Possible "Piston-Rock???"
 

Can anyone explain the purpose of the pistons skirts hanging out of the sleeve at BDC? I'm a registered drag racer, and I have been building race engines for more that 30 years. In a real race engine, pistons hanging out the bottom of the sleeve is not a good thing and causes piston-rock. Several pistons have broken in a few of my little nitro engines and I'm wondering if piston-rock (due to several mm of the piston hanging out of the sleeve at BDC) is a factor. What do you guys think? I'm thinking of machining the piston so that there is no part of the piston that extends beyond the sleeve. Has anyone ever did this?

Good question but are you sure its piston rock causing you issues. Seems to me that since there are no rings the piston would not move all that much especially at TDC. Where are your pistons breaking at?

The skirt is there so that at TDC it covers the exhaust port, otherwise the engine looses all the incoming mixture out the pipe. Are you setting the piston at TDC BEFORE you take out the backplate? A lot of engnies have a groove on the backplate because otherwise the skirt would hit it.

They are breaking at the pin. I just machined one so that it lines up with the sleeve at BDC and took the car out and ran it a little. Revs quicker and accelerates quicker. Don't know the long term effects yet, but so far seems to be a go. I'm not sure it's piston-rock. That is just something I'm wondering. Thanks for the response.

Okay, now that you pointed that out, I can see why the extended skirt is there. I got lucky and didn't take enough off the piston to uncover the exhaust port. I just can't figure out why the piston are breaking. All in the same spot. Detonation maybe? Running 30% Byron's with a #4 turbo plug at 7000 feet.
Thanks.

as long as the exhaust port stays covered at tdc it will be ok for awhile but remember it is a 2 stroke and the skirts get flexed into the ports under pumping compression on the down stroke all be it minimal because of the small displacements we are dealing with it is the cause of the majority of piston wear besides the liquid sand paper we run in them and the average piston speed at 40000 rpm is almost 4200 feet per second on a 16mm stroke just something to think about

You mentioned the engines are running much better than stock after modifications, several things are happening.

More performance does mean more forces. The piston has already to deal with a lot of G-forces and every extra bit of performance is dubbeling the load. A thin part like the skirt is the weakest part of a piston. Just for fun calculate the speed of the crankpin turning arround at top RPM, it can go up to 100 km/u, The piston is accalerating from 0 to 100 km/u in an half stroke and slows down to 0 in the other half stroke. The G-force on a few grams piston can go up to several kilograms.....

Then there is flex in the crankshaft, with more RPM the bending of the krankshaft is getting larger and it is possible that the skirt is touching the backplate or the piston is even touching the head, if that is the problem you should see a mark in the backplate and/or head.

And a last thing is heat. The skirt has to deal with a lot of heat and heat is affecting the strength of materials. It becomes weak and can deform or snap off due the high forces. In this case some extra cooling with better lubrication can do the trick.

Some things to look at.....

As a fellow machinist and engine builder (Subaru in my avatar is 589 to the wheels on E85!!) I wonder the same things! these engine mfg have been doing this for a long time with that being said I am sure there is a good reason for the skirt being so much longer. Personally i havent figured it out yet and will eventually get to it, right now I am concentrating on porting and port timing. In my 1:1 experience its not typically the high RPM's that kill an engine(unless you have oiling issues) its midrange torque that will send a rod through the block. Detonating while your in that range will break parts. once you get passed peak torque, momentum becomes your friend.

I have not actually looked at the relationship to the long skirt to the exhaust port but it seems to me if you were to bring the piston to TDC and then scribe the piston skirt from the exhaust port opening that would be the shortest that the skirt could possibly be. If that port opens up to the crankcase you would end up with exhaust waste in the crankcase which would in turn end up back in the CC. This would eventually turn everything inside the engine into a black nasty mess! You would also loose crankcase pressure. Keep in mind that the exhaust system sends pulses backwards.

