Which is best for fuel economy: Faster or slower acceleration

ford
acceleration
gasoline
f150

#1

Hi.



I searched the forum for quite some time and didn’t find a response to the following questions so, although I’m sure it’s been asked before and is in the archives somewhere, please excuse my asking it again. It’s still timely, I believe.



Which will provide the best fuel economy for an automatic transmission (not talking performance in any way here and there are plenty of posts regarding a stick shift but not automatics):



1. Accelerate quickly (but without flooring it) to the highest gear.



2. Accelerate slowly.



The real question is, do you use more fuel laboring the engine for a shorter time but using more torque or do you use more by laboring the engine for a longer period of time but with less torque?



Does this make sense or should I try to explain the question again?



Thanks ahead of time for your thoughts!



bc


#2

Accelerating slowly uses less fuel. I can’t explain why. I only know it is an easy hypothesis to test yourself, which makes me wonder why you are asking us to speculate instead of figuring it out for yourself. Try driving one week one way and measure your fuel economy. Then spend a week driving the other way and calculate your fuel usage. You should notice a difference.


#3

Well, that sounds simple but, in reality, the odds of being able to drive consistently one way or another and thus provide a valid test are way low. There are too many variables when driving that a driver cannot control.

I was hoping that, instead of speculation, someone who was knowledgeable about actual scientific analyses would provide a clear answer.

Additionally, I’m not so sure that accelerating slowly ALWAYS uses less fuel. Accelerating with a load means the engine is straining more than when the vehicle is running without a load. So it would seem to me from a physics standpoint, there could be a point of diminishing returns.

And I believe a “good thing” could be taken to extremes and loss the advantage.

For example, I could accelerate “slowly” taking a full minute to reach second gear and then do the same to reach third and likewise until I finally reach overdrive (if other drivers didn’t murder me) but it would seem to me that there’s a point at which a vehicle’s engine performs best – fuel economy-wise – that might not always be at the lowest RPM per gear. Starting out from a stop, for example, is more taxing to an engine than picking up speed from a crawl (such as one does when arriving at a light and having it change just before you get there). So if starting from a stop works the engine harder than accelerating from a crawl, why wouldn’t there be efficiencies to getting up to a higher gear faster (again, without going to the opposite extreme and punching it)?

I’m probably just wasting everyone’s time here, including my own. I apologize.


#4

We always get these type of questions but the OP never says “here is what I think and this is what I am going to do to test it”. Then the post turns into one of those “book smart but nothing practical type discussions” OH well,carry on.


#5

“An object at rest tends to STAY at rest unless disturbed by an outside force”…

Accelerating mass takes energy. Accelerating mass rapidly takes a tremendous amount of energy. This energy is then stored as kinetic energy in the moving mass. When you brake, you convert this energy to heat where it is wasted (except in hybrids, which capture much of it)

Gentle acceleration and cool brakes will produce the best fuel mileage…


#6

Accelerating a 3000 pound car from zero to 60 mph means the engine has to give the car 363,000 ft lb of kinetic energy, there is no way around it. That energy is exactly the same whether the engine gave that energy to the car in five seconds or whether it took all day to do the job. Why not do it with the engine producing the power at which it is most efficient?
That 363,000 ft lb of energy can also be expressed as 660 horsepower seconds of energy and you can supply that much energy by having the engine put out 660 horsepower for one second, or by putting out 66 horsepower for ten seconds, or by putting out 6.6 horsepower for 100 seconds. I bet the specific fuel consumption in pounds of fuel per horsepower hour is lower at 66 horsepower than it is at 6.6 horsepower for most car engines.

In my own driving, I find I can get excellent gas mileage in spite of or perhaps even because I don’t hesitate to accelerate briskly.

Another case for brisk acceleration is the fact that slow acceleration wastes a lot of time that would have been put to better use by lowering your cruising speed by a couple of mph.

Here’s an example:
Lets say you do a 10 mile trip and you slowly creep up to a 60 mph cruising speed taking the entire first mile of the trip to reach that cruising speed and let’s also assume the last mile of that trip is done coasting down to zero at the same rate.
Because the average speed of the first and last miles of this trip is 30 mph, it takes four minutes to do these two miles, the rest of the 8 miles of the trip take 8 minutes and it takes 12 minutes to do the 10 mile trip which is an average speed of only 50 mph.
wouldn’t it be better fuel economy wise to average 50 mph by using the first 1/10 mile of the trip to briskly accelerate to a 51 mph cruising speed and ending the trip the same way?


#7

My suggestion is: try a hypermiling forum such as www.cleanmpg.com Those folks eat breathe, and sleep mpg, and so are more likely to know the answer. My recollection is that moderate (not snail’s pace, not gun it) acceleration is likely to produce the best results.


#8

Generally speaking large throttle openings at low engine speeds yeilds the best fuel economy because of the lower pumping losses involved.


#9

Speaking of hypermilers, one of the proven methods for setting gas mileage records is a technique called pulse and glide, where the engine is run intermittently. Instead of calling on the engine to make a steady 5 or 6 horsepower, the driver calls on the engine to make 50 or 60 horsepower for about 10 seconds while the car accelerates and then shuts down the engine and lets the car coast for 90 seconds or so making an average of 5 or 6 horsepower.
The fact that this results in better gas mileage than a steady state cruise with the engine operating at 5 or 6 horsepower contradicts the idea that the slower you accelerate, the better your gas mileage will be.


