Author Topic: 30cc Flat Four (Puma)  (Read 19630 times)

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #180 on: September 26, 2019, 10:20:54 AM »
I'm sure there are those out there capable of making miniature oil control rings, I'm sure I've seen it done with a V8/V12 build. Itís not something I'd attempt.

As these O rings fit the 1mm x 1mm groove for CI rings they are just 1mm cross section. They don't seem to drag that much the only minor problem is a degree of stiction from stationary. As you've seen, this engine has run up to 8000rpm, albeit briefly. Weíll see how we go with longer runs.

I like the idea of quad rings and plan to fit them to my loco once the existing Viton O rings have had their day. I had thought they would be difficult to source but went to my local supplier and they had them in stock. As simple as deciding on the normal O ring youíd want and add a ĎQí on the front of the number.

Update - Crank case is out of the paint shop and redied for re-assembly. I'm going to take it easy to see how I can get it together and sealed up enought to stop the external oil leaks but able to dissassemble for maintenance.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Jasonb

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Re: 30cc Flat Four (Puma)
« Reply #181 on: September 26, 2019, 10:25:22 AM »
May be worth tapping a couple of holes in the sump and end cover flanges so you can use a jacking screw to help get things apart, often seen on old steam engines

Offline Vixen

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Re: 30cc Flat Four (Puma)
« Reply #182 on: September 26, 2019, 11:13:42 AM »
We are in danger of spoiling Doubletop's excellent Puma build with all this O ring piston ring discussion. Perhaps it would it be a good idea for Jo, or one of the other admins, create a new topic called "O rings as piston rings" or something similar. Then transfer the relevant replies #175 to #179 and this one, to the new topic.

Here are some thoughts about developing the O ring piston ring concept further.

An iron piston ring creates the initial wall seal by spring tension. The compression/ combustion  gas pressure then gets behind the ring and forces the ring tighter against the cylinder wall. The current thinking with O ring piston rings is to precompress, or nip,  the O ring by a few thou, by controlling the groove depth. This gives rise to higher static ring friction. Could an alternative approach be to use a slightly over-diameter O ring (say 26mm OD in a 25.4mm bore) provide the initial seal, with clearance on the ID. Gas pressure could get behind the O ring to increase the sealing force, in the same way as a conventional iron ring. Only a slight change in philosophy and dimensions, but it could have a beneficial effect.

The quad ring appears to have a number of advantages over the plain O ring both for both compression and oil control. The square section would appear to provide a better shape to scrape excess oil off the cylinder walls. The excess oil still needs to escape through the radial drain holes through the piston wall. These drain holes could be blocked if the O ring is precompressed, Perhaps an over-diameter ring with clearance on the ID would work better.

It would be very interesting to be able to make a direct comparison between an oversize O ring, with ID clearance, and a precompressed O ring, as a method of reducing ring friction.

My thoughts

Mike

#OringvCI

« Last Edit: September 26, 2019, 11:09:36 PM by Vixen »
It is the journey that matters, not the destination

Oh! sod the journey, lets hit the bar and pool instead.

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #183 on: September 26, 2019, 08:48:07 PM »

May be worth tapping a couple of holes in the sump and end cover flanges so you can use a jacking screw to help get things apart, often seen on old steam engines


Jason

I had the same thought and made a start yesterday and included them on the end cover flanges however the cylinders would also benefit so I'll do them as well. I'm using a paper gasket on the sump as that doesn't interfere with overall dimensions.

Pete


We are in danger of spoiling Doubletop's excellent Puma build with all this O ring piston ring discussion. .........................


Mike

Not at all, as far as I'm concerned stimulating a discussion is one of the purposes of these forums. Thats how we learn, or at least that's how I learn. I've always said I did my ME apprenticeship on these forums.

Maybe we should adopt the social media habit of including #hashtags in our posts so a search for say #Orings or #OringvCI will bring up all the threads on the subject (a search will find this one now  ;D ). I'm sure there will be a number of discussions that would benefit from retrospective #hashtagging. The mods could add a sticky on the topic #Hashtags and maintain a list of those to use so everything gets correctly grouped.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #184 on: September 26, 2019, 09:07:26 PM »
Viton o-rings do an incredible job of sealing a piston in a cylinder. Based on experience, I use a single 1/16" nominal cross section o-ring in a 0.094" wide slot x 0.058" deep in the piston


Continuing the Oring discussion I see from Brians dimensions that he is using the Orings as designed. A gap wider than the O ring cross section (50%) so the pressure can get behind it and deform the ring against the walls and a 7.2% squeeze. Also using only one ring, a second ring won't do anything as if the ring on the pressure side is doing its job the secod ring won't get any pressure to deform.

In my case I'm not following the rules. Two rings and a gap but not 50% and squeeze that is probably not more than 4%. With a 1mm ring thats only 0.04mm or 0.0015"

P.S; I've just asked my supplier and the smallest cross section for a quad Oring is 1.8mm. I won't be modifying my pistons to be able to use them and making 4 more pistons isn't on the cards at the moment.

