$($b$)$ A four$-$cylinder two$-$stroke engine running at $1500$ rpm has the crank of $50$ mm$.$ The length of connecting rods are $200$ mm and the cylinders are spaced $200$ mm apart. The cylinders are numbered $1$ to $4$ from one end, and the firing order is $1-3-2-4.$ The reciprocating mass per cylinder is $2$ kg$.$ Determine $($i$)$ Unbalanced primary and secondary forces, if any. $($ii$)$ Unbalanced primary and secondary couples about the central plane of the engine.