Neutral coasting with dual mass flywheel

sloinker · Aug 13, 2015

Been reading about coasting in gear versus coasting in neutral. I have some coasting opportunities leaving my house and going to work as it starts mostly down hill for a couple miles. I have tried both methods and the neutral coast returns better results over the same route and distance versus the in gear method. My concern is, does shifting into neutral, coasting in neutral, speed matching and shifting back into an appropriate gear cause any harm to the dual mass flywheel or throwout bearing. Premature wear? Seems silly to save a few bucks a week only to have to spend it on a clutch job some years/thousands of miles earlier than by not doing it at all. Any thoughts?

ezshift5 · Aug 14, 2015

...it's always a pleasure to address higher fuel efficiency methodology. Drifting along in neutral - for what seems to appear as little MPG increase - would seem contra productive to this sailor.

In other news, at 27,000 miles odo - my TDI is (seemingly) still continuing to break in.

Stay tuned! I can't really believe the numbers that have recently surfaced (sub term)

ez

turtleboy · Aug 15, 2015

I have a 6 speed manual and tried the difference between both when I first got my car and did not really notice much of a difference according to fuelly. If you have a scanguage it will tell you how much fuel you are actually using and you can calculate it out. As for wear and tear, I started showing signs of DMF a long time ago and I'm at 70k now. Dont think it has anything to do with coasting in neutral.

Franko6 · Aug 22, 2015

The DMF is only activated when idling. Any low-speed or long-term idling has a negative impact on the DMF, wearing it out. Once the DMF is above about 1200 rpm, it does not operate any different than a solid mass flywheel. The DMF's purpose is to smooth out the radical oscillations in crank rotation at idle, which protects other components, like the transmission itself.

We do not recommend idling the vehicle any more than necessary. In spite of the possibility of very minor fueling gains, the early replacement of the DMF, costing about $450 for the part and about $400 to replace, should be considered. After all, if operating the DMF correctly gets you 300k or incorrectly gets you 120k, that should be factored into your operating expenses.

turtleboy · Aug 22, 2015

Franko6 said:
The DMF is only activated when idling. Any low-speed or long-term idling has a negative impact on the DMF, wearing it out. Once the DMF is above about 1200 rpm, it does not operate any different than a solid mass flywheel. The DMF's purpose is to smooth out the radical oscillations in crank rotation at idle, which protects other components, like the transmission itself.

We do not recommend idling the vehicle any more than necessary. In spite of the possibility of very minor fueling gains, the early replacement of the DMF, costing about $450 for the part and about $400 to replace, should be considered. After all, if operating the DMF correctly gets you 300k or incorrectly gets you 120k, that should be factored into your operating expenses.

Thanks, I had no idea. I usually coast downhill in gear anyway, just feels safer.

Wankel7 · Aug 25, 2015

I would guess at idle the dmf is working hard to dampen the vibrations of the engine?

However I don't buy the dmf becomes a single mass at high rpms.

I am reminded of that fact when I perform a sloppy downshift and don't Rev match it perfectly. If a dmf driver doesn't notice this they have been driving a dmf too long and thinks that is normal

I would guess a sloppy downshift strains the dmf more than idling. Thoughts?

UFO · Aug 25, 2015

turtleboy said:
Thanks, I had no idea. I usually coast downhill in gear anyway, just feels safer.

What is coasting "in gear?" Do you mean engine braking?

turtleboy · Aug 25, 2015

I was just using the same terms that the OP used.

nkgagne · Aug 31, 2015

What Frank is saying is that at idle RPM, the heavy DMF's internal damping works hard to keep the clutch-mating portion of the flywheel at a constant speed while the crankshaft itself is speeding up in time with each fuel ignition and slowing down in the momentary lull in between (anybody with a lightened single-mass flywheel - or who's had a seized alternator pulley can attest to the existence of this variance in rotational speed at idle). This constant vibratory hammering on the springs and damping bits within the DMF cause wear to it.

Yes, certainly, torque shocks are hard on any rotational damper, but I'd hazard Frank is referring specifically to extra time spent idling vs being under steady torque at RPM higher than idle.

Gliding while out of gear is certainly conducive to MPG gains in the right circumstances (coasting to a stop sign that is WAY down the road, coasting on a mild downhill before terrain levels out again to gain some free momentum), but re-engaging gear is always a guessing game of where to "match" RPM back to, since your new speed is different than the speed when you slipped out of gear. This leads to more torque shocks, which also shortens DMF life.

