Abstract:
The air/fuel mixture ratio supplied to an internal combustion engine of a vehicle is modified to achieve a constant mass flow rate in spite of changes in atmospheric temperature and pressure conditions by employing an electronic compensation system. The system has sensors which detect air temperature and barometric pressure, from which signals are developed controlling the float bowl pressure in the engine carburettors, thus modifying the air/fuel mixture ratio as desired. The system also includes provision for enriching the fuel content of the mixture supplied to the engine to provide an oversupply of fuel in cold start situations.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     This invention relates to a new or improved fuel supply system for an internal combustion engine, to a manifold specifically designed for use in such system, to a kit of parts to enable retrofitting of such system in an existing engine and to a method of controlling the fuel supply to achieve the improved response to a number of environmental conditions. 
     B. Description of the Prior Art 
     In internal combustion engines having carburetor controlled fuel supplies, as is typical of engines used in vehicles such as snowmobiles and personal watercraft, it is well known that the rate of fuel flow in a fixed or variable venturi carburetor is dependent upon the pressure differential existing in the fuel system between the venturi and e.g. a fuel bowl (otherwise called a float bowl or a float chamber). In a conventional float bowl carburetor the pressure differential is measured between the pressure in the fluid float chamber (which is normally atmospheric pressure) and the pressure at the discharge orifice of the fuel metering system which is normally located in or adjacent the venturi in the induction passage. 
     For optimum combustion, the relationship between the mass air flow and the mass fuel flow delivered to the engine by the carburetor should be kept constant, and to achieve this the carburetor employs either a fixed or a variable venturi (or some equivalent structure) such that when air velocity in the induction passage is increased a pressure reduction (often called a vacuum) is created in the venturi zone. This pressure reduction creates a pressure differential between the induction passage and the fuel in the float chamber, causing fuel to be drawn into the induction passage at a flow rate that is proportional to the pressure differential. 
     The amount or level of the venturi underpressure or vacuum is mainly a function of air velocity through the induction passage, but as is well understood, at a given velocity, the mass air flow rate is affected by air density which in turn is mainly a function of barometric pressure and air temperature. 
     For example for a snowmobile operating at an altitude of 2000 meters, a given air velocity in the carburetor induction passage will deliver a very much reduced mass air flow to the engine as compared to the same air velocity when the snowmobile in operating at seal level, this being due to the reduced barometric pressure and air density at altitude. However since fuel flow is mostly a function of the venturi underpressure or vacuum, the engine when operating at altitude would tend to be supplied with a mixture that is over rich in fuel. This phenomenon is well understood. For example U.S. Pat. No. 5,021,198 Bostelmann discloses a carburetor system that is designed to adjust the fuel flow to maintain the mass air fuel mixture ratio constant despite changes in altitude. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a fuel supply control system and method which without the use of a choke or the like is adapted to adjust the air fuel mass flow ratio to provide a fuel enriched mixture in certain situations, e.g. for starting a cold engine. 
     The invention provides a method for modifying the air/fuel mixture ratio supplied to an internal combustion engine of a vehicle to achieve a constant mass flow ratio in spite of changes in atmospheric temperature conditions, said fuel being drawn from a float chamber into a venturi in a carburetor, wherein it is mixed with air before being delivered into the engine, said method comprising: (a) sensing the atmospheric temperature in the vicinity of said vehicle and generating a signal indicative of said sensed temperature; (b) supplying said signal to a control unit; (c) operating said control unit to modify pressure within said float chamber thus varying the pressure differential between the venturi and said float chamber so that the mass flow ratio of said mixture remains substantially constant. 
     The engine preferably also includes an air pressure sensor and an engine temperature sensor both of which feed signals to the electronic control unit which signals are also used in modifying the fuel/air ratio of the mixture. 
     From another aspect the invention provides a method for reducing the air/fuel mixture ratio supplied to an internal combustion engine in cold start situations, said fuel being drawn from a float chamber into a venturi in a carburetor where it is mixed with air and delivered into the engine, said method comprising: (a) sensing the temperature of the engine and generating a signal when said temperature is below a normal operating temperature range; (b) supplying said signal to a control unit; (c) operating said control unit to elevate the pressure within said float chamber to increase fuel flow into the venturi and thus increase the fuel content of said mixture during periods when said signal is received. 
