Patent Description:
An internal combustion engine is typically provided with a number of cylinders, each connected to an intake manifold and to an exhaust manifold, to which an exhaust duct is connected in order to feed the exhaust gases produced by the combustion to an exhaust system, which releases the exhaust gases produced by the combustion into the atmosphere.

An exhaust gas after-treatment system usually comprises a pre-catalytic converter, which is arranged along the exhaust duct; a particulate filter, which is also arranged along the exhaust duct, downstream of the pre-catalytic converter; and a catalytic converter, which is arranged along the exhaust duct, upstream of the particulate filter.

The exhaust gas after-treatment system finally comprises, in addition, a burner, which is suited to introduce exhaust gases (and, as a consequence, heat) into the exhaust duct in order to speed up the heating of the catalytic converter and so as to facilitate the regeneration of the particulate filter. Inside the burner there is defined a combustion chamber, which receives fresh air from an air feeding circuit and receives fuel from an injector, which is suited to inject fuel into the combustion chamber. Furthermore, a spark plug is coupled to the burner in order to ignite the mixture present inside the combustion chamber.

The exhaust gases produced by the burner have a pollutant concentration that proves to be particularly high during an initial burner starting phase. During this initial phase, the temperature of the catalytic converter is lower than the activation temperature and, therefore, the catalytic converter has an extremely low power of conversion of the pollutants produced by the burner.

<CIT> discloses a known method to control an exhaust gas after-treatment system for an exhaust system of an internal combustion engine.

Hence, the pollutants released into the atmosphere and produced both by the internal combustion engine and by the burner, especially during the burner starting phase, must be reduced.

The object of the invention is to provide a method to control a burner for an exhaust system of an internal combustion engine, said method not suffering from the drawbacks described above and, in particular, being easy and economic to be implemented.

According to the invention there is provided a method to control a burner for an exhaust system of an internal combustion engine according to the appended claims.

In <FIG>, number <NUM> indicates, as a whole, a supercharged internal combustion engine provided with an exhaust system <NUM> (not shown) and having a number of cylinders <NUM>, each connected to an intake manifold <NUM> and to an exhaust manifold <NUM> by means of at least one respective exhaust valve (not shown).

The intake manifold <NUM> receives a gas mixture comprising both exhaust gases and fresh air, i.e. air coming from the outside through an intake duct <NUM>, which is provided with an air filter for the fresh air flow and is regulated by a throttle valve <NUM>. Along the intake duct <NUM>, downstream of the air filter <NUM>, there also is an air flow meter <NUM>.

The exhaust manifold <NUM> is connected to an exhaust duct <NUM>, which feeds the exhaust gases produced by the combustion to the exhaust system <NUM>, which releases the gases produced by the combustion into the atmosphere.

The supercharged internal combustion engine <NUM> comprises supercharging system for the internal combustion engine <NUM> obtained by means of a turbocharger <NUM> provided with a turbine <NUM>, which is arranged along the exhaust duct <NUM> so as to rotate at a high speed due to the action of the exhaust gases expelled from the cylinders <NUM>, and a compressor <NUM>, which is arranged along the intake duct <NUM> and is mechanically connected to the turbine <NUM> so as to be caused to rotate by the turbine <NUM> itself in order to increase the pressure of the air present in the feeding duct <NUM>.

The exhaust system <NUM> is provided with an exhaust gas after-treatment system <NUM> comprising a pre-catalytic converter <NUM> arranged along the exhaust duct <NUM>, downstream of the turbocharger <NUM>, and a particulate filter <NUM> (also known as Gasoline Particulate Filter) also arranged along the exhaust duct <NUM>, downstream of the pre-catalytic converter <NUM>. According to a preferred variant, the exhaust gas after-treatment system <NUM> is provided with a catalytic converter <NUM> arranged along the exhaust duct <NUM>, upstream of the particulate filter <NUM>. According to a preferred embodiment, the catalytic converter <NUM> and the particulate filter <NUM> are arranged one after the other on the inside of a common tubular container.

