System for continuous control of air-fuel ratio with ionization current

Abstract

A control system for carburation of an internal combustion engine in use conditions comprising following activities: starting the engine with a value of equals .sub.0=.sub.T; constructing a curve c.sub.i() of the ionization current as a function of the angular position a of the crank shaft; selecting, on this curve c.sub.i(), a number of points at intervals of the rotation angle a; calculating value z, equal to integral from 0 to 360 of the curve c.sub.i(), is done by summing products c.sub.i for all preselected points; interrupting supply of fuel for some cycles in order to externally modify factor .sub.0 and take it to value .sub.1; for value .sub.1 constructing curve c.sub.i() and calculating value z.sub.1; calculating difference .sub.z=z.sub.1z.sub.0, and if the difference is >.sub.zref in absolute value, intervening on carburation by increasing the quantity of fuel injected in a case of a positive difference (lean mixture) and by reducing the quantity of fuel injected in a case of a negative difference (rich mixture).

Claims

1. A control system of carburation of a two-stroke internal combustion engine comprising: a. operating the engine with an air/fuel ratio of a predetermined initial value (.sub.0); b. for the initial value (.sub.0) of the air/fuel ratio, constructing a first curve (c.sub.i()) representing an ionization current (c.sub.i) as a function of the angular position () of a crankshaft of the engine, the ionization current (c.sub.i) being measured through spark plug electrodes; c. calculating a value (z.sub.0) equal to an integral of the first curve (c.sub.i()) in a predetermined angular interval of the crankshaft; d. interrupting fuel supply for a few engine cycles, thereby increasing the air/fuel ratio of the engine to a second value (.sub.1) larger than the initial value (.sub.0), the second value (.sub.1) of the air/fuel ratio being equal to the sum of the initial value (.sub.0) and a predetermined quantity (.sub.), the predetermined quantity (.sub.) being smaller than or equal to 0.05; e. for the second value (.sub.1) of the air/fuel ratio, constructing a second curve (c.sub.i()) representing the ionization current (c.sub.i) as a function of the angular position () of the crankshaft, the ionization current (c.sub.i) being measured through the spark plug electrodes; f. calculating a value (z.sub.1) equal to an integral of the second curve (c.sub.i()) in a predetermined angular interval of the crankshaft that is equal to the preceding angular interval; g. calculating a difference (.sub.z) as the value (z.sub.1) of the integral of the second curve minus the value (z.sub.0) of the integral of the first curve; h. if the difference (z) has an absolute value greater than a predetermined threshold value (.sub.zrif), intervening on the carburation to regulate a quantity of fuel injected, wherein the regulating of the quantity of fuel injected includes increasing the quantity of fuel injected in a case of a negative difference and reducing the quantity of fuel injected in a case of a positive difference, wherein the system provides a continuous and automatic intervention during an entire operating period of the engine and not only when operated at wide open throttle.

2. The control system of claim 1, wherein a first initial value (.sub.T) of the air/fuel ratio is comprised between 75% and 85% of the value of a stoichiometric ratio.

3. The control system of claim 1, wherein the predetermined angular interval on which the integral of the ionization current is calculated is 360 of a crankshaft rotation.

4. The control system of claim 1, wherein the threshold value (z.sub.rif) of the difference is equal to or less than 261 A*rad.

5. The control system of claim 1, wherein the value of the integral of each curve (c.sub.i()) representing the ionization current (c.sub.i) is calculated by: selecting, on the curve (c.sub.i()) a number of points (P.sub.1 . . . P.sub.n) at predetermined intervals (.sub.) of the rotation angle () of the crankshaft; calculating for each point (P.sub.1 . . . P.sub.n) the product of the value (c.sub.i) of the ionization current at that point for the respective angular interval (.sub.); and summing the products relative to all the points (P.sub.1 . . . P.sub.n).

6. The control system of claim 1, wherein the modification of the air/fuel ratio is carried out at regular time intervals.

7. The control system of claim 6, wherein the time intervals have a duration comprised between 10 and 20 seconds.

8. The control system of claim 1, wherein the value (z) of the integral of the ionization current is monitored at each engine cycle and the quantity of fuel injected is regulated so as to maintain a predetermined value of the engine velocity, if the value (z) of the integral of the ionization current exceeds a predetermined threshold value (z.sub.max).

9. The control system of claim 1, wherein the value (z) of the integral of the ionization current is monitored at each engine cycle and the engine is cut off if the value (z) of the integral of the ionization current is lower than a predetermined threshold value (z.sub.min).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and constructional characteristics of the invention will emerge from the detailed description that follows, which relates to a particular preferred embodiment of the invention given by way of non-limiting example.

