CA1276263C - System for controlling mass flow rates of two gases - Google Patents

Abstract

SYSTEM FOR CONTROLLING MASS FLOW RATES OF TWO GASES
Abstract of the Disclosure 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 indicat-ing the mass flow rate. A natural gas mass sensor is position-ed 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 subs-tantially 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 preselect-ed 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 opera-tion of the vehicle. A throttle body has a natural gas distri-bution ring in the throat of the body and provides a curtain flow of gas.

Description

De~criPtion SYS~EM FOR CONTROLLING MASS FLOW RATES OF TWO GASES

Technical Field The present invention relates to ~ ~y6tem for controlling the ma~s flow rates of two gases.

Backqround of the Invention In many situations, there iB a need to control the mass flow ratio of two gaseQ to maintain them in A pre-determined ratio. This need arises in operation of ga6eous fuel internal combustion engines, boilers and other indus-trial applications. There are other situations in which control of two gases is nece~ary, ~uch as ventilating greenhouses, in which it is desired to have a certain mix-ture of air and carbon dioxide. For internal combustion engine~, such as automobile engines operating on nntural gas, a stoichiometric ratio of the gaseous fuel and air is de~irable. Furthermore, it iB de~irable to achieve accur-ate control of the flow to provide the predetermined ratio by a method which iB not subject to error associated with pre~ure and temperature variations.
With an automobile internal combustion engine which iB operating on natural gas, the speed at which the engine is operating establishes an air flow into the engine for purposes of combustion. Similarly, the operating point of a boiler is determined by the blower used as the source of combustion air. Since in these applications the flow of air to the engine or boiler is relatively fixed for any particular operating condition of the engine, but varies significantly as the operating condition changes, it become6 neces~ary to regulate the flow of gaseous fuel to obtain optimum performance.

`~ , i' It will therefore be appreciated that there ha~
been a ~ignific~nt need for a control ~y~tem which 8u~tably regulate~ the m~ss f low r~te~ of two gase~ to maintain them in o predetermined ratio. For internal combustion engine~, boilers and the like, the flow rate of ga~eouo fuel ~u~t be respon6ive to the flow rate of the combustion air. The ~y~tem ~hould be relatively accurate ond reliable, and ~hould allow optimum performance of the engine or boiler, yet be inexpensive to manufacture.
When u9ed as a control sy~tem for a vehicle internal combustion engine where the engine i~ convertible during operation between natural ga~ and liquid ga~oline as alternative ~ources of power, the control 8y~tem ~hould provide means for a quick changeover from natural gas to ga~oline operation when the pre~ure of the natural gas fall~ below a predetermined level. In the past, vehicles ~et up to operate alternatively on both sources of fuel have experienced problems when ~witching from gasoline to natural ga~. The ~upply of gasoline in the float bowl had to be exhau~ted before the ~witch to natural ga~ could be made ~o that the engine would not be operating on both ga~oline and natural gas. ~hen ~witching from natural gas - to gasoline operation, such a~ when the pre~ure of the natural ga~ wa~ in~ufficient, it wa~ then nece~ary to terminate the flow of natural ga~ and wait for the gasoline float bowl to fill up again. This was often accompli~hed by coa~ting in gear or turning the starter motor to pump gasoline into the float bowl.- When an electric fuel pump was u~ed, the filling of the float bowl would occur auto-matically, but there wa6 a delay before a supply of gaso-line was present to commence operation of the engine on gasoline.
The present invention fulfills theqe needs and further provide~ other related advantages.

