Abstract:
A method is provided for reducing emissions behind a catalytic converter in an exhaust gas stream of the engine. The method includes, but is not limited to controlling a power supply to a glowplug of a compression-ignition engine. The glowplug is activated if a set of at least two input values remains in a first characteristic region of an input parameter space for at least a predetermined activation time. The first characteristic region consists of one ore more contiguous regions of the input parameter space.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to British Patent Application No. 0914481.7, filed Aug. 19, 2009, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The technical field is related to glowplugs, and more particularly to a glowplug temperature control method and device for the reduction of emissions from a diesel engine. 
     BACKGROUND 
     Diesel engines are typically equipped with a glowplug system. The glowplug system provides a general combustion aid during engine ignition and also during a warm-up phase of the running engine. A key component of this system is the glowplug whose tip can rise up to high temperatures of above 900° C. by means of an electrical to thermal power conversion. 
     Each cylinder is equipped with one glowplug which is turned on when needed on the base of engine and environmental conditions, typically in cold conditions. Glowplugs function as electrical resistors. Their resistance varies with temperature. As the temperature increases, the internal resistance increases, too. 
     Different technologies for glowplugs are in use. Glowplugs may be high or low voltage and they may be of different materials, such as metallic or ceramic glowplugs. High voltage glowplugs are typically supplied directly by a vehicle battery. Low voltage glowplugs in contrast, as they have a nominal voltage lower than the battery voltage, typically need a pulse width modulation (PWM) supply to get the correct voltage. Especially the low voltage glowplugs can be controlled easily by connecting the gates of MOSFETS of the PWM to an electronic control unit and controlling the duty cycle of the PWM. 
     For compression-ignition engines, the most commonly used catalytic converter is the diesel oxidation catalyst. This uses excess O2 (oxygen) in the exhaust gas stream to oxidize CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often reach 90% efficacy and help to reduce visible particulates (soot), however they are incapable of reducing NOx as chemical reactions always occur in the simplest possible way, and the existing O2 in the exhaust gas stream would react first. To reduce NOx on a compression ignition engine, the chemical composition of the exhaust must first be changed. Two main techniques are used: selective catalytic reduction (SCR) and NOx traps or NOx Absorbers. 
     An important development to increase the performance of a catalytic converter is to minimize emissions during the cold start by decreasing the catalyst light-off temperature. 
     During cold start, the temperature of the catalytic converter is low and the converter is not yet activated. Hence the catalyst light-off temperature at which the conversion of an exhaust gas component reaches 50% is not yet reached, hydrocarbons and CO are thus only not converted to a small extend which is why they contribute significantly to the total emissions in the legislated driving cycles during the first couple of minutes after the engine is started. Special techniques have been developed in order to minimize emissions during a cold start. These fast light-off techniques are either passive systems that employ changes in the exhaust system design, or they are active systems that rely on the controlled supply of additional energy to raise exhaust gas temperature during the cold start. 
     SUMMARY 
     According to the application, an improved glowplug control method for the reduction of exhaust gas emissions from a diesel engine is disclosed. Preferably, the emission reduction is achieved in conjunction with a catalytic converter for the diesel engine. According to the application, a method is disclosed for controlling a power supply to a glowplug in order to reduce emissions in an exhaust gas stream of the engine behind a catalytic converter. The glowplug is activated, or, in other words, supplied with power, if a set of at least two input values remains in a characteristic region of an input parameter space for at least a predetermined activation time. 
     The glowplug is deactivated again, or, in other words, the power supply to the glowplug is switched off, if the set of at least two input values remains outside a second characteristic region of the input parameter space for at least a predetermined deactivation time. The deactivation time may also be set to zero. 
     The first and second characteristic regions consist of one or more contiguous regions in the input parameter space. The input parameter space is defined by the input parameters and has as many dimensions as there are input parameters. The input values are the values that the input parameters take and are given by sensor output values or are derived from sensor output values by means of a computation. The first and second characteristic regions may be defined by specifying for each input value a range that is defined by a lower and an upper threshold. In this case, the characteristic region is given by a single contiguous region that takes the form of an n-dimensional cube. 
