Newby Question: Winter Fuel Economy; Color of Fuel

[RUSSELL5000]

I got my truck to tow a travel trailer; so we do that, as well as towing a utility trailer for the Boy Scout troop and our 21 foot boat around to different lakes. I only live two miles from work and commute in a 2008 Jeep Patriot. I don't have a lot of miles on the truck unloaded, but I have tracked the fuel mileage carefully. Also, the truck does not get used much in the winter months - we always take the truck on any trip somewhere, so it does get some use.

My question regards fuel economy => in the warmer months I would see somewhere between 17-19 mpg unloaded on the highway. In the colder months that has dropped to 15-17 mpg unloaded on the highway (the ambient temperatures on these trips have been from 5 degrees F to 30 degrees F). What causes this difference?

I buy fuel at a station right next to the interstate that always seems to be full of vehicles. I filled up on Sunday afternoon, and for some reason the pump did not click off and there was some overflow. The truck said is was 28 degrees F. outside. Our low temperatures over the past weeks have been in single digits or below zero, so I imagine the tanks down in the ground under the station are quite cold. I have never looked at the fuel before (after all, it was shooting down into the tank), but I noticed this fuel that overflowed was a somewhat white/blue color - very pale blue. It was not clear - kind of like a wispy cloud against the blue sky. Hard to describe. Is that normal?

Thanks!

 

[Powder Extreme]

Winter diesel is usually a blend of #1 and #2. #1 diesel does not have as much power per gallon (in basic terms) so it takes more fuel to do the same job. That's the biggest reason your mileage is worse. Add in small factors such as letting the truck warm up longer, denser air you are driving through, stiffer oil in the gearboxes because it's cold, etc. and they all add up to a decrease as well.
Diesel is sometimes clear, often green and sometimes blueish like you saw, it just depends on what the refinery did with additives and such. Color doesn't mean anything unless it is RED. Red is for off-road use only and no road tax is charged making it roughly 50 cents a gallon cheaper. If you get caught with red fuel on the highway, you are facing a potentially huge fine.

Hope that helps!

 

[cerberusiam]

The additives to help winterize the fuel cause the color change. The drop in mileage is simply the excess energy that needs expended to keep the engine at operating temp and move it around. Ambient temps make a big dent in diesel mileage because so much of the system is designed around using the BTU's generated from combustion. Cold intake air and cold air around the engine rob the BTU's that could be used to power the engine and impact efficiency. A winter front on the grill will sometimes help the efficiency in cold ambient temps but it is not a given. Of course the stiffer oil\grease in the drive train is going to rob power that could be used at the wheels also.

Most of the diesel fuel in the lower 48 is NOT a mix of #1 and #2, it is #2 with additives to keep it relative usable down to about 0 degrees. The cost of refining\storing #1 ULSD is so high that it is rare that mix is used. Keep this in mind when temps start dipping below 0 and start using an anti-gell additive or you may end up with issues. ULSD in below 0 condition is some of the wrost crap ever slated for vehicle use. The water holding capabilities of ULSD are about 50% greater the LSD and once the water starts freezing out it plugs the filters. In addition, the wax molecules are so uniform that they will start settling at a lot higher temps than LSD so you get a double whammy on filter plugging and fuel flow. Fuel heaters adn additives are your best friend in the cold.

 

[PC12Driver]

Winter fuel has less BTUs available (like, 4%), so more will need to be burnt to do the same work.

All your driveline lubes and oils are super thick in the low temps and will produce huge amounts of parasitic drag until they warm up and thin out.

And air resistance increases as temps decrease. From Cummins:
"Air becomes more dense as temperatures drop, which increases air resistance. For every 10° F drop in temperature, aerodynamic drag increases by 2%. Thus, fuel efficiency will drop by 1%. Overall, fuel economy tends to be higher in the summer than the winter. According to North American Truckload Fleet Data, driving in the summer increases fuel mileage by 8 to 12% over driving in the winter months."

