Explain "energy density" of fuels

Factual Questions
1

I want to be sure I know what is meant when I hear about “energy density”. Using units I’m familiar with, I found that Diesel has a density of about .0415 kW per gallon. I would take that to mean that if I could convert the chemical energy in a gallon of Diesel fuel directly into electrical energy, with 100% efficiency, I could only get .0415 kW-hours out of it. That seems ridiculously low… Can someone tell me where I’ve been a bonehead here?

2

I don’t know where you made your mistake, but you’re off by a factor of 1000.

38.5 x 10[sup]9[/sup] J/m[sup]3[/sup] x 10[sup]-3[/sup] m[sup]3[/sup]/L = 38.5 x 10[sup]6[/sup] J/L

38.5 x 10[sup]6[/sup] J/L x 10[sup]-3[/sup] kW-s/J = 38.5 x 10[sup]3[/sup] kW-s/L

38.5 x 10[sup]3[/sup] kW-s/L ÷ 3600 s/hr = 10.7 kW-hr/L

10.7 kW-hr/L x 3.785 L/gal = 40.5 kW-hr/gal

3

According to [url=http://www.vr-transport.de/transrapid-energy/n003.htmlthis page, diesel has an energy density of 9.8 kWh/l or roughly 39 kWh/gal., which seems more reasonable to me. On preview, this agrees well with Nametag’s figure.

4

Thank you. Must be a conversion error somewhere. So, energy density is what I think it is, then? Thank you very much.

5

Yep, pretty much. It’s just the amount of chemical energy available per unit of fuel; a convenient figure for comparing various fuels and their costs, and such.

6

I’m very comforted to know it’s merely my late-night arithmetical skills that are atrocious :smack: , and not my general grasp.

7

Let me guess: you were converting from table values in calories. The factor of 1000 makes me suspect that the small calorie/large C (kilocalorie) convention got you. In fact, you may not have made any mistake at all. I’ve seen tabular compilations that gave mistaken values, esp when converting diverse sources to a common unit, like joules/L or kWh/kg. The calorie capitalization convention is far from universal, and many editors refuse to use it. Alas, this doesn’t allow them to ignore it – they must still be eagle-eyed about the units/conventions used in each source.

8

Good guess, but not quite… I was trying to work out kWh/pound (to evaluate someon’s claims, which I can’t discuss here), with many merry adventures through units both imperial and metric, and lost it between grams and kilograms. Right factor, wrong unit. It was a late night.