     Especially, the ranges for the input values may be defined for two input parameters. In this case, the characteristic region takes the form of a square. In a specific example, the input parameters are given by a crankshaft revolution speed and a combustion intake. The combustion intake may be derived, for example, from a fuel intake, an air intake or an intake of an air-fuel mixture. 
     In other embodiments, more than one range may be specified for an input parameter. Other shapes of contiguous regions, for example triangles, circles, spheres and ellipsoids, are possible and different shapes of contiguous regions may be combined to form a characteristic region in the input parameter space. There may be different characteristic regions for switching on and switching off of the glowplugs. 
     The activation and deactivation times and the characteristic regions are stored in a memory of a glowplug control device. They may also be computed by the glowplug control device, which activates and deactivates the glowplugs. A precise control of the glowplug activation and deactivation that makes use of the activation and deactivation times and the characteristic region according to the application allows reducing emissions effectively. 
     A control of the combustion conditions via activation and deactivation of a glowplug exhibits hysteresis effects in that an effect of a glowplug activation may occur after the glowplug activation and an effect may also persist after a glowplug deactivation. According to the application, the hysteresis is taken into account by suitably chosen time intervals and by providing different thresholds for the activation and the deactivation of a glowplug. 
     Apart from the crankshaft revolution speed and the fuel intake, further input values, such as intake air, intake air-fuel mixture, motor torque, vehicle speed, coolant temperature, ambient air temperature and engine intake air temperature may be used to define a characteristic region in the input parameter space. A glowplug is activated when the input values remain in the characteristic region for a predetermined activation time. The glowplug is switched off again when the input values remain outside the characteristic region for a predetermined deactivation time. The glowplugs may be switched on and off together or also sequentially. 
     A maximum activation period may be provided, after which the glowplug is deactivated again. The time intervals, such as the activation time, the activation period and the deactivation time may depend on a combustion chamber temperature or any value which is dependent on the combustion chamber temperature. The activation and the deactivation of the glowplugs may be based on time averaged input values, such as time averaged sensor signals, to further reduce unwanted oscillations in the on/off signal. 
     The application further discloses a method for controlling the power supply to at least one glowplug in which after activating the at least one glowplug the at least one glowplug remains activated for at least a hold time. The hold time may depend on a combustion chamber temperature. 
     The supplied mean voltage during the activation period of the glowplug may be determined individually for each glowplug. Also, the timing parameters like the activation period of the glow plug may be determined individually for each glowplug. The glowplugs may be activated and deactivated together or sequentially. 
     Although the control method will be explained with respect to a pulse width modulation control of glowplugs via MOSFETS, different technologies may also be used such as other types of transistors or a glowplug relays. 
     The method according to the application can be employed without the use of an integrated sensor in the glowplug or a sensor in the combustion chamber, although additional sensors may be used. 
     A glowplug control method according to the invention is able to identify an acceleration phase of the motor and to support the combustion during the acceleration phase when the combustion is not effective. The combustion efficiency is improved and in some cases even the overall efficiency of the engine. This leads to a reduction of emissions. Furthermore, the activation of the glowplug warms up the exhaust gases such that a catalyst light-off effect sets in earlier. Thus, the emissions can be reduced effectively. 
     The reduction of exhaust gas emissions is especially pronounced when the combustion is ineffective, for example during acceleration phases. According to the application, conditions which allow efficient emission reduction by glowplug activation can be identified by measuring a simple set of parameters. The parameters, such as crankshaft revolution speed and fuel intake are readily accessible. 
     As compared to a measurement of the exhaust gas temperature for triggering a glowplug activation, a measurement of engine parameters according to the application is able to detect changed conditions in the combustion chamber directly. It can therefore react faster and reduce the emissions more effectively. However, the exhaust gas temperature may be used as an additional input value. 
     The use of at least two input parameters according to the application, such as crankshaft revolution speed and fuel intake, allows it to differentiate efficiently between different conditions, such as acceleration under load and acceleration during gear shifting. 
