driving technique: pulse & glide (long)
 

(i'm writing this more because it's interesting than because it's practical...)

pulse & glide in a prius: 109 mpg

this summer, a group of five prius fanatics drove an unmodified 2nd gen prius to a record 109.3 mpg (US) over a 1397 mile marathon on a "loop" of public roads in Pittsburgh. you may have heard about this already. if not, you can read about it here.

they used a driving technique called "pulse and glide" which initially i mistakenly thought was only applicable to the prius hybrid system. i was wrong - it works in theory on any car...

pulse & glide explained

first off, "pulse and glide" is a technique that would probably rarely be used in real world driving in a non-hybrid car. it works like this: let's say you're on a road where you wanted to go 36.5 mph. instead of driving along at a steady 36.5, you accelerate gently to 40 (that's the pulse), and then coast in neutral down to 33 (that's the glide). that's it. rinse and repeat. "pulse" up to 40, "glide" back to 33. repeat. and repeat. and repeat.

you're still averaging 36.5 mph, but it turns out that pulse and glide is significantly more efficient than driving at a steady 36.5. (i'm using 33 to 40 mph because it's the speed described in the hypermiling article linked above.)

wtf???

if you're like me, it's counter intuitive. you're asking, "how could that possibly be more efficient than maintaining a steady speed in the highest possible gear???" it violates one of the main commandments of efficient driving: conserving momentum.

how it works

the secret is in the glide. in the prius, when you release the accelerator below 40 mph, the gas engine shuts off and the CVT effectively freewheels in neutral (it's a little more complicated than that, see the above link if you want details). so, while coasting from 40 to 33, you're effectively getting infinite mpg - you're using no gas.

let's say the prius gets something like 75 mpg (US) at a steady 36.5 mph. then to achieve 109 mpg using P&G, you just have to achieve 54.5 mpg while accelerating from 33 to 40. and it turns out you can, in a prius.

so in the prius P&G trip, you "pulse" half the total distance while getting 54.5 mpg, and then you "glide" half the total distance using no fuel. it averages out to 109 mpg.

pulse & glide in a geo metro: 68 mpg (in theory)

skeptical, i went out in my metro on boxing day with the scangauge. i didn't believe it was going to work, but here are the numbers i got. i went to my "test course" - a nearly perfectly level stretch of 2-lane highway about 3 miles long. note: i used higher speeds, and there was a 20 mph tailwind (thus the high steady speed mpg); all runs were in one direction, and i didn't repeat it - so you should be skeptical of my data too.

- at a steady 80 km/h (about 50 mph) i was getting 59 mpg (US)

- "gliding" from 90 to 70 km/h took 16 seconds.

- "pulsing" from 70 to 90 km/h at a rate of acceleration that also took 16 seconds i was getting about 34 mpg (US).

- so my P&G average would be 68 mpg, vs. 59 at the same average speed. that's a 15% increase over the steady state mpg - theoretically.

i say theoretically, because the engine would have to be off in the glide to get that mpg. you could do it, but it makes it even less practical (whereas the ICE shuts off automatically in the prius).

so i took a couple more readings. with the engine idling, and the car in neutral, the average mpg shown on the scangauge in the "glide" from 90-70 km/h was 550 mpg. when you average that against the 34 mpg of the pulse, it works out to an average of 64 mpg. that's an 8% increase over the steady-state mpg.

that's it

so there you go. next time you're cruising along on a lonely road at a steady speed, you're not getting the best mileage you could. you could be pulsing & gliding to maintain the same average speed, and saving lots of gas in the process. to exceed your steady-state mpg, you just have to be able to "pulse" at a rate of fuel consumption that is greater than half of your current steady-state mpg (assuming equal length pulses & glides; you may be able to increase the proportion of glide to pulse - meaning faster acceleration in the pulse - and still beat your steady-state mpg for that average speed).

anyway... i thought it was a cool thing to learn. completely impractical if you don't have a prius (which handles the engine shutdown/startup and transmission neutral/re-engage all through the throttle pedal), but theoretically do-able in any car.

more, if you're interested: the physics of pulse & glide

.
www.MetroMPG.com
.