Wankel7 · Aug 31, 2015

nkgagne said:
What Frank is saying is that at idle RPM, the heavy DMF's internal damping works hard to keep the clutch-mating portion of the flywheel at a constant speed while the crankshaft itself is speeding up in time with each fuel ignition and slowing down in the momentary lull in between (anybody with a lightened single-mass flywheel - or who's had a seized alternator pulley can attest to the existence of this variance in rotational speed at idle). This constant vibratory hammering on the springs and damping bits within the DMF cause wear to it.

Yes, certainly, torque shocks are hard on any rotational damper, but I'd hazard Frank is referring specifically to extra time spent idling vs being under steady torque at RPM higher than idle.

Gliding while out of gear is certainly conducive to MPG gains in the right circumstances (coasting to a stop sign that is WAY down the road, coasting on a mild downhill before terrain levels out again to gain some free momentum), but re-engaging gear is always a guessing game of where to "match" RPM back to, since your new speed is different than the speed when you slipped out of gear. This leads to more torque shocks, which also shortens DMF life.

Once you get to know the car and gearing Rev matching the downshifts aren't that hard.

Although the dmf makes it a little hard to tell when it is done perfectly.

If one can't tell the influence the dmf has on the car at all rpms they aren't paying attention.

MichaelB · Aug 31, 2015

I think this thread is starting to wear thin. Many contributors have had this false notion that coasting in neutral (idle) saves all kinds of fuel and is the right thing to do. Well my opinion is it is just another game you play unnecessarily with your car. For those that think it saves money (not just fuel) "Show Me". Now it's to a point where you have to play rev matching games as not to harm you DMF. Come on guy's just drive your car like a normal person would and your fuel economy will not suffer and your DMF will last as long as it can. And for all the new comers to this thread that don't read back. I repost Franks comment.

Franko6 said:
The DMF is only activated when idling. Any low-speed or long-term idling has a negative impact on the DMF, wearing it out. Once the DMF is above about 1200 rpm, it does not operate any different than a solid mass flywheel. The DMF's purpose is to smooth out the radical oscillations in crank rotation at idle, which protects other components, like the transmission itself.

We do not recommend idling the vehicle any more than necessary. In spite of the possibility of very minor fueling gains, the early replacement of the DMF, costing about $450 for the part and about $400 to replace, should be considered. After all, if operating the DMF correctly gets you 300k or incorrectly gets you 120k, that should be factored into your operating expenses.

nkgagne · Sep 1, 2015

It definitely saves fuel when circumstances allow (no traffic behind, long coast up to a stop sign/red light, not in a hurry, etc). Money overall, probably not much. In real life in suburbia, you're going to piss a lot of people off trying to hit a fuel number. (Oddly, a lot of people around here lately seem to be competing in a "slow race". Most could be beaten to 40 km/h by a road bicycle).

sloinker · Sep 1, 2015

When the traffic lights are favorable, I can coast above the speed limit for over two miles before a slow right hand on ramp. This cannot be accomplished in gear without adding throttle at various spots. On a cold car direct from my garage to the freeway on ramp, coasting in neutral, the lie-o-meter indicates between 55-64 mpg. The same trip leaving the tranny in gear yields 34-42 mpg. At the end of the day, the fuel saved over this 2.4 mile comparison won't pay for an early clutch replacement, so I have stopped doing it per the advice of this board. I may continue the practice in the future dependent upon further research.

sloinker · Sep 1, 2015

Here's an added thought. If coasting in neutral with a 6 speed manual deteriorates the DMF/Clutch assembly more rapidly than not doing it. What do you folks do at extended length red lights? Keep the clutch pedal depressed, shift to neutral and idle with the clutch engaged.....?

Wankel7 · Sep 1, 2015

MichaelB said:
For those that think it saves money (not just fuel) "Show Me". .

I have to pay money for my fuel. So ..... fuel = money. If I save fuel I save money

What type of proof would you desire to show that coasting in idle uses less fuel than coasting in gear? This is assuming you are coasting goal is to extend the distance you are coasting.

Rev matching your downshifts is just a basic smooth driving technique. Rev matching is easier on your clutch friction material and I guess your dmf....I didn't realize that until this thread.

I wonder how many people do rev match their downshifts?

Wankel7 · Sep 1, 2015

sloinker said:
Here's an added thought. If coasting in neutral with a 6 speed manual deteriorates the DMF/Clutch assembly more rapidly than not doing it. What do you folks do at extended length red lights? Keep the clutch pedal depressed, shift to neutral and idle with the clutch engaged.....?

Neutral clutch out. I always figured sitting in neutral with the clutch pedal in put additional wear on the throwout bearing.

MichaelB · Sep 1, 2015

Wankel7 said:
I have to pay money for my fuel. So ..... fuel = money. If I save fuel I save money What type of proof would you desire to show that coasting in idle uses less fuel than coasting in gear? This is assuming you are coasting goal is to extend the distance you are coasting.

if you don't have any hard data showing the savings by comparing coasting in gear vs coasting in neutral it's just hearsay.

