System for controlling mass flow rates of two gases

Abstract: A control system for internal combustion engines, including a first conduit for the flow of intake air, and a second conduit for the flow of natural gas to be mixed with the air for combustion. The flow of the air is variable over a range of mass flow rates during operation of the engine and an air mass sensor is positioned to sense the mass flow rate in the first conduit and generate a first flow rate signal indicating the mass flow rate. A natural gas mass sensor is positioned to sense the mass flow rate of the gas in the second conduit and to generate a second flow rate signal indicating the mass flow rate of the natural gas. The first and second flow rate signals are compared in an electronic controller that generates a control signal if the first and second flow rates vary from a predetermined ratio. The ratio is selected to provide a substantially stoichiometric ratio of air and natural gas. A valve is provided to adjustably control the flow rate of the natural gas in response to the control signal to maintain the preselected ratio. A sensor and shutoff valve are also provided for terminating the flow of the natural gas when its pressure falls below a predetermined pressure, allowing immediate flow of gasoline to the engine, thus assuring smooth, continued operation of the vehicle. A throttle body has a natural gas distribution ring in the throat of the body and provides a curtain flow of gas.

Description: The invention relates to a control system according to the preamble part of Claim 1. US-A-4 537 172 and EP-A3-0071588 disclose control systems with first and second gas mass sensors provided in two separate conduits leading to the intake of an internal combustion engine. An electronic controller processes the output signals of both flow mass sensors and controls an adjustable valve in the second conduit for adjusting a predetermined ratio between the flow rates of the gases in said conduits. The flow mass sensors as well as the electronic controller are complicated and expensive. US-A-4 335 697 discloses an internal combustion engine dual fuel system for controllably running the internal combustion engine on a liquid fuel or at will on a gaseous fuel. Switching from one type of fuel to the other is carried out by means of a manually actuated switch means operated by the driver. In case of an insufficient supply of gaseous fuel the driver might switch over to the other fuel too soon or too late. US-A-4 523 461 discloses a hot wire gas mass sensor for sensing the flow rate of a gas flow in the intake system of an internal combustion engine. Said sensor generates non-linear output signals. It is a task of the invention to create a control system as disclosed which easily allows it to suitably regulate the mass flow rates of both gases to maintain them in a predetermined ratio. The system ought to be relatively accurate and reliable and should allow optimum performance of an internal combustion engine using at least a gaseous fuel, and yet be inexpensive to manufacture. This is achieved by the features contained in Claim 1. As the first and second conduits have interior flow cross-sectional areas at the points of mass flow measurement sized relative to each other to approximately correspond to the predetermined ratio of the mass flow rates of both gases and as both gas mass sensors have a substantially identical but non-linear output response relative to the mass flow being sensed, the output responses of both gas mass sensors will be generally scaled by the choice of the cross-sectional areas of both conduits. Even with the output responses of both gas mass sensors being non-linear, the first and second flow rate signals will remain comparable during operation over a wide range of flow rates without introducing unacceptable errors. Further preferred embodiments of the invention are contained in the depending claims. A preferred embodiment of the invention will be described in connection with the drawings. In the drawings is: Figure 1 a schematic diagram of the control system of the present invention, Figure 2 an isometric view of a flow conduit and a hot wire anemometer, Figure 3 a sectional view taken along lines 3-3 of Figure 2, Figure 4 an isometric view of a servo valve, Figure 5 a sectional view taken along the lines 5-5 of Figure 4, Figure 6 a fragmentary sectional view of the butterfly used in the servo valve of Figures 4 and 5, taken substantially along the lines 6-6 of Figure 5, and Figure 7 a fragmentary sectional view of a throttle body used with the control system. A control system 10 serves for controlling the relative mass flow rates of two gases. It may be used wherever it is necessary to control the relative mass flow rates of two gases, including ventilation of greenhouses, which require a certain mixture of air and carbon dioxide, and in the operation of gaseous fuel internal combustion engines and boilers. The system 10 is described herein for use with an internal combustion engine for a driven vehicle convertible during operation between liquid gas and gaseous fuel, such as natural gas, as