Pulse/Glide Acceleration Help

The reason people say not to drive EV from the battery is because for that energy to get there, it was to be converted, mostly from gas, and there are losses that will happen during the energy conversion, thus it is mathematically more efficient to use only the ICE when pulsing and gliding.

This seems to work well mathematically but varies wildly in real world driving.

I have found (in my purely non-scientific experience) at slow speeds the electric motor used sparingly - but still used! - is essential for good mileage, and the above argument is much more valid in the real world only when already in motion. I don't think anyone could argue that in stop and go traffic, for example, that it's better to give the pedal a good thumping to get the ICE going while crawling. At slower speeds like this the electric engine is more bang for the buck.

I have one route that averages 24 mph. With judicious electric engine use, I can get about 60 mpg or low-60s, if I run the ICE (as I tried to experiment with when I had read that on here, too), I get down in the low to mid 50s. Just yesterday there were two big obstructions during this commute (car stalled at top of a bridge stopping traffic both ways and an unfortunate three car accident at another spot), necessitating more use of EV than I prefer to use, but at one point during the trip I had 70 mpg showing, and ended up with 66, about 7-10% higher than I normally get on that trip, so I guess it was alright. I was able to largely regen the energy used or just stay easy on the accelerator when at speed, and the "gas debt" you'll hear of didn't seem to affect the mileage, just the opposite.

If I've had to stop at a light, I do use electric to get going just for a couple mph. If I know I have an opportunity for regen coming up, I'll use it more. I've also read people saying that electric acceleration at a stoplight is a gas debt, but since I try to maximize the regen at stops by decelerating relatively slowly, and not just slamming on the brakes, that I have a right to use that regenerated energy from my motion . In that case it's not a gas debt, per se, but reusing what was just captured from your motion - "free" energy.

With all the hills where you are, I would get up to the top as easily as possible, then regen the backside down if you know you have a stop coming up. If no one's behind you, foot lightly riding the brake will give you a bit more energy leading to the stop. If no stop, keep that momentum!

If, however, you are accelerating from a stop and know you will be going 35-40 mph continuously soon thereafter, just get right up to speed with the gas engine. A quarter of the way down on the pedal is an efficiency point to shoot for. You can also use instant mpg when accelerating - generally speaking, don't have the instant mpg be less than half of your current mph while accelerating as a limit.

 

Sorry to post a "me too", but... I also wonder about battery usage vs overall MPG. I've heard some call a low battery an MPG debt that eventually gets repaid via gas burning and losses through the system, but that doesn't sound right to me. What about regenerative braking? And can't the hybrid system tap the ICE when it is most appropriate to do so, thus not affecting MPG that much? Isn't the battery there to be used, dangit? These are the questions that keep me up at night.

I've been tempted to drive the thing at 10-20 MPH around my neighborhood using only electric power. It would be an interesting experiment to see what the reported MPG turned out to be when you factored in the ICE turning on. I'm not convinced that I could execute the test properly, though.

I usually glide with electric power, but I don't go higher than 1/4 up the bar (about halfway to the midpoint). I don't seem to get much more in the way of power when I go higher than that, so I don't bother. From a dead stop, the electric power seems to be enough up until almost 10 mph, and then I feel like the ICE is necessary for acceptable acceleration.

 

I have a similar route and I've found the same thing. I take surface streets to work about 22 miles one way. I don't do the whole pulse and glide thing, but try to maintain reasonably steady speeds. I've found that I get the best numbers if I try to remain in electric mode to maintain my speed at 40 or below. I do use the ICE to get up to speed, but I still accelerate gently. Doing it this way I can consistantly get numbers in the mid 60's and even get up into the low 70's on good days.

I think the concern here is that it's much more efficient not to have to stop at all. If you can see the light ahead of time and you start coasting when you see it turn red ideally you'll be close to the light and still moving when it turns green and you won't have to touch the brakes at all.

Regen is a good thing and it's very useful for lights that you can't anticipate and other things you have to stop for, but due to all of the losses in the system from the generator and the inverter and the batteries you'll only get back about 40% of the energy at best. So if you can start to slow down sooner and not have to stop at all you can use all of that energy for forward motion instead.

Brett

 

I've spent some time pondering this as well, and here are the thoughts I've come up with.

First, as everyone rightly points out, most of the power in the battery comes from the ICE. Granted you can get some power from regen, however, as I stated above, regen is at best about 40% efficient and unless you're driving down a mountain you really should be trying to avoid braking as puch as possible. So while regen is 'free' energy that would have otherwise been wasted, in an ideal world you won't really be getting much power from it.