~276263 Di~cloi3ure of the Invention .
The preient invention rei3ides in a control i~y~tem for controlling the relative mass flow rate~ of two gai~ei~.
The oy~tem includes a fir~t conduit for the flow of a fiei~t gas, and a firot gas maso oenoor positioned to ~en~e the mao~ flow rate of the first gas in the first conduit. The flow of the first gas iB variable over ~ range of maos flow rates, and the first gas maos oenoor generates a first flow rate ~ignal indicating the maos flow rate of the firot gas.
The ~y~tem ali30 includes a ~econd conduit for the flow of a ~econd ga~, and a ~econd gao mass ~en~or positioned to i~en~e the mass flow rate of the i~econd ga6 in the i~econd conduit. The flow of the second gas i8 adjustable over a range of ma~s flow rates, and the ~econd gas ma6s sen60r generates a oecond flow rate signal indicating the mass flow rate of the second gas. The syotem hao an electronic controller for comparing the fir~t and oecond flow rate signal~, and generating a control ~ignal if the fir6t and second flow rate signals vary from a predetermined ratio.
A valve adjustably control~ the flow rate of the ~econd gas in re~pon~e to the control signal to maintain a predeter-mined ratio. AB ~uch, the flow rate of the second gas is controlled re~pon~ive to variations in the flow rate of the fir~t ga~ to maintain the mass flow rateo of the fir~t and ~econd ga~eo in the de~ired ratio.
In the preferred embodiment of the invention, the fir~t and ~econd conduit6 have interior flow cross-~ectional areas sized relative to each other to approximate-ly corre~pond to the predetermined ratio of the mass flowrates of the first and second gases. The first and second ga6 mass sensor6 have a substantially identical non-linear output response relative to the mass flow being senoed. As such, the output re~ponse of the fir~t and ~econd gas mas6 ~en~or~ will be generally ~caled by the choice of cro~ ectional areas for the firi~t and second conduits.
Even with the output re~pon~e of the fir~t and ~econd gas i ~276263 ma~ ~en~or~ being non-linear, the firct and cecond flow rate ~ignals will remain comp~rable during operation over a wide range of flow rates without introducing unacceptable error.
The present invention also includes a method for controlling the relative mass flow rates of the two gases.
In one embodiment of the invention, the control ~y~tem i~ for ân internal combu~tion engine driven vehicle convertible during operation between liquid gasoline and gaseous fuel as alternative sourceg of power. The first conduit conduct~ intake air to the engine and a ~econd con-duit conducts gaseous fuel to the engine for mixture with the air. The gaseou~ fuel is ~upplied from a pressurized cource and a ~en~or ~en~e~ the pressure of the ga~eous fuel in the eource. A ~hutoff valve iB re~pon~ive to the ~en~or for terminating the flow of the gaseous fuel in the ~econd conduit in re~pon~e to the pres~ure of the source falling below a predetermined pressure.
Al~o provided is means for contolling the flow of gasoline into the engine. The control means i~ responsive to the pre~sure sen~or and allows the immediate flow of ga~oline to the engine in re~pon~e to the pre~ure of the ga~eouo fuel in the ~ource falling below the predetermined pre~ure. As auch, when a low ga~eou~ fuel pre~ure i~
realized, the control sy~tem switche~ to gasoline operation without delay and while avoiding supplying the engine with a mixture of both ga~oline and gaseous fuel.
Other features and advantages of the invention will become apparent in the following detailed description, taken in conjunction with the accompanying drawings.

Brief Descri~tion of the Drawings Figure 1 is a ~chematic diagram of the control ~y~tem of the present invention.

2~6263 Figure 2 i~ an ieometric view of ~ flow conduit and a hot wire anemometer mounted thereto of the type u~ed with the pre~ent invention.
Figure 3 iB a eectional view taken oubetantiolly along line~ 3-3 of Figure 2.
Figure 4 iB an icometric view of a ~ervo valve u~ed with the pre~ent invention.
Figure 5 i8 a ~ectional view taken sub~tantially 1~ along the line~ 5-5 of Figure 4.
Figure 6 i~ a fragmentary ~ectional view of the butterfly u~ed in the ~ervo valve of Figures 4 and 5, taken oubotantially along the line~ 6-6 of Figure 5.
Figùre 7 iB a fragmentary sectional view of a throttle body u~ed with the control oystem of the present invention.

