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From: ix1037@sdcc6.UUCP (Christopher Latham)
Newsgroups: net.auto
Subject: Re: Diesel efficiency
Message-ID: <1965@sdcc6.UUCP>
Date: Wed, 13-Mar-85 23:43:55 EST
Article-I.D.: sdcc6.1965
Posted: Wed Mar 13 23:43:55 1985
Date-Received: Sat, 16-Mar-85 01:04:35 EST
References: <216@cmu-ri-leg.ARPA> <436@terak.UUCP>
Organization: U.C. San Diego, Academic Computer Center
Lines: 24

> > Why does a diesel give better m.p.g. than a gasoline car.  Is it because the
> > effieciency is greater because of the higher compression ratio or because
> > diesel fuel gives out more energy when burnt?
> 
> My (admittedly unknowledgable) beliefs are:
> 1)  The Diesel cycle is slightly more efficient than the Otto cycle
>     in "normal" designs;
> 2)  Most Diesel engines are lower-powered than the gasoline engines
>     they're compared to;
> 3)  Gasoline engines are hampered by emission regulations from which
>     Diesels are exempt.
Actually the cycle in diesel engines is less efficient than that in
gasoline engines. However the efficiency in both cycles goes up as
compression ratio goes up. So since automotive diesels operate at around
20/1 as opposed to 7-10/1 for typical gasoline engines they get their
increased efficieny from the higher compression ratio. If gasoline
engines could be run at such high compression ratios they would get
much higher mileage.

Christopher Latham
U.C.San Diego
Dept. of Applied Mechanics
and Engineering Sciences
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