In a message dated 97-11-29 I wrote:
>>Given that Jerry is correct, and assuming that Christopher is also correct
(and I believe he is - thanks for the clarification, Chris), Just how much
more efficient can an engine be just by dropping the rpm by 820? How much
difference in throttle position is there? If it takes 15 - 20 hp to move the
car at freeway speeds in fourth, it still takes 15 -20 hp to move the same car
at the same speed in OD. Is there enough difference in efficiency to make a
real difference? How long will it take to pay for the cost of converting to an
OD if you don't already have one, particularly since most of the driving we do
in these cars is off the freeway?<<
To which Chris Delling replied:
>>What I think you are assuming is that your 15-20 h.p. figure is a constant.
It's not. The higher gearing provides a mechanical advantage. The engine does
not have to put out as much power to move the car down the road. 820 R.P.M.
is significant (about a 25% reduction from a 3500 R.P.M. cruise speed). If
you don't think so, check your R.P.M. at 40 in third, and again in fourth.<<
And:
>>As for fuel economy, I assure you that your assumptions are wrong. Consider
efficiency. It is simply the ratio of energy in to energy out. As you
correctly state, the engine is at it's most efficient point when operating at
it's power peak. This does not equate to the most economy, however, as
economy and efficiency are not directly tied to one another. Consider your
car. What throttle opening is required to achieve 60 MPH in 3rd. Now how
about 4th? Each cylinder has a constant displacement. If you are filling that
cylinder more completely (larger throttle opening), and more often (higher
R.P.M.) it's a given that you are using more gas. <<
Hi, Chris:
I'm going to disagree with you on your first point, and maybe the second as
well. The 15 -20 hp figure quoted is indeed a constant, as that was stated as
the hp REQUIRED to move the car. How much energy is consumed by the engine in
the form of gasoline is a variable, which is by definition efficiency. If it
takes 15 -20 hp to overcome friction, wind resistance, etc, at 60 mph, it
takes that 15 -20 hp regardless of the type of engine, fuel used, energy
conversion efficiency, etc. It doesn't matter if the engine is a nuclear
reactor or a brace of hyperactive chipmunks, the energy supplied to the rear
wheels is a constant, as long as the speed and road conditions remain
constant. Apply more energy, you go faster, apply less and you go slower. If
you were to leave the emergency brake on, for example, the engine may have to
produce 100 hp to get the required 15 -20 appled to actually moving the car.
The other 80 - 85 hp is spent in cooking the brakes, but that doesn't change
the 15 - 20 hp required - it is a constant.
As to the second point, it takes a SMALLER throttle opening to produce 60 mph
in 3rd than it does to reach 60 in fourth, not more (as long as you stay
within a reasonable rpm from the peak efficiency value). The fact that the
engine has a constant displacement is not a factor. Consider going downhill in
4th at 2500 rpm vs going uphill at the same speed in the same gear. If the
down hill is steep enough, you may have to apply negative energy in the form
of braking to maintain the 2500 rpm, whereas going up a steep hill, you may
put the pedal to the floor, and still not have enough power to maintain the
2500 rpm. In both cases, the engine displacement is the same, as is the rpm,
yet there is a drastic difference in throttle position. The difference is in
how efficiently the cylinders are being filled. With the throttle closed, they
may be only 10 % filled, yet in a race engine at full speed, they may be
filled at as much as 120 %! Not only that, but the air/fuel ratio will differ,
from a very lean condition in the first case, to a very rich condition in the
second.
As to economy and efficiency not being tied one to one, I will have to agree
with you in general, but I'm not sure how far apart they are within the
framework of our discussion as it applies to automobiles on the street.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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