Roelof , The crank is not going to flex it is Hardened steel. It is supported by one huge bearing and a smaller one meaning that the area supported by those bearings will not move unless the bearings move. So you are left with a very short amount of the crank sticking out past that point and connecting to the rod. Hardened steel will break before it flex's thats why they harden them! Only way the piston will hit the head is if the rod is either stretched or the bushing is worn out. Not sure which engine is being used but a billet 6061 or 7075 piston more than likely will not break but can deform slightly under stress but a hypereutectic piston will break and snap off as they have more silicon in them giving lighter weight and better friction coefficiency . more food for thought!

How about a pic?

You are overevving your engine. Ive had the exact same problem. The piston from the pin down basically comes apart. The engine is too lean on the top, but being maksed by too rich of a bottom end.

You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!

+1

Maybe you are confused on what flex is?? I dont dis agree with the fact that you need a certain amount of flex but not on a crank, you will get more play from the bearings than you will from the crank. Exactly where do you think the flex will come from? think about it, your not going to get any flex between the bearings ( if you did the crank would bind)meaning it can only happen on the largest area of the crank! secondly with a properly hardened crank at most you would see maybe .0002" and once again thats not enough to do any damage. now on top of that you have to think of what would cause the flex and that would be the force of the piston and rod moving the crank and both of those two parts are made of soft aluminum with bronze bushings! both of which will bend/flex before the crank ever would. As stated before i have been building real engines for most of my adult(I am 42 now:cry:) life ranging from 700hp methanol rotary engines to twin 100mm turbo charged 2500hp v8's!! Granted i dont have that kind of experience with these engines , the principle is still the same metal is metal! If you are breaking cranks i tend to believe it is more of a frequency issue than flex,40k RPM's is alot no matter what scale!

I am just giving my 20+ years of machine shop/engine building experience, i could be wrong as i have been before. But i dont see how the laws of physics change with nitro engines:D


Z, i think over revving and too lean is right and probably what caused the damage. Hoes is probably correct.

Marcus,

Roelof is correct, there is movement inside the engine. I'm not saying you're incorrect on some of your points but I DO have many years experience with designing, building, and tuning these small 2 strokes we use for our R/C cars.

With a top on road engine spinning at approx. 42-43000+ RPM there is deflection in the crank pin, piston pin, piston skirt, and rod at a minimum. There are also frequency movements in the bearings and crank body. Even the carb venturi resonates when the engine reaches peak power.

4 stroke engines like you are familiar with building have much lower piston and crank speeds even at full power/torque. Cylinder pressures are also lower due to the lower compression ratios typically run.

With the 2 stroke interference fit, schneurle port design we use the piston skirt is a smaller diameter than the top of the piston at the sealing area. Cutting the skirt, even if the exhaust port remains cover at TDC will result in a crankcase pressure loss due to venting around the smaller diameter skirt through the exhaust port. There is piston rock, but not enough to break the skirt or cause it to bind in the sleeve. The longer skirt also helps to maintain crankcase pressure.

Something else to ponder. The 2 stroke has no harmonic balancer or counter acting power pulses to nullify the shock induced into the reciprocating assembly at each combustion pulse. The counter weight on the crank is NOT the equal weight of the piston, rod, and pin/keepers. In most engines it is between 55 and 65% of the mass.

Long answer short, I would not recommend cutting the skirt shorter on the piston. I would also not recommend overly aggressive porting on existing engines for more performance. Too much material removed can reduce charge flow and cranking pressure which will net you less power and lower fuel mileage than if you left the engine stock. Not too mention the risk of damaging the chrome liner if you try changing port timing.

Movement or flex? I just find it hard to believe that the crank "flexes" more than it simply have movement from bearing play and what not. Flex in my mind is the bending of the crank which I just can't see happening in such a short distance , does that make sense ? Just trying to get the right terminology!

The amount of movement Roelof mentions is about the same amount as the bushing clearances from the top and bottom of the rod.

Just so there is no confusion I am not arguing the points just trying to verify them in my head! So take no offense to what I am saying.

Rick ,A lot of my experience stems from rotary engines which are not really four stroke !

The crank also flexes in addition to the radial play in the bearings/bushings. That's why the crank pin is surface hardened and not thru hardened. Thru hardened cranks snap due to the bending load imparted from the piston assembly.