So subtract 4% due to fuel, and another 4-7% due to temps, and your mileage is right where it should be.

 

[S.Morris]

Just returned from Florida and ran in very cold weather going down (like -34 the first 8hrs, slowly warming to just below freezing after entering Florida on trip down). Pulling a 38' tall flat backed TH and running 65-68 , I got 8.4-9.2 USMPG (hand calc) over 1500miles. This was 2-3mpg less than I normally get on same trip with same trailer annually. It was so cold the alt was charging batteries at 14.8-.9 volts most of the way.
My sons new 13 DRW Ram ran approx .5 mpg lower pulling a 42' 5th wheel with a much lower back end but a 3 axle trailer. Weights very similiar.

 

[slingshot383]

Did you have your winter covers on? According to the factory, anything below 50* F. they should be on, that would help the engines stay warmer and operate more efficiently.

 

[DonFitzwater]

Did you have your winter covers on? According to the factory, anything below 50* F. they should be on, that would help the engines stay warmer and operate more efficiently.

When your working your truck, no need for a winter front unless it's really cold out.........like -40 f.

 

[RUSSELL5000]

Thanks much to all who responded. Lots to know about these trucks!

So, if I understand this, we have things working in concert in cold temperatures to diminish mileage: (1) winter fuel mixes contain less energy (through mix of additives), (2) drive train lubricants are stiffer, (3) the fluid the vehicle is being pushed through - the air - is denser, (4) combustion is not quite as efficient in cold air, and (5) allowing the truck somewhat longer idle time to warm up upon starting. The color changes are due to additives designed to keep the fuel flowing freely in cold temperatures.

I notice the drop in fuel mileage in my wife's van and my Jeep Patriot, but I assumed that was solely due to me idling them while I shovel off the driveway and clean all the snow and ice off of them in the morning (the truck is kept inside the garage.

Thanks.

 

[rrl989]

When your working your truck, no need for a winter front unless it's really cold out.........like -40 f.

Probably true, but I wonder why they so OK to use up to 50*F ?

 

[PC12Driver]

So, if I understand this, we have things working in concert in cold temperatures to diminish mileage: (1) winter fuel mixes contain less energy (through mix of additives), (2) drive train lubricants are stiffer, (3) the fluid the vehicle is being pushed through - the air - is denser, (4) combustion is not quite as efficient in cold air, and (5) allowing the truck somewhat longer idle time to warm up upon starting. The color changes are due to additives designed to keep the fuel flowing freely in cold temperatures.

Strike #4 and you've got it. There's been a lot of effort put in to designing intake systems to make the air charge as cool as possible for more efficient combustion. Power comes from expanding air, and since how hot you can make it is limited, all you're left with is how cold you can make it before combustion starts. Cold air = good for internal combustion engines, whether diesel, otto, or brayton.

 

[KYOUNG]

my 2011 dually gets the same mileage on winter fuel as summer fuel 11-12 mpg..............

 

[RUSSELL5000]

PC12Driver: Well, now I am a little confused. I understood that we wanted to cool the air - that is why there is an intercooler, correct? But, can't you end up with too much colder air?

Why are there winter fronts if this is not the case?

Seems contradictory on wanting cold air and using a winter front. ??

 

[PC12Driver]

PC12Driver: Well, now I am a little confused. I understood that we wanted to cool the air - that is why there is an intercooler, correct? But, can't you end up with too much colder air?

Why are there winter fronts if this is not the case?

Seems contradictory on wanting cold air and using a winter front. ??

Two separate things going on here. Cold air for combustion is good, and a properly designed setup won't have the radiator cover blocking the air intake. The winter front is an attempt to heat up the water in the radiator. Most of the time if you're running around lightly loaded (the truck, not the driver) in sub-freezing temps, you can put your hand on the radiator cap after even a long drive and it will still be cold to the touch. Because of diesel efficiency and our huge cooling systems on these trucks, the water in the radiator never really heats up. So when the engine finally warms up to operating temp around 200* and the t-stat opens, the hot engine gets a shot of very cold water. The coolant in the block cools off, the t-stat slams shut, and the hot slug of water now in the radiator cools off again. I'll see 20*+ temp swings on cold days. Not good for metal things to repeatedly get those cold shocks. So you put the winter front on to bring the water temp in the radiator up and even out the coolant temps in the system. As a bonus, the cab heat evens out, too.

