Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

Project Number: P16318

GASEOUS MASS FLOW RATE CONTROLLER

Luke McKeanMechanical Engineering

Lianna DickeElectrical Engineering

Selden PorterMechanical Engineering

Schuyler WitschiElectrical Engineering

ABSTRACTOne possible solution in the search for an alternative fuel source to gasoline and diesel in the automotive

industry is natural gas, but a gaseous fuel requires a unique fuel system. To keep retrofitment costs to a minimum, a simple, accurate mass flow rate controller able to operate with a wide range of engine sizes is required. This project demonstrated that such a device is feasible on a limited budget using a rotational actuator supplied by CTS Corporation and a valve designed and built by the team. The P16318 gaseous mass flow rate controller (GMFRC) design was successful, meeting nearly all of the customer requirements and pinpointing key areas of design that need improvement.

INTRODUCTIONThe automotive industry and government continue to search for a cleaner and more efficient alternative to

gasoline as the primary power source for the transportation market. Natural gas is one possible solution due to its abundance and cleaner combustion process. Most modern engines require little redesign to run on natural gas. The implementation of natural gas into an internal combustion engines does, however, require a fuel system that is setup to handle a gaseous fuel.

Natural gas powered engines require precise and accurate control of the mass flow rate of the gas to ensure clean and efficient power generation. There are two main solutions to control the flow rate of natural gas currently being developed for the automotive industry, fuel injection and mass flow distribution systems (MFDS). Fuel injection is similar to current gasoline systems utilizing multiple devices to inject fuel in small bursts either into the cylinder head’s intake ports or directly into the cylinder. Due to the limited range of flow rates of injectors, they must be scaled to each individual application.

MFDS’ separate the fuel control (control valve) and fuel introduction systems (distribution plate). A distribution plate must be designed for different sized engines and applications, however the control valve which directly meters the fuel to the distribution plate can be designed to fit a wide range of automotive applications from a compact car to a large pickup truck. A valve capable of precisely controlling mass flow rate of natural gas over a large range of flows would enhance the marketability and simplify the manufacturing process by allowing a single valve to be fitted for numerous applications.

Copyright © 2016 Rochester Institute of Technology

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PROJECT REQUIREMENTSThroughout the project the requirements changed due to research and testing exposing feasibility concerns of

some of the initial criteria. The final objectives of the project required designing, building and testing a proof of concept valve that works with a gaseous fuel. The valve is required to control mass flow rates of a gas in an accurate and repeatable manner. The valve must have extremely low leak rate when fully closed, a high dynamic range of controllable flows and must utilize an available automotive actuator. The final design is a tool to provide data for future development of the product. The data required from testing is the response time of the valve, resolution of the valve, as well as the mass flow rate through the valve across its operating range at specified regulated static air pressures. As the device is a prototype, the design considered manufacturability, cost, and a small physical footprint, but these characteristics were not explicitly required, as demonstrating overall functionality of the device was the main objective.

DESIGN OVERVIEWThis project is a continuation of the first-generation control valve designed and tested by P15318 [1]. Their

design featured a slotted disk and a CTS 640-Series Rotational Actuator. The rotational actuator directly rotated the slotted disk which had a valve seat sprung against it. In the closed position the output hole in the seat would be completely sealed off by the solid disk; as the disk rotated, the slot would rotate past the seat creating a variable opening area and allowing natural gas to pass through the valve and into the mixing manifold. The rotational position of the disk was directly proportional to the opening area and therefore the mass flow rate. The two major pitfalls of this design were the sealing of the seat on the disk and the slow response rate of the system. Due to these major drawbacks, P15318’s design was scrapped in favor of a cam and ball valve design and a newer CTS GP 647 Rotational Actuator.

