Can pulse and glide work to improve electric vehicle highway range?
Wayne Gerdes explains hypermiling before the Ford Fusion 1,000 mile challenge
Recently we got a question from a reader regarding the benefit of using the common hypermiling technique of "pulse and glide" with a battery electric vehicle. Given the relative scarcity of pure electric vehicles, P&G has mostly been used by hybrid drivers as one of the techniques to squeeze every last mile out of every drop of gasoline. There's a reason hypermilers use the technique: it is very effective. Since my own professional driver training has more to do with vehicle dynamics and getting through corners with the least possible loss of momentum I decided to consult with a true expert in the field of hyper-miling, Wayne Gerdes.Wayne runs a web-site called CleanMPG.com that includes discussion boards where fuel sippers talk at length about getting more from less. At a recent media event we had a chance to talk with Wayne about what exactly is pulse and glide. At its core, it is precisely what the name implies, a pulse of acceleration to get the car up to some target speed, followed by a period of gliding, off the accelerator for as long as possible.
Find out more below the jump.
According to Wayne, there are two keys to getting the most out of pulse and glide: understanding the most efficient operating modes of the car you are driving, something that typically requires some practice, and testing. Hypermilers typically use a device like the Scangauge (right) to monitor the instantaneous fuel consumption of the vehicle. Getting the car up to target speed most efficiently will vary from car to car. Through a process of trial and error, drivers can find the most efficient rate of acceleration.Once up to speed, the goal is to stay off the accelerator as much as possible, this is where gliding comes in. Maximizing the glide means cutting drag as much as possible to keep the car rolling. This is where Gerdes complained about regenerative braking. With all the hybrids built to date, automakers have tried to emulate the behavior of traditional vehicles. That means some amount of regen is programmed in to simulate the effect of engine braking when the accelerator is released. This, of course, means the vehicle will slow down when the pedal is released, not exactly perfect for gliding. Gerdes would prefer to see automakers provide an option for drivers to control the regen or at least turn it off except when the brake pedal is applied. Without regen, cars could glide significantly farther between pulses.
Gallery: Hypermiling the Toyota iQ
The other aspect of P&G that requires experimentation by the driver is how far to glide. At some point the vehicle speed drops to a point where another pulse is needed. How slow the driver can allow the car to go depends on road traffic and the acceleration characteristics of the car. Obviously, when there are other cars around, a glider can't drop the car's speed as much. An experienced hyper-miler can extract huge jumps in efficiency under the right conditions. Last December during the media launch of the Honda Insight, Gerdes managed 79 mpg (the EPA rating is 40 mpg city / 43 mpg highway). Stories of Prius drivers hitting nearly 100 mpg are common. More recently, Ford took a Fusion hybrid on a hypermiling road trip that managed to squeeze 1,445 miles out of a single tank.
Gallery: Ford Fusion hybrid hypermiling test
Gerdes' limited experience with battery electric vehicles (BEVs) indicates that the improvements possible with P&G are likely much smaller than with hybrids or standard internal combustion vehicles, probably on the order of about 5-8 percent. The problem for P&G fans is that BEVs vehicles tend to be much more optimized for efficiency to start with, a benefit most of the time for something that leaves less room for improvement. In a hybrid, optimizing the use of the internal combustion engine can have a huge impact on the mileage numbers. Since a BEV has no engine, the driver only has the battery to work with. Clearly, driving style will influence BEV range, but it will likely only drop with aggressive driving rather than improve with careful driving.
Reader Comments (Page 1 of 2)
DP 3:05PM (6/25/2009)
I think the next 'big thing' would be to develop a cruise control that would do this for you. WIth a limited range for change of speed (2-4 mph) but that the cruise would be able to exploit, directly, this trick, pulse and glide.
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Robert 4:03PM (6/25/2009)
Especially if it could detect hills and modify it's behavior to take advantage of it.
downtoearth 4:24PM (6/25/2009)
DP:
> I think the next 'big thing' would be to develop a cruise
> control that would do this for you. WIth a limited range
> for change of speed (2-4 mph) but that the cruise would
> be able to exploit, directly, this trick, pulse and glide.
The reason why it hasn't come yet is very simple - engine durability.
Take a look at any specific fuel efficiency map (as a function of revs and load): http://images.google.com/images?q=bsfc+map
The isle of peak efficiency for nearly every internal combustion engine, be it a diesel or a gasser, is almost always at low revs and high engine load (almost wide open throttle).
To pulse and glide, you have to periodically load your engine hard then. This means high forces acting on pistons, connecting rods, the crankshaft and its bearings and the engine block. This will simply significantly lower engine life cause engines are not designed to sustain high load most of the time.
