Abstract:
In a method for controlling a composition of a fuel/air mixture supplied to an internal combustion engine, which engine has a speed control circuit maintaining an actual rotary speed of the crankshaft within preset limits and a controllable metering device for at least one component of the fuel/air mixture, a disruptive variable is introduced into the valve control circuit at a predeterminable point in time, the activity of an engine speed control circuit is monitored and evaluated immediately after introducing the disruptive variable, and, depending on the evaluation result, at least one parameter having an effect on the operation of the internal combustion engine is adjusted.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method for controlling a composition of a fuel/air mixture supplied to an internal combustion engine, in particular, an internal combustion engine embodied as a two-stroke engine. The internal combustion engine has a cylinder with a combustion chamber provided with a spark plug. The combustion chamber is delimited by a piston driving a crankshaft. An ignition control unit is connected to the spark plug and, depending on the rotary speed of the crankshaft, triggers at an adjusted ignition timing an ignition spark at the spark plug. A speed control circuit maintains the actual rotary speed of the crankshaft within preset limits. A controllable metering device is provided for at least one component of the fuel/air mixture. 
   Hand-held power tools such as motor chain saws, trimmers, blowers or similar devices are preferably driven by an internal combustion engine, for example, a two-stroke engine, that, when employing modern technology, is lively, powerful, and still can be operated in an environment-friendly way. In order to optimize operation of such a small-size engine with a displacement between approximately 20 cc and approximately 250 cc (cubic centimeters), microprocessors are used for engine control that, depending on operating parameters, control ignition of the internal combustion engine. 
   In regard to fuel systems it is also known to employ electromagnetic (solenoid) valves as disclosed in U.S. 2005/0168310 A1. Such a valve is used for controlling the fuel flow in order to adjust the quantity of fuel in regard to different operating parameters of the internal combustion engine. 
   When the engine speed of the internal combustion engine is monitored by a speed control circuit, the electronic system intervenes by changing the ignition timing in order to maintain the desired engine speed. In this way, the set idling speed during idling is controlled also. The controller used in this connection is usually a so-called PI or PID controller, i.e., a controller with a proportional term and an integral term. 
   Particularly in the case of a two-stroke engine, the engine speed is not only dependent on the ignition timing but also on the composition of the mixture. In case the mixture can also be varied as a control variable, a control intervention in regard to the mixture composition, on the one hand, and in regard to ignition timing, on the other hand, in particular during idling, can cause the system to undershoot or overshoot the control range. If this is the case, the engine can stall or the idle speed can no longer be maintained. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method for engine speed control in which, on the one hand, the composition of the mixture supplied to an internal combustion engine is adjustable and, on the other hand, the ignition timing can be changed without running the risk that the control system overshoots or undershoots the control range. 
   In accordance with the present invention, this is achieved in that at a predeterminable point in time a disruptive variable is introduced and, immediately after introduction of the disruptive variable, the activity of the engine speed control circuit is monitored and evaluated. Depending on the result of the evaluation, at least one variable or a parameter having an effect on the operation of the internal combustion engine is adjusted; preferably, one of the components of the fuel/air mixture is adjusted. Other operating parameters are also suitable as a variable, for example, ignition timing, compression rate or the like. 
   After introduction of the disruptive variable, the engine speed as a command variable of the control circuit will try to adjust. The engine speed control circuit however will immediately counteract in order to maintain the preset engine speed. The engine speed control prevents an increase or drop of the engine speed by appropriate control intervention in the ignition. This activity of the engine speed control circuit is mirrored in the control term and is monitored and evaluated in order to control, based on the result of evaluation, the metering device for the components of the mixture in such a way that one component of the mixture, depending on the result of the evaluation of the activity, is adjusted in order to obtain a mixture composition with an optimized air ratio λ. In this way, the control term of the control circuit, for example, the proportional term of the engine speed controller can be evaluated as a parameter for the control deviation and the control intervention to be carried out. The proportional term of the control loop is thus a measure of the momentary ratio of fuel and air in the mixture. By evaluating the magnitude of the proportional term, the fuel amount and/or the mixture amount can be changed for optimization of the mixture composition such that the control circuit is safely prevented from overshooting/undershooting the control range. 
