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-   -   Pulse and Glide? (https://www.fuelly.com/forums/f8/pulse-and-glide-857.html)

Pete7874 01-07-2009 04:52 AM

Pulse and Glide?
 
Has anyone heard of this?

Pulse and Glide

This technique sounds so counter-intuitive that I can't wrap my head around it. Maybe in case of hybrids, but all cars? Can someone offer me a viable explanation how this can result in better MPG than just cruising at constant speed?

pb 01-07-2009 04:58 AM

[Just a note--I edited the link in your post to point to a more descriptive explanation of pulse & glide at a less ad-laden site.]

cee 01-10-2009 07:21 AM

..Pete: i think in modern vehicles, the fuel injection system is advanced enough that, when you reach optimal speed for a given stretch of road and you then "glide" (ie. use no acceleration), you're essentially using very little gas on the "glide" portion of the ride..

..i have an automatic transmission, and i use this kind of tactic a lot on minor hills & inclines..for ex: i'll accelerate to speed coming up on a mild downhill slope & then coast just prior to cresting the hill, using gravity to maintain (or even increase) speed on the down side..

..or maybe that's not what "pulse & glide" is actually about (i dunno, being basically a car Dunce..heh!)..

..but it makes sense that if you accelerate modestly to speed and then let off the gas pedal & coast, you're basically running at idle speed during the coasting..and idle speed burns less gas even than trying to keep a steady speed using cruise control or manual foot adjustments (for the most part)..

..just my pennies for thought..others will have better, i'm sure..

l8r!

--c.

[owner of T'Pal; 2002 Subaru Legacy Outback]

bates 01-11-2009 08:12 AM

The theory works basically by the idea that you accelerate for a realtively short period of time then coast for a realitvely long time before accelerating again. In most modern fuel injected computer controlled automotive and truck engines when the throttle is closed (your foot is totally off the accelerator pedal) and cruise control is not operating will completely shut off the fuel delivery to the engine. So basically you are burning zero fuel for your coasting sections so if you can accelerate, admittedly buring more fuel than in a steady state cruise, for a short time say 15 seconds from 50 to 65 MPH on the freeway, then you coast for 45 seconds from 65 to 50 MPH. Then you do it again. You should see a fair increase in mileage from this tactic. Unfortunately it is somewhat dangerous and not very practical for most drivers in urban or suburban areas due to traffic congestion. It will also take you somewhat longer to travel the same distance as driving at the median speed would as it takes longer to decelrate than it does to accelerate, hence the fuel savings realized. Finally it isn't the best method for prolonging your engine life when the fuel is shut off the amount of oil lubricating your engine parts is substanitally less than it is a cruise RPM (oil delivery is a linear function of RPM from a cam or crank driven oil circulating pump) so bringing on the power will cause some additional wear to those parts when the engine goes from 700 RPM up to 2,500 RPM as soon as the accelerator pedal is depressed. Also this tends to be hard on the cylinders and head since they cool substantially during the coasting segments and reheat rapidly during the acceleration segments. Thermal cycles are very hard on internal combustion engines and can substantially reduce their life expectancy.

foxspit 01-12-2009 07:01 AM

I try to drive using a modified version of pulse-and-glide. City traffic doesn't allow pulse-and-glide very often, but in trying to drive that way I have found I can anticipate traffic better and cost a LOT more than I used to. This requires being able to see three or four cars in front of you, so if you're stuck behind a pickup or an SUV, then it won't work.

That said, I can't say I've significant increases in my in-town mileage. The most dramatic increases I've found have been moderate driving on highways and interstates. Drop 5 mph and watch the mileage increase.

VWJunky 01-12-2009 09:55 PM

I tried it a bit today on the high way. I could pulse about 2 seconds and glide 4 seconds. With my scan gauge the pulse was about 27 mpg and glide was with fuel shut off so I would think I am almost doubling my 27 mpg? fun to try but I migth wear out my pedlle and leg! LOL!

e

Pete7874 01-13-2009 03:27 AM

> In most modern fuel injected computer controlled automotive and

> truck engines when the throttle is closed (your foot is totally off the

> accelerator pedal) and cruise control is not operating will completely

> shut off the fuel delivery to the engine.

My car is manual, so if I leave the car in gear and the the foot off the accelerator - it's true, the fuel consumption is zero. But then the deceleration is rather quick. If I wanted to slow down the deceleration, I would have to put the car in neutral, and in neutral the car does consume fuel.

> So basically you are burning zero fuel for your coasting sections

> so if you can accelerate, admittedly buring more fuel than in a

> steady state cruise, for a short time say 15 seconds from 50 to

> 65 MPH on the freeway, then you coast for 45 seconds from 65 to
> 50 MPH.

See, that's the thing. If I coast in gear (so that fuel consumption is zero), going from 65 to 50 mph will take 15 seconds, not 45. So I don't think that will really save me gas compared to just cruising with constant speed.

