RE: The Great Castor vs Synthetic debate

RE: The Great Castor vs Synthetic debate
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RE: The Great Castor vs Synthetic debate - 5/26/2008 2:38:02 AM

combatpigg

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It might be easier for you to visualize the rod yanking the piston as it swings past BDC and the piston is traveling down against very little opposition. The piston moves past this point 300 times per second in a Cox engine swinging a 6x3 with random control of the ignition point. The engines you work on run much slower and they have precise ignition control with knock sensors, etc.
If you don't believe that the little Cox can pull itself apart, take a 5x3 prop, cut it down to 4 inches and run a TD on 40% nitro. Now the piston should be approaching 500 cps. Within an hour or two, you will have a good chance of finding the piston wedged into the combustion chamber and the ball socket completely pulled apart into little crumbs.
If the piston never operates in tension on any engine, then explain rod stretch to me.

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RE: The Great Castor vs Synthetic debate - 5/26/2008 3:07:54 AM

jeffie8696

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What I could see in a situation like that is the crankshaft distorting and the crank pin not staying square to the bore. This could cause a lot of extra stress on the rod.

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Post #: 52

RE: The Great Castor vs Synthetic debate - 5/26/2008 3:08:56 AM

digital_trucker

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I'm not saying the rod is in tension, I'm saying that the small amount of play allows the piston to 'float' slightly at TDC. Then when the power stroke comes along SLAP. Each one slightly increasing the play until you either reset the ball or the rod bashes through the top of the piston. Overcompression hurries it along a bit, as I found out. If you happen to have a piston that isn't quite up to snuff in the temper department it'll happen quicker too. It may only happen a microsopic amount with the ball properly set, but it WILL happen. Remember that the piston comes to a complete STOP at TDC, but the propeller and crankshaft (and thus the conrod) keep right on a-goin'.

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Post #: 53

RE: The Great Castor vs Synthetic debate - 5/26/2008 3:54:52 AM

makoman1860

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From: Oshkosh, WI, USA
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quote:

ORIGINAL: combatpigg

It might be easier for you to visualize the rod yanking the piston as it swings past BDC and the piston is traveling down against very little opposition. The piston moves past this point 300 times per second in a Cox engine swinging a 6x3 with random control of the ignition point. The engines you work on run much slower and they have precise ignition control with knock sensors, etc.
If you don't believe that the little Cox can pull itself apart, take a 5x3 prop, cut it down to 4 inches and run a TD on 40% nitro. Now the piston should be approaching 500 cps. Within an hour or two, you will have a good chance of finding the piston wedged into the combustion chamber and the ball socket completely pulled apart into little crumbs.
If the piston never operates in tension on any engine, then explain rod stretch to me.

Well rod stretch on 4 strokes is caused by the load reversal on the off cycle. As far as the TD with a 4 inch prop, there could be many causes. If its truly not having enough cylinder pressure to maintain rod compression as you cross TDC, then your CA50 burn must be way late, and not producing much power. But then if your not having a good burn, how can you maintain that rpm? How do you know its not side loading of the ball at high RPM? The piston speed on Tee Dee spinning 20K isnt that unreal. Anyway this is all acedemic, for the purpose of the engines I am testing, there is no evident tension on the rod.

P.S. I had a thought....you might be so underloading the engine that you are getting rod reversal if using a 4 inch prop, but that means your effeciency has gone to pot and half of your fuel is burning outside the exhaust ports. under propped Full size 2 strokes have this problem as it causes roller bearing skidding. This is what we term a form of abnormal combustion. If your running into this condition your not getting all you can from the given fuel and compression and may need to prop a little heavier. Just a thought.

< Message edited by makoman1860 -- 5/26/2008 4:53:48 AM >

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Post #: 54

RE: The Great Castor vs Synthetic debate - 5/26/2008 4:12:37 AM

makoman1860

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quote:

ORIGINAL: digital_trucker

I'm not saying the rod is in tension, I'm saying that the small amount of play allows the piston to 'float' slightly at TDC. Then when the power stroke comes along SLAP. Each one slightly increasing the play until you either reset the ball or the rod bashes through the top of the piston. Overcompression hurries it along a bit, as I found out. If you happen to have a piston that isn't quite up to snuff in the temper department it'll happen quicker too. It may only happen a microsopic amount with the ball properly set, but it WILL happen. Remember that the piston comes to a complete STOP at TDC, but the propeller and crankshaft (and thus the conrod) keep right on a-goin'.

Ok so you have a piston coming up againt compression, slowing down before TDC, ignition starts before TDC, cylinder pressure rises faster then compression pressure before TDC, Crank crosses TDC while cylinder pressure rises, piston slowly accelerates as cylinder pressure tops off about 45 degrees after TDC, piston start slowing down 90 degrees after TDC. Ok heres something to check, measure the distance from piston crown to bottom of big end of the rod on a new piston, tighten it up, run the engine, now remeasure. When I did that to mine, I fould ALL of the slop was from compression wear, none from tension. Now if your engines 4 cycling thats a whole different story as you have an off cycle. Ahh this is getting way off topic isnt it

The short of the story is, the sure start engines I am testing are wearing in compression only, not a cyclic pounding. The material does seem soft on the pistons, especially when compared to my older engines. You know Mccoy used to use a retaining clip to hold the piston ball in, and not a crimp? and funny thing is I never see any wear on that little retaining clip on those designs. So now I have a failure mode, what can be done to fix it?

