Bristol Jupiter VIII F., VIII F.P., XI F., XI F.P.
Part 4: Exhaust, and Ignition Systems
Compiled by Kimble D. McCutcheon
Published 5 Apr 2025
| Part 1: Specifications | Part 2: Description |
| Part 3: Lubrication and Induction Systems | Part 4: Exhaust and Ignition Systems |
Exhaust System
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| Overview | Flexible Mounting | Cylinder Branch Pipe |
The mild steel exhaust ring was made from two pressed circular sheet steel pieces that formed an annular chamber of streamline cross section. The steel edges were welded at the rear to form a flattened trailing edge. Three equally spaced saddles were welded to the ring inner surfaces and with four bolts each were attached to three box brackets that were bolted to three lugs on the reduction gear casing. The single bolt in each lug provided fore and aft ring adjustment to permit accurate alignment. The bracket- to- bolt attachment incorporated a flexible mounting that allowed for ring expansion and also absorbed vibration between the engine and ring. The end of each attachment bolt on the reduction gear casing had a transversely elongated hole. The cross bolt that passed through this and also through the forked bracket end carried a tubular distance piece upon which both were borne and which projected beyond the fork. The attachment bolt eye was flanked on each side by Ferodo washers (fender washers) that were compressed between the fork ends and attachment bolt by the cross bolt. A stiff spring washer, threaded on to a distance piece projecting end, was drawn up by the nut and washer on the bolt, and the pressure exerted by the nut (which was screwed down to the distance piece) was limited by the spring washer strength, leaving the exhaust ring free to move to the extent permitted by the attachment bolt hole elongation. The nut was secured with a split pin. Eighteen circular inlets to the ring, all swept in the same direction, were formed in its trailing edge, half of each being developed from its inner and half from its outer surface. Flanged adaptors, welded into the inlet mouths, had two bolt holes each for attaching flanged sleeves in which the exhaust branches from the cylinders were slideably mounted, thus providing telescopic joints that allowed for cylinder expansion. By sweeping all the inlets in one direction the entering gases circulated, promoting their easier escape by the extractor action that was thus set up. Two large swept outlet elbows or tail pipes were welded onto the ring outer surface in the lower segment. Each inlet branch consisted of a drawn steel tube bent at the cylinder end to a curvature that enabled the branch to meet the cylinder exhaust port in a faced-flanged joint. A spherical-sectioned seating flange was welded onto the branch end and engaged with a loose-ribbed four-holed flange, formed with a corresponding spherical chamfer, by which the branch was clamped to the four studs at the cylinder exhaust port. A steel seating washer was mounted on the studs between the flange and cylinder head and the central aperture edge was extended outward to enter the spherical flange mouth on the branch. While the loose flange clamped the washer rigidly to the cylinder head sufficient space was left between the spherical chamfer in the loose flange and the outward extension of the seating flange to permit the fixed flange on the pipe to be a sufficiently easy fit between the two to align itself as required when the cylinder expands under working conditions. The exhaust ring was treated with a process of heavy nickel deposit to prevent oxidization.
Ignition System
Magnetos
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| B.T.H. SC9-3B and Mounting | |
Two nine-cylinder British Thomson Houston (B.T.H.) SC9-3B magnetos furnished dual ignition. These polar inductor magnetos produced four sparks for each rotor revolution, making the rotor speed 9/8 times crankshaft speed in order to supply the required number of sparks to fire all nine cylinders during every two crankshaft revolutions. Both magnetos ran in an anticlockwise direction and were driven through laminated spring-dog couplings. A closely-serrated fine adjustment flange in the coupling driven element enabled accurate timing adjustment. A hand-operated advance and retard control was used on earlier engines while later engines used an automatic device in the coupling. The port magneto fired the sparking plugs nearer the gas starter non-return valves and the starboard magneto the remaining ones. The magnetos were shielded and the metal-braided high tension leads were bonded to the shields, which helped to eliminate radio interference.
