To Maximum RPM And Beyond

[timberwolf]

Been doing some thinking, reading and number crunching on internal engine loads when the RPM is taken up past factroy spec.

We know the rod is under significant compression as the fuel burns and transfers the energy down the rod into the crank, but many rod failures would indicate a tennsion failure rather than compression???

How much force is a rod subject too?

How much acceleration does the piston undergo?

What effect does a litened piston have?

What is the effect of a modded saw runing higher compression or higher rpm on the internal engine components?

I have come up with some caculations that give the force of acceleration on the piston through one revolution of the crank and compair it with the cylinder pressure for one revolution, working in all the rod geometry that is going on.


First graph is just charting piston velocity and acceleration in "G"s for a stock 066 running at a max spec of 13,500.

Second graph is showing the force on the rod caused by acceleration of the piston in pink, the force on the rod caused by cylinder pressure pushing down on the piston in blue, and the net force of the two forces (actual force placed on the rod) in yellow.

 

[taplinhill]

I'm not seeing the yellow. It looks like the 2 pics are the same.

 

[Pilsnaman]

I would love to see the calcs to figure these forces, accelerations, velocities, and compressions out.

 

[timberwolf]

I think there are a lot of interesting points in these two graphs.

On the first graph piston velocity is charted starting at TDC when the piston is sitting still for a millionth of a second or less, from this point the piston gains velocity up to about 80 deg ATDC, this is interesting, the obvious first thought is that if the piston moves from TDC to BDC (180 deg of crank rotation) the point of maximum velocity would be mid point at 90 deg. This would be true if the rod was a mile long and did not change angle, but because the rod changes angle and this change in angle affects piston velocity reducing it the closer the crank is to either 90 deg or 270 simply because the effective length of the rod is reduced to to it being slanted on an angle. The same is seen after BDC as the piston heads back up, but this time maximum piston velocity is not reached until 280 deg. The shorter the rod compaired to the stroke the closer maximum piston velocity moves towards TDC.

This diviation in piston velocity then affects the forces of acceleration, as the piston leaves and returns to TDC and speeds up or slows down faster than it does on the BDC portion of the cycle, accelerated forces are higher at TDC than BDC. From the Time the piston passes the point of maximum velocity as it goes down the rod must push up on it to slow it down all the way to BDC (~100 deg later), then from BDC the rod must push up on the piston to accelerate it back to maximum velocity (~100 deg later). Next after the piston reaches maximum velocity on the way back up to TDC the rod must pull down on the piston and slow it back to a stop over the next ~80 deg, and to finish the cycle off the rod must pull the piston back to maximum velocity in 80 degrees of crank rotation. This all is assuming there is no other forces of compression or combustion. This is where it gets tricky!

Graph 2 starts at BDC to show things beter.

The acceleration force (shown in pink given in lb of force)at BDC is pushing up on the piston to get it up to max velocity, From the Velocity and Acceleration graph we see that this force goes on until about 100 deg after BDC. During is time the exhaust port is open so cylinder pressure is basically zero. However as the exhasut port starts to close and the piston is moving up it starts trapping gas and increasing cylinder pressure slightly, this can be seen in the blue line between 240 and 280 deg. The net force on the rod is mostly compression. After max piston velocity and once the exhasut port closes cylinder pressure starts to rise slowly at first at the same time the rod must start to work to slow the piston down, the net force at this time then goes negative and the rod is placed under tension. About 340 deg (20 befor TDC there is a dramatic increase in the rise of cylinder pressure, combustion has started and is now increasing the push down on the piston, at the same time the volume of above the piston is being pushed into a smaler and smaler space increasing force down on the rod. Somewhere about 12 deg before TDC the pressure above the piston produces more force down than the force required to decelerate the piston to TDC, the rod is then placed into compression. About 10 deg after TDC combustion peeks producing the highest pressure down on the rod. Between 20-40 deg after TDC the fuel starts to burn out and as the cobustion space is increasing pressure on the piston starts to fall, meanwhile there is still a lot of energy being absorbed by the piston to get it accelerated back to maximum velocity 80 deg after TDC. in this cas the force down on the piston is still more than the force of acceleration and the rod remains under compressive force transfering energy into the flywheel or load. Once the Exhaust port opens about 100 deg after TDC the pressure quickly drops leaving the force of acceleration pushing down on the rod to slow the piston down befor BDC.

