In an earlier message, I say:
> After WWII, engine development has moved consistently toward the goal
> of maximizing RPM and BMEP.
Actually, that's not true. It was true roughly till 1968, after which
point engine development has been greatly affected by the redirection
of engineering dollars toward pollution control systems and fuel
economy optimization. Curiously enough, fuel consumption at least
has seen a return in many circumstances to a much more prewar-British
type of engine, one with lots of low-end throttle response and that
relies less on high-end operation. BMW's eta engine was one of the
first, coming on the heels of a BMW technical report which identified
sources of internal drag and friction as part of fuel consumption.
The worst culprits were found to be the valve springs and the throttle
plate. The valve springs of course press on the camshaft with equal
force to that with which they press on the valve collars, which
consumes power, generates heat and otherwise has a negative impact
on the engine's output. In an engine designed to run at high RPM,
the valve springs require a very high pressure, so this is yet another
reason to mazimize output at low RPM.
As for the throttle plate, it was determined that fuel use dropped
when drivers adopted a driving technique in which they used lots of
throttle opening but shifted very early, say at 2000--2200 RPM. The
other extreme, using shallow throttle positions but keeping it wound
out higher, invoked far worse fuel economy because of internal friction
and cycles per unit of travel. But surprisingly, one of the main
sources of the internal friction was the pumping loss that occurred
when the pistons tried to pull air past a barely-cracked throttle
butterfly. Opening the throttle past the 50-60% mark reduced this
source of drag, and as long as the driver shifted low enough in the
RPM range, overall fuel consumption was low. (At a fixed speed of
course the throttle has to be held at the position that holds the
speed, but the wide throttle/early shift technique holds true for
acceleration.)
I'd think that the wide-throttle/early shift would apply particularly
well to older British engines, which typically have a long stroke
and some kind of vacuum-dependent carburetion. Shifting early gets
an extra win on long-stroke motors because the piston speeds stay low;
higher piston speed increases the friction, saps even more power from
the engine, and requires much more fuel. I'm less sure now, however,
about the SUs than I was when I wrote the preceding sentence; it
seems now that the damper would keep the piston from raising too high
in the first place when you dropped the throttle, and the accelerator
pump effect that this causes would use more throttle anyway. Still,
if fuel efficiency is your goal, shifting early on early British cars
is probably a good idea.
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