Misinformation about effect of temp on MPG?

truckinwagen

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the biggest reason that I see for reduced mpg in cold temps
(winter) is more fuel burned at idle warming the engine up, and poor road conditions. any time you drive on a surface that is not flat, or has water on it or snow, or any loose material at all you are increasing rolling resistance to the tires. not to mention the energy that is expended warming up the transmission fluid and all the grease in the bearings. it is not the motor that is less efficient in the cold necicarily but rather all of the other environmental factors increasing load on the engine.
 

DFWDieselJet

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truckinwagen said:
the biggest reason that I see for reduced mpg in cold temps
(winter) is more fuel burned at idle warming the engine up
I just don't see this with my Scangauge. I see the lowest GPH (gallon per hour) readings when idling while cold. Typically around .25gph when cold, about .35 once warmed up.
 

nicklockard

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4u2do said:
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.

Thanks :) That is good input. We could all use some refresher on compressor efficiency from time to time. Do you have any graphs or charts describing/showing it?
 

2006DSGJETTA

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Winterized fuel and Biodiesel

Souzafone said:
Don't forget the effects of winterized fuel.
I have a total of 9 Deisel vehicles, one being my Jetta. I have not had a winter with it yet, what effect will this winter fuel have on it? If I use biodiesel in it, what effect will it have?
 

truckinwagen

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you will have to treat the biodiesel to keep it from gelling.
the winterized fuel has d1 and antigel addad to it to keep it from gelling.
the winter fuel will have no detramental effect on performance that is noticable.
 

jettawreck

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DFWDieselJet said:
I just don't see this with my Scangauge. I see the lowest GPH (gallon per hour) readings when idling while cold. Typically around .25gph when cold, about .35 once warmed up.
Just the opposite here. ScanGauge reports .2 gph when cold then down to .1 gph after warming up. (my SGII only reads out one decimal point in gph) Just a few minutes even when "cold", as in below 0F. Haven't VagCommed it to verify, but SG has been accurate in the rest of the fuel numbers.
There is a lot more fuel burn due to extra friction when tranny grease, axle joints and wheel bearing lubes are very stiff from the cold. And 30F is not "cold"!! Below 0F is just starting to get "cold".
My SG coolant temp is accually reading HIGHER when it's cold than it did in warm weather at hiway speeds just due to the extra frictional drag. Of course as soon as I coast or slow down below 55mph it drops right off.
 

jettawreck

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truckinwagen said:
the biggest reason that I see for reduced mpg in cold temps
(winter) is more fuel burned at idle warming the engine up, and poor road conditions. any time you drive on a surface that is not flat, or has water on it or snow, or any loose material at all you are increasing rolling resistance to the tires. not to mention the energy that is expended warming up the transmission fluid and all the grease in the bearings. it is not the motor that is less efficient in the cold necicarily but rather all of the other environmental factors increasing load on the engine.
Very true, I believe. Last Tues. nite 12/4/07 slogging home thru 6" fresh unplowed snow didn't make for a very good mileage run. For the next several days, or until the next snowfall, all the secondary roads will have slop from unremoved salted snow on them-lots of rolling resistance.
While it's not good to idle the VW TDIs, try not to and live up north. You won't be comfortable much.
 

TornadoRed

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On the evening of January 4 2007, I filled up in Odessa NE then headed toward Denver and points west. Almost immediately after crossing into Colorado it started to rain, which turned to light flurries, and soon was coming down about 2" per hour of wet snow. I napped for a couple hours while waiting for a snowplow to go through, then gave up and just slogged through the third of three major blizzards that hit Colorado in three weeks.

I got 48 mpg on that leg of my trip because, while the slick conditions hurt my mileage, being forced to travel so far at about 45-50 mph helped even more. I was also putting a major load on the charging system, what with lights, radio, fan, wipers, AC to keep the windows clear, etc.

Nothing scientific about this, and temps were only in the low 20s for the most part.
 

DFWDieselJet

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jettawreck said:
Just the opposite here. ScanGauge reports .2 gph when cold then down to .1 gph after warming up. ... And 30F is not "cold"!! Below 0F is just starting to get "cold".
Good point about the temps...proably need to qualify "cold" here (N Texas) is anything below 50! But I've noticed the best MPGs driving in the 30s. Haven't driven in anything colder than that yet.

