Hi Bob,
I agree that we are talking about two concepts. I thought that was
apparent, as I started out with "The other point ..."
The topics are:
1) Lower gas mileage in a hilly terrain.
2) Stretching your distance per amount of fuel by speeding up and
coasting to a stop.
Regarding Point 1) If you can't agree that going up a hill does not
require double the fuel in MPG, but only slightly more, and
secondly, that you burn no fuel at all going down hill with the
engine off, as per my first, most severe scenario, then there is no
point in discussing it further.
I also wonder what "thermodynamic" law here applies.
Since you consider it "unlikely". Why not give it a try? When we
were driving around in Austria and Switzerland, the gas mileage was
always better than say in Germany or France.
Rolf
Hi Rolf,
The problem with 'giving it a try' is controling all the variables.
Things like the wind, temperature, did I have the AC on, did I
keep the speed constant, etc. can change the outcome. I can tell you
that in my old Suburban, there are a lot of hills that would take 2 or 3
times the gas to go up as driving on level ground. I used to have a MPG
computer in it. I could see 90 MPG going down some hills, but 2 or 3
MPG going up. Average MPG is about 10. You don't want to see those
secondaries on the 4-barrel kick in if you are trying to save gas!
What I did do was use the data in Paul's earlier attachment to see what
interesting things I could learn. Someone went to a lot of effort to
collect that data, and our first assumption is, they did a good job
controlling the experiment and the data is accurate.
The first thing I had to do was figure out just exactly what bmep
(brake mean effective pressure) was and how it related to HP. I found a
link that was helpfull. (http://www.tsrsoftware.com/bmep.htm) From this
formula, it turns out that bmep is proportional to HP at a given
RPM, and I can ignore the constants involved.
Next we will assume that the extra bmep (or HP) required to go up a
given slope will be the same amount less needed to go down the slope.
This should be about right because the mass, distance and time are
constants.
First start with the bmep required for level ground, then get the bfsc
for that point, +20 bmep, -20 bmep, +40 bmep, -40 bmep, and additionally
the bmep required going up that allows coasting going down (double the
level value). Going up a grade any steeper than double will obviously be
less effecient.
Now I can get a number proportional to fuel usage by multiplying bmep
and bsfc for each point.
The first image shows the points I used, and the values assigned using
my extrodinary ability to interpolate (and extrapolate). In all
fairness, I tried to use slightly more favorable bsfc's for the up and
down values. Note the two 1.8 values shown on the map were changed to
1.7 in the table. I felt that I had violated the more favorable rule in
that highly non-linear area.
In the table summary, fuel usage is calculated by adding the + and -
hill values together, and multiplying the flat value by 2. Values where
the total for hills was better than flat are shown in green.
CONCLUSIONS:
At 40 MPH There is a clear advantage to running in hilly country. Could
I shift to a lower gear and do better? I don't know. I think I would
need another bsfc map.
at 55 MPH, the double/coasting value is better than flat. I'm somewhat
suspicious about this value, but maybe. Notice that the progression
from the +/-20 hill to the +/-40 hill is greater fuel usage, then it
gets a little steeper, and fuel usage goes down. Maybe.
For the 70 MPH case, I couldn't use the double value. I have no idea
how to extrapolate above that top line, and it doesn't look like a good
idea anyway. I did get curious and did a +/- 50 bmep. Using 0.57 bsfc
for the 119 bmep point, and 1.1 for the 19 bmep point, I get a fuel
number of 88.7. (the bsfc used for 109 was also 0.57, but that value is
actually at least 0.575. Changes are pretty slow in this area of the
map.
So, it does depend on how you are driving, and driving in hills can give
better gas milage than level ground under certain conditions. But, I in
the more normal driving ranges, even with better bsfc goin uphill,
better gas milage is obtained driving on the flat.
That was fun,
Bob White
--
http://www.bob-white.com
N93BD - Rotary Powered BD-4 (soon)
You did a great job Bob.
Paul Lamar
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/
Copyright 1998-2004 All world wide rights reserved.
