1、AEROSPACE AIR1466 . Issued 9-1-78 I N FO R MATI O N R EP O RT Revis4 iociety of Automotive Engineers, IN. IOQ COULWHIWEALTH ORIVE. WLR(NN0ALE. H. 1- HYDRAULIC ENERGY LIMITED ENGINE STARTING SYSTEMS 1. PUIIPOSE AND SCOPE This report presents information on energy-limited hydraulic starting systems fo
2、r gas turbine engines. hydraulic start system consists of a gas charged hydraulic accumu- lator to provide a fixed amount of stored energy and a positive displacement mechanical conversion device to do work. The informa- tion presented herein is intended to familiarize the aerospace industry with th
3、e design, and system performance characteristics of energy-limited hydraulic starting systems. An energy limited 2 . INTRODUCTION Self-sufficient starting of a gas turbine engine at cold ambient temperatures is- a very difficult design requirement. The batteries of electric start systems soaked at l
4、ow ambient temperatures do not supply sufficient current for engine starting. This has resulted in the application of energy-limited hydraulic starting systems where self sufficient starting at extreme cold ambient temperatures is mandatory . An energy-limited hydraulic start system was first employ
5、ed on the Boeing Vertol CH47 Chinook helicopter. Today, energy-limited hydraulic start systems are emp1oye.d with numerous applications, including the Boeing Vertol CH46, Sikorsky CH53, Lockheed C-141 and CSA, Rockwell International B-1, the McDonnell Douglas F-15, and the General Dynamics F-16. Tab
6、le I summarizes some of the system design parameters of these systems. Energy-limited hydraulic start systems are practical for starting small gas turbine engines such as auxiliary power units, jet fuel starters, and small propulsion and industrial engines. Weight and envelope restrictions and/or th
7、e requirement for hand charging the accumulator limit accumulator size from 100 to 1000 ine3. the application of energy-limited hydraulic start systems is restricted to small gas turbine engines, except where a large accumulator already exists in the aircraft hydraulic system. Therefore, 1978 ights
8、b Society Automotive reserve8. Printed in U.S.A. - - - SAE AIR*l,4bb 78 8357340 0004971 B m O O (v O O O M * w O b a I3 N W I E-i k rd rl O cn W * 8 s) 2 rl O ci k *li a, O a O O N O O O M u) a O 5 I3 N W I B k Id ri O VI fi * 8 3 rl O u k tn E: -4 a, O a O m cv O O O di ir) rn O Ei I3 N W I E-i k r
9、d ri O VI M m L, % 3 ?i k O VI O O m O O O M is w M O . 5 3rl 28 kl am OW mrl -4 I3 40 A rn u a a, a, A O I4 xu Page 2 m rl N O O N xrl N O O O M O m O 3 curl are always fixed displacement; whereas starter/pumps are either fixe or variable displacement. Both fixed and variable dis- placement starter
10、s in an energy-limited start system produce a similar torque characteristic which decreases while the speed increases and accumulator pressure decays. Starters A starter or starter/pump of sufficiently large displacement to produce the required torque must be selected. The torque output of a positiv
11、e displacement starter is directly pro- portional to displacement and differential pressure. The starter requires a mechanism to disengage the starter from the gas turbine after starter cutout so that the engine will not motor the starter. Continuous duty starter/pumps do not require a disengaging m
12、echanism. Page 5 I I- SAE AIR*KL4bb 78 8357340 OOOL1975 5 Hand Pump - A hand pump can be used to provide emergency filling of the accumulator and for topping-off the accumulator before cold starts or after a long storage period. Check Valve - Standard aircraft check valves are used to provide a mean
13、s of filling the accumulator from either the aircraft system or from the hand pump. Thermal Relief Valve - The pressurized accumulator may be subjected to high temperatures during storage. over-pressurizin.g the accumulator, a relief valve set to open at a maximum safe accumulator pressure is used b
14、etween the accumulator and the aircraft hydraulic reservoir. To prevent 3.2 System Operation Before an engine start can be initiated, the accumulator must first be precharged with gas and then charged with hydraulic fluid. The precharge is accomplished by pressurizing the accumulator with air or nit
15、rogen. Allowance for the increase in temperature during the precharging process must be made. It is standard practice to “top off“ the gas charge after waiting for the equilibrium temperature to be attained. If the system is not “topped off“, allowance for the change in temperature must be made, sin
16、ce, if the accumulator is not used immediately, the gas temperature will drop due to heat transfer to the surroundings and the pressure will be reduced. In most systems the accumulator is automatically charged by the aircraft high pressure hydraulic system after evry engine start. A hand pump is use
17、d as a backup to provide emergency charging of the accumulator in the event of an unsuccessful engine start attempt. In other systems where high pressure hydraulic pressure is not available, a hand pump or ground support equipment is required to pressurize the system. The charge.pressure or the star
18、t system pressure for most systems is 3000 to 4000 psi or the same pressure as the aircraft hydraulic system pressure. The precharge pressure is approximately one-half of the charge pressure. There is an optimum precharge pressure for each application and the required engine starting design point am
19、bient conditions. Page 6 - Operation of the system is initiated by opening the start valve which releases the pressurized oil to the hydraulic motor inlet. The hydraulic motor develops output torque due to the pressure differential between the inlet and outlet ports. motor output torque exceeds the