     The engine crankcase chamber is subject to pressure fluctuations during operation of the engine, and this chamber can be utilized as the pressure generator by including a line communicating the crankcase chamber to the control unit. At low speeds of rotation of the engine corresponding to cranking thereof this line will provide a sufficient flow of pressurized air. However at higher engine speeds and throttle openings the pressure will be insufficient so that an external pump may be required. Preferably such pump is a mechanical pump constructed to be driven by pressure pulse in the crankcase chamber. The pump is provided for delivering the flow of pressurized air at higher speeds of operation of the engine, i.e. at speeds of idling and above. Alternatively, the pressure generator may be a separate pump, for example electrically driven from a vehicle battery. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will further be described, by way of example only, with reference to the accompanying drawings wherein: 
     FIG. 1 is a schematic view of a first portion of a fuel supply control system in a snowmobile engine; 
     FIG. 2 is a graph showing the carburetor float chamber pressure as it varies with operating conditions; 
     FIG. 3 is a schematic view showing a second part of the fuel supply control system; 
     FIG. 4 is a schematic view of the overall fuel control system of a three cylinder two-stroke engine; 
     FIGS. 5 a  and  5   b  are perspective views showing two states of a manifold arrangement as included in the FIG. 4 embodiment of the fuel supply control system; and 
     FIG. 5 c  is a perspective view from the opposite side showing a portion of the manifold of FIGS. 5 a  and  5   b.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fuel flow control system incorporates an electronic control unit which is coupled to receive inputs from a series of sensors and provide output signals to control the fuel flow from the carburetor or carburetors. The invention as described concerns a fuel supply system in a snowmobile engine, but obviously is susceptible of many other applications. 
     Referring to FIG. 1, an electronic control unit (ECU)  10  is connected to receive input signals from a barometric pressure sensor  11  and an air temperature sensor  12 , these sensors being mounted at locations on the snowmobile where they are exposed to atmospheric conditions. The signals from the sensors  11  and  12  are processed by the ECU which produce an output signal that is sent to a solenoid  13  by means of which the fuel flow from a carburetor  14  is adjusted to compensate for air density at the location where a snowmobile is operating. As mentioned above, air density is mainly a function both of barometric pressure and of air temperature, and by measuring these parameters by means of the sensors  11  and  12  the ECU produces an output signal which modifies fuel flow to the snowmobile engine to compensate for changes in the measured parameters. 
     The engine has a carburetor  14  that is of a well known type having an induction passage  15  controlled by a spring loaded sliding piston  16  which carries a needle  17  slidably inserted in a fuel orifice  18  connected to draw fuel from a float chamber  19 . The induction passage comprises a venturi which creates an underpressure or vacuum in the air flowing therethrough, the pressure differential between the venturi and the float chamber  19  resulting in fuel being drawn into the induction passage through the orifice  18  and thereafter delivered to the engine in mixture with the air flow. 
     The solenoid valve  13  is designed to create a controlled reduction of the pressure in the float chamber so that the flow of fuel from the orifice  18  is modified in accordance with the atmospheric air density with the result that the mass air/fuel flow ratio is held substantially constant. 
     The solenoid valve has a valve closure  21  mounted in a manifold chamber  22  to which is coupled a first conduit  23  which is in communication with the venturi of the induction passage  15  adjacent the orifice  18 , and a second conduit  24  which is in communication with the carburetor float chamber  19 , this second conduit including an atmospheric vent  25 . 
     In operation, the above described system acts to compensate for the mass air flow diminution (which occurs when the snowmobile is operating at high altitudes) by reducing the pressure within the float chamber  19  which in turn reduces the pressure differential acting on the fuel thus reducing the fuel flow. To achieve the necessary reduction in pressure, the system utilizes the underpressure or vacuum in the induction passage venturi and applies this through the conduit  23 , the manifold  22 , and the conduit  24  to the float chamber. The extent to which the float chamber is exposed to this underpressure is determined by the solenoid valve  23  the closure  21  of which cooperates with the end of the conduit  13  to open this to a greater or lesser extent in accordance with the prevailing atmospheric conditions. For example the ECU  10  would be calibrated so that at some standard condition of temperature and barometric pressure, the closure  21  would completely seal the conduit  23  so that the float chamber would be exposed to only atmospheric pressure via the vent  25  and the conduit  24 . 
