maintains a tolerable engine temperature. Additionally, engines can be liquid cooled if

Transcription

1 maintain a lower temperature in sensitive components in the near surroundings of the engine. In rotary engines, the circumferential or axial cooling that occurs naturally from the engine motion maintains a tolerable engine temperature. Additionally, engines can be liquid cooled if necessary. 6.0 HARDWARE AND SENSORS The hardware and sensors for this test stand were chosen based on the small to medium size UAV engines that will be tested. These engines will be mounted on the test stand and then integrated with all the necessary sensors. The small to medium size UAV engines are found on such UAVS as the Pioneer, Predator, and Hermes. These engines have an average power range from hp and an RPM range of 5,000 8,000, which are the limiting factors for this test stand and the sensors chosen. Designing the UAV ETS involves several different sensors to obtain the data for analysis. Multiple sensors were researched and the optimal sensors for the range of tested engines were picked for each aspect of this project. Each sensor must meet the requirements set for our project and must be able to withstand the strain to which they are subjected during the engine performance experiments. More detailed specification sheets for each of these instruments can be found in Appendix A. 6.1 Dynamometer To measure the performance of the various engines being tested, a water brake dynamometer will be used. A dynamometer is a machine that is used to measure the torque and rotational speed of an engine while applying a constant load to the engine. Once both torque and RPM are known the power produced by the engine can be calculated. A dynamometer is made Page 18

2 up of an absorption unit, a torque arm with a connected load cell, and a RPM measurement device. Since only the torque and the RPM can be measured the power must be calculated, using the following equation:. The dynamometer chosen must be able to handle small to mid size UAV engines. These engines have a power range of hp and an RPM range of 5,000 8,000, which were the limiting factors when picking a dynamometer. The Kahn Series dynamometer, as seen in Figure 5, was chosen and met all the requirements set by the parameters of the engines that will be tested. 27 FIGURE 5 - THE KAHN SERIES DYNAMOMETER 27 This dynamometer has a maximum power capacity of 450 hp, max speed of 14,000 RPM, max torque capacity of 250 ft. lb. and a dry weight of 50 lbs. The figure below, Figure 6, illustrates the dynamometer operating range, and that the average small to mid size UAV engine Page 19

3 that will be tested will fall in the appropriate operating range. The series dynamometer is highlighted with the blue line in this figurefigure 6. FIGURE 6 - DYNAMOMETER OPERATING RANGE 27 This dynamometer works by having a single perforated disc that rotates in the housing between the stators. Cool water then enters from the center and is accelerated by the rotating disc and thrown outwards. An annulus forms in the rotor chambers rotating at about half the speed of the disc, and produces enough centrifugal pressure to force the water out of the rotor chambers. Power is then absorbed and converted into heat by the water vortices generated in the rotor and stator holes. This produces a drag that induces a moment to the dynamometer housing creating an urge to turn. However, the housing is held constantt by the load cell that is mounted to the torque arm. The torque between the housing and the torque arm is measured via the load cell. The power absorbed is a functionn of the water level and the speed, with a max power absorption Page 20

4 occurring when the rotor chambers are completely filled. The following illustration, Figure 7, is the schematic of Kahn dynamometer. FIGURE 7 - SCHEMATIC OF DYNAMOMETER 27 The dynamometer uses a water brake to provide the constant load and to act as the power absorber. Using a water brake has its benefits and its flaws. Water brakes are good because of their high power capability, small package, light weight, and relatively low manufacturing cost as compared to other systems such as hydraulic brakes, electromagnetic brakes, or friction brakes. The flaws of the dynamometer are that it needs a large amount of water to keep it cool as well as taking a long time to reach a constant stabilized load. A water brake works by adding water until the engine is held at a steady RPM and then kept at that level by constantly draining Page 21

5 and refilling the system. This action allows for cooling needed due to the heat created by absorbing the horsepower. The water system is discussed in Section Remote Throttle In order to ensure operator safety and maintain complete variable control throughout the testing procedure, it is necessary to remotely throttle the engine. With a system installed in line with a dynamometer, throttle control can be applied based on the engine RPM, torque, or manifold pressure. Additionally, the transient effects due to a change in mode or power state can be limited through the controller. Through the use of multiple sets of proportional-integrator-differentiator (PID) controller parameters, the control unit can be adjusted to variations in engine test response and operating standards. Most control units ideally employ both serial computer control, where data is passed from the main unit back and forth to a stand-alone computer, and manual control, where the operator enters a new setpoint through a numeric keypad. A DTC-1 Digital Throttle Controller, produced by DyneSystems, Inc., was selected for integrated throttle control in the ETS. 28 This particular single-loop system is capable of controlling RPM, torque, manifold absolute pressure (MAP), or manifold vacuum using a servo actuator system and sixteen sets of PID closed-loop coefficient parameters. The unit can be serially connected to a personal computer using a RS232 port, for PID parameter entry for finetuning control, mode changes, ramp rate, instilling a setpoint, monitoring data, and switching between manual and automated modes or power on/off. The onboard software is written in Dyn- Loc Basic, a language similar to BASIC. For stand-alone operation, programs can be downloaded to from a control PC to the DTC-1 unit over a serial link at up to 19.2K baud. Page 22

