standing still not in Km/h). In winter it takes longer to crank and therfore is fluctuation between 0.7 and 0.8 L/h! Not a big difference. And of course checking when it can whange: switching on some electricity equipment gives an maximum of 1.0L/h
The suggested numbers of injected fuel in warm and cold wheater situations are not reflected in the readings on my car. Maybe I make a logical fault in my thinking?

WOW... This was one of my last posts I made back in 2004 when I first joined... Now that the weather is going to start getting colder I guess this little "bump" is an acceptable thing ...

Man, 10 years... I've gotta look for other posts I made back then!

I don't think the fuel composition is such a big fator.
Cold viscosity of fuel (droplet size and vapour point) gear and engine oil is a big deal.
Low engine temperatures are a big deal because the cold cylinder walls and head absorb heat that would normally help expand the gas driving the piston down. A hot cylinder wall and head reject heat and it's used to drive the piston down.

In Ontario the winter fuel has 7% less btu per unit of fuel so at the very least you will see a 7% loss of economy hand calculated.

I see very little mpg difference between summer and spring/fall with my 02 Golf, negating winter fuel issues. I do see a 3-4 mpg difference between summer and spring/fall with my 12 Passat. If you look at my fuelly graph and compare it to air temps... you will see a very close correlation. I put snow tires on the Passat last winter and they were another 3-4 mpg hit vs no snow tires the winter before.

Higher electrical loads, emissions warm-up requirement, and winter fuel are all contributing factors to lower fuel economy. The newer TDIs suffer more of a penalty than the old TDIs primarily because of the electrical loads (AdBlue heaters, electric cabin heaters) and emissions warm-up (post-injection to create extra heat in the exhaust).

Short trips will make the impact much more noticeable than long trips. If you have a <10 mile commute, you're in for some low numbers come winter time (compared to summer).

Wow, a zombie thread!
If you live in an area with large summer to winter temperature differences, and do a decent amount of highway driving, than air drag variation due to air density change with temperature will be your largest effect. 30°F to 70°F is an 8% change in air density, which enters linearly into drag loss, so it is also an 8% change in drag loss, which dominates at highway speeds.

And if you forget to re-up your tire pressure in the Fall, you could be in for another few percent loss.