[Physics] Why does higher acceleration minimize a car’s fuel consumption

Question

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?

Answer 1

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

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