pulse & glide... accelerate fast or slow?

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

 

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).

 

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

 

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

 

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.

 

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.

 

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

Tom

 

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

 

OK, now I feel stupid. How exactly do you go in "stealth mode" without using the battery??

 

OK, I think I gotcha now. Using "stealth mode" to get to P&G is not beneficial as "stealth mode" alone.

 

Stealth mode is pretty much never beneficial. It's hard to get through your head, but repeat after me: "electric is bad, electric is bad, electric is bad...". Driving in EV mode is mostly good for impressing your friends. It's great fun, and the displayed MPG goes up while you are doing it, but in actuality you are reducing the overall MPG as it is going to cost more fuel to recharge the battery than it would have cost to just use the engine to power the vehicle. There are really only three cases where it makes sense to run in EV mode:

1) Moving a short distance, such as in a parking lot. In this case starting the engine is worse than using the battery. Walking is better.

2) Creeping in traffic. EV mode in this case allows engine starts and stops to be minimized. When the engine does come on, it will run for a bit and recharge the battery, making the restart more worth while.

3) Reducing the battery SOC before a long downhill. Battery capacity is finite, so there is a limit to how much energy you can recapture by regenerative braking. If you know you have a long downhill that will completely charge the battery, it makes sense to use the existing charge in the battery before starting down the hill to make room for the recaptured energy.

Other than these three cases, you are not going to help your mileage by running in EV or stealth mode. You may be confusing it with gliding, which is coasting without power - electric or otherwise. With P & G, you power up to speed (the pulse), and then coast until you slow (the glide). The pulse should be done without tapping the battery, and the glide should avoid using the engine or battery.

Tom

 

This has been an amazing revelation! I bought my used 2006 3+ weeks ago with 19k miles. I've added ~1,500 miles since then. My first tank got me 53 mpg when I admittedly knew little, if anything, about driving a hybrid. My 3 fillups since then, I have gotten 45 mpg. And that's after spending countless hours on PC reading up on all the cool features and techniques to maximize my [car's] driving pleasure and gas mileage. My weekly driving miles have been virtually identical on all 4 tanks of gas.

I can remember driving with that first tank and, while driving a little slower and slowing down long before getting to the red light line, found that I was driving the Prius very much like a non-hybrid car, except driving slower. Interestingly, these past 2+ weeks, I'm driving even slower and more judiciously watching the MFD to use EV mode more often in accordance with all the good advice on multiple PC boards. About 70% of my daily commute is on the freeway (about 9 miles) using cruise control pegged at 65 mph and the other 30% (about 4 miles) is city driving (each week, I have also had an extra 200 mile or so trip on the freeway). After reading your posts above, it now seems that I probably should run on EV less, run the ICE more, and I may get back up into the 50+ mpg range. Does that sound about right?

 

Yes, that is completely correct. Anticipate stops, keep your speed down, and avoid EV mode. That will get you great mileage.

Tom

 

As JimboK noted a while back, the sweet spot of engine operation
generally seems to occur when MPG reads between 1 * MPH
and 0.5 * MPH. That's about 1700 - 2400 RPM, all with a
fairly open throttle and high shaft load. This is a coincidence,
the numbers aren't supposed to work out that way by design,
but gives the driver something to shoot for.
.
Ideally, install a tach. Here's how.
.
_H*

 

I've noticed that on the scangauge, that "deadband" acceleration is more easily triggered then it seems.

Looking at the amps/hour values coming to/from battery, it seems that at any pedal position, except for the lightest or the hardest, the amperage will usually stick at close to 0 going in or out.

The only time that it really goes into "charge" mode is when I'm trying to trigger "super highway mode" by lifting my foot off the pedal to the borderline between warp stealth and 1300 rpm acceleration, which sometimes seems to trigger the inefficient "charge" at 15amps going into the battery instead of the deadband 0amps/hour going in or out.

If your accelerate at any moderate rate, chances are that your already accelerating at or close to "deadband", even if the mfd shows energy going into the battery.

 

I think that there is no risk of accelerating too hard with the accelerator pedal about halfway down.

Is there anyone with advanced instrumentation here who can tell me in what rpm range the ICE runs in with the accelerator pedal pressed about halfway down?

Ken1784 once said that 40% accelerator was considered to be the optimum by Japanese hypermilers. And hey, diamondlarry, now I see you were there too, and said that "Throttle Position Sensor reading of 25 seems to work pretty good for keeping the rpm's below 2200".