#10

For most cars (those with automatic transmissions) gentle acceleration results in shifts at low rpms, and lower average speed, increasing mpgs. For a manual, you can accelerate harder (open throttle = lower pumping losses), as long as you still shift at low rpms. I seem to remember one make (BMW?) saying that firm acceleration with shifts at around 3000 rpm worked well.

I do know with my AT car my mpgs are better when driven gently.


#11

As BLE said, accelerating to a certain speed takes a certain amount of energy no matter how hard you accelerate. If you’re accelerating briskly, you’re opening the throttle and allowing the engine to run efficiently. However, running the engine at high engine speed introduces friction losses. As FoDaddy said, accelerating with large throttle opening at low engine speed reduces pumping loss and it is best accomplished by shifting to top gear as quickly as possible. If my automatic is one of those automated manual variety, I would shift to the top gear as soon as I’m allowed to and accelerate with a heavy foot.

However, a conventional automatic transmission’s torque converter transmit torque by slipping. More torque = more slip = more wasted energy. And it is going to slip until it is cruising in top gear. Therefore, I have to vote slow acceleration for fuel efficiency.


#12

Yes, with automatic transmissions, you have to take into account the torque converter slip at high torque delivery.

I think there is another factor at play with manual transmissions and that is the energy it takes to accelerate the engine’s flywheel. When you accelerate to a cruising speed that has the engine turning 3000 rpm in fifth gear and you shift gears at 3000 rpm, in addition to accelerating the car to that cruising speed, you also had to accelerate the engine’s flywheel to 3000 rpm five times.

That, I believe, is why it is so important to shift into second gear almost as soon as the car is rolling fast enough for the engine to pull second gear without lugging. The rpm drop when shifting from 1st to 2nd is huge, nearly a 40% drop in a typical 5 speed transmission, in the higher gears, the ratio jumps tend to be closer, typically a 4th to 5th shift only drops the engine rpm by 10% or so.
Because of this, as you reach the higher gears, you can progressively shift at higher rpms without throwing away so much flywheel kinetic energy at each shift.

This is why I do everything possible to still be rolling when the light turns green, so I can skip 1st gear.

When I was studying physics in college, I looked up the gear ratios of a Harley Davidson Sportster and figuring a 28 pound eight inch diameter crank, I calculated the kinetic energy of the crankshaft in first gear and the flywheel kinetic energy accounted for about 30% of the total kinetic energy of the bike in first gear.
Because of the energy needed to accelerate the flywheel, even if your goal is winning a drag race, the ideal shift point is a somewhat lower rpm than the theoretical shift point you would get by looking at the engine’s horsepower curve measured on a dyno, especially in the lower gears.


#13

It varies. You need to factor in

  • Road conditions (including up or down hill)

  • The make, model, transmission type and year of the car

  • Traffic conditions

  • Your speed

  • A few others I have not thought of yet

    If you have all that data, then I suggest you take a refresh your college calculus to do the math.

    Of course you could just try measuring results each technique for a couple of tanks of fuel for each technique.


#14

It’s an interesting question and there’s a lot of good thinking going on here. I’d like to suggest one more variable.

Wiith an automatic transmission, you’ll notice that when you acelerate quickly the engine revs considerably higher than if you accelerate gently. In another thread here we discussed the losses inherant in higher RPMs. In addition to the engine internally having to expend more energy constantly reversing the direction of the reciprocating masses, there’s also a phenomenon called “pumping loss”. That’s basically the energy needed to pull air into the cylinders and push air out. At high RPMs, engines have to expend more energy internally starting and stopping all the reciprocating masses, and they have to move far more air into and out of the engine past inherant restrictions. A lot more energy goes into just running the engine.

Thus, if you keep the engine speed modest with gentle acceleration, you use less energy running the engine itself than if you wind the engine up as you accelerate.

In summary, as B.L.E. pointed out, the energy cost to get the vehicle moving to a specific speed stays the same, but the energy cost to run the engine changes.


#15

Agree that the energy needed to get up to speed is the same. HOWEVER, if you floor the gas pedal you are using more gas than necessary, as well as staying in the lower gears longer. In old style cars with carburetors, there was an accelerator PUMP, which literally pumped raw gas into the the carburetor, making for very poor gas utilzation. On very fast 1960s musacle cars, raw gas could actually be seen coming out if the exhaust.

On newer cars with precise fuel injection, the excess gas used is a lot less, but it’s still there. Gentle acceleration and getting in to high gear as quickly as possible is the most economic. Ask any driver who has particpated in economy runs.


#16

The instant fuel mileage readouts on either of our two GM cars with this feature seem to say that slow acceleration is best. The instant mileage goes down quite low with fast acceleration such as to 6 mpg. The best way to verify this might be to zero out the average MPG and then accelerate to a high speed first slowly and then do it all again quickly.


#17

Which is best for fuel economy: Faster or slower acceleration

Why the answer isn’t obvious alludes me. I suppose towing a trailer, carrying excess weight in the car and running on less than ideal air pressure could all give you better mileage if accelerating faster does as well…
Ask a train engineer, a pilot or boat captain when the most fuel is used for distance traveled (which is all that matters). It’s during acceleration and the more rapid, the greater the consumption.
Better still…don’t start the car on level ground and start pushing it. You’ll find out real quickly that accelerating takes lots more energy that maintaining speed. See how much faster you can accelerate the car by pushing it less hard ? NOT.


#18

Surely you aren’t serious!! Billcard.


#19

Wrong guy…:=)


#20

You need to learn the difference between power and energy.

If you really think it takes less energy to accelerate slowly than quickly, then you probably also think its cheaper to buy a house with a 30 year mortgage than a 10 year mortgage.