alternative sources of power. The control system 10 is shown schematically in Figure 1 operating with an engine for an automobile. The automobile includes a tank 12 for liquid gasoline, a gasoline filter 14 and an electric fuel pump 16. A pressure regulator 18 controls the pressure of the gasoline delivered by a fuel line 20 to injectors 22 (22A, 22B in Figure 7) in a throttle body 24 mounted to manifold 25. The throttle body 24 includes conventional butterfly valves 27A and 27B. A return fuel line 26 is provided between the pressure regulator 18 and the gasoline tank 12. The pressure regulator maintains a gasoline pressure of 2,76.10⁵Pa (40 psi). Mounted on top of the throttle body 24 is a throttle body cover 30 removably attached by a nut 32. A first conduit 34 communicates ambient air with the throttle body cover 30. The conduit 34 has an air filter 36 mounted at its end. A gas mass sensor 38 is positioned to sense the mass flow rate of the air in the air conduit 34, and to generate a flow rate signal indicating the mass flow rate of the air in the first conduit 34. A second conduit 40 communicates natural gas to the throttle body cover 30. It is important to mix the natural gas completely and in the same proportions for each cylinder of the engine. Mixing is a problem, since air and natural gas have different densities and mixing of the two is inherently difficult. As shown in Figure 7, mixing of the air and natural gas is accomplished in throttle body 24 first by splitting of the air and gas flows in the throttle body equally into two flows through throttle body throats 28A and 28B. The natural gas conduit 40 communicates with a central throttle body cavity 41 from which the gas is distributed evenly about the perimeter of the throats using distribution rings 43A and 43B at the venturi point for the throats and above the butterfly valves 27A and 27B. The distribution rings 43A and 43B have the shape of a one-quarter toroid and define circumferential distribution chambers 45A and 45B through which the natural gas flows from the central chamber 41 to a circumferential slot 47A and 47B defined by the space between the edge of the distribution ring and a corresponding edge of the throttle body wall. Due to the positioning of the distribution rings at the venturi point, the pressure in each of the distribution chambers 45A and 45B is dependent upon the vacuum in the throats, with both being proportional to the flow of air through the throats. As such, a very even flow in curtain form which is proportional to the air flow through the throats is provided. When the curtain flow reaches the area of the throat with the butterfly valves 27A and 27B, the high turbulence encountered there causes the natural gas and air to completely mix, avoiding previously encountered difficulties in achieving mixing of the different density air and natural gas. The described mixing system is adaptable to different engine sizes, requiring between one and four throttle body throats, by providing a distribution ring and venturi-slot in each throat. As shown in Figure 7, the air enters the throttle body 24 from above through a circumferential passageway 49, which communicates the air with the central opening of the distribution rings 43A and 43B. For operation on liquid gasoline, the throttle body 24 shown in Figure 7 is provided with two injectors 22A and 22B which are fed by a gasoline manifold 29 connected to the fuel line 20. The injectors extend downward through the central opening in the natural gas distribution rings 43A and 43B and inject the gasoline directly into the throttle body throats 28A and 28B. A natural gas mass sensor 42 is positioned to sense the mass flow rate of the natural gas in the gas conduit 40, and to generate a gas flow rate signal indicating the mass flow rate. The air flow rate signal and the gas flow rate signal are used to control operation of a servo valve 44 which controls the flow of natural gas to gas conduit 40. The natural gas is supplied to servo valve 44 from a pressurised storage tank 46. A manual shutoff valve 48 is provided for the tank. Piping communicates the natural gas to a pressure sensor 50 which operates in conjunction with a solenoid shutoff valve 52 to cut off the flow of natural gas to the throttle body 24 if the line pressure falls below a predetermined level indicative of insufficient pressure to operate the engine on natural gas. Positioned between the pressure sensor 50 and the valve 52 is a pressure regulator 54. The natural gas is piped from the solenoid shutoff valve 52 through a diaphragm pressure regulator 56 and then to servo valve 44. A control panel 58 is provided with a toggle switch 60 to allow an operator to manually select between operation on natural gas or liquid gasoline as alternative fuels. An indicator lamp 62 is provided to indicate when the vehicle is being operated on natural gas, and an indicator lamp 64 is provided to indicate when the engine is operating on gasoline. Control system 10 further includes an electronic controller 66, indicated by the phantom line box in Figure 1. The controller 66 receives signals from the pressure