Mikuni MX40

[Trilby Tim]

I recently (in November) fitted a Mikuni MX40. Now before the calls of "I told you so" I did this in the knowledge that I'd be using good quality fuel (get it from a garage forecourt, and it has "EN590" on the pump, and also mix it with bio diesel which I think is also clean burning) and that I'd be running it flat out for more than an hour at a time and not cycling it on and off. But now, it's just blown a glow plug However, on taking the plug out there's no evidence of soot or carbon deposits on the plug or in the boss, and the plug looks pristine except for one point where it's melted. There's some white deposits on it, it looks kind of like a spark plug is meant to look when you take it out an engine.

Full marks to Mikuni and the supplier for service, they're sending a replacement plug which should arrive tomorrow. Their opinion was that it was blown by a voltage spike, I was plugged into the mains shore line via my Victron combi at the time and the man from Mikuni thought this was the source of the spike and I should check my settings on the Victron. However the Victron's performed faultlessly for 2 1/2 years and nothing else has succumbed to voltage spikes (even cheap EBay LED lights), and I'm pretty certain I set it up correctly when I fitted it. I did however notice it had switched from Float to Absorption mode last night (I think it does this automatically periodically to condition the batteries), and it is possible that it did that switch while the Mikuni was igniting (so the plug was powered up).

What do people think, does voltage spike seem plausible, if so is there anything I can do to stop it happening or to protect the Mikuni. Are there any particular settings I should check on the Victron?

P.S. I also have a Smartgauge fitted, and over the summer when I had left the boat a few months it was showing a "high voltage" error code when I returned, would this be caused by a spike? It didn't show any such error code last night though.

[GUMPY]

Mikuni are correct and you wont see the voltage spikes as they are to quick!

Took me ages to get to the bottom of this with my battery charger.

In the end I binned it and bought another one!

 

Mikuni do do a device for the MX60 that limits the voltage to the plug it can be retro fitted to the 40 one of the guys on the moorings has one.

Edited January 3, 2012 by idleness

[Nickhlx]

Mikuni are correct and you wont see the voltage spikes as they are to quick!

Took me ages to get to the bottom of this with my battery charger.

In the end I binned it and bought another one!

Or you could fit a timer so its off when the mikuni is on.

 

I struggle to see how a voltage spike ( assuming spike is defined as a short duration pulse ( a few micro, or even milliseconds) of over voltage can kill a glow plug which is a pretty robust device...

 

Nick

[Trilby Tim]

I struggle to see how a voltage spike ( assuming spike is defined as a short duration pulse ( a few micro, or even milliseconds) of over voltage can kill a glow plug which is a pretty robust device...

 

Nick

That was my thinking too, but I'll post a photo of the old plug when I change it tomorrow (assuming the new one arrives as promptly as it should!)

[mrsmelly]

Mikuni are correct and you wont see the voltage spikes as they are to quick!

Took me ages to get to the bottom of this with my battery charger.

In the end I binned it and bought another one!

 

Mikuni do do a device for the MX60 that limits the voltage to the plug it can be retro fitted to the 40 one of the guys on the moorings has one.

 

My mx 60 was fitted with that device also.

[Chalky]

I struggle to see how a voltage spike ( assuming spike is defined as a short duration pulse ( a few micro, or even milliseconds) of over voltage can kill a glow plug which is a pretty robust device...

 

Nick

 

I agree. It's more likely to be a manufacturing fault in the glow plug that caused localised over heating and caused it to fail, especially if it's only a few months old. It's known as infant mortality in the trade and it's common to "burn in" / "stress test" parts to remove these from the units supplied to customers. Burn in isn't cheap, but it removes rogue parts from the supply chain. Google at weibull distribution and Burn in for more details.

[Nickhlx]

Perhaps it would be a good idea if we gave them a few cycles "on the bench" before installing ?

 

Any suggestions for an appropriate voltage / time if the glow plug is just clamped in a vice at the the nut ? I suppose that a heater could be seeing as much as on-charge battery volts if the engine is running or the charger is on float / absorption.

 

Nick

[Springer Skipper]

I dont buy glow plugs from mikuni,take the Bosch or Duraplug number and get the next spare from a commercial diesel shop.To my mind they are a bit fragile,I consider them as a consummable item,and we always have a couple in stock,I hate cold showers and as I have an aircooled engine I rely on my mikuni.