I would guess that the engines you base most of your experience on are multi rotor. Very smooth in operation with the counterbalancing effect. Think of our little 2 strokes as a whack-a-mole game every rotation of the crank, that illustrates the action better than anything else I can think of. :lol:

Here are two pictures, and the engines are OS 18TZs.

http://piston#2jpg

Well, I tried to upload two pictures but don't know how.

I have seen a video about Cosworth engines where the engineers did calculate the zero headclearance at max rpm and indeed you could see a complete clear squish band on the piston. Flex is there and it is needed to stay alive. I just calculated it, at 44.000rpm the (lineair) acceleration of the piston in an half stroke a 3 gram piston can create about 30kg of force. With the conrod making 5 to 6 grams in total it can go up to 50kg, that is huge! It is for sure pushing and pulling hard on the crankshaft,

You need to upload them....

I understand what you are saying but that movement is not soley from the crank but more from the other moving parts. The crank is only one part of the equation and since the crank is the hardest and most unlikley part to actually "flex" the movement you see is from the soft piston deflecting,rod deflecting, dual bushings and wrist pin all of which will move/flex more than a crank. Why would the actual crank flex before any of the other moving soft parts? I want to bring up the fact that movement and flex are two completely different things. A resonance can and will cause flex where as movement is just simply from tolerances on moving parts. I am a little confused on which one you are attempting to explain!!

Piston skirt
 

Just food for thought: I had the Same problem twice with same engine. Fortunately I was at a race which Rody Roem was attending. Rody explained to me that a broken skirt is usually caused by a worn crank pin. He explained that with a worn pin at high RPMs the resonance travels through the rod to the piston and skirt and will cause it to crack. Made since to me as it happened to twice to the same engine.

Flex is present and necessary. Every crank I've inspected that broke the crank pin was due to the hardening being done incorrectly. The crank pin was too stiff and did not flex enough when loaded by the piston assembly. If the piston assembly was a 'soft' as you're implying in your arguement, the bushings and piston would beat themselves to pieces in short order once the engine accelerated above 10000 rpm.

The rod bushings are made of the material they are for long service life and lack of friction on the steel crank and wrist pin. Yes they are 'softer' than the parts they ride on but hardly 'soft'. They are intended to be the wear items in the assembly to prevent failure of the more expensive and harder to produce crankshaft and wrist pin.

Per my calculations, an engine running at 44000 rpm (Roelof's example) with a 18mm stroke (typical average for a .21), gives 36mm of piston vertical travel per revolution. That works to 1,584,000mm per minute. This is equal to 62,362 in. per minute approx. That gives you a piston speed of 5196.850 ft/sec or 3543.307 mph or 4.654 mach. That's just an approximation covering the entire travel. Don't forget the piston comes to a stop twice per revolution and then accelerates the opposite direction to peak speed about mid stroke before starting to decelerate at the opposite end. Add to that cylinder pressure on the compression side of the stroke and the spike at combustion. You can be fairly certain that EVERY part in the reciprocating assembly is flexing/bending/moving, whatever you want to call it most if not all of the time. That sound you hear when the car goes by is the exhaust note, sure. It's also the sound of the piston breaking the sound barrier many times a second. Find a video from Lostallo, Switzerland when they run 1/8 open. The amount of time spent at WOT per lap is around 13-14 secs. The front straight alone account for over half that time. Listen to the cars as they pass the camera. Amazing does not describe it.

Yes, rods do stretch, bushings deflect, wristpin bend, bearings have runout, but the crank also flexes both in whole and at the crankpin when running. I've been studying these engines for a long time and I'm still amazed they actually run and don't destroy themselves instantly, especially they way they are treated by the majority of the users that have them. :lol:

I think you are wrong with your calculations... The max speed of the piston is the same as the rotating speed....

The stroke of current .21 engines is about 17mm, 1 round is 53.4mm in distance.
Lets say @46.000 rpm is 766rps. In 1 second it is running 766 x 0.0534 = 41 meter/sec which is 143km/u (no sonic sound)
I can remember a limit for lubrication has a magic number of 35m/s...

