Fortune7

I thought I'd post this here to get a wider 'turbine user' audience rather than in the specialist manufacturers forum (I'll go there next if needed!)

I have a Merlin MkII in a Eurosport with about 17 flights to date - very happy with the power and operation except I have gone thro at least a dozen glow plugs (yes, one at a time). I know what you're thinking: too high a plug voltage and/or too much gas. The Merlin FADEC ECU provides adjustment of both and since going through the first 8 plugs I must have put 12-15 start cycles on the engine in the workshop and have made adjustments to voltage and gas % (the gas valve pulses, with the ratio of on-off pulses determining the gas %) so that I get reliable starts with no thump as the gas ignites and gas-ignition within a couple of seconds of spool-up. It wasn't like this for the first few starts of the new engine in the model and I knew the long run of 3mm tubing from the gas valve in the nose to the motor would entail twiddling with the gas %, and so I did. Plugs were blowing then too, but only after a successful start; i.e. the next start: blown plug. All the time I was thinking it MUST be plug voltage and gas%, hence the exhaustive bench-checking. Today good starts but still one-plug-per flight being consumed. Once I'd bought a full card of 12 McCoy #9s I was convinced luck would change and I'd be keeping them forever. Not so - I'm half-way thro' them already.

Here's some other things tried:
a. different plugs: from OS TC3s, thru OS#8s, McCoy #9s, a couple of anonymous ones I had in my box and even a Rossi #5, super-cold with a thick filament in the hope it would withstand whatever was destroying the hotter ones - no joy; this one had started perfectly 8-10 times in the shop last week and blew the first flight today.
b. Teasing-out the element different amounts, from one-or-two coils (as per the Merlin instructions) to persuading-out the whole element. No discernable difference - they all blow within one cycle. At least I'm getting good at changing them quickly.
c. Using the plug washer or not (i.e. different reach) - nada

Each busted plug element is simply 'broken' on inspection, not 'evaporated', burned or missing.

So I'm thinking it is combustion gas in normal operation doing the damage, even though all other indications are normal on the engine. Could it be a combustion chamber or fuel stick issue? Before I aim the problem at Todd and / or Gaspar have I missed something obvious? I have a couple of things not yet tried: 1. hardly pulling the element out and adjusting gas% and voltage to get good starts, hoping the more-concealed element will stay intact thro' a run, and 2. Trying Keith Sievers' suggestion of a filter and restrictor in the gas line between the solenoid valve and motor (Thanks Keith, for the idea and the parts!). Keith having witnessed the death of many plugs today suggested this as he gets good starts with this setup on his Jet Central Rhino which is apparently close enough to a Merlin internals and has the same ECU.

Cheers; oh, and Happy New Year!

Andy

Robrow

Hi Andy, sorry to hear of the plug problems, give Todd a call, I'm sure he will sort your engine pronto, he has as much experience of Merlin operaton as anyone.

Current engines are supplied from the Barcelona factory with #8 plugs, these are very durable and giving reliable starting with a glow setting of around 35 and gas @60% As you have found these numbers are user adjustable to give the best set-up for your particular install.

Hope that helps, Rob.

Ehab

My Feedback: (44)

I am assuming that the ECU is retaining the plug setting after you program it. You must hit inter after changing the plug voltage or the ECU will not memorize it. Also, it could be your ecu is faulty and it has a big hysteresis loop that after the initial successful start, it somehow loses that last inial plug setting. This is all speculation on my part since reading your comments it looks like you did the usual inspection. Also try this: get it running then shut it down, then REDUCE the plug setting, then start it again and see what happens...

Another thing, your battery may have to high of a voltage and on restart it has too much voltage...again a pure guess!!!

joeflyer

My Feedback: (48)

My guess is that you have an ECU problem that's putting too much voltage to the plug. Use a voltmeter to read the plug voltage and to see if it changes when you reprogram the ECU glow setting. I'm not sure exactly what it should be, but I'd guess that anything over 2 volts is too much.

Joe

Eric D

I had the same problem with a home-built. Was not balanced well enough. Eric

Eric Banner

Just a thought:
Try removing the glow plug cap after start-up. Could be a faulty ECU not turning off the current to the plug.

Eric

George

My Feedback: (57)

Andy, send it back to Todd. He's close enough to you and will get you up and running.

It could be the ECU, but I do agree with Eric; it could be a balance issue that has arisen since the first runs.

George

Fortune7

Thanks Fellas - responses really appreciated. I hadn't thought of removing the plug cap after starting and will try this next time - good idea Eric!

Can someone explain the 'balance' issue please? Is this to do with the self-learning capability of the ECU and the possibility it may have led to unusual internal settings?