Incidently I use used veg oil as a a fuel,even in the road vehicle(merc indirect injection)HMRC allow 2500 litres a yr without duty ,so the boat runs on it too.

[cereal tiller]

That was my thinking too, but I'll post a photo of the old plug when I change it tomorrow (assuming the new one arrives as promptly as it should!)

Have seen this problem many times,the voltage control device for the glowplug should be fitted to these heaters as standard.

 

First came across this problem 20 years ago,would think MIKUNI would have sorted it by now?

 

A standard glowplug should be capable of approx. 300 start-up cycles.

[GUMPY]

I agree. It's more likely to be a manufacturing fault in the glow plug that caused localised over heating and caused it to fail, especially if it's only a few months old.

 

NO it is a voltage spike and I spent a long while tracking it down, it is caused by the sudden load on the battery and the charger responds however we are not talking plus one or two volts here we can be talking 5 to 10 volts.

I got through 5-10 plugs until I realised what the causes were and set the charger on a timer to only come on when the MX was off.

I now have an MX60 with the limiter and its not a problem.

[Chalky]

NO it is a voltage spike and I spent a long while tracking it down, it is caused by the sudden load on the battery and the charger responds however we are not talking plus one or two volts here we can be talking 5 to 10 volts.

I got through 5-10 plugs until I realised what the causes were and set the charger on a timer to only come on when the MX was off.

I now have an MX60 with the limiter and its not a problem.

 

If a 5 - 10V transient is enough to blow it then there's an issue with the design. Typically an automotive 12v system will be expected to survive a 24v start (jump start off a lorry battery) and a 60v load dump without damage as well as other transients.

 

I suspect that there's a fast high voltage spike that you're not seeing that's causing the damage. Ideally you'll need a fast storage scope to find it (and a box of glow plugs!)

Edited January 3, 2012 by Chalky

[GUMPY]

If a 5 - 10V transient is enough to blow it then there's an issue with the design. Typically an automotive 12v system will be expected to survive a 24v start (jump start off a lorry battery) and a 60v load dump without damage as well as other transients.

 

I suspect that there's a fast high voltage spike that you're not seeing that's causing the damage. Ideally you'll need a fast storage scope to find it (and a box of glow plugs!)

Yes it probably is higher

I just CBA to get out the scope and look once I had deduced the cause

The 5-10 was a stab I never measured it, but used those figures to show that it was outside the normal battery charging voltages.

[Nickhlx]

Yes it probably is higher

I just CBA to get out the scope and look once I had deduced the cause

The 5-10 was a stab I never measured it, but used those figures to show that it was outside the normal battery charging voltages.

 

Even so, I would be very surprised if a spike had enough energy to heat the glowplug significantly, as it's essentially a heating element within, unlike a piece of electronic kit which dies near instantly when its absolute maximum voltage has been exceeded...

 

Nick

[Trilby Tim]

By the way, does anyone know how these things actually work, it seems like magic to me how they manage to ignite! As I understand it, in an engine it's the very high temperature resulting from compression that ignites the fuel, the glow plug is just there to warm things up a bit to start with if the engine's cold. But in the heater, there's no compression, so what actually ignites the fuel? It seems hard to believe that the glow plug is hot enough to cause ignition without anything else? And also, in a diesel engine the injection pressures are extreme (hundreds of bar) to achieve atomisation of the fuel, but in the heater that little pump can't be supplying the fuel at more than 2 or 3 bar I wouldn't have thought....

[bargemast]

By the way, does anyone know how these things actually work, it seems like magic to me how they manage to ignite! As I understand it, in an engine it's the very high temperature resulting from compression that ignites the fuel, the glow plug is just there to warm things up a bit to start with if the engine's cold. But in the heater, there's no compression, so what actually ignites the fuel? It seems hard to believe that the glow plug is hot enough to cause ignition without anything else? And also, in a diesel engine the injection pressures are extreme (hundreds of bar) to achieve atomisation of the fuel, but in the heater that little pump can't be supplying the fuel at more than 2 or 3 bar I wouldn't have thought....