CaramelZappa 3:15PM (6/25/2009)
Hypermiling usually commonly takes fuel efficiency past the point of practicality. Many people who hypermile may be getting higher mpg for themselves, but a lot of the time that results in people behind them not expecting them to be speeding up and slowing down so much braking and accellerating unnecessarily. So while your personal gas efficiency is higher, the people behind you or around you are effected and their mpg can go down. On top of that people don't consider that fuel efficiency is important, but so does TIME. If we're crawling along all the time just to squeeze out an extra mile our two that's great for saving a few bucks, but how much time are you losing getting from place to place, and how much is your time worth? How much is time worth to the person behind you?
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Shane 3:40PM (6/25/2009)
Most hypermilers, including myself, are ultra-aware of their surroundings. They know when a car is closing in behind them, and if it's a single lane, they most likely will not be employing hypermiling techniques. I think the argument that it causes others around you to lower their MPG is, for the most part, flawed.
What I always find funny, is when I'm crusing into a red light, and someone rides up to my bumper, changes lane to pass, just in time to slam on the brakes for the light. I then coast up to them a few seconds later. In this situation, I don't feel bad for holding them up. I didn't waste any of their time, and it's their own fault for wasting gas to rush up to a red light. Anticipating lights in city driving can increase your MPG significantly, and it doesn't cost you any extra travel time.
ARC 9:49PM (6/25/2009)
Caramel, you raise great points and I agree 100% that P&G could cause lower MPG on other vehicles and also "waste" time. One approach to help manage your concerns is to use smarter pulse and glide "cruise control" under 2 conditions: sensors show there is no traffic behind you and second you are in not a time rush. Sometimes people do their best thinking in the car drive to or from work. Also it is often alone time to de-stresss. So it depends on your specifc situation. Clearly it adds MPG value to the car under control and it may even add value to the driver who stays in the car a little longer during their trip.
Kodos08 3:55PM (6/25/2009)
Or, on the other hand, the presence of aggressive, lead-footed tailgaters lead to an increased frequency of accidents. And when that happens in my area, HUNDREDS or THOUSANDS of people can get SIGNIFICANT delays.
So I prefer to consider that over concerning myself with inconveniencing 1 or 2 impatient people behind me. I know it's easy to forget the simple things, like the fact that the roads belong to Everyone and we All have to share.
Secondly, I think you are confusing inconsiderate driving with hypermiling. Any good hypermiler will not pulse-and-glide if it puts themselves and others at risk or significant delay in a crowded section of road.
CaramelZappa 4:04PM (6/25/2009)
Lead footed tailgaters are a huge problem too, and I'm not talking about people who use simple hypermiling techniques sensibly. I'm talking about the people in my town who pulse and glide ALL THE TIME. So on a road with a 40mph speed limit the person in front of me is sometimes going 35 and sometimes 45. There are a bunch of people who drive like that in my town and I see it cause just as much grief as tailgaters, as well as cause people to tailgate uninentionally.
Brent 4:37PM (6/25/2009)
I've been thinking. In my town its mainly rednecks and so-called patriot types(and young kids) who love tricking engines for gas guzzling. I was thinking, since most them waste more than I save, it would be more effective to speak to them.
I'm considering a "Racing funds terrorism" or a "By speeding you help buy weapons killing our soldiers", something along those lines. Though I myself don't believe in the whole oil nation terrorism thing(I feel its that we impeding on their homeland that causes this violence). I feel this could get through to those who like to call themselves patriots(a true patriot is about peace and freedom(and of religion) not murder and corpoate opportunist's suppression) then guzzle gas like its going out of style. By slowing them you save exponentially more than you conserving a tank can.
Gas guzzlers murder:
Cancer(30,000 related cases a year)
Environmentally(thousands of doomed species)
Terrorism(oil funds the side of the war opposite us)
Though most speeders won't read it unless we're at a stop(cause they're busy racing to the next stop). I think this could be an effective way of campaigning the conservation message.
Gray 4:07PM (6/25/2009)
So I hope to be getting my EV within a month and would be happy to run any trials. I will have a PHEV enhanced Prius, a Civic hybid, and a Toyota Yaris converted EV to compare/contrast.
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CaramelZappa 4:14PM (6/25/2009)
Is your Yaris a homemade-style conversion? If so I'm curious about what motor/controller/batteries you're using.
BlackbirdHighway 3:56PM (6/25/2009)
The physics of P&G only work because the ICE is most efficient at one particular operating point. So, you run the engine at this point for a while, accelerating, then drop the engine speed to near idle and let the car use the built up momentum to keep going. When speed drops too low, repeat the process.
