Bristol Jupiter IV
Ignition, Gas Starter, Exhaust Ring
Compiled by Kimble D. McCutcheon
Published 14 Mar 2025
| Part 1: Specifications | Part 2: Description |
| Part 3: Induction, Lubrication | Part 4: Ignition, Gas Starter, Exhaust Ring |
Ignition
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| Magnetos, Fine Adjustment Coupling, High-Tension Leads, Chater-Lee Terminals | |||
Magnetos: Dual ignition was provided by two British Thomson Houston (B.T.H.) S.V.9 magnetos, which as polar inductor types, produced four sparks for each armature revolution. The armature speed was therefore 9/8 times crankshaft speed so as to supply the required number of sparks to fire nine cylinders in two crankshaft revolutions. Both magnetos ran in an anticlockwise direction and were driven through laminated spring dog couplings. A closely serrated fine adjustment flange was incorporated in the coupling driven element and enabled a final spark timing setting to be obtained. No advance or retard mechanism was provided. The left magneto fired the front plugs and the right magneto fired the rear ones. The magnetos were shielded and the metal-braided high tension leads bonded to reduce radio interference.
The magnetos incorporated a spigot that engaged a hole in the rear cover. Two hollow spigots concentric with the magneto drive spindles projected from the magnetos and into machined rear-cover recesses surrounded by a flange, which was bolted to a. similar flange on the magneto. The hollow spigots served as housings for the couplings and collected drain oil from the driving spindles, returning it to the rear cover by two external oil pipes.
Both magnetos were shielded and bonded to reduce radio frequency interference. Metal covers enclosed the distributor and contact breaker. The distributor enclosure consisted of a circular brass cap secured by three screws. The contact breaker shielding was another, smaller cap retained in position by a leaf spring clip. The distributor face was bored with a circle of nine sockets into that received the high tension leads. At the center a tenth socket received the hand starter magneto lead. Brass segments conducted current from the high tension brushes through the distributor block to the bottoms of the nine high-tension terminal sockets, while the center terminal included a carbon brush at its inner end that made rubbing contact with the high tension brush holder.
Each spark plug lead consisted of an insulated plug and brass tip on one end; this threaded into the high tension distributor tower. At the opposite end a metal Chater-Lee terminal and insulating sleeve attached to the sparking plug. The plug end was engraved with the cylinder that it served. Each lead was covered with a metal shielding braid clamped firmly in place at each end. The hand starter magneto and distributor terminal construction was similar.
The high-tension leads were routed from the magneto using three brackets; the first was bolted across the magneto tops and the remaining two were bolted to the induction chamber cover back. These brackets were fitted with split aluminium blocks drilled with holes to receive the leads. The leads were carried to the engine front through holes drilled in the spigot flange by which the engine was mounted in the aircraft. From there the leads were clamped to the crankcase in split aluminium blocks and to the induction pipes by aluminium clips. Close to the sparking plug one end of a short stranded wire was clipped to the high tension lead braiding; the other end was attached to the cylinder by a small screw that entered a threaded boss for it on the cylinder. None of the cable retaining clamps were insulated, and since they were located at frequent intervals, the arrangement conformed to the bonding requirements.
Gas Starter
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| Distributor Detached from Oil Pump Drive |
Distributor Sectioned |
Crankcase Bolt Connections |
The distributor was spigoted into the oil pump cover top and driven through a dog-coupling by the oil pump spindle upper end. The distributor was of the hand-operated bypass valve type with a lever on its top cover connected to the aircraft control linkage. The mixture from the starter compressor cylinder was carried by a 1/4" steel pipe from the nine compressor unions on the distributor to screwed unions at the crankcase bolt rear ends. The crankcase bolts were hollow; their rear ends formed suitable junctions for the pipes running to the non-return valves in the cylinders. This arrangement firmly secured the pipe midway along its length, preventing vibration and chatter. The mixture entered the pipe screwed into the bolt end and left by the other end, which was entered radially through the flat on the crankcase The hole in the aluminium had sufficient clearance to allow the bolt to seat freely without exerting pressure on the union. The non-return valves in the cylinders were of the usual spring loaded type and were cut with a thread similar to that used on sparking plugs.
The distributor delivered the mixture from the starter's compressor cylinders to the appropriate engine cylinders in firing order. The distributor's rotor was geared to the oil pump spindle that revolved at half crankshaft speed and therefore provided the necessary speed ratio. The distributor was built up of two aluminium castings of which the lower one formed the distributor block and housed the driving spindle bearings and the upper one was a screw-cap body cover that formed a gas tight chamber for the incoming mixture. The block was machined with a faced surface with which the working face of the rotor was in rubbing contact. Nine equally spaced holes were drilled in the surface into passages to nine exterior unions for connecting the pipes to the cylinders. The phosphor bronze rotor was mounted on the main spindle upper end taper where it was secured by a nut and lock washer. This spindle end was drilled for a short distance with a small bore that was met at the bottom by a radially drilled hole intersecting a recess cut around the spindle half way down the taper. A passage drilled in the rotor coincided at the inner end with this recess and at the outer end with the distributor block holes pitch line. Here the passage opened into an elongated recess in the rotor surface, which enabled two holes in the block to be covered simultaneously.
The spindle's top was formed with a conical seat to receive a sliding valve operated by a striker fork and collar controlled from the cover's exterior by a small lever. The valve closed the passage, restricting the mixture flow through a slot on the rotor's opposite diameter. This slot was cut right through the rotor, permitting free communication between the gas chamber interior and two ports. When starting, the valve was left open, allowing the mixture to pass into the cylinders on both induction and explosion strokes. After the cylinder and induction system charging was complete, the lever was operated and the supply to the cylinders during the induction stroke was cut off. All pressure was then concentrated upon the cylinder firing stroke causing the engine to turn and eventually starts. The main spindle was borne in two ball bearings fitted in a steel sleeve housed in the distributor block underside and was located by a dowel screw. The sleeve interior had shoulders a short distance from each end that provided stops for the bearing outer rings. The spindle was bored for lightness up to a point short of the small bore on the top, and was formed with a shoulder at its upper end that abutted the upper bearing inner ring. An internal spacer separated this ring from the lower bearing spacer, and a nut and pin secured the whole assembly on the shaft. The nut's lower end was slotted to receive the oil pump spindle driving dog. When the distributor was bolted on the oil pump, the oil pump spigot abutted the lower bearing outer ring and retained its position in the housing. The hole into which the locating screw entered was large and permitted the steel sleeve to move axially within the aluminium housing, and since the sleeve was a loose fit, the rotor was able to seat freely on its track.
Exhaust Ring
The pressed sheet steel exhaust ring was built up of one front and one rear section that met in a circular lap joint. The front section was semi-circular and the rear of straight except it was slightly dished for rigidity. Nine elbows, each formed with two branches, were equally spaced around its periphery and these were riveted to the main body by flanges at their ends. The elbows were in two sections, which allowed telescopic movement and permitted cylinders expansion and contraction. Similarly there was a free joint between the elbow outer end amid the flange, which was bolted to the cylinder head. The ring's outlet was by two swept elbows in the lower segment. Each outlet carries a small branch pipe for connecting a bypass to the exhaust-heated induction jacket on the carburettors. The ring was secured to the engine at six points. Three pressed steel brackets were attached to studs by which the front cover was secured, and the other three points were secured by adjustable tubular supports, which were provided with brackets for fitting underneath three main crankcase bolts. Seven studs on the ring's outer periphery provide for attaching the aircraft cowling to the ring.
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