The Napier Dagger VIII is a 24-cylinder moderately-supercharged air-cooled aero engine having four banks of six cylinders disposed of in "H" formation on a common crankcase. Two crankshafts are used, each crankshaft being respectively coupled to the pistons in the diametrically opposed pairs of cylinder banks. The crankcase is split and houses the two crankshafts, each of which runs in seven lead-bronze bearings. The crankshafts are each geared to the airscrew shaft by means of a single individual spur gear. The axis of the airscrew shaft is located above the axes of the two crankshafts.

Cylinders, Pistons, and Connecting Rods

The separate steel cylinder barrels are provided with closely pitched fins. Forged Y-alloy heads are mounted on the camshaft casings, and the header units so formed are secured, together with the cylinders, to the crankcase by long studs. Each cylinder has one inlet and one sodium-cooled exhaust valve; the valves of each bank are operated by rockers actuated by a single overhead camshaft. Self-adjusting hydraulic tappets are incorporated in the rockers, which are mounted on hollow spindles above the camshafts. Separate cast induction manifolds equipped with branch pipes are located between the top and bottom pairs of cylinder banks. Forked and plain connecting rods employing lead-bronze lined big-end bearings are used. The pistons are of forged Y-alloy, and each has two gas and two oil scraper rings.

Magneto, Distributor, and Airscrew Governor Drives

The front cover houses the reduction gear, the drives for the magnetos and distributors, and the constant-speed airscrew governor unit. The magnetos and distributors are driven through individual layshafts from a large bevel gearwheel, which is freely mounted upon the airscrew shaft. This bevel gear also carries a spur gear, which is driven by a horizontal layshaft coupled to the forward end of the right-hand crankshaft through a servo-controlled differential type of ignition advance and retard control unit. The airscrew shaft embodies an oil-pressure feed duct for the actuation of the constant-speed airscrew; the constant-speed airscrew governor operates through this duct. The control of the airscrew governor is interconnected with the ignition servo-control unit.

Timing Gear and Auxiliary Drives

The timing gear, auxiliary and centrifugal supercharger drives, located at the rear of the engine, is taken through the centre of the crankcase by means of a long tubular drive shaft, which is driven by a secondary spur gear mounted on the forward end of the left-hand crankshaft. The camshaft vertical drives, timing gear, and auxiliary drives are incorporated in an intermediate casing located between the rear end of the crankcase and the front end of the supercharger casing. The timing gears and accessories are driven through idler gears and layshafts from one element of a large double spur gear coupled to the centre drive shaft; the other element of this double spur gear drives the supercharger impeller through the layshafts, which incorporate spring drives. Drives are available for a fuel pump, B.T.H. and R.A.E. air compressors, electric generator, electric turning gear, vacuum pump, and hydraulic pump.

Supercharger Impeller and Centrifugal Clutch

Excessive acceleration and deceleration impeller loads are absorbed by means of a centrifugal clutch incorporated with the impeller spindle. The impeller is of the double-entry type, each side being fed through one choke of the twin-choke carburettor. A hand and electric turning gear is fitted on the timing-gear intermediate casing, and is coupled to the rear end of the left-hand crankshaft through a layshaft.

Filters

The oil and pressure filter unit is mounted beside the carburettor on the underside of the supercharger casing. This unit embodies one pressure pump and two scavenge pumps, a double-element felt pressure filter, two gauze scavenge filters, a primary relief valve, and a secondary pressure-reducing valve for the low-pressure oil system.

Main and Connecting Rod Bearings

The main and big-end bearings are lubricated by high-pressure oil, while low-pressure oil is used for the lubrication of the plain bearings incorporated in the timing casing and the valve operating gear. A small vane-type oil scavenge pump is fitted at each end of the lower camshaft casings to ensure that the casings are properly scavenged of the surplus oil which reaches them from the front and timing casing. The small-end bearings and the gears in the timing casing and front cover are lubricated by splash and low-pressure oil directed through jets.

Carburettor and Boost Control

Mixture is supplied by a twin-bore vertical carburettor, which incorporates a variable datum automatic boost control and built-in automatic mixture and enrichment control units. The hollow butterfly throttle valves are oil heated to prevent the formation of ice. A combined slow-running mixture adjustment and cut-out is incorporated.

Bonding and Screening

The engine is completely bonded and employs a screened ignition harness. Cowling and cylinder baffles are attached to the engine for the purpose of effecting efficient cooling.

Photo album created with Web Album Generator