 

 

A diesel engine is self igniting because of the heat caused by the high compression when running, the air and very fine spray of diesel ignites just like that, and only need (in most cases) a glowplug for cold starting.

 

 

In the MIKUNI as you said, there's no compression, but a blower bows air in the combustion chamber, and at the same time a very fine spray of diesel fuel is pressed in this chamber while the sparkplug is constantly sparking during this cycle to ignite this easy to ignite mixture.

 

Peter.

[Albion]

By the way, does anyone know how these things actually work, it seems like magic to me how they manage to ignite! As I understand it, in an engine it's the very high temperature resulting from compression that ignites the fuel, the glow plug is just there to warm things up a bit to start with if the engine's cold. But in the heater, there's no compression, so what actually ignites the fuel? It seems hard to believe that the glow plug is hot enough to cause ignition without anything else? And also, in a diesel engine the injection pressures are extreme (hundreds of bar) to achieve atomisation of the fuel, but in the heater that little pump can't be supplying the fuel at more than 2 or 3 bar I wouldn't have thought....

 

Yep, it's pretty straightforward. When the unit kicks in firstly the air fan purges the combustion chamber of any unburnt fuel fume and smoke etc. Then, when the purging has finished, the heater plug is heated by switching on a high current to it before the fuel is injected via the small solenoid shuttle pump in-line to the fuel supply, and air is blown in through the two angled swirl holes in the heater plug holder. The swirl holes create a circular, swirling motion to the injected fuel/air mix which helps the ignition and subsequent burn. This ignited mixture is contained within the combustion chamber where the burn is completed and, as the combustion chamber is surrounded by a water jacket, the water is heated and circulated to the rads by a small water pump. The whole process is monitored by an optical flame sensor and temperature switches to ensure that all is well. IIRC the heater plug isn't required when the flame is well established in the combustion chamber because the current required drops considerably after the burn has been established. On the Mikuni the burn is switched to half heat when the unit reaches certain temperature conditions.

Roger

[Trilby Tim]

A diesel engine is self igniting because of the heat caused by the high compression when running, the air and very fine spray of diesel ignites just like that, and only need (in most cases) a glowplug for cold starting.

 

 

In the MIKUNI as you said, there's no compression, but a blower bows air in the combustion chamber, and at the same time a very fine spray of diesel fuel is pressed in this chamber while the sparkplug is constantly sparking during this cycle to ignite this easy to ignite mixture.

 

Peter.

But there isn't any sparkplug, it's a glow plug, and this is what I'm surprised by. I'd understand it if it had a spark plug to cause the ignition (maybe with a glow plug or similar to pre-heat the fuel when it's cold), but I'm surprised that a glow plug gets hot enough to cause ignition, although it sounds from Roger's description that it does! Maybe these glow plugs are hotter than in diesel engines? Maybe this is why they're more prone to blowing?

P.S. The replacement one hasn't arrived today, damn Royal Mail

[Albion]

But there isn't any sparkplug, it's a glow plug, and this is what I'm surprised by. I'd understand it if it had a spark plug to cause the ignition (maybe with a glow plug or similar to pre-heat the fuel when it's cold), but I'm surprised that a glow plug gets hot enough to cause ignition, although it sounds from Roger's description that it does! Maybe these glow plugs are hotter than in diesel engines? Maybe this is why they're more prone to blowing?

P.S. The replacement one hasn't arrived today, damn Royal Mail

 

You're correct, it isn't a spark plug it is a heater plug with a spiral wound winding that heats up when a high current is passed. It is ESSENTIAL to ensure that two angled air holes in the glow plug boss assembly (IIRC) are kept clear (use a small drill in between your fingers to drill them clear of any carbon build-up) otherwise you risk burning out your new plug. What seems to happen is that if the air swirl holes aren't passing air properly there is no swirl. Without air swirl the fuel dribbles down the underside of the plug holder and causes a carbon build-up and inter-turn carbon bridging of the glow plug coil. This tends to cause the burn out of the heater plug OR can cause the main supply fuse to blow as the current demanded by the heater plug is now increased to above the fuse rating.

