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Old 12-27-2005, 06:26 PM   #1
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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

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Old 12-27-2005, 06:39 PM   #2
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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.
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Old 12-27-2005, 06:46 PM   #3
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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.



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Old 12-27-2005, 06:54 PM   #4
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Re: i thought of that too. they

Quote:
Originally Posted by MetroMPG
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.


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.
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Old 12-27-2005, 07:02 PM   #5
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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.
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Old 12-27-2005, 07:49 PM   #6
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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).
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Old 12-27-2005, 07:56 PM   #7
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Quote:i say theoretically,

Quote:
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).
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."
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Old 12-27-2005, 08:10 PM   #8
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Re: Quote:i say theoretically,

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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 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?

Quote:
This technique is also quite similar to what Fran Giroux describes in his newsletters as what he calls "porpoising."
i had heard that term too, but haven't read anything about it. do you have a link?

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Old 12-27-2005, 08:47 PM   #9
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Quote:does yours continue to

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

Quote:
Quote:
This technique is also quite similar to what Fran Giroux describes in his newsletters as what he calls "porpoising."
i had heard that term too, but haven't read anything about it. do you have a link?
Here is a link to Fran's newsletters on Hydrogen-Boost.com: http://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.



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Old 12-28-2005, 07:39 AM   #10
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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).
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