Rod/Crank Geometry
Last updated 2007-09-18 11:17 PDT

Rod/Crank Geometry

A recent thread on the DIY EFI e-mail list was discussion PV (pressure/volume) plots as a useful tool in setting engine timing. I got to thinking that there were probably other useful plots, and wanted to know specifically if a PQ plot were useful, where Q is defined as:

Q =cos(alpha) * sin(beta)
alpha=Angle between rod and bore centerline, as the rod aligns with the bore, cos(alpha) goes to 1.0.
beta =Angle between rod and crank throw, as the rod/crank angle goes to 90d, sin(beta) goes to 1.0.

This first plot shows the value of Q versus crank angle for the power stroke. (CR is of no consequence in this plot, I just forgot to clean up the subtitle.) Q is function of rod:stroke ratio (L), which I varied to show some extreme values.

Here is how much of the total chamber volume (V) is exposed as the piston moves down the power stroke, again plotted as a function of crank angle.

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