A
Anonymous
Guest
Some people were getting confused as to what length rod increases the piston velocity and so forth so I have done 3 graphs to show, quantitatively, what happens when you change the rod length.
These graphs of the function,
y = -R*(1-cosx+(L/R)*(1-sqrt(1-((sinx)/(L/R))²)))
(which can be verified) and it's first derivative (velocity), and it's second derivative (acceleration), where
R = crank offset (stroke/2)
L = conn rod length.
The black curve represents the specs for a 221 ci engine (rod length 5.13, stroke 3.46);
the blue curve represents rod length 200 inches, stroke 3.46;
the red curve represents rod length 3.46, stroke 3.46;
and the grey curve represents rod length 1.73 (3.46/2; gudgeon pin axis would line up with CS axis at BDC) and stroke 3.46.
Acceleration and velocity graphs are with crank rotational speed 1 radian per second (2pi radians in 360 degrees), and rotational acceleration 0 radians per second.
Position vs CS angle:
http://www.users.bigpond.com/themilnefa ... stposi.gif
Velocity vs CS angle:
http://www.users.bigpond.com/themilnefa ... tvelo1.gif
Acceleration vs CS angle:
http://www.users.bigpond.com/themilnefa ... accel1.gif
It is interesting to notice that the shorter rod length actually means that there are two pulses but less peak acceleration toward BDC, but considerably higher peak acceleration at TDC. You can also see that with the long rod you have lower piston speeds, albeit very marginally; the shorter rod moves the piston further down the cylinder for the same CS angle, meaning better (speculative) torque characteristics.
These graphs of the function,
y = -R*(1-cosx+(L/R)*(1-sqrt(1-((sinx)/(L/R))²)))
(which can be verified) and it's first derivative (velocity), and it's second derivative (acceleration), where
R = crank offset (stroke/2)
L = conn rod length.
The black curve represents the specs for a 221 ci engine (rod length 5.13, stroke 3.46);
the blue curve represents rod length 200 inches, stroke 3.46;
the red curve represents rod length 3.46, stroke 3.46;
and the grey curve represents rod length 1.73 (3.46/2; gudgeon pin axis would line up with CS axis at BDC) and stroke 3.46.
Acceleration and velocity graphs are with crank rotational speed 1 radian per second (2pi radians in 360 degrees), and rotational acceleration 0 radians per second.
Position vs CS angle:
http://www.users.bigpond.com/themilnefa ... stposi.gif
Velocity vs CS angle:
http://www.users.bigpond.com/themilnefa ... tvelo1.gif
Acceleration vs CS angle:
http://www.users.bigpond.com/themilnefa ... accel1.gif
It is interesting to notice that the shorter rod length actually means that there are two pulses but less peak acceleration toward BDC, but considerably higher peak acceleration at TDC. You can also see that with the long rod you have lower piston speeds, albeit very marginally; the shorter rod moves the piston further down the cylinder for the same CS angle, meaning better (speculative) torque characteristics.