Coasting in neutral or with clutch down?

[KJSatz]

Either way (neutral/ICE-on* or DFCO**) you have to get the engine turning. In practice, neutral with it idling has been found to be more efficient. I can give you two reasons why that is the case.

i. Certain conditions must be met for DFCO, including a minimum RPM generally greater than idle speed (often the minimum is in the range of 1000-1500RPM). Thus the DFCO procedure involves rotating the engine more than the neutral procedure, and that extra rotation comes from extra momentum your car loses as you DFCO. Additionally, here you can see why a car can "creep" in first gear: eventually, your engine drops under the minimum threshold, and then fuel is injected, causing creeping.

ii. For neutral, you are burning gas to make the engine turn. For DFCO, you burnt gas in the past that (unfortunately with imperfect efficiency) contributed to your forward momentum that you are now using to make the engine turn. The additional step makes additional work-by-inefficiency inevitable, and so DFCO must be less efficient.

* Coasting in neutral with your engine running at idle.
** Deceleration fuel-injector cut off: under certain conditions, when the car is in gear and you take your foot off the gas, fuel injectors are shut off (zero fuel usage at that moment), and you "engine brake."

[DIMS]

Originally Posted by PaleMelanesian

That's true during that instant, but when you look at the whole trip, the balance tips the other way. I've tested pulse-and-DFCO against pulse-and-neutral idle and against steady speed driving. The results:

Best: pulse and EOC
Good: pulse and neutral idle glide
Ok: steady speed
Worst: pulse and DFCO

You only want to use DFCO if you're needing to lose speed, like coming up to a red light, or down a mountain. Any other time, it's a net loss in mileage. The drag from engine braking overwhelms the gains from no fuel being used.

My honda does DFCO down to 1200 rpm, and below that it restarts the fuel flow. You can feel a surge when this happens.

What is meant by Best, Good, Ok and Worst? Do you have data to help us understand the testing procedure and the end results data?

[PaleMelanesian]

No formal testing on this one. Just trying each enough until patterns start to emerge. Then, I focus on maximizing the one that works best. There is definitely value in controlled testing, but for my purposes of minimized fuel usage, I want to find the relative merits of different methods quickly and then move on.

For a given drive, like my daily commute, the different driving styles will give you gas mileage in the order listed. Pulse-and-glide gives me the best, while pulse and DFCO is the worst. As they say, Your Mileage May Vary. An automatic transmission car with decent gear ratios may do much better at steady speed, compared to my manual with short (high rpm) gearing.

[DIMS]

Originally Posted by PaleMelanesian

No formal testing on this one. Just trying each enough until patterns start to emerge. Then, I focus on maximizing the one that works best. There is definitely value in controlled testing, but for my purposes of minimized fuel usage, I want to find the relative merits of different methods quickly and then move on.

For a given drive, like my daily commute, the different driving styles will give you gas mileage in the order listed. Pulse-and-glide gives me the best, while pulse and DFCO is the worst. As they say, Your Mileage May Vary. An automatic transmission car with decent gear ratios may do much better at steady speed, compared to my manual with short (high rpm) gearing.

Thanks for being honest. Any chance that you remember what the difference between Best and Worst was?

[PaleMelanesian]

Best you can see in my signature, because that's how I drive all the time.

I've seen 45 mpg on the highway with the cruise control at 65. Pulse and DFCO was lower than that.

[cottonfox]

Originally Posted by MetroMPG

I'm with Leanburninating: I can't remember the last time I used the clutch to shift to neutral. Light pressure on the shifter as the transmission unloads, and it shifts out of gear like butter. I rarely glide with the clutch in.

Is that better/worse/the same for the clutch? You're not pressing your clutch in which seems like it would be good, but wouldn't it make your tranny more upset (or at least have more of a possibility if you miss your throttle blipping)?

[PaleMelanesian]

There's no problem or wear moving OUT of gear without the clutch. All you're doing is pulling the gears apart. It's going into gear where it might be an issue.

I can do clutchless shifting if I want to, but usually I use the clutch.

[falfa]

Thanks alot for all good comments.

That the engine goes into fuel cut-off when when coasting in gear, is that also for diesel?

I could never have figured out that putting a gearbox in neutral without clutch and then gear back to 6th after would be more healthy then holding the clutch down.

So for a diesel:

Coasting in gear for breaking
Coasting with clutch in for short distances
Coasting with neutral for long distances

[LeanBurninating]

My last car was an 89 BMW 325i. It was a higher compression motor then my honda, but at one point I had an issue with a fuel pump relay intermittently cutting off fuel to the motor for a second or two... it felt like I was hitting the brakes, it would really startle me. I have to guess that this "fuel cutoff" mode gives at least a tiny bit of gas... But Im not a doctor.

As with the whole shifting with or without clutch... like stated already, shifting out of gear and into neutral without the clutch is pretty easy and if you do it right, it feels no different than using the clutch. The shifter will slide into neutral with no bumping or grinding. I would say any wear this causes is negligible. With a little practice you can get it right every time.

Shifting back into gear from neutral is a bit more tricky but still can be mastered. If you said THAT was bad for the transmissions lifespan.. I would say, if done frequently and incorrectly, yes. You just have to get a bit more intimate with your tranny. haha. When in neutral, look at your tach and try to predict where your RPMs would be in Xth gear at your current speed, and if you are perfectly right, the shifter will slide right into that gear. This takes lots of finesse.

Any way you choose to do it, I don't think the time it takes to use the clutch or shifter is a big deal. I think trying to say one is better or safer is splitting hairs. But do try to avoid riding the clutch (.02)

[vinny1989]

Originally Posted by falfa

Thanks alot for all good comments.

That the engine goes into fuel cut-off when when coasting in gear, is that also for diesel?

I could never have figured out that putting a gearbox in neutral without clutch and then gear back to 6th after would be more healthy then holding the clutch down.

So for a diesel:

Coasting in gear for breaking
Coasting with clutch in for short distances
Coasting with neutral for long distances

Gas (Petrol), Diesel, LPG.. Im pretty sure its the same for everything. If your not accelerating the engine gets either idle fuel or no fuel (Depending on engine, electronics, make(Ford, dodge.. etc) etc).

As for the coasting, that sounds about right. I suggest keeping your foot over the clutch and a hand on the gear stick when coasting in neutral. Saves a second or so when having to go back in gear.