 

[blis]

Been doing some thinking, reading and number crunching on internal engine loads when the RPM is taken up past factroy spec.

We know the rod is under significant compression as the fuel burns and transfers the energy down the rod into the crank, but many rod failures would indicate a tennsion failure rather than compression???

How much force is a rod subject too?

How much acceleration does the piston undergo?

What effect does a litened piston have?

What is the effect of a modded saw runing higher compression or higher rpm on the internal engine components?

I have come up with some caculations that give the force of acceleration on the piston through one revolution of the crank and compair it with the cylinder pressure for one revolution, working in all the rod geometry that is going on.


First graph is just charting piston velocity and acceleration in "G"s for a stock 066 running at a max spec of 13,500.

Second graph is showing the force on the rod caused by acceleration of the piston in pink, the force on the rod caused by cylinder pressure pushing down on the piston in blue, and the net force of the two forces (actual force placed on the rod) in yellow.

Increased compression increases the pressure shock piston&rod receives during combustion, thats for sure... Increased rpm only affects piston speed and acceleration (which will lead to detonation when they pass certain set limits)

Heres some pics from jenkings 2-stroke tuners handbook, concerning compression, piston speed, acceleration and 2 charts...

 

[timberwolf]

Got the right Graph 2 uploaded now, oops, saved it under the wrong file name.

 

[timberwolf]

Here are a few more graphs with some different situations.

Stock 066 at @15000
Stock 066 at @15,000 with the thottle slammed shut
066 bb at @13500 assuming porportional output and head volume
Modified 066 improved compression, air flow and lightened piston @15,000

 

[manual]

Hey Timber wolf check your P.M.

 

[timberwolf]

Stock form going from 13,500 to 15,000 there is a significant jump in tension on the rod from about 500 lb to 700lb also the down force on the rod is reduced during the combustion phase but increased around BDC by about 200lb. One other thing that starts to happen is the rod is placed into tension at the end of the combustion phase, the piston is outrunning the flame.

Interesting, closing the throttle suddenly drops the cylinder pressure way down and puts a lot of tension on the rod, nearly double. Also the rod is then placed in tension at TDC so it would be streched and if squish was tight this could be a problem.

The BB puts a bit more stress on the rod with heavier piston and also the extra piston area increases force down on the piston during the combustion phase.

Modifications put more compression on the rod, but tension forces on the rod can be less than with a stock saw.

 

[blis]

Stock form going from 13,500 to 15,000 there is a significant jump in tension on the rod from about 500 lb to 700lb also the down force on the rod is reduced during the combustion phase but increased around BDC by about 200lb. One other thing that starts to happen is the rod is placed into tension at the end of the combustion phase, the piston is outrunning the flame.

Interesting, closing the throttle suddenly drops the cylinder pressure way down and puts a lot of tension on the rod, nearly double. Also the rod is then placed in tension at TDC so it would be streched and if squish was tight this could be a problem.

The BB puts a bit more stress on the rod with heavier piston and also the extra piston area increases force down on the piston during the combustion phase.

Modifications put more compression on the rod, but tension forces on the rod can be less than with a stock saw.

closing the throttle creates vacuum in intake and also above piston, therefore increasing tension on rod ... Also due the less volume in combustion phase decreases resistance in TDC thus allowing greater tension...

