How to accelerate?

[tangmu]

I'm a newbie and have been reading a lot about how to accelerate from a standstill, and getting a little confused because there seem to be two different ways: one is to accelerate slowly, use as much electricity as possible to bring it up to 40-45. The other is: bring it up quickly to 40-45, the computer will calculate the most efficient gas consumption, then start gliding. Both seem to make sense, so could you share your experience?

[efusco]

Well, perhaps I'll confuse you even more. I don't accept either of the options listed.

1)Using electricity only is likely the very least efficient. The most power hungry aspect of driving the prius is accelerating and thus you want to use the most efficient method. The electricity you're using was generated by the ice, went through an inverter where it lost some of it's energy, stored in the battery where it lost a bit more, then back through another inverter where it loses even more, and finally to the wheels. The loses are relatively small, but still real and thus the least efficient.

2)Hard acceleration pushes the ice outside it's most efficent range (~1700-2300 rpm), and may even use battery power (see #1 above) to get you up to speed. Since there's direct power transfer from the ICE to the wheels it's probably a bit more efficient, and a lot more street friendly, than #1, but not the best choice.

3) What I do and am convinced is the most efficient is to accelerate within that 1700-2300 rpm range with a target of 1850 rpm. (BTW, I'm speaking to the Prius II specifically, we may find that the most efficient RPM range for the Prius III is somewhat different). This will usually show as a "dead band" on the MFD Energy screen (no arrows to or from the battery) if you do not have a way to monitor your RPM.

I'm only getting 75mpg on my current tank, so someone getting 80+mpg may have a better suggestion, but with 91k miles in the Prius II I've become convinced that this is the best method of acceleration.

[tangmu]

Originally Posted by efusco

I'm only getting 75mpg on my current tank, so someone getting 80+mpg may have a better suggestion, but with 91k miles in the Prius II I've become convinced that this is the best method of acceleration.

Did you say ONLY getting 75 mpg? You must be doing something right. Thanks for sharing!

[JimboK]

I mostly agree with Evan. Where I disagree is with the suggestion that <1700 RPM is inefficient. Though the theoretical power curves for the Prius ICE suggest such inefficiency, many of us who routinely use lower RPM ranges are getting excellent results. My RPM off the line may be as little as 1200 RPM, assuming following traffic will tolerate it. Recent results are rather respectable.

How, you might wonder, will you know what your RPM is without a tachometer? Good question. Here's a rule of thumb: Watch your instantaneous MPG on your MFD during acceleration, and aim to keep it the same as your speed. That should keep it well within efficient ranges. If you need to push harder due to hills or impatient tailgaters (please do try to avoid inciting road rage), try to keep iMPG above MPH/2. That is the point, over a wide range of vehicle speeds, where RPM increases above ~2300-2400, which most everyone agrees is where a significant dropoff in efficiency begins.

Another small area of disagreement is in the significance of the "dead band" condition that he describes. Though theoretically ideal, it is hard to hit and even harder to maintain during acceleration, especially if hybrid battery SOC (state of charge) is low. If I'm watching the factory display, I focus on the iMPG/MPH relationship.

See this for a thorough discussion of the iMPG/MPH rule of thumb.

[tangmu]

Originally Posted by JimboK

I mostly agree with Evan. Where I disagree is with the suggestion that <1700 RPM is inefficient. Though the theoretical power curves for the Prius ICE suggest such inefficiency, many of us who routinely use lower RPM ranges are getting excellent results. My RPM off the line may be as little as 1200 RPM, assuming following traffic will tolerate it. Recent results are rather respectable.

How, you might wonder, will you know what your RPM is without a tachometer? Good question. Here's a rule of thumb: Watch your instantaneous MPG on your MFD during acceleration, and aim to keep it the same as your speed. That should keep it well within efficient ranges. If you need to push harder due to hills or impatient tailgaters (please do try to avoid inciting road rage), try to keep iMPG above MPH/2. That is the point, over a wide range of vehicle speeds, where RPM increases above ~2300-2400, which most everyone agrees is where a significant dropoff in efficiency begins.

Another small area of disagreement is in the significance of the "dead band" condition that he describes. Though theoretically ideal, it is hard to hit and even harder to maintain during acceleration, especially if hybrid battery SOC (state of charge) is low. If I'm watching the factory display, I focus on the iMPG/MPH relationship.

See this for a thorough discussion of the iMPG/MPH rule of thumb.

