Aviation of WWII
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Engines BMW VI - M-17 BMW 801 D Rolls-Royce Merlin Rolls-Royce Merlin 25 AM-35


Engine Maintenance Manual

Derby, England, 1946


The Merlin two-stage, two-speed engines covered by this publication are of basically similar construction and are of the geared, 12 cylinder, upright V type with liquid-cooled, two-stage, two-speed, supercharger and have two pressure-liquid cooled banks of six cylinders located on the upper side of the crankcase with an included angle of 60 degrees.

Each cylinder block comprises a light alloy skirt and detachable combustion head and contains six separate steel liners, the lower ends of which are free to float In the crankcase apertures. Four valves, two inlet and two sodium-filled exhaust valves are located in each cylinder and operated by a single, centrally-disposed overhead camshaft through individual rocker arms.

The forged light alloy pistons are supported on the connecting rods by fully-floating hardened-steel gudgeon pins, forked connecting rods operating in the "B" or left-hand cylinder bores and the plain type in the "A" or right-hand bores.

A forged, six-throw, crankshaft is mounted in seven lead-bronze lined bearings in the crankcase upper half, split bearing blocks retaining the bearing shells in position. The lower half of the crankcase is attached to the upper half along a horizontal joint face and forms a sump for the splash oil. Located fowards the rear end of the casing is the oil pumps unit, comprising one pressure pump and two scavenge pumps. Bolted to the forward end of the crankcase is the single spur reduction gear unit containing a small pinion driven from the front end of the crankshaft, which engages a large spur gear carrying the propeller shaft. When a cabin supercharger is fitted, however, as on Merlin 61, 71, 73, 77, 104 and 114 engines, an extension is provided on the starboard side on the gear casing to contain the cabin supercharger drive pinion which is driven from the large spur gear through a small idler pinion.

Reverse rotation of the propeller shaft on Merlin 131 engines is obtained by introducing into the reduction gear casing an idler pinion meshing with both the drive pinion and the propeller spur reduction gear.

The wheel case gears are driven from the rear end of the crankshaft through a torsion shaft. The wheel case is attached to the rear wall of the crankcase and provides mountings for the vertical drives, magnetos, main coolant pump, fuel and oil pumps, electric generator drive and hand and electric turning gear. This arrangement applies to all engines except the Merlin 130 and 131, which have the main coolant pump fitted to a horizontal drive housing on the crankcase port side in random with the intercooler coolant pump.

Three centrifugally loaded clutch wheels, which absorb the high inertia loading resulting from rapid acceleration and deceleration, are contained in the two-speed, two-stage, supercharger unit secured to the wheelcase. The drive to the supercharger is transmitted through only one of the clutch wheels in low gear and the remaining two in high gear, selection being performed by cam-actuated selector forks.

The supercharger is of the tandem, two-rotor, centrifugal type with a single point delivery volute.

A liquid cooled intercooler unit of the flattened tube type is located between the supercharger deliverv and the main central induction manifold for the purpose of lowering the temperature of the mixture delivered to the cylinders. The header tank may be formed integrally with the intercooler assembly, as on Merlin 71, 72, 73, 76, 77, 85, 100, 104, 113, 114, 131 and 131 engines, alternatively, be a separate tank (Merlin 61, 66, 67 and 70 installations only).

Carburation is effected in the following manner:

Engine Carburation
Merlin 61, 72 & 73 S.U. float carburettor
Merlin 66, 67, 70, 71, 76, 77 & 85 Bendix injector carburettor
Merlin 100, 104, 113, 114, 130 & 131 S.U. Injection pump

Both the S.U. float carburettor and the Rolls-Royce Bendix injection carburettor are designed to form a complete assembly secured to the double-entry updraught intake elbow. The former is a coolant jacketted, oil-heated duplex float carburettor and incorporates two mixture controls, one an atmospherically operated unit and the other a boost pressure controlled unit. A special device fitted within the float chambers ensures that a continuous flow of fuel is maintained to the carburettor jets irrespective of aircraft altitude.

The correct metering of the fuel in the Bendix single point injection type carburettor is determined by the weight, of air flowing through the intakes, inducing a pressure differential across a varies of rubber diaphragms contained within the regulator unit.

The S.U. variable stroke, fuel pump injects fuel through a single injection valve into the supercharger intake eye, the air supply being controlled by a single-plate type throttle contained within a casing which replaces the normal carburettor throttle body. The correct quantity of fuel is determined by the engine speed in conjunction with the pump plunger stroke, which is varied in accordance with mixture density within the induction manifolds, with suitable correction for altitude.

Each of the three types of carburation system operate in conjunction with a pneumatically actuated, variable datum, boost control.

The dry sump lubrication system employs two scavenge pumps, and one pressure pump supplying main pressure oil to lubricate the crankshaft bearings and to actuate the constant speed unit. A suitable relief valve provides low pressure oil for lubrication of the supercharger, reduction gear, camshaft mechanism and auxiliary drives.