7cylinder 35cc Radial engine

[Falcoray]

Hi I’m new to this forum briefly I’ve flown models and full size fixed wing aircraft since the early 80’s. The aircraft in question is a replica of a Pzlwilga104 which I flew for a from 1989 to 1992. 
I am having trouble with the radial  it is a 35cc 7 cylinder radial. The ignition system is powered be a single cell lipo battery and consists of a black box with power input from the receiver to the 7 glow plugs. The units cost in the region of £300 and I’m on my 3rd one which has just failed, they seem to last no more 10 or so flights. 
Has anyone on this forum had similar issues or ideas how to overcome this problem please. 

[Martin Harris - Moderator]

Welcome to the forum.  
 

I would have thought that your first call would be on the supplier (if you’ve bought direct, a well known and as far as I’m aware, reputable company) of the unit. If it is being installed and powered in accordance to their instructions you have a right under consumer law to expect it to work for a reasonable time and you should be entitled to free repair or replacement. 
 

Unfortunately, options for alternative commercial alternatives are likely to be limited - but hopefully someone may be aware of a better alternative if there isn’t an identifiable cause of these failures. 
 

There again, do you actually need such a system?  A glow engine is self sustaining once running so a simple 1.5 - 2 volt (depending on plugs used) source with sufficient current delivery for 7 plugs in parallel should suffice.  
 

Perhaps, to reduce the fairly hefty current demand (in the order of 20 amps) this could even be split down to start on, say, 4 cylinders and then switched via remote connectors to bring in the other 3 - or multiple supplies utilised. 

[Phil Green]

Hi I fly gliders so I've no right to an opinion, but just an observation - I see the label says 'HV', but you run it from a single cell - if you're running a switcher at half the voltage it expects, it will draw twice the current... possibly exceeding its current capability?   Have you looked inside a failed one, which components have blown?    

Have to say £300 (or in your case £900) is ridiculous for a glow driver, even with 7 outputs  🙂

 

[Martin Harris - Moderator]

Looking at the supplier’s site shows the units priced at £145 - have yours been supplied by a third party at a hefty mark up?

 

The site does specify a single “4.2V” LiPo source. 

[Phil Green]

Ah ok sorry   🙂

Maybe have a word with Stu Mackay who is or was the Moki importer, not the same engine but 7 glowplugs is 7 glowplugs...

[Falcoray]

Thanks Phil I’ll contact him. 

[Ron Gray]

Wrong post!

Edited October 20, 2024 by Ron Gray

[Falcoray]

Thanks for all the advice,

I will go back to the supplier as Martin suggests I will also try to find a source for a wiring diagram to make sure I’m not wiring it incorrectly. 

[Ron Gray]

The onboard glow unit looks very similar to the one that Just Engines supply @ £137.

[Falcoray]

Thanks Ron I’ll try them! 

[David P Williams]

Or go to Mike who makes them at Model Radio Workshop

[Martin Harris - Moderator]

I notice that there’s a warning not to short any output to the engine casing.  Perhaps this has happened to cause damage to the previous units?

 

I have to say that this seems to be a weakness in its design - in a multi connection heat and vibration rich environment coupled with wind blast effects, it would be only too easy to get a short circuit scenario - especially with the use of crocodile clips to connect to glow plugs. 

[Cuban8]

18 hours ago, Martin Harris - Moderator said:

Welcome to the forum.  
 

I would have thought that your first call would be on the supplier (if you’ve bought direct, a well known and as far as I’m aware, reputable company) of the unit. If it is being installed and powered in accordance to their instructions you have a right under consumer law to expect it to work for a reasonable time and you should be entitled to free repair or replacement. 
 

Unfortunately, options for alternative commercial alternatives are likely to be limited - but hopefully someone may be aware of a better alternative if there isn’t an identifiable cause of these failures. 
 

There again, do you actually need such a system?  A glow engine is self sustaining once running so a simple 1.5 - 2 volt (depending on plugs used) source with sufficient current delivery for 7 plugs in parallel should suffice.  
 

Perhaps, to reduce the fairly hefty current demand (in the order of 20 amps) this could even be split down to start on, say, 4 cylinders and then switched via remote connectors to bring in the other 3 - or multiple supplies utilised. 

Absolutely......keep things as uncomplicated as possible. Mention was made of not allowing the outputs to short to ground - good advice, although this is bound to happen at some time due to any number of reasons. If the unit is unable to cope with a dead short and shut itself down but blows up instead (not a difficult thing to design into the device to prevent) then I'd say that it isn't fit for purpose IMHO.

[Andy Stephenson]

Crock-clips are completely inadequate for this job, there's not enough insulation to prevent them from metal part from touching the cylinder head and the insulation could easily melt. I would recommend these, albeit expensive but safe.