20、engine torque resistance, the hydraulic motor begins to accelerate the engine. accelerates to starter cutout speed, the pressurized hydraulic oil stored in the accumulator is depleted at an increasing rate. of the gas expanding against the accumulator piston results in a reduction of the gas and oil
21、 pressure at the motor inlet. At khe same time, the increasing rate of hydraulic oil flow results in increasing pressure losses through the fluid lines. The combination of lower supply pressure and increased pressure losses reduces the pressure differential across the hydraulic motor and starter out
22、put power. The hydraulic starter assist terminates when the start valve is closed on a signal from the engine or when the accumulator oil is completely discharged. In either case the engine must have reached self-sustaining speed when starter assist is terminated. Following the start cycle, the accu
23、mulator(s) are recharged automatically from the aircraft hydraulic system or manually by a hand pump. When the hydraulic As the engine The depletion of oil from the accumulator due to the pressure 4. DESIGPJ COIJSIDERATIONS The sizing of an energy-limited hydraulic start system requires a knowledge
24、of a number of design considerations. For a minimum size system, consideration must be given to the characteristics of the various system components, including the engine being started. A knowledge of the thermodynamic properties of real gases is also required. At high pressure the properties of the
25、 accumulator pressurizing gas can not be based on the perfect gas law. 4 1 Component Characteristics The performance characteristics of the starter and the gas turbine starting characteristics must be defined in order to design the starting system. The starter performance can be defined based on tor
26、que efficiency as a function of speed and pressure, and volumetric efficiency as a function of pressure. Performance based on starter calibration test data rather than estimated data would be desirable. The engine starting characteristics must be considered as an integral part of the system in order
27、 to achieve an optimum design. The startiny characteristics of the small turbine engine like the large gas turbine are defined by a toryue/speed curve and the polar moment of inertia of the engine. Paye ? SAE AIRmLLlbb 78 M 8357340 OOOq477 7 M Some factors influencing the engine torque/speed curve a
28、re ambient air temperature and pressure, fuel scheduling, engine lubri- cation oil viscosity, acceleration rate, and gas turbine driven accessories. The low speed resisting torque of gas turbines is a combination of bearing and gear drag and of aerodynamic work asso- ciated with moving air through t
29、he engine during starting. In small gas turbines, the bearing and gear effects are a larger percentage of the total net resisting torque than in large engines due to the scaling effects of the mechanical elements. Because of this, the influence of lubrication oil viscosity is the most important fact
30、or in the low speed drag torque of small gas turbine engines. For rapid starting at minus 65Ft aerodynamic work does not become signifi- cant until after the engine exceeds approximately 25 percent speed. The heat added to oil films lubricating the gears and bearings is a significant factor. The amo
31、unt of heat addition is a function of the number of revolutions turned. The faster the start or accel- eration rate, the fewer are the number of starter revolutions, and the higher is the resulting gas turbine engine net resisting torque. This accounts for some past problems of extrapolating data be
32、tween one engine gearbox to another and between one starting system to another. The polar moment of inertia of the engine rotor is fixed by the mechanical and aerodynamic design of the engine rotating group. The rotating group is generally designed for minimum weight which also results in the lowest
33、 moment of inertia. The engine torque/speed curve is more amenable to modifica- tion than is the rotating group. The initial viscous resistance can be reduced, especially at low temperatures, with an engine de-oiling system. This system allows the engine oil system to be pumped dry and excess oil ce
34、ntrifuged from the bearings during roll-down follow- ing engine shut down. This system could also be used during the early stages of the start cycle to reduce the engine oil pump torque. Modification of the self-assist portion of the engine torque/ speed curve can be accomplished by designing the co
35、ntrol system to maintain the maximum safe turbine inlet temperature during the start cycle for all ambient temperatures. The penalties of this approach are increased control complexity and cost. The engine ignition point has a strong bearing on the design of the start system. The engine must be desi
36、gned to accomplish the necessary functions of fuel manifold filling, combustor spray estab- lishment, and spark generation in a timely manner. The minimum weight gas turbine starting system is one which provides the minimum start time. Care must be taken, however, that the system does not accelerate
37、 the engine through the engine lightoff window. Page 8 4.2 Thermodynamic Processes At the temperature and pressures encountered on hydraulic start system accumulators, nitrogen and air deviate considerably from ideal gas behavior. The significance of the real gas effects is shown on Figures 2 and 3.
