Does "Summer" Diesel Have an Impact on Fuel Economy?

I have routinely been getting ~50mpg after I completed the main maintenance on my Jetta after buying it last June (new LRR tires, vacuum hoses, MAF, ECT, thermostat, and turbo actuator). Prior to these items I was getting 39-42 mpg. In addition, I started adjusting my driving routine almost simultaneously. About 3k miles ago, my TB was changed and about 2 tanks ago (which is about a month), my FE dropped to about 45 mpg.

So it begs the question, does "summer" diesel (SD) have any impact on fuel economy? For SD, summertime means cracking home heating oil and likely more BD being added as winter diesel make have BD content not added in moderate to cooler climates (like the Midwest).

Now, I am asking this knowing that my drive time has changed and I have been driving a bit more aggressively, which is probably a bigger factor than anything else. But, I am still curious as to what others have experienced because last summer doesn't really count for me because my Jetta ran terribly then.

Thanks!

Usually - summer diesel has moe BTU in it, so the effect should be Positive. However, BD will lower that, and if you're driving more aggressively / using the A/C - the combination could account for your decrease in fuel economy. How the timing was set on the Timing belt change can also make a difference.

A/C, increase ventilation, overly high inlet temperatures (ie lower air density), even increased rolling resistance due to heat, even decreased energy volume in the fuel do to slightly higher fuel temperatures when tank is filled. Could be all, some or none of those factors. I know mine is going down, but most of that I figure is the A/C but possibly not all of it. MPG is a complex multivariable problem, but no, I don't think fuel chemistry has anything to do with the summer 'drop off'.

warm air, as near as I can figure, 70-90*f and warm fuel will give you the best mpg... I did some testing on my cummins ram and one of the tests showed a 3 mpg change warm vs cold during winter time testing...

some other reading... This thread gave some good info on the fuel temp side of things.

http://forums.tdiclub.com/showthread.php?t=27827&highlight=fuel+heater+mpg

I pick up around 3-4 MPG's during the warmer months here in Texas.
This is in my 92 ECOdiesel with no AC and no power steering.

From about November to April I average around 40-41.

May to October I'm getting around 44-46.

I am very anal about mileage and top off the tank to the exact spot every time, then divide trip meter by gallons put in.

Just topped off the tank at the Murphy's in Boerne, TX...
432.1 miles on 9.401 gallons = 45.96 MPG.
Not bad for a 21 year old smoky diesel with 392k original miles...

(In the UK at least) summer diesel has a higher energy density than winter and so you'd expect improved fuel economy. Running air conditioning is probably the most likely cause of the drop in mpg you've seen.

I always get noticeably better fuel economy in summer than winter, which I put down to three main factors:

1) It takes less time for the car to warm up to normal operating temperature in summer, which has more impact on economy for shorter trips.

2) Air density tends to be lower in summer due to the warmer temperatures, which reduces aerodynamic drag and therefore boosts fuel economy. This has more of an impact on longer, faster drives.

3) The higher energy density of summer diesel.

Ultrasonic said:

(In the UK at least) summer diesel has a higher energy density than winter ...

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How much higher? I couldn't find this information.


My personal feeling is that it is mostly due to quicker warm-up, based on my personal graph of tank-mpg versus temperature, but that is obviously only on a limited data set.

Well, I thought I could just chalk it up to more aggressive driving... so I calmed it down a bit and even added a long trip from Indianapolis to Cincinatti onto a tank. 50mpg... that is pretty sad from what I have been getting. The last time I added this same trip to my tank I had 48mpg and I was normally getting 379-39mpg. Prior to this recent drop in fuel economy, I had regularly been getting 50mpg just in my daily commute... so I would have expected this long trip to bring it closer to 55mpg.

I did have a boost issue on the trip. I have replaced most everything minor in that regards with the exception of the N75. It may finally be time for that. I should have just bought the full kit off of IDParts in the beginning... I have already spent more on the same parts... without the N75. It is also getting close to time for another oil change, and all filters (oil, air, and fuel).

One thought that I had was that maybe I am leaking fuel, just a little. I don't see anything in the drive way, but the lines don't look so great. I am going to replace them.

MikeMars said:

How much higher? I couldn't find this information.

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That is an excellent question, that like you I can't find an answer to from a reliable source . There are more references around for difference in BTU/gallon for North American diesel, but even that seems scarce. I take your I think implied point though that in the UK the difference probably isn't large (I suspect it would be easier to find data if it were...)

These don't properly answer your question at all, but in case you're interested the best 'data' I could find that's vaguely relevant for the UK market are these two discussions:

http://www.honestjohn.co.uk/forum/post/index.htm?t=93496
http://www.honestjohn.co.uk/forum/post/index.htm?t=80125

(Two thirds the way down the second thread a post claims 36-38MJ per kg for winter diesel vs. 44-45 MJ per kg for summer diesel, but I can't find these numbers in the reference cited, and what is really relevant would be MJ/L not MJ/kg, and I don't know the densities.)

My personal feeling is that it is mostly due to quicker warm-up, based on my personal graph of tank-mpg versus temperature, but that is obviously only on a limited data set.

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Very thorough of you trying to log mpg vs temperature. That sounds like interesting data, even with the obvious limitations due to temperature variations across the driving that make up a tank. As I posted above, I think warm-up is the dominant factor for short trips but for longer journeys I think air density changes will be more significant. It doesn't take that long to warm up, even when it's 0°C outside.

This Ecomodder post has a stab at calculating the relative effects of winter fuel and air density on fuel economy (the latter being bigger by far):

http://ecomodder.com/forum/showthread.php/winter-diesel-fuel-19705.html#post273835

So, nearly half way through this last tank of fuel, I decided I wanted to diagnose for potential vacuum issues. I hooked the might vac up to the hose to the actuator and could easily hold 25 psi. Next, I hooked it up to the far side of the N75 valve. It held 14 psi. I don't know if that is a good value, but since doing that, my fuel economy picked up and I ended the tank at 47 mpg, far better than my most recent 40-41 mpg. I think that some of the rust from my old actuator that I replaced last year could have been stuck in the N75 and I knocked it loose or something. Nevertheless, the N75 was the only item not replaced from the normal vacuum issues parts list, so I am ordering one tomorrow and will replace it anyhow.

I hate winterized diesel, it knocks about 4mpg off my results, and that's the only product they sell here in the winter...every pump / station has it. In the mid-west it's a blend of #1 and #2. None of the stations here know what the cetane rating of their fuel is...ever. I quit asking since all of them seem to be clueless about it.

Regardless of the weather you get winter blend starting Nov 30th and ending on March 30th...or when ever they run the stock down.

Winterized fuel is only necessary if you don't individually treat your own with PS or other additive. I think it only came about because the general populace didn't understand the concept of needing an additive for winter and probably felt that having to buy something to add to the fuel was an imposition. With winterized diesel people can just fill-n-go so they don't have to be bothered with remembering that they need to buy and add another product.

Personally I'd rather do my own additive and get back the lost 4 mpg....but that's just me.

Summer diesel is 2-D diesel, whereas winterized diesel has some 1-D diesel blended into it. The percentage is based on how cold it is likely to get in the area where the winterized diesel is intended to be sold. 1-D diesel has about 5% less BTU per unit volume, so obviously, winterized diesel has less BTU per unit volume than summer diesel.

http://www.ourexcellentadventures.com/diesel-fuel-101/