valve seat recession on LPG 2

[sierra4000]

Hello all,

What is cause valve seat recession on LPG ?
How to reduce this problem?

Thank You!
Radek

 

[GregLocock]

1 lack of lube in LPG
2 fit hardened valve seats

Cheers

Greg Locock


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[sierra4000]

Ok Greg, Thanks!

But

this 1. means, wear occurs when the valve is rotated?
or are related to high speed closing the valve at high RPM( valve to seat return)?
+ high temperatures etc.
I mean ,If for example are inappropriate cam lobe design ,clearance or spring preload, that more spring (or other) also may be help?

Radek

 

[GregLocock]

Sorry i don't know.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

There was a fairly long discussion about that and related subjects recently, started by a guy who paid for an expensive rebuild and declined the rebuilder's suggestion of hardened seats and learned the lesson the hard way.

I think the consensus was that there is no substitute for hardened valve seats, and no design phase magic bullet that can prevent valve seat recession on gaseous fuel with unhardened seats.


Mike Halloran
Pembroke Pines, FL, USA

 

[sierra4000]

Thank You!



Radek

 

[franzh]

Generally speaking, if the engine in question is capable of operating on conventional unleaded gasoline (petrol), and if the vehicle is driven in normal operation, no modifications are needed. I have operated several vehicles well in excess of 200,000 miles and never had the cylinder heads off. Conversely, I have had others complain about their exhaust valves receding after 60,000 miles. If the engine operates at stoichiometry using computer feedback, and ideally paralleling the original fuel injection system, I would expect good performance for quite a while. If the subject vehicle is driven at a heavier duty cycle, you might expect valve service may be required in the future.

One of your questions addressed valve rotation. Most of the engine builders who are familiar with gaseous fuels will disable the valve rotators by welding them, or exchanging them with another set of intake spring retainers. The valve tends to rotate slightly during lift and drop. It will tend to drop to the same place without rotators, but with them, the valve will turn slightly. A high speed spectroscopy study will show this process.

Franz

eng-tips, by professional engineers for professional engineers
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[Lou Scannon]

It is true that modern automotive engine exhaust valves are specifically engineered to deal with the relative lack of lubricity, if that is the right term in this context, of non-leaded fuel. Be that as it may, I'm aware of certain manufacturers that have specially engineered valves for engines that are intended for LPG or natural gas operation, and are warranteed as such. The non-gaseous fuel engines are not warranteed for extended gaseous fuel operation.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[sierra4000]

Thanks!

Also I wondered if it can be caused(partially) by improper ignition advance,
because I know that more advance = more torque on LPG.

If LPG burning slower, it might způsobit vyšší temperature of the exhaust gases?
Or even burn-in on start phase exhaust(when exhaust open)?
This could also explain the shorter life the catalyst?

I mean that the increased fire advance can help reduce the temperature of exhaust valves.

Radek

 

[swall]

Following up on what hemi said, if you take a look at SAE J1692, "Valve Seat Insert Information Report" you will find a table of the various commercially available seat materials and which type of engines/fuels they are recommended for.

 

[Lou Scannon]

Late combustion phasing will result in higher than normal exhaust temperatures irrespective of fuel. One would assume that a properly set up LPG application has proper combustion phasing. So, yes, I agree, improper ignition advance (retarded) could exacerbate valve seat recession.
I doubt that engine-out exhaust temperatures are a critical factor for longevity of under-floor catalysts, but I'm willing to be corrected as catalysts are not my area of expertise. I can see where engine-out temperatures could be a hazard to close-coupled catalysts.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[sierra4000]

In most engines that have these problems even after replacing the seats occurs repeatedly.

LPG equipment used in the rebuilding of modern mpi engines usually can (at least according to correction) to work in the stoichiometric mixture, also uses a partial parallel injection fuel, but does not address the requirement for different ignition timing for gasoline or LPG.

currently may be partially reduce recession only ancillary equipment for dispensing of special additives (eg. Flashlube), but at the expense of many other complications.

I'm trying to understand what may be causing these problems,
because others try to only eliminate its consequences.

Thank you all for your valuable posts

Radek Kolbábek - LPG servis Nedvědice

 

[Lou Scannon]

In a nutshell, when changing from gasoline to LPG, exhaust valves will tend to run hotter, especially under high load, as there is no evaporative cooling effect that helps mitigate peak temperatures. And LPG can be described as a "dry" gas fuel, i.e. there are no additives whatsoever that can form a protective barrier between the exhaust valve and its seat.
These two factors combine to accelerate exhaust valve recession on engines with exhaust valves that are not specifically engineered for LPG.
The recession phenomenon itself has been described a micro-welding process between the valve and seat, due to the high temperatures and contact pressures; that repeatedly tears away small amounts of material at the zone of contact, resulting in recession.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[sierra4000]

Thank You Hemi!

 

[turbocohen]

Allowing marginally increased exh valve oil loss will increase valve life a little but the erosion corrosion is mostly the result of higher static temperatures of the valve and seat along with the higher rate of erosion. I was involved with helping a company develop an additive optimized for liquid phase injectors that also addressed upper ring and valve erosion/corrosion but the LP industry really does not seem to give a damn about emissions, longevity or oem warranty costs associated with using non oem grade motor fuel for which the currents specs still do not address.

 

[winfieldblue]

Sierra4000, running too much lpg at low to middling revs can result in the mixture burning as it exits the exhaust valve, as can an advance curve that is set for unleaded fuel, it's easy to overfuel while using lpg as power can increase as the mixture gets overly rich in some circumstances. Pre-ignition also tends to pound the valves into their seats. You can try coating the valves with a thermal barrier, and you can regrind the valve and seat to get a much wider contact area which will allow more heat transfer from the valve to the head through the insert/seat (I have used seats 4 m.m.s wide with success, but airflow may suffer). If the valve seat problem is only with one particular cylinder then maybe the intake manifold isn't distributing even flow to all cylinders. You might want to look into an aftermarket lube system, there are a few available on ebay, try googling "valve saver"

 

[turbocohen]

Winfieldblue, how does preignition "pound" valves into their seats?

 

[winfieldblue]

Winfieldblue, how does preignition "pound" valves into their seats

My bad, I should have wrote " pre-ignition contributes to valve recession through the excess heat transferred to the valve and valve seat insert, which may be already running on the ragged edge".

 

[tbuelna]

turbocohen, good question.

Exhaust valve seat recession is caused by a repeated process of diffusion bonding/welding between the valve and seat contact at closing, that is then followed by the mechanical surface micro welds being torn apart when the valve is opened. The result is that a tiny amount of material is transferred from the seat face to the valve face during each cycle. The TEL added to leaded gasoline inhibited the diffusion bonding process between the exhaust valve face and the seat by forming an inert oxide coating on the contact surfaces.

The higher gas pressure force acting on the exhaust valve face from pre-ignition conditions would not result in any appreciable increase in contact stress levels at the exhaust valve/seat contact.

 

[sierra4000]

Thank You All for posts,

very interesting!!!

[Bigears]

Radek