Two hollow circular flanges on the rear cover received the magneto spigots with corresponding flanges for attachment and location to the rear cover. The magnetos were attached to the rear cover using a spring mounting to reduce the vibrations and shocks transmitted to the magneto and thereby lessen the strain on the rear cover spigot. The spring mounting comprised three stiff coil springs on the attachment studs projecting from the case flange, thereby providing the a slight resilience. A thick leather packing washer was fitted between the two flanges to serve as a buffer and absorb the friction generated by the slight relative movement between the two. The magneto flange holes were steel bushed, and the bushes had collars at their outer ends that located them axially in the magneto flange and provided spring seating. The spring was captured between the steel bush and a distance sleeve (spacer) fitted beneath the nut. The sleeve on the stud entered the washer, providing radial location for the washer and spring. When the nut was tightened the washer and distance piece were drawn up tight against the rear cover flange , leaving the leather washer and the magneto spigot free to float to the extent permitted by the spring. The nut was locked by a split pin. The springs were of considerable stiffness and practically no movement occurred, but enough resilience to reduce vibration and shocks.
A laminated spring dog fitted at the magneto drive spindle driving end in the rear cover constituted one element of the magneto coupling. The laminated spring dog engaged a pair of jaws formed at the forward end of the other coupling element, which was mounted on the magneto rotor spindle. The jaws projected from a plate that formed half of the fine adjustment coupling; the other half was a corresponding plate keyed to the magneto spindle. The two plates' contacting surfaces were finely serrated, enabling accurate timing setting. The boss on the magneto spindle plate projected forward and carried a sleeve formed in the center of the other plate, thereby locating the two centrally. When the desired adjustment was determined, the two were locked together by a washer and nut that screwed onto the rotor spindle end and was locked with a split pin. In later engines where an automatic advance and retard was fitted, this incorporated the fine adjustment coupling element at its forward end and the whole was accommodated in the hollow spigot mounting.
The hand-operated advance and retard control on earlier engines consisted of a common rocking shaft fitted with a lever at either end to which the control levers on the magnetos were connected by ball-jointed link rods. The rocking shaft was borne in a transverse sleeve, bushed at either end with phosphor-bronze bushes and mounted in a central bracket carried on two of the hand-turning gear casing studs. A loose coupling lever at the shaft port end was left free so it could be conveniently connected to the cockpit control. A centrifugally-operated spring-return automatic advance and retard scheme was employed on later engines. The VIII F., VIII F.P. and the XI F. engines ranged from 5° to 35° advance while the XI F.P. ranged from 20° to 35° advance; a stop in the coupling limited retardation. A modified brush holder, in which the starter pin was in a more advanced position, was fitted in magnetos with automatic advance and retard couplings. This was necessary because the automatic coupling achieved more retardation than the hand-operated control. Moreover, in the latter type of control the variation was effected by partial rotation of the contact breaker cam, which did not change the distributor position. With the automatic coupling the rotor shaft, and therefore the distributor, was actually retarded in relation to the crankshaft, thereby changing the position of the distributor brush in relation to the segments.
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| Automatic Control Coupling | |
The automatic advance and retard coupling consisted primarily of two elements of which one, the driving member, was coupled to the magneto drive spindle on the engine, and the other, the driven member, was mounted on the magneto rotor spindle. The two elements, which were able to rotate in respect of one other over a limited range of angular travel (approximately the timing advance/retard range), were coupled together via a spring-loaded centrifugally operated movement that, as the engine speed varied, changed the relative angular position of one member to the other, advancing or retarding the rotor spindle in relation to the drive spindle. The driven member on the rotor spindle consisted of a plate with a projecting central boss bored with a tapered hole and cut with a keyway for mounting on the rotor spindle upon which it was secured by a hex-headed sleeve nut. The projecting outer surface boss provided a journal for a sleeve on the coupling driving member, which was coupled to the magneto drive spindle by the laminated spring dog coupling already described. The driving member (coupling plate) consisted of a circular plate drilled with lightening holes and formed with a central sleeve on its inner face. Its outer face it was serrated to receive the corresponding serrated face of the spring dog coupling jaw. Two diametrically-opposed bob weights mounted on the driving member each carried a projecting roller that engaged with a cam plate riveted to the driven plate so that as the rotation speed increased and the bob weights moved outwards, the rollers operating on the cams on the driving plate caused it to move in relation to the driving spindle and thereby effect the appropriate timing advance. When the engine was stationary or the speed was insufficient to cause the bob weights to operate the two device elements were retained in their retarded position by a pair of coil springs, which opposed the outward weight movement. The springs, which were captured between projections on each element, threw the rollers back against the cam bottoms. Two small springs mounted on the bob weight pivots and provided with a 5-point adjustment, tended to throw the bob weights outward slightly to prevent these from chattering when the speed of rotation was in the intermediate ranges and the weights have not taken up their full load. The two main assembly elements were retained by the large central sleeve nut, which was fitted with a steel tab washer beneath its head and screwed down to the plate boss top. The washer projecting beyond the nut provided axial location for the coupling plate. The sleeve length was slightly less than the space between the driving plate and washer in order to provide the necessary end clearance. An oil groove machined on the driven plate surface ensured adequate lubrication. The coupling plate front face was serrated to receive corresponding serrations on the laminated spring coupling jaw element back; these serrations provided the fine adjustment by which the magneto was set. In the original hand operated control arrangement the two serrated fine adjustment elements were retained by the central sleeve nut but here they were secured together by two small studs that projected from the coupling plate through two semi-circular slots in the jaw member. Split-pinned nuts and washers on the studs secured the two elements together.
