Read Something Interesting - Diesel Forum

Dave Whitmer · 03-30-2006, 08:42 PM

We have all noticed the drop off in MPG during winter and wondered why. Well, we aren't alone.

If was reading through a EPA paper and the generally accepted reason is that cold weather air is denser than summer air. The vehicle has to push aside CFM of air but in winter that air is 10-15% denser and the truck has to do more work to bulldoze that heavier air.

There's your answer. Normally I don't trust the EPA, but checking against a psychrometric table confirms this.

lukecline · 03-30-2006, 08:51 PM

I would bet lower fuel economy is also due to winter blend fuel which has less btu's than straight #2.

WUNDERHOOT · 03-30-2006, 09:06 PM

i dont know i think its the feul.

sleepies · 03-30-2006, 09:18 PM

I get about 1 MPG less per tank during the cold months. Assumed it was the winter-blend (up to 40% diesel #1 is my understanding).

Dave Whitmer · 03-30-2006, 09:48 PM

It ain't the fuel. Guys in the southeast have the same problem (to a lesser degree) and they never see winter blend. even here in central Indiana our winter blend is pretty close to straight No. 2. It gels at 7 below. Ask me how I know that.

Consult you psychrometric charts. Winter air is 10-15% denser than summer air.

frashdog · 03-30-2006, 09:49 PM

[ QUOTE ]
If was reading through a EPA paper and the generally accepted reason is that cold weather air is denser than summer air. The vehicle has to push aside CFM of air but in winter that air is 10-15% denser and the truck has to do more work to bulldoze that heavier air.

[/ QUOTE ] now is this with the tailgate up or down? [img]/ubbthreads/images/graemlins/grin.gif[/img]

frashdog · 03-30-2006, 09:52 PM

[ QUOTE ]
Consult you psychrometric charts. Winter air is 10-15% denser than summer air.

[/ QUOTE ] how does humidity play here? winter really dry air and summer super humid? I know formula one cars have problems in really humid air due its high density.

**brett_kartracer** · 03-31-2006, 02:12 AM

Guys! What about letting your rig warm up longer (i.e. Idleing)? What about not being able to stay at a good MPH and RPM for optimum mileage?

Correct me if I'm wrong, but in diesels don't we want dense air for optimum horsepower/torque with a given fuel charge? Therefore, if we are getting more horespower/torque with a given fuel charge we give it LESS fuel to achieve the same road speeds. So technically wouldn't dense air give us better mileage?

ryaneverk2 · 03-31-2006, 02:43 AM

That's a good hypothesis, Brett. I think Dave's point about the denser air is in respect to wind resistance, rather than the air intake or engine performance. Sort of like driving into a headwind all winter long, just to a much lesser extent.

I don't really notice much difference either way with poorer mileage in the winter, but I guess I just don't pay attention or it doesn't bother me too much. [img]/ubbthreads/images/graemlins/ooo.gif[/img]

Suns_PSD · 03-31-2006, 09:14 AM

For years my friends and I use to drag our sport bikes off to secluded high altitude mountain roads for a weekend of racing, dragging knees, etc.

We all had noticed that even though the bikes were dramatically slower on the low end (about 20% less hp in many cases) many times the top speed would actually be slightly faster. We all reached the conclusion that since sport bikes are terribly unaerodynamic (about the same drag coefficient as the average p/u) the lessened air density/ resistance more than offset the hp loss.

You also notice that at very high speeds the wind does not knock you around nearly as much when you are at high altitude when your doing 170 on a sport bike.

At one time I tried explaining this to the COPSD people to help explain why their 1/4 mile times at altitude w/ a turbocharged diesel were not any slower than at low altitude but I was pretty much scoffed at. [img]/ubbthreads/images/graemlins/bleh.gif[/img]

shanba · 03-31-2006, 09:15 AM

This would impact highway MPG more than city. City MPG will be reduced due to cooler temps of the trans, axles, wheel bearings, tires, etc. causing higher parasitic losses and rolling resistance.

**brett_kartracer** · 03-31-2006, 10:59 AM

Very interesting. That does make sense (about more dense air having more aerodynamic drag). Good call on the thicker lubricants too Scott.

So I guess this means that the EPA hit on one point of why less fuel mileage is attained during winter driving, but they forgot to factor in several important contributers.

Dave Whitmer · 03-31-2006, 05:21 PM

Quote:
“now is this with the tailgate up or down?”
“…really humid air due its high density.”

Dave says:
Good grief!

Quote:
“…technically wouldn't dense air give us better mileage?”

Dave says:
That’s gas engine thinking. A diesel operates with excess air so denser air does not help burn the fuel more completely.

Quote:
“This would impact highway MPG more than city.”

Dave says:
Yup. Aero drag HP doesn’t really become much of a factor until over 40 MPH, but it goes up with the cube of speed.

If you look at the performance data on planes, most will make the same speed at a lower power setting at higher altitudes where the air is less dense. Trucks should get good MPG at higher altitudes but higher altitudes generally mean mountains and grades.

**Kanman** · 03-31-2006, 06:16 PM

Dave I think you have head your head hanging out the window in that cold air for just a lil too long lol. JK Glad to see you are paying attention to the aerodynamic thing these days.

Maybe your theory is a contributing factor. I think its probably one of many factors. Such as fuel blends, thickened lubricants. More drag on bearings and friction points. I do know that on a cool humid air day when the conditions are just right my truck feels like it has about an extra 80 horse under the hood.

frashdog · 03-31-2006, 06:53 PM

[ QUOTE ]
Quote:
“now is this with the tailgate up or down?”
“…really humid air due its high density.”

Dave says:
Good grief!

[/ QUOTE ]
The tail gate was a joke [img]/ubbthreads/images/graemlins/grin.gif[/img]

The formula one deal I misunderstood. Humid air is actually less dense than dry air. The issue had to do with the pressure differences generated off their air spoilers creating water vapor.

[ QUOTE ]
If you look at the performance data on planes, most will make the same speed at a lower power setting at higher altitudes where the air is less dense. Trucks should get good MPG at higher altitudes but higher altitudes generally mean mountains and grades.

[/ QUOTE ] as for planes doesn't the dense cold air make them super effecient, yes there is a trade off with the lower o2 and lower resistance at higher altitudes. I feel the low resistance being the winner at high altitudes.

Though, I have heard of jet planes making much much greater than normal thrust at low altitudes during -30ish weather.

Cold dense o2 rich air feed into our turbos, cheap horse power?

I could go either way on this one,
let's email mythbusters [img]/ubbthreads/images/graemlins/grin.gif[/img]