pulse & glide... accelerate fast or slow?

[prius_in_texas]

I have a question about pulse and glide. The advice on these posts is to get up to speed quickly, then glide. However... it seems that if you accelerate quickly, you're still ultimately are going to use more fuel than if you accelerate slowly, since you still need more energy to overcome inertia quickly (just as in a regular car). So I have been accelerating slowly (trying to keep the ICE from turning on as long as I can), then accelerating at a leisurely pace until I'm a few MPH over the speed limit, then taking my foot off the gas and gliding.

So... to get the best mileage, is it better to accelerate quickly or slowly before gliding?

[nosliw]

When you accelerate slowly and utilize the battery as long as possible, you are doing more harm to your MPG than you may think.

By draining the battery right off the bat, your ICE will run when you don't want it to to bring the charge back up. I find it harder to 'glide' when the battery is low. The battery isn't there to replace the job of the engine when accelerating normally.

Also, you can only come up to speed using the battery so many times before your ICE has to kick in. When you take off slowly, the ICE is not in it's optimum RPM range. These small 4-bangers love RPM and work more efficiently (read: MPG) with higher and shorter bursts of rev.

What I do for P&G is this...

From a stop, I depress the accelerator firmly enough to cause the ICE to kick in (with normal battery charge) at about 7 or 8 MPH. YMMV with this, but try and aim for a real-time MPG rating of approximately 1/2 your current speed in MPH.

I reach 5-10 MPH over the posted speed limit and let completely off the gas pedal. I then depress it slightly to engage 'glide' mode. I will run in 'glide' for however long it takes me to drop to 5-10 MPH under the posted speed limit.

Repeat.

I find it works best on off-highway roads with minimum traffic and a posted speed limit of 35-40. Flat land is when I usually use this method, because on more hilly areas it's either 25MPG or 99.9MPG.

My $.02

[efusco]

I try to pulse at 1850rpm...this equates to a moderate rate.
For your purposes you should try a 'dead band' acceleration...no arros to or from the battery on the Energy display. A little flow going to the battery is fine, but in no case should there be flow from the battery during the pulse.

[JimboK]

Originally Posted by nosliw

When you accelerate slowly and utilize the battery as long as possible, you are doing more harm to your MPG than you may think.

After the initial burst off the line, to accelerate on battery power only will be extremely slow, slower than most drivers (both those of the Prius and, especially, those behind them) would tolerate. I'm surprised the OP is able to.

Having said that, I think your advice is pretty much on target (although my top speed is not usually over the speed limit). I would modify it only to suggest keeping instantaneous MPG at least half the speed. Lower than that, and ICE RPM is more likely to be in [EDIT] inefficiently high ranges. In fact, iMPG anywhere between MPH*0.5 and MPH*1.0 will keep RPM in efficient ranges (Evan's 1850 is probably somewhere in the middle), and avoid drawing current from the battery.

As for Evan's "deadband" suggestion, that would be ideal; it suggests all the ICE's energy is going directly to the wheels. But it's tough to hit and even tougher to hold, and requires constant attention to the MFD. The car decides for itself anyway when to charge the battery and how much energy to send its way, and our control over that is quite limited. I pretty much gave up on deadbanding early on, and now I just watch RPM and current flow (both via CAN-View) with occasional glances. I suggest to folks without added instrumentation just to watch the iMPG/MPH relationship during a pulse (and steady-state cruising at higher speeds).

[qbee42]

Originally Posted by prius_in_texas

<snip> However... it seems that if you accelerate quickly, you're still ultimately are going to use more fuel than if you accelerate slowly, since you still need more energy to overcome inertia quickly (just as in a regular car). <snip>

This statement is simply wrong. The amount of energy needed to reach a given speed is independent of the acceleration. Overcoming inertia quickly takes no more energy that doing it over a couple of months. I think you are confusing energy and power. Power is energy over time. It takes more power to accelerate quickly, but you get to the target speed faster; time drops out of the equation and you use the same amount of energy either way.

Okay, you ask, if that is true, why does it matter how I accelerate? The answer is because other losses in the car vary with power. A gasoline engine running at low power wastes a lot of energy sucking air past the closed throttle plate. This is called pumping loss, because the loss comes from pumping air. As the throttle opens, there is less drag across the throttle plate (it opens), and the pumping losses decline. Engine efficiency continues to improve with RPM until some magic point where other losses increase faster than the pumping losses decline (friction, combustion efficiency, etc.), at which point the overall engine efficiency starts dropping with RPM. In an ideal world, an engine would always run at this sweet spot. The Prius does a better job than normal at keeping the engine running at an efficient speed, by virtue of the HSD with its virtual CVT and electric motors. The battery and motors help avoid inefficient engine speeds, as well as letting the Prius function with a smaller, more efficient engine. Still, there are a lot of losses associated with the electric motors. Energy must be converted from chemical (gas) to heat to mechanical to electrical to chemical (battery) to electrical and back to mechanical (motors). That's a lot of conversion loss. Despite all the losses, it helps with the overall system efficiency, but should be avoided whenever possible. In the ideal world you would never need the electrical part of the Prius.

So what is the most efficient way to accelerate: Keep the engine at its sweet spot, but don't draw energy from the battery. That amounts to a brisk acceleration, but not so fast that you see power coming from the battery.

Tom

[Fred_H]

KISS method: Press the accelerator pedal about one third down. (Edited: I first wrote "halfway")

[diamondlarry]

Originally Posted by Fred_H

KISS method: Press the accelerator pedal about halfway down.

You have to be careful with accelerating too hard though. As has been mentioned above, if you accelerate at a rate that requires energy from the battery, it will have an overall bad effect on mpg. I agree with Evan that "dead band" would be ideal but, as Jim says, that is difficult to hit. I generally accelerate up to 10-15 on battery then accelerate from there as gently as possible so as to not have any arrows showing from/out of the battery.

[richard schumacher]

It sounds like a paradox but you want to *avoid* using the battery. All energy conversions (kinetic, electrical, chemical) have unavoidable inefficiency, and moving electricity into/out of a battery is especially not so good.

The Prius has a battery because even an inefficient energy recapture is better than plain old friction braking.

[qbee42]

Originally Posted by richard schumacher

The Prius has a battery because even an inefficient energy recapture is better than plain old friction braking.

...and it lets us use a smaller, more efficient Atkinson cycle engine.

Tom

[nfschlaack]

Originally Posted by richard schumacher

It sounds like a paradox but you want to *avoid* using the battery.

Can I assume correctly that this means exclusively using battery power? AKA Stealth mode?