sensor 50 and in response thereto sends control signals to the solenoid shutoff valve 52. The controller also operates in conjunction with the control panel 58 to select the mode of operation. When the control system 10 is used to control gases for purposes of combustion, it is desirable to provide a stoichiometric ratio of air and gaseous fuel such as natural gas. To achieve an accurate flow rate of the gases, it is desirable to measure the mass flow rate by a method which is not subject to errors associated with pressure and temperature variation. This is accomplished by measuring the mass flow rate of the gases utilising hot wire anemometers 67 of the type shown in Figures 2 and 3. An anemometer of this type is described in U.S. Patent No. 4,523,461, which is incorporated by reference herein. By sensing mass flow, the measurement is not sensitive to normal pressure and temperature variations. The anemometer 67 includes a straight length of temperature dependent resistive wire 68 extending between a pair of electrically conductive posts 70. For purposes of illustration, the conduit will be described in Figures 2 and 3 as the air conduit 34; however, the arrangement is the same for the gas conduit 40 except for size as will be explained below. The posts extend through an opening 71 into conduit 34 and are rigidly supported by a support member 72. A circuit board 74 holds at least a portion of the anemometer electronic circuitry and is electrically connected to the posts. A block of closed-cell foam 76 is positioned over the opening 71 and held in place to prevent any air from exiting or entering the conduit. A cover 78, shown in phantom, covers the electronic circuitry. Upstream the anemometer 67 is positioned within the conduit 34 a honeycomb arrangement 80 with elongated cells which assist in providing laminar flow of the air in the conduit. The resistive wire 68 is one branch in a bridge circuit, and the electrical signal or voltage measured across the resistive wire generates an electrical output signal from the anemometer bridge which bears a specific and predictable, but non-linear, relationship to the mass flow of gas or air through the conduit with which the anemometer is operating. The output signal is dependent upon the molecular weight of the gas or air being sensed. The output signal of the air mass sensor 38 and the output bridge signal of the gas mass sensor 42, after appropriate amplification, are supplied to controller 66. The output signal of the air mass sensor is dependent upon the air mass flow rate established by the particular air intake of the internal combustion engine. The amount of air sucked in by the engine varies significantly between idle and high power operation. It is desirable to regulate the flow of the gaseous fuel, such as natural gas, to achieve a stoichiometric ratio of gaseous fuel to air in order to achieve optimum performance. Furthermore, this ratio should be maintained substantially constant over the full operating range of the engine, thus requiring the gas mass flow rate of the gaseous fuel to be varied depending upon sensed changes in the air mass flow rate. This is achieved by the controller 66. As shown in Figure 1, the output signal of the gas mass sensor 42 is connected to a signal conditioner 85. The output of the signal conditioner 85 is connected to the input of an inverting amplifier 86. The output of the inverting amplifier 86 is connected to a variable resistor 82 through a resistor 88. In the same manner, the output of air mass sensor 38 is also connected to the variable resistor 82 through signal conditioner 84 and resistor 92. The wiper arm of the variable resistor 82 is connected to the non-inverting input of an operation amplifier 90. The variable resistor 82 allows for periodic fine tuning mixture adjustment which can be done manually during normal maintenance. The inverting terminal of the operational amplifier 90 is connected to ground through a resistor 94. Effectively, the output signal of the air mass sensor 38 and the inverted output signal of the gas mass sensor 42 are summed at the non-inverting terminal of the operational amplifier 90, and the summation is compared to the signal on the inverting terminal. The output of the operational amplifier 90 is connected to servo valve 44 and provides a signal to adjustably control the servo valve and thereby the flow rate of natural gas to the natural gas conduit 40 to maintain the desired stoichiometric ratio of air and natural gas. With an automotive engine, the air mass flow typically varies from a low of 18,144 kg, (40 pounds) per hour at idle to a high of 771,12 kg (1,700 pounds) per hour at high speed operation. Natural gas is much less dense than air, and it is desirable to maintain a 17-to-1 ratio of air to natural gas for stoichiometric operation. In other words, it is desirable when the engine only requires 77,112 kg (170 pounds) per hour of air to provide 4,563 kg (10 pounds) per hour of natural gas, and when the engine requires 771,12 kg (1,700 pounds) per hour of air to provide 45,63 kg (100 pounds) per hour of natural gas. The particular ratio desired