So think of this. modify
 

So think of this. modify your cruise control to have two modes. If mode 1 is selected it will do pulse and glide. If mode 2 is selected it's normal cruise control. mode 1 is obviously for people like us, and you select mode 2 when your family is in the car and doesn't want to jerk back and forth for the entire trip.

i thought of that too. they
 

i thought of that too. they could easily do it in the prius - it's just a few more lines of code in its computer.

but on my non-prius car, the cruise would also have to select neutral at the start of the glide and re-engage it for the next pulse :(

neutral is crucial for getting a long glide. otherwise you've got engine braking, and the efficiency of the glide goes out the window.

Re: i thought of that too. they
 

Good point. then again a few people on here recently were saying that if you coast in gear instead of in neutral your fuel injectors actually turn off. Coasting in neutral will leave your fuel injectors going at idle speed (1000rpm-ish for me).

But then again your glide is MUCH longer in neutral than in gear.

i've heard about the
 

i've heard about the injectors-off mode as well, but according to the scangauge, it's not part of my car's programming.

when i simply released the throttle in 5th gear at 90km/h (around 2800 rpm), the gauge showed an instantaneous mpg of 255 - vs. 550 mpg at 800 rpm in neutral at the same speed.

if the scangauge is accurate, that tells me my injectors are still merrily injecting fuel at the higher rpm even with my foot off the throttle.

Very interesting indeed. Too
 

Very interesting indeed. Too bad I cannot practice this in my auto, as shifting into gear while driving takes 3 mph away. However, very interesting indeed. Bleh, coasting in gear is only good when stop, as it significantly slows you down, even though the injectors will be off (in some cars' cases).

Quote:i say theoretically,
 

Since my Saturn doesn't have power steering, I quite often coast with the engine off. At speeds of just over 50 mph, my Scanguage will read over 1,000 mpg with the engine off. I'm going to be doing some travelling from one side of the county where I live to the other tomorrow morning. I'll try to incorporate this technique as much as possible and see how well it works. I haven't used it so much between stops as this so it should prove interesting. Normally I coast from 1/2 to 3/4 of a mile before coming to a stop with the engine off. This technique is also quite similar to what Fran Giroux describes in his newsletters as what he calls "porpoising."

Re: Quote:i say theoretically,
 

i do that lots too - shut off the engine when coasting down to a stop. (no power steering either).

only in my car, the ScanGauge stops reporting instant mpg when the key is switched off and goes to "sleep" mode after a few seconds (even if i immediately switch the key position back to "on" again). though it appears to continue calculating current average mpg for a few seconds before it goes to sleep.

does yours continue to give you mpg info while you coast with the engine off the entire way to your stop?

i had heard that term too, but haven't read anything about it. do you have a link?

darin

Quote:does yours continue to
 

Yes, it does. I shut the engine off and as soon as it quits turning, I turn the key back on. I then get mileage readings all the way to a dead stop.

Here is a link to Fran's newsletters on Hydrogen-Boost.com: https://www.hydrogen-boost.com/newsletter.html As you scroll down, you'll see an alphabetical listing of topics. Porpoising is discussed in both the February and May 2001 newsletters. In the February newsletter he doesn't actually mention the name "porpoising" but describes the technique. Porpoising is mentioned in the May issue just after the part on hybrid vehicles.

thanks for that link.
 

thanks for that link. porpoising is similar to what i do already -- shutting the motor off and coasting in neutral (or clutch in) when approaching a stop.

pulse & glide to maintain a steady average speed however is a pain in the butt. i'd: clutch in, release gas, shut the car off, shift to neutral, clutch out, coast down (from 90-70 took 16 seconds). then "pulse": clutch in, key on, shift into gear, pop the clutch to restart, accelerate up to 90 km/h for 16 seconds.