(2) FIG. 1 is a curve c.sub.i() that shows the progression of the ionization current c.sub.i in the cylinder of an engine as a function of the angle of the crankshaft, for a certain value of the factor and cleaned of the irregularities which occur at the moment of the ignition discharge.

DETAILED DESCRIPTION

(3) The control system of the present invention is configured for controlling the carburation of an internal combustion engine, i.e. the air/fuel ratio or equivalently the factor at which the engine is operated.

(4) The factor is the quotient between the value of the air/fuel ratio at which the engine is operating and the stoichiometric ratio (i.e. 14.7).

(5) The engine is preferably a spark-ignited small engine (for example petrol), for example a two-stroke engine for portable tools, such as power saws, croppers, brush cutters or the like.

(6) The engine therefore further comprises at least a cylinder in which an alternating piston is housed, which is in turn connected to a crankshaft, so that the alternating motion of the piston, due to the combustion of the air/fuel mixture internally of the cylinder (i.e. the combustion chamber) is transformed into a rotation of the crankshaft.

(7) The engine is carburetor-fed. The carburetor is essentially made up of a main conduit which places the whole cylinder in communication with the outside. Along the progression of this conduit there are normally situated a valve, which can be a butterfly valve, which has the function of regulating the air flow internally of the conduit, and an inlet system of the fuel. The fuel inlet system generally comprises a choke in the main conduit and a nozzle located at the choke position, which is connected to a tank for the fuel. In this way, the nozzle is able to dispense the fuel which is aspirated by the tank thanks to the depression created by Venturi effect by the choke.

(8) The inflow of fuel to the dispensing nozzle can be controlled by means of a valve, for example a solenoid valve, which is positioned along the conduits connecting the tank to the dispensing nozzle. This valve can be an ON-OFF valve (i.e. able only to completely open or close the communication between the tank and the nozzle), which is piloted so as to open and close at high frequency, for example by means of a PWM signal (pulse-width modulation). In this way, by varying the average opening and closing time of the valve, for example by changing the duty-cycle of the PWM signal, it is advantageously possible to regulate the average quantity of fuel that is fed to the engine.

(9) The control system of the present invention is based on the measurement, while the engine is operating with a certain value of factor , the ionization current c.sub.i as a function of the rotation angle of the crankshaft at each cycle of the engine [c.sub.i=f()], where it is not the variation .sub.ci among the ionization current values being monitored, but the variation of a parameter z which expresses the value of the integral from 0 to 360 of the curve c.sub.i=f().

(10) The phenomenon of ionization arises internally of the combustion chamber, where ions are generated by effect of the oxidation reaction of the fuel and by the action of the heat generated by combustion.

(11) In the presence of two differently-charged poles located in the combustion chamber, between the poles a migration of ions takes place, giving rise to a passage of current which takes the name of ionization current c.sub.i.

(12) It is possible to use, as poles, the electrodes of the spark plug of the fuel mixture.

(13) The ionization current c.sub.i is the current transiting between the two electrodes, measured from outside the engine, i.e. through the electric circuit that heads to the spark plug.

(14) The current measuring systems a are known and therefore will not be described in detail.

(15) The system is conducive to a continuous monitoring of the carburation of the engine during its use, for example for calculating the value of the parameter z at each engine cycle.

(16) In this context consider the case, by way of example, in which, during the carrying out of a certain engine cycle, the engine is functioning with a carburation corresponding to the value of factor of .sub.0, where .sub.0 is a predetermined value which we shall call initial.

(17) The initial value (.sub.0) of factor usually corresponds to a predetermined duty-cycle of the valve.

(18) For this initial value (.sub.0) the curve [c.sub.i=f()] is constructed and the integral z.sub.0 from 0 to 360 of the curve c.sub.i=f() is calculated, i.e. the value of the integral of the ionization current during the current engine cycle.

(19) During a subsequent engine cycle, the value of factor is automatically modified and the value of the parameter z is recalculated, i.e. the value of the integral of the ionization current is recalculated during the engine cycle performed with the new value of factor .

(20) The modification of the value of factor is performed by interrupting the supply to the engine, or rather for a few cycles of the engine, for example for three or more consecutive engine cycles.

(21) In practice, the ON-OFF valve that connects the tank to the nozzle of the carburetor is kept closed for the above-mentioned time interval, for example for three or five consecutive engine cycles, so that the engine is supplied only with the quantity of fuel remaining in the circuit, overall causing an increase in the value of .