#OringvCI  ;D

Pete
« Last Edit: September 26, 2019, 09:44:31 PM by doubletop »
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Vixen

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Re: 30cc Flat Four (Puma)
« Reply #185 on: September 26, 2019, 11:17:10 PM »
Maybe we should adopt the social media habit of including #hashtags in our posts so a search for say #Orings or #OringvCI will bring up all the threads on the subject (a search will find this one now  ;D ). I'm sure there will be a number of discussions that would benefit from retrospective #hashtagging. The mods could add a sticky on the topic #Hashtags and maintain a list of those to use so everything gets correctly grouped.


Pete

The hashtag idea may or may not catch on with the MEM membership, only time will tell.

I have gone back and retrospectively added the #OringvCI hashtag to all my related posts, I cannot do that to everyone else's posts. At least I will be now be able to refer back to these posts, at a later date.

Mike
It is the journey that matters, not the destination

Oh! sod the journey, lets hit the bar and pool instead.

Offline Jasonb

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Re: 30cc Flat Four (Puma)
« Reply #186 on: September 27, 2019, 07:15:06 AM »
#Orings, #O-rings, #O rings

That should cover all eventualities ;)

I also use the usual slightly wider and lightly shallower groove sizes. Not the ones you will find published on most O ring suppliers sites but the ones in Model Engineers Handbook and Reeves paper catalogue which do not compress the ring as much so there is less friction.

For an actual 2.4 C/S ring that would be 2.6mm wide x 2.3mm deep. Also along the likes of Mike's thoughts that is a 19.6mm ID x 24.4mm OD ring in a 24mm bore and bottom of groove is 19.4mm dia

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #187 on: September 27, 2019, 10:25:28 AM »
The way the search engine works there only really needs to be one hashtag in the thread as the basic search just gives the most recent post that meets the search criteria.  So if a thread goes off in an interesting direction a single hashtag near the start which will get people to the beginning of the discussion.

Iíve started the re-assembly. The cylinders still complete with their valves and pistons, with my finger over the spark plug hole it is impossible to push the piston into the cylinder by hand. That tells me the O rings are working well, whether to recommendations or not

A bit back to front but as Iíve modified the timing to use a 160mm belt Iíve updated the drawings



As I said -re-assembly has started



The 160mm belt and tensioner. Now with 6 teeth of the crank pulley engaged rather than the previous 3 or 4. You can also see the tapped holes for the jacking screws to hopefully enable an easier disassembly.

And with the timing sensor loosely fitted.




Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #188 on: September 28, 2019, 08:58:16 AM »
Iíve got it all back together and gave it a quick run this afternoon.


I did fill it with oil and the oil baffles seem to be working. Only an occasional quick puff. I was trying to see how slowly I could get it to run.  I had to keep holding on he the throttle as the vibration keeps opening it up and the revs rise. It did run slowly and was remarkably quiet with the occasional loud crack and there was also an element of hunting. Iím assuming the cracks are a timing issue.

When I set it up this time the right-hand cam, cylinders 2 and 4, seemed to be out slightly. Setting the camshaft on exhaust valve #4 opening exhaust valve #2 was about 10degress late. I compromised and set it between the two. The left-hand camshaft was spot on. I canít remember if that was a problem the first time I set it up or just overlooked checking it. I had been very careful when I assembled the camshafts so there shouldnít be a problem. However, the only way to find out is strip it all down again. The only way to get the camshafts out is a complete strip down as the bearings are fixed in place.

Regardless of the possible camshaft problem I was happy with the way the oil baffle worked.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Jasonb

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Re: 30cc Flat Four (Puma)
« Reply #189 on: September 28, 2019, 10:09:38 AM »
That exhaust looks a lot cleaner now with the baffles in place

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #190 on: September 28, 2019, 11:52:14 AM »

That exhaust looks a lot cleaner now with the baffles in place


And there's no oil leaking out of the crankcase. Did I remember to put some in?........ ;D ;D

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline AlexS

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Re: 30cc Flat Four (Puma)
« Reply #191 on: September 28, 2019, 08:25:51 PM »
Great engine and build log!! What about a dyno run?

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #192 on: September 28, 2019, 10:55:36 PM »

Great engine and build log!! What about a dyno run?


Thanks Alex

Dyno run? I understand the theory but what is the method generally used for model engines?

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline Brian Rupnow

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Re: 30cc Flat Four (Puma)
« Reply #193 on: September 28, 2019, 11:04:33 PM »
Doubletop--Look up "Prony brake". That is a method used to calculate engine horsepower. There are Youtube videos of it being applied to model engines. ---Brian
« Last Edit: September 29, 2019, 12:04:13 AM by Brian Rupnow »

Offline doubletop

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Re: 30cc Flat Four (Puma)
« Reply #194 on: September 29, 2019, 02:43:10 AM »
Doubletop--Look up "Prony brake". That is a method used to calculate engine horsepower. There are Youtube videos of it being applied to model engines. ---Brian

Brian

Of course and would be easy to set up against the existing flywheel with webbing strap and a $10 digital baggage scale.

It could be on the 'to do' list once I've got the engine running a little better.

Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss