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Old 12-12-2010, 11:17 PM   #1 (permalink)
PrivatePilot
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Winter Diesel, Cetane, and decreased MPG.

In the last few weeks it seems that that there has been a lot of threads dealing with decreased fuel economy with the arrival of colder winter temperatures.

Although many of us know why, many new diesel owners, or those perhaps dealing with the joys of winter diesel or cold temperatures for the first time may not know exactly why their MPG has suddenly taken a nose-dive.

So, I thought I'd write a little FAQ. At least this way I can link to it from my signature and point people towards this (hopefully useful) information instead of typing it out over and over again.

- What's "Winter Diesel"?

First, it's important to know that diesel fuel normally has a very high paraffin wax content. In the summer this is really of no concern as your fuel is never cold enough for this wax to create problems, but in the winter, this wax content can lead to fuel clouding/gelling, a situation that can lead to a clogged fuel system and the inability to start your engine.

Winter diesel is effectively regular diesel fuel but with additives (and some suggest, a slightly different refining process) that help reduce clouding and gelling of the fuel making for a more carefree winter season once the weather gets cold.

- Why does it reduce my MPG?

It's well known that these additives and changes in the formulation of winter diesel reduces the fuels level of Cetane.

What's is "Cetane"? While gasoline's octane number signifies its ability to resist auto ignition (AKA detonation), diesel's cetane number is a measure of the fuel's delay of ignition time (the amount of time between the injection of fuel into the combustion chamber and the actual start of combustion of the fuel charge).

Because a diesel engine has no ignition system, the fuel itself must be able to ignite under pressure. The sooner the better.

A reduced Cetane rating reduces the ability of the fuel to auto-ignite, a process that eventually leads to lower fuel economy, sometimes only a slight change, but in the case of many of us here with larger pickups, sometime a drastic drop.

-What's the season for Winter Diesel?

Winter blend diesel is common in most northern US States and all Canadian provinces from about late October until late April or May. Depending on how large the tanks are at your choice of fueling location, and the volume of fuel they sell, sometimes winter diesel may linger in the stations tanks later into the spring until fresh "summer" diesel is delivered.

- What can I do to recover this lost cetane?

There are several options that can help your fuel mileage when dealing with winter and a diesel engine. Dealing directly with the lower cetane rating of winter diesel, you can use a cetane booster. A quality product such as Stanadyne Performance Formula not only significantly boosts the cetane level, but also provides added lubricity for the diesel engines fuel and injection pump systems.

There are many other cetane boosters out there, but many do not post any statistical information on how much they actually boost the cetane rating of the fuel, so your mileage may vary.

Although the per-tank costs may seem high for a quality additive like Stanadyne, when you sit down and crunch the numbers it can quickly more then pay for itself on a tank-by-tank basis. If you're going to spend $2/Tank on a lubricity additive anyways, why not spend the $3/$4 per tank for some quality additive during the winter months that will actually significantly raise your cetane while providing injection pump lubricity at the same time?

- What else can I do to get my MPG back up?!

There are lots of other options, although arguably none will have as much as an effect as dealing with the cetane rating of your fuel first.

1/ Plug in your truck. A few hours of block heater time before your morning commute will help your engine start easier and can lead to notably reduced fuel use while your engine heats up. If you use a timer to run the heater for only 2-3 hours (all that is needed) the electricity cost can be very little.

2/ Change to synthetic fluids. 0W40 or 5W40 diesel engine oil flows easier in the cold, and not making your engine pump molasses-like 15W40 will aid in fuel economy. If money is no object, converting to fully synthetic oils in your transmission, differential(s), and transfer case (if equipped) can also have a notable effect. Just be sure to switch back to regular 15W40 diesel oil come the spring!

3/ Check your tire pressures! Yep, many of us have forgotten about those tires, but low tires alone can mean +15% more fuel use! Yes, nobody likes crawling around in the howling wind or snow and checking your tires (especially for those of us with duallies) but you may find yourself shocked at how low your tires have gotten since you last checked them in the hot summer months.

4/ Drive conservatively. This really goes for all year round, but in the winter especially your engine works that much harder to make your truck move, and pushing that pedal harder and harder only ends up using more and more fuel.

5/ Don't idle! A surefire way to kill your MPG is to idle. An idling truck gets ZERO MPG. Contrary to old-school tales of years passed, you don't need to idle a modern diesel constantly for fear or it never re-starting in the cold, nor is idling your diesel for 20 or 30 minutes before driving away after a cold start necessary. The contrary, on some diesel engines, idling can actually be harmful - On the newer diesels, all idling serves to do is clog your DPF system, and on old diesels it can cause issues like wet-stacking. It's a waste of fuel and potentially harmful to your engine, so if you really care about $$$, simply don't idle. Start your engine, wait for your oil pressure to build and stabilize, and drive gently away. Wait a minute or three if it makes you feel better, but any more than that is really a waste. Take it easy for the first mile or three, and then drive normally.