Be~t Mode for Carrvinq Out the Invention AB ~hown in the drawings for purpose~ of illustra-tion, the pre~ent invention iB embodied in a control sy~-tem, indicated generally by reference numeral 10, for con-trolling the relative ma~ flow rate~ of two ga~es. The invention moy be u~ed wherever it i~ nece~ary to control the relative ~a-~ flow rate~ of two ga~e~, including venti-lation of greenhou~e~, which require a certain mixture ofair and carbon dioxide, and in the operation of ga~eou~
fuel internal combu~tion engine~ and boilers. The inven-tion i~ de~cribed herein for use with an internal com-bu~tion engine for a driven vehicle convertible during operation between liquid gas and ga~eous fuel, such as natural gas, as alternative sources of power.
The control system 10 is ~hown schematically in Figure 1 operating with an engine for an automobile. The automobile include~ a conventional tank 12 for liquid gaso-line and a ga~oline filter 14 through which the ga~oline pa~e~ to an electric fuel pump 16. A conventional pre~-cure regulator 18 control~ the pre~ure of the gasoline ~; 3L276263 delivered by a fuel line 20 from the pressure regulator to one or more injectors 22. The injector is mounted to inject gasoline into a throttle body 24 which may be mounted on the intake manifold 25 of the engine in place of the original equipment carburetor. The throttle body includes conventional butterfly valves positioned below the point at which the injector injeots fuel into the throttle body. A return fuel line 26 is provided between the pressure regulator 18 and the gasoline tank 12 for diverting excess fuel bac~ to the tank. In the preferred embodiment of the invention, the pressure regulator maintains a gasoline pressure of 40 psi.
Mounted on top of the throttle body 24 is a throttle body cover 30 removable attached to the throttle body by a wing nut and stem arrangement 32. An air conduit 34 communicates ambient air with the throttle body cover 30 to meet the volume of air demanded by the engine during operation for combustion purposes. The air conduit has an air filter 36 mounted at its end which is open to the ambient air and filters any particulate matter from the air flow passing into and through the conduit.
An air mass sensor 38 is positioned to sense the mass flow rate of the air in the air conduit 34, and to generate an air flow rate signal indicating the mass flow rate of the air in the conduit.
In much the same matter, a natural gas conduit 40 communicates natural gas to the throttle body cover 30 for mixture with the air in preparation for combustion when the engine is operating on natural gas rather than liquid gasoline.

~' 6A ~LZ76263 It is important to mix the natural gas completely and in the same proportions for each cyl inder 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 the throttle body 24 first by splitting of '~`
., ~ ,.

~27626 the air ~nd gas flow~ în the throttle body equally into two flows through the throttle body throats 28A ~nd 28B- The natural g~s conduit 40 communicates with a central throttle body caYity 41 from which the gas i~ distributed evenly about the perimeter of the two throttle body throats using di~tribution rings 43A and 43s positioned in the throttle body throat~ 28A and 28B, respectively, at the venturi point for the throat and above the butterfly valve 27A and 10 27B for the throat. The distribution rings have the shape of a one-quarter toroid and define circumferential distribu-tion 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 spPce between the edge of the distribution ring and a corre~p~nding edge of the throttle body wall. These slots 47A snd 47B extend circumferential-ly about the throttle body throats 28A and 28B, respective-ly, and distribute the natural gas equally around the throats and form a curtain flow downward in the throats.
Due to the positioning of the distribution ring at the vent~ri 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 10w reaches the area of the throat with the butterfly valves 27A and 27B, the hish turbulence encountered there causes the natural gas and air to completely mix, avoiding previously encount-ered difficulties in achieving mixing of the differentdensity air and natural gas. The described mixing system is adaptable t~ different engine sizes, requiring between one a-nd four throttle body throats, by providing a distribu-tion ring and venturi-slot in each throat.
As previously described, the throttle body 24 has the throttle body cover 30 positioned atop the body and the air conduit 34 communicates with the throttle body cover.
~::
: .

~ ~2~6~63 As shown in Figure 7, the air enters the throttle body from above through a circ,umferential 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 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 rin~s 43A and 43B and inject the gasoline directly into the throttle body throats 28A and 28B
above the butterfly valves 27A and 27B.
A natural sas 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 of the natural gas in the conduit. As will be described in more detail below, 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 the gas conduit 40.
The natural gas is supplied to the servo valve 44 in a conventional manner from a pressurized 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 in the flow of natural gas between the pressure sensor and the solenoid shutoff valve is a pressure regulator 54 to regulate line pressure. The natural gas is piped from the solenoid shutoff valve through a '~"
;, ~276263 ~A
diaphragm pressure servo 56 and then to the input of the servo valve 44. As previously noted, the output of the solenoid valve supplies the natural gas to the gas conduit 40.