I'm surprised that you get opposite results...and less than half of my idle GPH! Hoperfully more people will chime in.
 

jettawreck

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DFWDieselJet said:
Good point about the temps...proably need to qualify "cold" here (N Texas) is anything below 50! But I've noticed the best MPGs driving in the 30s. Haven't driven in anything colder than that yet.

I'm surprised that you get opposite results...and less than half of my idle GPH! Hoperfully more people will chime in.
Well, one reason for the higher gph at "cold" idle is that when it gets down below 40F somewhere (haven't paid that mch attention) when you fire it up they go into a high idle mode (aprov 1000 rpm instead of 903) for a while. So not only does the increased rpm account for some of the gph but also friction in the engine is higher (even synthetic oil thickens up) so it requires more fuel to achieve the rpm the ECU is calling for.

But, yes you would get great fuel mileage at those temps especially since you could still get away without winter blended fuel. Up here you wouldn't want to risk it because it can go from 30F to well.... I had -14 at home and saw -20F on the drive to work this AM.
 

N2TOH

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highender said:
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...
Wouldn't it just be easier if they sold gasoline and diesel the way they sell natural gas? by it's BTU content.
 

SCM.com

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DFWDieselJet said:
Good point about the temps...proably need to qualify "cold" here (N Texas) is anything below 50! But I've noticed the best MPGs driving in the 30s. Haven't driven in anything colder than that yet.

I'm surprised that you get opposite results...and less than half of my idle GPH! Hoperfully more people will chime in.
There is definitely a happy medium with diesel above freezing temps, as cooler air makes for better performance with most engines.

sidebar (As a long-time user ( 14yrs ) of diesel vehicles/ light-plants/ generators in some of the coldest temps in NAmerica ( oil biz ) once a larger displacement engine gets going, there is enough latent heat to make said engine happy. The tough part is getting the larger engines going.) /sidebar

The tough part about a smaller displacement engine, such as the mighty 1.9 TDI, is that even if it starts easier than the big boys, once warmed up, sometimes with those really cold temps, and also sitting in traffic, it's just not a big enough block of metal to stay warm.

50F is not cold for a diesel, but you may not have winterized diesel in TX either. Maybe you are running the heater more often? Or maybe idling the vehicle longer?

Air that is more dense + colder temps = more air. With the diesel, more air = more fuel, but unlike a gasser, the extra fuel may not be burnt as efficiently as a colder gas engine. ( ie:smoke )
Just thinking out loud.

-M. subscribed
 
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dhdenney

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SCM.com said:
sidebar (As a long-time user ( 14yrs ) of diesel vehicles/ light-plants/ generators in some of the coldest temps in NAmerica ( oil biz ) once a larger displacement engine gets going, there is enough latent heat to make said engine happy. The tough part is getting the larger engines going.) /
I have a friend who drills and I have heard some stories about getting 900+ cubic engines of Detroit rolling on a 10* day. :eek:
 

dhdenney

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Just an observation I made today (which I have made before but was reminded)....... I let the car idle for about 2 mins today while I ran into my apt and temp gauge lost a few tick marks. That was only at 32*.
 

damnyankee

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DFWDieselJet said:
Good point about the temps...proably need to qualify "cold" here (N Texas) is anything below 50! But I've noticed the best MPGs driving in the 30s. Haven't driven in anything colder than that yet.

I'm surprised that you get opposite results...and less than half of my idle GPH! Hoperfully more people will chime in.
Well, it was in the low 30's this weekend, and with 35+ mph headwinds from Houston the DFW on Saturday, and I still managed to get 39.3 mpg @ 70 mph. The return trip late Sunday night netted 52.6 mpg with tempertures in the upper 20's and no head winds.
 

