Coasting in gear vs coasting in neutral

thinker5555 · #1 03-08-2009, 04:48 PM

Hi all,

I've seen many instances where people are talking about coasting in gear versus coasting in neutral and how coasting in gear causes the car to go into this "DFCO" mode where it cuts off the fuel flow to the engine and winds up using less fuel than coasting in neutral. I have to admit that this doesn't make much sense to me. How can an engine running at 2500 rpm be using less fuel than an engine running at 750 rpm? I have trouble believing that the fuel is just "cut off" because, well, the engine is still running. The engine can't run without the combustible material to burn, so it's obviously using fuel. Soooo... can someone help me see the reasoning to people saying that the engine isn't burning any fuel during DFCO?

Thanks,
Jeremy

oldlar · #2 03-08-2009, 05:01 PM

I have tried coasting in neutral when there is no traffic around. If you have an instant mpg meter you can see what the mileage is as you coast. I have just the average mpg meter on my car, and have played by resetting the monitoring device and have seen dramatic increase in mpg. More than what I see by just coasting in gear.

The problem is that if you need to accelerate, you are in neutral and have no power which can be dangerous.

At 750 rpm, you are using less fuel and maintaining speed..works wonders going down hills, rather than 1500 to 2000 rpm when in gear and coasting in gear.

ksstathead · #3 03-08-2009, 05:03 PM

The computer cuts fuel flow when it senses you want to slow down as in approaching a stoplight. It can turn 2500 rpm without fuel because of the momentum of the car's mass. This is engine braking. It saves both fuel and brake wear.

It is not always the most fuel efficient technique, since the engine braking reduces glide distance markedly compared to neutral glides. But it is an ingeneous fuel saver in those cases where you need to throw away momentum (steep descents, light turns red as you approach, someone ahead slows unexpectedly, etc).

Most fuel injected cars have dfco. I've read that Hyundai's do not have it. And I'm sure there are other exceptions. My cars all have it.

seftonm · #4 03-08-2009, 05:05 PM

Hi Jeremy, the engine doesn't need to burn any fuel because the vehicle momentum is keeping it spinning. Your vehicle momentum causes your wheels to rotate, which is transferred through your transmission to your engine. So if you lift off the accelerator, your car does indeed cut fuel flow to nothing yet your engine can continue to spin.

MaxxMPG · #5 03-08-2009, 06:33 PM

As others confirmed above, DFCO is programmed into the computer of most cars today. Even in cars with an automatic, DFCO is available because the powertrain controller can keep the converter at least partially locked and maintain the connection between the wheels and the crankshaft. To get to fuel cut, a forward gear must be selected, the engine must be fully warmed up, and the engine speed needs to be above a specific threshold. Under those conditions, the fuel supply is cut off and the rotating wheels supply the twisting force back through the final drive, transmission, and clutch (or locked converter) to keep the engine spinning.

Generally speaking, you use neutral for gliding on level ground when the drag of the spinning engine would otherwise cause road speed to fall off too quickly. When coasting to a stop sign or red light or other obstruction, or coasting down hills, leave the car in gear and DFCO will allow you to cover that distance using no fuel as opposed to the thimble of fuel it would use to idle in N during that stretch. At lower speeds, or when I need to lose more speed on approach to a light that is due to change, or a traffic slowdown, I will downshift my auto-trans car to a lower gear to push the engine speed up and keep the fuel cut going. When decelerating in L1 at low speeds, I find I feel the "bump" of the fuel injectors switching on just under 1000rpm. At higher speeds, though, they seem to return at about 1100-1200rpm. I think the computer reads the rate of deceleration and will restore fuel flow at higher engine rpm when the speed is dropping faster in order to prevent stalling.

thinker5555 · #6 03-08-2009, 06:48 PM

Thanks for the replies everyone.

So if I understand it correctly, when coasting in gear, there's no spark in the piston chamber, and the pistons are moving only because the drivetrain has the rest of the engine engaged? Sounds like how I feel a lot of days: the wheel is turning, but the hamster is dead.

Jeremy

Jalen · #7 03-08-2009, 07:44 PM

I'm curious about something though, myself. Do even Standard transmissions to this? They're designed to be in neutral between shifts and when slowing down right? or am I waisting fuel by slipping between gears to coast into stops, etc?

ksstathead · #8 03-08-2009, 07:52 PM

Same rules apply to manual tranny's. If you need engine braking, leave it in a gear giving enough rpm to invoke and maintain dfco. I don't know if it sends fuel during shifting down to a lower gear, but I doubt it, provided shift occurs in a timely manner.

So, for glides, go to neutral. For slowing, leave in a gear and even downshift for dfco.

MaxxMPG · #9 03-08-2009, 08:27 PM

Quote:

Originally Posted by thinker5555

So if I understand it correctly, when coasting in gear, there's no spark in the piston chamber, and the pistons are moving only because the drivetrain has the rest of the engine engaged?

It's not the spark that is being switched off, it's the fuel supply. I tend to think they'd keep the spark plugs firing.

Quote:

Originally Posted by ksstathead

Same rules apply to manual tranny's. If you need engine braking, leave it in a gear giving enough rpm to invoke and maintain dfco. I don't know if it sends fuel during shifting down to a lower gear, but I doubt it, provided shift occurs in a timely manner. So, for glides, go to neutral. For slowing, leave in a gear and even downshift for dfco.

As soon as the clutch or brake pedals are pressed, the fuel supply is switched on. Upon selection of a forward gear, and a few seconds after the clutch pedal is released, the DFCO resumes. Ditto for brake pedal application in most cars - it shuts off the DFCO, although it will return after a few seconds, as long as all the other conditions are met. For that reason, I try to slow my AT-equipped car whenever possible by downshifting instead of pressing the brake pedal.

taylormah · #10 03-09-2009, 05:31 AM

Quote:

Originally Posted by MaxxMPG

As soon as the clutch or brake pedals are pressed, the fuel supply is switched on. Upon selection of a forward gear, and a few seconds after the clutch pedal is released, the DFCO resumes. Ditto for brake pedal application in most cars - it shuts off the DFCO, although it will return after a few seconds, as long as all the other conditions are met. For that reason, I try to slow my AT-equipped car whenever possible by downshifting instead of pressing the brake pedal.

So what is the best way to slow down on my AT car, making sure that DFCO stays on?? If it needs a threshold rpm or speed to keep dfco running, when we slow down to stop the car, wont the rpm/speed eventually fall below that threshold and turning Dfco off?
and How does downshifting to 3, 2 or L gear helps keep engine speed up?