gas vs. diesel
 

I was recently struck by something that I wanted to run by you all:

We always compare the mpg of gas and diesel with diesel generally coming out ahead, but if you look at the Btu content of gas and diesel it is an apples to oranges comparison really. Gas has 115,000 btu per gallon and diesel has 130,500 btu per gallon. This means that a gasser getting 44 mpg is getting the same miles per btu as a diesel getting 50 mpg.

The ratio of btu in gas vs diesel works out to 1:1.135. Using this same ratio, if you were paying the same price for fuel per btu, and the cost of gas was $2.50 per gallon, then the cost of diesel should be $2.84, which seems about right given current fuel prices.

This makes a CRX getting 50 mpg equivalent to a diesel getting almost 57 mpg, or, for that matter, a prius getting 60 mpg equivalent to a diesel getting 68 mpg!

Diesel always used to be less expensive than gas per gallon, but now with diesel prices running higher than reg. unleaded, the economic benefit of diesel isn't nearly so good.

Just curious to hear others' comments on this. . .

makes sense to me, i knew desil produced a tad more BTU's but i wasnt sure by how much.

Why is diesel more isnt it less refined so less work to make?

The city/highway ratings of diesel reflect this difference. Which is good in terms of driver mileage, since they have an extra ~15% more energy to work with, and seldom get less than the EPA figures, but from a thermodynamic POV a gasser like a CRX getting 50mpg is as efficient as a small diesel getting 60mpg in the old, old EPA tests. Give or take.

But traditional diesels are so dirty, hopefully honda and toyota have changed that for good, :)

I believe the summer diesel prices are better.... I know in past years they have been better than gasoline. This is because diesel and heating oil are produced at the same time (I think the key difference is sulfur content in diesel fuel). And in winter, there's a heavy demand for heating oil (except for me, I live in a warmer climate) :P

And that brings me to cost.... Sulfur content requirements have gone low -- a lot lower compared to 10 and 20 years ago. This means more refining. Then there's economy - if diesel was cheaper than heating oil. What would you buy? Heating oil or diesel? Because both would work :P

But again, the biggest thing is supply - demand -> competition.

As for efficiency of the diesel cycle v. the otto cycle.... Pick up a thermodynamics book and do a bit of reading, even follow example problems ;) Then you'll be asking yourself why our car's aren't inter cooled turbine powered with feed reheating that can run on any fuel that creates heat :P

Gasoline is lighter than diesel, i.e, it separates from the crude oil sooner in the distillation process. This is what is generally meant by gas being more highly refined. Being heavier than gas, it actually makes sense that diesel would have more btu content.

I don't know whether diesel is less work to make. From what I understand, it is essentially a by-product of making gasoline. In the early days of oil production, diesel/kerosene used to be dumped into rivers before people figured out a use for it.

All good points. The BTU of "winter diesel" drops between 5-10% when they cut it for winter with thinner fuels to keep it from gelling. Another reason you see diesel drop more MPG's in winter then gassers.

And as of last fall all diesel is required to ULSD or ultra low sulfur diesel, sort of like leaded vs unleaded gasoline. It will allow diesels to have a much cleaner burn and allow additional emissions equipment to be added. The reason I mention this is typically gasoline is more expensive then diesel, but adding the extra filtering or what ever they have to do to remove the sulfur drove prices up as well this last fall.

Also diesel typically has a higher road tax then gasoline. I know here in Wisconsin diesel is taxed about 15 cents more per gallon then gasoline is which pushes up the price as well.

An apple-a-day
 

The numbers I've seen were 115,500 and 128,500 or roughly 11% difference. Biodiesel (100%) has 117,090 or just 1.3% higher BTU per gallon than iso-octane gasoline. E85, for those interested, has 81,925 BTU per gallon.

Let's compare total (extraction/harvest, refining, transport, distribution, use) CO2 production with 115,500 BTU (apples to apples) heat production:
1 gallon of gasoline: 10,874 grams
.89 gallon of diesel, 10,963 grams
.99 gallons of B100, 2,746 grams
1.41 gallons of E85, 7,747 grams

What percentage of those 115,500 BTU were from renewable sources?
gasoline, 0.33%
diesel, 0.29%
B100, 72.12%
E85, 47.38

Another reason that diesel is more expensive relative to gasoline than it traditionally used to be is because of the increased demand of diesel fuel in Europe (thanks to the high popularity of diesel cars over there). When refining petroleum, you get a certain ratio between the amount of diesel and the amount of gasoline you can get out of each barrel of crude (19 gal of gasoline/10 gal of diesel per barrel of crude). In recent years there has been a surplus of refined gasoline coming out of Europe (a byproduct from diesel fuel production), a lot of which has been finding its way to North America and helping to depress the price of gasoline relative to diesel in our fuel markets.