38、 The ratio of specific heats (k) for air and nitro- gen at zero psi is 1.4. However, for an accumulator charged to 3000 psi, the real specific heat ratio is greater than 2.5 at initial temperatures below OF. During the expansion process the variation of the ratio of specific heats is too great to as
39、sume an average value. Therefore, a rigorous approach to the analysis of the expansion process requires the use of a temperature-entropy diagram or a similar phase diagram. In most applications, the gas expansion in the accumulator is very rapid and the heat transfer is very low. Therefore, the expa
40、nsion is assumed to be isentropic. However, a gas that expands non-isentropically produces more work than a gas expanding isentro- pically. Therefore, if the heat transferred to the gas during expansion is significant, the assumption of an isentropic process will result in an oversized accumulator.
41、5. SYSTEM ANALYSIS The analysis and design of an energy limited hydraulic start system is best accomplished by programming a model of the system for use with a digital computer. upon having accurate component characteristics and thermodynamic data for inclusion in the computer program model. Compone
42、nt data required are summarized as follows. Sizing of an accumulator is dependent O Accumulator - Gas phase diagram for expansion process and real gas equations. Starter - Torque efficiency as a function of spee and pressure, and volumetric efficiency as a function of pressure. O Engine - Polar mome
43、nt of inertia andengine starting characteristics curves. O Working Fluid - Viscosity as a function of temperature and pressure. O Fluid Lines - Length, diameter and friction factor as a function of Reynolds number. Page 9 -_ .- Ip - SAE AIR*/4bb 78 W 8357340 OOOLt777 2 W I ni u 4 O Co * . ni ,rl rl
44、rl rl rl O LW JtYZIl 3IdI39dS 33VXWV . PAGE 10 The computer program model calculates the starter torque and acceleration rate of the system between small speed increments. The program iterates at the end of each increment until the elapsed time from the previous speed point, oil consumed, starter in
45、let pressure, accumulator pressure and temperature, and the calculated starter torque are balanced before repeating the process for the subsequent speed increment. The use of a computer model allows the designer of the energy- limited hydraulic start system to rapidly size a system. variables such a
46、s accumulator volume, accumulator charge and pre- charge pressure, starter displacement, supply line length and diameter, reservoir backpressure, and ambient temperature can be varied to define the best system to satisfy engine starting requirements for the various ambient design points. Many The pr
47、imary variable influencing the start system weight is the accumulator charge pressure. This pressure determines the starter displacement or size required to develop sufficient torque to accel- erate the engine. The higher the charge pressure the smaller and lighter the starter required, since the di
48、splacement required to produce a given torque is inversely proportional to the pressure. The reduction in starter displacement reduces the accumulator volume since the oil usage is less. level might be expected to result in an increase in accumulator specific weight, but this can be offset by the us
49、e of high-strength materials and/or a composite-type structure. The increase-in charge pressure The length of the fluid lines between the accumulator, starter, and reservoir are generally dictated by the installation and, there- fore, the esigner has no control over the line lengths. However, the line diameters can be optimized by a tradeoff of line weight and reduced starter performance due to the fluid line pressure losses. At cold ambient temperatures, the fluid line pressure losses are very high due to high fluid viscosity. Special consideration must be given to the high
