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Discussion in 'Airheads' started by midlman, Feb 27, 2011.
Could you be more specific?
Supershaft - Good! Just wondered where you stood with that. No Googling?? C'mon! That's today's library! Not saying we should hang our hats on info we get from Forums, but there are various other reliable sources for tech info out there.
Lornce - I can elaborate a bit. It relates to the science of "kinematics", which is the study of relative motion. In this case, the study would be "Crank and Slider" relationships; the piston being the "slider". The reciprocating components part of a base engine design includes consideration of rod length, along with bore & stroke. In fact, race engine builders will change rod length depending on the track in some top-level series (like Sprint Cup) to change power delivery. Or...errrr...best power!
Basically, it goes like this. If you plotted the accelerations (positive and negative) of the piston as the crankshaft rotates at a constant speed, you know it stops at the top and bottom to change direction (the plot looks like that of a sine wave). The piston speed has accelerated to max velocity at 90º BTDC and ATDC in every case. Varying the rod length changes the periods of dwell at the top and bottom (longer rod, more dwell at the top and bottom), and max velocity at 90º.
We generally set or specify ignition timing in crank angle degrees. We do that because it's an easy dynamic reference using a strobe. What we really think about is time...time to combust percentages of the charge. Once we've established the environmental conditions for the combustion (fuel type, A/F ratio, temperature of the chamber and inlet, compression, etc.), the burn takes a fixed amount of time. That's why you need more advance as RPMs increase. Anyway, using a longer rod means that the piston is closer to the top of its travel as the crank passes from 10º BTDC to 10º ATDC (a fixed time at any given RPM). This means you're wasting less energy compressing the expanding gases BTDC, and building more pressure before the piston starts heading down. So, to get to Supershaft's point, you can set the timing later with respect to CA degrees because the time the piston dwells at TDC (and after) for each CA degree is longer at any given RPM.
There are also mechanical impacts to all this. First, leverage. Using a longer rod means the crankshaft has more mechanical advantage over the piston as it rotates toward TDC. This is good since you will lose less crankshaft speed and waste less energy pushing the piston up after the mixture lights and the gases start expanding. (Note: I guess it's time to say that crankshaft speed is never constant, so the time to turn the crank 10º isn't the same at 10º BTDC as it is at 90º ATDC). It also means the piston has more mechanical advantage over the crank immediately ATDC, also a good thing. However...there's the bad, too. The bad is that the piston speed at 90º BTDC and ATDC is higher...and piston speed is a limiting factor in engine design. It also means the piston accelerates at a higher rate to that higher speed, since it starts moving later in CA degrees ATDC. That means it begins to move away from that expanding charge faster (remember, there's inertia in that crankshaft), resulting in a more rapidly declining pressure in the chamber (and less force on the crankpin through those degrees) compared to a design with a shorter rod. That's not a good thing.
So it's all about compromise. When engineers design an engine, they design it for its intended use. These are basic piston engine fundamentals, and they can be adjusted. I hope this was worthwhile to someone...Lornce, did it make any sense???
Nope this is not a Beemer Geek thread.......nope...not-one-bit.
This thread is one step away from oil quality.
I need to defect to the KTM side.........all of their threads eventually morph into conversations about beer & strippers.
They say you should learn something new everyday, I think I am set for about a month now. Great information and discussion that I can comprehend.
I can easily drop into the beer and strippers theme...
A whole lotta theory in here. Everything works in theory...................but no one lives there.
Some engines benefit from long rods due to small bores limited by flame front speed and the longer rod will help. It carries a few negatives with it,higher weight,increased acceleration rates and a few geometry related items as well. Modifications of that type are best suited to those with the time,money and experience.
You can spew BMEP,MBT,A/F-R and all sorts of factors. They have merit however are not the end all in the quest for speed and power. In a everyday bike well tuned stock is the best compromise between power and longevity. It is truly rare that the designers and engineers get it that wrong.
Anotherguy - Agree. And I doubt the engineers really ever get it WRONG. They just might not have designed it exactly the way everyone wants. Engineering is definitly one of those pursuits where you can't please all the people all the time. BTW, it's really the BIG bore that's limited by flame front speed, not the small bore. More of a benefit of smaller bores, and part of the reason Ferrari builds V12s.
And let's not think about the times the bean counters win the war, and management tells the engineers to use the rod or other piece already in production.
I thought only the stroke changed max piston speed at 90 degrees. I have never seen anyone throwing rod length ratio into calculating max piston speed?
Some consider a longer rods increased acceleration an advantage since the increased acceleration is AFTER a slower start from a dead start. It's the initial jerk from dead still that actually effects the setup more adversely than increased acceleration after the piston is already rolling. It's kind of like how long ramps on a cam can prevent valve float.
Some also consider the increased acceleration an advantage in maximizing the flame front's energy since at higher RPM's by the time the Longer rod's piston starts really moving most of the flame fronts ability to push on the piston has passed anyway.
That same increased acceleration is seen by some as an advantage on the intake stroke as well.