(22) In this way a modification of the factor of the initial value .sub.0 ensues, to a different value .sub.1, still greater than .sub.0 to which the calculation of a new value z.sub.1 of the parameter z corresponds.

(23) For example, the value .sub.1 can be calculated as the sum between the initial value (.sub.0) and a predetermined quantity .sub., where this quantity .sub. can be equal to a constant or predetermined value of the factor , for example a positive value of less than or equal to 0.05.

(24) In this way it will ensue that value .sub.1 is always greater than value .sub.0, i.e. it will always correspond to a slightly leaner mixture.

(25) By comparing the values z.sub.0 e z.sub.1 the difference is calculated .sub.z=z.sub.1z.sub.0 and if the absolute value is greater than a certain reference value .sub.zrif this means that the carburation is not correct and needs modifying.

(26) For example, reference value .sub.zrif can be 8.32 A*rad, so that the difference between the two values z.sub.0 and z.sub.1 of the integral is acceptable only if it is lower than the reference value (.sub.z<8.32 A*rad).

(27) In particular if the absolute value of .sub.z is <.sub.zrif it is concluded that the carburation is correct. In this case, in the control system, in the subsequent engine cycles, the engine will return to operating with the initial value .sub.0 of factor , without interventions on the carburation.

(28) If the absolute value of .sub.z is >.sub.zrif and .sub.z is a negative value, this means that the air/fuel mixture is too lean. In this case, the control system of the engine proceeds to enrich the air/fuel mixture, for example by a fixed quantity. In other words, the control system acts so that in the following engine cycles, the engine is operated with a greater quantity of fuel with respect to the fuel corresponding to the initial value .sub.0, i.e. with a new value of factor which is lower than .sub.0.

(29) On the other hand, if the absolute value of .sub.z is >.sub.zrif and .sub.z is a positive value, this means that the air/fuel mixture is too rich. In this case, the control system of the engine proceeds to make the air/fuel mixture leaner, for example by a fixed quantity. In other words, the control system acts to that in the following engine cycles, the engine is operated with a lower quantity of fuel with respect to the fuel corresponding to the initial value .sub.0, i.e. with a new value of factor which is higher than the initial value .sub.0. This new value of factor might coincide with value .sub.1 but might also be different.

(30) In practice, the above-described operations represent an efficiency test on the initial value (.sub.0) of factor . If the difference between the two values of the integral of the ionization current has an absolute value that is lower than or equal to the threshold value, this means that the initial value .sub.0 of factor enables obtaining a good compromise between engine performance and quantity of polluting emissions, so that the engine can continue to operate with the initial value .sub.0. If on the other hand the difference between the two values of the integral of the ionization current has an absolute value that is greater than or equal to the threshold value, this means that the initial value .sub.0 of factor does not enable obtaining a good compromise between engine performance and quality of polluting emissions, so that the control system will then cause the engine to operate with a different quantity of fuel and therefore a new value of factor .

(31) This efficiency test is repeated several times during engine operation, using each time, as the initial value .sub.0 of factor , the value at which the engine was functioning immediately before, i.e. the one resulting (maintained constant or regulated) at the end of the last efficiency test previously-carried out.

(32) In particular, the above operations (i.e. the efficiency test in its entirety) can be repeated at regular intervals for the whole operating time of the engine, for example every 15-20 seconds; so that the carburation is continually adapted and maintained close to an optimal value of factor as a function of the conditions of use and the environmental situation in which the engine is operating.

(33) On first carrying out the efficiency test, for example on starting the engine, the initial value .sub.0 of factor can be equal to a predetermined calibration value .sub.T, which can be comprised between 0.75 and 0.85.

(34) Further characteristics and advantages of the invention will more fully emerge from a reading of the following example, for which a single-cylinder two-stroke engine was used having following characteristics:

(35) TABLE-US-00002 cubic capacity 40.2 cc max output 10,500 rpm max power 2.1 Hp Working output 8500 rpm

(36) The mapping of the engine was carried out at origin, assuming use of the engine at sea level, with an operating temperature of around 20 C.

(37) In these conditions a calibration value of factor was adopted of .sub.T=0.8, to which correspond CO emissions of 6% and a maximum value of the ionization current of c.sub.i=0.6 A.

(38) The first use of the engine was at a height of 2000 metres above sea level, with an operating temperature of close to 0 C.

(39) The carburation of the engine thus requires an adjustment, which is done in the following way.

(40) On first starting up the engine, the control system uses, as an initial value .sub.0 of the factor , the calibration value .sub.T (.sub.0=.sub.T=0.8).

