could someone explain DFCO and coasting to me?

[zero_gravity]

sounds dumb, but i have yet to find much detailed info on DFCO mainly and when to use as opposed to neutral coasting.

it was my understanding that the ECU will go into DFCO under higher rpm coasting...i.e. engine braking. any truth to this? what exactly triggers DFCO? when should i try to educe DFCO and when should i just neutral coast? i'm mostly a neutral coaster for now, but the more mpg's the better

btw i'm driving a '04 hyundai accent 5 spd. manual.

[theholycow]

In short:
You should DFCO when you were already planning on braking or slowing. If you intend to maintain your speed or slow just a little bit, you should neutral coast for best FE.

Longer explanation:
DFCO uses zero fuel, but the engine keeps turning at the same RPM. The energy to pump air through the engine, push past internal engine friction, pump oil and water, etc...that energy all has to come from somewhere. By DFCOing at 2000rpm (or whatever), your engine has to turn more revolutions to get to your destination -- and those revolutions cost energy as described above.

If you neutral coast, you spend a little gas idling at 850rpm (or whatever), but you save a lot of momentum. The total amount of energy spent is less and your MPG is more.

DFCO behavior:
Many people think that DFCO is dependable and happens any time you're over 1000rpm, in gear, and the throttle is closed. In reality, there are many conditions that must be met. Examples:

• My 2002 GMC 5.3l V8 automatic - After 8 seconds of >1500rpm, DFCO will begin, and will hold down to 1000rpm. I don't have more detail because DFCO is so useless on it.
• My 2008 VW 2.5l I5 manual - DFCO is immediate after steady-state cruising above 1000rpm. If I've just shifted gears, or was accelerating heavily, or just bump-started, it won't DFCO. If I'm below 15mph it won't DFCO no matter how many RPM.
• Others' Hondas - DFCO lower limit raises to 1300rpm with accessories (A/C, wipers, etc) on.

I highly recommend using something like the fuel rate meter linked in my sig for learning your car's DFCO behavior. It's the only sure and accurate way I've found so far. You can train your butt to detect it by feel but it's imperfect. A ScanGauge can tell but there's a delay before it's displayed, and that lag can make it difficult to truly understand.

[Jay2TheRescue]

MY 1998 GMC pickup with a 5.7 L V-8 will go into DFCO after 3 seconds of coasting in gear with the RPM staying over 1,500. DFCO will cancel at 1,000 RPM.

Here's my recommendation... If you want to slow down/stop then DFCO. I like to downshift and DFCO up to red lights. Moderate to steep hills, I'll DFCO all the way down. If you don't want to lose speed, like on a slight downward slope, neutral coast.

Also, on a somewhat related subject, I do not recommend EOC (Engine Off Coasting) unless the vehicle is rated as being "Flat Towable" otherwise transmission damage can occur.

[BEEF]

for your average person, DFCO is the way to go. it does slow you down somewhat but they are gas free miles and you have to do nothing to enduce it other than take your foot off of the gas pedal.

I have an auto so it is much better for me. I don't have the option of EOCing unless I want an injector kill switch and then I am not sure how I would start the engine back up.

anyway, until you get better info on your neutral coasting, I would say use DFCO. you could run your same daily run using DFCO and a second time using neutral coast and see what the scangauge says your average fuel economy is on each run. that would give you a better idea of which is better.

[zero_gravity]

hm looks like i'm going to be buying a scangauge when i have the cash. thanks guys!

[BEEF]

I just assumed you had one. most of us do.

[theholycow]

Originally Posted by BEEF

I have an auto so it is much better for me. I don't have the option of EOCing unless I want an injector kill switch and then I am not sure how I would start the engine back up.

I think we've had this discussion before, but...how would an injector kill switch help? Isn't the problem with the automatic transmission, which may get destroyed by EOC? You can kill/restart the engine with the key, just like you do when it's parked.

[BEEF]

I thought that depended on if it could be flat towed or not.

I didn't think you could start it back up while going down the road. it's monday and my mind isn't completely here yet.

[zero_gravity]

Originally Posted by BEEF

I just assumed you had one. most of us do.

been wanting to get one for a while now and i just don't have the cash at the moment. just got a house and did a big reno job so yeah....kinda broke.

