[mikesee]

I live on the edge of the rockies and I travel into/through them 3 to 4 times a month. In the interest of saving my trans and increasing fuel economy, i often shift over to N and let the van coast some, on some of the descents. Not always the best decision, especially if traffic is heavy or the grade is super steep, so I only do it when/where appropriate. If I don't know the road, I tend not to coast.

But for many of the CO highways a substantial mileage increase can be realized by coasting. "Hypermiling", as it were. Not to mention that you don't have the frustration of the trans constantly hunting for the right gear.

I've got 35,000 miles on the van now. Took the van in for an oil change and tire rotation yesterday and they rated the front brakes as 9/10, and the rears as 10/10. Even with the heavy use on the aforementioned descents, at this rate these brakes may last the life of the van!

I've read anecdotal (and vague) statements about how coasting is bad for the trans, or the catalytic system. Can anyone shed some real light on this, without repeating said vague and anecdotal info?

Thanks much.

 

[Wowbagger]

It shouldn't be bad for the transmission, all that happens is the drive shaft is disconnected from the gears. It is no different for the emissions system than extended idling - it could build up some particulate in the filter that will need to be burned off, but since the engine is basically not consuming any meaningful amount of fuel that shouldn't be a problem, and will burn off when you climb back up.

What it is bad for is control - in neutral the engine cannot contribute to the braking of the van, and you will have to ride the brake to control speed. That will use up the brake pad, and will heat the brake rotor, potentially leading to brake fade and rotor warping from the heat.

The mileage difference between coasting and using engine braking is minimal to zero - if you are doing it right you are idling, and allowing the engine friction to replace the brakes. The system is designed to do that; you aren't stressing anything or wearing anything out.

 

[Zoomyn]

Actually - these engines computer can completely shut off fuel supply at various times so while coasting and using engine braking there could/would be no idle fuel injected, no mechanically metered baseline amount supplied that would keep engine running smooth, the computer will feather fuel back in as RPMs drop down towards idle. It may be minuscule but it'd be higher mileage just letting it rev through its paces and not neutral coast.

One benefit I see for that would be helping dilute/displace any fuel slogging of the piston rings, get some fresh synthetic oil goodness higher up toward piston crown without the heat & pressure of combustion working against it to help keep diesel from a) diluting lubrication w/ increased cylinder wear, and b) dilute/soften diesel trying to carbonize in the rings and freeze up the rings in their grooves... Yeah, a tiny benefit trick that counts towards a 500,000-mile commercial motor life...

 

[papab]

I used to coast in neutral in my gas engine cars when appropriate. Gas engines have more engine braking than diesels so there is more to be gained. As zoomyn stated when the engine is idling (in neutral) you're using more fuel than if you're coasting in gear. Also more buildup in the DPF which results in more problems & more fuel used to burn it off. I use manual mode and keep it in 6th when appropriate for my downhills. The speed difference between neutral coasting and 6th gear is pretty small in this van.

 

[mikesee]

I have a hard time understanding how people can claim there is no difference in mileage between engine braking and coasting in neutral.

If engine braking, on the passes described in my original post, *best case* I'm at 1500rpm's. If in neutral I'm at 800rpm's. Multiply that by 15 to 20 minutes of descending.

Cutting revs in half uses a lot less fuel. I've kept track, it's a no brainer. And since I've already noted that in 35k of driving this way my brakes have shown essentially no wear, that's not a concern either.

Anyone have an answer to my original question?

 

[Zyzzyx]

I'm fairly sure that the PM diesel (and other modern diesels) behaves the same modern cars have for 20 years. When in an engine braking situation, any fuel input is turned off. The injectors simply do not inject fuel. I also figure this to be so by the way that the instantaneous MPG reading goes to 99.9 when in engine braking.

Whatever fuel you use while idling is going to be infinitely more than the zero fuel used while engine braking.

 

[RDinNHandAZ]

I agree with Zyzzyx on this one so I go over to manual and put it in 6th. I also like the idea I can have a gear if I needed to accelerate due to a runaway RV or other nutcase. Small matter perhaps. Idling is another thing all together. I don’t do it, if I can help it even shutting down when flaggers stop a line of traffic for what looks like 5 minutes or more. Nothing good can come from idling. Of course I am so cheap I do it with my gas vehicles too.
Sorry for the vague and anecdotal info. I haven’t seen any real data. It’s a good question.

 

[mikesee]

Whatever fuel you use while idling is going to be infinitely more than the zero fuel used while engine braking.

