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Old 09-29-2010, 07:24 PM   #1 (permalink)
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Neutral Coasting

I'm apologizing in advance if this topic as been beaten to death already, but if it has I couldn't find it on the site.

The car is a 2010 ford focus 2.0L 5 speed manual trans.
What I'd like to know is

1.) will coasting in neutral damage my car, I'm not concerned w/ excess shifter wear but I mean deep internal synchro or gear type damage??

2.) Is the fuel shut off during an In Gear Coast and while during Neutral Coast? or only one or the other?

3.) Any data or success anyone has been having w/ coasting or P&G in gear vs in neutral.

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Old 09-29-2010, 10:16 PM   #2 (permalink)
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1. I don't think so, but someone else might know otherwise

2. Only when in gear, as it requires that the momentum of the car is forcing the engine to turn. Switching off the injectors while in neutral would cause the engine to stall. Most cars with fuel injection will shut off the injectors while the the gas pedal is not being pressed and the RPMs are above a certain threshold (so the car doesn't stall). You can find your car's threshold by engine braking at low speed in 1st, there is a very obvious change (car stops slowing down/accelerates) when the revs get too low.

3. P&G gliding is usually done either in neutral (or with the clutch disengaged) with the engine on, or in neutral with the engine off. I think (but could be mistaken) that leaving the car in gear would mean that you decelerate too quickly for there to be a benefit over driving at a steady speed.
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Old 09-29-2010, 11:29 PM   #3 (permalink)
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Quote:
Originally Posted by dleefoto View Post
I'm apologizing in advance if this topic as been beaten to death already, but if it has I couldn't find it on the site.

The car is a 2010 ford focus 2.0L 5 speed manual trans.
What I'd like to know is

1.) will coasting in neutral damage my car, I'm not concerned w/ excess shifter wear but I mean deep internal synchro or gear type damage??
Absolutely not. Every time you shift, you are pulling it out of gear, then you clutch and re-engage it in another selected, appropriate gear. In the brief interim you are coasting in neutral.

You can even do this with an automatic transmission (providing it is not one of those that shouldn't be towed in neutral). Any potential problem or damage comes when putting the tranny back into gear, either by not clutching properly into an appropriate gear(with a manual) or, in the case of an A/T, shifting back into drive at a speed higher than the range of first gear (either at a full stop or at less than 10 MPH).

Quote:
3.) Any data or success anyone has been having w/ coasting or P&G in gear vs in neutral.
Either way you will save a small amount of fuel by doing it. Obviously the vehicle will coast or glide more if it is in neutral.
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Old 09-30-2010, 03:16 AM   #4 (permalink)
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I would doubt a manual will be damaged by neutral coasting Especially with the engine on.

In neutral if the engine is running it is using fue (but not much)l. In gear over 1000-1500 rpm with your foot off the gas you are using no fuel.

in gear pulse and glide will not save much if any fuel. With engine on neutral vs in gear glide, engine on neutral coast is better. If you are using in gear glide it means you glides are much shorter making you have to pulse and use lots of fuel more often.

You are better off driving at a constant, lower speed than using pulse and in gear glide.
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Old 10-05-2010, 02:21 PM   #5 (permalink)
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Quote:
Originally Posted by dleefoto View Post
1.) will coasting in neutral damage my car, I'm not concerned w/ excess shifter wear but I mean deep internal synchro or gear type damage??
Gear damages usually come from shifting under load.

Dunno about your driving expertise with manuals, but make sure you press down the clutch pedal completely in a quick push - quickly taking away the load - before shifting gears.

Over time, the bad habit of resting a hand on the shift lever when it need not be there, is another cause of damage as you load up the gearshift mechanism.



Quote:
2.) Is the fuel shut off during an In Gear Coast and while during Neutral Coast? or only one or the other?
Only in gear.
In neutral with the engine idling, you'll consume around 0.5L / 0.15 gallon per hour .

Quote:
3.) Any data or success anyone has been having w/ coasting or P&G in gear vs in neutral.
Coasting in gear means you slow down and effectively waste (kinetic) energy - unless you want to slow down.

On mostly level roads, coasting in neutral with the engine on is more efficient than driving at a constant speed for longer, then coasting in gear (i.e. engine braking) to slow down.

I used to be doing the engine braking thing a lot before joining ecomodder.
Coasting (and a grille block) is what really got my FE up.