According to a first variant, the internal combustion engine <NUM> is further provided with a UHEGO or UEGO linear oxygen sensor <NUM> housed along the exhaust duct <NUM> and interposed between the turbocharger <NUM> and the pre-catalytic converter <NUM> to detect the air/fuel ratio of the exhaust gases (providing a liner output that indicates the content of oxygen in the exhaust gases) downstream of the turbocharger <NUM> and upstream of the pre-catalytic converter <NUM>.

The internal combustion engine is further provided with a lambda sensor <NUM>, which is suited to provide an on/off binary output, which indicates whether the air/fuel ratio of the exhaust gases is higher or lower than the stoichiometric value, is housed along the exhaust duct <NUM> and is interposed between the pre-catalytic converter <NUM> and the assembly defined by the catalytic converter <NUM> and the particular filter <NUM> in order to detect the concentration of oxygen in the exhaust gases downstream of the pre-catalytic converter <NUM>; and, finally, a lambda sensor <NUM>, which is suited to provide an on/off binary output, which indicates whether the air/fuel ratio of the exhaust gases is higher or lower than the stoichiometric value, is housed along the exhaust duct <NUM> and is arranged downstream of the assembly defined by the catalytic converter <NUM> and the particular filter <NUM> in order to detect the concentration of oxygen in the exhaust gases downstream of the assembly defined by the catalytic converter <NUM> and the particular filter <NUM>.

The exhaust gas after-treatment system <NUM> further comprises a burner <NUM>, which is suited to introduce exhaust gases (and, as a consequence, heat) into the exhaust duct <NUM> in order to speed up the heating of the pre-catalytic converter <NUM> and/or of the catalytic converter <NUM> and so as to facilitate the regeneration of the particulate filter <NUM>. The burner <NUM> is arranged so as to introduce exhaust gases into the exhaust duct <NUM> upstream of the pre-catalytic converter <NUM> or downstream of the catalytic converter <NUM>.

According to <FIG>, inside the burner <NUM> there is defined a combustion chamber <NUM>, which receives fresh air (i.e. air coming from the outside) through an air feeding device <NUM>, which is provided with a pumping device <NUM> pumping from a tank <NUM>, preferably with the interposition of a manifold air filtering element, and feeds air by means of a duct <NUM>.

The combustion chamber <NUM> further receives fuel from an injector <NUM>, which is suited to inject fuel into the combustion chamber <NUM>. Furthermore, a spark plug <NUM> is coupled to the burner <NUM> in order to ignite the mixture present inside the combustion chamber <NUM>. The internal combustion engine <NUM> also comprises a fuel feeding circuit <NUM> provided with a pumping device <NUM>, which feeds fuel by means of a duct <NUM>, which is adjusted by a valve <NUM>.

The internal combustion engine <NUM> finally comprises a control system <NUM>, which is designed to control the operation of the internal combustion engine <NUM>. The control system <NUM> comprises at least one electronic control unit (also known as "ECU"), which controls the operation of the different components of the internal combustion engine <NUM>. It is evident that the electronic control unit ECU disclosed in the description above can be a dedicated control unit ECU, which controls the operation of the burner <NUM>, or can be the electronic control unit ECU controlling the operation of the internal combustion engine <NUM>. The spark plug <NUM> is controlled by the electronic control unit ECU so as to generate a spark between its electrodes, thus determining the ignition of the gases compressed inside the combustion chamber <NUM>. The control system <NUM> further comprises a plurality of sensors connected to the electronic control uni ECU.

The sensors comprise, in particular, a temperature and pressure sensor <NUM> for the air flow fed to the burner <NUM>, which is preferably housed along the duct <NUM>; a temperature and pressure sensor <NUM> for the exhaust gases flowing out of the burner <NUM>, which is housed along an outlet duct <NUM>; a pressure sensor <NUM> for the fuel fed to the burner <NUM>, which is housed along the duct <NUM>. The electronic control unit ECU is further connected to the UHEGO or UEGO linear oxygen sensor <NUM> and to the lambda sensors <NUM>, <NUM>, from which it receives signals indicative of the air/fuel ratio of the exhaust gases.