[dkjones96]

I hardly ever DFCO in the Durango, it waits too long. The Tracker almost always did it, if you were on the freeway and let off completely it would DFCO the instant the torque converter unlocked. The Tracker would DFCO down Sandia like crazy and I'd even lose heat because of it. The Durango would coast down that mountain beautifully if it didn't DFCO, there is a noticeable difference in engine braking when it activates and then it goes too slow and I have to change gears or hit the gas. 2nd gear is too little engine braking(and not enough rpm for it to activate the cutoff) and 1st is too much with DFCO.

I don't have a Scanguage either. I have thought of getting one of those KIWI gauges tho.

[PaleMelanesian]

My take on the options:

DFCO - It's just another form of braking, that uses less fuel than regular braking. It's still throwing away TONS of momentum. Avoid by planning ahead.

Coasting - better than DFCO, because you're not throwing away momentum. There's a lot of energy stored in a moving car. If you start a coast sooner, you'll have less need to brake, be it dfco or regular. It does use a little fuel, unless you EOC, but the momentum savings are much larger. Do this whenever possible.

[theholycow]

Originally Posted by BEEF

I thought that depended on if it could be flat towed or not.

Yup, if it can be flat-towed without damaging the transmission, you can EOC. Most automatics cannot.

I didn't think you could start it back up while going down the road. it's monday and my mind isn't completely here yet.

I've definitely started these vehicles during EOC in neutral using the key:
- 1987 Cadillac Deville, HT4100 V8, 4 speed automatic
- 2008 VW Rabbit, 2.5l I5, 5 speed manual

I've almost certainly done it on these, though I can't specifically remember doing it:
- 1997 Pontiac Grand Am, 3100 V6, 4 speed automatic
- 2002 GMC Sierra, 5.3l V8, 4 speed automatic

I've also started the GMC in neutral while stopped or going very slow. The GMC and the Cadillac seemed to crank faster than normal when in neutral. I figure maybe the system is designed to put in extra effort when cranking in neutral or something, or maybe it's just my imagination.

I was experimenting with this stuff long before I ever thought I might have any interest in saving gas, back when I had money to blow, gas was under a dollar a gallon, and I was a reckless kid looking for adventure...

[Jay2TheRescue]

As a reckless kid looking for adventure I'd EOC in my mother's 1988 LeBaron just to see the instant MPG read 99 MPG.

[dkjones96]

In the Tracker, before I realized that it did dfco so willingly on its own, I used to turn the key one click back while in drive on the freeway to induce DFCO. As long as the rpms stayed above about 1200 there was enough line pressure for the transmission to keep the converter locked and in overdrive. Or, I would let off the gas and let the TC unlock before clicking it back.

There is a drawback to leaving the TC locked, the torque converter staying locked gives you less speed before the transmission lets go from a lack of line pressure but it also means the engine is able to apply more braking force to the wheels. As long as you stay above that critical point the engine can simply be restarted by turning the key back to on. This shouldn't have any adverse effects on the transmission but those of you that can't EOC will want to avoid doing this since once the speed falls enough for the transmission to let go you are EOCing.

As long as the line pressure is up I could downshift as well. The hydraulic system in the transmission was limited but it worked. With key off you only had OD 2nd and 1st. Fortunately, even if the converter was locked the hydraulic system in the transmission didn't allow for the TC to stay locked in first so even without computer control it unlocked.

[theholycow]

Originally Posted by dkjones96

used to turn the key one click back while in drive on the freeway to induce DFCO.

When doing that, it would make sense to floor the gas pedal the whole time. A closed throttle at those speeds represents a lot of pumping loss.

[dkjones96]

Originally Posted by theholycow

When doing that, it would make sense to floor the gas pedal the whole time. A closed throttle at those speeds represents a lot of pumping loss.

Actually, I once tested that by accelerating to the top of first and clicking the key off. It made no difference whatsoever what throttle position it was at the engine braking was the same.

That, however, is not indicative of pumping losses at any position being the same all the time. When an engine is running the engine pumping against a vacuum is a total loss, except for the work it does in the brake booster and other vacuum operated devices. At WOT, pumping against the large amount of air and fuel in the chamber goes towards the next power stroke and isn't a total loss. Now it is possible that the vacuum in the chamber at the bottom of the intake stroke does suck up the piston at the beginning of the compression stroke too and BSFC increasing with load is merely related to dynamic compression ratio and cylinder charge volume vs cylinder wall area ratios changing.