Terminal Fittings, Bonding and Screening
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| High-Tension Lead Details | Spring Ball Terminal |
Magneto screening (shielding) consisted of enclosing the distributor and contact breaker in metal covers. The distributor cover consisted of a circular nickel-plated brass cap that enclosed the distributor block. The contact breaker shield was another but smaller cap retained in position by a cross bar attached to studs on each side of the cap. The fittings at each end provided lead bonding connections as well as terminals for the leads themselves. The distributor face was bored with a circle of nine sockets into which the high tension terminals entered; a tenth accommodated the lead from the hand starter magneto by which current was fed into the main magneto when the engine was being started. The brass segments that received the high- tension current were carried through the block to the bottom of each socket for contact with the high tension lead terminal ends. The contact at the center terminal bottom carried the carbon brush at its rear end that made rubbing contact with the high tension brush holder. The distributor cover, which was drilled with ten holes corresponding with the ten sockets, was bolted to the magneto body. Each hole in the shield had a projecting threaded spigot that received the clamping nut by which the lead was held in position. The terminals consisted of metal thimbles soldered onto the bare lead end. A short length of the lead's metal braiding was stripped and a metal sleeve formed with a shoulder in the middle was slipped onto the lead's fabric cover prior to attaching the brass cap. The metal braiding end was opened over the sleeve where it was clamped firmly in position by an outer sleeve pressed onto it. The terminal was held in the socket by a clamping nut that engaged the collar on the outer sleeve and screwed onto the spigot projecting from the shield, thereby clamping the terminal firmly in its socket with the collar of the inner sleeve engaging the spigot end and thus forming connection for both the high tension lead and the braiding to their respective members. The outer sleeve was extended to form a right-angled fairlead that protected the lead at the bend to the distributor. The connection for the low tension switch lead to the contact breaker cover was similar to the above except that the lead terminal was an eye that attached to a terminal screw within the contact breaker cover, instead of the soldered-on thimble. A snap-on ball terminal was fitted at the sparking plug end. This consisted of an insulated sleeve fitted with a spring-loaded plunger in a short tubular metal socket, one side of which was cut away exposing the plunger. The top of the sparking plug electrode was formed with a ball end, which was inserted in the open side of the socket by forcing the ball against the spring loaded plunger by hand pressure thus constituting an attachment that was secure and quickly detachable. The opposite end of the insulated sleeve had a socket that was essentially similar to those in the distributor to receive the fittings on the metal braiding, but the fairlead was omitted. The clamping nut was locked with a wire circlip. Connection between the plunger socket was by a coarse-thread taper screw that projected from the socket end and screwed into the lead end. The sleeve bore the cylinder number that it served.
From the magneto the high tension leads were routed through four clamping blocks, the first two of which were bolted to brackets mounted on the gas starter distributor sides and the remaining two to the induction chamber cover back. The clamping blocks consisted of split aluminium blocks drilled with chamfered holes to receive the leads; the chamfer prevented metal braiding damage. The leads were routed to the engine front through holes drilled in the spigot flange by which the engine was mounted in the aircraft, after which they were clamped to the induction pipes by brass clips and were finally supported by a clip attached to a lug provided on the induction branch flange. Two sheet brass shields bolted to the induction chamber cover on either side of the hand turning gear casing kept the leads clear of the crank handle shafts and prevented the bonding from being chafed. None of the cable retaining clamps were insulated and since these were located at least every 18", this scheme met the electrical bonding requirements.
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