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Post #: 55

RE: The Great Castor vs Synthetic debate - 5/26/2008 5:46:36 AM

makoman1860

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Ok I think I get where you guys are coming from. In an underloaded condition you will get a tension reversal at TDC. Much like if you had a flywheel on the the engine but no load, and wond the engine up as fast as it would run. In the case of running out of breathing ability and flame speed before reaching the mechanical limits of the engine you get this condition. We get this in full size engines, usually with failure as a result, therefore its avoided. If the engine develops peak power at say 18K , why would you prop it for higher?

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Post #: 56

RE: The Great Castor vs Synthetic debate - 5/26/2008 5:54:40 AM

combatpigg

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Rod stretch is from too much tension on the rod, period. If the rod is always loaded in compression you would never see a stretched rod.
This is taken from Carillo, the maker of the finest steel drag racing rods in the world for the past 40+ years:
"All connecting rods stretch to a certain degree during the maximum acceleration and deceleration points in the stroke. This results in a pulling and pushing action that distorts the big end of the rod."

< Message edited by combatpigg -- 5/26/2008 6:07:00 AM >

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Post #: 57

RE: The Great Castor vs Synthetic debate - 5/26/2008 6:18:16 AM

jeffie8696

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4 stroke versus 2 stroke. Apples and oranges. 2 stroke rods never fully unload like a 4 stroke. In a 4 stroke there is an intake stroke that places the cylinder under vacuum, this never happens with a 2 stroke since the intake and exhaust stroke is combined.

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Castor, its like Vitamin C for glow engines. I am not Dom from Airwolf but I do resemble him.......Unfortunately

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Post #: 58

RE: The Great Castor vs Synthetic debate - 5/26/2008 7:01:27 AM

combatpigg

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quote:

ORIGINAL: makoman1860

Ehhhh no offense but im not buying that Ive never seen any engine, 2 or 4 stroke where the connecting rod was put into tension ............

sounds like we're talking about both types of engines to me.

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Post #: 59

RE: The Great Castor vs Synthetic debate - 5/26/2008 5:02:49 PM

gcb

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From: Port Ewen, NY, USA
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quote:

ORIGINAL: makoman1860

George,
That break in for CI piston engines is pretty common I think. CI grows for the first few times you heat and quench it, so as the surfaces wear, and work harden, if you heat and cool the piston rapidly your also taking up the wear. PAW diesels love being broken in by a few quick hot lean runs and pinching the fuel line so the engine dies quick and can air quench. All the engines I build with CI pistons get broken in this way now.
-Aaron

I have been using that particular break-in procedure on my diesels ever since I found the Canadian Combat flyers' site: http://www3.bc.sympatico.ca/dieselcombat/index.htm

Although that method may work for glow, I'm not sure it is the best method for glow because of the different fuel properties. Whereas kerosene supplies some lubricating properties, I'm not sure alcohol does. For an I/S glow, I usually start out rich with short runs, and lean it out progressively every couple of runs until it runs well at peak.

The method I mentioned above about running until it seizes for that particular engine is a method I have never used, and probably never will.

One additional thought, I always break-in a new engine on a test stand so I have full control over it...even an ABC. Of course these are only MY preferences. There is seldom only one way.

George

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Post #: 60

RE: The Great Castor vs Synthetic debate - 5/26/2008 5:35:42 PM

gcb

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Aaron,

I can't believe that these are typical of Cox engines unless they GREATLY reduced quality near the end.

To give you a comparison with other engine tests I have read in model mags, an Enya .19 was broken-in, measured, then run for 300 hours. Upon tear down and re-inspection...no additional wear. Factory machining marks were still there. Another test was by Fox. They hooked an engine (.35 I think) to a 55 gal drum of fuel and let it run. If I remember correctly, they stopped the test at the end of the week because people got tired of running it. Engine was still good.

All of this proves nothing because those engines were constantly run. I think most wear occurrs with starting and stopping (as in your phase 2).

Some questions:
When you cleaned the engines before running, did you clean the whole engine or just the piston? Sometimes some junk remains in the engine.

Were the crankcases the cast type or bar-stock type?

When the castor engine slowed down was it from varnishing?

What type of castor did you use?

George

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Post #: 61

RE: The Great Castor vs Synthetic debate - 5/26/2008 5:49:16 PM

jeffie8696

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Typically engines produce the most wear during startup when the engine parts have not been lubed properly yet. This is the case with typical 4 stroke applictions. 2 strokes are a whole other animal. Since the lube is supplied from the incoming fuel the startup probably should be rich and at a decent rpm to provide lube as soon as possible. Yes this is hard on cylinder walls and I would agree that a lube should be used that sticks around (literally) to the cylinder walls for protection. If the cylinder walls and piston are made from the same material they will grow at the same rate and minimize damage but in the typical car engine running an aluminum piston inside a cast iron cylinder you have to treat it differently. This is why I like the K&B Sporster sreies engines, they have aluminum pistons (chrome plated) in an aluminum bore so the parts expand at the same rate. ( hopefully since cast aluminum and forged aluminum expand at slightly different rates. )

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Castor, its like Vitamin C for glow engines. I am not Dom from Airwolf but I do resemble him.......Unfortunately

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Post #: 62

RE: The Great Castor vs Synthetic debate - 5/26/2008 6:45:24 PM

Mr Cox

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quote:

ORIGINAL: jeffie8696
This is why I like the K&B Sporster sreies engines, they have aluminum pistons (chrome plated) in an aluminum bore so the parts expand at the same rate. ( hopefully since cast aluminum and forged aluminum expand at slightly different rates. )

Uhhh, I though it didn't work that well....
http://www.rcuniverse.com/forum/m_7480043/anchors_7481085/mpage_1/key_/anchor/tm.htm#7481085

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Post #: 63

RE: The Great Castor vs Synthetic debate - 5/26/2008 7:30:47 PM

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