Cheers, Andy

George

My Feedback: (57)

Andy, no. It's just the possibility that the turbine has become unbalanced (just slightly) after the first runs when you were not seeing any problems. Although not common, I have seen it on a different turbine brand. These engines are assembled by hand and the parts are not always perfect. I'm not saying this is what is wrong with yours, but it is in the realm of possibility.

I have/had a Merlin MKI in my Euro and the only problem I had was that I had to constantly mess with the glow plug wire because I was not making good contact with the glow plug. I would get a good start, fly, shut down, then have to "wiggle" the wire on the plug for the following start. Replacing the plug with one that had a "ball" on the stem fixed that problem.

Do you have pics of your installation?

George

Jet Doctor

Just a silly thought, some turbine manufacturers suggest the best position for the glow plug e.g. 2 o'clock for the best ignition. If yours is low, you might need to be driving it too hard to get the gas to ignite.

Happy New Year to you all from sunny South Africa!

Zane

davo580

take it back to where you got it from if it is new you should have some warrity. I got a wren 54mk3 and the person who got the contract to put together for the hobby shop is looking at mine. Mine when first started runs fine 160,000 revs flat out. When you go to taxi it flame out , do the cool down sequence then start it again and it will not rev up from idle untill about a minute the it revs up so if any one got some advise will be appreiciated as well

A343

My Feedback: (4)

Andy
I had the same problem as you with my MkII.
There has been an issue. Contact Gaspar or Todd (I´m sure he´has been updated). I got an imediate response.
Regards
Thomas

SAP_2000

I've also had the problem on my MK II. Plugs have typically lasted for 1-2 flights. I'm currently so low on the glow-power setting that I'm having difficulty getting ignition.

Posted a question about this on the Merlin forum on the Jets-Munt page last fall, but got no response before the forum disappeared.
Sent Gaspar a mail about the forum being down and got the response that he had more important things to worry about....[sm=confused.gif]

So if anyone know what makes the plugs blow and how it's fixed, I'm would be happy to know

Fortune7

Hi Staale,

I spoke with Todd Witkoff (the USA distributor) today and he says its a rare problem. He's taking it up with Gaspar and will let me know what the outcome is. Todd thinks there may be an issue with the position of the plug in the combustion chamber (apparently they changed it on later models) so that combustion gas is taking-out the plug. This certainly agrees with my findings; the plugs are blowing after startup, not during.

I'll post further information here when recieved - thanks for the input.

Andy

grbaker

My Feedback: (29)

Please do, Andy. I have a Merlin 160 mkII, but have not experienced what you are going through.

My bet, is that Todd and Gaspar will get it fixed for you. Good luck.

SAP_2000

Thanks Andy!
I look forward to it.

I have had the same suspicions as you.
I can start and stop the engine ten times in a row without the glow plug blowing, but it blows after just one or two full flights...

digitech

My Feedback: (10)

do me a favour

get a FS type plug from OS max
set the power lower until it just ignites
you should be ok

lov2flyrc

My Feedback: (24)

Gaspar has traced this problem to the glow plug hole in the CC being a bit small, and a small mis-alignement from the cover to the CC. The misalignment causes the plug push the CC wall inside, creating a gap. This allows air to enter the glow plug hole in the chamber, creating a hot point directed at the plug.
A fairly simple adjustment to the chamber will fix the problem.
Staale,
Contact me if you need assistance with your Merlin

Andy,
will give you a call...
Todd

abqflyer

Try annealing the glow plug element before "teasing it out". Heat the element a few times before pulling it to soften it a bit. It's still takes a bit of skill.

jclittle

My Feedback: (99)

I'm having the same issue with a Rhino in that the plug is blowing after start up; plugs are working for only 1 start. All parameters on the ECU were set just yesterday by the factory rep but I'm still having an issue. Did you ever find out what was the problem with yours?

Todd what adjustment to the chamber is needed?

Ruizmilton

My Feedback: (29)

I had the same problem on a RAM 750, the issue was exactly the one Todd mentions, caused by a missaligned combustion chamber plug hole, the CC was touching the coil, ajusted it and has worked well ever since

jclittle

My Feedback: (99)

Is this somethign that the turbine needs to be sent in to have done? If not, anyone have some instructions for? Thanks.

jclittle

My Feedback: (99)

Is this something that the turbine needs to be sent in to have done? If not, anyone have some instructions for doing it? Thanks.

Ruizmilton

My Feedback: (29)

I'd suggest you contact Todd at Dreamworks, he should be able to tell you how's it in the Rhino..... In my case since there were no warranty coverage issues, I just removed the turbine body cover and realigned the bolt holes that were a bit to the side

SinCityJets

JC,

Try turrning the glow plug power down a little in the ECU. Eric recommends lowering it by three each time. Give that a try.