AndyW

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The longer stroke in the Picco/Norvel stroked engine will need a longer rod to clear the piston. Found an article relating to rod length.

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Rod Length



Rod Length Relationships

You are invited to participate in this attempt to understand a part of internal combustion engines. I invite any/all criticisms, suggestions, thoughts, analogies, etc. written preferred, phone calls accepted from those too feeble or who have arthritis. Contributors are invited to request special computer printouts for specific combinations of interest to them.

In general, most observations relate to engines used for some type of competition event and will in general produce peak power higher than 6000 RPM with good compression ring seal as defined by no more than 3/16 CFM blowby per cylinder.

Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.

I. LONG ROD

A. Intake Stroke will draw harder on cyl head from 90-o ATDC to BDC.

B. Compression Stroke Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDCmay change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPMmeasure CO, CO2? Find out!!

C. Power Stroke Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDCallows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

D. Exhaust Stroke see above.

II. Short Rod

A. Intake Stroke Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

B. Compression Stroke Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closesie. may permit longer intake lobe and/or later intake closing than a long rod.

C. Power Stroke Short rod exerts more force to the crank pin at any crank angle that counts ie.20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

D. Exhaust Stroke Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.

III. NOTES

A. Rod Length Changes Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

B. Short Rod in Power Stroke Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

C. Stroke Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversionEGR on a street engine.

D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.

E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will beie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.

IV. DEFINITIONS

Short Rod Min Rod/Stroke Ratio 1.60 Max Rod/Stroke Ratio 1.80

Long Rod Min Rod/Stroke Ratio 1.81 Max Rod/Stroke Ratio 2.00

Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.

Contributors to Date: Bill Clemmons, Jere Stahl

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Connecting Rod Length Influence on Power

by William B. Clemmens

A spark ignition (SI) engine and a steam engine are very similar in principle. Both rely on pressure above the piston to produce rotary power. Pressure above the piston times the area of the bore acts to create a force that acts through the connecting rod to rotate the crankshaft. If the crankshaft is looked at as a simple lever with which to gain mechanical advantage, the greatest advantage would occur when the force was applied at right angles to the crankshaft. If this analogy is carried to the connecting rod crankshaft interface, it would suggest that the most efficient mechanical use of the cylinder pressure would occur when the crank and the connecting rod are at right angles. Changing the connecting rod length relative to the stroke changes the time in crank angle degrees necessary to reach the right angle condition.

A short connecting rod achieves this right angle condition sooner than a long rod. Therefore from a "time" perspective, a short rod would always be the choice for maximum torque. The shorter rod achieves the right angle position sooner and it does so with the piston slightly farther up in the bore. This means that the cyl pressure (or force on the piston) in the cylinder is slightly higher in the short rod engine compared to the long rod engine (relative to time).

Table 1
ROD LENGTH RELATIONSHIPS*
(with Crank @ 90 deg ATDC)

Piston Position Crankpin/Rod Angle

Stroke Rod Length Rod Angle from TDC ATDC
3.5 5.70 17.88 2.025 72.12
3.5 5.85 17.40 2.018 72.59
3.5 6.00 16.96 2.011 73.04
3.5 6.20 16.39 2.002 73.60

Table 2
ROD LENGTH RELATIONSHIPS with CRANKPIN/ROD centerline @ 90o @ 7500 rpm

Stroke Rod Length Rod Angle Piston Distance Crank Angle Piston Accel
3.5 5.70 17.07 1.487 72.93 2728.35
3.5 5.85 16.65 1.494 73.35 2504.72
3.5 6.00 16.26 1.500 73.74 2324.26
3.5 6.20 15.76 1.508 74.24 2097.27

*data from Jere Stahl

Another concern in selecting the rod length is the effects of mechanical stress imposed by increasing engine speed. Typically, the concept of mean piston speed is used to express the level of mechanical stress. However, the word "mean" refers to the average speed of the piston in going from the top of the bore to the bottom of the bore and back to the top of the bore. This distance is a linear distance and is a function of the engine stroke and engine speed, not rod length. Therefore, the mean piston speed would be the same for each rod length listed in Table 1.

Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.

Classical textbooks by Obert ( ) and C.F. Taylor ( ) provide little guidance on the rod length selection for passenger or commercial vehicles other than to list the ratios of rod length to crank radiuses that have been used by various engine designs. Race engine builders using production blocks have done quite a bit of experimentation and have found many drivers are capable of telling the difference and making clear choices along with similar results from motorcycle flat track racers/builders.

