Hot engines, Hot air, high speed driving

[C M Smith <cmsmith@domain.elided>]

Hot engines run more efficiently because less heat is lost to the coolant, 
leaving more heat energy for the expansion stroke. Also, hot engines run 
tighter so there's less blow by. Clearly this applies only to engines 
designed for the temperature you're running at. Modern engines run hot and 
are designed to run that way.

Hot intake air is a bad thing, lower density means less oxygen per unit of 
indrawn air. With modern FI less fuel is injected making less power. A 
minor effect is the higher the intake temperature the sooner the engine 
begins to knock, further reducing power. Modern engines are designed to run 
at the widest possible range of ambient temperatures without knocking so 
this only affects cars with knock sensors, particularly supercharged 
engines with intercoolers.

Fuel also changes density. In Canada, fuel pumps are calibrated to deliver 
the stated volume as if delivered at 15 degrees Celsius (pumps are posted 
with a tiny notification that "volume corrected to 15 degrees C"). This 
means that maximum fuel mileage can be obtained by purchasing fuel on a hot 
day and using it in the coolest part of the day, with the engine running as 
hot as possible.  Wow, eh?

Drive fast at night and fill up at the end of the summer day, just before 
driving off into the desert after midnight.

Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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