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?
..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..
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.
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.
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!
> 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.
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.
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!
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.