And from Cummins:
"Cold Weather Grille Cover
A winter front, or cold weather cover, is designed to be used in ambient temperatures below 30 degrees Fahrenheit; especially during extended idle conditions, to reduce condensation build-up within the engine crankcase. The cover will also drastically increase the warm rate inside the cab."

Running around unloaded, the intercooler isn't doing much work anyway (low boost = not much of a temp rise), so blocking it isn't really impeding engine performance. When it's time to pull, roll back the cover to let the radiator and intercooler work.

 

[RUSSELL5000]

OK - that follows. Thanks.

 

[RUSSELL5000]

As a follow up - went to watch my son play soccer today - a little over 100 miles away. Temperature was 45 degrees F when we left at 6:00 AM. The EVIC said trip mileage was 16.8 mpg. Watched soccer from 8:00 AM until about 1:00 PM; ate lunch, got back on road. Temperature had dropped; was now 30 degrees F. EVIC said mileage back was 16.1 mpg.

I would have to check the winds, but it seemed calm. Since it was the same tank of fuel, same road, same load, I assume the difference was the density of the air.

In another thread I asked about mileage pulling trailers; the resistance of moving through the air is much more significant to fuel economy than I assumed. It looks like air density changes with temperature are quite significant.

 

[PC12Driver]

In another thread I asked about mileage pulling trailers; the resistance of moving through the air is much more significant to fuel economy than I assumed. It looks like air density changes with temperature are quite significant.

Dragging my 3,000 lbs flat-faced box trailer around empty gets close to the same mpg as my 13,000 lbs rounded-front 5th wheel. And my flat bed barely changes the mpg of my unloaded truck if it's empty or has a couple tons on it. On the open road, wind resistance drives mpg.

 

[NIsaacs]

I would have to check the winds, but it seemed calm. Since it was the same tank of fuel, same road, same load, I assume the difference was the density of the air.

Another possibility for the change in MPG, is a possible elevation change, down hill overall over, and up hill back.

Nick

 

[cerberusiam]

I would have to check the winds, but it seemed calm. Since it was the same tank of fuel, same road, same load, I assume the difference was the density of the air.

Probably not much differenc ein air density at those temp differences, neglible at the most. 40 degrees seems to be the cutoff where mileage differences start to accelerate. Simply takes more fuel to make the same BTU's to warm the engine and drive combustion which is power. The more BTU's sucked away to warm the air charge and keep water temp up is less that goes to force on the piston which equates to less efficiency. As a general rule under 30 down to about 0 F is the biggest drop noticed. Under 0 not much difference. 40-70 shows drop but not as steep. Ambient over 70 is best observed conditions.

At those temps a winter front may have given the lost mileage back. I know in the single digits the winter front makes a huge difference.

 

[PC12Driver]

If you burn a gallon of diesel, you'll get about 140k BTUs released. It doesn't matter if it's 90* or -20* out. Ambient temps do not change the amount of energy stored in the fuel.

The source of motive power is at the point of combustion, inside the cylinder. Once the cylinder walls are up to temp, the process has no idea what the ambient temps are outside the engine. Any heat left over from combustion is waste heat anyway, so how fast it is dissipated from the block (a rate which is dependent on ambient outside temps) is irrelevant. So yes, more BTUs are released from the exterior of the engine block as ambient temps drop, but they are post-combustion leftovers, so no change in efficiency. Just less heat going out the radiator.

It could be argued that colder temps do increase efficiency due to the improved scavenging resulting from the increased temperature delta between intake and exhaust, but I'd have no data to back that up.