~., ~L276263 A control panel 58 i~ provided with o toggle ~witch 60 to allow an operator to manually ~elect between operation on natural ga~ or liquid ga~oline as alternative fuels. An indicator lamp 62 iB provided to indicate when 5 the vehicle i6 being operated on natural gas, and an indica-tor lamp 64 is provided to indicate when the engine iB
operating on ga~oline. The control ~y~tem 10 of the pre~ent invention further includes an electronic controller 10 66, indicated by the phantom line box in Figure 1. The controller receives signals from the pressure sen~or 50 and in respon6e thereto sends control signal~ to the ~olenoid shutoff valve 52. The controller also operates in conjunc-tion with the control panel 58 to select the mode of opera-15 tion. As will be de~cribed below, the controller controls operation of the ~ervo valve 44.
When the control ~ystem 10 of the pre~ent applica-tion i~ u~ed to control gases for purpoQes of combustion, it iB desirable to provide a stoichiometric ratio of air 20 and gaseous fuel such as natural gas. To achieve an accurate flow rate of the gases, it iB desirable to measure the mas~ flow rate by a method which iB not ~ubject to ,~
rror- a~-ociated with pre~ure and temperature variation.
With the present invention, thi~ i~ accompli-hed by ~ea~ur-25 ing the ma-s flow rate of the gase~ utilizing hot wire anemometers 67 of the type shown in Figures 2 and 3. An t anemometer of this type i~ described in U.S. Patent No.
4,523,461, which i~ incorporated by reference herein. By ~en~ing mass flow, the measurement is not ~ensitive to 30 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. An end portion of the po~ts between which the resistive wire extends is 35 positioned within the conduit through which the air or gas being eensed pas~e~. For purpo~es of illustration, the conduit will be described in Figure~ 2 and 3 a~ the air iL276263 conduit 34; however, the arrangement is the same for the gas conduit 40 except for size, as will be explained below.

The opposite end portion of the posts extend through an opening 71 in the conduit wall 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. The circuit board is attached to the support member, and both are rigidly mounted on the exterior of the conduit.
A block of closed-cell foam 76 is positioned over the opening 71 and held in place between the circuit board 74 and support member 72 and th conduit 34 to prevent any air from exiting or entering the conduit through the opening around the posts. The block is compressed to seal the opening, but is provided with cuts to receive the posts. A cover 78, shown in phantom, is provided to cover the electronic circuitry mounted on the circuit board. Upstream from the hot wire anemometer 67 is positioned within the conduit a honeycomb arrangement of elongated cells 80 which assist in providing laminar flow of the air in the conduit at the resistive wire 68 of the anemometer.
In a conventional manner, the resistive wire 68 is one branch in a bridge circuit of the anemometer, 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 ;~

