Abstract:
A method of measuring the particle content, in particular the soot particle content, in the exhaust gases of an internal combustion engine using a sensor arranged in contact with the exhaust gases. The sensor detects in at least one combustion chamber of the internal combustion engine an electrical quantity, the size of which is dependent on the particle content in the exhaust gases. The level of a measurement signal output from the sensor, the signal is dependent on the detected quantity, is determined and compared with a desired value level of a signal which corresponds to a desired particle content in the exhaust gases.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of measuring the particle content, in particular the soot particle content, in the exhaust gases of an internal combustion engine by means of a sensor arranged in contact with the exhaust gases. 
     In piston engines, in which the fuel is injected directly into the combustion chamber, such as GDI (Gasoline Direct Injection) engines and diesel engines, problems of soot formation in the exhaust gases occur under certain operating conditions. This is due inter alia to the fact that the air/fuel mixture in the combustion chamber is what is referred to as layered or, more usually, stratified. A further reason for soot formation in the exhaust gases is that, under certain operating conditions, the fuel injected directly into the cylinder strikes the piston and the surrounding cylinder walls. In GDI engines, it is desirable under certain operating conditions to work with a stratified air/fuel mixture because a lower fuel consumption can then be obtained. In order to ignite a lean air fuel/mixture in the combustion chamber using a spark plug, a locally richer mixture must be produced around the region where the spark plug is located. This state with the mixture being divided into a lean and a rich mixture in the combustion chamber is consequently referred to as stratified. The opposite of a stratified air/fuel mixture is a homogeneous air/fuel mixture. 
     When soot is formed in the exhaust gases, the combustion of the fuel injected into the combustion chamber becomes incomplete, which means that substances that have an effect on the environment are formed in the exhaust gases. In order to prevent soot formation in the exhaust gases, various measures can be taken. For example, the moment of fuel injection can be regulated and/or the moment of ignition of the air/fuel mixture in the combustion chamber can be regulated. It is also possible to regulate the ratio of air to fuel in the air/fuel mixture. However, the problem is detecting when soot formation in the exhaust gases is taking place and how much soot is being formed. 
     A method and an arrangement for measuring the soot particle content in the exhaust gases of an internal combustion engine are described in DE-A1-33 04 548. An insulator in the form of a ceramic plate is located in the exhaust pipe of the internal combustion engine. Two electrodes are arranged on the insulator at a distance from one another so that a gap is formed between the electrodes. When soot is formed in the exhaust gases, the soot adheres to the ceramic plate between the insulators. The soot then acts as an electric conductor between the electrodes. The quantity of soot on the ceramic plate is proportional to the resistance between the electrodes. The more soot that adheres to the ceramic plate, the smaller the resistance becomes. The resistance can then be measured and an idea of the soot particle content can thus be obtained. 
     In the case of a GDI engine and a diesel engine which, under certain operating conditions, work with a stratified air/fuel mixture, however, a measurement of the soot particle content in the exhaust pipe does not give an accurate picture of how complete the combustion of the fuel in the combustion chamber is. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a method of the type indicated in the introduction which gives a picture of the particle content in the exhaust gases in the combustion chamber of an internal combustion engine. 
     A further object of the present invention is to provide a method of continuously measuring during operation of the engine the particle content in the exhaust gases in the combustion chamber of an internal combustion engine and, against the background of measurement values obtained, regulating the internal combustion engine so that a lower particle content in the exhaust gases is. obtained. 
     According to the invention, this is achieved by virtue of the fact that the sensor detects in at least one combustion chamber of the internal combustion engine a quantity, the size of which is dependent on the particle content in the exhaust gases, and that the level of a measurement signal output from the sensor, which signal is dependent on the detected quantity, is determined and compared with a desired value level of a signal which corresponds to a desired particle content in the exhaust gases. 
