Bristol Jupiter IV
Induction, Lubrication
Compiled by Kimble D. McCutcheon
Published 10 Mar 2025
| Part 1: Specifications | Part 2: Description |
| Part 3: Induction, Lubrication | Part 4: Ignition, Gas Starter, Exhaust Ring |
Induction System
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| Carburettor and Air Intake; Induction Elbow; Float; Pressure Balance Flange | |||
Three Bristol Claudel Carburettors supplied air/fuel mixture to the induction elbow, which was bolted on the lowest point of the induction chamber cover circumference. This carburettor was a modified form of the Claudel Hobson H.C. 8; only the modification details are covered here.
The Float Chamber employed a crescent-shaped cork float treated with several coats of Cellon dope to render it benzol- or petrol-proof. (Cellon Ltd. was a company that produced the cellulose acetate lacquer used to coat aircraft fabric) To stiffen the float, a semi-circular brass plate was fixed to its top by wood screws. A toggle lever mounted on the float upper surface was secured by three long rivets that passed through the cork to a washer plate on the lower surface.
The Diffuser Tube was modified by placing the row of holes usually located around its base at the top; another row was drilled half-way down the length. This had the effect of reducing the petrol head demanded during acceleration and of admitting considerably more air, preventing the excessively rich mixture that previously occurred with the H.C. 8 carburettor when the throttle was opened quickly.
The Pressure Balance Flange was fitted between the carburettor base and the induction elbow. This flange surrounded the air intake and fitted over the four studs by which it was clamped between the carburettor body and the intake flange. The pressure balance flange helped to obtain more constant pressure balance system conditions and also provided a housing for an easily-detachable air jet through which air entering the pressure-balance pipe passed. The calibrated jet passed a constant air quantity into the pressure balance duct. The original H.C.8 had a threaded hole receiving the union that carried the pressure balance pipe that entered the carburettor body in the needle chamber that protruded from the float chamber side. Normally the pipe from this union was either carried down to the mouth of the intake pipe or into the intake pipe immediately below the choke tube entrance. Of the two, the latter method was more suitable for the Jupiter IV, where a common air intake was standard, but it was found that the pipe frequently became bent or otherwise misaligned in the induction pipe, causing uncertain pressure balance system operation. The pressure balance flange was an aluminium casting approximately 3/8" thick. A radial duct in the flange terminated in a short integral downwardly-disposed nozzle, which was located at the desired point in the air intake. The other end of the duct registered with the pressure balance hole in the main casting. A hollow threaded plug, which was a clearance fit in the ring, entered the threaded portion of the pressure balance duct in the main carburettor casting and carried in its upper end a detachable brass air jet. Four radially drilled holes in the plug below the thread corresponded with an annular recess formed around the inside of the large hole in the ring, and constituted an annular union, allowing the passage of air from the nozzle to the main duct leading up to the altitude control cock. Removal of the plug gave easy access to the air jet, and enabled a much finer pressure balance inlet adjustment.
The exhaust-heated Induction Elbow consisted of three annular chambers surrounding a steel pipe through which some of the exhaust gases were bypassed. The three chambers were a one-piece aluminium casting divided from each other by partitions. A central core ran through the whole, and was reamed to receive the steel pipe, which was shrunk into it. The elbow ends were formed to receive steel extension pipes that extended outside the fuselage to join the main exhaust pipes. A self-aligning spherical joint formed the junction between the extension pipe and the manifold. An aluminium guard pipe surrounded the pipe to protect it from the heat, and this was located axially by collars on the flanges riveted to the extension pipe ends.
Three short branches led from the induction elbow to three apertures in the circular induction chamber cover. The branches were appropriately marked with the cylinder numbers that each carburettor supplied. Facing the rear of the engine, the left-hand unit supplied cylinders 2, 5, and 8; the center, 3, 9, and 6; and the right-hand unit, 1, 4 and 7. The mixture from each carburettor was isolated from the remaining ones by a three-start spiral housed within the induction chamber. The spiral carried the mixture in each of its three channels to three equally spaced cylinders. This arrangement ensured an equal mixture distribution to all cylinders, a more homogeneous mixture, and provided for more even running if one carburettor failed. On entering the induction chamber the mixture was deflected in one direction around the spiral three baffles riveted to the spiral on the left of each aperture. This not only assisted in equal distribution to all cylinders, but by maintaining the gas flow in one direction a more even and continuous induction was obtained. This baffle was shaped to fit into the spiral's cross section except for a 1/8" clearance between the baffle and cover.