     By arranging that the conduit  23  opens into the induction passage  15  at a location very close to the fuel orifice  18  it is ensured that the compensation is essentially linear at any throttle opening condition, as illustrated in FIG. 2 which shows the float chamber pressure as a percentage of the pressure at the fuel orifice  18  throughout the duty cycle activation of the solenoid valve  13 . In other words the float chamber pressure is directly related to the underpressure or vacuum around the discharge fuel orifice  18  in the induction passage. 
     Thus if the snowmobile is operating at high altitude, the ECU will respond to the signals received from the sensors  11  and  12  to activate the solenoid valve  13  in such a duty cycle that the float chamber pressure is reduced to ensure that a constant mass air/fuel mixture is delivered by the carburettor to the engine. 
     By “duty cycle” is meant the percentage of the opening time of the solenoid valve  13  in relation to its fixed cycle time. For example if the cycle time of the solenoid valve  13  is 0.1 seconds, and the duty cycle is 50%, then the opening time of the solenoid valve  13  during each cycle will be 0.05 seconds. 
     Referring to FIG. 2, at standard atmospheric pressure and temperature conditions, the ECU does not deliver any signal to the solenoid valve  13  which therefore remains closed and the float chamber  19  is at atmospheric pressure, this corresponding to a duty cycle percentage of 0 at the solenoid valve  13 . At increasing altitude, the air density is reduced so that the ECU  10  in response to signals received from the sensors  11  and  12  will deliver a signal to the solenoid valve  13  opening it for a percentage of its duty cycle corresponding to the specific atmospheric conditions of pressure and temperature that have been sensed so that the float chamber through the conduit  24  is exposed to an under pressure or vacuum as indicated by the graph in FIG.  2 . This system is calibrated such that at a 100% duty cycle of the solenoid vale  13  (corresponding to the minimum air density atmospheric conditions which will be encountered) the float chamber under pressure will as shown be approximately 40% of the vacuum in the induction passage  15  at the location of the fuel orifice  18 . Between these two extremes the change is essentially linear. 
     It will be understood that it is at all times possible to alter the mass air/fuel ratio in response to the above discussed or other parameters by feeding appropriate signals to the ECU  10 . 
     In some circumstances it is desirable to provide a fuel-enriched air/fuel mixture to the engine, e.g. during cold starting of the engine. Traditionally this has been done by use of a manual or automatic choke or primer. In the present invention however this function is also included in the fuel supply control system, and is also monitored by the electronic control unit which acts to increase the pressure within the float chamber  19  to provide the mixture enrichment required during engine start-up and during warming up of the engine from a cold start. 
     Referring to FIG. 3 there is shown a snowmobile engine  30  is a two stroke internal combustion engine having a crankcase  31  in which pre-compression of the air/fuel discharge is carried out prior to the latter being delivered into the engine cylinders. During low speed rotation of the engine crankshaft (e.g. between 200 and 900 rpm) the pressure changes that occur during pre compression of the charge in the crankcase can be utilized, and to this end a pressure line  26  communicates with the crankcase interior and through a check valve  20  and a pressure line  34  supplies crankcase gases to a pressure regulator  35 , the latter supplying a regulated pressure flow to a pressure line  36 . 
     This first pressure source as mentioned is useful only at low engine rpm because for a given throttle opening the available pressure decreases with increasing rpm, as is well understood in the technology of two stroke engine applications. In effect, this pressure source is only useful during the cranking stage of operation of the snowmobile engine under consideration, the cranking speed being of the order of 500 rpm. The idle speed of the engine is about 1,700 rpm which is well above the range when any useful pressure output can be obtained from the above described pressure source. Therefore at higher speeds, the second pressure source is provided by utilizing the pressure pulsations occurring in the crankcase to drive a diaphragm air pump  33 . Thus as seen in FIG. 3 the air pump  33  is divided by a movable diaphragm  37 , the chamber  38  on the upper side of the diaphragm being in communication with a branch  27  of the pressure line  26 . On the underside of the diaphragm there is a pumping chamber  39  designed to draw air from a line  32  (connected to the interior of the crankcase) through a plenum  40  and a non-return valve  42  and to deliver air under pressure past a second non return valve  42  into an output chamber  43  which communicates with the pressure line  34 . 
