temp-mpg relationship

[John Shost]

Why do you think the less thermally efficient diesel engine gets so much
better fuel economy under cruising conditions than an Otto cycle (spark
ignition) engine? It doesn’t take much power to sustain 60 mph on a level
roadway. A significant portion of power is lost by pumping a vacuum. This
greatly reduces both the mechanical & thermal efficiency of the Otto cycle,
resulting in poor fuel economy relative to the diesel engine for less than
full load operation. When you choke the engine with the throttle to limit
road speed you reduce the amount that the cylinder is filled so the
effective compression ratio is much lower than when the throttle is wide
open, thermal efficiency is reduced. During the intake stroke the choked
engine is expanding the air/fuel & residual mixture in the cylinder. Energy
is lost by doing this expansion, mechanical efficiency is reduced. This
occurs in SI engines (including Audi’s) and is not limited to “academics”.

On Sat, 7 Apr 2001 04:41:15 -0700 (PDT), mike wrote:

>  looks like academic fluid mechanics to me...only a
>  tenuous link to car engines especially Audis
>  
>  --- Huw Powell <audi at mediaone.net> wrote:
>  > huh?
>  > 
>  > John Shost wrote:
>  > > 
>  > > If you are using a throttle to regulate power then
>  > hotter (less dense) air
>  > > will result in less pumping losses.
>  > > 
>  > >
>  >
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>  > Huw Powell
>  > 
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>  > 
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>  
>  
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