04-04-2019, 08:55 AM
(This post was last modified: 04-04-2019, 08:58 AM by Bob Culver.)
Few appreciate just how great the inertia loads are. The firing load at full throttle and med revs is reckoned at about 100x the cr in psi so the load on piston can be approximated.
The big end, crankpin, and not directly counterbalnced parts of the crank follow basic formula for centrifugal force. The accel/decel piston and little end is a bit more complex but is about 11/4 and 3/4 the corres centrifugal force at top and bottom stroke. (For a jowett engine the piston assembly exerts 800x weight upward at 4,000 rpm). So at small throttle and revs the total upward force from two piston assemblies, big ends crankpins etc soon exceeds the net down push from one firing piston under load. The inertia loads increase as the square of revs. So at 6000 more than double at 4000.
It is reckoned that at 7:1 cr for a typical square engine, full firing load down and upward load on conrod bearing are about the same at 4,000 rpm on our small bore low cr engines would occur at much lower revs..
The big end, crankpin, and not directly counterbalnced parts of the crank follow basic formula for centrifugal force. The accel/decel piston and little end is a bit more complex but is about 11/4 and 3/4 the corres centrifugal force at top and bottom stroke. (For a jowett engine the piston assembly exerts 800x weight upward at 4,000 rpm). So at small throttle and revs the total upward force from two piston assemblies, big ends crankpins etc soon exceeds the net down push from one firing piston under load. The inertia loads increase as the square of revs. So at 6000 more than double at 4000.
It is reckoned that at 7:1 cr for a typical square engine, full firing load down and upward load on conrod bearing are about the same at 4,000 rpm on our small bore low cr engines would occur at much lower revs..