<FIG> shows the development of the air/fuel ratio λ of the exhaust gases, the pressure signal P<NUM> generated by the combustion inside the burner <NUM> and detected by the sensor <NUM>, the high-frequency content P<NUM>_HF of the pressure signal generated by the combustion inside the burner <NUM> and detected by the sensor <NUM>, the low-frequency content P33_LF of the pressure signal generated by the combustion inside the burner <NUM> and detected by the sensor <NUM> and the temperature T<NUM> detected by the sensor <NUM>.

Experiments have shown that the pressure signal generated by the combustion inside the burner <NUM> and detected by the temperature and pressure sensor <NUM> is rich in information. More in detail, the high-frequency content P<NUM>_HF of the pressure signal generated by the combustion inside the burner <NUM> and detected by the sensor <NUM> is particularly rich in information.

In particular, the Applicant found out, through experiments, that the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> identifies the start of combustion instant SOC represented by the instant in which the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> is greater than a threshold value TVSOC. The threshold value TVSOC is determined in a preliminary set-up phase.

The high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> further identifies the starting time SOC_T through the difference between the start of injection instant SOI (namely, the instant in which the injection of fuel into the combustion chamber <NUM> starts, which is known to the electronic control unit ECU) and the start of combustion instant SOC.

Then, the peak value PKP2HF of the high-frequency content P33_HF of the pressure signal detected by the sensor <NUM> (i.e. the maximum value assumed by the high-frequency content of the pressure signal detected by the sensor <NUM>) is measured.

Finally, the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> controls - by means of a feedback control - the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed. In particular, both aforesaid quantities (i.e. the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed are determined as a function of the starting time SOC_T and of the peak value PKP2HF of the high-frequency content of the signal detected by the sensor <NUM>.

According to a preferred embodiment, the management of the burner <NUM> is divided into distinct steps shown in <FIG>, in which the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed are controlled in a differentiated manner so as to optimize the combustion inside the burner <NUM> and minimize the production of pollutants. <FIG> shows the development of the fuel injection time tINJ, the pressure signal P<NUM> generated by the combustion inside the burner <NUM> and detected by the sensor <NUM>, the air/fuel ratio λ of the exhaust gases, the objective air/fuel ratio λOBJ of the exhaust gases and the objective quantity ṁAIR-OBJ of air to be fed.

The step indicated with P<NUM> starts in the instant in which the pumping device <NUM> is started; the objective fuel quantity ṁFUEL-OBJ is zero, whereas the objective quantity ṁAIR-OBJ of air to be fed and the duration of the step indicated with P<NUM> are variable as a function of the environmental conditions.

During the following step indicated with P<NUM>, the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed are variable as a function of the environmental conditions, whereas the duration of step P<NUM> is variable as a function of the start of combustion instant SOC. The step indicated with P<NUM> has a duration corresponding to the starting time SOC_T. During the following step indicated with P<NUM>, the spark plug <NUM> coupled to the burner <NUM> to ignite the mixture present inside the combustion chamber <NUM> is started; furthermore, during the following step indicated with P<NUM>, the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> is analysed so as to determined the quantities described in the description above, namely the start of combustion instant SOC, the starting time SOC_T and the peak value PKP2HF of the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM>.

As to the following step indicated with P<NUM>, on the other hand, the duration of the step itself, the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed are variable as a function of the starting time SOC_T and of peak value PKP2HF of the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM>.

The duration of the last step indicated with P<NUM>, the objective quantity ṁFUEL-OBJ of fuel to be injected and the objective quantity ṁAIR-OBJ of air to be fed, on the contrary, are variable as a function of the environmental conditions and of the requested thermal power.