Because of recent developments in computer modeling of the engine cycle by R.D. Rabbitt ( ), another factor may be critical in selecting a given connecting rod length. This new factor is the cylinder head flow capability versus connecting rod length over stroke ratio (l/r) versus engine speed. To understand this relationship, let us first review previous techniques used to model air flow during the engine cycle which as Rabbitt points out is founded on principles initiated in 1862 and refined in 1920. These theories are documented in Taylor's textbook ( ). To calculate air flow throughout the cycle these models use such parameters as mean or average inlet mach number for the port velocity and an average inlet valve discharge coefficient which compensate for valve lift and duration. In these models a control volume is used to define the boundaries of the combustion chamber. The air flow determined by the previous parameters crosses this boundary to provide air (and fuel) for the combustion process within the control volume.

However, this control volume has historically been drawn in a manner that defines the boundaries of the combustion chamber in the area of the inlet and exhaust valves as if the valves were removed from the cylinder head (ie. a straight line across the port). With the valves effectively removed, the previously mentioned average port flow and valve discharge coefficient (ie. valve restriction) are multiplied within current computer models to quantify the air flow (and fuel) delivered for each intake stroke. But, as Rabbitt points out, this approach totally ignores the effect of the air flow direction and the real effect of valve lift on the total air flow that can be ingested on each intake stroke.

Rabbitt reaches two important conclusions from his study. One, because of the direction of the air flow (angle and swirl) entering the combustion chamber, three dimentional vorticies are set up during the intake stroke. Two, that above a certain piston speed, density of the mixture at the piston face is a function of valve geometry and valve speed. Rabbitt further discusses the effect of the first conclusion as it relates to the mass of air that is allowed to flow through the port and by the valve. Vorticies can exhibit different characteristics and in general conform to two general typeslarge scale bulk vorticies that could be described as smooth in nature and small scale eddies that are highly turbulant.

If one can consider that the vacuum produced by the piston on its downward travel to be the energy that causes the air to flow through the port when energy losses throughout the intake tract (including losses at the valve) are at a minimum, the flow delivered to the chamber will be maximized. If the area between the piston face and the valve is also included in the consideration of flow losses, the effect of the type of vorticies created can be more easily understood. Large scale bulk vorticies comsume less energy than highly turbulent eddy vorticies. Thus, more of the initial energy from the piston's downward movement is available at the port-valve-combustion chamber interface with which to draw the intake charge into the chamber. Small scale eddies eat up energy which reduces the amount of the initial energy that reaches the port-valve-combustion chamber interface which in turn, reduces the port flow.

Rabbitt's second conclusion follows that at some higher piston speed, the vorticies within the combustion chamber (which are assumed to be large scale bulk type at low speeds) transition from the bulk type to the small scale eddy type. At this point the flow into the combustion chamber ceases to increase in proportion to increases in engine speed. It is theorized that this flow transition point can be observed on the engine power curve as the point at which the power curve begins to fall off with increasing engine speed.

As indicated earlier, piston speed is normally viewed as mean or average piston speed. Thus for a given engine, the mean piston speed increases as the rotational engine speed increases. However, in Rabbitt's model the piston speed of concern is the instantaneous piston speed during the intake stroke near TDC. For any given engine, changing the rod length to stroke (l/r) ratio changes the instantaneous piston speed near TDC. For the purposes of flow visualization, the type of vortex formed should not care whether a given instantaneous piston speed had been achieved by a given rotational speed or changing the (l/r) ratio and operating at a new rotational speed. As long as the instantaneous piston velocity is the same, the type of vorticies formed should be the same and the amount of air inducted into the cylinder should be the same.

If other factors influenced by rotational speed such as the time distance between slug of intake air flow and valve opening rates relative to the acceleration of the air slugs were ignored, one should be able to predict the location (RPM) of the peak power as a result of a change in the (l/r) ratio. Note, that even though power is a funtion of air flow and air flow should be roughly constant for the same instantaneous piston speed (neglecting the afore mentioned factors), the power may not be the same because of the lever arm effect between the crank radius and the connecting rod. (As we noted earlier, the shorter rod should have the advantage in the lever arm effect.)