~276263 lOA
anemometer i5 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 the controller 66. The output signal of the air mass sensor is dependent upon the air mass flow rate established by the particular ~276263 air intake of the internal combustion engine for the ~peed at which it is operating, or when u~ed with other type~ of combuotion, by the combustion air ~ource. With a boiler, the air blower would determine the air mass flow rate. Of cour~e, as the ~peed of the engine or the blower varies, the mass flow rate of the air varies. In the case of an internal combustion engine, the amount of air sucked in by the engine varies significantly between idle and high power 10 operation.
In view of this, it iB desirable to regulate the flow of the ga~eous fuel, ~uch as natural ga~, to achieve a ~toichiometric ratio of ga~eous 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 ~en~ed change~ in the air mass flow rate.
This iB achieved by the controller 66. As shown in Figure 1, the output 6ignal of the gas mass sen~or 42 is connected to a ~ignal conditioner 85. The output of the ~ignal conditioner 85 i8 connected to the input of an inverting amplifier 86. The output of the inverting ampli-fier 86 i~ connected to a variable resistor 82 through a re~i~tor 88. In the ~ame manner, the output of air ma~
~en~or 38 i~ al~o connected to the variable re~i~tor 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 resi~tor 82 allows for periodic fine tuning mixture adjust-ment 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 ~en~or 38 and the inverted output ~ignal of the gas mass ~en~or 42 are ~ummed at the non-inverting terminal of the ~276263 operation~l amplifier 90, and the ~ummotion i~ compared to the signal on the inverting terminal.
The output of the operational amplifier 90 i~
connected to the servo valve 44 and provides a ~ignal to adjustably control the electrically activated ~ervo valve and thereby the flow rate of natural gas to the natural gas conduit 40 to maintain the desired stoichiometric rotio of air and natural gas. As previou~ly noted, the volume or flow of gas allowed by the servo valve is dependent upon the air mass flow for the speed at which the engine iB
operating. With an automotive engine, the air mass flow typically varies from a low of 40 pounds per hour at idle to a high of 1,700 pounds per hour at high ~peed operation.
The controller 66 provides a means for controlling the flow rate of the natural gas in response to these variations in air flow rate.
Natural gas is much le~s dense than air, and it iB desirable to maintain a 17-to-1 ratio of air to natural ga~ for stoichiometric operation. In other words, it is desirable when the engine only requires 170 pounds per hour of air to provide 10 pounds per hour of natural gas, and when the engine require~ 1700 pounds per hour of air to provide 100 pounds per hour of natural gas. The particular ratio de~ired i~ preferably the one which produces a mix of air and natural gas that provide~ the desired operating power.
It iB alBo desirable to utilize identical hot wire anemometers for the air mass sensor 38 and the gas ma~s ~ensor 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 ~ensors will be operating at different points along their output response curves since the mass flow rate~ ~ary by about 17-to-1. Al~o, changes in the air output ~ignal if used directly will not produce proportional changes to the gas ma~s flow. Thi~ presents a ~j , .
i 13 ~2 7 62 63 problem when the output ~ignal~ of the air ma~ ~en~or and ga~ mass ~en~or are being compared by the controller 66 For example, the proportional changes in the air mass and gas ma~s flow~ needed to maintain the predetermined flow ratio will not produce correspondingly proportional changes in the output ~ignals of the air mass and gas mass ~en~ors due to their non-linear response characteri~tic~. One ~olution i8 to linearize the output 6ignals of the ~ensors In the present embodiment of the invention, the electronic controller 66 provides the means for linearizing the output ~ignal~ of both ~nemometers, thus allowing the two signals to be accurately compared and a precise ratio of the two gase~ to be maintained.
One embodiment of the servo valve 44 which i9 u~able with the pre~ent invention i6 shown in Figures 4 and 5. The servo valve has a body 98 with an interior passage-way 100. In the pas~ageway i8 positioned a butterfly valve 102 mounted on a rotatable ~tem 104. The stem extends through the body and is connected to a permanent magnet linear ~ervo motor 106 of conventional de~ign mounted on top of the valve body. The operational amplifier 90 pro-vide~ the drive to the ~ervo motor to rotate the valve stem 104 and change the po~ition of the butterfly 102 within the body to increase or decrease the flow of natural ga6 there-through. A return ~pring 108 has one end fixed to the ca~e of the ~ervo motor and the other end attached to the valve ~tem for rotation of the valve ~tem to bias and return the butterfly to the clo~ed position when the ~ervo motor i6 deactivated.
It will be appreciated that, although ~pecific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without departing from the spirit and ~cope of the inven-tion. Accordingly, the invention is not limited except bythe appended claim~.

Claims (4)

1. A control system for controlling the relative mass flow rates of two gases without appreciably altering the temperature of the gases, comprising:
a first conduit for the flow of a first gas comprising air, the flow being variable over a range of mass flow rates;
a first hot wire anemometer having a hot wire operating at a substantially constant temperature or current without appreciably altering the temperature of said first gas and being substantially insensitive to pressure and temperature variations of said first gas, said first hot wire anemometer being positioned to directly sense the mass flow rate of said first gas in said first conduit when in a gaseous state, and to generate a first flow rate signal indicating the mass flow rate of said first gas;
a second conduit for the flow of a second gas comprising a combustible fuel in a gaseous state, said second gas differing in composition from said first gas, the flow being adjustable over a range of mass flow rates;
a second hot wire anemometer having a hot wire operating at a substantially constant temperature without appreciably altering the temperature of said second gas and being substantially insensitive to pressure and temperature variations of said second gas, said second hot wire anemometer being positioned to directly sense the mass flow rate of said second gas in said second conduit when in a gaseous state, and to generate a second flow rate signal indicating the mass flow rate of said second gas, said first and second hot wire anemometers having a substantially identical nonlinear output characteristic relative to the mass flow being sensed;
an electronic controller 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, said first and second conduits having 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, whereby the output characteristic of said first and second hot wire anemometers will be generally scaled by the choice of cross-sectional areas for said first and second conduits such that both said first and second hot wire anemometers operate in substantially the same region of their nonlinear output characteristic curve, and even though the output characteristic of said first and second hot wire anemometers is nonlinear, said first and second flow rate signals will remain comparable during operation over a wide range of flow rates without introducing unacceptable error; and a valve positioned in said second conduit and adjustably controlling the flow rate of said second gas when in a gaseous state 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.