But I do have data to back up the fact that I get my best mileage on the warmest days and worst on the coldest. I don't know where your generalizations come from, but the temperatures you're throwing out don't line up with anything I've experienced up here where we have sub-zero winters and 90*+ summers. And as a winter front user, I haven't seen a "huge difference" in MPG with it on or off.

 

[cerberusiam]

1. Yes, ambient temps do affect the available BTU's in a fixed amount of diesel. Ambient temperature directly effects the temp of the fuel because of the exposure to the ambient temps in the tank and lines. The longer the cold soak the more the effect. Even with dual heater elements all they can do in cold temps is keep the fuel above 40 F to keep it flowing, that is they have capacity for. The 40 F is about when you start seeing the standard ULSD start clouding and separating the waxes out that give it the actual BTU's. Waxed that have separated won't go thru filters easily and tend to settle to the bottom of the tank where they are picked up. The colder the temps the colder the fuel and less rise heaters provide. Diesel fuel is most efficient at 80-110 degrees and once you drop under 40 F ambient you start seeing the results in reduced mpg. The gains in efficiency from 40 F to 90 F could be as much as 20%.

2. Ambient temps also directly effect the the amount of combustion heat available to drive the piston. It is not the BTU's that generate the power it is the expansion of the gasses that push the piston. The BTU's in the combustion event are directly absorbed by the cylinder walls and into the coolant then directly to the outside portion of the block. With the block loosing more heat due to lower ambient temps that loss is drawn from the only source available, combustion. The whole engine and cooling system is loosing heat at a much higher rate due to exposure to ambient temps. To bring the coolant and engine up to operating temps takes more BTU's and the ONLY source for that is the combustion event. No, "once the cylinder walls are up to temp they don't care what the ambient temp" is wrong. They do care and it does make a difference becasue they NEVER get to the temp they would in warmer ambient temps due the accelerated heat loss encountered.

3. "It could be argued that colder temps do increase efficiency due to the improved scavenging resulting from the increased temperature delta between intake and exhaust". Total BS when applied to a forced induction engine. Scavenging efficiency is the result of cam design and drive pressure, there is little if any contribution by temp differences. The temperature of the intake air at the cylinder is ONLY relative to the DENSITY of the air. Cold air with the same amount of oxygen with show a distinct LOSS in power becasue too many BTU's are directed to warming the air to increase expansion. X amount of fuel will create Y expansion at Z temp. To maintain Y expansion at Z-n temp will require X+t fuel. Simple physics defines the relationship. The concept of colder air is better ONLY applies to forced induction engines when the compression temp of the air exceeds the ability to hold enough oxygen for the fuel charge. THEN and only then is air temp significant. Think the difference between 300 degree intake air and 150 degree intake air, and, the equation is NOT linear. Think bell curve with the tails of the bell below the x-axis, net power loss. That is the relaionship when it moves from 100 degrees down to 50-60 degrees IAT.

4. Typical cruising speeds with only 4-5 lbs of boost and low ambient temps result in low IAT temps. The turbo does not warm the air that much in those conditions and what it does is sucked out by the CAC in the cold air flow. Net result agian is efficiency loss due to power loss in the cylinder which requires additional fuel to compensate.

"But I do have data to back up the fact that I get my best mileage on the warmest days and worst on the coldest" You just validated every single comment and description I have made on this very subject, and, contradicted every thing you were trying to say. :-laf

Anybody that has done any amont of cold weather driving is well aware that in cold ambient temps you completely block air flow to the engine bay and draw intake air from there not the outside. If you don't see a solid 10% increase you need to reasses the setup and driving consistency. The difference is significant and shows more the colder it gets. The gen1's were a lot easier to see the difference because they had less power to start with. It was normal to go from 18-19 mpg in good conditions to 9-10 mpg in really cold weather. We could get about half or a little better back depending on conditions with a winter front and warm air intake.

Not significant indeed.