Why no fast acceleration?
 

I have just started reading some articles about fuel efficient driving. While I cannot say that I am committed to doing everything that hypermilers seem to do, I have already started backing off my speeds, and staying more off the gas pedal when I see that I need to break up ahead; in just a few days I am now automatically picking out ideal points from which I can coast. I am surprised by how much I’m enjoying this—its almost as if I’m working at a puzzle or something. It sure is making me a safer driver. And a more relaxed one--as well as saving some gas and $.

OK—down to my question. I saw one suggestion, that urged drivers not to accelerate too quickly. What is the reason for not accelerating too quickly? Is it simply that a rapid acceleration chews up more gas than a slower one? Or is it that a rapid acceleration will make it more likely that you will have to break soon?

For example, if I have a clean shot for 1/2 mile—no lights or cars ahead of me or anything—would I still be consuming gas unnecessarily by accelerating rapidly?

If you accelerate too quickly the fuel mixture will be enriched to prevent detonation, either by the carburetor acceleration pump or by the ECU operating off its load map. When the mixture drops to 14.7:1 or lower, the engine efficiency drops.

Don't let "don't accelerate too quickly" confuse you into thinking it means "accelerate slowly." It means not to so it too quickly.

Harder acceleration means less acceleration, and that's a good thing- until the computers decide to start dumping extra fuel into the mix.

OK--I realize each car/engine might be different, but are there some decent rules of the thumb---like take 5 seconds for accelerating each additional 10 MPH, or something like that? Articles discussing this have been incredibly detailed---a bit too much for my eyes, given that I'm not a mechanic or a physicist.

Let's keep it simple. I found this quote in another thread here:

It's been debated back and forth as fas as what rate of acceleration is best. Personaly, I've found it hard to notice a difference either way. What really kills your mileage is unecessary acceleration. Hard acceleration is generally thought of as bad becuase, in many cases, people accelerate hard only to have to brake hard at the stop that's right up ahead. It's really the braking that is wasting all the energy.

If this is the best policy to take, then the only thing I need to worry about is speeding up so fast that 5 secs later I'm breaking. Was that advice (in the quote) on target?

Haha, I startrd hypermiling the same way as you. Not to commited and now I try to find every way possible to ecomod my truck so I'll get better gas millage.

Best thing to have to get the best possible gas millage without heavy ecomods, just start with a Vacuum gauge. With it you'll know if you're accelerating to hard or to soft. There is the Ultragauge that is pretty awesome to monitor your car's efficiency.

High rpm acceleration is bad and consumes lots of gas. If you have a manual transmission you can do decent acceleration at not-high rpm, like 2000. If you press the pedal that same amount in an automatic it will push the rpm higher and consume more fuel. You can accelerate efficiently faster in a manual than an automatic.

High-throttle enrichment varies widely from car to car. Some you have to actually have the pedal to the floor and high rpm, like 4000+. Some cars, anything above 80% throttle will enrich the mix. So to be safe, I'd say keep the gas pedal at or below 3/4 if you don't know otherwise.

That's a nice rule of the thumb. Both vehicles are automatic. I'll try to get that in a sig on the left if I can figure out how.

Look up "BSFC charts". Most automotive engines are most efficient at lowish RPMs and high (but not 100%) loads. As others have said, the computer will often enrich the fuel mixture as you near 100% load, which is bad. In my 5MT, I get the best economy when I shoot for about 80% load (as reported by the computer) at less than 2500RPM. The pedal position required to accelerate at that load changes based on RPM, however, so you can't just rely on pushing it a set amount. You really need instrumentation to get close to "perfection ", but I figure a good rule in an automatic is to accelerate at near the max rate you can without causing a downshift, and let it upshift as soon as possible.

That said, so long as you aren't revving it up, the biggest gains are too be found in the art of slowing down.

One of the easiest examples to understand vehicle acceleration is with one of the least popular(at least in the beginning) automatic transmissions, a Continuously Variable Ratio Tranny(CVT). My own CVT was a Nissan CVT, that was adapted to be used in a 2007 Dodge Caliber. The CVT was said to be most efficient when driving at 2000 rpms. At 60mph, rpms were 2000. When accelerating, set the throttle so your rpms are 2000, & you could accelerate slowly to 60mph. When both conditions could be met, cruising at 2000rpm & slow acceleration at 2000rpm, Caliber would give pretty good mpg, despite the car itself, not aerodynamic or the engine, particularly thrifty. Any divergence from 2000rpms & you should expect less thrifty mpg.