is preferably the one which produces a mix of air and natural gas that provides the desired operating power. It is also desirable to utilise identical hot wire anemometers for the air mass sensor 38 and the gas mass sensor 42. Unfortunately, as previously noted, the output signal from a hot wire anemometer bears a non-linear relationship to the mass flow being measured. As such, the two mass sensors will be operating at different points along their output response curves since the mass flow rates vary by about 17-to-1. Also, changes in the air output signal if used directly will not produce proportional changes to the gas mass flow. This presents a problem when the output signals of the air mass sensor and gas mass sensor are being compared by the controller 66. For example, the proportional changes in the air mass and gas mass flows needed to maintain the predetermined flow ratio will not produce correspondingly proportional changes in the output signals of the air mass and gas mass sensors due to their non-linear response characteristics. The first and second conduits 34, 40 have interior flow, cross-sectional areas sized relative to each other to correspond approximately to said predetermined ratio, e.g. 17-to-1, of the mass flow rates of the air and the natural gas. Consequently, the output signals of both mass sensors 38, 42 are scaled by the choice of said cross-sectional areas. The electronic controller 66 processes the output signals of both anemometers, thus allowing the two signals to be accurately compared and a precise ratio of the two gases to be maintained. The servo valve 44 (Figures 4 and 5) has a body 98 with an interior passageway 100. In the passageway is positioned a butterfly valve 102 mounted on a rotatable stem 104. The stem is connected to a permanent magnet linear servo motor 106. The operational amplifier 90 provides the drive to the servo motor 106 to rotate the valve stem 104 and change the position of the butterfly 102 within the body to increase or decrease the flow of natural gas therethrough. A return spring 108 serves to return the butterfly 102 to the closed position when the servo motor 106 is deactivated.

Claims: A control system for controlling the relative mass flow rates of two gases, comprising: a first conduit (34) for the flow of a first gas, the flow being variable over a range of mass flow rates; a first gas mass sensor (38) positioned to sense the mass flow rate of said first gas in said first conduit (34), and to generate a first flow rate signal indicating the mass flow rate of said first gas; a second conduit (40) for the flow of a second gas, the flow being adjustable over a range of mass flow rates; a second gas mass sensor (42) positioned to sense the mass flow rate of said second gas in said second conduit (40), and to generate a second flow rate signal indicating the mass flow rate of said second gas; an electronic controller (66) for comparing said first and second flow rate signals, and generating a control signal if said first and second flow rate signals vary from a predetermined ratio; and a valve (44) adjustably controlling the flow rate of said second gas in response to said control signal to maintain said predetermined ratio, whereby the flow rate of said second gas is controlled responsive to variations in the flow rate of said first gas to maintain the mass flow rates of said first and second gases in the desired ratio, characterised in that said first and second conduits (34, 40) have interior flow cross-sectional areas sized relative to each other to correspond approximately to said predetermined ratio of the mass flow rates of said first and second gases, and that said first and second gas mass sensors (38, 42) have a substantially identical non-linear output response relative to the mass flow being sensed, whereby said first and second conduits (34, 40) have at the points of flow mass measurement interior flow cross-sectional areas sized to provide approximately equal output responses of said first and second gas mass sensors (38, 42) when the mass flow rates of said first and second gases are in said predetermined ratio. so that first and second flow rate signals will remain comparable during operation over a wide range of flow rates without introducing unacceptable error. A control system according to claim 1 characterised in that said first and second gas mass sensors (38, 42) are first and second hot wire anemometers. A control system according to one of claims 1 or 2, characterised in that it is used as air gaseous fuel control system for an internal combustion engine. A control system according to at least one of claims 1 to 3, characterised in that it is used as air/fuel control system for an internal combustion engine of a driven vehicle, the internal combustion engine of which is convertible during operation between liquid gasoline and gaseous fuel as alternative sources of power.

Inventors: MILLER KENNETH E, SMITH DAVID H, MILLER, KENNETH E., SMITH, DAVID H.
Applicants: AIR SENSORS INC, AIR SENSORS, INC.
Application year: 1986
Main classification: F02D19
IPC codes: G05D11, F02B43, F23N1, F02M21, F02D19