56. Item labeled «R» as shown in Fig. 5 of the illustration is identified as the __________

Item R in Fig. 5 is the aftercooler, a heat exchanger that cools compressed air before it enters the cylinder. Its position between the turbocharger/blower outlet and the scavenge air receiver confirms its identification, as exhaust manifolds, air filters, and non-return scavenge valves are located elsewhere in the engine's operating cycle.