Multidisciplinary Senior Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York 14623 Project Number: P15318 GASEOUS MASS FLOW RATE CONTROLLER Brian Church Rochester Institute of Technology Mechanical Engineer (Co-Team Lead) Anthony Salmin Rochester Institute of Technology Electrical Engineer (Co-Team Lead) Tyler Breitung Rochester Institute of Technology Mechanical Engineer Ross Bluth Rochester Institute of Technology Electrical Engineer Michael Oplinger Rochester Institute of Technology Mechanical Engineer Stephen Mroz Rochester Institute of Technology Electrical Engineer ABSTRACT A Gaseous Mass Flow Rate Controller (GMFRC) is a mass flow regulating device capable of metering precise amounts of compressed natural gas (CNG). In the early 2000s, prototypes of such a device were first developed and tested by Dr. Roman Press. Although the prototypes demonstrated high accuracy and fast response times, the main functionality that they both lacked was that they were not stand-alone devices and had to be operated with external equipment. The main objectives of this project are to make a fully functional, stand-alone prototype that further increases the overall accuracy and performance, as well as develop a means to mix the regulated fuel with air and deliver it for use in an internal combustion engine. INTRODUCTION As oil and gasoline prices continue to rise, along with their environmental concerns, the discovery of natural gas sources in the United States creates the opportunity to utilize natural gas as an alternative for transportation fuel. In order to harness the energy in CNG, a system that can precisely regulate and deliver the fuel to an internal combustion engine is needed. Currently, there are a few manufacturers of systems that utilize CNG for use in an internal combustion engine. Bosch’s Bifuel CNG-System was first introduced in 2004, when it became available with the Chevrolet Astra in Brazil . More recently, Ford’s Bi-Fuel CNG/LPG Engine Package became available on their F-Series line of trucks in 2014 . The advantages of both systems are that they can operate on either CNG or gasoline alone and do not suffer noticeable differences in torque or horsepower between the two fuels. The major disadvantage of the systems is that they are complicated and expensive. Ford’s Bi-Fuel CNG/LPG Engine Package, the more Copyright © 2015 Rochester Institute of Technology expensive of the two, costs between $6000-9500 on top of the cost of the truck. The reason for the elevated cost is that these systems utilize separate fuel injectors and fuel lines for each of the fuels, which leads to increased costs due to added components and more machining time that needs to go into the engine block. This also means that these systems cannot be retrofitted to an existing engine platform without a large number of modifications. With the intention to make a simpler and less expensive fuel regulating system for CNG, the original GMFRC prototype was designed and developed by Dr. Roman Press in 2001. The main objective of the prototype was to show proof of concept of a device that could regulate fuel just as well as the more complicated and expensive competition. This prototype was very successful, as it was able to regulate the fuel with an accuracy of ± 1 % of full scale and with a response time of less than 70 ms . However, this design also had a few deficiencies; mainly the fact that it was not ready to be used with an internal combustion engine. It did not have its own dedicated controller and did not have any means of mixing the regulated fuel with air or delivering it for use in an internal combustion engine. Also, the device was not designed with the intention to be mass produced. The main objectives of this project are to make a fully functional, stand-alone GMFRC prototype that further increases the overall accuracy and performance, as well as develop a means to mix the regulated fuel with air and deliver it for use in an internal combustion engine. PROCESS The team was tasked with developing the next generation of the GMFRC over the planning and design stages of Multidisciplinary Senior Design. The planning stage involved using input from the customer, Dr. Roman Press, to draft a list of customer requirements and quantifiable engineering requirements. Once the scope of the project was understood through the requirements from the customer, concept generation started. Several concepts were developed for subsystems using various different tools learned throughout the early portions of the semester. Once concepts were generated, a design review was held with our team guide. From the design review, optimal concepts for the subsystem were chosen. Once the subsystems were developed, the integration of subsystems into a complete system started with detailed design. Several weeks of engineering discussions and analysis allowed for complete system integration. After the team had a final design review with our guide and customer, we obtained approval to move onto the build phase. rqmt. # Importance Customer Rqmt. # CR1 