   For introducing the disruptive variable, advantageously a change of the fuel/air mixture is carried out wherein the fuel proportion and/or the mixture proportion can be changed. 
   Expediently, as a disruptive variable the fuel amount is changed by means of the electromagnetically controlled valve (solenoid valve) in order to adjust, depending on the detected activity of the control loop, the composition of the fuel/air mixture in regard to an optimal air ratio λ (lambda value). This interaction between disruptive variable, control action, and change of the mixture proportions is done until an optimal value for the air ratio λ is reached. 
   Expediently, the parameter to be changed for adjusting λ is the fuel quantity of the fuel/air mixture; however, it can also be advantageous to change the air quantity instead of the fuel quantity. 
   The valve is operated as a synchronized valve that expediently is open in the currentless state. Also, a linear valve or a proportional valve can be used that is to be controlled appropriately. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic illustration of an internal combustion engine with intake part and schematically illustrated control circuit. 
       FIG. 2  is a schematic illustration of a basic circuit diagram of a carburetor arrangement with solenoid valve. 
       FIG. 3  is a flowchart illustrating schematically the method according to the invention. 
       FIG. 4  is a schematic illustration of an engine speed curve plotted against the air ratio λ. 
       FIG. 5  is a schematic illustration of another carburetor arrangement with a solenoid valve for switching the idle jets. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows an internal combustion engine  1  that is a two-stroke engine in the illustrated embodiment. The internal combustion engine can also be embodied as a four-stroke engine, a two-stroke engine with scavenging air or a similar engine. The internal combustion engine  1  illustrated in  FIG. 1  has a cylinder  2  with a reciprocating piston  3  that drives in rotation a crankshaft  5  supported in crankcase  4 . For this purpose, the crankshaft  5  is connected by means of a connecting rod  6  to the piston  3 . 
   The piston  3  controls a mixture intake  10  that takes in combustion air through intake passage  11  and air filter  17  when underpressure is present within the crankcase  4 . In the carburetor  12 , fuel is admixed to the incoming combustion air. By means of the mixture intake  10  controlled by the piston  3  a fuel/air mixture is taken into the crankcase  4  and, as the piston moves downwardly, is conveyed through transfer passages  7  formed in the cylinder  2  into the combustion chamber  8 . By means of ignition control unit  13  an ignition spark is triggered at the spark plug  9 ; the spark ignites the fuel/air mixture compressed in the internal combustion chamber  8  by the upwardly moving piston  3  and, upon downward stroke of the piston  3  as a result of combustion, the crankshaft  5  is driven. The combustion gases are exhausted by means of an exhaust port  18  that is preferably controlled by the piston  3 . 
   A wheel  14 , indicated in dashed lines, is connected to the crankshaft  5 ; it can be a flywheel, a fan wheel or a similar wheel. A magnet is arranged on the wheel  14  and induces a voltage in a coil  15  arranged stationarily relative to the circumference of the wheel  14 . Moreover, on the wheel  14  an engine speed sensor  16  is arranged that monitors the engine speed of the internal combustion engine. The engine speed sensor  16  delivers a pulse sequence wherein for each revolution of the crankshaft  5  preferably several pulses are delivered. The pulse sequence is supplied by signal line  19  to the ignition control unit  13  and processed therein, preferably by using a processor. 
   The voltage of the stationary coil  15  is supplied to the ignition control unit  13  that provides the ignition energy for the spark at the spark plug  9 . Moreover, by means of the induced voltage of the coil  15  a valve circuit  20  that is provided for controlling the solenoid valve  21  is operated also. The valve  21  is provided in the fuel system  30  and serves for controlling the fuel supply. For this purpose, the solenoid valve  21  is connected by control line  22  to the valve circuit  20 . 