[R.I.D.E.]

Let your car decelerate in 3rd gear (when it is safe to do so) and when DFCO shuts off you can feel the engine apply power to the vehicle. This tells you how low your RPMs will be when DFCO stops working.

As an experiment use 3rd gear to slow down from about 30 MPH. Don't touch the brakes, and have all of your accessories off. Let it slow down until it stops slowing down. It will probably be somewhere near 1000 RPM when it stops slowing down. Don't touch the gas.

The car should slow down and when the engine gets near idle speed, you will feel the engine start to apply power to the vehicle, even though you have not touched the gas pedal. It will be like your car is idling in gear.

When you learn the threshold at which DFCO stops, then you can use DFCO instead of your brakes. Try to time your deceleration so you never have to come to a complete stop. You can downshift and maintain DFCO from higher speeds. Use the higher gears and keep your RPMs low, no need to have it go over 3000 RPM in DFCO.

AS Pale stated it is better to never give up your inertia if you can avoid it. DFCO is your choice when you have no option but to give up some inertia, and it saves your brakes. Do not engage in high rev downshifting. It's not necessary. When you get it down pat you will be in the correct gear to accelerate from the unwanted deceleration.

Example;

The light is changing red 500 meters ahead, use downshifting to slow down enough where you can go through the light after it changes back to green, at the highest speed practical.

If the light is 1000 meters ahead try to coast.

Use the brakes as your last resort, ususally when you get caught by a light and have no choice but to stop quickly. Learn the light timing on your route so you can maintain a constant speed if possible. I go through 50 lights on my daily drive, but I seldom have to stop for more than 5-8 of those lights.

regards
gary

[theholycow]

Originally Posted by R.I.D.E.

When you learn the threshold at which DFCO stops, then you can use DFCO instead of your brakes. Try to time your deceleration so you never have to come to a complete stop. You can downshift and maintain DFCO from higher speeds. Use the higher gears and keep your RPMs low, no need to have it go over 3000 RPM in DFCO.

I wouldn't be surprised if his 2004 Hyundai would refuse to DFCO after a shift, as my VW does.

[ecofarmer]

I have a semi off topic question but do think it’s pertinent to this subject.

When you let off the gas and if your injectors shut off or not don’t you start pulling air thought your PCV valve witch bypass the almost closed throttle body. If there was a way to let more air by the throttle body then would the engine spin with less resistance?

[zero_gravity]

oh she hits DFCO alright

thanks for the help guys, the info here got me playing on my lunch break. just let off the gas and coast in gear and i'm pretty sure it came on. took about a second and i felt a very small lurch and the engine noise change a tad. wasn't touching the clutch or anything so its gotta be.

i figured the basics, i was just confused by people saying 'coast in gear' as i figured it was only high rpms that would do it. but since i do a lot of city driving this could be useful. i've got the planning and coasting down, this is just the next step

thanks again!

[R.I.D.E.]

Understand pumping losses.

The more you pump the more you loose.

Opening the throttle increases the "effective" compression of your engine.

A perfect vacuum is O. Atmospheric pressure is 14.7 Psi. No engine creates a perfect vacuum.

Now if you have maximum compression, in most modern engines that 180-200 PSI. On the top of a 4 inch diameter piston that is 200X4X3.1416 PSI.
A perfect vacuum (which doesnt exist) would be -14.7X4X3.1416 PSI

Most modern engines only produce about 22 inches of vacuum. When you let off the throttle a bypass keeps the vacuum lower than a perfect vacuum, because if you had a perfect vacuum and fuel was being injected, it would not combust and you would dump the raw fuel into your catalytic converter.

DFCO eliminates the problem by eliminting the fuel. I dont know how you can not have DFCO in some form or the other, but I guess it's possible.

The pumping losses would be higher if you opened the throttle because you would have more compression. This is easily understood if you realize you must open the throttle when doing a compression test, in order to get the proper reading.

When you open the throttle during a compression test, the craking speed of the engine drops off, becasue the engine is doing more work creating higher compression. A compression test is the same as decelerating without fuel, just at higher revolutions.