SCM.com

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damnyankee said:
Well, it was in the low 30's this weekend, and with 35+ mph headwinds from Houston the DFW on Saturday, and I still managed to get 39.3 mpg @ 70 mph. The return trip late Sunday night netted 52.6 mpg with tempertures in the upper 20's and no head winds.
One of my points, exactly. There is a happy medium with every diesel that does not run with winterized fuel. DieselJet probably has another problem.

dhdenney said:
I have a friend who drills and I have heard some stories about getting 900+ cubic engines of Detroit rolling on a 10* day.
10˚F is not cold where I come from. LOL

-M.
 

dhdenney

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SCM.com said:
10˚F is not cold where I come from. LOL

-M.
In British Columbia, I could imagine! If big diesels are hard to start on a 10*F day in KY, I would say they're a little harder to start in -40*F in BC.
 

jettawreck

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Most of the logging/construction contractors up here have quick couplers on their work vehicles that they drive out to the (diesel) equipment on the jobsite or woods and couple into the cold engine's cooling system so that the hot coolant circulates into the cold one, warms them up pretty quick. Lots better/faster than having to heat the engine from the outside with propane torch heaters and such. Most of this kind of equipment does not have GPs either, so ether (starting fluid) is used quite a bit. Lots of smoke and rattling diesel noise on start-up.
 

dhdenney

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jettawreck said:
Most of the logging/construction contractors up here have quick couplers on their work vehicles that they drive out to the (diesel) equipment on the jobsite or woods and couple into the cold engine's cooling system so that the hot coolant circulates into the cold one, warms them up pretty quick. Lots better/faster than having to heat the engine from the outside with propane torch heaters and such. Most of this kind of equipment does not have GPs either, so ether (starting fluid) is used quite a bit. Lots of smoke and rattling diesel noise on start-up.
Sounds pretty neat. I saw some guys at the drag strip with quick couplers on their drag cars and they had little mobile refrigeration units they hooked into them. I knew a guy with an old Ford diesel and ether was a must. You filled the air cleaner with it and hopped in and tried to start it before it all evaporated. That was on a 30* day.
 

jettawreck

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Yep, the old over the road semi-tractors had (maybe some still do) ether cups on the intake air stack. Fill the little cup with spray ether, close the lid, and crank away. My uncle had a cab-over GMC with a 318hp V8 Detroit Diesel (2 stroke sytle diesel engine) years ago that used that system. Wow, talk about rattle when cold! Twice as many firing strokes as normal 4 stroke engine, they really sound like they're coming apart.

When they drag race snowmobiles on grass up here they do the ice chest/cooler trick to cool them since there is no snow for the engine coolers up under the tracks. They get hot pretty fast. You would think it would be quite a cooling shock to go from HOT to COLD coolant that fast, but they do it all the time-'course they get rebuilt fairly often, too.
 

SCM.com

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ether is really dangerous... but also very entertaining! I've seen a couple of large cylinder heads destroyed.

the recirculating heater is a really easy fit for the Golf/ Jetta platform, and if I move back into the colder climes, I'll install one.

-M.
 

dhdenney

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I shot some ether with a rifle once. The explosion was faster than my mind's eye could comprehend. Very neat. I thought the damn can disappeared.
 

DFWDieselJet

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SCM.com said:
DieselJet probably has another problem.
No no, not at all! Damnyankee's results are exactly what I'm seeing! The "colder*" it is, the better the MPG!

SCM.com said:
There is a happy medium with every diesel that does not run with winterized fuel.
You're right on here. So somewhere in the 20's or 30's F seem to deliver optimum fuel economy, as long as you're not running winterized fuel. Colder than that, and the engine can't warm up fully, warmer than that and you start losing intake air density.

* - The other conclusion we're seeing here is that "COLD" in Texas barely qualifies as "COOL" in other places. ;)
 

10then34

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DFWDieselJet said:
* - The other conclusion we're seeing here is that "COLD" in Texas barely qualifies as "COOL" in other places. ;)
Your 'cold' qualifies as my 'balmy' (-14F two days ago in the morning).

And yes, since it is really cold, my mileage has gone down by 5mpg. I go 95miles one way, cold-start penalty should be minimal.
 

dhdenney

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That's probably the winterization of the fuel more than anything, correct?
 

10then34

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dhdenney said:
That's probably the winterization of the fuel more than anything, correct?
Maybe a change in the 'level' of winterization. Around here we get winterized fuel starting october, mileage at that point was still in the 45+ range. Only now that it stays below 10 for my morning commute pretty consistently I have dropped to 40.x for the last 2-3 tanks.
 
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