     A method according to the invention may even be effective in reducing emissions when it is used in the ‘warm condition’ when a glowplug has already reached its steady state temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and. 
         FIG. 1  illustrates a glowplug control device and controlled glowplugs; 
         FIG. 2  illustrates an applied voltage to a glowplug during an engine warm up phase; 
         FIG. 3  illustrates measurement data and a first glowplug activation pattern; 
         FIG. 4  illustrates measurement data and a second glowplug activation pattern; 
         FIG. 5  illustrates a comparison of engine CO emissions for the glowplug activation patterns of  FIG. 3  and  FIG. 4 ; 
         FIG. 6  illustrates a comparison of exhaust CO emissions for the glowplug activation patterns of  FIG. 3  and  FIG. 4 ; 
         FIG. 7  illustrates a method for glowplug activation; 
         FIG. 8  illustrates a method for glowplug deactivation, 
         FIG. 9  illustrates a first characteristic region; and 
         FIG. 10  illustrates a second characteristic region. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
       FIG. 1  shows a glowplug control device  11  for electric glowplugs  12  which are symbolized by heating coils. The glowplugs  12  are connected to a power supply  13  via field effect transistors (MOSFETS)  14 . A gate of each of the MOSFETS  14  is connected to a corresponding output of a gate drive unit  16  within the glowplug control device  11 . Sense resistors  17  are provided between the drain of each MOSFET and the corresponding glowplug  12 . An input and an output of each of the sense resistors  17  is connected to a corresponding output and a corresponding input of a diagnosis unit  19  within the glowplug control device  11 . 
     The glowplug control device  11  further comprises a logic unit  20  which in turn comprises a diagnostic logic and control logic. A diagnosis output  12  of the logic unit  20  is connected to an engine control unit (ECU) which is not shown. A control input  23  of the logic unit  20  is connected to the ECU. Furthermore, the glowplug control device  11  comprises a mode programming unit  15 . The mode programming unit  15  is connected to sensor outputs via an input  26 . A voltage sensing input  28  of the glowplug control device  11  is connected to the power supply  13  and a power input  29  of the glowplug control device  11  is connected to a supply voltage. 
     During operation, the logic unit  20  receives control input from the ECU and the mode programming unit  15  receives sensor values via the input  26 . Based on the sensor values the mode programming unit  15  determines an operation mode and sends corresponding output values to the logic unit  20 . The sensor values may include, among others, the temperature of an engine coolant, for example of the cooling water, the engine speed, the injected fuel and the output torque of the engine. The ECU makes use of a suitable model to derive a combustion chamber temperature from sensor values and provides the derived combustion chamber temperature at the input  26 . The ECU may also provide further information to the glowplug control device  11 , for example the length of a previous idle phase of the engine motor. 
     The control logic of the logic unit  20  computes a desired effective voltage for each of the glowplugs  12  which is based on the input values to the glowplug control device  11 . The gate drive unit  6  uses the desired effective voltages to compute a length of a duty cycle of a pulse width modulation for each of the glowplugs  12  and controls the gates of the MOSFETS  14  according to the duty cycle. 
     Via the inputs and outputs to the sense resistors  17 , the diagnosis unit  19  derives a voltage drop for each of the sense resistors  17 . From the voltage drops, the diagnostic unit derives supply currents for each of the glowplugs  2 . The diagnostic unit  19  provides the values of the derived supply currents to the mode programming unit  25 . Furthermore, the diagnostic unit  19  generates an error condition if the derived supply current is higher or lower than specified boundary values. 
       FIG. 2  shows the average supply voltages of a glowplug current supply during a preglow phase of a glowplug. During a fast heat up phase  30  from time t0 to time t2, the glowplug is heated at an elevated voltage. The fast heat up phase is subdivided into a first fast heat up phase  31  from time t0 to time t1 in which an average voltage of 11 V is supplied and a second fast heat up phase during which an average voltage of 9 V is supplied. During a heating phase  33 , the glowplug is supplied with its nominal voltage. The length of the heating phase is not drawn to scale, which is symbolized by a gap. After the heating phase, an after-glow phase  34  starts in which the glowplug is only activated from time to time. 