each transition has at least 5 physical steps and involves 3 of my limbs... every 16 seconds. i could reduce it to 3 steps if i just pressed the clutch in and held it (skipping the gear-to-neutral part).

or i could just market it as an exercise program: "lose weight, tone up AND save money! while you drive!"

as for the ScanGauge, you're lucky your OBD2 protocol supports the data feed after engine-off. apparently there are 2 protocols (ISO bus vs. CAN bus). both shut down the ScanGauge at engine-off, but ISO takes much longer than CAN to re-initialize when switching the key back on, effectively giving ISO (me) no coast-down data (and skewing my mpg averages down as well if i do much engine-off coasting).

Quote:as for the ScanGauge,
 

In some ways my Scanguage is almost annoying. I have already been sitting in my driveway with the key on and engine off right after a trip and I can push the button on the Scanguage to bring it out of sleep mode and I can watch the trip mpg go down.:-( Although in some ways it's kind of interesting to watch how fast the engine cools down when it's not running.

I was thinking about this
 

I was thinking about this tonight. I realized that hooking up the stock honda cruise control mechanically would not be difficult at all.

Here is how the honda cruise control works. Instead of the cruise control actuator pulling on the throttle cable like in other cars it is actually hooked up to the gas pedal. The pedal has an extra bracket on it. It's all cable driven.

So if we could figure out a way to change the code of the cruise control ECU we could actually add a second actuator that is hooked up to the clutch pedal or clutch cable. When it is gliding it will engage the clutch and it will actually be in nuetral.

The downside of course is extra use of the clutch. if I could get 80mpg out of it however I think changing a clutch more often than usual is worth it.

Pulse and Glide Driving Technique
 

Decided to try a modified version of the pulse and glide in my F150 truck for the past two days. I used to always keep the cruise control on 55 and drive to and from work like that. Now I accelerate up to 60 then let off the gas until it drops down to 50. Then I do it over again. I don't turn the engine off. My rpm drops from 1550 to 1100 when I glide. Going up hills, I just keep it at a steady 55. It really seems to help. I don't have a scangauge (saving my pennies for one) but the farthest I could go on a 1/2 tank of gas was 319 miles. Today the odometer read 349 miles on a 1/2 tank. I started doing this after I had already driven 100 miles on the existing tank so I will have to redo the test next week when I fill up. But anyway it shows promise! Thanks for the tip!

Is the f150 an automatic?
 

Is the f150 an automatic? Diesel? Two tanks? That's exciting news though, I may need to try this on my road trip tomorrow night and to pick up and engine on monday, :)

it would be better to go to
 

it would be better to go to neutral for the glide than just to let off the gas. letting off in gear means you've got engine braking, so the glide may not be long enough to compensate for the reduced mpg of the pulse.

just a thought - i didn't try it with my scangauge.

my very non-scientific results showed an 8% efficiency gain gliding in neutral with the engine running. it would be significantly less than that with engine braking factored in.

though i suppose even a 1 or 2% improvement is better than nothing...

The reason I asked about
 

The reason I asked about auto is from your first comment. If he was auto shifting into neutral then back into gear is no good, you lose a few mph from the initial getting the engine spinning again stuff.

i don't know about auto
 

i don't know about auto transmissions, but in the manual, you should blip the throttle as you release the clutch to re-engage the gear at the proper rpm. that way there's no sudden engine braking, drivetrain shock or clutch wear... "matched-rev shifting" - same principle as heel & toe downshifting.

you could probably do the same with an auto tranny, but i'm not sure. (in reviews of sports cars with paddle-shift slushboxes, you often read about how the ECU blips the engine RPM up to the proper speed as you engage a lower gear, so it's out there.)

but i'd personally be reluctant to shift in and out of gear at highway speeds with an auto unless i knew it was OK for it.