   In the illustrated embodiment, by an appropriate control of the solenoid valve  21  the fuel supply into the intake passage  11  is adjustable. It can also be expedient to supply air instead of fuel through a solenoid valve. In the illustrated embodiment, the fuel amount of the mixture is changed for adjusting the mixture. A control for changing the air quantity of the mixture operates in the same way. 
   The fuel system  30  comprises a control chamber  23  of carburetor  12  as well as fuel tank  24  from where fuel is supplied to the control chamber via fuel line  25 . As shown in  FIG. 2 , in the fuel line  25  a fuel pump  26  is arranged for this purpose. The fuel pump  26  can be driven, for example, by the alternating crankcase pressure in the crankcase  4  of the internal combustion engine  1 . The pressure side of the fuel pump  26  is connected by intake valve  27  to the control chamber  23  that is delimited by a diaphragm  28 . The diaphragm  28  controls by means of control lever  29  the intake valve  27  against the force of a spring  29   a.    
   A fuel line  25   a  is connected to the control chamber  23  and branches off to a main jet HD, expediently a partial load jet TD and several idle jets L 0 , L 1 , and L 2  opening into the intake passage  11 . 
   In the embodiment according to  FIG. 2 , in the fuel line  25   a  between the control chamber  23  and the jets a switchable solenoid valve  21  is arranged that, as shown in  FIG. 2 , is open preferably in the currentless state. For this purpose, a spring  21   a  is provided, for example, that force-loads the valve member  21   b  into the open position. For closing the valve  21  and for thus blocking the fuel supply, a control coil of the solenoid valve  21  is to be supplied by valve control circuit  20  with supply voltage. When the supply voltage is supplied to the control coil of the valve  21 , the valve member  21   b  is retracted against the force of the spring  21   a  and fuel supply is blocked. 
     FIG. 2  shows in an exemplary fashion a signal sequence  50  that can be supplied via the control line  22  to the valve  21 . The valve  21 , over time t, is either switched on or off. At the time when a signal is present, the valve  21  is always switched off. The valve is currentless and open. The width of the individual signals  51 ,  52 , and  53  is adjusted in accordance with preset values of the valve control circuit  20  so that the valve  21  is controlled by a pulse width modulated signal sequence  50 . It can also be expedient to configure the control pulses  51 ,  52 ,  53  to be of the same width and to supply a frequency-modulated signal sequence to the valve  21 . Also, the use of an appropriately controlled linear valve or proportional valve can be expedient. 
   Downstream of the valve  21  the main jet path  31  is provided that is connected by means of adjusting element  32  as well as a throttle  33  and a check valve  34  to the main jet HD in the area of the venturi section  35  of the intake passage  11 . 
   Parallel to the main jet path  31 , there is a partial load jet path  36  that supplies by means of a fixed throttle  37  and a check valve  37   a  the partial load valve TD. The partial load valve TD in the flow direction  38  is downstream of the venturi section  35  approximately at the level of the idle jet L 1 . In the schematic illustrations of  FIG. 2  and  FIG. 5 , the jets are illustrated adjacent to one another for the purpose of simplifying the drawing. 
   Downstream of the venturi section  35  and downstream of the main jet HD, a pivotable throttle valve  44  is positioned in the intake passage  11 . In the pivot range of the throttle valve  44 , the idle jets L 1  and L 2  open into the intake passage  11  and, downstream of the throttle valve  44 , the exit port L 0  is positioned. All ports L 0 , L 1 , L 2  are supplied by means of a common idle chamber  40 . The idle jet L 1  is positioned approximately at the level of the axis  45  of the throttle valve  44  and is connected by throttle  42  to the idle chamber  40 . 
   The idle jet L 2  opens upstream of the idle jet L 1  into the intake passage  11  and is connected also by means of throttle  43  to the idle chamber  40 . The idle chamber  40  is connected by idle adjusting screw  39  and preferably a check valve  39   a  to the main jet path  31 . 