(41) Operating the engine with this value, .sub.0=.sub.T of factor , the ionization current c.sub.i is measured and the curve c.sub.i() plotted, which represents the ionization current c.sub.i as a function of the angle of rotation of the crankshaft for a cycle of the engine.

(42) As illustrated in FIG. 1, on this curve a number of points (P1 . . . Pn) is chosen at regular intervals .sub. of the angle of rotation of the crankshaft and for each point P.sub.1 . . . P.sub.n the corresponding value of di c.sub.i1 . . . c.sub.in is read.

(43) For each point, product c.sub.i.sub. is applied and thereafter the sum of all the products is calculated.

(44) The summing of the products c.sub.i.sub. represents the value z.sub.0 of the parameter z, i.e. the integral from 0 to 360 of the curve c.sub.i().

(45) This value z.sub.0 of the parameter z remains substantially constant for all the engine cycles in which the engine is operated with an initial value (0) .sub.0=.sub.T of factor .

(46) At this point, the value of factor is modified, interrupting the supply of the fuel, for example for five cycles, thus taking the initial value .sub.0=.sub.T to a new value .sub.1.

(47) The variation of factor , consequent to the interruption of the supply, i.e. the difference between the values .sub.1 and .sub.0 is a positive value .sub. lower than or equal to 0.05 (.sub.0.05).

(48) Therefore the calculation of the parameter z is repeated on a curve c.sub.i() obtained by making the engine function with value .sub.1 of factor , in this way calculating the new value z.sub.1.

(49) Then the value of the difference .sub.z is calculated between two values of the parameter z, i.e. .sub.z=z.sub.1z.sub.0.

(50) If .sub.z is >.sub.zrif (for example 477 A*ms) in absolute value, the regulating system of the engine automatically enriches or weakens the air/fuel mixture according to whether the value of .sub.z is negative (lean mixture) or respectively positive (rich mixture).

(51) The operations are repeated at regular intervals for the whole period of operation of the engine.

(52) The above-described method can be implemented by means of electronic measuring devices known to technical experts in the sector.

(53) With the above-described method the value z of the integral of the ionization current is further constantly monitored, which can in fact be calculated at each engine cycle.

(54) If the value z is too high or too low, the system can activate safety procedures which enable safeguarding the engine.

(55) In particular the system can activate a first safety procedure if the calculated value z of the integral of the ionization current exceeds a predetermined threshold value z.sub.max, for example a value of 261 A*rad, for a time (i.e. a number of engine cycles) of greater than a predetermined value t, for example 20 engine cycles.

(56) This first safety procedure can include deactivating the control procedure described in the foregoing, and controlling the carburation on the basis of the velocity of the engine.

(57) For example, this first safety procedure can include measuring the velocity of the engine, typically the number of revolutions completed by the crankshaft in the time unit, and regulating the quantity of fuel injected so that the velocity of the engine is maintained constant at or nearly so at a predetermined target value .sub.tar, for example 10000 rpm.

(58) To do this, the system can use a feedback control which includes calculating, for each engine cycle, the difference between a measured value W of the engine velocity, and the target value W.sub.tar, and regulating the supply of fuel so as to minimize the difference, for example via a proportional control (P), a proportional-integral control (PI) or a proportional-integral-derivative control (PID) which as input uses the difference between the measured value W and the target value W.sub.tar of the engine velocity.

(59) This first safety procedure can also be activated in other circumstances, for example should the temperature of the engine exceed a predetermined maximum value, for example 270 C., or should the number of engine revolutions exceed a respective maximum value, for example 10000 rpm.

(60) To activate the safety procedure, it is possible for each of these further conditions to be verified for at least a predetermined length of time.

(61) The control system can further activate a second safety procedure if the calculated value z of the integral of the ionization current falls below a predetermined threshold value z.sub.min, for example a value of 1.75 A*rad, for a time (i.e. a number of engine cycles) greater than a predetermined value t, for example 20 engine cycles.

(62) In these circumstances it is probable that the spark plug is very dirty and that it is actually not able to provide reliable value of the ionization current, which does not enable controlling the carburation effectively.

(63) In this case, the second safety procedure can simply include causing the shutting down of the engine, for example by supplying the engine with the greatest quantity of fuel possible, i.e. leaving the ON-OFF valve connecting the fuel tank with the injecting nozzle of the carburetor constantly open.

(64) In this way the quantity of fuel injected becomes so high as to rapidly flood the engine which consequently shuts down.

(65) The invention is understood not to be limited to the above-described example, and any variations and improvements can be made thereto without its forsaking the scope of the claims that follow.