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Old 01-15-2009, 01:52 PM   #1 (permalink)
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"Pulse and DFCO" (deceleration fuel cut off) doesn't work

I've seen a few descriptions of people mis-applying the pulse and glide technique on various web sites & forums.

More than once I've read of people describing the technique as simply repeatedly accelerating and letting off the accelerator (to achieve fuel cut). The logic being, "if I'm in fuel cut, it must be more efficient!"

Of course the penalty of being in fuel cut is that you've got engine braking, and the vehicle won't travel nearly as far as it will in a neutral, engine-off glide.

One person did a "pulse and DFCO" experiment and posted the results of his efforts on his web site. He noted 2-4% worse fuel economy based on the before/after "A" portions of his on-road A-B-A comparison over 575 miles for the "B" portion (using p & dfco in the "B").

Full details (see Part 5 of the page): http://sites.google.com/site/diyover...er-experiments

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Old 01-15-2009, 03:26 PM   #2 (permalink)
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Additional sources of this kind of misinformation:

- Wikipedia: Fuel economy-maximizing behaviors - Wikipedia, the free encyclopedia

which leads to:

- Ecomiling.com: ecomiling | slow down - save fuel | Lesson 5
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Old 01-15-2009, 03:42 PM   #3 (permalink)
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Can you imagine riding in the car with them as they drive like that? I'd pull my freeking hair out if I had any.
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Old 01-15-2009, 04:47 PM   #4 (permalink)
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What does DFCO stand for?

EDIT - nevermind, I found it...

I have to say that this isn't necessarily "misinformation" though... this might be a safer practice for those less inclined to pay attention while driving, like most of America, for instance. I can see myself that properly P&G could cause a dangerous situation for a person who isn't experienced enough to handle it, and might possibly even cause one for someone who has more experience than any of us do... we know the odds of that happening aren't very likely, but a large part of that has to do with skill level of the operator too.

I also have to mention that pulsing and DFCO don't work together... at all, except in some automatic cars (some of them basically unlock the transmission if you let off the gas, adding very little resistance via engine braking, although it probably doesn't provide for fuel cutoff either.)

However - DFCO alone does work, which is what those articles are both describing... neither of them says to accelerate like a jack rabbit then let off the gas and keep it in fuel cut mode, they simply say that in scenarios where you should be decelerating anyway, it's better to use fuel cut than hit the brakes, like in the case where you're going from a 55 to a 45, instead of slowing down, just let off the gas, and make sure your engine is above it's fuel cut RPM.

I really don't think either of those sites is spreading "misinformation" so much as warning people of the dangers (and there are dangers) of coasting in neutral and shutting off the engine while coasting. (in a vehicle not designed for this to happen.)
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Last edited by Christ; 01-15-2009 at 05:17 PM..
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Old 01-15-2009, 05:10 PM   #5 (permalink)
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DFCO = Deceleration Fuel Cut Off

There is a 2 mile section of road that I drive on between Baltimore and Washington that is slightly down hill. When I stay in gear in this area, I typically loose about 5 mph and gain 4 mpg. When I stick the car in neutral, I gain 2 mph and gain 4 mpg. I bet DFCO would be far less useful on a flat stretch of road.
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Old 01-15-2009, 05:19 PM   #6 (permalink)
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Does that stretch of road have a decreased speed limit on the downhill side? That is a situation where DFCO is worthwhile, when you'd have to slow down on the other end of your zero-throttle coast. Granted, coasting in neutral is going to net you a longer, higher speed coast, but you're still going to have to use the brakes to maintain legality due to speed, which is wasteful.

That's all I'm saying.
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Old 01-15-2009, 06:55 PM   #7 (permalink)
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The driver was going way to fast to see an improvement. At 70mph they aren't going to pick up much if anything in the way of a difference between optimal engine efficiency and engine efficiency cruising at that speed.

That said, it does work AFAIK, but the only way to get decent testing conditions for most vehicles is to "Pulse" up a hill, and let DFCO do the job on the way down, w/ much lower average speeds. Unfortunately this requires a pretty steep hill, but it works AFAIK, it's just that there needs to be a significant difference in engine efficiency cruising in gear at whatever speed compared to optimal engine efficiency.
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Old 01-15-2009, 07:19 PM   #8 (permalink)
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For the modder - fuel cut is merely a switch away.. since your fuel cut is partially controlled by the TPS, which is only a small pot anyway, you might be able to rig up another pot that shows the ECU 0 throttle whenever you want it to.

This probably won't work on speed/density (mass air flow) systems.

Anyway - the idea is that if you MUST leave it in gear, and leave the engine on, set it up so that you can manually cut the fuel, then open the throttle to 100% (least pumping losses for the engine). In this way, you can partially modulate the amount of engine braking that is actually being done.

This could be achieved using some type of switch and a rotary pot.
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Old 01-15-2009, 07:36 PM   #9 (permalink)
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is that thing about engine oil amount true? only fill up to the lower limit and you get less oil pumping losses?
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Old 01-15-2009, 07:54 PM   #10 (permalink)
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I only use DFCO when I need to slow down. It only happens above ~47 MPH in fourth gear, but if I'm going to end up slowing more, I'll turn off OD which drops to third and keeps it going down to ~35 MPH.

Sometimes it doesn't catch, in which case I can turn off OD, tap the brake to make it happen, then turn OD back on and it keeps the DFCO going. Strange, but it works.

Neutral is definitely the way to go, though.

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