6 FIGURE 8 - DYNESYSTEMS DTC-1 DIGITAL THROTTLE CONTROLLER 28 In addition, the DTC-1, Figure 8, features an 80-character display and a numeric keypad so that the operator can quickly enter setpoints, change modes, and modify other setting ranges. The upper and lower limits of operation and speed of the actuator are controllable from both the remote operator s station and the front panel. For the ramp rate, setpoints can be entered from.1% to 200% per second for each mode of control. Functions can operate on adjustable auxiliary control inputs with an analog reference input range of 0 to +10VDC or 0 to 10,000 units maximum. This analog feedback input can be sued for torque or RPM feedback. Digital torque position control can be manipulated up to 10,000 lb-ft, digital RPM control to 25,000 RPM. Manifold vacuum control ranges from 0 to 29 in-hg and MAP control from 0 to kpa. 28 The actuator unit is comprised of a DC servo motor, gearhead, and encoder assembly, which is totally enclosed in a waterproof housing. The throttle linkage is provided by a five inch lever arm with an adjustable rod assembly; therefore, the throttle stroke ranges from 1 to 4 inches according to the adjustment of the rod end position. Position is sensed by an optical encoder with a 0.005% FS resolution. In order to prevent motor damage, software stall detection and Page 23

7 shutdown procedures are in place. Furthermore, upon power failure or the issue of emergency stop, either manually by the operator or by the automated system, the throttle is returned to the closed position as a safety feature. 6.3 O2 Sensors- The oxygen sensor will be located in the exhaust pipe. However, the oxygen sensor module will be located outside the exhaust mounted to the ETS. As the engine is running the oxygen sensor will measure the oxygen content of the exhaust and determine the air-fuel ratio. Oxygen sensors determine if the air-fuel ratio exiting a gas-combustion engine is rich, with unburnt fuel vapor, or lean, with excess oxygen. The sensors work by measuring the difference in oxygen between the exhaust gas and ambient air. The sensors generate a voltage or change in resistance based upon the difference between the gas and air. For the test stand a Bosch wide band lambda sensor LSU 4.9 will be used in conjunction with an ETAS ES430 Lambda Module, Figure 9. 29,30 The LSU 4.9 is a planar ZrO 2 dual cell limiting current sensor with an integrated heater. The oxygen sensor can withstand the exhaust gas and pressure up to 4 bars with an operating exhaust gas temperature range up to 930 C. The sensor should be placed where the gas is the hottest. This will mean that the exhaust will be sufficiently far from the inlet to get the most accurate results. The oxygen sensor must be used in combination with a special LSU-IC such as the lambda module mentioned above. This module determines the air-fuel ratio, fuel-air ratio, and oxygen content of the exhaust. One of the advantages of this module is that it is easily configurable to support different fuels or combustion types, has an automatic detection of sensor or wiring failure, and also is capable of easy Page 24

8 integration into the DAQ system. The sampling rate of this device is approximately 0.5 samples/s to 2000 samples/s. More detailed information can be found in Appendix A.3. FIGURE 9 - LSU 4.9 O2 SENSOR AND LAMBDA MODULE 29, Thermocouples- The temperature will be measured at the heads, the block, and in the exhaust pipe using multiple thermocouples. Thermocouples work by using a pair of dissimilar metal wires joined at one end. This configuration creates a net thermoelectric voltage between the pair. That voltage will increase with temperature and will usually be in the range of 1 70 mv/ C. The temperature and the voltage obtained from the wires can be related by the equation:, (1) where the coefficients a n are given for n from zero to between five and nine. 31 The thermocouples that will be used for this test stand will be the Omega 5TC Ready- Made Insulated Thermocouple, as seen in Figure 10. The calibration that was chosen was K, due to the fact that it is the most common and offers a temperature range of -270 to 1372 C (-454 to 2501 C). This thermocouple is made of aluminum-chromium and has a standard accuracy of 2.2 C. 32 Page 25

9 FIGURE 10-5TC READY-MADE INSULATED THERMOCOUPLE Pressure Sensor There are two different pressures that will be measured during these tests, the ambient pressure and the manifold absolute pressure. The ambient pressure will be measured using a simple barometer or other simple measurement device that will be read and recorded at the time of the testing. The manifold absolute pressure will also be measured because it is a good indicator of the engine performance. The higher the manifold pressure, the more oxygen there is, and the more fuel can be added, which increases the amount of horsepower produced. The manifold absolute pressure will be measured using two MPX4115A Integrated Silicon Pressure Sensors, made by Motorola. 33 These transducers are monolithic, signal conditioned, silicon pressure sensors that provide an accurate, high level analog output signal that is proportional to applied pressure. They will be able to measure pressure from 15 to 115 kpa and have an accuracy of ±1.5 %V FSS. They have a fast response time at 1 ms, and a small warm up time of just 20 ms, which means that there will be a very small time after the engine is running before the sensor is measuring accurately. 33 Page 26

