Ideal rate of acceleration?

[theghodfather]

What is the ideal rate of acceleration, in regards to fuel economy?
I'm looking for statistics.

Thanks.

[SVOboy]

crxMPG - Gas mileage never looked so good » Acceleration and Fuel Economy Tested

But then there's so much variation from car to car and all

[theghodfather]

Thanks man, this is perfect.

[jonr]

You want to accelerate at whatever power output is best BSFC for your engine. Typically this is much faster acceleration than people think would give them the best FE. I call it "moderate" acceleration.

[MechEngVT]

I have read that BMW instructs drivers to accelerate at 75% engine load and shift when the engine's mean piston speed reaches 1200-1500 feet per minute. Mean piston speed is a function of engine stroke and RPM.

Vp=RPM*S/6

Vp= mean piston speed
RPM= engine speed
S= engine stroke length, inches

This keeps you in closed-loop operation (for EFI) at high volumetric efficiency with low frictional horsepower loss for as short a time as possible before you can achieve steady-state cruise and/or EOC. It is what I like to say is accelerating "smartly."

[sohl]

MechEngVT: That's kind of funny... when I first started to try hypermiling, I tried to get my automatic to downshift below 2000 RPM (by letting up on the gas a bit), but it was kinda hard to do without lurching in my Protege, not to mention holding up people behind me in congested traffic. So I've gone to using 2000 to 2500 RPM as my downshift target. When I solve your equation for the 1200 to 1500 fps range I get .... 1988 to 2495 fps. Now I don't feel quite as guilty about letting the rev get up to 2400 to 2500. ;-)

[Daox]

Interest info MechEngVT. Where did you find this info?

[rjacob]

I am trying to figure out the best way also. I have read easy acceleration. How easy is easy. The link above seems to suggest that going out fast is better than going out easy. I have ready to accelerate as fast as possible without going open loop. I have read that accelerating with a high LOD is the best way. The RPM stuff doesn't mean much to me since I drive an automatic and have little control over what RPMs the car shifts at. I have no clue what BSFC is.
Can someone post in easy to follow instructions the best way to accelerate for the best fuel efficiency. Pretend that I am kind of slow (um...yeah...pretend :P ).

Thanks,
RJ

[PA32R]

This is a very difficult problem. The data on the linked site above is convincing for the vehicle and conditions tested, but EOC isn't a viable option for me in my LR3.

Why is it difficult? You'd think that it would be a matter of running at the most fuel efficient portion of the engine map (for the poster above, BSFC is "brake specific fuel consumption," see here for a pretty good article) up to the intended speed, presumably that which yields best fuel economy.

But the wild card is the fact that, when accelerating, you're using fuel both to overcome external resistance (drag, rolling resistance) and to add kinetic energy. It's easy to show that the more slowly you accelerate, the greater distance covered in the process of adding a fixed amount of kinetic energy (the kinetic energy of the vehicle at the selected final speed). So, there are three considerations: slowly adding kinetic energy, spending as much time as possible at the most efficient speed, and operating in the most efficient portion of the engine map.

I've rigged a "generic" engine map (they all kind of look similar) and used known points for my engine to try to use optimization techniques to find the most efficient acceleration but, mathematically, it's a difficult problem. Especially for an old fart like me (math is a young person's game). But if and when I reach a conclusion, I'll post it.

Man, that's a long post for no particular conclusion, but I did want to point out the kinetic energy aspect, which I don't see anyone considering in discussing this issue.

[bgd73]

torque is the work . non-japanese inline four bangers find it naturally.
I made a funny. anybody get it?

your foot finds it for your car: the slowest rpm to get what you want. Screaming weirdos enignes don't understand and called it moderate to heavy acceleration...

a big diesel at 40 tons loaded is getting 224 mtpg (mile tons per gallon) all while never seeing much more than 2k rpm. The ultimate workers has no concept of this "perfect throttle".It is never a concern. all else who ask have pansy driveline. "Statistics" you seek will only reveal the most common driveline. Most likely that dumbo inline four cylinder...and those are in the moderate to heavy acceleration to get a thing called torque doing the work...
the concept of hypermiler would not exist if we had real engines....

