Merlin "Twenty"

Details of the latest Rolls-Royce Merlin XX model, of which Packard Motor Car Company is to build 9,000 — one-third for the Army Air Forces, and two-thirds for England — have been reported by our British contemporary, The Aeroplane. The American-built engine is similar in design to that in production in Britain, except that in this country the model is built with a two-piece cylinder block.

As reported in The Aeroplane, the Merlin XX differs from earlier types chiefly in its two-speed supercharger, which makes it slightly longer. The rated output is 1,260 hp at 12,250 ft, which represents an increase in power of 16.3 percent over the previous Merlin X model at an altitude 18.3 percent higher, while the weight increase is 8.5 percent. The dry weight of the engine is 1.450 lb.

The gain in power is said to be a result of better performance from the supercharger and a longer carburetor. Comprising 11.000 separate parts, including 4,500 different varieties of parts, the Merlin XX, like others of the series is a 12-cylinder upright 60° Vee-type arranged as a right-hand tractor. Each cylinder block is a single aluminum casting, comprising the head and coolant jacket. Wet cylinder liners in high quality steel are inserted into it and are provided with suitable joints at top and bottom to prevent leakage of the coolant into the cylinder, or gas leakage from the cylinders into coolant system. Fourteen long studs, which extend to the top of the block, attach each cylinder block assembly to the crankcase.

Two inlet and two exhaust valves, operated through rockers, are provided each cylinder. All valves are parallel with the center line of the block and are fitted with two concentric coil return springs per valve. One camshaft is used for each cylinder block and is carried in seven forged aluminum bearing pedestals mounted on the top face of the cylinder casting. The camshafts are driven through bevel gearing by two inclined shafts, one at the rear of each.

The valve seatings are made from special heat-resisting steel, and screwed into the aluminum cylinder head. The valve guides, in cast iron for the inlet valves and phosphor bronze for the exhaust valves, have taper shanks and are pressed into suitably machined holes in the cylinder casting.

Bolted at the front end to the top half crankcase, which is a rigid aluminum casting, is another aluminum casting which houses the propeller reduction gearing and its shafts and bearings. The lower half crankcase contains the oil pumps and filters and is a shallow casting of aluminum. At the rear end is another casting, known as the wheelcase, which carries gearings, bearings and driving shafts for the supercharger, magnetos, starter, dynamo, camshafts, coolant and fuel pumps.

A balanced hollow crankshaft is used, made from a one-piece steel forging, having a flange at its forward end which transmits the drive through a splined coupling to the propeller reduction gear. All other main units, including supercharger, magnetos and camshafts, are driven from the rear end of the crankshaft through a spring drive shaft. The crankshaft is carried in seven bearings, lined with special lead bronze, housed in the top half crankcase.

The connecting rods are H-section steel forgings, machined all over, and are of the forked marine type. The pistons are aluminum forgings and have three pressure rings and two drilled scraper rings. Hardened steel floating gudgeon pins are employed.

The two-speed supercharger used is the centrifugal type which delivers pressures above atmospheric through a large diameter pipe led between the cylinder blocks to the inlet ports in the cylinders. It is driven from the rear end of the crankshaft through clutches by a two-speed mechanism, the gear ratios of which are arranged so that maximum engine power is available at operational heights to meet service conditions. The gear change mechanism for the supercharger is operated by the pilot through a servo cylinder deriving its power from scavenge oil pressure. Maximum operational speed of the supercharger rotor, which is made of an aluminum alloy forging, is said to be about 29,000 rpm.

A comparison between the Merlin X of 1938, and the present Merlin XX, both of which are equipped with two-speed superchargers, is given as follows:
  Merlin X  Merlin XX
Max power (low gear)  1,130 hp at 3,000 rpm at 5,200 ft  1,260 hp at 3,000 rpm at 12,250 ft
Max power (high gear)  1,010 hp at 3,000 rpm at 17,750 ft  1,175 hp at 3.000 rpm at 21.000 ft

This article was originally printed in the June, 1942, issue of Aviation magazine, vol 41, no 6, pp 207, 255.
The original article includes two photos of engines on the production line.
Photos credited to International.