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Originally Posted by bruce.augenstein@comcast.
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Originally Posted by Eddien123
Lower revs does not always mean better mpg. If the engine can't turn the gear without dumping more fuel into the cylinder then that doesn't help your economy. So it comes down to less fuel with more combustions (at 3,000 RPM) or more fuel and less combustions (at 2,000 RPM) it would essentially even out. That is why economy cars like accords don't have extremely low revs. The engine just struggles and uses more fuel.
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Look. Without rancor, you need to do some more studying on the topic.
First, lower revs (meaning lower revs at cruise) results in reduced fuel consumption - for the indisputable reasons I've already mentioned.
Second, of course you'll need more fuel per power pulse in order to make the same power to maintain 80, but this in no way addresses the topics of friction and pumping losses. Do you dispute my claim that best mpg will be at full throttle at whatever cruise speed? If you do, you are disagreeing with any engineering tome on the topic, written in any language.
Do you disagree that one of the two* key reasons that diesels get better mpg than gas engines is because they effectively run at full throttle all the time, with power regulated only by fuel flow? If you do, you again need to do more reading.
*The other reason is that diesel fuel contains more energy per gallon than gasoline.
Lastly, do a little reading on pumping losses, would you? On the intake stroke, each piston has to pull down against a significant vacuum at cruise (because the throttle is nearly closed), and that effort robs power from the cylinder that's firing at the time. These losses may account for a 10-15 horsepower loss at cruise (out of the 50-60 you'll need to cruise at 80), meaning you're using fuel to do nothing more than power the next piston on its intake stroke. At full throttle, pumping losses are minimized (perhaps two or three HP), so you're effectively having to feed fewer HP to maintain cruise, thus maximizing fuel economy.
BMW obliquely addressed this topic more than 30 years ago, when they said that best MPG would be obtained by accelerating at full throttle and shifting at absolute minimum rpm - meaning actual acceleration would be leisurely, but most effective, mpg-wise.
Bruce |
I wasn't saying a drop in RPM from 3,000 to 2,500 would be bad for economy. But most people assume getting a highway cruise at 1500 will do the trick and it will not. Plus this car isn't even designed to be an economy car so M has no reason to drop the highway revs.