The real hardcore pulse&glide people suggest to put the car in neutral and completely turn off ignition during the 'glide' phase, but to me, that's not only illegal (in some states) but also dangerous - you lose your power brakes and power steering. Plus the added wear&tear on your started/alternator.

rem83 01-13-2009 09:30 PM

Pete - pulse and glide is generally considered a dangerous technique that probably shouldn't be used under normal driving conditions. i think odds are pretty good that you would reduce fuel consumption just by shifting into neutral and coasting, since you're getting rid of the majority of engine drag by keeping the revs low like that. i think, on older cars anyway, if you're moving, you can actually restart the engine by putting it into gear, but that's probably even worse for your clutch and transmission than using the starter is on your starter / alternator / electrical system. I guess the best solution would be to get a mild hybrid which is actually designed to cut off fuel / ignition during those low demand situations.

VWJunky 01-14-2009 12:26 AM

cuting the car off while move would be suicide in my car due to the fact that the steering wheel locks, not worth the pennies in savings in fuel costs. Want to save like that take the bus or ride a bike.

The savings in insurance and not owning a car is greater than pulse driving! LOL!

e

rem83 01-14-2009 04:38 AM

usually cars have 2 off positions - there's a kill switch position (just turn the key a little) and a full off position, which is when the steering locks. i discovered this the hard way on my e30 - the accelerator got stuck on the floor mats when i was on the highway and i put the car into full off, the steering locked, i lost power brakes, and i eventually went into the guard rail on a gentle bend, although i was only going about 20 at the time.

VWJunky 01-14-2009 05:31 AM

yep you are right REM83, I was just remembering a close call with a similar situation except in a 1973 vw thing on I-85 N just north of South Hill VA when my ignition switch died. I rigged up a push button for a start and a pull swith to turn it on. Yea I know I had lots of spare parts in the car that I could make that thing work with a shoe string. Worked good. I did this on the side of the highway, A highway patrol man even stopped to see what was going on but I did not need it. any way I got it going down the high way with traffic for about 20 mins when my exit came up, I pulled off and rigth at the top of the ramp my steering wheel locked!!! I had driven 20 mins with the key in my pocket and it could have locked at any sudden change of lane and thank god the road was strait the whole way! man I tell you that my life just about flashed in front of me! LOL! I went off the road only at 15 mph and stopped ok. what a day!

carlsr 01-14-2009 11:32 PM

After reading these very informative posts, I thought I'd contribute a little bit.

Firstly, if your engine is on, the fuel is NEVER completely cut off...remember there is something called the power (or combustion) stroke in the 4-cycle engine that we all use. This is the process of fuel exploding in the cylinder thus forcing it down and creating the necessary revolutions of the driveshaft that moves us. I'm sure we all know that, but the general speak was that fuel is completely cut off when your foot is removed from the gas pedal. Not true, gas will always be used as long the engine is on.

The Pulse and Glide method attempts to capitalize on the kinetic energy created by the small bursts of acceleration and works best on flat roads. Addressing the concern of cylinder temperatures...well, we don't typically keep our foot on the gas pedal while we are driving (or coasting) downhill...thus the pulse and glide method is almost inherent in some of our typical driving. Some drivers just take it a step further. This process of coasting downhill would have a far greater effect on the temperature of the cylinder heads than a pulse and glide on flat land as the cylinder heads would cool much quicker. And again, the temperature of the cylinder heads would not cool much using the pulse and glide method due to the fact that there is still combustion occurring keeping the cylinders heated.

Some serious hyper-milers will take the pulse and glide method too far by turning off the engine and coasting for as long as they can...but you can see why. The fuel is truly off and they are then achieving infinite MPG at that point. Still, definitely not recommended. However, the pulse and glide method can be good as a speed check as well. I know I can get a heavy foot the longer I leave my foot on the gas pedal to "maintain" a constant speed. The pulse and glide method can allow you to check your speed and re-evaluate your driving methods. We tend to forget how inefficient we are driving...how many times have thought your were driving 65 mph and then look down...whoa! 80mph!

rem83 01-15-2009 12:59 AM

Are you positive that on a car with computer controlled fuel injection, the computer doesn't turn off the injectors while the car is coasting in gear? It's not actually necessary to have fuel in the cylinder during the power stroke if the engine is kept spinning by the rest of the drive-train (using the car's kinetic energy). With a carb, you're definitely still letting fuel into the engine through the idle circuit.

VWJunky 01-15-2009 02:38 AM

I agree with Rem83 on the fuel shut off. At least on my Jetta TDI it will shut the fuel off all together when coasting in gear, the motor is moving but with the help of the wheels turning the transmission. There have been a lot of discussion with folks in the TDI world about this. I would think most newer cars with fuel injection this would also be the case. The Hemi cars and trucks shut down cylinders and I am sure they are not puting fuel in cylinders that are not running.

e

Pete7874 01-16-2009 07:29 AM

> I'm sure we all know that, but the general speak was that fuel is

> completely cut off when your foot is removed from the gas pedal.

> Not true, gas will always be used as long the engine is on.

