Author Topic: Help wanted with piston design  (Read 881 times)

Offline Steve Crow

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Help wanted with piston design
« on: November 28, 2021, 05:42:25 PM »
Hello, I'm 16 months into building a 1/12 scale V8, powered by compressed air/gas. The bore is 7.2mm and the stroke is 5mm. I will not be using rings. There is thread on here about it but I haven't updated it in ages.

I've made nearly all the parts but I haven't even started designing the pistons yet. I made the con rods and the crank nearly a year ago!

I haven't been able to find any help on the net as regards proportions of the pistons. I'm sure somebody here can help.

Here's a diagram to help me explain:-



My questions are-

Is there a rule of thumb for the proportion of D to x? I realise that having a bigger x will help keep the piston parallel.

Is it more advantageous to have the centre of the gudgeon pin (p) between a and b as in the first diagram or is it ok below as in the second?

Grooves as shown on the left. Are they a good thing?

Finally, materials. I haven't made the bore liners yet and have been thinking of using brass. For the pistons, I have no idea. I would like to keep the weight down, how about ali with a steel sleeve for bore contact?

Any ideas, advice or experience would be very welcome.

Cheers

Steve

Offline Roger B

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Re: Help wanted with piston design
« Reply #1 on: November 28, 2021, 06:06:41 PM »
Some thoughts:

The maximum length of the piston below the gudgeon pin is limited by the length of the con rod and the diameter of the crankshaft.

Modern high performance engines have very short pistons but also very tight tolerances.

If you are not using rings a longer piston with some labyrinth sealing grooves will probably give the best seal.

I have a 16mm bore and stroke petrol engine with a brass liner and aluminium piston with a single cast iron ring. This revs to 7-8000 rpm and seems to work ok.
Best regards

Roger

Offline Jasonb

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Re: Help wanted with piston design
« Reply #2 on: November 28, 2021, 06:27:03 PM »
A lot will depend on how much clearance you have before the piston skirt starts to collide with crank or conrod but I tend to try and get the pin al least in the middle if not slightly closer to the head of the piston. You can always cut away part of the skirt if there are issues but still keep some length to help line things up.

I would tend to use V grooves to retain a little oil rather than the rectangular notches you show.

Aluminium will run fine with a small amount of oil in the air supply or a drop or two every few minutes of running

I've just made this one for the current high speed steam engine that will only run on air, 18mm dia x 15mm long the pin is 8.25mm up from the bottom.


Online Jo

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Re: Help wanted with piston design
« Reply #3 on: November 28, 2021, 06:54:00 PM »
On the little compression ignition engines we use CI pistons which have smooth outsides - no labyrinth seals or rings  :hellno:

When Eric Whittle designed his V8 engine (10cc air cooled glow plug) back in ME in 1995 he used HE15 with no seal/rings. The pistons are about 10mm tall and slightly over 10.5mm high but that includes the curve where it goes over the crankshaft at BTC.


If you have not read Eric's article it is well worth a read it will give you lots of ideas - including how to make the tiny bits.

Jo
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Offline Jasonb

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Re: Help wanted with piston design
« Reply #4 on: November 28, 2021, 07:12:03 PM »
You do have the advantage of a high oil content of the fuel on the compression ignition engines to keep every thing slipery

Offline Steve Crow

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Re: Help wanted with piston design
« Reply #5 on: November 28, 2021, 10:22:19 PM »

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.

Steve

Offline derekwarner

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Re: Help wanted with piston design
« Reply #6 on: November 29, 2021, 06:17:55 AM »
Steve...you 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

Derek
« Last Edit: November 29, 2021, 06:35:56 AM by derekwarner »
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Offline Steve Crow

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Re: Help wanted with piston design
« Reply #7 on: November 29, 2021, 06:52:33 PM »
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.

Cheers

Steve

Offline TonyM

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Re: Help wanted with piston design
« Reply #8 on: November 30, 2021, 08:20:45 AM »
You could use an inline lubricator either separate to regulator or combined like this Tailonz-Pneumatic-AFC2000-Compressor-Lubricator in Amazon.

Offline cnr6400

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Re: Help wanted with piston design
« Reply #9 on: November 30, 2021, 12:46:24 PM »
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.
« Last Edit: November 30, 2021, 12:50:34 PM by cnr6400 »

Offline Jasonb

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Re: Help wanted with piston design
« Reply #10 on: November 30, 2021, 01:01:47 PM »
It's an air powered engine with a wrist pin that is probably no more than 1.5mm diameter with M1.0 thread, your a better man than me if you can fit snap rings to retain that :o

I've suggested an air tool oiler on another forum but they don't work so well if pressure is low so just adding a drop every few minutes into the intake would be a good alternative and what I use. needs to be a light oil to keep the sticktion down such as this

https://www.eileensemporium.com/materials-for-modellers/product/superfine-clock-oil-bottle-15ml/category_pathway-1109

Offline Steve Crow

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Re: Help wanted with piston design
« Reply #11 on: November 30, 2021, 02:33:48 PM »
Pretty close with the wrist pin dimensions Jason! It's 1/16" with M1.2 thread.

I take it that the clock oil is very low viscosity?

Offline Jasonb

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Re: Help wanted with piston design
« Reply #12 on: November 30, 2021, 02:48:35 PM »
Yes, On a couple of engines I used something a bit thicker like 32wt hydralic oil and the light springs on the piston valves were not enough to keep them in contact with the cams and several hours were wasted trying to work out what was wrong. Once I cleaned off the old oil and used the light stuff the engine ran great. Could have put on heavier springs but that just adds more load for the engine to overcome and that reduces smooth slow speed running

I bought a syringe and parallel needle from the same source and use that to apply the oil.