Quote:
Originally Posted by drmiller100
But, if take a 500cc engine, in theory we could find an RPM and run it at WOT to get our 22 horsepower.
In theory, we could take a 1500cc engine, and run it at 1/3 the RPM, at WOT, and get our 22 horsepower. So, given this example, out 1500 cc engine would be as efficient as the 500cc engine, right????
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By using the Saturn chart for both, we are effectively saying that the engines are identical, even down to cylinder size: one cylinder of a four cylinder engine produces 1/3 the hp at exactly the same efficiency, as the other three. (In real life this is unlikely to happen that you find a single with exactly the same traits as a 4 cylinder.)
In this case, the cylinder volume to cylinder area difference disappears. Then the issue that arises is the ~ 3:1 difference in rpm to achieve the same hp. Due to acoustics, inertia effects, combustion time vs stroke time available, heat influx vs expansion rate, etc there is a speed at which the engine is most efficient under a particular high load. In the example I'd used, in one engine, the speed is below this point, and in the other it is above. Coincidentally, it's about the same distance above and below, and the engine is pretty flexible (broad torque band) so the actual efficiencies are about the same.
In practice, the general shape of the power band and BSFC map of two engines, one at 500cc and one at 1500cc are likely to be quite different. And the cylinder sizes are likely to be different too: the 500 could be a thumper, or a twin, or could be a four-cylinder motorcycle engine with peak power at 12,000 rpm.
So the reason most basic reason a small engine is
typically more efficient is mainly because the applied is closer to the engine torque peak, where BSFC is at a minimum. The effect shows up more dramatically under low loads, and especially dramatically when idling when (if the accessories are not cycled on) the engine is producing no useful work, but is overcoming internal friction. Then, the difference can be pretty close to the difference in displacement: a 500cc engine uses about 1/3 the fuel of a 1500 cc engine when idling.
Going beyond those generalities really requires the specific BSFC maps of the engines in question. After selecting a good fit, a designer would then tweak everything to keep the engine close the sweet spot as much as possible... balanced against marketing demands. It is a rare design in which all the stops are pulled out in the interests of fuel efficiency, with the Prius being the best example of heading in that direction. But even there, it could certainly be much lighter, could have better aero, could have lower frontal area (if comfort can be sacrificed a little), the cargo area could be smaller, etc.
But then the market would likely be a little smaller.