In reality, the analysis must be viewed by stroke (ie intake, compression, exhaust, power) the selection of exhaust valve opening time combined with the exhaust system backpressure and degree of turbulance the exhaust port experiences. If the exhaust port has good turbulance control then you may run a shorter rod which allows you to use more exhaust lobe which reduces pumping losses on the exhaust stroke.

Japanman

That is, without a doubt, the longest post ever [8D]



Thanks Andy.

GrahamC

Long, informative and lots of food for thought. This reference has been noted before but for those that haven't seen it, here is a bit on Glen Dye had to say about making connecting rods:

http://www.go-cl.se/dye-rods.html (pdf added to this post for quick access)

cheers, Graham

AndyW

My Feedback: (1)

Yow,

Now we know why the Picco rod is so expensive. Did I see 65 dollars in their parts list? Worth every penny considering the work involved.

Geez, soft crank pins. Why would they do that?

The way I make rods is to use flat blanks from 6061 and just hack and grind till it looks like a rod. Then I jig it up in the vise and drill the holes to take bushings. Then I make bushings with pilot holes that get installed in the rod before final drilling using incrementally larger bits. The second last bit is just a hair too small and the final bit acts more like a reamer than a drill. These have various tolerances and when I find one delivering exactly the hole I need for a particular cank, it gets labelled and put away in a special spot away from the bit rack.

The trick is to do the final drilling of both holes in one sitting. That ensures that they'll be square to each other. I make the bushings a light press fit and use red Loctite to keep it all together. I've only had one rod failure so far.

forsakenrider

The snows melting and its beautiful out so I figured today would be a great day to break in my picco.
KT16 4.6x2.8 prop from Larry, 30% car fuel with added Castor (ends about 25% nitro I hope).

[youtube]http://www.youtube.com/watch?v=Fj1_JJExw1I[/youtube]

Toad

Well, so far I have broken 3 cranks, one stock and two annealed stock cranks. With the first race of the season this coming weekend, I am really disappointed in the stock cranks. so. I got busy and here is a pic of two 8mm cranks I am working on for this engine. Of corse these are not done yet or heat treated and ground. I need to cut the counter weight cuts also, I hope to get these treated and to the grinder and done by the weekend.. That probably wont happen as the grinder may not have room in his schedule this week. anyway, these two actually have a bit different opening between them. it will be interesting to see which one turns better. sorry about the picture quality. so I sent two. Note these will take a 4mm prop screw, which will fit the glass props nicely, gone is the stock threaded stub. You can see I have not polished up the port yet, these are fresh off my mill

AndyW

My Feedback: (1)

Good work guys. Wish I could make up a header to take one of my tuned pipes. [X(]

forsakenrider

Toad, those look great!
How are you breaking the cranks? starting? while running? I was scared as crap at first so I really stood back, but it just seemed to start and run so smoothly I got a bit more confidence in it. Is it one of those things thats just going to jump out and bite me?

Toad

The stock one broke after about 6 oz of fuel, it broke about 1 minute after start, waiting for the flag to drop for a race, 1 annealed one ran almost a qt of fuel, then broke in the air, 2nd annealed broke within 10 sec of start as I reached for my tach. First break on the ground, the prop pulled half the crank out the front and landed about 12ft away, straight forward, second crank on the ground just suddenly stopped and the prop kept spinning, but did not leave the engine.
Scares me too. one in the air...I have no idea where the prop and driver went...

AndyW

My Feedback: (1)

Never mind on that tuned pipe. [:@]

forsakenrider

What happened about the tuned pipe??? I was looking forward to this!

man, I really hope my crank doesnt break this engine seems soo much easier to run then my norvels, they are picky!. What kinda nitro content are you running Toad?

AndyW

My Feedback: (1)

Well, the problem with the Picco is making up a proper header. I get a decent 1500 RPM boost with the pipe on a Norvel and VA with a 6 x 3 prop, no problems,, never got them up to 27K on small props, though. So maybe on the Picco on a 6 x 3, a pipe would be alright. But are we dealing with an unusually fragile crank? That's not like Picco. But cars don't seem to load an engine as much as aircraft. Cars do the WFO thing for seconds, we need it full time, most times,, maybe that's the deal.

But a 1/2A on a pipe sure sounds sweet. Going to have to do a video of that soon.

Oh yes, the VA comes with rear exhaust and I have a Norvel converted to rear exhaust. Not easy to do on the Picco.

