Download Design specifications

This document is intended to list all the requirements of the gaseous mass flow rate controller and give definitions to all the requirements. It
includes what can and what can't be tested.
Device Definition
The device is intended to control the mass flow rate of natural gas to an engine. It receives natural gas from an input fitting at a regulated
pressure of about 4 bar. It will be designed to output to atmospheric conditions, though in actual use it would output to a naturally aspirated
engine's intake tract which typically operates at a pressure below atmospheric. It receives output requirements from a 128PWM (0-5V)
signal which updates at a frequency of 100 Hz.
Open Items
This is a list of all things that need to be addressed by the team, guide, and/or customer:
● If the device needs to be powered closed to achieve the acceptable Close Response time, is there a required Failsafe Response time?
● Are there Flow Response time targets (i.e. does the device need to be able to get to a commanded value within so much time and
meet the Flow Accuracy/Repeatability Targets)?
◦
This will be important to figure out sample rate requirements for the sensors as well as PID control.
● Package (Envelope) Size needs to be redefined.
● Number of Assemblies needs to be redefined.
● The relevancy of burst pressure needs to be discussed.
Table
Error! No sequence specified.: Current Engineering Requirements
Requirement Definitions
Flow Accuracy
The device must control the mass flow rate of natural gas accurately. The accuracy will be determined by comparing the actual output of the
device against the commanded value as received by the 128PWM signal. A ±5% difference in actual mass flow rate from commanded flow
rate is acceptable; ±1% is the target value; anything better is ideal.
Flow Repeatability
The device must output flow in a repeatable manner. The repeatability will be determined by comparing the output flow rate of the device
against the previous flow rate outputs with the same commanded value. A ±5% difference in mass flow rate from previous flow rates is
acceptable; ±1% is the target value; anything better is ideal.
Gas Leak
There are two versions of gas leak that must be considered: Leak Rate of the device and Total Leak of device.
Leak Rate is the flow rate of natural gas from the input to output fitting when the valve is either in a commanded closed or an unpowered
state. This will be determined by measuring the flow rate at the output of the device when in both of the states mentioned above. A leak rate
of 25 sccm is both the acceptable and target value for the upper limit; anything below this is ideal.
Total Leak is a pressure leak in the device. This will be determined in two ways: measuring the flow rate at the inlet of the device when in an
unpowered state or submerging the device in water in an unpowered state and visually inspecting for bubble production. No leaks can be
present other than what is accepted through the outlet of the device by the leak rate, above.
Operating Voltage
The device must work with typical voltage variations associated with an automotive environment. This will be determined by either
physically changing the voltage and physically testing the device or simply by checking the manufacturer recommended operating
conditions on boughten electrical components. It is acceptable that the device work only at 13.5 V; the target is that the device work from 12
to 16 volts; increasing the range limits is ideal.
Flow Control Requirements
A key feature of the device is that it can work with a wide range of engines. It is expected that the device should be able to handle the mass
flow rates of not only small displacement engines, but engines with displacements of up to 5.7L. This brings in two aspects: the Maximum
Output flow rate and the Dynamic Range of flow rates.
The Maximum Output flow rate is the flow rate output of by the device when the inlet receives 4 bar,g of pressure and is commanded to give
the maximum flow rate by the 128PWM signal. This will be determined by measuring the actual flow rate output by the device to the
maximum flow rate commanded by the 128PWM signal. A maximum flow rate in the range of 12 to 18 g/s is both the acceptable and target
values; a flow rate as close as possible to 15 g/s is ideal.
The Dynamic Range of flow rates is the ratio of the maximum commandable flow rate to the minimum commandable flow rate of the
device. This is important for the device to be applicable to a wide range of engine displacements (i.e. from idling a small displacement
engine up to full throttle of a 5.7L). This can be determined by comparing the actual flow rates at both the largest commanded value and the
smallest commanded value that actually produces flow. A dynamic range of 50:1 is acceptable; a range of 100:1 is the target; anything
greater is ideal.
Dynamic Response
The device must react quickly if either the ECU demands wide open or if it requires the device to completely close. These two separate
events are the Open Response Time and Close Response Time.
The Open Response Time is the amount of time required for the device to actuate from a closed position to a full open position once it
receives a command from the 128PWM signal. 50 ms is both the acceptable and target value; anything faster is ideal.
The Close Response Time is the amount of time required for the device to actuate from a full open position to a closed position once it
receives a command from the 128PWM signal. 25 ms is both the acceptable and target value; anything faster is ideal.
Operating Inlet Pressure Range
The device receives pressure regulated flow and output flow must be insensitive to any expected variations in that inlet pressure. This can be
determined by varying the inlet pressure of the device within the expected range of variations from the regulator. The nominal value of the
regulated flow is 4 bar,g; the expected variation is ±4%.
Total Weight
The total weight of the system needs to be as low as possible to not add stress to automotive components and to not measurably affect the
efficiency of the vehicle. The acceptable weight of all components is 5 lbs; the target is 4 lbs; anything less is ideal.
Package Size
The device is expected to consist of two housings: the electrical component housing and the valve housing.
The current values give 96.5 in3 as acceptable; 86.5 in3 as a target; 72 in3 as ideal.
Production Cost
The cost to produce the device will be important in later stages. For now, there just needs to be something to shoot for. It is acceptable for the
device to cost $200; the target is for the device to cost $150.
Number of Assemblies
The greater the number of assemblies the more complex and typically the more expensive the device will be. The current acceptable value is
3; the target is 2; 1 is ideal.
Operational Temperature Range
The device is intended to be used in an automotive environment. This means that it could see Alaskan Winters or Death Valley Summers.
The device is also expected to be under the hood which receives radiation and convection from the exhaust, engine block, cylinder heads,
etc. and radiation from the sun. The expected temperature range is -40 to 85ºC.
Burst Pressure
The device works with high pressure gases. The current burst pressure requirements are listed TBD.
Operating Life
The device is intended for the automotive industry and must be designed to last an expected amount of time in order to be competitive. Two
versions of life must be considered: hours and cycles.
The anticipated life cycle is 10,000 hours long. The anticipated cycle life is 106 (1 million) cycles.