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Help wanted with piston design

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Steve Crow:

Thank you Jason, you answered pretty much all of my questions there!

I intend to introduce a bit of oil so I'll go for the aluminium.


derekwarner: have mentioned the use of one, or a number?.....of 'grooves' or labyrinth rings in the piston

Successive labyrinth rings in pistons of a single acting engine, introduces Boyles Law P1 * V1 = P2 * V2, so whilst the labyrinth cavitiy rings will hold lubricating oil, they will also progressively reduce the final [bypass pressure] to the crank rod side of the piston

The point here is with this type of single-acting engine, positive crankcase ventilation is required


Steve Crow:
After taking on board your advice, I have come up with this design -

Can anyone see any potential problems or means of improvement with this?

Also, I would be interested in any methods for introducing oil to the air supply.

In the intake manifold (plennum?) I have made, I've left space for bleeding off some air in case I need to inject some oil any where but mixing with the air supply sounds a lot simpler.



You could use an inline lubricator either separate to regulator or combined like this Tailonz-Pneumatic-AFC2000-Compressor-Lubricator in Amazon.

Hi Steve, Based on seeing thousands of small engine, motorcycle, and car / truck pistons, and pistons in many diesels tiny up to huge, I make these comments:

1. several tiny V grooves on the piston will likely work much better than one or two big ones to gas-seal the piston.
2. several grooves will likely improve lubrication of the piston since oil will be retained better in a small groove than a big one.
3. I would not recommend using a thread to secure a piston pin. You need all the area you can get in a piston to support the piston pin against combustion force which can be very large. By having a thread retain the pin, and having a head cutout for the head of the pin, you lose valuable piston pin support area. I'd suggest the pin be placed in a bored hole as far across the piston as you can get, and be as large a pin diameter as you can fit. It can be retained by snap rings or better yet by bronze or PTFE pads or pads with a pin turned onto them that engages in the hollow piston pin. The pads would be used at the ends of the pin to keep the pin from pushing out into the cylinder bore.
4. be sure to provide a way for oil in the fuel to find its way to the small end of the con rod to lube it. Some engines have a hole or holes, others like motorbikes have a slot, to give oil the best chance to get to the piston pin and/or piston pin bearing if one is present. If you can only use a tiny hole or holes, a bronze piston pin in a steel con rod might be a better choice as these materials will run well together.

Re oiling - mixing the oil into the fuel is a crude but simple and well proven way to get consistent mixing of oil and fuel and air. You really do not need fancy gizmos and instruments for this. Millions and millions of chainsaws weed trimmers scooters mopeds RC aircraft and cars etc with two stroke engines cant be all wrong.

Just food for thought.


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