The following quotes are from "Souping the Stock Engine" by Roger Huntington and
published by Floyd Clymer Publications. No copyright is given, but the address
has a zip code, so I would guess it was published after the mid 60's. Engines
being referenced are the typical American 6 or V8 of the period. The power
curves show peak horsepower at 4000 RPM, Peak torque at 2000 RPM and minimum
fuel consumption at 2300 RPM.
"As we all know, the cylinder pressures are converted into torque at the
flywheel through the piston-rod-crankshaft power train. What we may not be
aware of is that a lot of power is lost in friction during this conversion of
gas pressure into torque. Included in these mechanical losses are the "pumping"
losses caused by the piston moving against a vacuum and a pressure during the
intake and exhaust strokes.
"A typical stock engine with full throttle will deliver about 90% of the power
developed in the cylinders to the flywheel at 1000 rpm; this "mechanical
efficiency" drops to around 70% at 4000 rpm. Now engine friction multiples
rapidly with speed, and we can't get away from it. By using ball or roller
bearings for the rods and crankshaft mains, we can raise these efficiency
figures to around 80% at high speeds, but these types of bearings are unsuitable
for production engines for other reasons.
"The efficiency figures quoted above are for full throttle. With the throttle
partly closed when cruising, things are even worse because the HP lost in
friction at a given RPM is nearly constant-regardless of engine load. For
example, suppose we have a car traveling 50 mph with the engine turning 2000
rpm. At this speed, the car will require about 25 hp at the clutch to drive it
on a level road, but the engine could develop say 70 hp at 2,000 rpm with full
throttle.
"Now the power lost in engine friction at this speed will be say 14 hp at all
loads (this is typical). Obviously at full throttle, the cylinders a producing
70+14=84 hp, and the efficiency is 70/84=83%. But throttled at 50 mph cruising,
the cylinders are producing 25+14=39 hp and the efficiency is only 64%. This
explains why an overdrive increases your gas mileage by holding the RPM down and
making the engine work at a greater load."
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"Now consider the fuel consumption curves. Fuel consumption isn't too important
if you're souping your engine for track sprint racing, but good gas mileage is
awfully hand in a road car. In engineering work, fuel consumption is usually
expressed in terms of lbs. of fuel consumed per HP produced per hour. This is
convenient since the energy in a fuel is given in heat units (B.T.U.'s) per lb.
"Now notice how the fuel consumption curves reach a minimum near the peak torque
RPM (with full throttle), and increase at lower and higher speeds. We can lay
the main blame for this increasing consumption on only two things - engine
friction at high RPM and low compression pressure at low speed. Obviously, with
the friction increasing rapidly with RPM, a greater amount of fuel must be
burned to overcome it, and this causes the "specific" fuel consumption
(lbs./HP-hour) to go up at high RPM.
"At low speeds, however, friction is of little importance and in this case, it's
the low compression pressure and high heat loss through the cylinder walls that
causes the specific compression (sic: consumption) to rise. You can easily see
that, with the lower consumption (sic: compression) pressures, the cylinder
pressures on the power stroke will be lower for a given weight of fuel mixture
burned. Also, with more time per stroke at low speeds, more heat is lost to the
cooling water and this all contributes to a very low combustion efficiency. Not
only this, but with the usual exhaust-inlet valve timing overlap, some of the
fuel is pumped out of the exhaust valve on compression. The net result is a lot
of fuel mixture being burned for the amount of power appearing at the flywheel.
And when we combine all these effects, we find that the minimum fuel consumption
comes near the same RPM as the peak torque - which is what we'd expect since
this is the most efficient speed for the engine.
"As we mentioned earlier, closing the throttle has the effect of increasing the
specific fuel consumption because then the friction losses remain constant while
the total power produced in the cylinders goes down. Add to this the usual low
efficiency of operation at low RPM, and we get a very high specific consumption
when running throttled at low speed.
"Numerically, the usual stock engine will have a specific consumption ranging
around 0.60 lbs./HP-hour under the best conditions at full throttle, and running
up to about 1.2 lbs./HP-hour when throttled way down to say 800 rpm. This will
be a blow to some of you fellows who thought an engine's most efficient
operation was at low throttle and low RPM. Admittiedly it's burning fewer
gallons per hour, but for the HP produced, it's a glutton under these
conditions! This all means that, for best fuel economy, we should increase the
engine load at moderate RPM (around 2,000 rpm)."
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NOTE: The 2000 RPM stated above is at the peak torque and close to the minimum
fuel consumption of 2300 RPM for the typical American V8.
Blake
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