     In operation, at low engine rpm as during cranking, as described above a supply pressurized air is delivered through the line  26  and through the check valve  20  and the pressure line  34  to the regulator  35 . 
     At higher engine speeds, e.g. at the idling speed of 1,500 rpm, as explained, the line  26  no longer delivers an adequate flow of pressurized air. However in these circumstances the pulsations from the crankcase through the lines  26 ,  27  produce a rapid fluctuation in the position of the diaphragm  37  against the force of its return spring  44 . These fluctuations of the diaphragm cause small amounts of air from the line  32  to be drawn in past the one way valve  41  when the diaphragm moves upwards, and then to be driven out of the pumping chamber  39  past the one way valve  42  when the diaphragm is moved downwards thus supplying a pressurized air flow to the line  34  and the regulator  35 . 
     FIG. 4 shows a fuel flow control system which incorporates elements from not FIGS. 1 and 3, and where possible like reference numerals are used to illustrate like parts. 
     The electronic control unit  10  is coupled to receive signals from the barometric pressure sensor  11 , the air temperature sensor  12  and an engine temperature sensor  50  and utilizes signals received from these sensors to control the fuel supply to the engine  30  in the desired manner. As described in relation to FIG. 1, the ECU  10  delivers a control signal to a solenoid  13  which is mounted in a manifold  122  the interior of which communicates with the floor chambers  19  of each of three carburetors  14  through conduits  124  and which communicates with atmosphere through a vent orifice  125  (FIG. 5 c ). A vacuum conduit system  123  is exposed to the pressure within the induction passage venturi of each of the carburetors and communicates this pressure to the manifold  122  under control of the closure  121  of the solenoid  13 . The manifold  122  also carries a second solenoid  51  which is connected to the ECU  10  and controls the supply of pressurized air from the line  136  to the manifold  122  in accordance with signals received from the engine temperature sensor  50 . Although not shown in FIG. 4, the system for generating pressure from the engine crankcase as shown in FIG. 3 is included, and the output pressure line  136  therefrom is connected to the interior of the manifold  122 , this connection being regulated by the solenoid  51 . 
     From the foregoing it will be appreciated that the pressure in the carburetor float chambers  10  is regulated under the control of the ECU  10  in response to signals received from the sensors  11 ,  12  and  50  to provide a mass air/fuel flow mixture having the desired characteristics in relation to various operating conditions of the engine. 
     Referring to FIGS. 4 and 5 a  and  5   b , the manifold  122  is shown as constituting a pair of closed end tubes  120 ,  121 , access to the interior of which is controlled through a number of tubular connectors. The manifold  122  is designed for use with the three cylinder engine  30 . Specifically, on the lower tube  121  at opposite ends thereof and in the middle are three tubular connectors  123   a  for communication with the vacuum conduits  123  that connect to the venturi of the respective carburettors  14 . Three further pairs of tubular connectors  124  provide communication between the interior of the manifold upper tube  120  and the float chambers of the carburettors  14 . 
     In an intermediate position in its length the manifold  122  carries a block  130  in which are received the solenoid valves  13  and  51  and the associated valve structure (not shown in FIG.  4 ). The block  130  also carries the atmospheric vent  125  and a further tubular connector  136  (FIG. 5C) to receive the pressure line  134 . 
     As will be understood, within the block  130  the solenoid  13  controls communication of vacuum from the lower tube  121  to the upper tube  120  of the manifold, and thus application of pressure reduction to the carburetor float chambers. This is the condition represented by the arrows in FIG. 5 b.    
     The solenoid  51  on the other hand controls communication of pressurized air flow from the connector  136  to the interior of the upper tube  120 , and thus controls application of overpressure to the carburetor float bowls. 