Also, using the ICE to charge the battery isn't 100% efficient. As with regen there will be losses from within the system. I haven't seen any numbers for this, but I imagine it's much better than regen... maybe more along the lines of 80% or maybe even more.

Now, also keep in mind that the ICE has an RPM range where it gets the best efficiency... and the Atkinson engine used in the Prius has a particularly narrow band. If the engine runs faster or slower than that ideal range the efficiency suffers. The trick is to use the electric motor to replace or suppliment the ICE when it would otherwise be operating outside of the maximum efficiency.

For example, when you're going slowly the ICE isn't particularly efficient. However, the HSD can start to take power from the generator making the ICE spin faster and get into the efficient range. It can take this power that it's getting from the generator and send it directly to the electric motor to suppliment the ICE and/or send it to the battery to help recharge it.

In stop and go traffic when you're just inching along it's most effecient to stay in all electric mode... you'll be draining the battery, of course, however, the battery can be recharged while the ICE is running in it's efficient range... either just running the generator if you're stuck in traffic for too long, or as you're driving down the road once you get out of traffic.

Along the same lines, there are times when it doesn't make sense to use just the electric motor. Going from 0 to 30 (or more) is one of those times. It takes quite a lot of power to accelerate with just the electric motor and acceleration is one of those times when the ICE could be running in it's efficient range, so the hit you take by discharging the batteries and then needing to recharge them later is a big one. In this case you'll get better mileage overall if you allow it to start the ICE during acceleration.

Clearly Toyota thought it was worthwhile to suppliment the ICE with an electric motor despite the energy conversion losses or they wouldn't have spent millions of dollars on R&D and kept the prius line around for more than 10 years, so just trying to run on the ICE alone doesn't really make sense either.

They built the HSD to try to maximize efficiency, so you can get pretty good numbers just by driving normally and letting the system choose when to start and stop the ICE. However, they also wanted to make sure that the car drove very much like a regular car so some compromises were made. You can tweak your driving style a bit to get it to force the ICE on or off and if you do so at the right times you can definitely get better mileage.

 

Yeah, definitely, very much in agreement. I do this all the time, but no matter what sometimes we have to stop our cars (although I dream of pausing time to get through red lights unfettered...Hiro Nakumura style). In those cases I maximize regen and then use it. If a stop is inevitable, it's not an energy loss to use regen, it's an energy gain. The energy loss comes from not using regen in such a case.

I posted my experience(s) with regennin' to the max here. 76 mpg potential over 3.0 miles average speed over 40 mph, including accelerating to speed from a stop/5 mph with the normal flow of traffic, climbing a hill, and climbing a relatively steep bridge (and admittedly going back down the other side of it . It's an extreme example, but from that I've taken that it should be done regularly to a lesser extent when those darn stop signs won't get out of the way.

Thanks, too, for nice writeup in that second post.

 

Oh, and to comment on your commute:

A steep hill may be a good time to take your foot off the gas and let gravity take you up to speed. A feathering of the pedal to little or no electric power can maximize the effect of gravity. If you go too fast as you crest the hill, then you might be in a situation where you have to coast or brake (preferably regenerative) to keep your speed reasonable. A section of my commute is like this: a little under the speed limit on the top of the hill, glide all the way down to above the limit, keep gliding and lose speed on the level ground afterwards. It's kinda fun. It feels like a roller coaster.

35-40 MPH on level or slightly downhill terrain is typically where I can use electric mode to maintain speed. I would think that the ICE usage would be mostly optional on such terrain.

Some people recommend letting the car sit while it goes through its warmup cycle. If you have a minute, you could try that, and see if it makes a difference.

 

Does regen braking happen if you are really pushing on the brakes hard, like in an emergency stop?

 

You get a small amount of regen from an emergency stop, but a good portion of the stopping power there is from the standard brakes. You'll get the most regen from a more controlled stop with moderate braking.

 

There's a good post about Gen2 regen braking by Patrick Wong here (specifically, brakings.pdf, chart on page CH-37). It looks like brake pressure in excess of the capabilities of regenerative braking will cause the vehicle to use a blend of regen and friction braking. So, clearly, light braking would generally be preferred.

 

I wouldn't worry too much on whether to use the ICE earlier or later when coming off of a start.

To accelerate up to 15 mph takes about 40-60 KJ of energy (factoring in estimated losses, 32.6 KJ without losses).

For comparison, the HV battery useful range is 600 Wh, or 2160 KJ, and the entire battery stores 1300 Wh, or 4680 KJ. So you are hardly making a dent in the HV battery (about 2.5-3% of useful range). Of course if you need to accelerate up to 40 mph anyways, you may as well start the ICE, making sure you can keep the ECO light on as much as possible.