Patent GB-658780-A (GB)
Title: Improvements relating to fuel systems for gas-turbine engines
Inventor: DAVIES KENNETH ROY
IPC: F02C9/16
Filed: 1948-10-29
Abstract: 658,780. Gas turbine plant. ROLLS-ROYCE, Ltd. Oct. 29, 1948, No. 28191. [Class 110(iii)] [Also in Group XXIX] A fuel supply system for a gas turbine engine having a speed governor with a selectively variable speed datum comprises a member movable by the governor means to control the fuel flow, which member may be a throttle valve, and means, sensitive to a selected value of an operating variable which varies during acceleration which controls a stop limiting the maximum opening movement of the member to a value less than that set by the normal control at least until the selected valve of the operating variable is passed through during acceleration and to reimpose the limit when the selected value is passed through during deceleration. The operating variable may be the speed. of the engine, the pressure difference between the intake and discharge of the compressor of the engine, the compressor delivery pressure, the compression ratio or the actual fuel flow to the engine. This invention is described as applied to a fuel system of the type described in Specification 649,287. The fuel system shown in Fig. 1 differs from that described with reference to Fig. 1 of the Specification referred to in the following respects. Firstly the controls regulating the movements of the control valve 142 are interchanged, i.e. the axial movement of the valve 142 is controlled by the second hydraulic governor and the rotational movement by the altitude sensitive device 158. Secondly, the diaphragm 143 is responsive to the pressure drop across a variable restriction 172 under the control of the throttle instead of using the throttle to vary the spring loading on the diaphragm. Thirdly, an emergency changeover valve 205 and manual fuel throttle 206 is provided for use in the event of failure of the hydraulic governor control. Fourthly, a bleed 209 is provided to ensure a minimum fuel flow for idling. The present invention consists in the embodiment in the fuel system of a stop 210, which limits the movement of the diaphragm 143 in the opening direction of the control valve 142, during acceleration and is described with reference to Fig. 2A. The stop 210 which co-operates with an abutment 211 attached to the diaphragm 143 is mounted on a rod 212 which is continuous through and connected to two further diaphragms 213 214. The diaphragm 214 is loaded on one side by the pressure at the upstream side of a fixed orifice 125 and on the other side by a spring 219 and the pressure on the downstream side of the orifice. Fuel is passed through the fixed orifice 125 by a constant volumetric capacity pump 122 driven by the engine so that the pressure drop across the orifice is proportional to the square of the engine speed. The movements of the diaphragm 214 are limited by manually adjustable stops 215, 216. Neglecting, for the purposes of explanation, the interaction of the hydraulic loadings on the diaphragms 214, 143, it will be seen that at low engine speeds when the pressure drop is small, the diaphragm 214 is urged against the left hand stop 215 so that the stop 210 will be in its extreme left hand position, thus restricting the maximum area of the port 166 in the slide valve 142. At a preselected engine rotational speed, the pressure drop across the restriction 125 loads the diaphragm 214 to overcome the loading of the spring 219 moving the diaphragm against the right hand stop 216. This allows a greater maximum opening of the valve 142 as compared with maximum opening below the preselected speed. When the stop 210 is operative against the abutment 211 the loading on the diaphragm 143 must be balanced in order to prevent this loading affecting the position of the stop. For this purpose, a third diaphragm 213 of equal area to the diaphragm 143 is mounted on the rod 212. This diaphragm is loaded by a spring 222 equal in strength to spring 187 and by the pressure loading which acts on diaphragm 143. Both these spring and pressure loadings operate in the opposite direction to those acting on the diaphragm 143. The arrangement shown in Fig. 2A may be simplified by making the diaphragm 214 equal in area to diaphragm 213. In this case the pressure loadings in the chambers 218, 220 cancel out and therefore may be omitted and the two diaphragms 213, 214 combined into single diaphragm loaded on one side by the pressure upstream of the orifice 125 and on the other side by two springs 219. 222 and the pressure downstream of the variable restriction 172. This simplified arrangement, is the arrangement shown embodied in Fig. 1. Modified arrangements in which the variable restriction 172 is arranged before the fixed restriction 125 in the discharge conduit from the pump 122 are also described. In another arrangement, Fig. 5, the variable restriction 172 is replaced by a fixed restriction 231 and the speed at which the hydraulic governor controls is selected by varying the loading on the diaphragm 143 by the spring 187 by means of a cam 233 acting on a slidable sleeve 232. To maintain the balance of the spring loads on the diaphragms 143, 214, this cam also acts on a sleeve 332 to adjust the load of the spring 222. The movement of the diaphragm 214 to the right is limited in this arrangement by an abutment 216A co-operating with the stop 215 The arrangement shown in Fig. 2A may be used to position the stop 210 when a predetermined compressor pressure rise is reached by connecting the chamber 217 to the compressor discharge and chamber 218 to the compressor suction. Other arrangements in which the stop is positioned when a predetermined compression ratio or discharge pressure of the compressor of the engine or a given fuel flow to the engine is reached are also described. The application of the invention to a fuel system in which the main fuel pump is a constant volumetric capacity pump which delivers the fuel past the fixed restriction 125, the variable restriction 172 and a relief valve to the supply conduit 117 is also described. The fuel system may also include a temperature control device. A thermocouple or resistance thermometer located in the turbine exhaust duct may be used to control an auxiliary variable restriction in series with the restriction 172. When applied to a fuel system of a gas turbine plant driving a variable external load the thermocouple may be used to control the load. In a further modification the fixed restriction 125 is made variable and the area of the restriction is varied in accordance with the ambient air temperature. In all the above arrangements the diaphragms may be replaced by pistons tight in cylinders or by a bellows. According to the Provisional Specification the increase of fuel, when the preselected valve of the engine operating variable is reached, may be permitted by opening a throttle in the fuel .supply to the engine. It is also stated that the adjustable stop may be used to vary the datum with respect to which the altitude sensitive capsule operates.