10 These sensors will also be helpful in determining the air density and calculating the engine's air mass flow rate, which, in turn, will be used to find the appropriate fuel flow. Knowing this information will help determine how the engine operates with different fuels, which is one of the main goal of this project. 6.6 Ambient Temperature and Humidity To compare the measurements made during test, the ambient characteristics of the environment, such as the pressure, the temperature, and the humidity must be known. Knowing these characteristics will allow for calculations to be made concerning the efficiency of the fuel being used. To measure the ambient temperature and humidity the Thermo-Hydrometer RH411, made by Omega, will be used. This device will measure and display the temperature and humidity at the time of testing. The Thermo-Hydrometer has an accuracy of 3% with a range of 2 98 % for the humidity and an accuracy of 0.5 C and range of -17 to 48 C for the temperature. This device will be useful for precise measurements at the time of testing Air Flow Meter An air flow meter measures the amount of air that is flowing through the intake pipe and through the engine. Many engines need this data so that it knows how much fuel to inject into the intake manifold. To measure air flow on this test stand, a redundant system will be used, utilizing both a hot-wire anemometer and a digital monometer which will be attached to a pipe which is sealed to the engine s air intake. A hot wire anemometer will be placed in the intake pipe to measure the velocity of the air flow into the engine from which the mass flow rate will be calculated as (2) Page 27

11 where A is the cross sectional area of the intake pipe. A hot-wire anemometer has a platinum wire that is heated about the incoming temperature, and then as the air flowing by cools the wire, is heated to keep a constant temperature. This change in current to keep the wire at a constant temperature is proportional to the air mass entering the engine. The anemometer that was chosen was the Extech Instruments Heavy Duty Hot Wire CFM Thermo-Anemometer. 35 This sensor will accurately measure the air flow, and velocity, and the temperature of the intake. This anemometer has a velocity range of 0.2 to 17 m/s, a temperature range of 0 to 50 C, and a flow range of 0 to 36,000 m 3 /min; which are all within the ranges required that this test stand. A redundant system will be used due to the fact the anemometer is a fragile instrument and if it breaks, there will be a need for another way to measure the air flow into the engine. To do this a digital manometer will be used to measure the pressure inside the intake pipe. Then the air velocity can be found using Bernoulli s equation: (3). Then after finding the air velocity, the mass air flow can be found using the Equation (2). The manometer that will be used for this project is the Love Series HM28 Handheld Digital manometer. 36 It has a high accuracy, 0.05% to 2%, selectable scales, can measure differential, gage, or absolute pressure, and can take 20 readings/sec when outputted to the RS-232, which has a adjustable baud rate of 1200, 2400, 4800, or 9600 baud. 6.8 Fuel Flow Meter Though the O 2 sensor mentioned above would give a speculative fuel/air ratio, a hydrocarbon fuel flow meter will be placed on the fuel line for more precise results. This fuel flow meter will allow the stoichiometric fuel/air ratio to be accurately measured. For this Page 28

12 measurement, a FloCat C-SF45 A001 series turbine flow sensor will be used, as seen in Figure A turbine flow sensor works by measuring the velocity of the fuel passing through proportional to the flow rate. Then a neutrally buoyant rotor spins with the liquid and the rotor movement is sensed when the notches in the rotor interrupt an infrared light beam. This particular sensor can measure the flow of a variety of hydrocarbon fuels, which will be useful when different fuels are experimented with. The flow sensor will allow for measurements of fuel from 0.3 to 30 GPH, has a working pressure of 200 psi, and a temperature range of -65 to 100 C. Some advantages that this meter has over other similar products is that it has a very low pressure drop, at less than 1 psi and has a very long life span of 10,000 hr. FIGURE 11 - HYDROCARBON FUEL FLOW METER DAQ System A large part of the project is taking the data from the sensors and hardware mentioned above, and sending them to a PC so that it can be analyzed. To do this a data acquisition system (DAQ) will be required to obtain and record the data. The DAQ system will provide signal conditioning, getting rid of erroneous data, as well as real time processing. The DAQ will then Page 29

13 send those signals to a computer where the data can be seen in a more user-friendly manner using software such as LabView. The DAQ system used for this engine test stand will be the National Instruments CompactDAQ system, shown in Figure This DAQ is an easy to use plug and play instrument that will provide fast, accurate measurements. It also has a modular design, so that in the future if there is a need for new tests or new equipment it can be added without having to buy a new DAQ system. For now the test stand will have 32 channels for the variety of inputs of the different sensors, which will be more than adequate for our purposes. For more detailed information on this DAQ system, see the specification sheet in Appendix A.10. LabView software will need to be created in the future work done on this test stand to properly examine the data and form conclusions on what is found. 38 FIGURE 12 - NATIONAL INSTRUMENTS COMPACTDAQ SYSTEM CURRENT DESIGN 7.1 Background The engine test stand has been used in engineering practices for as long as engines have been developed. The engine test stand is still an integral part of design process for modern Page 30