i made this quick video to illustrate the effect of longer rods on an engine.
it was a quick effort, so dont critique my model, the point is to show the effect of the rods.

http://www.youtube.com/watch?v=iYEIUVHccrg

hope its helpful for someone.


the one closest to the camera is the short rod version
the one in back is the long rod

i didnt feel like making two rods and pistons so i made extra holes in each.

 

Cool visual of the kinematics involved.

As Smokey Yunick said, the ideal connecting rod is infinitely long.

 

yeah.
one thing i didnt think about before the video is the dwell at BDC with a shorter rod.
as you can see, the piston stays down for a very long time. i was more thinking about the TDC side, but now i see that the longer rods effect BDC more.

btw, the whole thing is upside down incase someone was confused about that.
sorry

 

Thats pretty interesting. Never really thought about the effects of rod leingth in thoes kinda terms. Its interesting though that the shorter rod does spend more time at BDC than its longer rod equivalent and also the rate at which the piston travles from TDC to BDC seems a bit quicker. This would this lead the the shorter rod set up to be better? This is because it would seem to me that because the pistons rate of travle is faster and it spends more time at BDC that would indicate to me it would generate a greater average vacuum than the longer rod set up which in turn would lead to better cylinder filling?

 

almost, but in RL, the piston drives the crank, not the other way around like i have modeled.
in reality, the pistons would travel at the same speed and the cranks would be changing speed (to a degree at least)
the longer the rod, the more linearly the rod travels throughout the stroke.
also, if you think about it, the short rod motor would make it "easier" for the combustion gasses to expand, so you get less torque.
also, note the rod angle, in the short one, there would be a lot of side loads.

the short rods length in my animation is 5.5 inches with a 3.5 stroke (basically a 5.7 rod 350 motor, i made the rods shorter to make it easier to see)
the long rod motor is 6.5 inches (i started out making it 6.0, but the difference between the two was pretty small.

 

Interesting. One other question if you dont min. Now assumeing that both pistons move at the same speed and the angular velocity of the crank is changeing to account for this then i can see your point about how there would be no real advantage in terms of vacuum. I can also see your point about how because the piston can only move up and down the more linear the movement of the rod the better. However what about the trasfer to the crank? I mean you always want the force in the direction of the movement like the piston can only move up and down so you want all the forces ideally moveing in an up and down fashion. However looking at the crank it would seem haveing the forces acting as mignt not be such a good thing? For example If the motor was not moveing and the piston was a TDC and that cylinder fired all of the energy would theoretically be transfered into the crank and none would be transfered into movement because the forces are acting perfectly perpendicular to the direction of movement. That dosnt really matter so much because that will be the case eather way but lets take a look at another case at 45 degrees of rotation a rod that was equal to the stroke would have the forces perfectly in line with the direction of movement of the crank where as a longer rod would not. Now at 90 degrees a long rod that was acting as verticle as possable would have most of its forces in the direction of motion where the shorter rod would now be transfering some of its energy to the crank. So when you think about it would seem that although your reduceing sidewall wear on the piston/ cylinder and better transfering the forces to the rod transfering thoes forces to crank in some cases a shorter rod is better in some the longer is better. So basically what im trying to ask is there some balance between these factors to give the ideal rod leingth for a given stroke or am i missing something again?

 

you know, there are a lot of dynamics involved that neither you nor i completely understand.

if i understand what you are asking, which is, where is the ideal rod length, then im sure there is a way to calculate it.

it has more to do than just the forces acting on the crank though
its true that an infinitely long rod will push with the most force on the crank at 90 past TDC, and that a shorter rod will push most sooner, but as far as i know, the magnitude of the force on the crank is less with the shorter rod because some of the force of the piston is exerted sideways into the cylinder walls.
so then it becomes less an issue of optimizing the rod length and more an issue of simply stuffing the longest rod you can in the engine.

if this is completely untrue then hopefully someone will speak up, but that is my understanding.

also, we can assume that the cranks rotation will be more or less constant at a given time.
in dynamics, we tend to look at things in what is called a quasi-equilibrium state, meaning that although the crank is rotating at 5000 rpms and accelerating to 7000 rpms in less than a second, we can pretend that it is simply rotating at 5000, which makes the math a lot simpler.

anyway, the more time the piston spends at tdc means more torque (to a degree i believe)
my understanding of this is that you want you ignition timing as advanced as possible while preventing detonation.
the timing goes off the cam, which goes off the crank, so if the crank moves 18*, but the piston is still pretty close to TDC, then you will get more total energy out of the spark than if the crank has moved 18* and the piston is already halfway down the stroke...
get my drift?

now, the cam moves off the crank as well, and if the crank has to move 4* (say) for the intake valves to open, and the piston is still at TDC, then there will potentially be less vaccuum created as a result. then again, the longer the piston stays at TDC, the faster it will accelerate down, creating more of a cyclone and atomizing the fuel better.
this accleration also could create more vacuum.
who knows.

again, i cannot begin to pretend i understand everything that is at work here, and i would venture a guess that probably nobody on this board, or anywhere completely understands all the physics involved in a combustion engine.
there are simply too many variables that affect each other to optimize a single thing.

hope that helped.

 

Yea well thats ok we cant know it all but we can try lol. Its interesting to discuss it though and try to learn the fundamentles because like you say its just one big jigsaw puzzle of variable. Its a great thread though and a nice video to illistrate your point.