 

[timberwolf]

I am not seeing that it produces vacuume above the piston. On the intake yes. But above the piston is open to the outside air through the exhaust port, also even with the throttle closed there is a surprising amount of fuel and air sucked through the notch in the throttle plate and through the small amount it is lifted around the edge as it is adjusted for idle. What does get compressed under a full speed throttle drop is highly diluted with exhaust gas, but stil there is enough fuel and air to get some remnent combustion.

The force down on the piston at TDC is mainly a function of the dynamic compression ratio at the full RPM and some due to what ever heat and combustion still taking place. Suddenly closing the throttle plate causes peek cylinder pressure to drop from about 35 atmoshperes on a stock saw to about 15.

There can be a slight vacuume over the piston with tuned exhaust just at the moment when the transfers open, but that vaccume is long gone before the exhaust port closes and compression starts.

 

[timberwolf]

Here it is run at 7000 RPM, the rod realy never gets much tension on it. If the saw was run any slower or if compression or volumetric efficiency was higher the rod would be under some compressive force throughout the whole revolution.

 

[joesawer]

Do these models take piston and ring friction into consideration?

 

[motosierra]

Been doing some thinking, reading and number crunching on internal engine loads when the RPM is taken up past factroy spec.

We know the rod is under significant compression as the fuel burns and transfers the energy down the rod into the crank, but many rod failures would indicate a tennsion failure rather than compression???

How much force is a rod subject too?

How much acceleration does the piston undergo?

What effect does a litened piston have?

What is the effect of a modded saw runing higher compression or higher rpm on the internal engine components?I have come up with some caculations that give the force of acceleration on the piston through one revolution of the crank and compair it with the cylinder pressure for one revolution, working in all the rod geometry that is going on.


First graph is just charting piston velocity and acceleration in "G"s for a stock 066 running at a max spec of 13,500.

Second graph is showing the force on the rod caused by acceleration of the piston in pink, the force on the rod caused by cylinder pressure pushing down on the piston in blue, and the net force of the two forces (actual force placed on the rod) in yellow.

Hey Timberwolf...."with all the thinking, reading and number crunching on internal engine loads" ..what are the units of measure on your x and y coordinates??

 

[timberwolf]

lb force on the vertical deg on the horizantal.

I did not take ring fricton into it, that would be fairly easy to aproximate.

 

[joesawer]

lb force on the vertical deg on the horizantal.

I did not take ring fricton into it, that would be fairly easy to aproximate.

Your work amazes me.

 

[wipikas1]

Is all this research geared toward mechanics or is there something the average saw user might want to know about prolonging the life of their saw.
Sum it up alittle if there is something the average saw user should know about your research and their saw.

 

[timberwolf]

I think it mostly applies to those who modify saws or run modified saws.

If you take a factory saw the factory has done the testing and if you keep within their guidlines you should expect good life out of the product.

But even opening the muffler up and gaining 1000 RPM or so is going to start to put some extra loads on the rod and bearings.

In this respect modding a saw to be a screamer with high exhaust porting numbers and faily low compression is going to put a lot of tension loading and unloading on the bearings and is likely not good for the long run.

Throttle blipping puts a lot of tension on the rod and bearings, and might be the cause of piston/ head clearance problems if squish is run too tight.

Reducing piston weight keeps bearing loads down. Overbore situations increase bearing loads even without increasing output or RPM.

 

[Al Smith]

In this respect modding a saw to be a screamer with high exhaust porting numbers and faily low compression is going to put a lot of tension loading and unloading on the bearings and is likely not good for the long run.

Now,on that point,I tend to agree.It would seem to me that during a portion of the blow down that the downward forces on the piston would be niether compression or power thus could in fact be subjected to forces of tension.

With that factor in mind though,do the tension forces overcome the inertia forces already present in the downward movement in the piston/rod?

For what it is worth,I've never seen a rod fail,most of the time it is the piston that gives up the ghost ,for varied reasons.

 

[Lakeside53]

rod bearings do wear out (subjective between "new" and "worn" though), and fail...