Electronic Fuel Metering Apparatus For Internal Combustion Engine

Abstract

Electronic fuel metering apparatus is described for use with an internal combustion engine having an air inlet passage for supplying air to one or more combustion chambers. The apparatus includes an electrically controlled fuel metering device, a circuit for accumulating a count of air entering the combustion chamber (s), and a circuit for controlling, in accordance with the air count, the amount of fuel discharged by the fuel metering device. An up/down counter may be employed, and the rate at which fuel is discharged may be varied depending upon the air count accumulated on the up/down counter.

Description

DESCRIPTION OF THE INVENTION

This invention relates to an Otto cycle internal combustion engine and to fuel metering apparatus for it.

According to the invention, such an engine has the following features:

A. An electronic logic circuit is responsive to sensing means disposed in an inlet passage to produce electrical signals representative of the mass of air flowing through the passage into a cylinder of the engine;

B. THE LOGIC CIRCUIT CONTROLS THE MASS OF FUEL INJECTED AT A LOCATION DOWNSTREAM OF THE SENSING MEANS IN ACCORDANCE WITH SAID AIR MASS FLOW SIGNALS.

Preferably the mass of fuel injected during each induction stroke of the cylinder is in a predetermined constant ratio to the mass of air entering the cylinder during the induction cycle as indicated by said air mass flow signals.

In the embodiment described below the fuel is injected directly into the cylinder but the injector may alternatively be located in the inlet manifold downstream of the air flow sensors.

The invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a section of one cylinder of a multicylinder Otto cycle internal combustion engine embodying the invention;

FIG. 2 is a circuit diagram of the injection control system of the engine of FIG. 1;

FIG. 3 is a graph illustrating the variation of the different parameters affecting the mass of air entering the cylinder over one cycle of the engine; and

FIGS. 4 and 5 are graphs showing injection timing in relation to air mass in a cylinder.

A multi-cylinder Otto cycle internal combustion engine includes an aluminum alloy integral cylinder block and cylinder head 10. A piston 11 in each cylinder 12 is connected by a connecting rod 13 to a crankshaft (not shown). In each cylinder, an inlet valve 14 and an exhaust valve (behind the inlet valve in FIG. 1) are actuated by a single overhead camshaft driven by the crankshaft.

A spark plug is mounted in spark plug bore 27. The timing of the valve openings and ignition follows conventional practice in modern Otto cycle engines.

The inlet valve 14 of each cylinder is mounted in a passage 15 which connects to an inlet manifold 16 common to all cylinders. A water carburetor 17 is mounted on to the manifold 16 and a conventional air cleaner (not shown) is mounted on the water carburetor. The water carburetor is arranged to humidify the air passing through it to the engine to a substantially constant value, for example 100 percent relative humidity.

A butterfly valve 18 in the water carburetor is connected to a throttle control pedal and regulates the air flow to the engine in accordance with throttle pedal movement.

A low pressure fuel injector 19 is located in the wall of each cylinder 12. The exact location of the injector is chosen having regard to a need for a reasonably low (e.g., less than 100.degree.C) and reasonably uniform temperature of fuel passing through the injector to prevent premature fuel vaporization and to facilitate measurement of the mass of fuel passing through the injector. The water jacket 9 of the block 10 completely surrounds the injector for effective injector cooling. The injector is preferably mounted as low as possible in the cylinder wall consistant with being uncovered by the piston for a sufficiently long period during inlet valve opening to permit substantially all fuel to be injected when the inlet valve is open. This is to permit use of low pressure injectors and to allow fuel/air mixing during the compression stroke.

The fuel injectors may be conventional solenoid operated low pressure injectors as used in many existing injection systems which inject fuel into the inlet passage of the cylinder. The valve should however, open outwards in view of the high pressure which the injector will have to withstand during combustion cycles.

The fuel injectors 19 are connected to a fuel system in which a constant pressure of the order of 100 p.s.i. is maintained by a fuel pump.

The soleoids of the fuel injectors are energized by an electronic logic circuit. The circuit of FIG. 2 shows the logic associated with one cylinder. Identical independent circuits may be used for the other cylinders but preferably at least part of the circuit is time-shared with the other cylinders as described later in this specification.

The inputs to the circuit are provided by sensors or transducers 20 through 26 which are responsive, respectively, to air velocity in the intake passage 15, air pressure in the intake passage, air temperature in the intake passage, humidity, crankshaft position, fuel velocity and fuel temperature.

