Glow Plugs or Spark Plugs

I am trying to complete a part-built 3 cylinder 4 stroke aero engine; to give an indication of size, bore and stroke are 24mm each.

One of the missing bits is the ignition system and there seems to be no provision for it so far in terms of space for any timing mechanism. So I was wondering if the it had been planned to use glow plugs; I had originally assumed a spark plug system would be employed.

Now my knowledge on glow plugs is zilch, other than they need a battery source to get going (glowing) and possibly to keep the plugs glowing when the engine is on slow running. Also that with glow plugs there seems to be no provision/requirement for timing.

Could anyone with knowledge of glow plugs please give me some advice on how to employ them, is the timing or lack of it an issue and what plugs would be suitable, or should I bin the idea and go for spark plugs and it's associated equipment instead?

As I am now trying to design the valve timing gear at the moment now is also the time to sort out the ignition system as crankcase space will very much be an issue if a timing actuation has to be incorporated as well, as I had originally intended.

Thanks in anticipation,

Chris.

BobHi Chris,

I would suggest your engine would probably run on either spark ignition or Glow plug. However for the sake of simplicity I would go with a glow plug. Try an O.S No. 3 as a general start, if all is correct with the engine and carbonation you should be able to run the engine at tick over speeds without the power supply. I know that Glow plugs come in various types, cold plugs are generally used in a high performance engine and hot plugs in a low performance engine. The thicker the wire of the plug the hotter it is. If you find the engine will not keep the engine ticking over try a hotter plug.

Glow plugs use a 1.5 Volt battery supply and can be obtained fairly cheaply via most Aero modelling suppers, however for Multi cylinder work you might want to think about a small lead acid accumulator so that you can run all three at once via hard wiring rather than fiddling about with three individual starters, by doing this you will be keeping your fingers clear of the prop too.

I Presume you have a carburettor set up for glow fuel, go for a four stroke pre mixed fuel with a couple of percent of Nitro in it and either synthetic oil or Casor oil. ( I am assuming it doesn't have a closed, re circulatory lubrication system)

If you go for Spark ignition you can run on either Glow or Petroil mix, however it will run hotter on petrol mix. You would have to use a carburettor suitable for petrol if you go for petrol as a fuel choice Also, you will have to provide some means of providing a spark to each cylinder at the correct time. Commercial systems are available but can be expensive.

Sure it's 1.5volts and not 1.2volts for standard glo-plugs? The 1.2volts is derived from a single lead/acid cell. I think the "Cyclon" cells sold to heat glo-plugs are 1.2 volts. NiCads and other re-chargeables also produce about 1.2v.

Steve

Not so I'm afraid; electrochemistry dictates that the nominal voltage for a single lead-acid cell is about 2V, as are the single Cyclon cells.

Andrew

It seems that none of you are model flyers as glo-plugs are obtainable in both 1.5v and 2v - the latter being operated from a single lead acid cell. Most 1.5v plugs are heated with a single sub-C cell which provides more current and is very adequate.

Thomas,

I think Andy is correct. I was a model flyer but mainly a model builder last time I flew a model would have been 2004. My advice is based upon experience with commercial four stroke & two stroke engines as well as from my own home built engines which were all four stroke, petrol,glow or diesel engines single and multi-cylinder.

Bob

Hello Chris.

If you haven't done already, why not take a look on the 'Just Engines' website. They list all sorts of parts and accessories for small engines, including reasonably priced miniature spark plugs with the 1/4x32 glow plug thread.

As well as glow plugs they also offer miniature CDI ignition systems, including one for a three cylinder engine, and also the tiny magnets that are required for them.

My only connection with them is that of a previous customer. I bought a few miniature spark plugs and a CDI ignition kit off them a few years ago, but have not yet used them. In fact I wonder where the hell I have put them.

For the glow plug engine to work nicely, a real compression ratio of about 9:1 to 10:1 works very well.

The glow plugs only need enough to glow about a bright red, Not orange. Most glowplugs only require about 0.8 to 1 volt dc with a current draw of 2.5 to 8 amps, depending on the wire thickness and type.

Nicad batteries work well, but so do NiMh. I would recommend if you are using the 2volt cyclon batteries, that you use 0.15 ohm resistor if it is a higher current draw glow plug or a 0.2 ohm resistor if you are using the lower current draw glow plugs. Generally the higher heat range glow plugs have a low current draw, and the colder heat range have a higher current draw.

Running the engine on glow fuel will generally result in a cooler running engine. There are glow plugs designed to make the engines run with petrol instead of the methanol based glow fuel. These engines make a little over 1/2 the power of a glow fuel engine, and generally run a little hotter as well, but do have a good range on the tank capacity.

There is no adverse effect of leaving power on the glowplugs while the engine is running. It just flattens a battery. An enelope AA rechargeable NiMh battery will keep an engine running and restarting for about 10 to 15 mins depending on the heat range of the plug.

Neil Lickfold

Thanks for all the responses. Certainly gave a lot of food for thought.

I will probably try a glow plug system initially although some of the model spark plug ignition systems as marketed by Just Engines, MiniMag and Hemmingway Kits look attractive. It's just the cost! Now where have I heard that before?

Chris

I bought a couple of the spark gas ignitors to have a go at Jan Ridders "Blokker" ignitions a couple of months ago, look promising but have not done the modifications yet, you could have one for each plug and still get change from £20.

S/S in the state salso have a very good range of ignitions which are what I mostly use.

Jason, I had a go at Jan Ridders 'Blokker" ignition system a few months ago and got it all wrong and it didn't work. I then bought another spark gas lighter and different capacitors but partly I have not had the time to redo it plus I also lost confidence to have another go, but another go I will have sometime soon.

Will look at S/S in the USA too - thanks for that.

Chris

Not so I'm afraid; electrochemistry dictates that the nominal voltage for a single lead-acid cell is about 2V, as are the single Cyclon cells.

Andrew

True! brain fade, my plugs are 2volt types!

Steve