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
Hello Bob, Sorry for being late, Paul answered it all, also regarding bsfc being
different at different engine operating conditions.
However, consider also this; you drive up a long hill, then turn around and drive
down again with the engine off. You covered twice the distance with only slightly
more fuel used going up. Your average fuel consumption is only a little more than
half compared to the same distance traveled on a level road.
Of course, this is an extreme case. Normally the engine is not switched off. The
closest you come to above scenario is when the road has a slope (or grade), that
allows you on the downhill leg to be completely off the accelerator without
needing to step on the brakes and going the same normal travel speed. The air
butterfly will be closed and the fuel supply is nearly equal to idle. Very little
fuel is used going down. The total fuel used is more than per the above scenario,
but certainly less then you would need if the road was level and you drive the
same distance (equal to up and down).
The total fuel consumption goes up from this minimum if the road is less steep or
if steeper.
The other point mentioned by Paul to increase fuel consumption dramatically is to
accelerate to say 40 or 50 mph, then shutting the engine down and letting the car
run down to a stop or near stop, then start up the engine and repeat this
procedure. You can?t do that in traffic of course, but at night on a lonely road
when you are low on fuel and there is no gas station in sight for miles around.
The gas mileage you get is in the range of being double.
Now, wouldn?t that apply to flying as well? I wonder if it would really work too.
Assuming you fly level at a given cruise altitude. Now you apply more power and
climb, perhaps 2000 ft. Then you throttle back to say 1500 or less rpm (~2500 rpm
for rotaries) but maintain speed with the elevator. After you are back to the same
cruise altitude, you apply power and climb up 2000 ft again and then you throttle
back. You repeat this as often as you want. Mind you, if you have passengers, they
may ask what the hell are you doing? Nevertheless, it should get you further for
the same amount of fuel. Are there any takers out there to try that out? If would
be nice to have that theory tested.
Best Regards
Rolf Pfeiffer
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