Eccentric big end bearing 1

[Tim_S]

A Google trawl uncovered a few avenues that have been explored using an Eccentric bearing on the big end journal which gives the big end of the connecting rod a secondary motion (on top of the simple rotary motion of the crankshaft).
Most have had something to do with Variable Compression Ratio test bed engines. So I was having a tinker with the idea and thought maybe there were gains to be had from a continuously rotating eccentric bearing in terms of increasing leverage when the piston is near TDC. I thought that the trajectory of the piston could be delayed by, say, 10 degrees or so, getting the crankshaft rotated past the 12 o'clock position before igniting the mix.
A criticism was levied that friction arising from the additional bearing surface area would more than wipe out any potential gains.
Does anyone have any strong reasons why an eccentric bearing would be a really good or conversely a really bad idea ?

 

[gruntguru]

The eccentric bearing is an OK concept for variable compression or variable displacement.

Weird mechanisms for "increasing leverage" or asymmetric piston motion offer ZERO improvement in terms of converting cylinder pressure and volume into mechanical work.

W = integral P.dV

je suis charlie

 

[Tim_S]

It is a weird mechanism, agreed - I've attached a picture which tries to show the piston coming down later and therefore cylinder pressure being preserved until more leverage is available - happy to have the idea shot down in flames just so long as I can follow why

 

[gruntguru]

Work = Integral of Pressure.dVolume = Integral of Force.dDisplacement = Integral of Torque.dAngle

Basically if the eccentricity of the crank is increased, the instantaneous torque increases but the piston descends faster so the pressure decreases more rapidly causing the force to decrease more rapidly. No free lunch.

je suis charlie

 

[mohawk2]

The idea of variable compression is great for diesels where the ignition is fixed & thus the revs are limited due to the ignition point being fixed. Varying the compression would allow for the ignition point to be moved & or the you of two stage turbo/superchargers, allowing for more revs which helps efficiency &/or more power from higher boost ratios.

Eccentric cam bearing have been used as one means to control 2 of 4 valves see Honda CBR400F3 & or to provide variable cam timing or lift, so the idea has merit, but the forces involved are much higher on the crank, plus double the number of bearings & clearance issues, may have its own impact on the longevity of such a system.

There is also the dual/bent conrod solution that provides the same effect, where the main rod is connected to the crank via a shorter secondary rod that allows the piston to dwell, it looks like an elbow.