Consumption at idle vs non-dfco

[Kodak]

When coming to a stop, engine braking consumes no fuel and is the best way to slow down if you have enough momentum to make it.

But DFCO only works above a certain rpm. I think mine is around 1800, because I can kind of feel a very tiny bump when the revs fall below that, and the engine braking starts to diminish. (I could be wrong about the rpm, but it's my best guess).

Here's my question: If you can make it to a stop by either: coasting in neutral, or coasting in gear at an rpm too low for fuel-cutoff, which consumes less fuel? Do both options simply consume the same gph as idling?

[UFO]

That's something that may depend on the vehicle and the distance to the stop. Best way to answer that for yourself is to monitor instantaneous mileage or fuel consumption while trying either technique.

[groar]

It will depend how much is injected at each injection in both modes and the rpm at both modes.

If the same amount is injected in both modes then the one with lower rpm will consume less.

IMHO when in engine break under DFCO's min RPM then the injection is certainly bigger to avoid the engine to stall.

I tried once to see consumption in neutral and in gear at low rpm. In 1st, 2nd & 3rd gear (not tested over) idle uses less gas. IIRC being in gear at lowest speed the car could roll without touching to any pedal, 1st was using 20L/100 (12mpg) and 2nd 10L/100 (24mpg); while idle at same speeds was using much less.

Have fun,

Denis.

[Kodak]

Originally Posted by UFO

That's something that may depend on the vehicle and the distance to the stop. Best way to answer that for yourself is to monitor instantaneous mileage or fuel consumption while trying either technique.

The option of a Scan Gauge has been on my mind a little bit. I know I'd break even with it within a fairly short amount of time. (I calculated just shy of $8 savings for even a modest 5% gain in FE per 1000 miles).

But to be honest, I'm weary of another gauge to look at. I've already done a good job of weaning myself off my tachometer, and am trying to get better at holding a constant speed without the speedo (I sometimes vary within a few mph).

I feel that while I might gain in FE, my eyes would be in the wrong place.

[euromodder]

Originally Posted by Kodak

Here's my question: If you can make it to a stop by either: coasting in neutral, or coasting in gear at an rpm too low for fuel-cutoff, which consumes less fuel? Do both options simply consume the same gph as idling?

Start from your idle fuel consumption.
Mine is 0.5L/100 km
Which means that when coasting at 10km/h (6mph), I'm consuming 5L/100km (47mpg, what the car is rated for) , so it simply isn't worthwhile anymore to coast with the engine on at these slow speeds - I don't use it much below say 15-20mph.

I try to arrive at that speed bracket to make a turn, or to brake to a stop if necessary.

My fuel consumption started to drop significantly when I started coasting (engine on) instead of using DFCO.

[user removed]

My Insight and my Altima both had instant FE bar graphs. The Insight would indicate 150 MPG coasting at 16 MPH. The Altima indicates 60 MPG at 16 MPH.
Both were max readings on each vehicles individual bar graphs. I coasted down to the speed where the graph just started to drop off the max reading.

Assuming these readings are fairly accurate, the Insight uses 16/150 (.106 GPH) gallon of fuel at idle. The Altima uses 16/60 (.267) gallon at idle (both are engine idle speeds when coasting).

I found that you can pulse and coast (engine on) and significantly reduce your average fuel consumption even when the engine is cold, and you can read the consumption when cold by coasting and measuring the speed at which you FE goes below the max gauge reading (Altima 60 max, Insight 150 Max).

I don't use engine off tactics in either car, due to the CVT transmissions.

@euromodder

Idle consumption should be in GPH or LPH since you can be idling and moving at different speeds.

@kodak

1800 RPM for DFCO sounds high to me. It does go to a higher RPM when you have any additional electrical loads, especially AC. Try decelerating in 3rd gear with no additional loads applied. Not saying you are wrong but DFCO on some cars works down to 800 RPM.

regards
Mech

[Kodak]

Originally Posted by Old Mechanic

@kodak

1800 RPM for DFCO sounds high to me. It does go to a higher RPM when you have any additional electrical loads, especially AC. Try decelerating in 3rd gear with no additional loads applied. Not saying you are wrong but DFCO on some cars works down to 800 RPM.

regards
Mech

Here's how I tested it last time, maybe I did something wrong.

I put it in second gear and gave it some healthy throttle. Then, I let it decelerate on its own. Around 1800 I felt a slight bump, so I assumed that was the injectors giving it gas.

I'll try it in 3rd to see if it behaves differently.

Is the DFCO rpm the same in every gear?

[Red_Liner740]

Mine kicks in at 1500...there is a definitive change in deceleration exactly at 1500...its at that point that injectors kick in to slow down the RPM drop as well as your Idle Speed Solenoid starts to open to allow idle air to start coming in.

its set at a higher RPM to "catch" the rapidly declining RPM that you would see in neutral if you blipped the throttle and let it fall back down. Without this feature, a lot of cars would stall because by the time idle speed comes the flywheel momentum would overcome the gas given and it blamo....stalled car.

[groar]

Anticipation + idle in neutral is the best option. The more you anticipate, the more you save, and the more you are safe. Don't forget that driving without brakes means you loose each time you use breaking, with the brake pedal or the engine break

Engine break is interesting only if you have been pushed to go too fast and you have to break, not only slow down.

Have fun,

Denis.

[SentraSE-R]

DFCO is definitely programmed differently for different cars.

From the repair manual, and confirmed by observation, my 2002 Nissan Sentra SE-R went into DFCO when engine rpms were >2500 rpm, while my 2006 Scion xB goes into DFCO with rpms >1500 rpm. In effect, the Sentra wouldn't enter DFCO at speeds below 65 mph in top (6th) gear. The Scion, OTOH, enters DFCO at any speed above 35 mph in top (5th) gear.

Worse yet, once in DFCO, the Sentra stayed in open loop for a full four minutes after exiting DFCO. So any fuel saved by coasting in gear was wasted during the next four minutes of open loop driving. The Scion, OTOH, resumes in closed loop after DFCO. It has no DFCO penalty. I don't think the difference is Nissan v. Scion philosophy as it's 2002 v. 2006 programming progress.

My wife's 2006 AT Hyundai Elantra doesn't have DFCO, from what I can see.