I have noticed the same results with my Prius. The first week we had the car, I noticed that whether you accelerate gently or hard, the fuel economy is about the same.

Just for fun last weekend, I 'borrowed' our Prius from my wife to go somewhere, and drove about 22 miles round trip. I practically floored the car from each traffic light or stop sign until I reached the desired speed. I did not change any of my other driving habits (I do as much coasting as traffic safely permits). The results? 64mpg.

We have a HCH2, and the same technique does not work with the Honda.

Most people I have mentioned this 'trick' to don't believe it, so it will have to be our little sectret.

Happy hypermiling!

 

Well, I sincerely hope our inference is correct and is verified by some other data points .. anyone else tried this??

 

Thanks GeekGal for this useful link .. I need to go and check the max pressure rating of the new Prius II that I got. I plan to fill it about 5 psi less than the max rating under cold conditions ...

 

Most Prius drivers in my neck o' the woods seem to know it, 'cuz I'm always being passed up by them.

 

That's funny because that technique seems to more or less work in the Insight. I don't "floor it", but I accelerate fairly quickly. Maybe I will do some comparisons, but my perception in the past was, a short burst of low MPG w/high assist and then maintaining speed worked better than a longer burst of higher (but still low due to acceleration) MPG and then the same. Maybe it has something to do with the Insight's lightness and the fact it only has a 3 cylinder engine. Maybe the electrical assist adds more assist in comparison to engine output.

 

The technique of accelerating fast from stops then backing off early I believe is designed to minimize engine pumping losses due to having a partially-closed throttle during the acceleration, so the goal is to have a completely open throttle body, but (on a manual transmission) still shift as soon as possible. (I suspect this would have a more profound difference on a "normal" car with an oversized engine than a hybrid) Since it requires take the same amount of total energy to accelerate a given weight to a given speed regardless of weight, the best results will be found by maximizing the efficiency of the engine -- staying at near the optimal RPMs with a completely open throttle body.

Also, as the CVT systems in the Honda Hybrids tend to kick in more IMA assist to "hold" the engine in that efficient range as long as possible, the effect is even greater - you get the maximum power available at the optimum RPM + lots of electric boost.

The HCH-II drive-by-wire system probably already automatically incorporates this technique even at lower throttle settings moreso than the Insight/HCH-I, and opening it up more may have the trigger the less efficient set of cams (new on HCH-II) to generate more power.

 

WOT does not give optimum gas savings. From all the stuff I have seen, gentle acceleration is best.

One thing people forget when they figure on hard acceleration is that they may get to a speed they don't have to get to and have to use the brakes more which takes away any gains.

I recomend acceleration rates of slowly applied throttle up to around 1900 rpms (I wouldn't even go that high if I don't have traffic behind me)

 

Well, the comparison has to do with accelerating rapidly up to the same given speed, all else equal after that point. Also, mostly open throttle body an engine with drive-by-wire doesn't necessarily mean pedal floored (which will tend to "tell" the computer to drop the gear ratio for greater power). Also, another point of note with respect ot hybrids is that they have downsized engines to begin with to reduce partial throttle loss, so they will hit minimul throttle loss at a much lower acceleration rate than on a non-hybrid with an oversized engine.

 

My shift points are in the range of 2000RPM under moderate (50-65%) load. While cruising is the most efficient state for the car, it is impossible to make up for a very inefficient (in my case 10-20mpg) start with a few hundred extra feet of cruising. Remember that you can't un-burn that fuel, only average it out slowly.

As far as pumping losses are concerned, I think that's really the wrong way to look at it. Going WOT for a few seconds may be a more efficient way to produce power, but that doesn't mean that you are using the extra power efficiently. In addition to paying for the kinetic energy gained, you also have to pay for significantly increased drivetrain losses if you end up at high RPM (a factor especially for automatic and CVT-equipped cars) and slightly more aero drag due to slightly more distance travelled at high speed. Finally, many vehicles run rich at WOT under open-loop fuel management in order to protect the engine from pinging and detonation, which is akin to dumping fuel directly out your tailpipe. So I don't think there's anything to be gained except thrills, engine wear, and two seconds off of your trip time.

 

Ending up at a higher RPM is a valid point, which is why I mentioned the technique specifically for manual transmisisons/shifting at the lowest RPMs possible. Combined with other mitigating factors, your acceleration strategy is close to optimal.

It's also true that average speed will be slightly higher, leading to slightly more drag. If I wanted to nitpick I suppose you could compare accelerating at a slower rate to a slightly faster top speed, so that you'd end up at the same overall average speed, for the sake of comparison

These are some good points, which would mean the optimum efficiency in the engine would be found at the widest throttle position before these mitigating factors (fuel mixture, etc) come into effect, probably about 2/3rd throttle at 2000rpm, which is generally where I'll accelerate.

The only other case where I'd say it's more efficient overall to accelerate as fast as possible is if accelerating more slowly would impede traffic behind me and reduce their fuel efficiency. For example, if I am the only car in my lane stopped at a traffic light that has just turned green, with traffic at full-speed approaching me, acclerating very rapidly will mean that all those cars behind me can simply maintain their speed. Accelerating at a slower rate, would force them to slow down more, then re-accelerate -- a huge amount of fuel when multiplied across several larger vehicles, though I personally don't see any of those savings, it is a reduction in overall emissions.