temp-mpg relationship

[John Shost]

True, if you look at the efficiency for a fixed maximum pressure the diesel
is more efficient. I was trying to make a point on how the throttle effects
gas mileage. For any given expansion ratio and heat input the thermal
efficiency is highest for the Otto cycle. 

On Sat, 7 Apr 2001 09:03:37 -0600, James Marriott wrote:

>  >Why do you think the less thermally efficient diesel engine gets
>  so much
>  >better fuel economy under cruising conditions than an Otto cycle
>  
>  Errr, because it's _more_ thermally effiecient? A higher
>  compression ratio leads to better efficiency (defined here as
>  turning heat energy into work energy), either in power or
>  economy.
>  
>  And because it has no throttle, and because virtually all
>  hydrocarbons have about 19500 Btu/lb of combustion energy; we buy
>  vehicle fuel by the gallon, and diesel (oil) is a bit more dense
>  than gasoline.
>  
>  cu, James Marriott
>  '87 4kq (alias "late-B2 90q") with rare NG engine, 181k
>  '89 200q (MC1, ProconTen/no bag, 1.8 blah blah), 132k
>  Boise, ID, USA     http://www.webpak.net/~marriott/
>  
>  
>





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