Patent GB-625889-A (GB)
Title: Density responsive control device
IPC: F02D9/00
Filed: 1945-05-03
Abstract: 625,889. Fluid-pressure servomotors. BENDIX AVIATION CORPORATION. May 3, 1945. Convention date, March 6, 1944. [Class 135] [Also in Group XXVII] In a control device having an expanding and contracting bellows of resilient material partially evacuated, which is contracted by external pressure against the internal and expansive action of a spring, the expansion and contractions of the bellows with variations of external pressure operating a valve which varies the rate of flow of fluid through a conduit, the bellows is evacuated to such a degree that the forces due to external pressure, to internal pressure and to the spring, maintain the bellows in a state of compression throughout the major part of its stroke. As shown, the capsule comprises a resilient bellows 10 sealed to a sleeve 11 fixed to a housing 22. At the free end the bellows is connected to a member 12 having a hollow cylindrical portion 13 extending within the bellows. The cylindrical portion, in conjunction with a piston 16a, fixed to a shoulder 15 of the sleeve 11, forms a guide for the free end of the bellows and also acts as a dashpot for damping out vibrations of the bellows. Fixed within the member 12 is a valve 26 which is held in place by a spring ring 29 and a plunger 30 and spring 30a. A spring 14 within the bellows resists collapse thereof under atmospheric pressure when the interior is evacuated. The interior of the bellows is filled with an inert gas such as nitrogen and the pressure reduced below atmospheric to a pressure such as that of the air at an altitude of 20,000 feet. For damping the bellows contains a fluid such as high grade petroleum oil and holes 21 are provided in the portion 13 through which the fluid can be drawn or expelled during operation of the capsule. Filling takes place through holes 18, 18a. Fins 20 are provided at the outside of sleeve 11 to facilitate heat transfer. A suitable material for the bellows is beryllium copper. The advantage of reducing the pressure to an intermediate value is that if the starting pressure were atmospheric pressure, at altitudes the difference in pressure between the interior and exterior of the bellows would cause over-compensation for temperature changes which is undesirable. By reducing the interior pressure at ground level the pressures are made nearer at altitude with consequent less error. At ground level undercompensation for temperature occurs, which is preferable to over-compensation. The application of the capsule to a carburetter is shown in Fig. 1. In this arrangement the valve 26 of the capsule controls the pressure in a diaphragm chamber 40 which together with chambers 41, 42, 43 and diaphragms 44, 45 enables the position of a fuel valve (not shown) to be controlled. The valve 26 varies the opening of ports 32 arranged in a passage from pipes 51 in the air inlet to the chamber 40. Chamber 41 is connected to the throat of a venturi 39 and the two chambers are inter-connected by a small passage 56. Chambers 42 and 43 are operated on respectively by metered and. unmetered fuel pressures. The housing surrounding the bellows capsule 10 is connected by holes 36 to a passage which opens into a venturi 38 and further holes 36a in the housing are connected to the air scoop. As the air pressure in the housing drops, the bellows expands and restricts ports 32, thus reducing the pressure in the chamber 40, since the leak passage 56 will have a greater effect, and reducing the fuel supply. A groove 33 is provided in the valve to enable flow through the ports 32 in the event of a failure of the bellows. In this event, the spring 14 moves the valve so that the groove provides a passage of capacity similar to that provided at low altitudes. Curves are given in the Specification showing the relationship between travel of the bellows and air density up to a density corresponding to an altitude of approximately 45,000 feet.