driving technique: pulse & glide (long)

[jamescartagena]

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.

[MetroMPG]

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.

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Exploring 'Pulse and Glide'

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[GasSavers_Diemaster]

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.

[MetroMPG]

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).

[JanGeo]

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.

[Matt Timion]

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.

[MetroMPG]

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?

[Matt Timion]

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.

[MetroMPG]

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.

[MetroMPG]

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.



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