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Why you get better gas mileage going up and down hills.
sentences preceeded with > were part of previous comments.
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The engine efficiency running uphill can approach 40% and the efficiency running downhill can easily be 0%, if the grade is steep enough to allow the throttle to be completely closed. (It only takes about a 4% downgrade to result in zero power required to maintain 60 mph.)
The efficiency of an Otto cycle engine is primarily a function of its compression ratio, its internal friction and parasitic losses (e.g., the oil pump), and its "pumping losses" or throttling losses. Assuming fuel octane limits the compression ratio to 10:1, the peak theoretical efficiency of an Otto cycle engine is 45%, assuming a stoichiometric air-fuel mixture, zero throttling, and zero friction/parasitic loss. When friction and parasitics are considered real engines can achieve something close to 40% efficiency. As throttle is applied to reduce engine output, efficiency decreases and reaches 0% efficiency at no load.
Using real engine efficiency maps I've obtained from GM and others, the a typical 3500 pound passenger car engine running up a 4% grade at 60 mph burns about 17 pounds/hour (21 mpg). Down a 4% grade, the engine consumes 2 pounds/hour (even less if you take the transmission out of gear). At 2 pounds/hour, the vehicle is getting 180 mpg. The average consumption up and down hill is 9.5 pounds/hour (38 mpg). (Note that 21 mpg + 180 mpg doe NOT equal 100.5 mpg.) Running on a zero grade at the same speed, the engine consumes 10 pounds/hour (36 mpg). As a result, the vehicle gets 5% better fuel economy going up and down 4% grades than running on a dead level roadway.
> Going uphill, you are using a lot more
> total energy (working against gravity, all else being equal) and you
> are getting that energy at only a modest improvement in efficiency.
> They still don't balance out.
All of the increased energy required to climb the hill is returned going downhill and since there is a huge difference in fuel consumption, it is possible to achieve a net benefit.
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Tom Austin
http://www.sierraresearch.com/
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