Anyway,,,,

MJD

My Feedback: (1)

I have three VA's, I'd love to run pipes on them. But exhaust adapters are the issue. I recall someone was making some a while ago, has that avenue dried up or still available special order?

MJD

BAGB

One way to increase the crank life might be to grind off the sharp edges around the intake hole. My crank had typical fatigue fracture starting from the corner of the intake.

I bought 2 Piccos and my first snapped after the 5th run doing 28000 rpm.

Otherwise I think it's a great engine!.

Third photo shows polished edges.

Toad

I have seen one radiused stock crank that broke in early life also.... I hope it works for the masses as a whole though.

uliner

My Feedback: (2)

Doug G claims he can keep a crank in the ones he sells, has anyone been sucessful at keeping a runner? I have 2 to convert or will drip some doouhg for a fora

MJD

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Uliner - if you can find a Profi .049 or .061 you will not be disappointed. My .061 rich 2c on first run on Driskill combat prop 34k and change. Easy to run too if you know how to do bladders. Or do you want an RC carb?

uliner

My Feedback: (2)

Wow I did not ask that, but thanks for what?

I can get profi's, cyclons and foras with one phone call. But they are not cheap.

The low cost of the picco makes it attractive to me, I bought a few.

http://www.the-printer.net/DookCat.html#picco

Doug Galbreth can get the Picco to hang together. His engine set the national record in 1/2a proto speed. Has anyone else figured out how he keeps it together?

Mr Cox

He's making his own cranks I believe. That's the only week point in these engines.
I haven't flown mine much but staying below 20000rpm it has not faltered yet.

MJD

My Feedback: (1)

His converted Piccos sell for about the same as a Profi. I noted he makes a comment about the crankshafts still being weak.

My only point was that insane performance is available somewhat off the shelf, but of course that takes all the fun out of making a low cost engine work for you doesn't it?

combatpigg

My Feedback: (3)

When you stop to consider the many trouble free gallons of fuel you will pour through a Fora..I think they are the best bang for the buck at $140-150 bucks [the last time I heard].
I'll take a single Fora anyday over 3 Piccos...no question about that.
I owned a Fora .049 for 7 years, put 100s of flights on it and never had the head or backplate off that engine. I ended up trading it for a RC version VA NIB and the Fora was still running strong.

MJD

My Feedback: (1)

Cool. Of the Cyclon/Profi/Fora crowd I only know the Profi, and don't have enough time on it to assess durability. I just know it blew me away since the hottest 1/2A I had ever run before that was a VA. Runs like stink with very little effort - my first 30k+ engine if you don't count clipping the prop on a TD .010.

I must say there is a unique appeal to the real small high performance engines. I should own more. Santa.. do you hear me??!!

So you have one of the 100 or so VA MkII R/C's that were ever made? I wish I bought a handful when I bought the one example I have now. Fun little thing, I was quite pleased with the throttling. It needs out of the Blink and into a new home, like a little aerobatic bipe.

combatpigg

My Feedback: (3)

MJD, ironically the VA broke the crank after just a couple dozen flights........
I should have known better...because I DID know better....[:@]
Ptulmer is the proud owner of that old Fora now.
The reason why I rank the Fora highest is it gets you into "The Game" for only about $150 and will outlast a coffee can full of the cheaper engines.
It all depends on your performance expectations and the amount of trouble free usage you demand..and then amortize the cost of all that thrilling fun into what that engine ended up costing you per flight, per year, etc.

forsakenrider

FYI doug is NOT making his own cranks. Doug de-strokes the engines cutting a little off the piston crown, turning down the crank pin closer to the center, and re-bushing the conrod. My theory in his success is hand starting and the slightly less leverage on the crank. Doug is making the engines into .049's or less to fit into the control line category.

I got a picco from doug before he started selling them I sent him mine to have the mods done. I havent ran it yet because I havent ran anything since may

hopefully soon I will have a chance.

Before that I had a picco that ran a good while until I zipped it with the electric starter while flooded, oops.


PS- I miss you 1/2a guys, we need to liven this place up!

proptop

My Feedback: (8)

Rather than start a new thread ...I thought I'd drag this one back...

I stumbled across this while cruzin around: Picco .05 for $29.99
http://www.nitrohouse.com/Picco-05-S...ad_p_8703.html

Any of you guys still doing any modifying of these little boogers?