"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

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

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. :mad:

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.)

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.

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.

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.

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.

is that thing about engine oil amount true? only fill up to the lower limit and you get less oil pumping losses?

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.

I tried the open throttle on the Clunker when doing my 13 mile downhill. I switched off the key, and then tried going downhill in gear with the throttle both closed and open. It changed the sound of the engine, but didn't reduce engine drag at all. I think the only way to make that work would be to also hold the intake valves open, and my engine's an "interference" engine, so I think I'd end up with bent valves anyway.

Not oil pumping losses... but it does create less work for the engine to do, if the crank is churning that oil... you can get away from doing this by installing a windage tray... not only do you get the benefits of NOT risking oil starvation, but you will actually gain a few HP at the flywheel from the windage tray shaving oil off the crank, reducing the rotating weight (and thus the power needed to spin it) of the crankshaft. It's kinda like adding a lighter flywheel... it doesn't give you more horsepower, it just lets you use more of what's already being created.

I think this has alot to do with the engine size and type, among other considerations. To accurately see how it affects coastdown times, you'd have to AABBAA test it, honestly. It's not by and large a huge difference, but in some cars, there is a difference.

My loose-engined Civic, for instance - I would be able to modulate 3-5 MPH difference at the end of a long coast on the same stretch of flat road, when coasting from the 45mph stop sign to the same curve (really tight curve, have to slow down to like 20 or so) Usually, by the time I hit that curve, I'd be around 30 MPH... but using the throttle with the engine off, I could keep it around 33-35 MPH over that same 1000ft stretch. Of course, engine RPM also has alot to do with it, and I would never take it out of 3rd gear while doing this.

Ah, okay. I think my expectations were too high, and maybe it does make a difference, but not as much difference as I expected.

Frictional losses are most definitely the biggest culprit for engine braking... Imagine it this way -

You've got a 300 lb person sitting on your chest, and you're flat on your back on a hardwood floor, with your mouth and nose covered by a slightly porous wet rag. You're laboring to breathe at this point...

Remove that rag, but leave the fat guy. Now you can breathe freely, but your chest has trouble rising to get an "easy" breath...

When you open the throttle, you're just taking the rag off your engine's face - get it?

Great imagery; now I'm going to have nightmares. :)

I love DFCO mode. This mode helped me go down about 1 litre on 100km. ( 5 to ~4).
Verified by refueling, driven over 100km (105) with average consumption 4,12 l/100km. (my personal record).

Driven at highland ( hills and downhills) and flat road.

Pulse and dfco mode at flat roads i tried also. But i have not data yet.


Engine-off glide i don't use because i'm too much scary :-/

So do I!
That said, I only use DFCO as a substitute for my service brakes. For Decelerating and controlling my speed on a downgrade. From my point of view, those are the only scenarios that warrant its use.
It's helped me get fairly decent fuel economy from my "pony". It saves brake pads and fuel.
YMMV