Cut off the red wire with the cap and wire it to the outlet on the OBG unit.

https://www.arrowmodels.com/jp-remote-glow-lead-adaptor-mount-deluxe-for-rc-models

[Paul De Tourtoulon]

Is it worth wiring up a 2 v lead acid or the like battery on a car type switch relay ?.

[GrumpyGnome]

https://www.aliexpress.com/w/wholesale-remote-glow-plug.html?spm=a2g0o.productlist.search.0

 

Possibly off the same production line as the one above. I believe you can get them without the 90 degree bend as well ...

[Ron Gray]

Personally I wouldn't use commercial glow connectors, just make your own from bits of 'curtain wire' with a bit of heat shrink to insulate.

[Chris Walby]

9 hours ago, Martin Harris - Moderator said:

I notice that there’s a warning not to short any output to the engine casing.  Perhaps this has happened to cause damage to the previous units?

 

I have to say that this seems to be a weakness in its design - in a multi connection heat and vibration rich environment coupled with wind blast effects, it would be only too easy to get a short circuit scenario - especially with the use of crocodile clips to connect to glow plugs. 

Typical maximum glow plug current is 3A at 1.5V so it has a resistance of 0.5 ohm, the driver circuit has to have a very internal low resistance. The issue with this is that if you then short circuit the glow plug lead an excessive current will flow and damage the control unit. Probably why the warning states to avoid shorting the glow plug lead as its not a weakness in design, but a function of the design of the glow plug which is outside of their control. It would need a far more complex design/cost unit to be short circuit protected for what is a failure beyond their control.

 

Good news if you can call it that, but that's probably why the other units have failed, short the glow lead and the unit will be damaged.

 

 

[Falcoray]

Thanks very much for the advice and suggestions I think I’m going to bite the bullet and buy another unit, hopefully for around the £150 mark. A couple of things which have been suggested were the high voltage and shorting, I use a single cell 5000mah lipo battery 3.7 volts and the glow plug connectors are the push and twist type as shown in my picture, which shouldn’t be susceptible to shorting.
The other thing I have always done is to leave the power on  to the glow plugs up to 90% of the throttle range I’m wondering this might be the cause? I can’t think of anything else to do just hope the next new one lasts a bit longer

The second picture is of the internals of the ignition unit, as was suggested earlier I opened it and the part circled looks to have burnt out, it has 7 outlets one to each flow lead. 

Thanks again for the help and advice. 

[Chris Walby]

I could be wrong! However if the devices that go to each glow lead look undamaged then its not a shorting issue and the device circled is something to do with switching the glows on or off (as its a single common device) might be able to confirm this by following the "servo" input lead that connected to the RX throttle channel.

 

IMO it could be supply voltage or the way the supply voltage and RX inputs are configured that is causing the problem.

 

Could you post the box external leads and indicate how they are connected up, cheers

[Jon H]

28 minutes ago, Galcoray said:

The other thing I have always done is to leave the power on  to the glow plugs up to 90% of the throttle range

 

errr...bad. Glow plug engines do not need constant glow support like this and it can be extremely detrimental to their performance as well as making them impossible to tune. Glow drivers are only intended for use at idle and for starting, so its likely you have simply fried the units by massively exceeding their designed power on time. 

 

I recommend you set up the next one so you can switch it off completely, fully tune up the engine with it off, and then turn it back on at the bottom 10-20% of the throttle travel if you need it to keep the plugs lit. My OS 4 cylinder has no need for any glow support once running and i never loose cylinders. My ASP, Enya, Magnum and Laser twin cylinders are the same, and i have never had an issue with the saito 3 cylinder radials either. The only multi cylinder engine to give me trouble was an  SC 5 cylinder radial, but it has such a collection of problems its not a great benchmark and it is awaiting another rebuild. 

 

I would also make sure you are using the right fuel (5% nitro and 7 or 8% oil in these i think if my memory is right) and then tune it up nicely with the glow off. 

[Falcoray]

Thanks for that Jon I’ll send a picture of the fitting shortly. I think you are right I have been frying them by leaving them on up to about 90% throttle, I was always afraid of losing power at low  revs. Fuel wise I use methanol 85% oil 8% and nitro 7% it is a little oily but safe! I have used Optimex  premixed before but prefer to mix my own. 

[Falcoray]

Firstly thanks for all of the help and advice. Someone mentioned opening the unit to see if there’s any obvious damage, the picture attached shows what I think is the damaged part which sends the current to the individual glow plugs. This could be either voltage too high or shorting of the plug leads I think it’s voltage as I always change the crocodile clips for the much safer push and twist ones as shown in the pictures. 

[Falcoray]

This one of the fittings which are difficult to short out as opposed to the crocodile clips which are fitted as standard. 

[Frank Skilbeck]

Might be worth contacting Mike Ridley at Model Radio Workshop to see if he can repair one of your units.