2. The control system of claim 1 for an internal-combustion engine convertible during operation between liquid gasoline and gaseous fuel as alternative sources of power, wherein the first conduit is an air induction conduit leading to the intake of the engine for the flow of air thereto, the flow of said air being variable over a range of mass flow rates during operation of the engine, and wherein the second conduit is a fuel conduit, the control system further including:
a pressurized source supplying said gaseous fuel to said fuel conduit, said fuel conduit leading from said source to the intake of the engine for the flow of said gaseous fuel thereto and mixture with said air;
a pressure sensor for determining the pressure of said gaseous fuel in said source a shutoff valve responsive to said pressure sensor for terminating the flow of said gaseous fuel in said fuel conduit in response to the pressure of said source falling below a predetermined pressure; and means for controlling the flow of gasoline to the engine, said control means being responsive to said pressure sensor and allowing the immediate flow of gasoline to the engine in response to the pressure of said source falling below said predetermined pressure, whereby when a low gaseous fuel pressure is realized, the control system switches to gasoline operation without delay and while avoiding supplying the engine with a mixture of both gasoline and gaseous fuel.

3. The control system of claim 2 in which the engine has an electric fuel pump for said gasoline and at least one gasoline injector receiving fuel from said fuel pump without requiring the use of a fuel bowl, wherein said control means for controlling the flow of gasoline to the engine includes switch means responsive to said pressure sensor to switch on the electric fuel pump and provide pressurized gasoline to the injector.

4. A method for controlling the relative mass flow rates of two gases, comprising:
providing a first conduit for the flow of a first gas comprising air, the flow being variable over a range of mass flow rates;
providing a first hot wire anemometer operating at a substantially constant temperature without appreciably altering the temperature of said first gas, said first hot wire anemometer being positioned to directly sense the mass flow rate of said first gas in said first conduit;
sensing the mass flow rate of said first gas in said first conduit with said first hot wire anemometer and generating a first flow rate signal indicating the mass flow rate of said first gas;

providing a second conduit for the flow of a second gas comprising a combustible gaseous fuel, the flow being adjustable over a range of mass flow rates;
providing a second hot wire anemometer operating at a substantially constant temperature without appreciably altering the temperature of said second gas, said second hot wire anemometer being positioned to directly sense the mass flow rate of said second gas in said second conduit, said first and second hot wire anemometers having a substantially identical nonlinear output characteristic relative to the mass flow being sensed;
sensing the mass flow rate of said second gas in said second conduit with said second hot wire anemometer and generating a second flow rate signal indicating the mass flow rate of said second gas;
selecting a predetermined ratio for the mass flow rates of said first and second gases;
providing said first and second conduits with 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, whereby the output characteristic of said first and second hot wire anemometers will be generally scaled by the choice of cross-sectional areas for said first and second conduits such that both said first and second hot wire anemometers operate in substantially the same region of their nonlinear output characteristic curves, and even though the output characteristic of said first and second hot wire anemometers is nonlinear, said first and second flow rate signals will remain comparable during operation over a wide range of flow rates without introducing unacceptable error;
comparing said first and second flow rate signals, and generating a control signal if said first and second flow rate signals indicate variation in the mass flow rates of said first and second gases from said predetermined ratio;
providing a valve positioned in said second conduit and adjustable for controlling the flow rate of said second gas; and controlling said valve in response to said control signal to maintain said predetermined ratio of mass flow rates, 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.

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