     The full operating range of the system is calibrated such that for fuel enrichment (corresponding to cold start/warm up conditions) a 100% duty cycle for the solenoid  51  is provided at a predetermined ratio between atmospheric pressure and the pressure provided by the air pump  33 . For reduction of the proportion of fuel in the air fuel mixture ratio (compensation for low atmospheric pressure or altitude) this system is calibrate to give 100% duty cycle operation of the solenoid valve  13  at a predetermined maximum ratio of negative (vacuum) pressure to atmospheric pressure. The effects of the duty cycle operation of the two solenoid valves  13  and  51  can to some extent offset one another e.g. for high altitude cold start situations. 
     Instead of the mechanical pump  33  described in relation to FIG. 3, it would of course be possible to utilize various other pump arrangements, and in particular a battery driven electric pump. 
     As is well understood, when an engine is cold it is difficult to vaporize a sufficient amount of the liquid fuel in the combustion chamber for the engine to operate properly. Vaporization and atomization are adversely affected by low temperature, and therefore in cold start conditions it is necessary to increase the quantity of fuel in order to compensate for poor atomization, and this is typically done by using a primer such as a choke or other enrichment system at the carburetor. As the engine gradually warms up during operation, the air fuel mixture atomizes and vaporizes more readily, and if the enriched mixture ratio is maintained, the engine performance will be reduced and the spark plugs may become fouled. The control system described herein and illustrated in the drawings overcomes this difficulty and will operate to enrich the air fuel mixture in cold start conditions, and automatically to reduce and remove the enrichment when the engine warms up. This is done by the electronic control unit  10  which receives signals from the engine temperature sensor  50  and modifies the pressure in the float bowls of the carburetors  14  as required to provide the necessary degree of mixture enrichment. The sensor  50  can be mounted at any convenient location on the engine  30 , e.g. for a liquid cooled engine, within the engine coolant jacket. As described above in relation to FIG. 2, the pump  33  is driven by pressure pulses in the engine crankcase as communicated through the line  27  to deliver a flow of pressurized air through the line  134 . This pressurized air is delivered to the block  130  through the connection  136  and enters the upper tube  120  of the manifold under control of the solenoid  13  which is driven by the ECU  10 . Pressure from the tube  120  is communicated to the float bowls of the carburetors  14  through the tubes  124  to increase the pressure differential between the float bowls and the carburetor venturi and thus increase fuel flow to the extent required. As the engine temperature increases, the ECU  10  responds to signals from the sensor  50  to reduce the duty cycle of the solenoid  51 , and thus reduce the overpressure applied to the carburetor float bowls until normal engine operating temperature is reached and this overpresure is completely eliminated. 
     The ECU  10  also controls the solenoid  13  in accordance with signals received from the air temperature sensor  12  which is conveniently located in the engine air filter (not shown) and from the barometric pressure sensor  11 . The duty cycle of the solenoid  13  is controlled such that underpressure or vacuum from the lower tube  121  of the manifold (which communicates with the venturis of the carburettors through the lines  123 ) enters through the block  130  into the upper manifold tube  120  and hence acts to reduce the float bowl pressure of the carburetors  14  producing a leaner air fuel mixture corresponding to the reduced air density that occurs for example at increased altitudes. 
     The use of the manifold  122  as shown particularly in FIGS. 5 a  and  5   b  make it possible to use a single pair of solenoids  51 ,  13  to control the fuel flow in a number of carburettors (three as shown in the three cylinder engine of FIG.  4 ). Without the manifold, individual pairs of solenoids  13  and  51  would have to be provided for each respective carburettor  14 . 
     The fuel control system as described in the foregoing can readily be provided as a retrofit on existing engines, and conveniently is provided in kit form the kit including 
     a) the electronic control unit  10  together with the atmospheric pressure and temperature sensors and the engine temperature sensor; 
     b) the manifold  122  together with the block  130  including the connections for the various lines as described above; 
     c) the pump  33 ; 
     d) modified carburetors  14  including connections to the float bowls and venturis thereof; and 
     e) electrical connections and pneumatic connections for coupling the various parts of the system.