Why no fast acceleration?

[Fat Charlie]

Wasted noise, be it a fart can or Justin Bieber, is 100% waste. Turning the noise into something useful like forward movement or or just about anything Carlos Santana did will still involve waste, but you get a whole lot more out of it.

[jamesqf]

Originally Posted by ConnClark

You will here the riceboy's fart can muffler long before you will here their stereo.

Not true, in my experience. Though of course you mostly hear the thump, thump of the bass notes from the subwoofers.

[skrotbil]

Originally Posted by Zach

Sorry to bump a thread kinda old but isn't that the opposite of what the 100+ hypermiling tips states for DWL (56)? Accelerating on downhills right before an uphill climb since you burn less on the downhill? I am still trying to learn so some clarification woold be great.

I found UFO's advice posted on 11-16-2015 to be useful and true. Rather than accelerating on the downhill, I seem to get better mileage when I apply only the minimum acceleration needed to maintain speed on the downhill, using DFCO coasting whenever the gradient alone is sufficient to maintain speed. I then start applying increased acceleration in the level transition area between the downhill and uphill, using DWL on the uphill while monitoring instant MPG gauge as feedback, gradually easing off the accelerator as I crest the hill.

[ProDigit]

Originally Posted by UFO

Yes, don't do a running start before you get to the hill; accelerating downhill is not efficient. Save your engine power for going up hills and don't back off until you crest the hill, unless you know you will need to scrub speed on the downhill.

I don't know what most people say about the fuel cutoff; most cars continue to supply fuel even when coasting in gear, with foot off the accelerator.
The car also will slow down faster.
It's much better to stay in neutral when coasting downhill, for as long as the car maintains speed, or accelerates.

Also, going 100% open throttle is really bad for the engine, and fuel economy.
Acceleration should be spirited, but not too aggressive, 50-80% of engine power. Higher with smaller engines and/or heavier vehicles, lower with larger engines and/or lighter weight vehicles.

[ProDigit]

Originally Posted by ConnClark

The biggest argument against fast acceleration is that excess pressure is left in the cylinder when you lay into the gas and it just ends up as noise when the exhaust valve opens. Its called blow down and with the otto cycle it is un avoidable. The more gas you give the engine the more that goes to waste.

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.

[Ecky]

Originally Posted by ProDigit

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.