   The exit port L 0  opens downstream of the throttle valve  44  into the intake passage  11  and is connected by throttle  41  to the idle chamber  40 . 
   Upstream of the venturi section  35  in the intake passage  11   a  choke  46  is arranged that, as is known in the art, is closed during cold start of the internal combustion engine in order to enrich the mixture. 
   The configuration of the fuel system  30  with solenoid valve  21  serves for providing a modified fuel supply in order to effect, independent of the vacuum in the intake passage  11 , the fuel quantity. For this purpose, the valve control circuit  20  is provided that, through control line  48 , is supplied with control signals of the ignition control unit  13 . 
   The valve control illustrated in  FIG. 3  comprises an engine speed control loop (circuit)  55  which is schematically illustrated by solid lines. After starting the control loop, depending on the load state of the internal combustion engine, engine speed limits are set within which the engine speed is to be maintained by the control loop. Expediently, a lower engine speed n L  and an upper engine speed n H  are set so that, for example, at idle the engine speed control loop  55  maintains the engine speed of the internal combustion engine precisely between the lower engine speed n L  and the upper engine speed n H . For example, an idle speed with a fluctuation width of 400/m can be preset. For recognizing the load state of the internal combustion engine, a load switch  60  can be provided whose output signal is evaluated in a setting unit  61  and converted to appropriate engine speed limits. The load switch  60  recognizes idle and load situations. 
   The preset engine speed limits are compared in the comparators  62  and  63  with the actual engine speed n actual  that is supplied by means of the engine speed sensor  16 . In the first comparator  62 , the actual engine speed n actual  is checked to see whether it is greater than the lower engine speed limit n L . If this is the case, the second comparator  63  is activated which checks whether the actual engine speed n actual  is greater than the upper engine speed limit n H . Is this not the case, the loop is closed by means of branch  64  and another pass through the engine speed loop is carried out depending on the load state (idle/full load). 
   When in the first comparison in the comparator  62  it is determined that the actual engine speed n actual  is below the lower engine speed limit n L , branch  65  is activated by means of which an intervention in the ignition control unit  13  can be done in such a way that the engine speed is increased. In the same way, the comparator  63  is connected to branch  66  when the actual engine speed n actual  is greater than the upper engine speed limit n H . A pass through branch  66  causes an intervention in the ignition control in such a way that the engine speed is lowered. In both branches  65  and  66  this is done by adjusting the ignition timing to “advanced” or “retarded” or, for the purpose of lowering the engine speed, also by suppressing ignition. Both branches  65  and  66  return to the start of the control loop as does branch  64 . 
   A constant idle speed and thus idle quality greatly depend on the composition of the mixture. Therefore, the solenoid valve  21  or  21   b  is provided for controlling the fuel supply. In order to achieve an optimal idle speed that is maintained as uniform as possible based on the engine speed control circuit, the mixture must be adjusted appropriately.  FIG. 4  shows the dependency of the engine speed n on the mixture composition (air ratio λ). 
     FIG. 4  shows clearly that for a mixture that is too lean the engine speed greatly increases and, in an extreme situation, can no longer be lowered by the engine speed control circuit  55  to the desired idle speed. The system is then outside of the control range. On the other hand,  FIG. 4  shows clearly that a mixture that is too rich leads to stalling of the internal combustion engine. The engine speed control circuit  55  can no longer compensate the engine speed drop caused by the mixture being too rich; the engine will stall. 
   In order to ensure a satisfactory control range of the engine speed control circuit  55  upwardly as well as downwardly, it is provided in accordance with the invention to introduce, in regular intervals or stochastically, a disruptive variable Δm s  into the control loop, for example, by changing the mixture composition toward too rich or too lean, as is illustrated by dotted lines in the illustration of  FIG. 3 . At idle, the control can also be permanently active while in the other engine speed range it can operate stochastically. 
   The disruptive variable Δm s  is supplied in accordance with criteria of the ignition control at predetermined timing; this is realized by an appropriate control of the valve control circuit  20 . This can be done, for example, in that a signal  51  is made wider with regard to its pulse width for making the mixture more lean or is reduced with regard to its width for making the mixture richer. 
