Ignition Spark

Hello,

I'm after some help again! I am in the process of making a small 4 stroke Petrol Engine with a 3/4" bore. I'm almost at the stage where I need to fabricate the points. Does anyone know where I would be able to buy power pack/coil? I have been told that you can buy ones which run off AA batteries but as yet I have not been able to source one. I'm using a CM 6 spark plug.... the type used in the smallest Honda generators.

Any help would be appreciated.

Regards

Tom

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These guys **LINK** will be able to help you with your needs be it for a traditional system that uses points or a CDI based ignition unit.

This company http://cncengines.com/ic.html is also excellent.

Nick

Edited By Nick_G on 17/08/2017 20:14:12

What Nick says, I mostly use the stuff from S/S but have also used coils from Minimag. If you have not made the head yet then one of the smaller 1/4" rimfire Plugs or far eastern copies would suit that bore and give you a bit more room around the Valves

If you are using the conventional Kettering type cam operated points and coil system, it is important to have the dwell angle correct for the coil being used. This will ensure optimum magnetic saturation of the coil, and result in the biggest spark when the points open to collapse the magnetic field.

The dwell angle will be determined by the shape of the cam and the points gap at maximum opening.

Presumably, if using solid state "contact breakers" the comments re dwell angle to optimise magnetic saturation of the coil will still apply, but be managed electronically, rather than mechanically, and hopefully be more consistent and reliable.

Howard

**LINK**

Gentlemen,

Sorry for the delay in answering your responses. I've contacted Minimags and bought one of their setups.

Thank you all for your comments.

Regards

Tom

Hello Tom,

I don't know what you have purchased from Minimag, a Kettering system or Magneto. These should work first time. Bu if not, do yourself a favour and buy a modern model aircraft ignition system These work! I would further go on to say use either an optical switch or a magnetic sensor instead of points. These systems are cheap and reliable and while you are sorting out any problems (there usually are!) at least you can rule out the ignition part of the system.

If you want to use the old fashioned Kettering system and the 1930 / 40 model aircraft system then do so after all the problems are sorted . I use them all the time for my many ignition engines. I note some one says that the coil becomes saturated after a short time with primary current. This is bunkum and is perpetuated down the years. The coil reaches its maximum magnetic field for the given current, but the core is nowhere near being saturated. Apologies to the gent that claimed saturation, it is a common mistake.

If you have any problems, PM me, I think I have seen just about every failure mode for the small coil / capacitor type arrangement. Indeed I have made most of the common mistakes myself. Once sorted the old fashioned Kettering system is delightfully simple.

Regards,

Andrew.

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Hi Andrew,

You obviously are far more knowledgeable than me in this area having had considerable success over the years with them.

Maybe an 'urban myth' but I read that 'Some' of the ignition setups for high revving R/C engines can cause issues with the much lower revving display type engines. Something to do with the inbuilt automatic advance and retard on 'Some' makers of them.

Never having used them I only know what I have read.

Nick

The auto advance becomes more of a problem when people use them on 4-strokes and fit the sensor to the cam shaft so the unit thinks it is going twice as fast as it should.

Both Minimag and S/S units don't have auto advance though S/S do also sell ones with it for conversion of 2-stroke model engines where the sensor reads off the crank.

Jason, should that say half as fast? When I eventually get back to my 4 stroke I was going to fit 2 magnets on te camshaft and have a wasted spark. Am I barking up the wrong tree.

.

I think Duncan you only need 1 magnet. (providing it's a single cylinder) - If it's on the camshaft you will still have a wasted spark with a 4 stroke.

Edit :- Sorry that is wrong.! But you still only need one magnet. (providing it's a single cylinder)

Nick

Edited By Nick_G on 25/08/2017 10:51:44

I think the argument is that the ignition systems are designed for 2 strokes, and so they work out the rpm based on one pulse per rev from the sensor. If you have one magnet on the camshaft of a 4 stroke it thinks it is only going at half speed and gets the advance wrong. Either 2 magnets on the camshaft or one on the crank sorts out the rpm issue, but gives you wasted sparks.

Edited By duncan webster on 25/08/2017 10:54:36

Edited By duncan webster on 25/08/2017 10:55:25

The time when a wasted spark is an advantage is when a magneto is used. By driving the magneto at crankshaft speed, a higher voltage is attained than if at camshaft speed. For a coil and battery, no advantage, or disadvantage.

Ian S C

Yes other way round Duncan, typed as I was about to go out.

Easier just to pay a couple of quid more and get one without auto advance, they also tend to have much neater looking spark plug leads and caps than the typical steel braided ones you find on the Rcexl units which use the braiding as the earth wire.

The modern aircraft ignition modules (maybe £20 or £30), have the option of 2 or 4 stroke ignition, Usually just a switch or a wire moved. An awful lot of big model aircraft are 4 stroke powered and the manufacturers cater for them in their modern units.

Don't get me wrong, I love the simple two stroke set ups of the 1940s, when set up they are very reliable. I thing that your supplier of minimags or whatever also sell the small spark plugs (that go into a glow thread). These Rimfire plugs are far superior to the old plugs. If it is a larger plug (I think they are 6mm thread?), make sure you get a reputable make, a lot of the cheap Chinese plugs can give problems. I think Bosch do them so no excuse for skimping on that item!

Best of luck with your experimenting. If you are using a magneto, then you are one up on me! When I tried to buy one, the guy was I think retiring? There obviously is someone new taking over the set up. I may even get to using a magneto after all. From a guy who made his own "They are not easy to make at this scale and you need to know exactly what you are doing". So I paid heed!

Andrew.

Glow plug thread is 1/4 x 32 UNEF so 6mm would not be smaller. The CM-6 is M10x1.0 which may be what you are thinking of. Or maybe the #10-40 Rimfires with the 6mm hex.

Gentlemen,

I had no idea that my post would produce so much interest... once again thank you for all your words of wisdom. This is my first Petrol Engine build so I need all the help I can get!

After contacting Minimag and speaking to Julian and Bernadette, the proprietors, I chose their MCL-1 Low Power Capacitor Ignition kit (this is powered by two AA batteries). I found Julian and Bernadette to be most helpful and the kit was delivered the next day. At this point I should say that other than being a satisfied customer, I have no other affiliation to Minimag.

Mounting tha 3mm x 2mm neodymium magnet on the camshaft on such a small engine may be problematic. I think my best option is probably to go for the crankshaft and take a hit on the battery power which is being wasted.

I tested the kit this evening and it works fine on the CM-6 spark plug. Julian told me that he has used this ignition kit to run a Lister D type.

Regards

Tom

You can buy minature spark plugs, ignition coils and points sets from the specialty hobby shops that supply parts for RC internal combustion racing cars, I've not bought any, but I have seen them there when looking for other small stuff.

I'm fairly certain they cater for both 2 stroke and 4 stroke.engines.

Edited By Bob Gitsham on 29/08/2017 06:06:20