Yeah, if I "blip" it
 

Yeah, if I "blip" it shifting into gear it doesn't do anything. It starts out in first gear then shifts up is the problem. Mehbe it's just my piece of crap, need to swap to manual.

another potential downside
 

another potential downside to pulse & glide: seems logical to me that it's harder on machinery to run it at a constantly changing speeds, vs. at a constant speed.

repeatedly changing a car's speed with this technique, while it may use less energy, strikes me as more likely to promote wear in the drivetrain.

i have no idea what the difference may be. just putting the thought out there.

My wife said that in her
 

My wife said that in her Mitsubishi Galant the cruise control was set up to behave in this manner. She said it always went above the target speed and then coasted down below it to only climb again. It might be worth looking into.

As for the wear on the engine, you might be right. This would be a good reason to not do this all of the time.

Sorry for getting back late.
 

Sorry for getting back late. The F150 is a 4.6 L gasoline with automatic and overdrive. It only has a single tank, don't think 97 and later came with dual tanks, but I wish it would. The trick I am finding is to accelerate slowly back up to speed (V8 Engines love gas especially when accelerating). I was a little to anxious on the acceleration on Friday and I could notice it. Shifting into neutral and back costs a few mph and I think it is harder on the drivetrain.

i just posted a review of
 

for anyone interested, i just posted a review of pulse and glide on my site. much the same info as mentioned above, just presented a little more coherently.

...

Exploring 'Pulse and Glide'

...

https://www.metrompg.com/posts/photos/pulse-chart-z.gif

my autoshop teacher says
 

my autoshop teacher says that of the cars that do this technique stock, they have had there trannies a part more than often. This eats your trannies b/c in gliding evreything's spining but not being lubed. the trannies are being replaced or rebuilt 3x times a non-glide cc.

i'm not a transmission
 

i'm not a transmission expert, but i'd be wary about generalizing that coasting = damage for all cars.

automatics: my understanding is that, yes, automatics can be damaged by coasting with the engine off since fluid pressure from a spinning torque converter is needed for proper lubrication.

but... not all automatics are built the same. why are saturn cars so popular to tow behind rv's? because they're one of the few automatics that can freewheel in neutral without damage. they're built differently. are those tow-behinds subject to more rebuilds than non-tow-behinds? i don't know.

i'm not educated enough about manual transmisisons, but i believe you could coast all you want with no extra damage because they don't rely on fluid pressure for lubrication - it's more of a gear bath.

i know i couldn't coast (engine off) on my motorcycle because the engine and gearbox shared the same lubrication, and the engine needed to run to circulate it thru the gear box.

so it would seem to depend on the design.

the prius is obviously designed to be able to coast - engine on or off.

any of you vx tranny rebuilders care to comment?

having said all that, i'm NOT arguing that pulse & glide is OK for your car. just looking to clarify the issue of coasting & transmission lubrication.

i still feel it would be somewhat harder on the whole car (not just the transmission, but the engine, clutch, drivetrain components) if you pulse and glide instead of just driving along at a steady speed (e.g. for highway driving).

new guy pulse and glide
 

Another thing you should consider when you turn off the engine for such a short time is that you cool the exhost system - O2 sensor and Cat which means that the fuel mixtures may be a little off when you start up again reducing efficency. Better to just let the engine idle and keep the wear and tear on the starter to a minimum and keep the battery charged up. One other thing you may be forgetting is that the brakes are powered by engine vacuum so you have a couple to three pumps before you loose power brakes and that is pretty much on all cars today. I like to just coast down sections of road that are slightly downhill often half a mile to a mile in some instances. Coming off the Newport Bridge about 200 feet above water is a great coast and speed builds up over the posted speed limit a little.

Excellent points JanGeo.
 

Excellent points JanGeo. I've never been a big fan of turning the engine off to coast. Something just doesn't seem right about it to me. The loss of power brakes is another great reason to avoid this.

Welcome to the site! Please introduce yourself in the introductions forum.

i'm going to leap to the
 

i'm going to leap to the defense of shutting off.