Electric motors have a pretty flat efficiency curve, so P&G is not an effective strategy. The dominant source of drag at highway speed is wind resistance, which increases with the square of the speed, so the best strategy with an electric car is to simply slow down. The squared effect means that even a small difference in speed can dramatically affect the range. Going 75 mph is much worse that going 70, and going 60 is very much better than going 65. Whatever speed you choose, you are better off keeping it steady, except on hills.
The other technique that is quite effective for electrics is drafting, but that's not recommended because of all sorts of safety issues. Don't try this at home! (or on the road).
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James 4:26PM (6/25/2009)
It's actually even more significant than a 'squared effect'. Power consumption due to drag increases cubically with speed: http://bit.ly/solarcaraero
downtoearth 4:27PM (6/25/2009)
BlackbirdHighway:
> The physics of P&G only work because the ICE is most efficient
> at one particular operating point. So, you run the engine at this
> point for a while, accelerating, then drop the engine speed to near
> idle and let the car use the built up momentum to keep going. When
> speed drops too low, repeat the process.
> Electric motors have a pretty flat efficiency curve, so P&G is not
> an effective strategy.
BlackbirdHighway is bang on accurate and what's above is all that needs to be said about pulse and glide and electric motors.
Chris M 10:37PM (6/25/2009)
That's pretty much what I was going to say. "Pulse and Glide" is most beneficial for IC engines where it maximizes efficiency on the "pulse" and can shut off on the "glide". Full Hybrids are particularly well adapted to that technique, because of the way they run their IC engine.
But BEVs wouldn't get any benefit at all from "P&G", they are efficient over a very wide range of power output and speed. The way to maximize economy on a BEV is to slow down a bit and drive as close to the optimum speed as possible. Turns out the "maximum efficiency" speed of the Tesla Roadster is 18 to 20 mph, at that speed the range could reach over 400 miles per charge! Of course, it would be very uncharacteristic to drive a Roadster so slowly!
Snowdog 3:42PM (6/25/2009)
P&G works in ICE automobiles because the are more efficient when
putting out more power, not very efficient in typical cruise range
and obviously have zero cost when they are off. So you use the fatter
part of the curve by using more power in a short burst in the more
efficient range then shut them off entirely.
I doubt you will get much if anything at all trying this with
electric as they are likely quite efficient while in cruising speed
and depending how they are set up will either glide on little energy
or actually capture energy while slowing down. Turning them off
entirely in the glide phase will likely save next to nothing. Going
faster than target speed in the pulse phase will lose more energy to
air resistance.
Dedicated Hypermilers will probably make their gains on some other
practices like not slowing for corners and other nitwit behaviors but
I seriously doubt there is much to be gained with P&G in an electric.
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CaramelZappa 3:55PM (6/25/2009)
You also have to consider that one of the better advantages electric cars have is regen braking, which you would have to turn off in order to pulse and glide.
Snowdog 4:08PM (6/25/2009)
As I said. Depends how the car is setup.
Many will have regen on the brake pedal and none on the accellerator, so you won't have to do anything special to "glide".
The technique was made to address ICE efficiency issues, it really doesn't apply to EVs.
Brent 4:15PM (6/25/2009)
Partially responding to Zappa and partially just commenting,
The time factor is a thing to pay attention to, but I've noticed a way without wasting time to get about +60mpg out of my 2010 prius. You are able to use battery only at any speed by pressing lightly on the gas, so if you're ever on a slight or major decline you can capitalize on this well without slowing or hindering anyone behind you. At a major incline its a good time to let the regen soak up some, and at a minor give it just enough to keep you rolling the speed you want(and cancelling the regen). This might sound like common sense, and thats cause it is. Also, I've found that most acceleration is used going up slight to major inclines. I avoid most of this by over accelerating when I am going down the hil or flat surfacel before it(when I have room) often going 7-10 over the speed limit(which u can blame on hills by law) and then with far less gas head up the incline. My ride home through city is quite hilly and I manage about 70mph without impeding traffic by just using slight front and back gaps when they're there. The majority of my driving is probly averaging about 55mph(I take the interstate down every morning for work(about 20min drive(65mph) and go home via city (35-50mph) thusfar I got about a 60mpg average with 1474 miles so far. This is month 1 though, thru this year i'll become veteraned and get maybe 5mpg more on average.
Anyways, Like zappa said, don't impede and make others use more gas. They often will use more than you save(as they're less efficient). If you're not just penny pinching, like I'm not(and care about the air we breathe and every species on the planet you're dooming). The far greener thing to do is to keep up with the flow and not aim for egocentric figures. Just use your battery as much as you can when its feasible.
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dhymers 7:36PM (6/25/2009)
Well said Shane.
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