Pulse and Glide with the engine running? - Fuelly Forums

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Old 12-02-2008, 06:26 AM   #1
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Pulse and Glide with the engine running?

I'm talking about the VX here but other cars may apply.

If you were to pulse to 70mph, leave the engine running, car in fifth gear and glide down to 50mph, clutch engaged, the injectors will shut off during the glide with engine braking. Which would still be a fuel savings. The thing is that the car will slow down to 50 mph faster. But how much faster? Or how much of a distance difference?

I'm asking because I live in a congested area and pulse and glide is not an option.

I was wondering if someone may test this and let me know of the results?

Just curious. Seems like a safer, easier pulse and glide method.

Thanks.

, Bill
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Old 12-02-2008, 06:44 AM   #2
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That's called DFCO - Deceleration Fuel Cut Off. It's great if you already intend to slow down.

However, it doesn't save any gas at all if you intended to keep going at speed. That's because all the same work is done - the car is moved through the air, the drivetrain is turned the entire time, and the engine is turned the same quantity of revolutions.

Normal engine-on neutral-coasting P&G saves gas because the engine idles at low RPM while the car continues to coast without slowing much. Of course you should never exceed your comfort level, do anything you think is dangerous, or do anything that is illegal; but if it's not illegal or uncomfortable, you should be able to P&G safely with the engine on.

Turning the engine off during a P&G cycle's coasting segment, a strategy that I call P&EOC (a term which I hope will catch on), additionally saves gas for the obvious reason that you're not idling the engine during that time. As with

The 70 to 50 mph cycles you're talking about must be very unpleasant. I tried the same strategy, which I called P&DFCO, for my car on the highway due to P&G being difficult for me to do at those speeds when I first started driving it -- I had difficulty rev-matching the idle->3000rpm shift. Indeed, it was unpleasant, jumping on and off the gas, accelerating and then engine braking repeatedly. However, it did not save gas; in fact, it cost me a few MPG.

Tangent note: I use the term P&EOC to differentiate it from P&G which usually means leaving the engine on, and to be more specific than just EOC which isn't necessarily part of a P&G cycle -- one could EOC when descending without ever having pulsed and without intending to pulse afterwards.
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Old 12-02-2008, 07:03 AM   #3
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Thanks for the info!

Quote:
The 70 to 50 mph cycles you're talking about must be very unpleasant.
What would be an optimal speed range?

, Bill
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Old 12-02-2008, 07:23 AM   #4
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Originally Posted by neverendz View Post
Thanks for the info!



What would be an optimal speed range?

, Bill
You got a VX... I got a 1990 Civic with DPFI. P&G'ing, I found out that 50-60 is better than 50-65 or 50-70. Now I'm trying to figure out the correct amount of throttle to give it on the pulse. Initially I tried using my aftermarket cruise control unit that gently accelerates. But now I'm trying to give it heavier throttle to pulse up to 60 to see what works better since some other Honda owners have seen success with that.

Whether you got some mileage instrumentation or not it takes a lot of experimentation. Good luck.
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Old 12-02-2008, 07:46 AM   #5
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The momentum lost due to engine braking will outweigh the fuel not used due to DFCO. You'll do better by maintaining a steady speed than trying to pulse-and-dfco.

Best: P&G, engine off glide
Good: P&G, engine on, neutral glide
Ok: steady speed
Bad: Pulse-and-dfco

With the VX, though, you have the option of running in lean-burn at moderate speed, which should be at least as good, or possibly better. That might move steady speed up into 2nd place on the list above.
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Old 12-02-2008, 08:45 AM   #6
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Yes, Lean burn will complicate things.
My car has lean burn, although it works differently than the lean burn mode in a VX.

I noticed no benefit to P+G on flat ground, regardless if the engine was on or off during the glide. This is because holding a steady/light throttle lets the ecu use very lean A/F ratios. The 'pulse' portion of P+G uses heavier throttle and the ecu uses a richer A/F ratio at this time.
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Old 12-22-2008, 09:04 AM   #7
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Yes, Lean burn will complicate things.
My car has lean burn, although it works differently than the lean burn mode in a VX.

I noticed no benefit to P+G on flat ground, regardless if the engine was on or off during the glide. This is because holding a steady/light throttle lets the ecu use very lean A/F ratios. The 'pulse' portion of P+G uses heavier throttle and the ecu uses a richer A/F ratio at this time.
When i have no draft targets insight i just eoc from 62 to 52 mph or if lazy just use the cruise control at 57 mph. Using eoc yields slightly better results for my car but takes a lot of effort. If there are no cars even close i will slow to 45 mph and pulse to 55. The freeways here have a lot of humps for the overpasses so depending how i feel that day i use the above methods.
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Old 12-22-2008, 11:15 AM   #8
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I think the original idea might be made to work, although I doubt the FE gains would be impressive. Instead of a full DFCO throttle chop, apply just enough gas to roughly offset the engine drag. Your car will slow at a rate close to it's coast down deceleration. During this glide phase, you'll be using a lot more gas than at idle, but less than you would at steady-state.

Overall efficiency would depend on the battle between the peak BSFC pulse, and the low efficiency (but low power) semi-powered coast period. I'm guessing that FE at highway speeds would be slightly improved over steady-state.

At slower speeds, where you can gear the engine to run at low revs, this technique should work pretty well. Engine drag is vastly reduced and you're probably not using a whole lot more gas than at idle.

(A slipper clutch would sure be nice, eh?)

-Moo
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Old 12-23-2008, 01:52 PM   #9
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Quote:
Originally Posted by Geonerd View Post
I think the original idea might be made to work, although I doubt the FE gains would be impressive. Instead of a full DFCO throttle chop, apply just enough gas to roughly offset the engine drag. Your car will slow at a rate close to it's coast down deceleration. During this glide phase, you'll be using a lot more gas than at idle, but less than you would at steady-state.

Overall efficiency would depend on the battle between the peak BSFC pulse, and the low efficiency (but low power) semi-powered coast period. I'm guessing that FE at highway speeds would be slightly improved over steady-state.
Bad guess.
PaleMelanesian already gave the correct answer in post #5
Quote:
Originally Posted by PaleMelanesian
The momentum lost due to engine braking will outweigh the fuel not used due to DFCO. You'll do better by maintaining a steady speed than trying to pulse-and-dfco.

Best: P&G, engine off glide
Good: P&G, engine on, neutral glide
Ok: steady speed
Bad: Pulse-and-dfco

With the VX, though, you have the option of running in lean-burn at moderate speed, which should be at least as good, or possibly better. That might move steady speed up into 2nd place on the list above.
To add a 5th option to the above list, 'worse than bad' would be to decel using very light throttle. In fact, that would be worse than normal driving since you're using gas to keep the engine turning against it's own drag. If you're not using gas to propel the vehicle, then you're wasting gas.
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Old 12-23-2008, 09:43 PM   #10
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Originally Posted by DRW View Post
Bad guess.
PaleMelanesian already gave the correct answer in post #5
Brave (and rather rude) talk, but do you have the data to prove it?
If your car has a small engine and tall gears,the penalty for 'keeping the engine running' will not be that great. At the same time, the large benefits of a peak efficiency pulse are well established.

Quote:
If you're not using gas to propel the vehicle, then you're wasting gas.
No kidding, Sherlock. I never said it was an ideal solution.
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