   Any change of the mixture, as shown in  FIG. 4 , has direct effects on the engine speed n of the internal combustion engine. As schematically illustrated in  FIG. 4 , when the mixture is made leaner, the engine speed n will increase. This increase of the engine speed is suppressed by the control loop. The internal combustion engine itself continues to operate substantially disruption-free wherein the engine speed branch  65  or  66  is used for compensation of the disruptive variable Δm s . The activity of the engine speed controller  55  is reflected in the control term of the control loop so that information can be provided in regard to an expected engine speed deviation, without intervention of the control circuit, by monitoring the control term, e.g. the proportional term or the integral term or even a derivative term. Preferably, the proportional term of the control loop is monitored by a detection device  70  and evaluated. It will calculate, based on the magnitude of the monitored control term, a required change of a proportion of the mixture and supply an appropriate control signal to the valve control circuit  20  in order to change the mixture proportion by a control variable plus or minus Δm i . The valve control circuit  20  changes in accordance with the obtained control command (control variable) the opening characteristics of the valve  21 ; this can be done by pulse width modulation or frequency modulation. The proportion of the fuel in the mixture is thus changed so that the air ratio λ for idle control is optimized, for example, and the engine speed is prevented from overshooting/undershooting the control range. 
   After a new adjustment of the mixture has been performed and the engine speed control loop  55  has compensated a possible engine speed change caused by the change in the mixture, a disruptive variable is supplied again after elapse of a rest period and the work of the engine speed control circuit  55  is evaluated again by monitoring the control term that is necessary for controlling the engine speed change expected as a result of the disruptive variable Δm s . The overall arrangement is designed such that in the respective operating state an air ratio λ is adjusted to be within the range of an optimum  69  shown in  FIG. 4 . 
   In the embodiment according to  FIG. 2 , the valve is provided in the main fuel supply line  25   a . In the embodiment according to  FIG. 5 , the preferably synchronized valve  21  is used for fuel supply into the idle system  40 ,  41 ,  42 ,  43 . The configuration of the fuel system in  FIG. 5  is essentially identical to that of  FIG. 2  so that same parts are referenced by same reference numerals. In the main fuel supply path  25   a  to the fuel system  30   a , an adjusting screw  39   b  for controlling the fuel supply is provided. The control of the valve  21  is realized again by valve control circuit  20  that receives appropriate control signals from the ignition control unit  13 . 
   The arrangement according to the invention can be utilized in a simple way in connection with the control loop  55  of  FIG. 3  or separate therefrom in order to adjust by appropriate control of the valve  21  the idle mixture with regard to its richness. When the load switch  60  indicates idle, the corresponding signal is processed and the valve control circuit  20  is controlled such that the richness of the mixture, i.e., the air ratio λ (lambda value) is constant. The mixture composition can be adjusted optimally wherein the engine speed itself is maintained exclusively by means of the ignition timing at the nominal value or within the preset nominal value limits. 
   In accordance with a different aspect of the invention, instead of maintaining the engine speed constant by changing the ignition timing by means of the engine speed control circuit, the engine speed can also be adjusted by changing the richness of the mixture. The control of the engine speed can also be realized by controlling the valve  21  and the adjustment of the proportions of fuel and air in the fuel/air mixture. 
   When for such a control action the engine switches from idle to load conditions, the valve  21  can be used in order to enrich the fuel/air mixture for improved acceleration. When the operation of the internal combustion engine causes a drop from high engine speed to idle (rich-come-down), the valve  21  can be controlled so as to provide a leaner mixture for avoiding the rich-come-down effect. 
   The arrangement according to  FIG. 5  serves thus also for idle mixture control by means of the solenoid valve  21 . 
   The specification incorporates by reference the entire disclosure of German priority document 10 2006 038 277.3 having a filing date of 16 Aug. 2006. 
   While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.