     The diagram of  FIG. 3  illustrates a glowplug activation pattern which arises when a glowplug  12  is activated according to a control algorithm according to the application and the motor is driven according to Urban Drive Cycles (UDC) of the New European Drive Cycle (NEDC). The control algorithm is explained below with respect to  FIGS. 7 and 8 . 
     The diagram of  FIG. 3  shows a square wave on/off signal  36  of a glowplug  12 , a crankshaft revolution speed signal  37 , a fuel intake signal  38  and a cooling water temperature signal  39 . The signals are measured in volt, revolutions per minute, cubic millimeter per stroke degrees Celsius, respectively. A time scale is given in seconds. The diagram shows a time window from about 69 seconds after a cold start of the diesel engine to 690 seconds after the cold start of the diesel engine. 
     According to the temperature signal  39 , the cooling water temperature rises continually in a logarithm like pattern until a final temperature of about 60° C. is almost reached. The continuous activation of the glowplug  12  ends at about 120 seconds. After the continuous activation, the glowplug  12  is controlled by a control algorithm according to the application and remains switched on for an activation period  41  only. In the case of the UDC an activation pattern of activation periods  40  results that has the periodicity of the UDC phases. This can be seen particularly well in comparison with the crankshaft revolution signal  37 . In the example of  FIG. 3 , a total activation duration of 90 seconds of the glowplug results. 
     The maxima of the crankshaft revolution signal  39  reflect the three velocity phases of an UDC. In the diagram, the pattern of the three maxima repeats itself almost four times, which means that the diagram covers almost four UD cycles. The glowplug activation starts approximately with the UDC phase. Furthermore, the idling speed of the motor between the phases decreases slightly. 
     The periodicity of the crankshaft revolution signal  37  is also reflected in the pattern of the fuel intake signal  38 . However, the fuel intake signal  38  is modified by the changing conditions in the combustion chambers. The fuel consumption decreases and the spikes before the first and the third UDC phase almost disappear. The spike before the second UDC phase decreases. Furthermore, several negative spikes of the fuel intake signal mark times when the fuel consumption goes down to very low values due to reduced load during gear switching. 
       FIG. 4  shows a diagram with a second glowplug activation pattern which is due to a simplified algorithm. According to the simplified method, the glowplug is activated when the crankshaft revolution speed is above a threshold value for a certain minimum time. The glowplug is deactivated if the crankshaft revolution speed falls below the threshold value. According to the diagram, the glowplug is only activated during the second phase of the UDC. In the first UDC phase the glowplug is not activated due to low velocity and in the third UDC phase it is not activated due to gear shifting. As a result, the glowplug is activated for 3×20=60 seconds. 
       FIG. 5  and  FIG. 6  show a comparison of CO emissions for a glowplug control method according to the application and for the second control method. In  FIG. 5 , the raw emission of CO from the engine is shown while in  FIG. 6  the cleansed emission of CO behind a catalytic converter is shown. 
     In the diagram of  FIG. 5 , a velocity curve  50 , a first raw emission curve  51  and a second raw emission curve  52  is shown. Scales are in km/h and grams CO/second. The velocity curve comprises four UD cycles which have a first phase  53 , a second phase  54  and a third phase  55  respectively. The first raw emission  51  curve differs from the second raw emission curve  52  mainly in the emission peaks where emissions from the first raw emission curve are lower. The differences are indicated by distances  56 . 
     In the diagram of  FIG. 6 , a velocity curve  50 ′, a first emission curve  51 ′ and a second emission curve  52 ′ are shown. Scales are indicated in km/h and grams CO/second. As in  FIG. 5 , the first raw emission curve  51 ′ differs from the second raw emission curve  52 ′ in the emission peaks where emissions from the first raw emission curve are lower. In addition, emissions of the first raw emission curve  51 ′ during the second UD cycle are also significantly lower. All in all, this results in a significant reduction of CO emission when the glowplugs are heated by a method according to the application as compared to the second control method. This result holds despite the fact that also according to the second control method the glowplug is activated during the second phase of the UD cycle. 
