Hard acceleration and fuel economy?

Everybody knows the well known “fact” that hard acceleration and deceleration is awful for your automobile fuel economy. However, hard acceleration by itself is not really accurately addressed in anything I have come across using Google.

The idea that accelerating rapidly would give you overall worse fuel economy is not sitting right with me. A water-cooler discussion with some coworkers have resulted in a few “:dubious:” looks and pretty much everybody except for me thinks that accelerating smoother will be better.

Let’s consider the situation – you have 2 miles to drive on a level, straight empty highway. My car will ideally get around 28 mpg coasting at 70 mph, so let’s use those numbers. Assuming you have to be traveling at 70mph at the end (disregarding various alternatives for coasting in gear/stopping with brakes/etc.) let’s consider the following possibilities

  1. Accelerate to 70mph smoothly at 1mph per second, shifting up as soon as possible to prevent lugging (more open throttle)

  2. Accelerate to 70mph smoothly at 1mph per second, shifting up at high RPM (less open throttle)

  3. Accelerate to 70mph with about 3/4 throttle open shifting up at about 3/4 of redline. Assume this will take about 10 seconds.
    Now, my coworkers claim (1) will get best fuel economy, (2) will get slightly worse and (3) will get worst over those 2 miles. That doesn’t sit right with me, assuming a modern ECU controlled car, I would think (3) will get best fuel economy, (2) will be second and (1) will be worst.
    My reasoning is that in (3), the car will start going at an even rate of 70mph(28mpg) after 10 seconds, and will arrive at the destination after about a total of 1 minute and 46 seconds, which is a total of 0.07 gallons + whatever it burned during the 10 second acceleration burst.

In (1) and (2), they will spend one minute and 10 seconds accelerating, and will arrive at the destination at 2 minutes and 18 seconds. The 1 minute and 8 seconds spent at 70mph is about 0.05 gallons of gas. So the total trip is guaranteed to be at least 0.1 gallons, probably a little more.

To get worse fuel economy than (1) or (2), (3) would have to have burnt more than 0.03 gallons in the 10 seconds accelerating, which is about 110 cc of gasoline, and means my fuel injectors were open a lot longer than the intake valves. It’s certainly possible, but somehow it feels… unlikely.

Which I why I turn to you, dopers.

What do you say?

My guess is that 2 would be by far the worst of them. The high throttle is great for getting you up to speed quickly, but it’s wasted if you’re not accelerating…you’re just running your car in an improper gear.

The fact is that #1 will get you the best fuel economy, and it’s been shown in numerous studies. I know it seems counterintuitive, but the fact is that smooth, slow acceleration gets a lot closer to cruising speed mileage than the gas that’s guzzled on a hard acceleration. It’s not a linear gas consumption curve. I do think that a medium acceleration (say, shifting at half of redline and applying moderate throttle) does get me pretty darn close to the slow acceleration for the reasons you note. (I can guess this from the instantaneous fuel gauge I had in my hybrid for 2 years). Plus it’s not annoying. I do not do slow takeoffs because my normal start was so close and got me up to speed about 80% faster.

Under hard acceleration, you induce losses you cannot get back. Slippage of the transmission is just one. Under heavy ‘load’, a transmission is going to waste power. If you have a tachometer, watch the revs. Under hard accel, your engine will rev above the required RPMs normally needed for light acceleration, because the transfer of engine to transmission is hydraulic, and slippage is natural and necessary to keep parts from snapping (simplification here).

If you arrive at your destination after 30,000 engine revolutions, and I arrive at the destination after 25,000 engine revolutions, you burned more gas. There are factors that mitigate this, but as a rule of thumb you want light throttle to minimize losses and keep RPMs down. Your losses from hard accel and your extra RPMs will help ensure you lose the fuel economy battle to a light footed driver who shifts early (or causes his automatic to shift early by having a light foot).

Keep losses and revs and bay.

This is as I understand it, the essence of it. Lower gears use more revolutions of the engine to travel the same distance as fewer revolutions at a higher gear. Therfore hard accelleration using more time in lower gears = more revolutions over the same distance.

Engine fuel use per revolution is actually fairly consistent.

Some cites would be nice for some of these claims. It seems there’s got to be an optimization point where you gain from not changing your momentum the whole time.

I don’t have a cite, but here’s one technique advocated by both Mercedes and a european driving school for situations where no traffic or stop lights threaten ahead:

  1. Give heavy gas (1/2 to 1/3 throttle) during acceleration in first gear.
  2. Before hitting 3000 RPM in first gear, back off of the gas such that the vehicle shifts into second gear.
  3. Give heavy gas (1/2 to 1/3 throttle)
  4. Before hitting 3000 RPM in second gear, back off of the gas such that the vehicle shifts into third (or sometimes fourth) gear.

My personal driving style is consistent with the above, except for the fact that I try to get my vehicle to shift at or about 2000 RPM.
I’ve seen charts of MPG by RPM, and it seems that the best mileage can be found somewhere between 1500 and 2000 RPM.

from what I remember, lowest Brake-specific fuel consumption (max efficiency) of an IC engine is at near full throttle, at the RPM level where the torque peaks.

Folks, digest all the info and then decide what is practical for drivers to do, in order to obtain the optimum fuel efficiency of their vehicle.

Light foot, keep the revs down, don’t waste energy on losses induced by a heavy foot and be realistic when assessing your in/ability to keep the engine at it’s optimum RPM under all circumstances.

Throw in hills, traffic, engine/tranny combos and you will lose your mind trying to figure it all out.

That’s simply not true. The amount of gas used in a modern car is (simplistically) determined by the total duration the fuel injectors stayed open multiplied by the flow rate of the injectors. In a normal gasoline car, for each revolution of the engine, half of the fuel injectors will be fired by the ECU. The duration (technically called pulse width) is determined automatically by the ECU based on many factors.

If the pulse width stays exactly the same, then 30,000 engine revolutions will burn more gas than 25,000. Pulse width will normally not be the same, and if you arrive at your destination after 25,000 engine revolutions with 10ms average IPW, and I arrive after 30,000 revolutions with 5ms average IPW (assuming the exact same car), then I would’ve used less gas.

This is the best answer so far.

We know from basic physics that it’s the weight of the car and the final speed that will determine how much power is needed, acceleration is not part of the equation. So that leaves drag, friction and drivetrain efficiency as the determining factors. Accelerating quickly to the target speed and then coasting as much as possible will give you the best mileage. Modern cars will also turn off the injectors as you coast.

The deceleration part is where the most gas savings occur. Except for electric and hybrid vehicles, all braking is converting your gasoline into useless heat. Coasting to a stop without using the breaks at all will yield the greatest gain. Turning off your engine as you coast is even better, but not recommended because it is more dangerous.

Operating an automatic at speed without the engine running will damage the transmission in many modern cars, so that dubious strategy is right out if you’ve not got a stick.
I’m unconvinced of the pulsing theory unless it’s done in such a fashion that you finish coasting at some point where you had to stop.

Getting up to 30 and then coasting to the stopsign at the corner, such that you need not use your brakes seems like a dandy idea.
Getting on traffic-free two-lane blacktop where I can avoid the brakes for three miles, punching it up to 40, then coasting to 15, then punching it up to 40 over
and over again just seems silly.
Is that actually going to help?