ProDarwin said:alfadriver said:I've never come across an normal production engine that has it's best efficiency point much more than 2300rpm, and on a pretty conservatively geared car, that will be about 70 mph. Most cars will be above that at 70. Let alone 90. So if you are getting better mileage the faster you are going (which I'm not doubting is possible), something is wrong at the lower speeds. Or at least not correct.
Yes. And what I was trying to point out was that even if you are right at peak engine efficiency, the lower power requirements of lower speeds almost always outweigh the difference in efficiency and result in better fuel economy. Notice how much engine efficiency changes over the course of RPM when loaded with typical vehicle road loads (dotted line) - very little.
Random engine BSFC:
I can't find a rotary BSFC, but I highly doubt its very far off this in terms of shape.
Rotary BSFC is basically shamefully bad at anything lower than WOT. I used to have some charts. Basically, a side port engine will have a WOT BSFC that muddles around from 1.0+ lb/hp/hr at low loads to maybe .55-50 at WOT. I haven't seen a similar chart for peripheral port engines, since they were only intended for racing, but the WOT BSFC on them will dip down to .40. The curve looks like an inverse of the torque curve, basically flat with the minimum at 6800 or so.
I will allow that on my bridge ported engine, my fuel injector duty cycle at idle is roughly half of what it is at 60mph. Maybe 7% duty cycle at idle and 15-16% at 60mph. At, since I can lean the engine out under higher aerodynamic load, it's pretty much the same at 85mph as it is at 70. It's unhappy at light cruise at anything leaner than ..88-.9 Lambda but under higher loads it will happily run at 1.1-1.2.
Stuff like that is why I sometimes think I want a Volvo 240 wagon with a bridge ported 13B, as a tow vehicle. Rotaries are like draft horses, they need to be run hard to be happy.
