That is more the result of diesel having a higher energy content per liter (volume) then gasoline ; not that the diesel engine is so much more efficient in energy conversion (although it is a little more efficient). On the other hand diesel also weights more. It is very interesting to note that the 18% higher energy content of Diesel is mirrored by it also weighting 18% more per given volume.


But even more dependable sources are a little bit confused (see the second paragraph)

From wikipedia : http://en.wikipedia.org/wiki/Diesel

The density of petroleum diesel is about 850 grams per litre whereas petrol (American English: gasoline) has a density of about 720 g/L, about 15% less. When burnt, diesel typically releases about 40.9 megajoules (MJ) per litre, whereas gasoline releases 34.8 MJ/L, about 15% less. Diesel is generally simpler to refine from petroleum than gasoline and often costs less ... Also, due to its high level of pollutants, diesel fuel must undergo additional filtration which contributes to a sometimes higher cost. In many parts of the United States and throughout the whole of the UK, diesel is higher priced than petrol.[1] Reasons for higher priced diesel include the shutdown of some refineries in the Gulf of Mexico, and the switch to ultra-low sulfur diesel (ULSD), which causes infrastructural complications.[2]

Diesel-powered cars generally have a better fuel economy than equivalent gasoline engines and produce less greenhouse gas pollution. This greater fuel economy is due to the higher energy per-litre content of diesel fuel and also to the intrinsic efficiency of the diesel engine. While diesel's 15% higher density results in 15% higher greenhouse gas emissions per litre compared to gasoline,[3] the 20–40% better fuel economy achieved by modern diesel-engined automobiles offsets the higher-per-liter emissions of greenhouse gases, resulting in significantly lower carbon dioxide emissions per kilometre.[4][5]


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First they define fuel economy as per liter of fuel, which will get the author lose his engineering title if he ever tries to pass it as energy conversion efficiency in any scientific publication. Still, a common reader will definate equate the two and thus be errornously informed. Then they produce another gem. Then they say :

"... the 20–40% better fuel economy achieved by modern diesel-engined automobiles offsets ... "

So first we need to subtract the 15% better fuel economy (as they define it) due to the higher energy content per volume. So we end up with the Diesel engine being 5% to 25% more energy conversion efficient then a gasoline engine which is typically about 20-25% in non stationary operation. Of course this means that a diesel engine will have the following efficiency range in stationary operation : 105%*20% to 125%*25% = 21% to 32%

I know that their "20–40% better fuel economy" sounds alot better (in marketing) but the real SCIENTIFICALLY sound numbers (- energy conversion efficiency) are only 21%-32% for Diesel compared to 20%-25% for gasoline. A difference indeed, but not a huge difference.

I think Hybrid-cars get better ratios then both, mostly because in that setup the engine can be run stationary which will be more efficient still. Even better would be a practical fuel cel setup as that totally circumvents the thermodynamic processes and heat loses that are implicit in combustion engines. As a result a fuel cel is not limited to the max theoretical Carnot energy conversion efficiency of 55%-65%. As a result the practical efficiency can be much higher as well. I had expected a practical fuel cet setup by now but something is holding it up.

Wouter