Hmm... it's getting deeper and deeper, a little beyond my senile comprehension... I'll wait for that light bulb to come on...

Thanks

[TheSpoils]

You will get 10 different answers from 10 different drivers. I don't think any of those methods are wrong. Use what works for you. For me, I like to accelerate on all electric until 40-45 because I believe an ICE is more efficient at higher rpm or speeds (thats why most cars get better economy on highway. The most fuel is used on starting from a standstill. My method is to use all electric to start and stop the vehicle and only use the ICE when cruising at a steady speed.

[efusco]

Whoa! Who's this Jim guy with the audacity to disagree with me...I will smite thee!

Listen to Jim, his comments are excellent and on target, I suspect we less disagree than have slightly different emphesis on various points that ultimately comes down to splitting of very fine hairs that matter mostly just to hypermiling geeks like us!

Originally Posted by JimboK

I mostly agree with Evan. Where I disagree is with the suggestion that <1700 RPM is inefficient. Though the theoretical power curves for the Prius ICE suggest such inefficiency, many of us who routinely use lower RPM ranges are getting excellent results. My RPM off the line may be as little as 1200 RPM, assuming following traffic will tolerate it. Recent results are rather respectable.

Indeed lower RPM ranges are fine and I'm comfortable going with somewhat lower RPM in some conditions. I, personally, have vestiges of my Type-A driver personality and those painfully slow accelerations make me crazy. At 1850 rpm I'm usually ahead of traffic up to about 25-30mph where they then pass me by. But if I'm in traffic that won't tolerate the 1850 rpm rate then I'll go slower. I also like being able to reduce my total ICE-on time and get into my glide more quickly. But we're getting into some pretty advance hypermiler dialogue at this point.

In any case, I think the take-away message is to use the ICE at some moderate rate of acceleration that you are comfortable with. (any disagreement Jim?)

How, you might wonder, will you know what your RPM is without a tachometer? Good question. Here's a rule of thumb: Watch your instantaneous MPG on your MFD during acceleration, and aim to keep it the same as your speed. That should keep it well within efficient ranges. If you need to push harder due to hills or impatient tailgaters (please do try to avoid inciting road rage), try to keep iMPG above MPH/2. That is the point, over a wide range of vehicle speeds, where RPM increases above ~2300-2400, which most everyone agrees is where a significant dropoff in efficiency begins.

No disagreement at all here. My only problem here is that I always had a hard time keeping an eye b/w speed, iMPG, and that annoying traffic!

Another small area of disagreement is in the significance of the "dead band" condition that he describes. Though theoretically ideal, it is hard to hit and even harder to maintain during acceleration, especially if hybrid battery SOC (state of charge) is low. If I'm watching the factory display, I focus on the iMPG/MPH relationship.

Let me say that I make little effort to stay exactly in the dead-band, but rather I try to stay close and I'm OK with slight energy flow to/from the battery. I find this easier, personally, to monitor with a glance than the iMPG/MPH ratio thing. YMMV!

Try both and see which method works best for you and your driving style while still keeping safety foremost.

[efusco]

Originally Posted by TheSpoils

You will get 10 different answers from 10 different drivers. I don't think any of those methods are wrong. Use what works for you. For me, I like to accelerate on all electric until 40-45 because I believe an ICE is more efficient at higher rpm or speeds (thats why most cars get better economy on highway. The most fuel is used on starting from a standstill. My method is to use all electric to start and stop the vehicle and only use the ICE when cruising at a steady speed.

I think if you ask those of us who've spent way more time than any normal human should you'll find a solid concensus very close to what Jim and I are proposing.

Physics, math and experience all side against your method...and here's why....
You're forgetting that any electricity you use to get up to 40MPH uses MORE gas than using the ICE. All energy your prius uses comes from gas..period.

So while the car may be quiet and there is no gas being consumed during your acceleration phase, you will consume, ultimately more gasoline to replentish that batter charge due to the losses I described in my first post above.

[Bob.H]

Answer #6 here.

I use moderate acceleration from a stop to 40 MPH and glide. Then anticipate traffic and stops decelerating to regenerate. I usually get >65 MPG doing that and it's almost by feel now without the need to constantly look at that screen.

Bob

[TheSpoils]

Understood, I am fairly happy with my 57 mpg, however I will try your method in the next couple tanks. 65-70 mpg is better than 57.