IC Engine without piston rings.

[Tomo1]

Recently I have read somwhere that some small model aircraft engines do not run piston rings. I had not thought that this was possible. Do they run due to extremly low tolerence between bore and piston? Would have thought that heat expansion would cause the piston to bind?

Must be missing something??

 

[Tomo1]

Wondering if maybe it is the oil in the fuel that helps create a seal?

Have heard that some IC model engines use oil in the blow-by to help lubricate the bottom end?

 

[Jasonb]

Thats right you just need suitable materials usually a steel liner and iron piston and lap one to fit the other. The little Firefly 0.46 that I made was like that and good for 8500rpm.

Yes both glow and diesel aero engines use oil in the mix for lubrication.

J

 

[Tomo1]

Thats right you just need suitable materials usually a steel liner and iron piston and lap one to fit the other. The little Firefly 0.46 that I made was like that and good for 8500rpm.

Yes both glow and diesel aero engines use oil in the mix for lubrication.

J

Thanks Jason B for answering that one for me.

I wonder if this could be possible on a 10cc single cylinder 4 stroke?

Then again with a aprox 2cm wide piston a set of rings would probly be easier to machine and fit?

 

[dmac]

The way these engines work without rings is to have a slightly tapered bore in the cylinder. The piston just nips up at top dead centre. So yes they do bind but all in the name of compression.

Dave.

 

[stevehuckss396]

Bobby Shutt built his Peewee V4 without rings. It's been running som 5-6 years now and still no rings. He used cast iron pistons and liners. No taper in the sleeves. Runs good.

 

[waynes world]

the high performance marine racing engines use brass, chromed cylinder liners in combo with high silicon alloy piston with all the thermal properties of the metal combinations well and truly soted out.

oh and yes a slight or a very fine taper above the exh port to tdc to trap the comppression ,the difference of good and bad is very minimal with them but last for years upon good managment.

 

[Tomo1]

Thanks everyone for your help.

I think I will have to give the no rings idea a go on my next engine.

 

[Tomo1]

Just had a thought. I wonder if fuel mix would have to be different?

 

[user 10585]

Very many model A/C engines do not use rings. They rely on the composition of the piston and cylinder and piston fit. Pistons are generally high silicon Al and cylinders are a variety of materials. Many new engines are AAC, ABC or AAN construction. AAC-aluminum piston, Aluminum cylinder, Chrome lined. ABC Al piston, Brass cylinder, Chrome lined. AAN aluminum piston, Al cylinder, Nicasil lined. All require close fits and have a tight fit at TDC when cold. Glow engines run methanol, oil and nitromethane. Oil content from around 10% up to 25% depending on construction. Older engines ran CI pistons and steel sleeves, no rings and baffle on top of the piston. These run higher oil content fuels and mostly if not all castor oil for lube.

 

[mu38&Bg#]

Ringless internal combustion model engines have been around for since the 1930's maybe? Modern designs utilizing specific aluminum and brass alloys for the piston and liner were first used in the 1970's. The ringless design requires excellent roundness tolerance achieved through lapping, honing, or grinding. The concept relies on predicted expansion of the piston and liner along with carefully selected tapers (400-600:1) of the liner and piston to liner fits. The piston has an interference fit when cold near TDC on the order of a few microns. At operating temperature all this comes together to give a tight seal that's not so tight it wears the parts. The bore must be hard in order to run a high silicon content aluminum piston without wear. This hardness usually comes from chrome or nickel plating on the brass or aluminum sleeve. When we say brass or aluminum, these are general terms. You will not build a successful ringless engine using any of the common aluminum alloys. The good alloys used for pistons are often priced at 10USD per inch for one inch round stock in small quantities. The oil requirement is really limited by the rod big end bearing. A recently released 10cc gasoline two stroke uses only 5% oil with a ringless piston, but has a needle bearing big end. Ringless designs using cast iron on steel or steel on steel piston/sleeves require more oil unless the sleeve is plated.

There have been commercially produced ringless fourstrokes. The some of the first production model four strokes made by Enya and Saito were ringless. The most recent was an OS model FL-70 about 10 years ago.

Greg

 

[jack620]

I'm currently building a pair of 1cc ring-less 'diesel' engines. I found getting the correct fit between the CI piston and the steel liner to be challenging. At this stage I don't even know if I have succeeded. I assumed a piston & ring combination would be easier to make. Have I got that wrong?
Chris

 

[RLWP]

I'm currently building a pair of 1cc ring-less 'diesel' engines. I found getting the correct fit between the CI piston and the steel liner to be challenging. At this stage I don't even know if I have succeeded. I assumed a piston & ring combination would be easier to make. Have I got that wrong?
Chris

You are right, which is why every car engine in the world is built with piston rings

Richard

 

[Lawijt]

Hello friends,

Like other said before.....The cilinder is a little bit conisch. That is for the compression. When a motor is new , it is almost possible to turn him around. You need to heat him up with a heat gun before starting!!! Then you may start & good running in!!! Very rich mixture!!
When the running in is done , you still feel that he wil blocked on the top dead center. Ok,a little bit. When the motor is warm , you don't feel that anymore.
In competition the motor have a life for about 8 - 12 hours.

Here a picture from a Novarossi 3.5cc kit. You see the oil grooves in the piston. This kit was tune by JP in France.

And here I try to take a picture from the inside of the cilinder. The shiny spot that you can see was the TDC of the piston. Not more for use anymore.

By the way , this is not a ABC kit , but a AAC kit. so the cilinder is from aluminium. Supergood , superfast & very high RPM's , but the exhaust port will be very hot & the cilinder is not more round , but more the form like a "EGG"....
I hope you understand it a little & sorry for my bad english writing .