- as JanGeo pointed out, you don't "lose" your brakes. you still have 2-3 full-assist pedal applications in reserve with the engine off (depending on the car). that said, i would not be comfortable doing this in a heavy car, where assist is critical. in my car though, i could safely stop it with no assist at all because it's light enough. besides that, the pedal effort isn't ridiculously high without vacuum assist.

- the loss of power steering is a bigger safety concern. and doesn't apply to my vehicle, since it's manual anyway

- as for cooling down the cat and losing efficiency on re-start, i think that's stretching a little bit. we'll be seeing auto shut-off coming to more and more production cars, not just hybrids (gm tries to get away with calling its pick-ups with auto shut-off "hybrids", but they aren't really hybrids). nevertheless *all* current hybrid models currently shut down their engines when not needed (depending on a number of variables, including that they have already reached normal operating temps). once fully warmed, the cool-down is minimal in shut-off, or else they wouldn't be doing it.

- wear and tear on the starter is absolutely a valid point. it's why i would be much less likely to switch off the engine in a car with an automatic transmission, since restarting electrically is your only option (not to mention potential loss of lubrication, depending on tranny design, described in an earlier post).

- in my manual shift car, i don't usually use the starter if i have been coasting. i almost always re-start with the clutch before i lose momentum. select a high gear (4th or 5th) at a low road speed, partially engage the clutch - just for a split second - and the motor spools up with hardly a "bump" felt.

- as for not "feeling" right, i can't argue against that. you have to do what you're comfortable with.

welcome to the site JanGeo! can i assume from your name that you're a metro owner, sent to dilute the general honda-ness of things around here? :)

You make all very valid
 

You make all very valid points MetroMPG. I think I would be more inclinded to turn off the car and restart if I lived in an area with more hills. The only hills around here are the mountains leading up to Park City, UT.

good point about the hills -
 

good point about the hills - i tend to shut off (a) when decelerating from high speeds (i.e. the coast down lasts for a while), (b) going down a grade, or (c) if i'm going to be stopped for more than 30 seconds or so (e.g. train crossing, really long traffic lights, etc). i don't usually do it in normal stop & go driving.

for those who are still
 

for those who are still skeptical about the potential gains to be made from p&g...

the image shown below is a photo of the fuel economy screen (comes with the NAVI option) on a NON-hybrid 2005 Honda Accord PZEV EX-L w/ Auto and NAVI.

https://www.greenhybrid.com/share/fil...01-19-2006.jpg

yes, that's an average of 64 mpg over 180 miles.

the car is owned by an expert efficiency driver. his combined lifetime mileage on that car is an incredible 47.6 mpg. (higher than my firefly/metro).

he shuts his motor off when he coasts doing pulse & glide, so he's undoubtedly wearing out his starter faster than normal.

p&g thread that includes that photo

coasting and cooling
 

Yep I have no power steering on the Geo also but the brakes you should try first before you THINK you can stop without vacuum - the pedal gets really hard to push. I also have a new Scion xB - Geo rotted out at the right lower A Arm on Thanksgiving morning and on the xB you really don't want to turn the engine off and try to steer - the power assist in the rack and pinion steering is really tought and the owners manual also does NOT recommend it.

But the cooling issue is valid because stopping the engine when stationary is a lot different than while moving - air cooling is much greater at speeds and the sudden change in temperatures can cause some problems to the metal over time. Also the fuel injection may use different mixtures until the oxygen sensor and the exhost manifold warms up again so that you may be affecting the fuel economy. These things are taken into account on the Prius already as is the starter motor. I am not saying it is REALLY BAD for the motor but long term it could be.

I can't run the ScanGauge on the Geo of course but the xB has been really interesting to watch - still waiting for a tank of gas to run down more before I can reset the gas used calibration and get more accurate readings on the MPG. Getting a bit of a pump shock with the 20mpg DECREASE in the new car but I needed something a little bigger to carry the electic vehicle I am building. I am also thinking of adding a motor or two to the rear wheels of the xB to make it a parallel hybrid - with the extra weight on the downhills I could recapture more of the energy to assist on the level and slight downhills with the ICE in neutral.