The sensors for air velocity and pressure are mounted closely adjacent to each other in the inlet passage 15. Temperature and humidity sensors may be mounted at other locations in the inlet tract and shared with the other cylinders since temperature and humidity will not vary significantly within a particular engine cycle.

The fuel velocity and temperature sensors may be mounted either in the fuel injector 19 or in the fuel supply line connected to the injector.

The crankshaft position transducer 24 may be an electro-magnetic device coupled to the engine fly-wheels. It produces a signal at the point in the cycle when fuel injection may begin, i.e., the point in the induction stroke when the piston uncovers the injector. Signals from the crankshaft position sensor may also control an electronic ignition system.

The measurement of air velocity and fuel velocity accurately and at high speed is essential for the system to operate effectively. One example of a flow meter which could be used is described in our British Patent No. 1,127,568. Alternatively hot-wire or hot-film anemometers may be used. Such devices are described in the following references:

1. Hot-Wire Anemometers. The Review Of Scientific Instruments, May 1967 page (677).

2. P. O. A. L. Davies, M. R. Davis and I. Wold, `Operation of the Constant Resistance Hot-Wire Anemometers` Institute of Sound And Vibration University of Southampton, Report No. 189;

3. J. C. Wyngaard and J. L. Lumley, `A Constant Temperature Hot-Wire Anemometer`, Journal of Scientific Instruments, Vol. 44, 1967, page (363);

4. B. J. Bellhouse and D. L. Schultz, `The Determination of Fluctuating Velocity in Air with Heated Thin Film Gauges`. Journal Of Fluid Mechanics, Vol. 2, part 2, 1967 page (289).

The variation of the different parameters effecting the mass of air entering the cylinders during the induction stroke is illustrated diagrammatically in FIG. 3.

The mass rate of flow of air into the cylinder is a function of the air flow velocity, air temperature, air pressure and humidity, through a known area. This function is generated by an air mass flow function generator 28, the output signal of which is representative of the rate of mass flow of air into the cylinder. The function generator is connected to a voltage to frequency converter 29. An up and down counting register 30 has an up counting input 31 connected to the voltage to frequency converter 29 so that the register accumulates a count representative of the mass of air which has entered the cylinder.

Gate circuits 32 to 34 each connect a respective stage of the register 30 to a pulse generator 35. A signal from the first gate 32 causes the pulse generator to generate pulses with a relatively low mark/space ratio. A signal from the second gate 33 produces an intermediate mark/space ratio and a signal from the third gate 34 produces an either a high mark/space ratio or a continuous signal (i.e., infinite mark/space ratio). The output of the pulse generator is applied to an amplifier 36 connected to the solenoid 37 of the fuel injector 19.

The amplifier 36 has sufficient gain that the amplitude of the pulses which it produces is sufficient to fully open the fuel injector. However, the frequency of the pulses is greater than the fuel valve can follow so that the mark/space ratio of the pulse generator determines the position assumed by the fuel injector and hence the three available mark/space ratios provide three rates of flow through the valve.

The actual fuel flow (mass flow) is represented by the output signal of a fuel flow function generator 38, responsive to fuel velocity and fuel temperatures. The output signal of function generator 38 is converted to a frequency modulated signal in a voltage to frequency converter 39 and applied to a down counting input 40 of the register 30.

The circuit is such that the frequency at input 31 for a given rate of air mass flow is in a predetermined ratio to the frequency at input 40 for the same rate of mass flow of fuel. This predetermined ratio is the same as the fuel/air ratio produced by the system. A lean air/fuel ratio of the order of 20:1 is used in order to minimize harmful exhaust emissions (i.e., unburnt hydrocarbons and carbon monoxide, and oxides of nitrogen).

Thus, for a 20:1 air/fuel, a certain mass of fuel passing into the cylinder causes a count down 20 times greater than the count up produced when the same mass of air flows into the engine.

At any point in the cycle, the register thus stores a count representative of the mass of air in the cylinder for which fuel has not yet been injected in the required air/fuel ratio.

Gates 32 to 34 are opened and closed by a bistable circuit 41. The crankshaft position sensing circuit turns on the bistable circuit through a monostable circuit 42 at the point in the engine cycle when fuel injection may begin. The bistable circuit 41 then opens the gates 32 to 34.

A level detection circuit 43 is connected to the output of air mass flow function generator 28 and produces an output signal when the air mass falls below a predetermined level indicative of the closing of the inlet valve. A time delay circuit 44 responsive to the level detector circuit 43 turns off the bistable circuit 41 thereby closing the gates 32 to 34 after a short time delay. This time delay is long enough to ensure that the inlet valve is fully closed before injection stops but short enough to avoid the system attempting to inject fuel after pressure in the cylinder has begun to rise due to the compression stroke of the piston or when the piston covers the injector.