[PA32R]

Originally Posted by bgd73

torque is the work . non-japanese inline four bangers find it naturally.
I made a funny. anybody get it?

your foot finds it for your car: the slowest rpm to get what you want. Screaming weirdos enignes don't understand and called it moderate to heavy acceleration...

a big diesel at 40 tons loaded is getting 224 mtpg (mile tons per gallon) all while never seeing much more than 2k rpm. The ultimate workers has no concept of this "perfect throttle".It is never a concern. all else who ask have pansy driveline. "Statistics" you seek will only reveal the most common driveline. Most likely that dumbo inline four cylinder...and those are in the moderate to heavy acceleration to get a thing called torque doing the work...
the concept of hypermiler would not exist if we had real engines....

[rjacob]

Originally Posted by bgd73

torque is the work . non-japanese inline four bangers find it naturally.
I made a funny. anybody get it?

your foot finds it for your car: the slowest rpm to get what you want. Screaming weirdos enignes don't understand and called it moderate to heavy acceleration...

a big diesel at 40 tons loaded is getting 224 mtpg (mile tons per gallon) all while never seeing much more than 2k rpm. The ultimate workers has no concept of this "perfect throttle".It is never a concern. all else who ask have pansy driveline. "Statistics" you seek will only reveal the most common driveline. Most likely that dumbo inline four cylinder...and those are in the moderate to heavy acceleration to get a thing called torque doing the work...
the concept of hypermiler would not exist if we had real engines....

"Glorious sunset of my heart was fading. Soon the super karate monkey death car would park in my space. But Jimmy has fancy plans... and pants to match. The monkey clown horrible karate round and yummy like cute small baby chick would beat the donkey." - Jimmy James

[mwpiper]

The "best acceleration" is also complicated by real conditions. You only need to accelerate downhill enough to prevent the onset of road rage in people behind you. Going uphill, you just need to punch it. Crawling uphill means staying in low gears for longer. The name of the game is getting into high gears as soon as possible.

[azraelswrd]

I've been accelerating with loads between 70 and 80 and I'm seeing better or rather more efficient use of my MPGs by the time I reach 4th gear. I might have broken 43mpg on my last trip if I wasn't stuck with traffic. Had to settle for 41.

[MechEngVT]

Daox:

As I stated I've read that BMW recommends this, but so far I have yet to find it as a direct source. I have read it from a gentleman who posts on Dodge Truck forums and lists who has spent much time experimenting with fuel efficiency on his truck and researching efficiency and aerodynamics. The feet/minute number is a rule of thumb as far as I know so it may not be optimized for any specific engine design, but all of us here probably lack the means to determine the optimum number.

I find that the 1200 fpm target correlates well with my personal experience. My engine, with a 3.40" stroke, hits 1200 fpm at 2117 RPM. I have gotten my best fuel efficiency lately shifting at 2100. I get my best steady-cruise efficiency at 2100 in 5th (75 mph) with another "island" of efficiency ~1800-1900 in 5th (65 mph) both achieving identical MPGs. If I wound out to 1500 fpm that would put me at 2647 RPM and I know it's not worth my while to drive that fast on the interstates (would have to put me near 95 mph), when accelerating from a stop up a hill I will let it rev to 2500 before shifting and find that not only does it work better in the elevated torque band, it keeps me out of open-loop enrichment under the load of uphill acceleration.

There is a direct correlation between an engine's stroke and the speed at which that engine will naturally produce a torque peak. As noted above the engine's stroke directly correlates an engine's speed to a mean piston velocity. As engines produce more low-speed torque they need to be run slower to maintain efficiency, and as engines produce more high-speed torque then can be run faster and still be efficient. All you folks out there with Honda b16s with a 3.05" stroke should let 'er rev up to 2360 instead of dogging it down below 2000 rpms (Wiki says the D16A3 in a Civic HF has a 3.405" stroke, putting you guys at 2100 rpms like me).