Does Coasting Reduce Piston Ring Contamination?

[jtay2024]

I was thinking about this on the way to work this morning as I coasted up to a set of traffic lights.

Do you think cars that are coasted a lot are less likely to clog piston rings. (Some cars seem to be more likely to develop this issue than others from what I've read).

My logic being that during coasting

A) You are getting 0 contamination added to the rings, only cleaning.
B) On gasoline engines you create a vacuum in the cylinders which I'd think draws oil up into the ring packs on the cylinders giving it chance to clean out more.

I am just wondering whether this would help people with stuck / clogged piston rings?

 

[Oro_O]

That’s the polar opposite of the theory and practice of the “Italian tune-up.”

Somewhere at this moment, Jeremy Clarkson is shedding a tear.

https://www.motorbiscuit.com/italian-tune-up-redlining-remove-car-engine-carbon-deposits/

 

[Kira]

A) If the engine is running, it's still burning fuel.
B) The vacuum increase is in the intake system, not the cylinders.

Where the drive side and coast side of ring and pinion gear wear can be observed, I don't know if the rings shift (up and down in their grooves) any between the two conditions.

 

[MRC01]

A) If the engine is running, it's still burning fuel.

Not necessarily. Modern cars can shut off the fuel when the gas pedal is all the way up and the car is coasting. Especially with a manual transmission, where the wheels will mechanically drive the engine. An automatic may need a tiny bit of fuel to keep the engine spinning.

B) The vacuum increase is in the intake system, not the cylinders.

Agreed. When coasting and no fuel, the cylinders still have compression, though obviously less than when fuel is introduced and power is generated.

 

[IndyIan]

I was thinking about this on the way to work this morning as I coasted up to a set of traffic lights.

Do you think cars that are coasted a lot are less likely to clog piston rings. (Some cars seem to be more likely to develop this issue than others from what I've read).

My logic being that during coasting

A) You are getting 0 contamination added to the rings, only cleaning.
B) On gasoline engines you create a vacuum in the cylinders which I'd think draws oil up into the ring packs on the cylinders giving it chance to clean out more.

I am just wondering whether this would help people with stuck / clogged piston rings?

I don't think it helps too much? I've never heard of the manual transmission cars of known oil burner engines having less chance of oil burning? For piston rings sealing on a new engine I think its a good idea to do long engine braking sessions, so maybe it helps a bit on carbon'd up rings too?

 

[Fordiesel69]

With non turbo, modern oils ring clogging should not be an issue.

 

[JohnnyG]

According to a GM engineer, high vacuum downshifts do indeed create large amounts of vacuum in the cylinders, thereby causing the rings to "flutter" in their grooves. This helps keep the carbon build to a minimum and helps free them up.

 

[Oro_O]

With non turbo, modern oils ring clogging should not be an issue.

It’s design-specific. Poor ring design or inadequate drainage will make any engine do it, NA or turbo.

 

[totegoat]

More fun to wind my engines up once in awhile.

 

[ARCOgraphite]

I was thinking about this on the way to work this morning as I coasted up to a set of traffic lights.

Do you think cars that are coasted a lot are less likely to clog piston rings. (Some cars seem to be more likely to develop this issue than others from what I've read).

My logic being that during coasting

A) You are getting 0 contamination added to the rings, only cleaning.
B) On gasoline engines you create a vacuum in the cylinders which I'd think draws oil up into the ring packs on the cylinders giving it chance to clean out more.

I am just wondering whether this would help people with stuck / clogged piston rings?

I don't know what you mean by coasting, you might leave it in gear and foot off the throttle. I would define "coasting" as out of gear, in other words - placed in N. Not engine braking, Not effective.

Can't hurt at all pulling some clean oil up into the ring lands with engine braking.

And to refute what others have stated (above), there is less than atmospheric pressure in the cylinders during intake stroke. OTW, Q: How do you think they might become filled with air? A: By atmospheric air pressure > cylinder pressure. Open system equalisation of a Δ P.