     Such a method of measuring the particle content in the exhaust gases provides a true picture of how complete the combustion of the fuel in the combustion chamber is. With a sensor located in the combustion chamber, such as the spark plug of a GDI engine and the heater plug of a diesel engine, simple and economical measuring equipment is obtained for measuring the particle content in the exhaust gases in the combustion chamber in conventional GDI and diesel engines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail below with reference to exemplary embodiments shown in the appended drawings, in which: 
     FIG. 1 relates to a coordinate system which shows the ionic current as a function of the crankshaft angle in the case of low soot particle content in the exhaust gases in the combustion chamber, 
     FIG. 2 relates to a coordinate system which shows the ionic current as a function of the crankshaft angle in the case of a soot particle content in the exhaust gases in the combustion chamber which is greater than the soot particle content represented by the function in FIG. 1, 
     FIG. 3 relates to a diagrammatically represented sensor and measuring unit in the case of a GDI engine, 
     FIG. 4 relates to a diagrammatically represented sensor and measuring unit in the case of a diesel engine, and 
     FIG. 5 shows a diagrammatic view of a measuring unit connected to a spark plug. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a graph  1  which represents the ionic current I J  that is formed in combustion chamber of a piston engine in connection with the combustion stoke. The graph  1  is drawn in a coordinate system where the X axis represents the crankshaft angle α and the Y axis represents the ionic current I J . By arranging a sensor, which is connected to a measuring unit, in the combustion chamber, the ionic current signal shown in FIGS. 1 and 2 can be sampled in the measuring unit. 
     FIG. 3 shows an embodiment in which a measuring unit  2  is connected to the spark plug  3  of a GDI engine  4 . The spark plug  3  therefore constitutes a sensor for providing signals to the measuring unit  2 . 
     FIG. 4 shows another embodiment, in which a measuring unit  2 ′ is connected to a heater plug  5  of a diesel engine  6  of the prechamber type. The heater plug  5  in this case constitutes a sensor which provides signals to the measuring unit  2 ′. 
     The graph for the ionic current signals shown in FIG. 1 consists of three different stages, namely the ignition stage S, the flame ionization stage F and the post-ionization stage P. During the ignition stage S, the charging action of the current in the ignition coil produces an interference which gives rise to a small current peak. A spark from the spark plug  3  then produces a strong pulse with a negative sign (broken line in FIG.  1 ). This negative pulse is filtered out. A large pulse then follows, which arises when the current to the spark plug  3  is broken. During the flame ionization stage F, a burning core of fuel/air mixture in or at the spark plug gap produces a pulse peak. During the post-ionization stage P, the combustion pressure in the combustion chamber  7  rises to its highest value and the flame front has then reached the walls of the combustion chamber  7 . 
     According to the invention, a number of graphs for different operating conditions and different air/fuel, mixtures of an “ideal” internal combustion engine can be stored, in respect of which operating conditions and air/fuel mixtures a desired soot particle content in the exhaust gases is obtained. The level of these graphs is calculated and desired values are obtained for different operating conditions and air/fuel mixtures. 
     The level of the graph  1  means the position of the entire graph  1  above the X axis in the coordinate system. If the ionic current I J  increases for each crankshaft angle α during the combustion stroke, the entire graph  1  is shifted upwards in the coordinate system, that is to say the level increases. 
     The level of the graph  1 , which represents the ionic current I J  that is detected by the sensor  3 ,  5  arranged in the combustion chamber  7 , can be calculated in different ways in the measuring unit  2 ,  2 ′ connected to the sensor. For example, one or more points of the sampled measurement signal, where the first derivative changes the sign from positive to negative, can be detected. Consequently, a value of the ionic current I J  at the peaks of each stage S, F and P is then obtained. These values provide a measure of the level of the graph  1 . Another way of obtaining the level of the graph  1  is to integrate the ionic current I J  as a function of the time during the combustion stroke or a part thereof. The integral consequently provides the area under the graph  1 , which is a measure of the level of the graph  1 . 