On downhills, coasting in gear is the most efficient technique if the slope is steep enough to require continuous engine braking (no fuel being used at all).
On lesser slopes coasting in neutral is also highly efficient - I need a 4% gradient to maintain 100kph @ over 470mpg


I combine both techniques.
Coasting in neutral, engine on, to slow down slowly, then when I need to slow down quicker or have to stop, I switch to engine braking (coasting in gear).
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Last edited by euromodder; 10-06-2010 at 09:58 AM.. Reason: spelling mistakes
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Old 10-05-2010, 03:08 PM   #6 (permalink)
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1. Absolutely no damage with a manual transmission. Mine's fine after 181,000 miles and LOTS of neutral coasting. (nearly 4 years hypermiling)

2. Fuel cut off only happens in gear when you're engine braking.

3. If you let off the gas in gear, you're not coasting / gliding. You are engine braking. It's useful at times, when you must slow down. It's not very useful in a P&G routine. Rolling in neutral will make the most of your momentum, while only consuming a very small amount of fuel.
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Old 10-05-2010, 04:12 PM   #7 (permalink)
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1. As others have said I haven't had any yet (TDI with 6sp - 60K miles). Others have 3x that I see

2. When you are accelerating your engine is pushing the car, you use fuel. When you decelerate your car is turning the engine, you use no fuel - fuel is cut off. It is tricky if not impossible to get to the neutral state in between the two for any length of time.

3. You have to experiment to select when to you the different techniques. If you P&G on the flat, coast out of gear as you will coast longer. Same for shallow declines.
EDIT - in this state the engine is idling and it is burning fuel to idle.

For steeper declines you can use in gear coasting to control your speed and enjoy fuel cut off.

Even for relatively shallow declines you can use a higher gear and get the car to turn the engine over to get to fuel cut off.

Or you can shut the engine off on long out of gear coasts but remember your engine also provides breaking assist, power steering etc. which will also go away. I don't use this as I have a turbo.

If you have an instant MPG readout or an SG then you can see the effect of each one.

Good luck!
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Last edited by Arragonis; 10-05-2010 at 04:14 PM.. Reason: Idling, I am so Idle.
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Old 10-05-2010, 10:23 PM   #8 (permalink)
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You are good to go, no demage to your tranny, coast as much as you want
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Old 10-05-2010, 11:58 PM   #9 (permalink)
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I coast all the time. No damage. I like the strange looks I get when I roll through town at the speed limit with the engine off.....you can push my truck with one hand on level pavement.
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Old 10-10-2010, 02:22 PM   #10 (permalink)
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Euromodder's post was accurate and covered most of the bases. While it does seem to cover what you asked (transmission issues with coasting), it is important to keep in mind that the transmission is only one part of the system.

If we boil down the issue, the main issues are friction and shock. Friction will be absorbed by the tranny, just as designed. But, shock can have significant impact throughout the system- from the engine to the universal joints, along with any other parts that are under torque. The main things to worry about are universal joints and engine mounts. Engine mounts are great for absorbing vibration and small shock from proper gear changing, but they will not stand up to repeated improper gear changes or other sources of significant shock. The same goes for universal joints (and other parts that serve similar functions). Engine mounts, UJ's and other such parts don't usually catastrophically fail-- they cause increasing mounts of vibration and noise, while increase wear and tear on other parts of the vehicle until they are replaced. And, of course, the more wear such parts have, the lower your fuel efficiency-- not to mention that they cost money to replace (and even more if you pay someone else to do it).

Basically, cars are designed with the assumption that the driver knows what they are doing, with some leeway given for occasional mistakes.

So what can you do to avoid this? Basically, make sure there is no shock to the system, and the only thing the vehicle has to absorb is a little friction in the tranny. This is done by simply matching the engine RPM to whatever is needed whenever you re-engage the clutch (which varies depending on speed and gear, of course). If you feel (or hear) shock go through the vehicle, or see a sudden change in the tach, you did it wrong. It takes practice to get it right while the vehicle is moving, but it is easy once you get the hang of it.

I'm new to these forums, so I don't know what is common and not with driving techniques. But some of the posts above implied that some might try turning off the engine while coasting (in neutral). That also seems to imply that they will restart the engine at the end of a coast-- or, worse yet, restart the engine by simply re-engaging the clutch. If anyone does that-- STOP! Even disregarding obvious safety issues, any form of coasting while the engine is off causes far more harm than good. If the intention of doing so is to restart the engine and continue on your way, that is. I suppose it could do less harm than good (for your car) if your intention is to, say, coast into your parking space at the bottom of a hill (while the engine remains off), perhaps. I still wouldn't recommend that because of safety issues, though. The only exceptions are for vehicles (hybrids and the like) that are designed to do that automatically-- and that is only because the process is automatic (computer-controlled), and the engines are specifically designed for it. But you can't really not do that in those vehicles. So, if you're having to make a conscious decision to do it-- make sure that decision is "no."

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