Wankel7 · Sep 1, 2015

MichaelB said:
if you don't have any hard data showing the savings by comparing coasting in gear vs coasting in neutral it's just hearsay.

So if I find a mile of pavement I can have to myself....get the car up to say 60 when I start the mile....and then let the car idle neutral coast for one run and than try rev matched downshifts for the next one....then see which one used less fuel....would that do?

MichaelB · Sep 1, 2015

Wankel7 said:
So if I find a mile of pavement I can have to myself....get the car up to say 60 when I start the mile....and then let the car idle neutral coast for one run and than try rev matched downshifts for the next one....then see which one used less fuel....would that do?

What are you going to use to measure the amount of fuel used? The MFD? We all know that is not accurate it's been called the "lie o meter" by many here. I suggest that over a period of time and a couple of tanks of fuel, you do your coasting thing and on another tank you don't. Then hand calculate the amount of fuel used. To actually have accurate results you should repeat that several times and that would give you an average under numerous conditions I know that will take some time but it would be a much better method for confirming results. You don't have anything to prove to me but, you may prove something to yourself. Then several months from now you can post your results and I think all of us here will be very receptive to your results and much more inclined to believe them. One data point does not make a line.

Wankel7 · Sep 2, 2015

MichaelB said:
What are you going to use to measure the amount of fuel used? The MFD? We all know that is not accurate it's been called the "lie o meter" by many here. I suggest that over a period of time and a couple of tanks of fuel, you do your coasting thing and on another tank you don't. Then hand calculate the amount of fuel used. To actually have accurate results you should repeat that several times and that would give you an average under numerous conditions I know that will take some time but it would be a much better method for confirming results. You don't have anything to prove to me but, you may prove something to yourself. Then several months from now you can post your results and I think all of us here will be very receptive to your results and much more inclined to believe them. One data point does not make a line.

I would say my SG would be sufficient. It is normally correct within 0.2 gallons on a fill up.

UFO · Sep 16, 2015

MichaelB said:
if you don't have any hard data showing the savings by comparing coasting in gear vs coasting in neutral it's just hearsay.

It's all about momentum. You will preserve it if you coast, and lose it if you engine brake. So if you can use the momentum, it's going to be worth the 0.03gph the engine consumes at idle, but if you have to stop for a light you will be better off engine braking.

I've been coasting in my Toyota pickup for years, and I get 40mpg with it commuting. I'll be posting data in the fuel mileage threads now that I converted my 2001 Beetle to a 02J.

By the way, I got a brand new DMF and clutch (with bearing and pressure plate) for less than $200. I am not concerned about idling, it's designed to support that.

MichaelB · Sep 16, 2015

UFO said:
It's all about momentum. You will preserve it if you coast, and lose it if you engine brake. So if you can use the momentum, it's going to be worth the 0.03gph the engine consumes at idle, but if you have to stop for a light you will be better off engine braking.

I believe it's more like .3gph or about 1Lph at idle. No way does the car use 3.84 oz of fuel per hour at idle. Dream on.

UFO · Sep 16, 2015

MichaelB said:
I believe it's more like .3gph or about 1Lph at idle. No way does the car use 3.84 oz of fuel per hour at idle. Dream on.

I was off, but it's not 0.3 gal/hour, it's 0.1gal/hr. There are some VCDS graphs posted that show consumption at idle between 0.3 and 0.4 liters/hour, so that is at most 0.1 gal/hour.

My Scangauge shows 0.03 gallons/ hour, that's where I got the number. It may be wrong because I have an 11mm pump and automatic nozzles, with an ECM re-coded to manual and my adaptation has not yet been adjusted.

MichaelB · Sep 16, 2015

UFO said:
I was off, but it's not 0.3 gal/hour, it's 0.1gal/hr. There are some VCDS graphs posted that show consumption at idle between 0.3 and 0.4 liters/hour, so that is at most 0.1 gal/hour.

My Scangauge shows 0.03 gallons/ hour, that's where I got the number. It may be wrong because I have an 11mm pump and automatic nozzles, with an ECM re-coded to manual and my adaptation has not yet been adjusted.

So I can idle for ten hours on one gallon of fuel? If I understand what your telling me.

UFO · Sep 16, 2015

MichaelB said:
So I can idle for ten hours on one gallon of fuel? If I understand what your telling me.

You do not have to believe me. Here is the data I mentioned:
http://forums.tdiclub.com/showthread.php?t=200694&page=5

Neutral coasting with dual mass flywheel

Veteran Member

Veteran Member

Well-known member

Vendor , w/Business number

Well-known member

Veteran Member

Veteran Member

Well-known member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member

Veteran Member