Originally Posted by
Hudson
No and no.
The torque input from cylinder #1 is the same as the torque from cylinder #4. These torque inputs are cyclic and not cumulative in the manner you suggest.
There are four problems that you need to understand in designing a crankshaft:
1) The bending stresses on the individual crankshaft components such as the pins, journals and crank webs. These are the result of the piston and rod assembly loads and the centrifugal force in the crank bits themselves. There are gas forces and inertia forces to consider.
Peak stress in the filet area is a big consideration.
2) After you determine the loads on one crank throw and size the bits accordingly, the next step is to select a firing order and calculate the necessary counterweights to counteract the primary imbalance and/or to reduce the bending moments in the main bearing areas.
3) The cyclic nature of the firing pulses and the flexibility of the crankshaft and driveline can result in the accumulation of harmful levels of torsional stress when the engine is operated at a speed where there is a resonance. Engines of more than two cylinders should have an estimate made of the resonant points in the engine operating range. This is where vibration damper designss and flywheel modifications are made to move these resonant points or eliminate them.
4) The lubrication of the crankshaft and associated bearings should be considered. Sufficient oil for hot idle will determine the pump size and a review of the way in which the load moves around the journals will show if there are any half speed vectors in dangerous locations.
You will repeat these stages a few times as the design process is iterative.
If, when you complete all of your calculations you still feel that progressive journal diameters are the hot setup, a savage beating by the manufacturing engineers will straighten you out. You see, they will not want to tool up four different sized connecting rods. Purchasing will not want to buy four different bearings and will stall your project until you are replaced.