Quote:
Originally Posted by alvaro84
That might be true, yet it's probably a very bad idea to cruise at that rpm: an 1200cc bike engine at 4200rpm needs ridiculously light throttle which gives very bad efficiency, even if the engine could theoretically do its best there. (May not be true at 90mph anymore, but most of its energy will be absorbed by the aero drag then, so it'll be inefficient again.)
Having it spin slower can mean a higher load which, eventually, almost surely leads to much better efficiency.
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"Efficiency" can mean many things. It depends on how it's defined
To a racer it can mean how much power can this thing put out, measured in horsepower. It can also mean how full can the cylinder be filled on each stroke, measured in maximum torque. It could also be defined as the area under the torque curve that shows how full the cylinder fills over a useable or defined RPM range. All of these are measured at full throttle, meaning there is little restriction in the intake system. We usually don't ride full throttle for extened periods on the street and expect fuel efficiency so these numbers probably don't apply for our purposes.
Efficiency is also expressed as BSFC or Brake Specific Fuel Consumption. Gasoline ICE engines are most fuel efficient where BSFC is lowest. This can occur at RPM and load points other than the torque maximum (at WOT), usually at a lower RPM than the torque peak, that I've seen. The BSFC chart also considers less than maximum throttle openings so it's directly applicable to real-world loads, not WOT maximum outputs. A small engine running at high RPM and WOT producing 20hp is not always more efficient than a larger engine running slower and producing 20hp. There are factors of mechanical friction at high RPM for the small engine and the energy required to overcome throttling loss at less than WOT for the big engine. Then there is the efficiency of the combustion event itsself.
To manage these factors for the maximum fuel efficiency possible while producing ONLY the power required there is a balance to be found having:
a) a wide throttle opening to reduce throttling loss (tall gearing)
b) a low RPM to reduce mechanical friction
c) an efficient combustion event to completely burn the mixture
Finding an engine with the smallest BSFC for the power required will get you the highest fuel efficiency. Simply operating the engine you have at it's lowest BSFC will maximize your efficiency as we define it. Finding the BSFC for your engine is elusive though.
I suspect the Harley in question has maximum torque (cylinder filling efficiency) at 4200 rpm. Alvaro and most of the rest of us efficiency geeks define efficiency as how much fuel is used to actually propel the vehicle at a given speed.
There are other factors as well, most of which is aerodynamic drag over about 50mph or so.