V=a x t , a = V/t

t is 1 round /4 (the moment from 0 to max speed) so that is 1/766 and again /4 = 0.326msec

a = 41/0.000326 = 125624 m/s^2

10m/s^2 = 1G so you are talking about 12562.4G

I know a piston+rod is about 7.5 gram so it will create 94kg force @46.000rpm

This is pure theoretic because a 90 degrees travel of the crakshaft is not giving a halfway of the piston travel. A 90 degree exhaust timing is somewhere arround 7mm so there is a 7mm and a 10mm acceleration and not exactly half. And these calculations are in a lineair movement but actually is is more like a sinus shape so actually the 94 kg is a bit higer.

As you can see with such forces manufacturers are searching the limits of the mechanical strength.

By the way, some of the noise of an engine is comming from the cooling head, the vibrating fins can produce some noise....

The term soft is in relation to the crank which is WAY harder than the rods or piston which are both made of aluminum! So what i am trying to understand is regardless of the speed of the piston and what not, when you are at the point when the flex is supposed to happen for the crank to flex means that the rod and or piston is fully flexed/stretched?? reason being is that it seems to me that in order for the crank to flex via the force of the combustion cycle all of that force must travel through the piston and rod before it even gets to the crank, therefore THEY take a fair amount of the abuse. Now with the crank pin sticking out with no support obviously makes that the weak link more so than the say middle of the crank, so i can see how an overhardened crank can lead to destruction. I have been machining metal for over 25years from aluminum to inconel, nitronix 50 and anything in between! So what you know in nitro engines i know in the machine process of creating them( my 4 axis CNC mill is running as i type this!!)! taking a wild guess but i assume the pistons are a form of 6k series aluminum (6061) rod a 7k series (7075) the bushing are more than likely made of a self lubricating bronze-oil lite and since the cranks are being hardened I am sure they are some sort of carbon based alloy but I am not sure on that. Now taking all of that into consideration the crank is easily 10-15 times harder (maybe more) than the aluminum parts and being the "caboose" on the line of fire is why i am having a hard time seeing the crank flexing (not moving). I think the reason the over hardened crank broke is simply shear force and shock and the softer crank can absorb that shear force/resonance. For something to flex and break implies that it bent before breaking and everytime I have seen a crank break it is sheared off straight at the pin. Make sense? I am not saying that the crank never flex's but the amount it does is multiple times less than the rest of the assembly.

I can see the logic in this and all of the motors were in need of new rods but I didn't think they were that bad. It's a shame no one make after market pistons and rods for these little motors.

and max piston speed is also in relation to conrod length as to at what degree of rotation it is achieved and is also the point of maximum side load on the piston when rod angularity to crank centerline is at 90 degrees

hi that was good reading but I am a carpenter/rc came later and I am just getting my head around changing /rod before it all goes bang one nats== new rod and since then no bang but it was good reading///64 not out but it still makes me think when you look what is in there what makes them go like hell and not explode

Without getting in the actual 'fight'. :)
I used a quick calculator to get some basic numbers for a Novarossi, both 16.8 and 17.6 mm stroke at 45000 rpm. :tire:

Attachment 1044032

Nice! Which program is that?

With a simple straight calculation I was not far off :)

You where quite close! :)

I used this (you find it if you scroll down); http://2.3liter.com/Calc2.htm :tire:

I´m not sure how accurate it is, but for a quick calculation I think it´s accurate enough. :)
For more in-depth analysis I use other programs.

You are correct, I did those calculations in a hurry last night and forgot to do the last conversion. That gives me a more realistic piston speed of 25.226 m/sec.

I know the fins do resonate. More of the noise anymore is not from the exhaust but the intake, even with INS boxes. Does EFRA still require the 3 chamber mufflers? I remember Frankie doing noise testing at Lostallo at the WC in '09. Didn't see much difference in dB output and the engines ran worse if I remember correctly.

Intake does make some noise but mounting an extra mufler on the already excisting exhaust system is doing more to lower the sondlevel than an insbox.

The 30xx 3 chamber exhaust were crap, no one is using them in the onroad although the 3036 I still have was a good exhaust. Now we have still the old 20xx type 3 chamber (with internal extra holes and larger holes) but some were taken off the list. There were some experiments with a small extra snap on silencer but no succes and now all eyes are focust on the new Hipex exhaust wich seems to have a dubble wall acting as a 4th chamber.