Roger

 

Edited to add that this Mikuni link is handy to see what parts are involved MX 40 and show the dished washer (part number 33 <although incorrectly labelled as 31 when you get to the final diagram>)that should, ideally, be replaced on the underside of the heater plug holder if you remove it for swirl hole checking purposes.

Edited January 4, 2012 by Albion

[bargemast]

But there isn't any sparkplug, it's a glow plug, and this is what I'm surprised by. I'd understand it if it had a spark plug to cause the ignition (maybe with a glow plug or similar to pre-heat the fuel when it's cold), but I'm surprised that a glow plug gets hot enough to cause ignition, although it sounds from Roger's description that it does! Maybe these glow plugs are hotter than in diesel engines? Maybe this is why they're more prone to blowing?

P.S. The replacement one hasn't arrived today, damn Royal Mail

 

 

Thank you very much Tim and Roger for your information, I assumed wrongly that the MIKUNI works like my own diesel central heating system, which is a domestic SOMY boiler with a WEISHAUBT burner that works with a sparkpug instead of a glowplug, and is a trouble free

and easy to maintain setup that is working really well on my liveboard.

 

I hope that your problems will soon belong to the past.

 

Cheers,

 

Peter.

[Trilby Tim]

The new plug arrived, I've fitted it and it's working again. Here's a photo of the old one. As you can see no carbon build up, no thinning of the element, pretty much pristine except for the point on it where it blew, which you can see about half way along the coil.

 

[Biggles]

Interesting the hear you are using Bio as well in the heating fuel. What percentage added? as I think all forecourt diesel has 5% already.

 

The reason I ask is my car has a Webasto for when the temp drops below 3c and definitively didn't like it even at 20%

[Trilby Tim]

Interesting the hear you are using Bio as well in the heating fuel. What percentage added? as I think all forecourt diesel has 5% already.

 

The reason I ask is my car has a Webasto for when the temp drops below 3c and definitively didn't like it even at 20%

Forecourt diesel can have up to 5% (up to 5% is allowed in the specification), which doesn't necessarily mean it all does. I mix 50 % bio with regular diesel.At the moment I buy my bio from a company that makes it, but I do plan to make my own. Until blowing this glow plug in the Mikuni I've not had any problems with my engine or heater.

What problems has your Webasto had? Are you sure they were caused by the fuel? Where did you get your bio from?

[Nickhlx]

Forecourt diesel can have up to 5% (up to 5% is allowed in the specification), which doesn't necessarily mean it all does. I mix 50 % bio with regular diesel.At the moment I buy my bio from a company that makes it, but I do plan to make my own. Until blowing this glow plug in the Mikuni I've not had any problems with my engine or heater.

What problems has your Webasto had? Are you sure they were caused by the fuel? Where did you get your bio from?

 

 

Interesting on the fuels...

 

My first tank was red, but ever since I have run mine on home heating / 28 second oil as I have that and it is / was 45p/litre until recently, although refills will now be around 55p/litre...

 

I use that as it is known fresh and burns cleanly...

 

Nick

 

 

 

[Trilby Tim]

Arrgh, it's gone and blown the new glow plug , exactly the same as the first. Again, no signs of any carbon deposits, or unburnt fuel fouling just a slightly whitish tinge and a melted spot in the middle, in exactly the same place as before.

Like Nickhlx, I was a little bit sceptical of it being the electricity supply that's doing it, I would have thought that a glow plug was quite tough against very short voltage spikes, but having two fail in a row like this does seem to point to it, so I guess idleness was right. Also the whole coil on the second one appears very slightly deformed, as though its been hotter than it should be. Does anyone have any suggestions for some sort of voltage regulator I could put in to protect the glow plug when I get a new one? The problem is that it draws quite a lot of current (25A) so it would need to be pretty beefy.

Tim

[Trilby Tim]

I suppose I could fit a relay into the shore line and take a feed from the glow plug so that when the glow plug is energised the shore line is disconnected (the Victron will automatically switch to inverter mode). Actually, I had been thinking of putting a relay into the shore line this summer anyway. I'm going to fit solar panels, so I was going to have the shore line normally disconnected and only switch back in if the state of charge of the batteries falls below a certain limit (I'll use the alarm from the Smartgauge), so it might not be too much extra effort to have a connection for the glow plug being powered to turn off the relay.....