Thursday, April 28, 2011

Hypermiling attempt #2, 92.4 mpg over 14.9 miles

Attempt numbah two:

92.4 "real" mpg
from Scangauge

With the warmer weather and recent talk of hypermiling on PriusChat, I couldn't help but go out (even on an otherwise busy day!) for a little under an hour of fuel economy practice. With my limited time, the only place nearby was up the big hill in my town, on some back roads with low speed limits and relatively sparse traffic. The biggest problem was the route: I could only drive for about 5 minutes before coming to the end of the road, and would have to do a 180 to go the other way. There was also a stop sign in the middle. This means I had to stop every 2.5 minutes.

I didn't glean too much from this run, other than some useful ideas regarding pulsing and gliding on slight hills. I actually tend to get better mileage in the middle of one of my commutes - I can usually get over 100 mpg for fifteen minutes straight if I don't get stopped at too many lights. This run was interesting for me since I was trying to use lower speeds for a change, not more than 35 mph, sometimes lower, and also seeing what's possible while stopping so much.

While the best results on flat ground historically tend to be between about 18-20 mph on the low end and around 34-35 mph on the high end, I found slowing to 18 mph on a hill and then accelerating to be a bad idea. You're still trying to accelerate with a gas engine going at relatively high RPM but low torque with the slow speeds. This results in a longer burn time to reach the desired speed.

The simple solution is - well, there isn't one, but at least in the particular terrain I was on, I did try to go a little faster, keep momentum, and make sure I came into the hills with some speed, which I would sort "artificially" maintain with a very short pulse to not destroy momentum while cresting at the top. Using a small amount of EV while heading into the hill (not more than 10 amps, so it's less than warp stealth use) will help keep it moving a tad more than jumping straight to gas, but without a huge drain on the battery.

I don't have specific segment mpgs, but the 5 minute segments approaching the hills with more speed, air resistance be damned, easily yielded better mileage visually than the other bars.

Finishing up the stop and go section of the trip, I had about 88 mpg on the Scangauge. I tried an interesting, circuitous, and mostly downhill route home to get it up to 92 :-)

Meanwhile, still searching for an appropriate road for pulsing and gliding near my home - without stop signs or lights!


  1. Mike I'm trying to make heads or tails of all this. I've read your priusing with hobbit post and his two sweet spot posts, and after all that I was under the impression that keeping the RPMs more around 2k (+/-200) up to as much as 3k if you need more power was really the indicator of the sweet spot. At least it seems that is how hobbit drives? But then you're talking about maintaining 15kw. Why look at that rather than RPMs? Am I misunderstanding it all?

  2. Hey Randy,

    The reason is that the specific fuel consumption that Hobbit talks about is all about finding the most power produced for the least fuel consumed. Every car has a sweet spot where this occurs, where you get maximum turning force of the wheels for minimal revolutions per minute. In the Prius, this happens to be at 15 kW (20 horsepower). At a steady 15 kW, RPMs are not fixed - they'll change as you accelerate while maintaining the same power output. That's why RPMs could perhaps give you an idea of where to be in some cases, but not really, since they'll vary wildly over different speeds. Between 1600 and 2400 generally is efficient (per Hobbit), but it's still somewhat secondary (or even useless) as a consideration. I can tell you as a benchmark, though, that 2400 RPM is basically your speed divided by 2 for the instant mpg on the display.

    By the way, don't aim for 3000 RPM. That's extremely rare up a very steep hill. I've never had to use RPMs that high in five months of owning the car.

    I just tried a sort of combo of techniques today for highway driving, you can see the results here. It's much more straightforward for me, it might be of some help.