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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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    $\begingroup$ 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! :-) $\endgroup$
    – Steve Byrnes
    Commented Mar 24, 2013 at 21:38
  • $\begingroup$ 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. $\endgroup$
    – John Alexiou
    Commented Jul 11, 2013 at 19:02

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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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    $\begingroup$ 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)? $\endgroup$
    – OSE
    Commented Mar 25, 2013 at 20:28
  • $\begingroup$ The question in my previous comment is referring to vehicles with automatic transmissions. $\endgroup$
    – OSE
    Commented Mar 25, 2013 at 20:34
  • $\begingroup$ @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. $\endgroup$
    – Řídící
    Commented Mar 25, 2013 at 20:47
  • $\begingroup$ @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... $\endgroup$
    – Řídící
    Commented Mar 25, 2013 at 20:56
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    $\begingroup$ 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. $\endgroup$
    – Colin K
    Commented Jul 12, 2013 at 3:29
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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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  • $\begingroup$ 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. $\endgroup$
    – Jeff Axelrod
    Commented Mar 24, 2013 at 17:09
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    $\begingroup$ 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. $\endgroup$
    – Xiao-Gang Wen
    Commented Mar 24, 2013 at 17:21
  • $\begingroup$ 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. $\endgroup$
    – Jeff Axelrod
    Commented Mar 24, 2013 at 17:22
  • $\begingroup$ 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. $\endgroup$
    – Xiao-Gang Wen
    Commented Mar 24, 2013 at 17:26
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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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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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    $\begingroup$ Good point about the constant energy used by systems regardless of the vehicle acceleration. $\endgroup$
    – Jeff Axelrod
    Commented Mar 24, 2013 at 19:34
  • $\begingroup$ The throttle in an engine exists to waste energy; flooring the accelerator pedal will minimize the amount of energy wasted in the throttle, though in many cases will cause the engine's rotational speed to increase enough that the fuel doesn't have enough time to burn optimally. I would expect that given a continuously-variable transmission, optimal fuel efficiency at a given speed would be obtained at a gear ratio that cause the engine to rotate at a speed where its wide-open-throttle power output would match what was necessary to maintain that speed. $\endgroup$
    – supercat
    Commented Nov 29, 2014 at 5:40
  • $\begingroup$ Unfortunately, from a practical perspective, (1) at slower travel speeds, an engine's power output couldn't be reduced low enough without using the throttle or lugging the engine; (2) without the throttle, the relationship between speed, torque, and power would be unstable; if the engine generated any excess torque, that would cause its speed to increase, which would in turn cause it to generate even more excess torque. $\endgroup$
    – supercat
    Commented Nov 29, 2014 at 5:42
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If an engine is being used for a task which can make use of all the mechanical energy it produces (such as charging a battery or pumping water as quickly as possible from a supply that's sufficient to avoid cavitation), optimal efficiency will be obtained by having the engine run at wide open throttle but having the load torque match the engine torque when the engine is rotating fast enough to allow "smooth" combustion without detonation, but slowly enough that all the fuel can burn during the power stroke. If the engine is allowed to run faster than that, fuel consumption will increase roughly proportional to speed, but power output will not, since an increasing portion of the fuel won't burn during the power stroke. If the engine isn't allowed to run fast enough, then cylinder pressures will get excessively high. Because fuel burn rate increases with cylinder pressures, this will quickly cause pressures to get even higher, resulting in a process called detonation. The excessive forces generated during detonation often end up getting converted not into useful energy, but rather to accelerated wear and tear on the bearings.

While wide-open throttle operation may be optimal for applications that can adjust the load to control engine speed (e.g. an alternator whose purpose is to charge a battery may have its field windings continuously adjusted so that the amount of energy sent to the battery per revolution increases with speed), it doesn't work so well in a car, since the amount of power that is needed to make a car move at a desired speed will vary markedly with driving conditions. Most automatic-transmission cars are designed so that when the driver floors the accelerator pedal, the motor will be operated at a speed which is considerably faster than its maximum-efficiency point. Operating a motor at 6,000rpm wide-open throttle will likely consume fuel more than twice as fast as would operation at 2,500rpm, but will not generate twice as much mechanical power.

In a manual-transmission car, once the vehicle was moving, one could accelerate efficiently by flooring the accelerator between shifts, but upshifting as early as possible without lugging the engine. If one shifts an economy car into 5th gear at 40mph, reaching 65mph will take a lot longer than if one had stayed in 3rd gear to 50mph and 4th until 65mph, but the total fuel required to accelerate to 65mph and drive to a point some distance from the starting line would be reduced [the early-shifted vehicle might take so much longer to reach 65mph that it uses more fuel during that time, but the distance traveled during that time would reduce the amount of fuel required to drive the rest of the way to the destination].

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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;-)

Ok. I will add friction which is interaction with the electromagnetic field. Or should I word it the electric field and the magnetic field?

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  • $\begingroup$ Clearly not the dominant effect... $\endgroup$
    – Kyle Oman
    Commented Jul 12, 2013 at 19:07
  • $\begingroup$ So I get minus votes because there are 2 things that effect acceleration? Friction and inertia. $\endgroup$
    – Jitter
    Commented Jul 19, 2015 at 4:02
  • $\begingroup$ I am assuming the negative votes were because it doesn't seem to answer the question. Your initial answer mentions that higher acceleration takes more energy. But the question is asking why higher acceleration appears to take less fuel. $\endgroup$
    – BowlOfRed
    Commented Jul 19, 2015 at 5:03

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