It is an important feature of the engine that injection is complete at the beginning of the compression stroke because the system relies on the compression stroke to properly mix the air and fuel in the cylinder.

The time delay circuit 44 also activates a monostable circuit 45 at the completion of air induction and fuel injection. The monostable circuit 45 resets the register 30 to zero ready for the next cycle.

The operation of the circuit is illustrated in FIGS. 4 and 5. At the beginning of the induction stroke of the piston, air begins to flow into the cylinder and the register 30 accumulates a count representative of the air mass in the cylinder. Fuel injection does not start immediately but is delayed until the injector is uncovered by the piston. At this time gates 32 to 34 are opened by the crankshaft piston sensor 24. In low power operation (FIG. 4) when the butterfly valve 18 is only partially open, the count which has accumulated by the time the gates 32 to 34 are opened will be of intermediate value, sufficient to turn on gate 33, thereby producing an intermediate value 48 of fuel flow. This intermediate value of fuel flow continues until a sufficient number of fuel flow units have been counted out of the register to reduce the count to a level at which only gate 32 is on. Fuel is then injected at a low rate 46.

In high power operation (FIG. 5) the count in the register at the beginning of fuel injection is sufficient to produce a high rate 47 of fuel flow through gate 34. The fuel flow reduces through intermediate rate 48 to low level rate 46 as the fuel injection catches up with the air induction and the count in register 30 decreases.

The correct functioning of the circuit described above depends upon the speed with which the various operations can be performed. At maximum r.p.m. of the engine, the duration of the induction cycle is about 2 milliseconds. If this system does not operate sufficiently quickly there will be a significant count left in the register 30 at the point when fuel injection ceases, representing excess air in the cylinder thereby making the mixture weaker than the design value.

One way of overcoming this problem is to measure the air injected in one cycle and inject a corresponding measured quantity of fuel into the next cycle. The difference between the successive cycles may not be sufficient to upset the desired air/fuel ratio. Such a system could operate in an `in-cycle` mode (i.e., injecting the measured fuel into the same cycle as the air measurement takes place) at low r.p.m. where the operation is slower but cycle to cycle variation is greater and switch to `following cycle` mode as the speed of the engine rises.

An alternative type of system attempts to anticipate or extrapolate from the first part of the induction cycle what the total air mass will be at the end of the cycle and to inject fuel in accordance with such a computed value.

A simple modification to the circuit of FIG. 2 uses the count remaining in the register at the end of the cycle to compensate the next cycle and thereby maintain the design mixture strength at high r.p.m. The monostable circuit 45 is omitted so that the register is not reset to zero at the end of the induction cycle. Thus, any count which remains in the register is added into the next cycle and fuel injection begins at higher rate or remains at the higher rate for a longer period. At the end of this next cycle, the register will still hold a residue, which will be approximately equal to the residue at the end of the previous cycle so that the correct mass of fuel will have been injected. As the speed of the engine increases, the residue in the register 31 at the end of each cycle becomes greater as the operation effectively changes from `in-cycle` mode to `following cycle` mode.

The whole of the circuit of FIG. 2 may be provided for each cylinder but considerable economies of circuitry can be achieved by time-sharing at least part of the circuit between the different cylinders.

This is possible because the induction cycles take place sequentially. In a four cylinder engine, for example, two logic circuits each time shared between two cylinders would be adequate.

All of the circuit of FIG. 2 except the air flow sensor, the air pressure sensor, the fuel velocity sensor and the injection solenoid can be time shared in this way.

If the modified circuit omitting monostable 45 is used, separate registers 30 for each cylinder may be necessary.

Claims

We claim:

1. Electronic fuel metering apparatus for an internal combustion engine having an air inlet passage for supplying air to at least one combustion chamber in said engine, said apparatus comprising: an electronically controlled fuel metering device; first circuit means for accumulating a count of air entering said engine combustion chamber; second circuit means, connected to said electrically controlled fuel metering device, for controlling the amount of fuel metered therefrom in accordance with the air count accumulated on said first circuit means; said first and second circuit means being interconnected and including means for reducing said accumulated air count by an amount determined by the amount of fuel metered by said fuel metering device, said first and second circuit means further including means for varying the rate at which fuel is discharged from said fuel metering device, said rate being reduced in steps as said air count is reduced by said means for reducing said accumulated air count.