After researching and evaluating control valve designs from a myriad of sources, the P16318 team decided to base the design on a sketch provided by the customer and from modern automotive EGR designs, featuring a cam, precision ground ball, and valve seat. The team felt it would be straightforward to implement and would result in an inexpensive valve design. Initially, the design was going to reuse the rotational actuator from P15318 with updated software, but the customer acquired two newer CTS GP 647 actuators, which featured an internal control software dealing with many of the low-level control requirements necessary for real world operation, such as voltage variation compensation. With this as a basis, the proof of concept control valve was designed featuring the cam and ball valve design actuated by the GP 647.

VALVE DESIGNThe valve design can be broken down into two components: gas dynamics theory and mechanical design. Gas

dynamics theory was used to determine the approximate sizes of the valve components that directly control or limit the mass flow rate of natural gas, e.g. ball diameter, seat diameter, cam lift, etc. The mechanical design covers the rest of the components, e.g. sealing surfaces, the housing, cam shape, etc.

Gas DynamicsChoked flow is a phenomena that can occur when a gas is accelerated to the speed of sound in a converging

geometry and is a consequence of both the geometry and the fluid properties of the gas. When a flow becomes choked, the mass flow rate of the gas cannot be increased due to decreasing the pressure on the downstream side of the choked section, hence its name. A flow becomes choked when the ratio of the total pressure to the static pressure at the throat (minimum dynamic flow area) of a stream exceeds a critical value:

(1)

where P0 is the total pressure, P* is the static pressure, and γ is the adiabatic ratio of the gas all calculated at the throat; the right half of the equation makes up the critical value. To simplify the theory, it was assumed that the flow was steady state (not changing with time) and isentropic (inviscid gas with no external heat loss or gain). This allowed the total pressure used to be the compressed gas tank pressure that supplies the natural gas at its minimum condition. The valve throat static pressure was assumed to be atmospheric pressure which is close to the maximum expected to be seen in the distribution plate. Using the fluid properties of pure methane as a substitute for natural gas, the critical pressure ratio was calculated to be 1.840; the pressure ratio at the lowest expected tank pressure of 4

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bar (58 psig) was calculated to be 5.082, which far exceeds the critical value. This means that under all anticipated conditions the flow should be choked.

With this in mind, mass conservation of an ideal, choked flow was used to approximate the effective opening area necessary to achieve the flow requirements of the valve:

(2)

where T0 is the total temperature at the throat, R is the specific gas constant, m* is the mass flow rate through the throat, and A0 is the effective throat area. Again, the flow was considered to be isentropic with all total properties evaluated at the supply tank. It was assumed that pressure in the tank was at 4 bar,g (58 psig) with an ambient temperature of 22°C (72°F) and a barometric pressure of 101.3 kPa (14.7 psia). The results of this calculation are plotted in Fig. 1. It is necessary in the design to remember that a flow will be choked at the minimum dynamic area within its flow path, so all areas through which a flow travels must exceed the area required to meet the maximum flow and restrictions such as turns and long lengths must be considered with a critical view.

Figure 1

Mechanical DesignTaking into consideration the issues with the previous design, the new design attempts to solve the leak and

high friction problems caused by the slotted disk design. The final implementation of the cam and ball valve is illustrated in Fig. 2.

Copyright © 2016 Rochester Institute of Technology

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Figure 2: Valve Cross Section

As seen in Fig. 2, the light blue cam is rotated by the actuator and displaces the pushrod linearly. This allows the ball to lift off the seat. By having an equation driven cam profile, the lift height of the ball can be calculated based on the actuator position. The choking point of the flow is the smallest cross sectional area which, in this case, was the conic frustum formed from the nearest point on the ball to the edge of the seat. The design allows for simple changes in the cam design equation to accommodate different lift profiles and different flow requirements. The spring provides constant pressure on the ball so it maintains a linear lift path. The pushrod never loses contact with the cam except at the full closed position.

The flow of natural gas in Fig. 2 follows the path from left to right. The pressure differential from the input to the output increases the force of the ball onto the seat. This, in turn, aids in preventing leak past the valve and also means that at higher pressures the leak rate of the valve will, if anything, improve.

