In aircraft engines, the explosion pressures are maintained within safe limits at low altitudes by means of a stop device which is automatically adjusted according to the altitude by an aneroid device operating through a relay. The control lever a moves in a slotted member f2 adjustable by a rod f1 connected to a relay e controlled by an aneroid device d. In a modification, the control lever simultaneously adjusts the throttle and the ignition timing, only the latter being subject to the aneroid control. Fig. 3 shows an aneroid device h connected by a lever h2 to the relay comprising a valve j1 controlling the action of the lubricating‐oil pressure on a piston k1 connected to the ignition control by a lever f and rod f1. When the aneroid chamber h expands, the valve j1 is raised to allow oil admitted at l to pass by way of passages j2, k4 to the underside of the piston. When the piston is raised sufficiently, the oil pressure passes through a port k5 to the upper side of the piston, which is then lowered again by a spring m1. Instead of oil pressure, the engine suction or the pressure in the water‐cooling system may be used to actuate the relay. Specification 247,304, [Class 7 (iv), Internal‐combustion engines, Igniting in], is referred to.
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