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I generally try to optimize my car's fuel consumption when driving, using my car's real-time MPG gauge and average-trip MPG indicator.

Until recently, I believed the slower the acceleration, the better the fuel economy. However, my observations seem to contradict this.

Generally, I notice that the acceleration seems to be directly proportional to fuel economy, at least in a new BMW 328i. This is within the bounds of "normal" driving; I haven't experimented much with very hard acceleration.

(Assuming an internal combustion engine,) what factors are causing quicker acceleration to a fixed target speed to result in higher average fuel economy than slower acceleration to the same speed?

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Just so that people don't get the wrong idea --- Even if this observation is true, it is still possible that, all things considered, telling people to accelerate slowly will lead them to better fuel economy, because it indirectly leads them to use lower average speed and/or less braking. Much depends on the situation, e.g. stop-and-go city traffic is very different than accelerating onto an open highway. Anyway, I think this is a very interesting observation and good question! :-) –  Steve B Mar 24 '13 at 21:38
    
This is also known as the bang-bang method of racing long distances on limited gas. Full throttle to high speed, and the coast for a long time, only to go full throttle again. Of course there is a Top Gear episode about it also. –  ja72 Jul 11 '13 at 19:02
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5 Answers 5

Some of the gasoline savings came from changing the patterns of gear shifting. ''People were shifting too late from first to second, and from second to third,'' Dr. [V]an der Voort said. People saved the most gasoline when they pushed down on the accelerator briskly and then shifted quickly, keeping the revolutions per minute low -- not by accelerating very gently.

''It's not commonly understood by people who drive,'' Dr. Dougherty said. ''They think that the way to get best fuel economy is to accelerate very gently, but that proves not to be the case. The best thing is to accelerate briskly and shift.

''Don't give it everything the car has, but push down when you're going to shift, using maybe two-thirds of the available power, and change through the gears relatively quickly.''

Source

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Source: interesting related paper

Fuel efficiency varies with the vehicle. Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque (brake specific fuel consumption.) However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration. Experts recommend accelerating quickly, but smoothly.

Source

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I always assumed minimum engine RPM $\rightarrow$ minimum fuel consumption. This is a very interesting compilation of data indicating otherwise. I especially like the fuel consumption map. It would be nice to have that plot for my specific vehicle. Are most cars designed to shift at the best time for fuel consumption or are the shift timings optimized for performance (or some other compromise between the two)? –  OSE Mar 25 '13 at 20:28
    
The question in my previous comment is referring to vehicles with automatic transmissions. –  OSE Mar 25 '13 at 20:34
    
@user2018790 1) This possibly not only depends on the make/model/engine/tuning of the car, but also its current state. I would like it too. :) 2) I can only guess, but I don't know. –  Glen The Udderboat Mar 25 '13 at 20:47
    
@user2018790 I assume that the kickdown regime is for (attempted) maximal acceleration, and that therefore, the normal regime optimizes something else, but what that is, I don't know... –  Glen The Udderboat Mar 25 '13 at 20:56
    
It strongly depends on the model and age of the car. Modern engine control does an astounding amount of data processing and clever decision making. It's not simply the "clockwork" logic of earlier electronic engine control. If it detects "aggressive" behavior on the accelerator pedal, in may adjust shift timing for performance. Gentle driving may bias it towards economy. It's impossible to make a blanket statement. –  Colin K Jul 12 '13 at 3:29
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Engines generally provide better fuel economy at higher RPMs until a point near the peak torque. The more quickly the engine reaches this optimal point, the less time your engine will have spent performing less efficient work, and the better the overall gas mileage.

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Here are two practical points:

To the zero-th order approximation, if you accelerate the car in such a way that minimize the amount of time that you push the break, you may minimize the fuel consumption (for non electric car). (Or more precisely, to minimize the amount of heat generated at the break pad).

On high way, use cruise control may help.

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Rate of acceleration has no impact on the amount of time I push the brakes; and increasing the rate of acceleration certainly does not reduce the amount of braking needed. Cruise control also has nothing to do with acceleration, unless you're implying that one should use cruise control to increase the speed rather than your foot. Regardless, my question isn't about how to improve fuel economy, it is about the physics factors at play causing quicker acceleration to result in better fuel economy. –  Jeff Axelrod Mar 24 '13 at 17:09
    
Sorry, my first point does not address the rate of acceleration, but address the speed reached after an acceleration. If the speed is not too high which reduce the need of breaking, you may save fuel. The second point is about the rate of acceleration. Even to maintain an even speed on highway, a drive may press on the gas paddle unevenly. Cruise control remove this uneveness. –  Xiao-Gang Wen Mar 24 '13 at 17:21
    
I'm not asking about what happens after you reach a target speed--meaning for the purposes of the question, braking is irrelevant. Also, the question is not about maintaining speed--it's about reaching a speed. You may just want to delete your answer. –  Jeff Axelrod Mar 24 '13 at 17:22
    
As to "the physics factors at play causing quicker acceleration to result in better fuel economy", I guess there must be an optimal rate of acceleration which maximizes the ratio of kinetic energy gained by the car and the fuel spend. –  Xiao-Gang Wen Mar 24 '13 at 17:26
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Your hunch is true for older cars where the fuel dosage would be in linear relation to angle of accelerator pedal - you floor it - you pump the richest mix, and even if a lot of it doesn't burn, or burns while the valves are open, you get maximum acceleration, like 100% power at 180% fuel usage.

New cars have this regulated by computer following calculations of engineers who built the car - while the pedal is to the floor, that doesn't mean the mix is at maximum richness. That just means the mix will be best for quickly accelerating without being wasteful.

If you do it yourself, you're trying to outsmart the computer. That never works very well - you don't let it calculate optimal curve to reaching the desired power by constantly changing the position of the pedal.

There's one more factor: constant overhead. Whether you stop in a jam or drive 100mph, or jump from one traffic light to another, there's always the energy that is constant over time used up by lights, electronics, friction of the gearbox and engine itself, all its pumps etc. The less amount of time elapsed during the trip the less of that you use up. If you drive at economy speed most of the time, you use less fuel.

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Good point about the constant energy used by systems regardless of the vehicle acceleration. –  Jeff Axelrod Mar 24 '13 at 19:34
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The faster you accelerate the more you interact with the Higgs field. This interaction requires more energy.

Correct me or delete me if I'm wrong;-)

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Clearly not the dominant effect... –  Kyle Jul 12 '13 at 19:07
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protected by Qmechanic Jul 11 '13 at 15:06

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