CR2 CR3 CR4 CR5 CR6 CR7 CR8 CR9 CR10 CR11 CR12 Importance 9 9 9 9 9 3 3 3 3 3 1 1 Description Accurate and repeatable mass flow rate control Operate on the nominal OEM control board power Usable with gaseous fuels Wide dynamic range of flow control Low production cost Integrated package design Long life in an automotive environment Stable to shock and vibration Small and lightweight Fast opening/closing response time of actuator Distribution plate efficiently mixes fuel with air Usable with a midsize engine TABLE 1: CUSTOMER REQUIREMENTS ER1 ER2 ER3 ER4 ER5 ER6 ER7 ER8 ER9 ER10 ER11 ER12 ER13 ER14 9 9 9 9 9 9 9 3 3 3 3 3 1 1 Source CR1 CR1 CR1 CR2 CR3 CR4 CR5 CR6 CR7 CR9 CR9 CR10 CR11 CR12 Engr. Requirement Accuracy of mass flow rate control Repeatability of mass flow rate control Leak rate of device Operating voltage Usability with gaseous fuels Dynamic range of flow control Cost of production device Number of separate assemblies Temperature limit Size envelope of production unit Total weight of production unit Open/close response time Fuel mixing shape Compatible engine size Unit of Marginal Target Measure Value Value % ±5 ±1 % ±5 ±1 cc/min 25 25 Volts 8-16 6-18 Yes/No Yes Yes Ratio 50:1 100:1 $ 100 100 # 3 2 °C 75 80 in3 96.5 86.5 lbs 5 4 ms 50 50 Target No Vortex Vortex Liters 4.7 4.7 Ideal Value ± 0.5 ±0 0 5-24 Yes >100:1 <100 1 85 72 3 <50 Vortex All Will be acknowledged in the design, but not testable: ER15 3 CR7 Operating life ER16 3 CR7 Number of full cycles ER17 3 CR8 Shock resistance ER18 3 CR8 Vibration resistance TABLE 2: ENGINEERING REQUIREMENTS There are three major components that contribute to the design of a successful GMFRC device. The first is the design of the fuel metering mechanism to allow gaseous flow through the device. The second is the design of the controller and supportive electronics to accurately control the flow metering mechanism. The last is the fuel delivery system that will be used to mix the fuel with air and deliver it for use in an internal combustion engine. MECHANICAL DESIGN To satisfy the first component of this project, thermodynamics and gas dynamics must be understood in order to develop a method to regulate mass flow. From a thermodynamic aspect, it is known that pressure and temperature have a direct relationship with each other if there is a control volume, meaning that volume is constant in the situation being analyzed. With the knowledge of how pressure and temperature act together, it is imperative to understand how they will affect mass flow rate, which brings in gas dynamics to explain mass flow rate. Proceedings of the Multi-Disciplinary Senior Design Conference Mass flow rate at a high level is the product of density, velocity, and area. When breaking these components up, it is recognized that velocity is proportional to the square root of temperature and density is proportional to pressure. The equation for mass flow rate can be derived by combining the equations of velocity and density when in terms of pressure and temperature, as shown in Equation (1). ̇ √ ( ) ( ) ( ) One last thing to consider when designing the valve to control mass flow is the possibility that the flow will become choked. Choked flow occurs when any gas property exceeds the critical value. These properties include the density and pressure of the flow as well as the area of the opening that the flow will be going through. Once a flow is choked, the resultant mass flow is equal to the product of density, area, and the speed of sound ( =1). In this project, choked flow can be achieved through the variation of pressure. Now that there is an understanding of how the gas will behave, it is critical to design a valve that will allow for accurate and repeatable mass flow rate control. The valve on the GMFRC needed to be designed so that it could provide a tight seal in the fully closed position in order to reduce the leak rate of unwanted fuel being delivered to the engine. To accomplish this, an aluminum rotating disk along with a delrin output port were used. Figures 1a and 1b show the valve in the fully open and fully closed positions, respectively. When the slot in the disk overlaps with the hole through the output port, flow is allowed through the output port. When there is no overlap, the flow is restricted from entering through the output port. The mating surfaces between the rotating disk and output port both had spherical profiles defined by the same radius. To achieve a sealing configuration, the rotating disk used an outside radius and the output port used an inside radius so that they would match up when pressed together. Aluminum was chosen for the disk since it had a rather complex profile that had to be Computer Numerical Control (CNC) machined. The material of the output port needed to be softer than aluminum to promote the wear of the output port instead of the disk. It also had to be a material with a relatively low coefficient of friction against aluminum so that the valve would have as little friction as possible. For that reason, delrin was the material of