If your compression ratio is 10 to 1 the pressure on the top of the piston is 10times greater than could ever be created by even a perfect vacuum. In fact it is almost 15 times greater becasue you do no have a perfect vacuum at idle.

Pumping losses consist of all energy losses relative to moving air through an engine. The actual individual losses are numerous, each contributing some percentage of total pumping losses. I think I can name something like 20 differect individual events that are all factors, and their percentages change relative to engine speed.

regards
gary

[R.I.D.E.]

One example. Many years ago Ford discovered that they could reduce the oil pressure in their new engines by simply making the pressure relief valve weaker. Any more than 35 PSI or pressure was a waste of energy, and they found that they could get an average of .5 MPG better mileage by just reducing the strength of the spring in the oil pressure relief valve.

Modern lower viscosity oils also reduce the energy needed to push oil through the engine, a fact we all know.

regards
gary

[theholycow]

Interesting. I thought that, when not burning fuel, air getting compressed gives most of the energy back on the power stroke, effectively acting as a spring; and I thought that all pumping losses are at the throttle, intake mainfold, valves, and exhaust.

I would think you need to open the throttle during a compression test to make sure pumping losses don't prevent the cylinder from filling properly.

[R.I.D.E.]

If your engine has port fuel injection, you can actually hear DFCO. Take a screwdriver and place it on the injector. Put the handle up to your ear and listen for the ticking as the injector cycles.

Rev up the engine and listen to the cycling frequency increase. Then let off the throttle and see if the ticking of the injector stops altogether.

Thats DFCO

regards
gary

[R.I.D.E.]

It does to a point Holy Cow, but consider this. When the manufacturers started using cylinder deactivation, they found they only increased mileage by 10 %.

Since the rotational inertai of the engine is already pulling the piston down, and the reciprocating masses are fighting the rotating masses every inch of the way, the benefit of the "air spring" effect is relatively small.

Thats my opinion, but I think the evidence supports it.

regards
gary

[R.I.D.E.]

Your point is valid HC, I think you are to smart to make any invalid points.

Without combustion the same air spring effect applies to pushing the piston down, during the normal power stroke.

In fact the only power produced by an engine is during that phase of the cycle, but you know that already.

Power during combustion is produced by the differential pressure due to combustion. The overall ratio is about 7 to one, the comparison of the kelvin temperature of the compressed charge to the kelvin temperature of the ignited mixture.

When you create a vacuum, the temperature of the air is reduced, so the air spring effect of that same vacuum on pulling the piston up is reduced as well.

When compressed the heat content of the compressed air is increased, which would increase its effective pressure somewhat. This may help its push on the piston on the downstroke, but it also hurts on the upstroke.

This can get complicated easily .

regards
gary

[theholycow]

LOL, I am definitely not too smart to make invalid points. I do it all the time.

I only cited the air spring effect as minimizing the energy losses involved in compressing more air; I didn't mean to imply that you'd get any net energy increase out of it during DFCO. If I could make that work I'd invent a perpetual motion machine and give you a cut of the profits!

[dkjones96]

A couple of points.

Not many of us here have 4" bores. It's more like 3" for the majority of vehicles on here so the calculation is 200*2.25*pi.

Secondly, you have to press the throttle when doing a compression test because the throttle plate being closed creates a vacuum which lowers the amount of air in your cylinders and thus your readings.

Also, raw fuel in the cat wasn't why they started DFCO or the bypass. An EFI vehicle can meter fuel however low it wants to so dumping raw fuel into the cylinders is a non-issue. Only a carbureted vehicle would need DFCO and it obviously can't get it. .

The bypass is used on a few different vehicles for different reasons. Toyota, for example, uses it to reduce afterfiring during engine braking. You can have a perfect stoich mixture but at high manifold vacuums exhaust gets sucked back into the cylinder during valve overlap and it dilutes the mixture to a point that it won't reliably fire without excessive amounts of oxygen.

[dkjones96]

Originally Posted by R.I.D.E.

When you create a vacuum, the temperature of the air is reduced, so the air spring effect of that same vacuum on pulling the piston up is reduced as well.

As temperature decreases density increases. This would increase the vacuum, not decrease it.

[ecofarmer]

I would think there would be a too much vacuum with it only being able to pull the air thought a half inch to three quarter inch line. This is what caused my earlier question.