The Applicant found out, through experiments, that the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> identifies the end of combustion instant EOC represented by the instant in which the high-frequency content P33_HF of the pressure signal detected by the sensor <NUM> is smaller than a threshold value TVEOC. The threshold value TVEOC is determined in a preliminary set-up phase. The end of combustion instant EOC is used for the control and the diagnosis of the correct operation of the burner <NUM>.

The Applicant further found out that, in case the burner <NUM> and the internal combustion engine <NUM> work simultaneously, the pressure signal detected by the sensor <NUM> is also affected by the exhaust stroke of the cylinder <NUM> (i.e. pressure components generated by the exhaust strokes of the cylinders <NUM> are detected). In particular, experiments revealed that, in the turning-off phase of the burner <NUM>, the simple analysis of the high-frequency content P33_HF of the pressure signal detected by the sensor <NUM> is not sufficient to identify the end of combustion instant EOC in a reliable manner. Hence, in this case, the pressure signal detected by the sensor <NUM>, at first, is filtered with a band-pass filter in the neighbourhood of the injection frequency of the burner <NUM> and, subsequently, the high-frequency content P33_HF of the pressure signal detected by the sensor <NUM> and filtered identifies the end of combustion instant EOC represented by the instant in which the high-frequency content P<NUM>_HF of the pressure signal detected by the sensor <NUM> and filtered is smaller than a threshold value. The injection frequency of the burner <NUM> preferably is <NUM>.

Finally, according to <FIG>, which shows a comparison between the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> and the development of the air/fuel ratio λ of the exhaust gases, the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> is also rich in formation. More in detail, the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> is correlated with the development of the air/fuel ratio λ of the exhaust gases. In particular, the Applicant found out, through experiments, that the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> is strongly correlated with the signal detected by the UHEGO or UEGO linear oxygen sensor <NUM>. The electronic control unit is suited to use the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> in order to control the air/fuel ratio of the exhaust gases; the low-frequency content P33_LF of the pressure signal detected by the sensor <NUM> is used to determine a correction factor, which allows the objective quantity ṁFUEL-OBJ of fuel to be injected to be corrected, thus making the mixture richer or leaner.

Claim 1:
A method to control an exhaust gas after-treatment system (<NUM>) for an exhaust system (<NUM>) of an internal combustion engine (<NUM>) having an exhaust duct (<NUM>); the system (<NUM>) comprises at least one catalytic converter (<NUM>, <NUM>) arranged along the exhaust duct (<NUM>) and a burner (<NUM>), which is suited to introduce exhaust gases into the exhaust duct (<NUM>) to speed up the heating of said at least one catalytic converter (<NUM>, <NUM>), wherein inside the burner (<NUM>) there is defined a combustion chamber (<NUM>), which receives fresh air through an air feeding circuit (<NUM>), which is provided with a pumping device (<NUM>) housed along a first duct (<NUM>), and receives fuel from an injector (<NUM>) for injecting fuel into the combustion chamber (<NUM>), and a spark plug (<NUM>) coupled to the burner (<NUM>) for the ignition of the mixture present inside the combustion chamber (<NUM>); the method comprises the steps of:
housing a pressure sensor (<NUM>, <NUM>) along the first duct (<NUM>) interposed between the pumping device (<NUM>) and the burner (<NUM>) or along a second duct (<NUM>) leaving the burner (<NUM>) ;
acquiring the pressure signal (P<NUM>) detected by said pressure sensor (<NUM>, <NUM>) following the combustion inside the combustion chamber (<NUM>); and
controlling the combustion inside the combustion chamber (<NUM>), by controlling the objective quantity (ṁFUEL-OBJ) of fuel to be injected into the combustion chamber (<NUM>) and the objective quantity (ṁAIR-OBJ) of air to be fed to the combustion chamber (<NUM>), as a function of the high-frequency content (P33_HF) of the pressure signal (P<NUM>) detected by said pressure sensor (<NUM>, <NUM>).