I don't know about automatics, but manuals definitely cut off all fuel when coasting in gear. The engine is running because the wheels (car's momentum) are turning it, not because it burns gas.

If you have a trip computer with momentary fuel consumptions, you will notice that the consumption drops to nothing when you stay in gear but take the foot off the gas pedal.

Check the response next to last in this thread:

https://action.publicbroadcasting.net...t/1242810.page

Also this:

https://wikicars.org/en/Manual_transmission#Advantages

"Fuel cut-off. The torque converter of the automatic transmission is designed for transmitting power from the engine to the wheels. Its ability to transmit power in the reverse direction is limited. During deceleration, if the torque converter's rotation drops beneath its stall speed, the momentum of the car can no longer turn the engine, requiring the engine to be idled. By contrast, a manual transmission, with the clutch engaged, can use the car's momentum to keep the engine turning, in principle, all the way down to zero RPM. This means that there are better opportunities, in a manual car, for the electronic control unit (ECU) to impose deceleration fuel cut-off (DFCO), a fuel-saving mode whereby the fuel injectors are turned off if the throttle is closed (foot off the accelerator pedal) and the engine is being driven by the momentum of the vehicle. Automatics further reduce opportunities for DFCO by shifting to a higher gear when the accelerator pedal is released, causing the RPM to drop."

rem83 01-16-2009 10:06 AM

I know that on my manuals (I guess the bike's dont count because they use carbs) the engine starts to sputter and stall if I let the revs get below idle while coasting.

VWJunky 01-16-2009 09:42 PM

good one Pete, I would also think that if it gets below idle rpm that it should turn the fuel back on. Carbutrator are a different matter. ;)

Pete7874 01-17-2009 03:42 AM

Yup, you need to be above certain (rather low) rpm level for the fuel cut-off to work.

dkruitz 01-26-2009 09:10 PM

From what I'm reading this fancy term of pulse and glide (really over accelerating and coasting) seems impractical for all but a few driving conditions with certain cars. Additionally it appears dangerous - turning off your engine disables your power steering too. Hardly seems worth the risks. If you're that worried about your mileage drive conservatively with less junk in the trunk and plan your trips to avoid extra miles. I've hit a high of 28.1MPG with a V8 loaded with the family - that is unusual - 25-26mpg is more common - keep the speed constant and at the limit.

VWJunky 01-26-2009 09:20 PM

dkruitz, you are correct. I think just using the accelerator pedal only would be ok but not the turn the car off thing. To crazy for a few cents in savings.

Pete7874 01-27-2009 09:50 AM

dkruitz, VWJunky, that's basically my take on it as well. The reason I started this subject was because I thought I was missing something here. You'd be surprised how many P&G hardcore fanatics are out there though. This isn't something I'd be able to try out here in Chicago area, even if I wanted to... I'd get shot by other drivers probably.

VWJunky 01-27-2009 08:56 PM

yes you would. I think with the lower the price on gas the less people will do this but of course if prices go back to where they were, somepeople can squeeze blood out of a turnip or at least another quater miles out of a gallion of fuel! LOL!

With the economy like it is I just don't drive that much anymore. That saves lots of fuel!

rem83 01-27-2009 09:36 PM

P&G may be a relatively main-stream technique (compared to some of the other stuff that hyper-milers do), but I think a lot of the hyper-milers aren't in it to save money, they just want to see how efficient they can be (kind of like people who race just want to see how fast they can go, not how much time they can save). I doubt those folks have stopped just because gas prices have dropped off. While this seems pretty dangerous, stuff like removing your external mirrors, taping up body gaps and intake holes and removing excess weight from the vehicle, etc... is also quite risky.

jhu 02-22-2009 01:57 PM

Pulse and glide - something that I was skeptical of until I actually tried it. It really does work. So what does this entail? The idea is to accelerate to a higher speed at about 3/4 throttle, then coast in neutral to a lower speed. Once the lower speed is achieved, put the car in gear and accelerate again to the set higher speed (eg, accelerate to 70mph, coast to 60mph to get an average speed of about 65 mph).

Now when the car's coasting in neutral, you can keep the engine on or off if you're driving a manual. If you're driving an automatic, turning off the engine will damage (most likely depending on your car, check your owner's manual) your car's transmission. If you're driving a manual and you coast with engine off, you can bump-start the engine instead of using the starter. Be aware that with the engine off, you'll lose power steering and lose power brakes after 3 full pumps of the brake so don't do this until you get some practice in low traffic areas such as an empty parking lot.

So, why does this work? Well, at cruising highway speeds your vehicle will really only need to use 20hp. But your car's engine is most efficient at low rpms and high engine load.

Results: I used to get about 30mpg on the highway. Now getting low-to-mid 40mpg on the highway is common.

coolbreeze 02-26-2009 11:11 AM

with a stick p$g is no more dangerous than shifting gears. it works and is as natural as shifting and causes no inherit danger in normal driving conditions. i would not recommend in city driving but for rural roads and quiet highways it is a natural practice that bears great mpg results. cutting off your engine is a different matter altogether.


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