could someone explain DFCO and coasting to me?

[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!