 

[jtay2024]

I don't know what you mean by coasting, you might leave it in gear and foot off the throttle. I would define "coasting" as out of gear, in other words - placed in N. Not engine braking, Not effective.

Can't hurt at all pulling some clean oil up into the ring lands with engine braking.

And to refute what others have stated (above), there is less than atmospheric pressure in the cylinders during intake stroke. OTW, Q: How do you think they might become filled with air? A: By atmospheric air pressure > cylinder pressure. Open system equalisation of a Δ P.

Engine braking yeah. It's called coastin regardless of in or out of gear I believe here in the UK.

 

[jtay2024]

Not necessarily. Modern cars can shut off the fuel when the gas pedal is all the way up and the car is coasting. Especially with a manual transmission, where the wheels will mechanically drive the engine. An automatic may need a tiny bit of fuel to keep the engine spinning.



Agreed. When coasting and no fuel, the cylinders still have compression, though obviously less than when fuel is introduced and power is generated.

I believe even in an automatic it still turns the engine via momentum of the car as the torque converter basically just acts in the other direction.

 

[PontiacHO]

High vacuum scenario with throttle closed definitely creates the opportunity to draw oil/crankcase vapor past the rings and into the cylinder. More of that to burn without the typical shot of fuel/air. I view that is more opportunity for ring land carbon buildup.

 

[DGXR]

With more vacuum in the cylinder during deceleration, coupled with the potential lack of a combustion event (DFCO), blow-by will be reduced or possibly eliminated. This presents the possibility of more oil getting up into the ring packs... along with more fluttering of the rings within the lands during high cylinder vacuum, the result being the rings do not get stuck or less likely to get stuck/carboned up. That's what I am understanding here... someone school me.

 

[dbias]

Living in WV i always do a lot of coasting down the hills and mountains. I have never had any vehicles that burned oil and have always emphasized engine braking when breaking in a new engine.
I don’t know if it helps but I’ve never had an oil burner.

 

[PontiacHO]

Ignition still firing that mixture (oil) in the high vacuum - closed throttle scenario and there’s the potential for carbon…

 

[SHOZ]

Modern cars if you coast in gear will reduce the fuel injector pulse width to idle feed so the engine doesn't die. Coasting in neutral will not do this. With autos the torque converter should lock until you touch the gas again depending on speed.

I always consider coasting in gear as a means if getting wear on the 'other' side of the normal motion. Especially with a manual transmission.

 

[MRC01]

I'm not sure that coasting actually increases the amount of pressure fluctuation (?) because the pressure is always fluctuating.

When coasting in gear (for example down a hill) with the throttle closed, the cylinder will have more negative pressure (stronger vacuum) during the intake stroke than it does during normal cruise, then positive pressure during the compression stroke simply due to the engine's compression ratio. So the cylinder pressure will snap back and forth between negative & positive.

However, during normal cruise part-throttle operation, the same thing happens of course: negative pressure during intake stroke and positive pressure during compression & combustion. The negative pressure won't be as strong but the positive pressure will be much stronger.

In a NA engine, pressure during the intake stroke is always less than ambient, though at full throttle it can approach ambient pressure.

So either way, full throttle, normal cruise or coasting, the cylinder pressure is always snapping back and forth over a wide range from intake to compression stroke. Yet during coasting, the average pressure (BMEP) is much lower, though I'm not sure whether it actually goes negative. It seems that would depend on the intake, throttle, valve timing & compression ratio.

But even if the average pressure never goes negative during coasting, it would still be much less than normal operation, which may change the forces on the piston rings. So the answer to the OP question is ... maybe? It at least seems plausible.

 

[tcp71]

Higher (than idle) RPM with the throttle closed and injectors off creates the highest vacuum in the cylinders. Happens all the time in the MT CX-5. Off throttle down a hill the instantaneous fuel consumption readout goes to zero.

 

[JohnnyG]

More fun to wind my engines up once in awhile.

That's not the same, and doesn't work for the ring pack. Tops of pistons, maybe.