> that assume gasoline and run like they are lean. This doen't explain why
> gas mileage gets worse, as you pointed out. The reason gas mileage goes down
> is because there is less energy available, per unit volume, in the fuel. To
> get the perfornmance you expect, you have to open the throttle further.
> Our friends with gag, gag, cough, hack diesels would like us to believe that 
> their engines are just way more efficient than gas engines (and they are more
> efficient) and that's why they use less fuel, but that's not the whole story.
> Diesel fuel is the opposite of ethanol, it has more energy per unit volume 
than
> gasoline. There's just more zip per drop in the fuel itself.
>

Ethanol has a specific heating value of 29.7MJ/kg, compared with gasoline's 
47.3MJ/kg. But diesel is 44.8MJ/kg. Diesel engines use less fuel because 
they run at much higher compression ratios than gasoline engines, and 
operate (at least in theory) on a constant pressure cycle which is more 
efficient than a constant volume cycle. Thermal efficiency is directly 
proportional expansion ratio. You get more bang per drop out of diesel 
because you can squeeze the heck out of it before burning it and then expand 
it more completely. 

Andy E.

A.C.Erskine@lut.ac.uk