The Induction Pipes were of light-gauge aluminium tubing and were held between the inlet branch pipe at the cylinder head and a spigot at the crankcase The attachment at each pipe end consisted of a loose flange recessed to receive an India rubber composition gland that encircled the pipe. When the flange's four nuts were tightened the rubber gland was compressed against the pipe and formed an airtight joint, while at the same time allowing for slight movement due to thermal expansion. A small steel bonding clip on one bolt at each induction pipe end established an earth connection, preventing the pipe from being insulated at each end by the rubber rings. Two branches at the pipe outer end served the two inlet ports in each cylinder; two studs secured each branch to the cylinder port.
The Common Air Intake was a cylindrical light aluminium casting with three large flanged apertures on its upper surface, by which it was bolted to the carburettor bases. Immediately below and opposite to these were three smaller threaded apertures normally closed by threaded plugs that allowed access to the diffuser caps and diffusers by means of a long box spanner. The center plug was fitted with a union for a drain pipe that carried away accumulated petrol, and the casting was formed with a gutter on each side by which the petrol was drained. Flanges on the casting attached extension pipes leading outside the fuselage. Three priming jets were fitted in the induction chamber top. Each of these entered one of the three spiral starts. The joints between the carburettors and manifolds and the manifolds and induction chamber were made with thin vulcanized fiber washers. Joints between the induction pipes and cylinders used paper washers.
Three throttle levers were connected by short adjustable link rods and bellcranks to a transverse rocker shaft, which was carried in two brackets bolted to the induction manifold rear. The three altitude control cocks were similarly connected to another rocker shaft situated behind the first, while a lever at the end of each shaft connected the controls to the cockpit. Short interconnected levers on each rocker shaft automatically returned the altitude control to the closed position when the throttle was shut.
Lubrication System
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| Oil Pump Unit Disassembled, Sectioned and Plan Views | Lubrication Diagram | ||
The lubrication system provided oil under pressure to the cam gear; master rod big end, magneto drives and oil pump drives. Indirect or splash oil lubricated the pistons, connecting rod small ends, articulated rods wrist pins, main crankshaft bearings, thrust bearing, tappets, cam faces, and revolution indicator drive. The valves received a certain amount of lubrication by oil that passed the pistons. The rocker arm bearings were oiled by hand at intervals.
All oil that drained from the engine interior was ultimately collected in the small sump at the crankcase bottom and was withdrawn by a scavenge (scavenger) pump, which passed it back to the aircraft oil tank. Gravity supplied oil from the oil tank to the pressure pump. In order to ensure a dry sump, scavenge pump capacity was approximately one-third greater than the pressure pump.
The pump unit comprised an aluminium casing containing the pressure and scavenge pumps, which were housed one above the other. The pump unit was bolted to a faced surface on the rear crankcase cover exterior and secured by six studs and nuts. The tachometer and gas starter distributor drives were also embodied in the pump unit. The two upper pump unit sections together formed a chamber for the driving bevel, the housing for the pump driving spindle upper bearing, the revolution indicator drive housing, and the gas starter distributor mounting pad. The next and largest pump unit section constituted the actual pump body and housed the pressure and scavenge gears. It contained the passages by which oil entered and left the pumps and housed the relief valve. The last and lowest pump unit section was a cover plate that closed the assembly bottom.
The pump body was partitioned horizontally into two compartments. The larger upper compartment housed the scavenge pump and the lower the pressure pump. The driving gears of each gear pair were made of steel while the driven gears were of bronze. The scavenge pump drive was formed with an extended shank that passed through the partition dividing the two pumps, and was coupled to the pressure pump drive by a tongue-and-slot joint. The driven gear journals of both pumps were formed by a hollow steel spindle that was blanked off in the middle. It was pressed into a reamed hole in the partition where it was secured by a cotter pin, and carried the scavenge pump pinion on its upper portion and the pressure pinion on its lower.
A bevel at the crankshaft rear end drove a bevel integral with the pump driving spindle upper end; the spindle ran in a duralumin bush housed in the upper casting. The spindle upper part above the driving bevel carried a sleeve machined with a worm for the revolution indicator drive; at the sleeve top were dogs that engaged the gas starter distributor spindle. The bevel and the sleeve were both secured by a nut at the spindle's upper end. The pressure pump drive short spindles were carried in a phosphor bronze bush in the partition and in an unbushed hole in the aluminium cover plate.
The passages through which oil entered and left exited formed in extrusions that surround the pumps. The passages registered with other rear cover passages that completed the circuit to the crankshaft and filters. The two casing upper sections (i.e., the chamber for the driving bevels) were held together by three bolts and nuts, which had suitable lugs provided for them around the joint. The lower casting bottom surface was faced and formed the scavenge pump pinion chamber top. Five long studs and one loose bolt extend downwards from it and passed through reamed holes in the pump body to the cover plate, beneath where they were secured by nuts.
