Umran

Total Torque Consideration

For 111cc twin engine, all need to be done is to sum up 2 cylinder torque and add the prop inertia torque, 14.1 + 14.1 + 0.7(for 111cc prop inertia is slightly less) = 28.9Nm. This is basically the amount of maximum TORQUE needed to be overcome by the starting system and it peaked at every 360 degrees of rotation. And for a successful start system, we need to design it to have at least twice this amount of torque i.e. at 57.8Nm.

However for 222cc engine even though the maximum torque will be the same (pause and think for a while) but the frequency of its occurring is every 180 degrees. Statically this doesn’t change anything, but dynamically it changes a lot. Consider the rotation momentum, for a normal twin; the shaft will rotate almost freely thru 270 degrees before being hit by compression stroke. These lengthy free rotations will buildup both speed and momentum and it will definitely help to overcome the compression load. However for 222, the free rotation is very minimal and the system doesn’t have the luxury of building up its momentum. For this, we have to ensure the system have ample of power on continuous basis.

For conservative approach, we may need to consider doubling the amount of torque, 57.8 x 2 = 115.6Nm.

This value (115.6Nm) however does not actually reflect our actual need and since we are considering time base loading of the system (frequency of loading), we may consider the root mean square of this value as a continuous amount of torque required for the system to operate, 115.6N/SQRT [2] = 81.7Nm. This deduction can be analogous to AC current apparent power calculation.