For the prototype valve the housing was machined from a billet aluminum block that used O-ring face seals on all static sealing surfaces [2]. The O-rings provided a cheap and highly effective seal while maintaining ease of assembly. The cam was a CNC part that used a linear lift profile reaching a maximum of 3.175 mm (1/8 in.) of lift at full open. This profile was based on the aforementioned flow theory.

ACTUATOR AND VALVE CONTROLThe initial design made use of the CTS 640-Series Rotational Actuator and a Teensy 3.1 microcontroller. The

microcontroller was necessary to process the pressure, temperature, and position sensor data required to measure the flow of gas through the valve. Initial testing of the 640-Series showed that it was simple to actuate using a PWM signal, but showed a high deadzone voltage, resulting in control difficulties over small movements, and slow response times making the positional feedback control of the system challenging to implement. However during the controls design process, the customer acquired the two GP 647 Rotational Actuators from CTS, and requested that the team consider switching to the new actuators. In addition to the new actuator, the customer decided that only position and not true mass flow rate control was required. The GP 647 had built in software for calibration, positional feedback control, and compensation for high and low system voltage. Since response time wasn’t externally controllable with the GP 647, it would only need to be measured. The GP 647 is a stepper motor so though it had more operational range (155° compared to 75°), it has a finite step size. Because the actuator would reduce the electronics development time and provide a quicker transition to testing the system as a whole, it was implemented in place of the 640-Series.

The GP 647 actuator communicates using the J1939 Controller Area Network (CAN) communication protocols. CAN is a multi-master serial bus communication scheme and a standard within the automotive industry due to its highly robust nature and resistance to electrical noise. Initial attempts at communication used an Arduino Uno R3

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with a Sparkfun CAN-Bus shield backed by open source software. It was found however that the software was inadequate with the J1939 specification and an alternative solution was found by using a PCAN controller supplied by CTS. The PCAN has canned software and hardware that allowed position control of the actuator from a computer with the accompanied software.

FLOW AND LEAK TESTING

Figure 3: Flow and Leak Testing Setup

Flow and leak testing was performed using two flow meters: an Omega FL7317 Rotameter and a TSI 4040 Thermal Mass Flow Meter. Due to the limited range of flows that each individual meter could measure, they were connected in parallel to the outlet of the flow controller. The readings were processed as necessary to convert from standard volumetric flow rates to mass flow rates and summed together. The customer required that the flow be measured across a range of valve openings with inlet static pressures regulated to 25, 45, and 60 psig, 1.7, 3.1, and 4.1 bar,g respectively. Since the thermal mass flow meter maxed out before the rotameter, a simple throttle was added to the outlet of the thermal mass flow meter to direct more flow to the rotameter and allow for more flow points to be measured. Though the throttle was effective in proportioning the amount of flow going through each flow meter, it is likely a source of error. The rotameter’s output needed to be adjusted to the pressure and temperature of the actual flow passing through it. The pressure and temperature used for correction were taken from the display of the thermal mass flow meter which was likely similar to that of the rotameter; however, the throttle increased the pressure reading of the thermal mass flow meter which the rotameter most likely didn’t experience resulting in the error. To measure leak, the rotameter was capped off and a reading was taken from the thermal mass flow meter with the valve in the closed position; it should be noted that at this low flow and atmospheric conditions the meter isn’t very accurate--on the order of about 50 sccm, twice the required specification.

RESPONSE TIME TESTINGTo determine the time response of the actuator, a test plate and bracket was manufactured that allowed a

potentiometer based position sensor to be mounted on the shaft of the actuator. The small size of the actuator shaft meant only the time response of the actuator was measured, not the whole GMFRC system. Due to the high output torque provided by the actuator and the low sliding friction of the pushrod on the cam, it was concluded that the friction force would not affect the response time significantly.

