Diesel v petrol fuel economy

On the subject of fuel economy I wonder if anybody on the ng can answer this question. Do diesels give more mpg because there's simply more energy per
unit volume in diesel fuel compared with petrol, or is a diesel engine intrinsically more efficient at turning hydrocarbons into kinetic energy (or perhaps I should phrase it "less inefficient")?
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In thermodynamic terms, they're totally different things. Best of all would be the Carnot Cycle (I'm not even going to get started - just Google ;o)
Funnily enough, Diesel engines use the Compression Ignition Cycle, which isn't quite as good as the Diesel Cycle. AFAIK, the "proper" Diesel engine remains pretty much theoretical.
Depending upon how you look at it, Diesel was either a visionary or a bit of an engineering idiot: he designed something to utilise waste coal and cotton dust, but, given the state of materials technology at the time, the only thing his engines powered were themselves - at high speed, and over an expanding area of real estate. 400 bar and wrought iron just.. doesn't.
His design improved on steam engines of the day, in terms of fuel efficiency, but lacked the power to weight ratio required for use in a car. That took several decades' of development by slightly more switched-on engineers.
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Hairy One Kenobi

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Noisy bacon wrote:

130 TDi was the most thermally efficient production car engine available, if that's any help. I've read somewhere that it's acpable of transferring 43% of the fuel energy into power, so "less inefficient" may apply here, but that's much more efficient than, for example, an ordinary lightbulb.
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Sounds like someone's dream - the mere age of the fuel would make several percent difference in conversion efficiency, let alone the fact that they're using the same thermodynamic cycle as everyone else!
It's been a couple of decades since College, but 43% sounds like the Carnot Cycle (i.e. theoretically perfect), rather than something produced by a typical petrol engine. The hs tables are undoubtedly in the loft, but, well, um ;o)
Standing-by to be corrected...
H1K
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Hairy One Kenobi wrote:

Hmmm. A diesel-cycle, with an expansion ratio of 5, a CR of 19 and using eta=1.4 gives a theoretical efficiency of 57%, even ignoring the increased effective CR due to the turbo.
IIRC modern low NOx diesels have flame temperatures of under 2000K, and exhaust temperatures of around 500K, so the equivalent Carnot cycle efficiency would be around 75%
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Wowsa.. things have changed since 1988 ;o) For some reason, 43% rang a bell.. I stand/sit corrected!
The age of the fuel still makes a difference, though - enough so that F1 fuel is overrated when transported, but down to pump values when used a few days later.
H1K
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Hello Hairy. Don't you hang around uk.telecom.mobile too? Think I've seen your name in there somewhere.
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Nope, sorry...
H1K
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Hairy One Kenobi wrote:

Well how about ntl.discussion.broadband then?
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Been there, but not recently.. the servers have to be working, first ;o)
H1K
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