choice. To keep the output port pushed up against the rotating disk, a standard compression spring was used. The benefit of this is that the valve will still have a tight seal, even as the delrin wears over time. FIGURE 1a: VALVE IN OPEN POSITION FIGURE 1b: VALVE IN CLOSED POSITION The distribution plate needed to be designed with a few key requirements in mind. It had to be able to evenly mix the regulated CNG with air coming into a combustion engine through the air intake. This device should be located between the throttle body and the air intake, mounted in a way that allows it to mix the fuel with the air passing through the throttle body. To limit the amount redesigning that would be needed to retrofit an existing engine, the overall thickness of the device had to be smaller than 0.5 in. The design we used is a ring-shaped plate that directs the working fluid through eight small nozzles. The nozzles are angled radially and tangentially in order to induce a “swirling” effect, as shown in Figure 2a. This effect was first evaluated using SolidWorks’ built in Computational Fluid Dynamics (CFD) simulation toolbox. The results, shown in Figure 2b, were very promising and provided a significant “swirling” effect. Copyright © 2015 Rochester Institute of Technology FIGURE 2a: DISTRIBUTION PLATE FIGURE 2b: CFD ANALYSIS ELECTRICAL DESIGN On the original GMFRC, Dr. Press used the CTS 640 series Rotary Actuator to open and close the valve used to regulate mass flow rate of the fuel. The actuator is a 4-pole torque motor with a return spring feature, a milled male “D” shaped keyway, and can be operated with 0-5 A. The actuator is commonly used in automotive transmissions, which makes it manufacturable in large quantities, inexpensive, and able to operate in the -40-85° C temperature range. The torsion return spring acts as a safety feature to close the actuator in the event of power loss and cut the flow of fuel. Based on the preceding features, it was decided to incorporate the CTS Rotary Actuator into the new design. To control the flow of the GMFRC, it was decided that a controller with proportional feedback would be implemented. The controller needed to have interface circuitry to read a pressure sensor, a temperature sensor, and a position sensor for feedback control as well as supporting circuitry to power these sensors, filter the sensor outputs and control the actuator. The circuit architecture, shown in Figure 3, is basic layout for the circuit hardware and signal paths. FIGURE 3: CIRCUIT ARCHITECTURE Proceedings of the Multi-Disciplinary Senior Design Conference Proportional feedback was selected since it is a simple to implement and is not computationally intensive, which would help preserve the response time of the actuator. To implement proportional feedback, the Teensy 3.1 development board was selected for a few reasons. The first being that a microcontroller was needed to make the GMFRC a stand-alone device. The second being that the development board includes interface circuitry, making it easier to use and design supporting electronics. The third reason is that the Teensy 3.1 has a clock speed of 72 MHz, 256 KB of flash memory, USB interface, and 8 Timer interrupts. These features made the Teensy 3.1 much simpler to interface with and program compared to the other development boards and stand-alone microcontrollers that were researched. The fourth reason is that the Teensy 3.1 could operate in the desired -40-85° C temperature range. The last reason is that it is relatively inexpensive for a development board. For the rotary position feedback, the Delphi 514 Series rotary position sensor was selected. The position sensor is an analog output, 5 kΩ ± 40% potentiometer that has a female receptacle that accepts the “D” shaped keyway in the shaft of the rotary actuator. This sensor is commonly used in throttle control for vehicles, making it manufacturable in large quantities, inexpensive, and able to operate in the -40-85° C temperature range requirement. The position sensor can take a wide range of input voltage with an output of 0.925% of Vin per degree, but is recommended to operate at 5V or higher. To ensure proper interfacing with the Teensy 3.1, the position sensor was powered with a 5V supply from an On Semiconductor LM317MBSTT3G adjustable voltage regulator and the output was run to a one half voltage divider. This is to guarantee the sensor output is 2.5V or less and ensures that this will not damage the microcontroller during operation. For the pressure sensor feedback, the Measurement Specialties M7139 Pressure Transducer was selected for its suitability to work in liquid and gaseous media. This sensor is commonly used in vehicle and industrial applications, making it manufacturable in large quantities, inexpensive, and able to tolerate the -40-85° C temperature range requirement. The M7139 is an analog output, gauge pressure transducer capable of operating up to 200 PSI (13.79 Bar) with an accuracy of ± 0.25% of full-scale. This operating range well exceeds our requirement of 90 PSI (6 Bar). The M7139 is powered with a 5V input from the LM317MBSTT3G and its output ranges from 05V. Therefore, the output of this sensor was run through a one half voltage divider to guarantee a sensor output of 2.5V or less to ensure no damage is done to the Teensy 3.1. For the temperature sensor feedback, the Delphi 150 Series Coolant Temperature Sensor was selected for its easy to mount package design. Since this sensor is specifically designed for automotive applications, it is manufacturable in large quantities, inexpensive, and able to operate in the -40-85° C temperature range requirement. The coolant temperature sensor has an accuracy of ± 0.6° C in the desired temperature range. It uses a thermistor pulled up to 5V power through a resistor to the LM317MBSTT3G and the output ranges from an analog 0-5V. The output of this sensor was ran through a one half voltage divider to guarantee a sensor output or 2.5V or less to ensure no damage is done to the Teensy 3.1. To ensure the Teensy 3.1 properly reads output signals of the feedback sensors, the position, pressure, and temperature sensors were run through a unity gain operational amplifier (op-amp) to isolate the output resistance of the voltage divider and the input resistance of the Teensy 3.1. Since these resistances are on the same order of magnitude, directly connecting the feedback sensor voltage dividers to the Teensy 3.1 would cause a loading effect on the output signals and reduce the signal range. The unity gain op-amp does not amplify the signal output, but provides a high output resistance, which renders the loading effect on the sensors as negligible. A MCP6004 quad operational amplifier was used to make the unity gain op-amps for the feedback sensors and input voltage sampler. The MCP6004 was selected for its rail to rail operation, its operating temperature range of -40-85°C, and its low cost. The Teensy 3.1 uses the feedback sensors above to control the actuator position of the rotary actuator by pulse width modulating (PWM) the input voltage to the actuator. The larger the “on time” of the PWM signal, the more current the actuator draws and the more the shaft rotates. The shorter the “on time” of the PWM signal, the less current the actuator draws and the less the shaft rotates. To meet the current demand of the actuator, an H-bridge was used as a digital relay to PWM the actuator input voltage, while also isolating the Teensy 3.1 from the back EMF generated from operating the actuator. The Freescale MC33931EK was the selected H-bridge for a few reasons. The first is that the MC33931EK can handle up to 5A, which exceeds the maximum required current needed to operate the actuator. The second is that it continuously operates from 0-40V, which easily includes the input voltage range specified in the customer requirements. The third reason is that it can operate in the desired temperature range of -40-85°C. The last reason is that it can be operated using 3.3V, which is the logic output of the Copyright © 2015 Rochester Institute of Technology Teensy 3.1. Figure 4, below, summarizes the electrical design for both the circuit architecture and electronic components being used. FIGURE 4: CIRCUIT SCHEMATIC FINAL SYSTEM PROTOTYPE Combining all of the subsystems together yielded the final system prototype shown in Figure 5. The pressure and temperature sensors are connected to the input of the device on the left, while the output port subassembly is connected on the right. The rotary actuator’s shaft is connected to the rotating disk through the top, while the position sensor mates with the end of the shaft from the bottom. To consolidate space and provide adequate heat dissipation for the H-bridge, the electronics were placed on a double sided printed circuit board (PCB), as shown in Figure 6. FIGURE 5: EXPLODED VIEW Proceedings of the Multi-Disciplinary Senior Design Conference FIGURE 6: PRINTED CIRCUIT BOARD RESULTS AND DISCUSSION The major engineering requirements that we were unable to meet were the leak rate of the device in the closed position, the cost of a production device and the response time of the device, as shown in Table 3. Although we could meet the leak rate requirement of less than 25cc/min for pressures below 10 psi, the leak rate increased significantly at pressure above this. The reason for this was because a compromise between leak rate and being able to turn the actuator was needed. A strong spring that kept the leak rate very low caused too much opposing torque for the actuator to overcome. A weak spring allowed the actuator to easily move, but caused a very high leak rate at any pressure. Next, we were unable keep the estimated cost of the production GMFRC below the $100 metric. However, this is a conservative estimate since many components in the bill of materials we interpolated to yield a reasonable estimate. It is possible to reduce the price of the production GMFRC by obtaining proper quotes on prices, which was out of the scope of this project. Lastly, we were unable to meet the response time requirement