The New Help and Assistance Topic

[brickyParticipant @bricky]

Building an engine with a verticle cylinder with two opposed pistons.The cylinder head is octagonal with valves ,spark plug and compresion adjuster at opposing positions around the cylinder head.The depth from outside of the cylinder head to inside of the cylinder is 25mm,what i need is to know the distance from the bottom of the plug to the inside of the cylinder to give a good ignition please.

Frank

[JasonBModerator @jasonb]

It can vary quite a lot from engine to engine, some I have made have the tip of the plug right inside the combustion chamber others the plug is in a completely different casting and 40mm or so from the chamber.

 

Can you not use the image in part 1 to give you an idea of where it should be

[HopperParticipant @hopper]

Generally speaking, it is a good idea to have the sparkplug located so the cool intake charge passes over it and cools it off. If you put it next to the exhaust valve, it will get overheated by the exhaust gasses flowing over it.

Found that out on old sidevalve motorbike engines with the sparkplug in one of the screw-in valve access plugs above the valve head. Get the access plugs mixed up and put the spark plug above the exhaust valve instead of inlet, the sparkplug will have a short but heated life.

[brickyParticipant @bricky]

Jason,I will take your advice and having worked out a scale to fit the drawing ,I get 6mm but to ensure the top of the plug clears the bore I have increased it to 7.5mm .

Hopper,I find what you say interesting,the inlet is opposite, would you think this is correct.

Thank you both for your assitance.

Frank

[JasonBModerator @jasonb]

Position is probably irrelevent on a model engine like this you are not going to be using high revs for any length of time as a bike or car engine would

[bricky]

On bricky Said:

 

Hopper,I find what you say interesting,the inlet is opposite, would you think this is correct.

Thank you both for your assitance.

Frank

Knowing nothing about what engine you are building, I could not really say. If you are working to an established design, best stick with that.

That said, if the valve opposite is the intake then I suppose the incoming charge will be directed generally at the sparkplug, albeit perhaps somewhat masked by the valve head itself.

As Jason says, probably not critical on a model running short demonstration runs under no load. Bit different from a vintage motorbike engine lugging up mountain roads dragging a couple of hundredweight of bike and rider along.

[JAParticipant @ja]

Directing the incoming charge at the spark plug definately makes starting easier.

JA

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