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Originally Posted by kircher
Another thing I read when I was doing my own research on rod:stroke ratios was that with a longer rod (higher rod ratio) the piston dwells longer at TDC, but shorter at BDC, and a shorter rod (lower rod ratio) will dwell longer at BDC, yet shorter at TDC, relative to the longer rod combo. This doesn't make sense mathematically to me. The piston travels in one dimension, so is always travelling on the one plane, and the crank journal rotates in circular motion, with the rod joining and converting the circular motion into the linear motion of the piston. With the piston always on the one plane, and the rod length and stroke being constant, shouldn't the dwell at BDC and TDC be identical?
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You probably need to plot it on some graph paper to appreciate what happens. On short rod ratio's the acceleration curve in particular is gnarly around TDC, while long rod ratio's it is smooth. Remember that although the crank may be travelling at a fix speed, the piston isn't because the rod is not operating at a fixed angle.