Re: Why Can't A Fuel injected Petrol Engine be as Efficient as a Diesel?
- From: Cyril <Meynier.Cyril@xxxxxxxxxxxxxxxxx>
- Date: Fri, 20 May 2005 21:35:06 +0200
"Fritz Schlunder" <me@xxxxxxxxxxx> a utilisé son clavier pour dire :
>
>Compression ratio. Higher=better for efficiency. Diesel engines use higher
>compression ratios.
Yes. That's because Otto engines compress an air-fuel mixture. If one
compresses it too much, the mixture may explode during the
compression. This limits the compression ratio (to about 10 iirc).
diesel engines compress air only and add the fuel after that. Since
there is no fuel during the compression, there isno risk ofpremature
ignition, and compression ratio can safelybe set to much higher
values.
>However, this is only part of the reason why a diesel fuelled vehicle may
>appear to get better gas mileage. Diesel fuel also normally contains
>somewhat more energy content that gasoline on a volume basis.
This shows in factthat measuring fuel efficiencyin milesper gallon
(here in europe, we do the opposite, we count how much litters thecar
use to drive 100 km,butr the problem remainsthe same) is not relevant
to compare cars that uses different fuels.
A LPG car can seem inefficient because it has not much mpg's, but this
is only because LPG is 30% less lighter than gasoline.
The best measure for fuel efficiency is not Miles Per Gallons, but
miles per Megajoule of fuel.
This unit would be relevant to compare various (existing or planned)
car using diesel oil, gasoline, LPG, natural gas, hydrogen, ethanol,
biodiesel, butanol or electricity.
This is still not the whole story, because hydrogen and electricity
are not primary sources of energy : unfortunaly, there is no hydrogen
well, and no natural electric plug; so the mileage of cars using H2or
electricity should be decreased bya ratio reflecting what is lost when
these forms ofenergy are produced from primary energy.
But we could go yet a little bit further : gasoline does not occur
naturally either, and refineries consume some energy to make it from
crude.
to sum up, it is very difficult to compare fuel efficiency of cars
that doesn't use the same fuel !
--
"We do not consider that aeroplanes will be of any possible use for war purposes"
Richard Haldane, ministre à la guerre brittanique, 1910
.
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