A Diesel engine is conventionally equipped with an aftertreatment system that comprises an exhaust pipe, for leading the exhaust gas from the engine to the environment, and a plurality of aftertreatment devices located in the exhaust pipe, for degrading and/or removing pollutants from the exhaust gas before discharging it in the environment. In greater detail, a conventional aftertreatment system generally comprises a Diesel Oxidation Catalyst (DOC), for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), and a Diesel Particulate Filter (DPF), located in the exhaust pipe downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas. In order to reduce NOx emissions, most aftertreatment systems further comprise a Selective Reduction Catalyst (SCR), which is located in the exhaust pipe downstream the DPF.
The SCR is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are reduced into diatonic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3), that is absorbed inside the catalyst. The ammonia is obtained through thermo-hydrolysis of a Diesel Exhaust Fluid (DEF), typically urea (CH4N2O), that is injected into the exhaust pipe through a dedicated injector located between the DPF and the SCR.
These aftertreatment systems are generally controlled by an engine control unit (ECU), with the aid of an Universal Exhaust Gas Oxygen (UEGO) sensor, located in the exhaust pipe upstream the DOC, and at least a NOx sensor, located in the exhaust pipe downstream the DPF and upstream the DEF injector. The UEGO sensor is provided for the ECU to measure the oxygen (O2) concentration in the exhaust gas, in order to determine the air to fuel ratio λ within the engine cylinders.
The NOx sensor is provided for the ECU to measure the NOx concentration in the exhaust gas, in order to calculate the quantity of DEF to be injected in the exhaust pipe for achieving an adequate NOx reduction inside the SCR. However, while the conventional NOx sensors provide a measure of the concentration of NOx as a whole, namely without distinction between NO and NO2, the reduction reactions occurring in the SCR depend on the individual concentrations of NO2 and NO, so that the calculation of the DEF quantity can sometime result unreliable.
In greater detail, the NOx reduction reactions in the SCR can happen according to the following chemical equations:4NH3+4NO+O2=4N2+6H2O  (1)2NH3+NO+NO2=2N2+3H2O  (2)8NH3+6NO2=7N2+12H2O  (3)Where the reaction (1) prevails when the NO2 concentration is lower than NO concentration, the reaction (2) prevails when the NO2 concentration is quite similar to the NO concentration, and the reaction (3) prevails when the NO2 concentration is greater than NO concentration.
Notwithstanding the above mentioned reactions involve different DEF consumptions, since the NOx sensors are not able to discriminate NO2 from NO, the calculation of the DEF quantity is currently based on average ANR (Ammonia to NOx Ratio) in the chemical reactions, thereby returning a rough estimation that can lead to an excessive DEF consumption or to a reduced SCR reduction efficiency, depending on exhaust gas conditions. Moreover, the conventional NOx sensors are generally affected by an high NO2 cross sensitivity, which can deviate the measured NOx concentration from the real one. NO2 cross sensitivity is an expression used to indicate that the response signal generated by a conventional NOx sensor is influenced also by the NO2 concentration alone, so that different response signals can be generated by the NOx sensor, even if the actual NOx concentration in the exhaust gas is the same.
In view of the above, it is at leats one object to provide a method for estimating the NO2 concentration in the exhaust gas flowing through an aftertreatment system, so as for example to compensate the NOx sensor readings and/or to perform a more accurate estimation of the DEF quantity to be injected into the exhaust pipe, in order to achieve a reduced DEF consumption and an improved SCR reduction efficiency. At least another object is to reach the above mentioned goal with a simple, rational and rather inexpensive solution. Furthermore, 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.