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Old 01-22-2016, 09:37 AM   #88 (permalink)
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Originally Posted by ProDigit View Post
I think the biggest argument against fast acceleration, is mainly high RPM.
When having a CVT, best results come from running the engine at the lowest RPM needed for the load; eg: low acceleration, keep RPM at 1500RPM.
Fast acceleration, at 1500RPM it might be lugging; so RPMs go up to where the engine has enough torque to pull it off.

Ever heard of advancing the timing of an engine?

In a way, it's doing exactly the same thing as increasing the load (or increasing the gearing).
It's running the engine in a much more efficient state.
Too much, and the engine will lug, ping, or shut down.
Too little load, and the engine will run inefficiently.
Most cars are tuned to run fine when towing it's own weight up a 25 degrees hill in 5th or 6th gear. That makes for good acceleration, but also lower MPG.
Reducing that buffer, may make the engine much more fuel efficient, but also less able to tow heavy loads uphill.
Some interesting analogies here!

I think you'll find some contention about the existence of lugging in modern engines, but regardless, peak BSFC is not always at the lowest possible RPM, and maximum economy will be found by producing as much forward energy as possible at peak BSFC, and as little as possible away from peak BSFC. Peak BSFC is often quite low in the rev range for large engines, but for many small 4 cylinders, it's somewhere around 2,500-3,500RPM at high (but less than 100%) load.

Still, it's probably a safe rule to follow, to shift as soon as possible. Slightly better is pulse and glide (with EoC) at peak BSFC.

In the case of my particular car, 3rd gear is slightly less than 1:1, and I have two overdrive gears. With the tiny 3 cylinder, all but the shallowest of hills require a downshift, and when towing close to its own weight, it's not uncommon that I find myself in 3rd or even 2nd. Increasing the amount of engine would probably increase its efficiency when towing, because I could keep RPMs down and BSFC up, but it would decrease efficiency when not towing. For best economy, you really want just enough engine that, while cruising up a shallow grade at highway speeds, you're at high load near peak BSFC in the rev range. It makes me wonder why more cars don't include a gear for that. Yes, you're reducing your buffer in top gear, but downshifting is always an option.
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