gradual crankshaft

This Forum is dedicated to the Hobbyist I.C. (Internal Combustion) Engine Community.

Moderators: Harold_V, JackF

Posts: 884
Joined: Wed Jun 07, 2006 8:36 am
Location: Toronto, Canada

Re: gradual crankshaft

Post by RET » Mon Dec 05, 2016 8:20 pm

Hi gilteva,

First, welcome to Chaski. What you say would be true if there were no other forces to consider and if your engine only ran at one or two hundred RPM.

Just one of the reasons you don't have varying crank & journal sizes in an engine is that the centrifugal forces on the crank and inertial forces on the pistons and rods from the crank rotational speeds are significantly higher than any forces that may be generated by burning the fuel in the cylinders. Obviously these same forces exist for all the cylinders. These forces also vary by the square of the RPM so changes in RPM are quite significant. This is still true even with model engines because they typically run at a much higher RPM.

About the only exception I could think of to this idea would be one of the early John Deere 2 cylinder tractors. You could always pick them out by their distinctive sound (Put, Put, Put, etc.). They ran at a very low RPM, but still had lots of torque.

Lubrication (maintaining an oil film) is another.

The short answer is, it isn't worth it to vary the crank journal sizes.

Richard Trounce.

Posts: 532
Joined: Thu May 30, 2013 4:40 am

Re: gradual crankshaft

Post by Magicniner » Mon Dec 26, 2016 8:04 am

BigDumbDinosaur wrote:
NP317 wrote:This discussion sure makes radial engines sound like a good idea!
Well, they were, as long as you were willing to put up with their idiosyncrasies and large, bluff frontal area. The latter wasn't too much a problem in aircraft after the NACA cowling was developed, but it sure created some interesting design challenges for the few folks who used a radial to power an automobile.
M4 Sherman anyone?

Post Reply