Two horizontal cylindrical filter chambers, one on each side of the pump unit, extended rearwards from the rear cover and each contained a removable gauze filter. The chamber ends were closed by screw caps that give access to the filters. The left hand chamber was the scavenge filter and carried a union at its end for a short extension pipe that connected the pipeline from the sump. It also had a union for the attachment of an oil thermometer. The right hand or pressure filter chamber had a large union formed at its end for connection to the supply pipe from the tank. The relief valve, fitted in an extension of the pressure pump inlet passage, was the usual spring-loaded type and was adjustable by a screw that varied the spring pressure. An aluminium cap, sealed by a wire to prevent unauthorized adjustment, covered the screw.
Pressure Circuit. Oil from the tank entered the right filter and passed through a passage formed in the rear cover, across to the lower left pump body passage. A duct in the side of this passage led to the pressure pump inlet, and oil passing through the pump was delivered into the lower right pump body passage, then to the crankshaft through a passage in the rear cover. Both the inlet and outlet passages were at the pump rear but both communicated with the relief valve, which opened then the oil pressure became too high and dumped surplus oil to the pump inlet. A small branch in the pressure passage was fitted with a union for connection to a pressure gauge.
Scavenge Circuit. Oil was withdrawn from the sump by a pipe connecting it to the union on the left or scavenge filter chamber. A duct in the filter chamber wall led into the upper left passage in the pump body, which was the scavenge pump inlet passage. From the scavenge pump delivery side oil flowed into the upper right pump body passage to a screw union to a pipe running to the aircraft oil tank.
Circulation in the Engine. The pressure pump rear cover passage opened into a white-metal bearing that carried the magneto bevel sleeve. A pressure recess was formed around the bearing interior to receive the oil, which then flowed into the crankshaft via four slots in the sleeve and holes in the crankshaft that the sleeve surrounded. The sleeve was a loose fit on the crankshaft and allowed some oil to escape and lubricates the bevels. Two holes in the rear cover bypassed a certain quantity of oil to the recess formed between the magneto spindle bushes; the oil from there drained back from the bottom of the spigots surrounding the vernier coupling and back to the crankcase through two exterior copper pipes.
Oil entering the crankshaft filled its rear end and flowed through drilled passages in the rear crank web to the hollow crankpin. Two holes in the crankpin, joined by a flat, provided lubrication for the master rod big end bearing. Oil flow continued down a passage in the front web and fillrf the shaft front end. The only outlet here was a hole drilled through the web on the side opposite the crank throw and close to the shaft. The hole opened on to an oil way cut along the shaft to a point beyond the cam sleeve. The components stacked on this length of the shaft (i.e, the distance or radius washer, the roller bearing inner ring, the cam sleeve and eccentric) formed a cover to the oil way and converted it into a reservoir. The cam sleeve was lubricated by oil from this reservoir, which reached it through a groove and holes in the crankshaft sleeve; these supplied the recess between the two halves of the cam sleeve bushes from where the oil was distributed over the bush surfaces and the spiral grooves cut in them. Part of the oil thrown from here lubricated the intermediate main crankshaft bearing, which was therefore oiled by splash. The compound pinion received its oil supply through a radial drilling in the eccentric that communicated with the shaft oil way. The drilling terminated at its upper end in a shelving groove cut along the eccentric's length. The cast iron pinion sleeve was grooved and freely drilled to facilitate the passage of oil to the pinion surface. Oil thrown out here, combined with that from the cam sleeve, lubricated the remainder of the gear, and also the thrust and front main crankshaft bearings. The thrust nut outer surface was machined with an oil-retaining spiral that served to prevent oil from working out at the crankshaft front end. Surplus oil from the thrust and front main bearings drained back from the thrust cover to the front cover by a duct and a short pipe located at the housing low point. From the front cover oil passed into the cam chamber by two holes in the fixed internal gear lower part. The cam chamber was drained by an external corrugated pipe that led into the sump front. The articulated rod ends, gudgeon pins, pistons, intermediate and rear roller bearings were lubricated by oil thrown from the master rod big end. Oil draining into the rear cover flowed to the crankcase through a hole drilled at the crankcase rear wall lowest point, where it combined with drain oil from the connecting rods, cylinders, etc., into the sump through its hollow limbs. The hole in the rear wall was shielded by a small aluminium louver to prevent the oil thrown from the revolving crank web from obstructing the passage of drained oil.
Oil Cooling. The Jupiter Series IV engine circulated oil at 55 – 65 gph, which assisted in dissipating the heat generated in, or transmitted to, the engine interior parts. However, only a normal oil supply was carried on the aircraft and no advantage accrued from this circulation unless the oil temperature leaving the sump (which in some installations was been found to be as high as 100°C) was reduced to the normal temperature (60°C) before it again entered the engine. An oil cooler was thus a necessary lubrication system component and was installed in the scavenge pipeline between the scavenge pump and oil tank in a position where it was exposed to the airscrew slipstream.
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