The combustion process in an internal combustion engine produces NOX (principally NO and NO2), CO, CO2, HC (HydroCarbons), and PM (Particulate Matter). The amount of CO2 depends on the amount of fuel injected into the cylinders and the amount of CO and HC depends on the combustion efficiency of the internal combustion engine. The amount of NOX depends on the combustion temperature and on the amount of oxygen introduced into the cylinders, while the amount of PM is strictly dependent on the air to fuel ratio (λ).
To optimize the amount of PM and NOX produced, combustion engines are provided with an EGR (Exhaust Gas Recirculation) circuit. The EGR system recirculates exhaust gas from the exhaust manifold to the intake manifold in order to dilute the fresh air introduced into the engine. This leads to emission optimization during the combustion process, because higher amount of H2O and CO2 are introduced, which have a high heat capacity that reduces the combustion temperature. Another effect of diluting the intake flow is that it is possible to control the amount of O2 in the intake flow. The counter effect of this system is that the more the fresh air is diluted, the more the air to fuel ratio (λ) is reduced. This leads to higher amount of PM emissions. The quantity of exhaust gas flowing into the cylinders is controlled by an EGR valve.
In conventional internal combustion engines there are also an air mass sensor (or air flow meter), an air pressure sensor, an air temperature sensor and an oxygen sensor at the intake manifold. The air mass sensor is adapted to measure the fresh air flow entering the intake manifold through a throttle valve. The air pressure and temperature sensors are adapted to measure the pressure and the temperature of the gas entering into the cylinders, respectively. They are placed in the intake manifold downstream the mixing point between the fresh air flow and the recirculated gas flow.
In conventional engines there is an electronic control unit (ECU) arranged to estimate the gas flow entering into the cylinders and to control the exhaust gas recirculation in the intake manifold. In order to detect a failure in the engine operation, the ECU performs a deviation error monitoring by calculating the difference between a requested (or setpoint) value for a given entity and a corresponding measured value taken from a sensor, so as to detect a deviation of the air system behavior due to failures inside it.
It has been demonstrated that the emissions can be limited by the introduction of the oxygen concentration monitoring in the control of the exhaust gas recirculation in the intake manifold. However, oxygen sensors adapted to measure an actual oxygen quantity in the intake manifold are expensive and do not provide data quickly, this resulting in a delay in obtaining an indication of the deviation of the actual oxygen quantity from a predetermined oxygen setpoint.
In view of the above, it is at least one object of the present invention to provide an improved method for detecting faults in the air system which takes into account the oxygen concentration in the intake manifold without using data directly provided by an oxygen sensor. In addition, it other objects, desirable features and characteristics, will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.
This and other objects are achieved according to the present invention by a method for detecting faults in an air system of an internal combustion engine having an intake manifold and an exhaust manifold. The method comprising the steps of measuring an oxygen concentration ([O2]em_UEGO) of a gas flowing in the exhaust manifold; a) estimating an intake oxygen control value ([O2]im_control) in the intake manifold, estimating an intake oxygen reference value ([O2]im_ECU) in the intake manifold based on the oxygen concentration ([O2]em_UEGO) of the gas flowing in the exhaust manifold, calculating an intake deviation value ([O2]im_dev) as a difference between the intake oxygen control value ([O2]im_control) and the intake oxygen reference value ([O2]im_ECU), and comparing if the intake deviation value ([O2]im_dev) with a predetermined first threshold (TH1). The method further comprising the steps of b) estimating an exhaust oxygen control value ([O2]em_control) in the exhaust manifold, calculating an exhaust deviation value ([O2]em_dev) as a difference between the exhaust oxygen control value ([O2]em_control) and the oxygen concentration ([O2]em_UEGO) of the gas flowing in the exhaust manifold; and comparing said exhaust deviation value ([O2]em_dev) with a predetermined second threshold (TH2). The method also including the steps of c) determining an exhaust oxygen concentration setpoint ([O2]spEM) indicative of the oxygen concentration in the exhaust manifold, calculating a fresh airflow setpoint (Airreference) as a function of the exhaust oxygen concentration setpoint ([O2]spEM), measuring (400) a fresh air mass flow value (mMAF), calculating (1400) an airflow deviation (Airflowdev) as difference between said fresh airflow setpoint (Airreference) and the fresh air mass flow value (mMAF), and comparing said airflow deviation (Airflowdev) with a third predetermined threshold (TH3) and a fourth predetermined threshold (TH4), and finally detecting faults in the air system as a function of the combination of results of comparisons at step a), b) and/or c).