Summer vs winter diesel: different energy density?

Ultrasonic

This is probably a long shot, but let's see if anyone here happens to know this!

I believe that summer diesel has a higher volumetric energy density (BTU/gallon, MJ/L or the like) than winter diesel, but when someone actaully asked me yesterday by how much I spent a fruitless hour searching online for any reliable data for the UK market. Does anyone happen to know about this?

vaio

I think the same as you but thought that it was a result of lower temperatures increasing the density of the fuel.

more density = more hydrocarbon molecules per unit volume = higher volumetric energy

facade

In foreign lands, where they simply mix a lighter fuel like kerosene with the diesel, there is quite a reduction in winter. Over here, we use cheaper additives, I recall reading somewhere that it is upto 4% lower in calorific value.
Bear in mind how much water they can put into diesel and I doubt if you would notice.
The increased air density will probably make the engine work harder, and fuel a bit more to compensate.

Ultrasonic

vaio wrote: »

I think the same as you but thought that it was a result of lower temperatures increasing the density of the fuel.

more density = more hydrocarbon molecules per unit volume = higher volumetric energy

Thanks for replying. Interesting point. Although actually that would be the opposite way round to what I was thinking, in that I thought winter diesel had a lower energy density? You're right though that there is an appreciable density change, according to this graph at least.

For the US I can find references to the blending of what is termed diesel 1 (kerosene) and diesel 2 (what I think is normally put in road vehicles) in order to reduce the plugging point (CFPP) temperature, so that it is suitable for lower temperatures in winter. For example, here:

http://en.wikipedia.org/wiki/Winter_diesel_fuel#Blending

Diesel 1 has a lower energy density that diesel 2 though, so this blending reduces the energy density. (See e.g. this post from the Ecomodder forum.)

I guess that a similar blending proces is used in the UK, but I can find no reference to confirm this. Certainly something has to be added or changed compared to summer diesel to reduce the CFPP...

Ultrasonic

facade wrote: »

In foreign lands, where they simply mix a lighter fuel like kerosene with the diesel, there is quite a reduction in winter. Over here, we use cheaper additives, I recall reading somewhere that it is upto 4% lower in calorific value.
Bear in mind how much water they can put into diesel and I doubt if you would notice.
The increased air density will probably make the engine work harder, and fuel a bit more to compensate.

Thanks (you posted that while I was writing my last reply about the blending you have referred to). Yes, I've seen the up to 4% figure quoted too. As you might guess I was trying to separate out the fuel effect from other factors determining seasonal mpg variations.

Ultrasonic

Bump, just in case anyone else has any info.

colino

The old methods of increasing the cold filter plugging point was achieved by adding kerosene and that would have increased the overall energy density of a fixed mass of diesel fuel. However since that blending was outlawed here to reduce sulphur emissions and prevent modern high speed diesels going bang, they actually use a more sophisticated additive to achieve the result differently. The new additive works with the diesel to prevent waxing (the particles repel each other to prevent clumping) but I don't believe itself adds to the cetane value of the mixed fuel, then intuitively for the same mass of mixed winter diesel, the overall energy must be less. Unfortunately I don't recall the additives details, but I'm sure if you trawl BP and the likes websites, they'll give you a pointer.

Ultrasonic

Thanks Colino, interesting post.

colino wrote: »

The old methods of increasing the cold filter plugging point was achieved by adding kerosene and that would have increased the overall energy density of a fixed mass of diesel fuel.

Are you sure about that? As posted above, I've certainly read that adding kerosene decreases the volumetric energy density (as posted above). Although I suppose the specific energy density (energy per unit mass) could increase as you say provided there was an associated larger decrease in density. (It's energy per unit volume that is really of interest to me in relation to fuel economy, since we buy fuel by volume, and measure economy in mpg.)

However since that blending was outlawed here to reduce sulphur emissions and prevent modern high speed diesels going bang, they actually use a more sophisticated additive to achieve the result differently.

That clears that up, thanks!

The new additive works with the diesel to prevent waxing (the particles repel each other to prevent clumping) but I don't believe itself adds to the cetane value of the mixed fuel, then intuitively for the same mass of mixed winter diesel, the overall energy must be less.

Does Cetane number correlate with energy density? I thought it was a measure of ignition delay.

Unfortunately I don't recall the additives details, but I'm sure if you trawl BP and the likes websites, they'll give you a pointer.

No joy so far on that front...

colino

Your absolutely right on my mistook on energy levels, diesel has more than kerosene and the overall "mix" would reduce in energy. Being merely a humble BP Engineer in a past life, never gave any of this half remembered stuff much thought. Cetane is as you say, just the value given in the test engine for its anti-knock value, just was always spoke of in short-hand as an energy value (which it isn't!).
Try API in the states for more info.

Ultrasonic

colino wrote: »

Try API in the states for more info.

Thanks for the tip, I'll have a nose .

westwood68

& then in winter the thermal efficiency of the engine is greater but the engine oil is thicker on engine starting,the heater is on, the tyre rolling resistance is greater, the air is denser so provides a greater air resistance and you stop and start more because of all the numpties who can't drive in icy conditions etc etc etc

Tons of variables so exactly how important is the cetane value compared to everything else? Bugg*&ed if I know..