Prechamber spark plugs in MWM 2016 TCG biogas engine - failing early - pics

[USAeng]

Hey guys

A little over a year ago we made post on here on looking for aftermarket pre-chamber plugs since we are going through a lot of plugs on our biogas engines - we were trying to save some money. At the time we decided to stick with the OEM plugs and have been continuing to use them to date.

Since then we have had some plugs go 3000 hours plus and other fail with less than 500 hours. Failure is usually due to low cylinder temp on the cylinder with bad plug - then we must replace the plug to get the engine to continue to run

Recently we have cut open the plugs to look at the electrodes. Here are some pics

The forward pointing legs of the electrode are gone in most of the failed plugs - so far that is our only observation. We have cut open the rest of the plug and inspected but so far no other findings.

We are still trying to figure out why some plugs are failing so quickly - and appreciate any comments and suggestions

 

[dgallup]

Do the legs appear to be eroding away from sparking or fracturing? Is there a correlation between erosion and polarity? I.E. some engines use a coil to spark two cylinders with a waste spark and the polarity is different between the two cylinders. Not sure what the scale is here but those look like some pretty fragile electrodes.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Hoxton]

I remember your post last year. Questions:

1. What sort of biogas is it? I guess digester gas? What is the feedstock? (animal effluent, which sort of animal, waste from household collections, a mixture? Is the feedstock mix consistent?

2. Have you sent a gas sample off to a lab to be analysed against the requirements of the MWM Document 'TR 0199-99-3017, Combustion Gas Technical Circular'? You must use the latest edition!

3. Other Technical Circulars which would be useful are TR 0199-26-2141-0 Inspecting spark plugs and TR 0199-26-2145-0 OEM Spark plugs (This warns of the perils of using third party spark plugs (but they would say that, wouldn't they!))

 

[USAeng]

Dgallup - There is a single coil for each cylinder. 12 cylinders total on this engine. They are not connected

 

[USAeng]

Hey Hoxton

It is Digester gas with human waste as the feedstock

We do have a gas analyzer in place and have checked that in the past - we will get the latest copy of that TR and check it again though. I'm pretty sure we are OK there. We also scrub our gas and check regularly for H2S and Silixanes

We have asked our supplier for those technical circulars to review them if they are of any help

We did get one that didn't make much sense to me. We were told that if the spark plugs pass this test then we cannot get warranty credit for one that fails prematurely. It seems to me that there would always be infinite resistance though unless something crazy happens inside

 

[Lou Scannon]

The external body of the plug is the ground. There needs to be not only high (near infinite) resistance, but also high dielectric strength between the ground and the electrode (which is connected to the lead of the spark plug, 4, in the diagram). Otherwise, instead of jumping the gap in the prechamber to initiate ignition, the high voltage will find a lower resistance path to ground and the plug will misfire. In a plug that is deteriorating in this respect, it will misfire at high load first, and the misfire can progress to lower and lower loads, as the weakened dielectric barrier progressively breaks down.
By the same token, eroding electrodes drive up the breakdown voltage needed to initiate a spark, which can cause the increasing breakdown voltage to find a different path to ground, i.e. puncture the formerly intact dielectric barrier.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[enginesrus]

So the engine is an open chamber? It looks like federal mogul markets those type of plugs, maybe talk to someone there about the problem, maybe suggest testing some of theirs to see if they hold up. IMO those plugs look like they are designed not to last with that thin wire electrode.

 

[Lou Scannon]

Maybe I missed something... if you are running the engine with fuels or with spark plugs not approved by the OEM, good luck to you. If you are operating within the OEM guidelines, the OEM should be your best resource for troubleshooting issues.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[USAeng]

We are asking them for help but so far Hoxton seems to know more than anyone we've talked to. We are using OEM plug

We did figure out the spark plug testing procedure above - so we are clear on that now

 

[TugboatEng]

hemi, I have not had the same experience. The OEM explaining the cause of the problem is the same as admitting that they know there is a problem. The best I have gotten from an engine OEM is, "we've never heard of that" despite us having 100% failure rates across the entire fleet in multiple different installations. Currently the manufacturer no longer provides that configuration with no explanation why.

 

[Lou Scannon]

Well, I certainly can't vouch for the customer perception; my experience has been with an OEM, and with Tier 1 suppliers to various OEMs, all very respectable ones, and when field issues have occurred, they were taken very seriously. Now the 7-step, 8D or what-have-you issue resolution processes take some time to get to a permanent corrective action, but one of the first steps is always "protect the customer", or something to that effect, and by God, that is what we tried to do.
I will add, that as a curious engineer, I have found these field issue investigations to be very educational and rewarding, and have always been grateful to my employers for paying me to pursue these activities when they have come up.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[BigClive]

The fuel for this engine sounds as if it would be a very pleasant substance.

 

[LEO LIU]

Hi @ USAeng, we are manufacturer of spark plugs for more than 60 years. We supply good quality spark plugs for MWM engines. Maybe if you send me more details about your experiences, I could discuss with our engineers and help you out.

Leo

 

[RodRico]

I'm rather surprised to see such delicate structures in a spark plug tip. Assuming the design works and all 12 cylinders show equal propensity for the same failure mode, I'd look at coolant temperature in and out of the radiator and common coil voltage. If all 12 cylinders do *not* show equal propensity for the same failure mode, then it's a matter of comparing parameters of good vs bad cylinders including the running temperature of each (which can be measured on the spark plug body for purposes of comparison).

 

[btrueblood]

That's a very long spark plug. Long, cantilevered things like to bend due to vibrations more than short things, and ceramic insulators don't like to be bent. Any chance you have some methods to measure vibrations during operation on the engine, and could you correlate high vibration transients to plug failures?

I'd wonder, as an R+D kinda guy, if a silicone sleeve supporting the proximal end of the plug (filling the gap between the unsupported end of the plug and whatever bore it is reaching down) might help.