adm@cbneb.UUCP (06/18/84)
#R:pertec:-14800:cbneb:8400004:000:1959 cbneb!wdg Jun 18 09:09:00 1984 Engine efficiency at a particular rpm is a factor of dozens of parameters. Several major ones come to mind: engine temperature, compression ratio, fuel mixture, cam timing, engine head design (including valve size and shape), ignition timing, fuel quality, etc. For several reasons small engines today are usually more efficient at high rpms than large "American V8's". This is due to the smaller engines' usually having an overhead camshaft and hence better breathing capability at higher rpms and larger valves per cc of combustion chamber. Since each engine is different, and the load and environment is unique to each there is no set answer. However, I have looked at several dyno figures on brake fuel comsumption (the amount of fuel per horsepower comsumed) and would venture a guess that smaller engines are probably most efficient in the 3500-4500rpm range whereas larger engines are most efficient in the 2000-3500rpm range. Of course these are guesses. If you discuss say a BMW or a souped up American car then these figures could be quite higher. However, this is really not what you need to know. The engine is only one factor in our MPG figure. The transmission and rear end gearing play just as important a role. Engine efficiency does not nesessarily mean a high MPG. In fact usually the higher rpm a engine is running the less the MPG. This is due to the fact that an engine may be putting out say 75 horsepower at 4000 rpm, though at say 55 MPH it may only take 45 horsepower to overcome the friction caused by drag. If in the above case the car were in say 3rd gear then the driver should shift into forth, thus lowering the rpm and horsepower and fuel consumption; but still delivering plenty of power to drive at 55. One point, don't run a engine too slow causing it to "lug". This hurts fuel economy and is hard on the engine. Also running the engine at too high an rpm will cause abnormally fast wear on the internal components.
tron@fluke.UUCP (Peter F. Barbee) (06/18/84)
Two statements about fuel economy: 1) Thermodynamics tells us that an IC engine is most efficient when operating at maximum torque 2) A study done by Porsche about 4 years ago showed that all of their cars got their best mileage when driven in high gear such that they were almost lugging. At first glance these 2 statements appear contradictory because no engine develops max torgue just above lug speed but they're not. The torque ratings shown in your owners manuel are in a no load situation. This indicates when your engine can develop the most torque for a given load, but does not account for the interaction of load and torque. Thus the Porsche study does not "violate" the laws studied in Thermo. Naturally the empirical Porsche study must be correct. Peter Barbee decvax-+-uw-beaver-+ ihnp4--+ allegra-+ ucbvax----lbl-csam-+--fluke!tron sun-+ ssc-vax-+
jackh@zehntel.UUCP (jack hagerty) (06/18/84)
Most smaller engines have their POWER peak at higher revs (and, until
recently, their torque peak too) meaning that if you need fast acceleration
to pass or to merge on the freeway, etc., then you *should* use higher
revs.
However, if you're after maximum economy, then you should use the lowest
engine speed possible. There are at least two reasons for this:
First, despite the best efforts of Castrol, Mobil, Valvoline, et. al.,
engine friction between all of the sliding, rubbing, rotating parts of
your engine increases with speed. This is, in part, why your engine will
wear out faster if you keep the revs up all the time.
Second, the so-called pumping losses are lower at lower engine speeds.
Your engine is just a large pump, sucking air and fuel in, burning it and
pushing it out the tailpipe. A certain level of power requires a certain
amout of fuel/air mixture. If you force the engine to turn slower (by
using higher gears) then it must draw in a larger amount of mixture each
revolution. The amount of mixture is controlled by a movable restrictor
in the intake passage called a throttle plate. This is what your "gas"
pedal is really attached to. Therefore, at lower engine speeds the throttle
plate is opened more lessening the restriction and reducing the amount of
work your engine must do to pump the air through.
The conventional wisdom says that you should never "lug" your engine. This
is a vaguely defined term that has to do with operating your engine below
the "green line", i.e. the lowest reccomended engine speed, which could
result in all kinds of nasty things happening to your engine. This has
been largely discredited and current thinking says that if your engine is
not pinging or knocking and you're still getting enough power to do the
job, then you're not running too slow.
-- Jack Hagerty, Zehntel Inc.
!ihnp4!zehntel!jackh