Help me understand how twice as many RPM's can equate to less fuel used?

 

[RDinNHandAZ]

When the engine is braking no fuel is injected as no energy needs to be created to make the engine run. It is holding the vehicle back by pumping losses which all engines have as they are glorified pumps. In idling a small amount of fuel is injected to keep the engine turning as power is required to do that. It has nothing to do with RPMs (try to put that non-factor out of your head). That being the case putting the engine in neutral to coast will use more fuel than engine braking.


https://en.wikipedia.org/wiki/Engine_braking
An opinion:
https://www.quora.com/Does-a-car-consume-more-fuel-when-coasting-downhill-than-when-idle-and-why

 

[Ziggy Stardog]

Makes me wonder what they do in regenerative braking systems for hybrids.

They must have decision points when it's best to use the engine to brake or charge the batteries, and when to shut the engine off or have it idle etc.

 

[mikesee]

When the engine is braking no fuel is injected as no energy needs to be created to make the engine run. It is holding the vehicle back by pumping losses which all engines have as they are glorified pumps. In idling a small amount of fuel is injected to keep the engine turning as power is required to do that. It has nothing to do with RPMs (try to put that non-factor out of your head). That being the case putting the engine in neutral to coast will use more fuel than engine braking.


https://en.wikipedia.org/wiki/Engine_braking
An opinion:
https://www.quora.com/Does-a-car-consume-more-fuel-when-coasting-downhill-than-when-idle-and-why

I'd still love an answer to the original question.

As for coasting vs. engine braking fuel economy, I tested it repeatedly in the first year of ownership. Coasting down gains me 1.5 to 2 mpg over engine braking. It's not even close.

 

[RDinNHandAZ]

I was thinking the discussion following your OP had done that but reviewing your question I am not sure it has or hasn’t so here goes. Coasting in neutral is the same as idling. 1. Our EGR, and DPF do not handle it well. 2. Internal wear is greater idling than running the engine. 3. The diesel produces very little wasted engine heat so the engine may not stay up to temperature.

From your Diesel Manual
"Engine Idling
Avoid prolonged idling, long periods of idling may be harmful to your engine because combustion chamber temperatures can drop so low that the fuel may not burn completely. Incomplete combustion allows carbon and varnish to form on piston rings, cylinder head valves, and injector nozzles. Also, the unburned fuel can enter the crankcase, diluting the oil and causing rapid wear to the engine."

http://www.teletrac.com/teletrac.com/a-assets/the-real-effects-of-engine-idle-time-time03.pdf

 

[mikesee]

Coasting in neutral is the same as idling. 1. Our EGR, and DPF do not handle it well. 2. Internal wear is greater idling than running the engine. 3. The diesel produces very little wasted engine heat so the engine may not stay up to temperature.

From your Diesel Manual
"Engine Idling
Avoid prolonged idling, long periods of idling may be harmful to your engine because combustion chamber temperatures can drop so low that the fuel may not burn completely. Incomplete combustion allows carbon and varnish to form on piston rings, cylinder head valves, and injector nozzles. Also, the unburned fuel can enter the crankcase, diluting the oil and causing rapid wear to the engine."

http://www.teletrac.com/teletrac.com/a-assets/the-real-effects-of-engine-idle-time-time03.pdf

Closest thing to a concrete response thus far. Thank you.

 

[RDinNHandAZ]

It is a good question and interesting from an engineering point of view. We need to get rid of the energy being gained from descending in a gravitational field and we normally do it by producing heat in several ways. Running the engine without fuel so the pumping occurs which essentially heats the air on compression and extracts heat during the closed off intake stroke of the engine is one. Some mechanical heat is lost to the internal moving parts of the engine too. Much of the energy is turned to heat through friction, a small but significant amount in the rubbing surfaces in the engine, transmission, bearings, tires, etc., but most by the brakes' friction. In regenerative braking as much of that energy as possible is recovered by having the electric motor act as a generator and the electricity is then pushed into the vehicle’s substantial battery. Generally the engine is disconnected and the low rolling resistance tires allow more to be captured. The motor is capturing the mechanical motion and a computer raises the motor's recovery to balance the need to maintain speed. No need to friction away the energy- recover it electrically.
So that we speak the same language a motor coverts electricity energy to mechanical motion and an engine converts chemical energy to mechanical motion. A hybrid has both of them and to say my van has a motor is not technically correct, (it has an engine) although we all use the words out of place.