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
Hairy One Kenobi
Disclaimer: the opinions expressed in this opinion do not necessarily
Well, there was a rumour that at the time of it's launch, the VW-Audi PD
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
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,
Standing-by to be corrected...
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%
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
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