I finished my pulse and
 

I finished my pulse and glide tank and here are the results. I had to do some calculations to compensate for a highway trip I went on. By estimating my highway mpg at 33mpg based off my previous tests and temperature I was able to calculate fuel used on that trip (130 miles) and subtract it from the total. Remaining miles and fuel would represent my pulse and glide efforts. Using this method I calculated that I got 29.38mpg, up from about 27 of my normal driving habits (a good punch every once in a while, but normally steady and coasting downhill), and 25 if I'm really feeling aggressive. Overall I wasn't too impressed. Maybe my pulses were taking up too much gas or my idle wasn't lean enough. No way to know until I make a mpg gauge. I don't think I'll continue this method because it can sometimes be annoying to always having to concentrate on the speedo when it's easier to just maintain constant speed. It's also incredibly annoying to other drivers, so you can either piss people off, or only do it when no one else is around. I really enjoy driving my car the way I normally do so I don't consider a 2 to 3mpg gain to be sufficient enough to alter my driving habits. I figure I probably have to buy an extra gallon of gas to get the same mileage as if I used pulse and glide all the time. But for me, when money's not tight right now, and gas isn't outrageously high, I'd gladly pay the extra $2.50 to be able to enjoy all 170hp every so often without having to think about how it's going to mess up my pulse and glide habits. I'll revisit this idea when I have a way to measure mpg instantaneously.

One benefit to coasting this much is that I found some really great roads to coast on that I normally didn't think would sustain a coast very well. I got one to last out to a mile. Some roads are ever so slightly downhill and it's hard to tell without rolling in neutral for a little while.

FYI...
 

FYI...

i just came across another description of the theory of pulse and glide and why it works the way it does. the writer has an enviable ability with words and i think he explains it more clearly than i have done.

he makes the added point that the advantages to be gained from pulse and glide are most evident at speeds below where aerodynamic losses are more prevalent.

it also indirectly addresses the issue of what type of acceleration is most efficient: slow or quick.

Re: FYI...
 

'



how can you tell on the rev band oncce you reach "peak torque power". With stepless gears how do we know which gear to extract most of the power from?

your car's power
 

your car's power specifications usually tell the RPM where peak torque and peak horsepower are achieved. if it's not in the owner's manual, it's out there on the web somewhere, or in car reviews.

Yeah, should be in the specs
 

Yeah, should be in the specs from the dealer somewhere or just randomly online.

https://www.psytronix.org/myride/mpfi...dyno_white.gif

Blue and red are my car now, black and green when I get it running with this swap finished.

but... be careful.
 

but... be careful. efficiency peak doesn't necessarily = torque or hp peak. but i also don't know any more about it to say more than that.

quoted above (my emphasis):

too bad magazines/manufacturers don't also publish their engine's efficiency peaks along with the other performance info.

You said torque and
 

You said torque and horsepower peak, so that's what I posted, :p

115 horse @ 6100 110 lbs/ft@
 

115 horse @ 6100
110 lbs/ft@ 4500


at 50 km/h (28 mph)

1st gear (max out)
2nd gear (4500)
3rd gear (2500)
4th gear (1800)
5th gear (1200)

I'll get my scangauge soon enough, then i'll know if its better to pulse agressively in 3rd (3000+ to 70km/h) OR switch to 4th and pulse slowly towards 70 km/h (38mph) until rpms reach (2400). I won't bother with 5th -_-;

You're average will actually be less than 36.5. Think about it, your glide time is longer at lower speeds than it is at higher speeds (ie. the amount of time you're gliding at speeds between 36.5mph and 40mph is shorter than the time you are gliding between speeds of 33 and 36.5) Because the slower you go, the less you are being slowed down by wind resistance and rolling resistance. You'd prolly average closer to 35mph, but yeah, that is the general gist of it. But the math is a little more complicated. I also mistakenly thought P&G was exclusively only for complete engine shut off hybrids (ie, the prius) I only recently found out otherwise.