     If the soot particle content in the exhaust gases increases, the level of the graph  1  above the X axis will increase, as shown in FIG.  2 . The more the soot particle content increases, the higher the level of the graph  1  becomes. 
     This will be explained in greater detail below. 
     FIG. 5 shows a measuring unit  2  connected to a conventional spark plug  3 . The measuring unit  2  is connected between the positive and negative (earth potential) electrodes  8  and  9  respectively of the spark plug  3 . Between the positive electrode  8  and the housing  10  of the spark plug  3 , in which housing the negative electrode  9  is arranged, there is an insulator  11  made of an electrically insulating material. FIG. 5 also shows diagrammatically how an ignition coil  13  is connected to the spark plug  3 . 
     When soot particles are formed during the combustion stroke, these adhere to the insulator  11 . The more soot particles that are formed, the thicker the soot deposit on the insulator  11  becomes. The soot particles conduct current, as a result of which a leakage current I L  arises between the positive electrode  8  and the housing  10  which is connected to earth. The size of the leakage current I L  is a measure of the soot particle content in the exhaust gases. 
     The leakage current affects the measurement of the ionic current I J  in such a manner that when soot particles adhere to the insulator  11 , the leakage current I L  increases and thus the level of the ionic current signal also increases during measurement of the ionic current I J . The more soot that adheres to the insulator  11 , the more the level of the graph  1  representing the ionic current I J  is raised. 
     The leakage current I L  can itself be measured by a predetermined current being induced between the electrode  8  and the housing  10 . The current that will then be transmitted between the electrode  8  and the housing  10  constitutes the leakage current I L . FIG. 2 shows this leakage current I L  as a straight line at the end of and after the post-ionization stage P and also before and during the ignition stage S. The leakage current I L  is suitably measured at given defined crankshaft angles α or points in time, that is to say in given measurement windows, in which measurement windows the predetermined current is induced between the electrode  8  and the housing  10 . The predetermined current that is induced between the electrode  8  and the housing  10  can be induced continuously during the cycle of the whole engine, that is to say during all strokes, for which a suitable measurement window can be defined by detecting the leakage current I L  when the first derivative of the latter is zero for a continuous time interval. 
     Instead of measuring the leakage current I L , it is possible to measure the impedance Z between the electrode  8  and the housing  10 . As the soot particles that adhere to the insulator  11  conduct current, the impedance Z between the electrode  8  and the housing  10  will decrease when soot particles adhere to the insulator  11 . When the impedance Z between the electrode  8  and the housing  10  is measured, the resistance, inductance and capacitance between the electrode  8  and the housing  10  will be taken into consideration at one and the same time of measurement. Instead of measuring the impedance Z, the resistance, inductance and capacitance can be measured individually. 
     A suitable measurement window for measuring the soot particle content in the combustion chamber  7 ,  7 ′ may be during charging of the ignition coil  13  during the ignition stage F. This can be advantageous when the soot particle content is to be measured in a number of combustion chambers  7 ,  7 ′ of the internal combustion engine  4 ,  6  because it then becomes simple to coordinate the measurements between the various sensors  3 ,  5 . 
     By connecting the measuring unit  2  to the control unit  12  (see FIG. 3) of the internal combustion engine  4 , the moment of ignition, the moment of fuel injection and/or the air/fuel mixture can be regulated so that the soot particle content in the exhaust gases decreases. When the soot particle content in the exhaust gases has decreased, the soot deposited on the insulator  11  will be burnt off during the combustion stroke. 
     Measurement of the soot particle content in a diesel engine  6  takes place in a similar manner to that described above. The heater plug  5  has an insulator (not shown), to which soot particles can adhere. The diesel engine  6  is also regulated by the measuring unit  2 ′ being connected to a control unit  12 ′ which adjusts the moment of fuel injection and the volume of fuel injected. 
     Instead of using the spark plug  3  or the heater plug  5  as the sensor, it is also possible for a separate sensor to be arranged in the combustion chamber  7 ,  7 ′, which sensor has an insulator to which soot particles can adhere.