Once the test plate was attached, an oscilloscope sampling at 500hz was used to monitor the DC signal voltage output of the potentiometer. A 6 point moving average filter was applied to the data to reduce noise. The response measurement was taken from the difference in time of the initial increase or decrease in voltage to the sharp change in slope associated with the slow-down and braking phases of the internal feedback loop. This response test was effective at measuring the quick response time but ineffective at measuring the braking period. A more accurate response time measurement should be acquired from the manufacturer.

Copyright © 2016 Rochester Institute of Technology

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RESULTS

Figure 4 Figure 5

Figure 6

Percentage Travel Opening Response Closing Response100% 342 ms 324 ms80% 268 ms 284 ms60% 186 ms 208 ms40% 140 ms 114 ms20% 74 ms 72 ms

Table 1: Opening and closing response times with actuator calibrated to 135°

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CONCLUSIONS

Table 2: Performance vs. Engineering Requirements

All the engineering requirements set by the customer in the last iteration of the design phase were met or exceeded, except for the leak rate of the device. Leak rate is high because of the limited manufacturing facilities at RIT and time constraints. Though above the required specification, the leak rate achieved is a major improvement over the P15318 design, and it is expected that an improved seat design will yield a more acceptable leak rate. It should be noted that the repeatability figure in Table 1 only refers to the repeatability of the position of the actuator, not the repeatability of flow. The positional repeatability of the GP 647 easily meets the engineering requirements, but it should be mentioned that the GMFRC design showed a relatively poor flow repeatability as seen in Fig. 5. This is due to a combination of factors, but since nearly all the flow points taken on May 5 are higher than those taken on April 27, the two most significant factors are likely flow conditions upstream of the GMFRC which were outside the control of the test setup and how tightly the outlet fitting of the valve was screwed in. If the outlet valve wasn’t located the same distance from the face of the housing during both tests, then the pushrod was not in the same position relative to the center of the cam resulting in an offset of valve lift for each angular position of the actuator.

There are four major areas where the mass flow rate controller could be improved: actuator gearing, valve seat design, spring choke, and production manufacturing design. The actuator is geared toward resolution and torque; the controller’s needs would be better met with gearing more focused on speed to improve response time. The valve seat material currently used is Delrin; sealing and lifetime of the valve could be improved if a hardened steel seat with a precisely ground surface were used. Currently at around 60% throttle opening, the valve chokes at what is expected to be the spring coils; though the valve meets the flow requirements at around 30% opening as seen in Fig. 4, optimizing the design with a spring with less densely packed coils and a different cam design could result in a more responsive system that doesn’t choke at high lifts. The prototype design is not ready for production; to improve the number of lifecycles the valve can achieve, it is recommended the cam material be changed to a hard steel and use a small bearing on the interface of the cam and pushrod to reduce friction. The housing in the current design was made to allow for a variety of cams and output fittings to be tested; to improve manufacturability and overall size and weight, a new housing should be designed that minimizes the wetted volume of the housing and reduces the wall thickness to meet a burst specification. The design would also need to locate the output fitting in a better manner to improve the flow repeatability of the device.

Overall the GMFRC design met the goals of the design team despite the many challenges presented by scope drift and time and budget constraints. The design was successful in satisfying the customer and proves the device to be feasible.

Copyright © 2016 Rochester Institute of Technology

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REFERENCES[1] Church, B., Salmin, A., Breitung, T., Bluth, R., Oplinger, M., and Mroz, S., 2015, “Gaseous Mass Flow Rate

Controller,” Rochester Institute of Technology, Rochester, New York.[2] 2007, "Parker O-Ring Handbook," Parker Hannifin Corperation, ORD 5700, pp. 135-140.

ACKNOWLEDGMENTSDr. Roman Press - Alphacon LLC - CustomerCTS Corporation - Sponsor George Slack - MSD GuideDr. Amitabha Ghosh - Fluid Dynamics AdviceDr. Lynn Fuller - Controls and Electronics AdviceCraig Piccarreto – Fabrication Advice

Project P16318