The cranks and connecting rods of a 4-cylinder in-line engine running at 1800 rpm are 50 mm and 200 mm long respectively. The cylinders are spaced 150 mm apart. If the cylinders are numbered 1 to 4 in sequence from one end, the cranks appear at intervals of 90° in an end view in the order 1–4–2–3. Reciprocating mass corresponding to each cylinder is 1.5 kg. Determine

(a) unbalanced primary and secondary forces, and

(b) unbalanced primary and secondary couples with reference to central plane of engine.