Whether or not it actually works out like that depends on the entire situation. Sometimes engines really respond to different rod length ratios and sometimes they don't. Personally, I think in some cases that they don't respond well to longer rods are instances where the longer rods were not long enough but that is another story. Airheads respond very well to longer rod length ratios. I have seen it. Comparatively speaking, the Carrillo rods and Venolia pistons that the mod takes are very inexpensive bang for buck wise. Since Dr. Curve pioneered the setup in our airheads, the mod has proven to be very reliable.
Yeah, but they start the same...delusional Dynojet numbers.
Yea, I mis-spoke myself on the piston speed issue. The max velocity remains the same. I sure wish I still had the kinematic spreadsheet I made years ago when I was heavily into this stuff. It would have told us everything, including the forces on the components, including bearings, etc.
The initial acceleration of the piston off TDC is slower, but the rate increases to beyond that of a shorter rod configuration as you approach 90º ATDC. The burn is still happening well past 90º ATDC, BTW, particularly at speed. That's why you see the flame out the exhaust port. That's a negative to a longer rod actually because the pressure is diminishing more than it otherwise would at the point where the piston has greater leverge over the crank. But as we know, many factors will determine whether the diminishing pressures are hurting the power output. There's a lot to consider, obviously. Again, it's all about the area under the curve! This time it's a cylinder pressure curve relative to CA degrees.
Airheads could likely be an engine design that likes longer rods. With the low RPM ceiling they have, you sure wouldn't think the mass increase would be much of a concern, especially with their heavy pistons. And their cylinder head design makes for some very abrupt 90º flow direction changes. Supershaft, your experience here is good. Would like to see some data.
Where do you find the Carillo/Venolia combo? Sounds interesting, and I'd sure like a higher CR for the riding I do. Plus, I don't have to burn soft coal where I live! Remember, BMW was considering many factors when they made their choices.
This is an interesting read, mostly when you scroll down to the "Connecting Rod Influence On Power" part...good ol' Google! http://www.stahlheaders.com/Lit_Rod%20Length.htm
Oh...one other thing regarding dynos. The comment was made that meaningful data can't be gained below 3000 RPM. That's only true for the poor folks who have to use hydraulic brakes. The guys developing the engines initially at the OEM manufacturer can get excellent data across the entire range, because they're using AC or DC motoring dynos. WOT or 10% throttle, 10,000 or 500 RPM, it all works for the guys with the good equipment.
As always, it's all about money.
As you mentioned, the burn is often going on clear out the exhaust pipe. That obviously isn't pushing on the piston any since the exhaust valve has been open for some time. At higher rpm's, the flame front's usefulness pushing on the piston is actually pretty much over LONG before the exhaust valve opens. The piston is already in a hurry and the flame front has no idea. Many tuners including Yunick, Curve, and Hodgeson think that at higher rpm that piston hanging around longer and waiting for the flame fronts crucial first grunt on the piston is the main source of longer rod ratio's higher rpm torque gains. The advantages and disadvantages of different RLR's vary with rpm. Longer RLR's favor high rpm torque. The physics of longer RLR's also favor higher rpm. Most tuners report longer RLR's netting HIGHER rpm limits despite the heavier rod. Most attribute this in practice finding again to the longer RLR's slower decel and acceleration on both sides of the dead stop. Longer RLR's have greater acceleration and deceleration further into the stokes at TDC but they also have less herk and jerk right at TDC and that is what really effects the situation negatively.
You can get Carrillo rods from Carrillo, San Jose BMW, and Siebenrocks as far as I know. Venolia pistons are available from Venolia and San Jose BMW. Mahle and others make applicable pistons as well.
More data? This narcissist has never taken a photo of any of his work or others that I have worked with. I never owned a camera till just recently and I still haven't taken many photos with it. I never posed with a race bike at the track. I never saved any dyno printouts of experiences I have spoke of on this topic except for ONE of my own. It's all been in the interests of eventually controlling the minds and bodies of airheads! I think all of Yunick's books are a good source. I have never looked to see if any of them are googleable. I will read your link when I get time.
My bike was dynoed on an electric dyno. CC Products dyno was electric if I remember right. It might have been hydraulic. It was a brake dyno. That I remember for sure! My dad's used water. I have never seen any of them read down right off idle for some reason or another. Dynojet doesn't JUST make inertia dynos.
Thanks, that made perfect sense.
I was previously aware of the leverage benefits but wasn't well enough versed in "crank and slider" mechanisms to appreciate the added dwell effect.
Am keen to hear the flip side of the equation. Does a long rod result in more or less net torque transfer? Or is it a matter of maximizing an efficiency at specific rpm's due to burn rates?
Thanks for taking the time to explain.
Think about it. If the dwell at either end's increasing at any given RPM, the time spent traveling between bdc and tdc must be reduced. Right? If that time between bdc and tdc is reduced, piston speed must be increased.
I'm still getting my head around this but the speed max does not have to increase.
An increase in speed average will do it. Increasing acceleration would achieve this.
Yes, it's the acceleration that increases.
So does ring flutter become a factor with higher piston acceleration or does the slow start from stopped tdc help here?
Except to boneheads, dynos are not about the numbers themselves. They are about comparatively analyzing runs on the same dyno.
Lets try and keep to the theory and forget the "bonehead" remarks please.
That makes sense, but I don't know enough to know if that's the case.
Out esteemed colleague has suggested otherwise.
edit: okay, just getting caght up in this thread and noticed WM's additional information.