of 50 ms or less. Since the actuator response time of fully opening or closing the actuator was less the 10ms, the problem lies within how the control algorithm is being implemented in the code on the microcontroller. The response time can be slowed by an additional 1-2 seconds depending on the amount of friction between the output port and rotating disk and by the magnitude of the normal force that keeps the output port pushed against the rotating disk. There were also a few engineering requirements in particular that we were able to meet the marginal value for but not the target value. We were not able to directly test the accuracy and repeatability of the device with regards to flow rate control, as a measurement device more accurate than our specification was not able to be obtained. Therefore, we evaluated these metrics based on the device’s ability to be controlled to a specified angular position since that directly correlates to the flow rate. Since the control algorithm decides when the angular position is settled by seeing if it is within a specified percentage of the commanded position, it was expected that the accuracy would be constant and the repeatability would vary. Contrary to these expectations, our test data showed that the repeatability was very high and the accuracy was only marginally acceptable. Most of this error attributed to the accuracy was accumulated in the higher ranges of operation, being 50° or higher. Since the observed positions were much further from the commanded positions than in the lower range, this problem could be caused by the position sensor not being properly calibrated at the higher position ranges, and thus led to the increase in the error for this test. Lastly, the dynamic range that we obtained from the device was just over 75:1. This problem came from the fact that the slot in the disk was made by a grinding process instead of Electrical discharge machining (EDM), as original planned. The grinding process was not able to produce a sharp point at the end of the slot as an EDM process would. This resulted in a higher minimum flow rate and therefore a decreased dynamic range. Copyright © 2015 Rochester Institute of Technology TABLE 3: PERFORMANCE VS ENGINEERING REQUIREMENTS CONCLUSIONS AND RECOMMENDATIONS A closed-loop feedback control was successfully developed and implemented, allowing the GMFRC to operate as a standalone device. The system offered control with a reasonable accuracy and a high precision, however its response time was much slower than required. From a mechanical standpoint, the tolerances and surface finishes on the components were not sufficient to reduce the leak rate and sliding friction to acceptable values. The distribution plate, on the other hand, successfully demonstrated proof of concept as a method of introducing and mixing CNG into the intake air of an automobile. Looking to the future, there are several things that can be done to improve the current design, both electrically and mechanically. On the electrical side; the code implementation can be modified to improve timing and accuracy. The idea of using a proportional-integral (PI) controller instead of the current proportional (P) controller could be pursued. The pressure and temperature sensors also need to be integrated into the control algorithm. The PCB can be redesigned to reduce size and cost, and improve overall capability of the device. Looking at the mechanical design; a new housing should be made that incorporates the sensors and PCB, reducing the overall size of the device. Given the shortcomings of the current design, a valve design that uses a cam and ball may be worth pursuing. Further development of the distribution plate can include making it suitable for mass production and implementing it in tests involving a throttle body and air manifold from an internal combustion engine. Lastly, a new calibration of the device would be needed to make sure the device works properly with varying input pressure and temperature. REFERENCES  Bosch Mobility Solutions, n.d., “Bifuel CNG-Systems.” From http://products.bosch-mobilitysolutions.com/en/de/powertrain/powertrain_systems_for_passenger_cars_1/bifuel_cng_saugrohreinspritzung_1/ bifuel_cng_systeme.html#  Ford Motor Company, 2013, “First CNG-Capable 2014 Ford F-150 Rolls Off The Line in Kansas City.” From https://media.ford.com/content/fordmedia/fna/us/en/news/2013/11/21/first-cng-capable-2014-ford-f-150-rollsoff-the-line-in-kansas-c.html  Press, R.J., 2001, “Quantum Fuel Metering Valve Controller Development,”n.p., n.p. ACKNOWLEDGEMENTS We would like to thank our guide, Edward Hanzlik, for all of his help and support throughout the entire design process. We would also like to thank our customer, Dr. Roman Press, for his assistance with our system design, Dr. Lynn Fuller for his manufacturing services and assistance with our electrical hardware, and both the Mechanical Engineering Machine Shop and Brinkman Lab staff for their assistance with machining our components.
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