It's important to note that your winter MPG, even taking everything above into consideration, will likely never equal your summer MPG. Small things like increased rolling resistance of cold tires, the fact that fluids never reach the same temperatures as they would in summer, amongst other things *always* take a permanent toll on your fuel economy in the winter, but with some knowledge (and some Cetane!) you can put up a valiant fight none the less!

Comments or additional suggestions welcome.
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Last edited by PrivatePilot; 12-12-2010 at 11:24 PM.
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Old 12-13-2010, 08:40 AM   #2 (permalink)
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Very informative thread - I enjoyed the read and learned some stuff!
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Old 12-13-2010, 02:33 PM   #3 (permalink)
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Has anyone got any before/after cetane booster info on mileage?
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Old 12-13-2010, 08:13 PM   #4 (permalink)
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Hey don't forget to add drafting as a fuel saving method!! lol, dangerous it can be for sure, but it's a fuel saver. Just find the balance between your safety/risk level and you're wanted fuel savings!!
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Old 12-14-2010, 08:07 PM   #5 (permalink)
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good write up, thanks
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Old 12-14-2010, 08:57 PM   #6 (permalink)
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Quote:
Originally Posted by mgray1982 View Post
Hey don't forget to add drafting as a fuel saving method!! lol, dangerous it can be for sure, but it's a fuel saver. Just find the balance between your safety/risk level and you're wanted fuel savings!!
I think that's more on the hypermiling side of fuel savings. Of course there are a lot more drastic things you can do to save fuel, but I wanted to cover the basics, focusing primarily on the effects of winter diesel and cold weather.

Thanks to all those those who have commented and found this useful, thanks!
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Rebuilt with Navistar 506 block spring 2012
Special thanks to Racer55 and WhiteK2500 for their invaluable assistance!
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Find my list of reference information HERE
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Old 12-15-2010, 12:10 AM   #7 (permalink)
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Another thing to think about is the temperature-pressure-energy relationship. I'm not a physics major, nor have I stepped foot in a college classroom, but I know how engines work so you'll have to forgive my crude representation.

To produce power you must first produce pressure. It a vehicle requires 50hp to drive at 50mph, then we can say that it needs "X" cylinder pressure to achieve that power rating.

We cannot achieve X without adding fuel energy. So if we start at 70*F and have to add "Y" amount of fuel to achieve X, then @ 70* X=Y.

Since pressure is proportional to temperature, and we need X pressure to achieve the desired power, if we drop the temp to 0*F then we would need an additional 70 degrees worth of fuel to achieve the same pressure. So @ 0*F X=Y+70*

You use more fuel in lower temp to create the same amount of energy that you had in warmer temps.


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Edit: It all sounds great in my head, but probably reads like garbage...So, K.I.S.S. method:

It takes heat to create pressure. If you start with less heat, you'll need more fuel to create the same pressure.
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Old 12-15-2010, 02:28 AM   #8 (permalink)
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I don't get this part though: "Just be sure to switch back to regular 15W40 diesel oil come the spring"

I see things like this said on here a lot. What does 5W-40 hurt in the summer? I though it has the same warm viscosity either way. I'm not arguing; it just confuses me. Is there any less protection with 5W-40?
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Old 12-15-2010, 05:00 AM   #9 (permalink)
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Actually, it really doesn't make sense in logical form. It only makes sense to some people who don't understand oil grade ratings and how viscosity reacts to temps, and how VIIs (viscosity index improvers) are desgined to function in the additive package.

5w-40 will pump easier than 15w-40 only when cold. Once they are warmed up, they have the same viscosity.

Also, if you really want to save fuel, you'll use a 5w-30 or 10w-30. Less pumping loss at start up and also less pumping loss when at full temp.

For reference, Shell's Rotella 5w-40 synthetic T6 has the same cold-crank-simulator rating as their semi-synthetic Rotella 10w-30 T5. If you use the T5, you get the same starting ease in winter, less fuel consumption when warmed up (being a 30 grade versus a 40 grade), and more money left in your wallet because T5 is one heck of a lot cheaper than T6 by about 40%! Think about that for a moment. Same cold start protection, better fuel economy, less purchase price.

And yet some people still think "thicker is better" ...
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Old 12-15-2010, 06:50 AM   #10 (permalink)
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Quote:
Originally Posted by sigepsb View Post
I don't get this part though: "Just be sure to switch back to regular 15W40 diesel oil come the spring"

I see things like this said on here a lot. What does 5W-40 hurt in the summer? I though it has the same warm viscosity either way. I'm not arguing; it just confuses me. Is there any less protection with 5W-40?
I think I was primarily referring to what oil the 6.5 calls for by default in the summer - sometimes I forget that I've stepped outside the 6.5TD forum here at DP.
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'97 C3500 Crew Cab Long Box Dually 6.5 Turbo Diesel.
Rebuilt with Navistar 506 block spring 2012
Special thanks to Racer55 and WhiteK2500 for their invaluable assistance!
Commercial OTR Driver Since '95

Find my list of reference information HERE
New members, please introduce yourself HERE
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