One thing though Paul is, that when driving on a stretch with high
hills up and down again, you always burn less fuel on average for
the total trip as the extra fuel for climbing is less than the fuel
you save going down hill.

I must respectuflly disagree.  It is just the opposite, particularly if
you maintain a constant speed.  Allowing the car to coast up the hill is
comparing apples and oranges.  You would have to speed up and coast on
the level stretch also. It's a rule of nature that you never come out
ahead.  The best you can do is almost even.  (One of those termodynamic
laws.)

Bob White

--
http://www.bob-white.com
N93BD - Rotary Powered BD-4 (soon)

On the surface of things Bob you would be right.
If you assumed the BSFC was constant with HP consumed.
What happens is the engine is a lot more efficient going up the hill
at a constant speed than it is going down the hill at the same constant 
speed. A lot depends on the exact BSFC map of the engine and the total drag 
of the car. A low drag car would position the engine operation in a much 
worse part of the BSFC map while at the same time reducing the HP requirements
both going up the hill and down the hill. It all depends on how fast the
BSFC gets worse as the HP required gets less.

The HP required to climb the hill at a given speed depends on the weight 
of the car and the total drag both rolling and aero.

One must accurately model mathematically the entire system to know for sure.
You cannot assume the BSFC is constant.

Paul Lamar
 
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