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

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.


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:


Also this:


"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.

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