       FIG. 5  and  FIG. 6  show that generally the raw emissions decrease as the cooling water and hence the combustion chambers reach its final temperature.  FIG. 6  shows in addition that the efficiency of the catalytic converter improves significantly as the combustion chambers heat up. A similar result as for  FIG. 5  and  FIG. 6  is also valid for the NOx emissions. 
       FIG. 7  and  FIG. 8  show a glowplug control algorithm according to the application.  FIG. 7  illustrates an activation of a glowplug. It also refers to the activation of several glowplugs which may be activated simultaneously or sequentially. In decision steps  60  it is tested whether the crankshaft revolution speed is between a lower threshold v1_on and an upper threshold v2_on. If this is the case, it is tested in a further decision step  61 , if the fuel intake is between a lower threshold q1_on and an upper threshold q2_on. If the crankshaft revolution speed and the fuel intake lie in the respective ranges, a timer is started in step  62 , otherwise decision steps  60 ,  61  are repeated. 
     After start of the timer, it is again tested in decision steps  63  and  64  if the crankshaft revolution speed and the fuel intake lie in their respective ranges. If this is the case, it is tested in decision step  65  whether an activation time t_activate has been reached. Otherwise, the timer is reset in step  67  and the algorithm loops back to decision step  60 . If, in decision step  65 , it is determined that the activation time has been reached, the glowplug is activated in step  65 . Otherwise, the algorithm loops back to decision step  63 . 
       FIG. 8  illustrates a deactivation of a glowplug. It also refers to the deactivation of several glowplugs which may be deactivated simultaneously or sequentially. In a decision step  68  it is tested whether hold time t_hold has already been reached. If this is the case, it is tested in decision step  69  whether the crankshaft revolution speed lies between a lower threshold v1_off and an upper threshold v2_off. In a decision step  70  it is tested whether the fuel intake lies between a lower threshold q1_off and an upper threshold q2_off. If the crankshaft revolution speed and the fuel intake lie in their respective ranges, the algorithm loops back to decision step  69 . Otherwise a timer is started in step  71 . 
     In a decision step  72  it is again tested whether the crankshaft revolution speed lies between the lower threshold v1_off and the upper threshold v2_off. In a decision step  73  it is again tested whether the fuel intake lies between the lower threshold q1_off and the upper threshold q2_off. If the crankshaft revolution speed and the fuel intake lie within their respective ranges, the timer is reset in step  74  and the algorithm loops back to decision step  69 . Otherwise, it is tested in decision step  75  whether a deactivation time t_deactivate has been reached. If this is the case, the glowplug is deactivated in step  76 . Otherwise, the algorithm loops back to decision step  72 . 
     The engine control unit may—on the basis of data such as cooling water temperature—decide to suspend the glowplug activation. Otherwise, the decision step  60  of  FIG. 8  is executed after deactivation of the glowplug  12 . 
     According to the application, ranges for fuel intake and crankshaft speed are defined by calibratable upper and lower thresholds which may be calibrated at the production facility or at a workshop. After the deactivation step  76 , the glowplug or the glowplugs may remain deactivated for a predetermined deactivation period until step  60  is repeated again. 
       FIG. 9  illustrates the definition of a characteristic region  78  in an input parameter space  79 . The characteristic region  78  is defined by the ranges [q1_on, q2_on] and [v1_on, v2_on]. The definition of ranges leads to a box shape of the characteristic region or, in the case of more then three input parameters, to a multidimensional cube. 
       FIG. 10  illustrates the definition of another characteristic region  78 ′ in an input parameter spaces which is oval shaped. In the case the more general shape of  FIG. 10 , the test for ranges [q1_on, q2_on] and [v1_on, v2_on] of the input parameters q and v must be replaced by a test whether the value (q,v) lies within the characteristic region  78 ′. Therefore, for a general shape of the characteristic region  78 ′, previously explained decision steps like for example the steps  60 ,  61  must be modified accordingly. Like characteristic region  78 , the characteristic region  78 ′ forms a contiguous region as opposed to several disconnected regions. 
     While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.