It is known that an internal combustion engine generally comprises an engine block having at least a cylinder that accommodates a reciprocating piston coupled to rotate a crankshaft. A cylinder head closes the cylinder and cooperates with the piston to define a combustion chamber. An air and fuel mixture is inducted into the combustion chamber and ignited to produce hot expanding exhaust gasses that cause the movements of the piston.
The exhaust gasses exit the combustion chamber and are directed into an exhaust system, which includes an exhaust pipe coupled to the internal combustion engine and one or more exhaust aftertreatment devices configured to change the composition of the exhaust gasses flowing in the exhaust pipe. These aftertreatment devices generally include at least an oxidation catalyst (DOC) provided for oxidizing hydrocarbons and carbon monoxide into carbon dioxide and water, and a particulate filter (DPF), which is located downstream of the oxidation catalyst for trapping particulate matter or soot from the exhaust gasses.
More particularly, the particulate filter comprises a porous substrate structure that collects liquid and solid particles, while allowing the exhaust gasses to flow through. As a consequence, during the normal operation of the internal combustion engine, the quantity of soot accumulated inside the particulate filter progressively increases.
The quantity of soot accumulated inside the particulate filter, which is also conventionally referred as soot loading level, is constantly monitored by an electronic control unit (ECU).
More particularly, the ECU is generally configured to repeatedly measure a variation of the pressure drop across the diesel particulate filter, to apply the variation of the pressure drop to a mathematical model of the diesel particulate filter that yields an estimated variation of the soot loading level, and then to increase the soot loading level with the estimated variation.
As the soot loading level exceeds a predetermined threshold, which represents the nominal storage capacity of the diesel particulate filter, the ECU operates the internal combustion engine so as to promote an “active” regeneration of the diesel particulate filter, during which the accumulated soot is burned off and the original efficiency of the diesel particulate filter is restored.
More particularly, the ECU generally operates the internal combustion engine so as to increase the amount of hydrocarbons (HC, unburned diesel fuel) contained in the exhaust gasses. This increased amount of hydrocarbons is oxidized inside the diesel oxidation catalyst, thereby raising the temperature of the exhaust gasses flowing through the diesel particulate filter. In this way, the diesel particulate filter is heated up to a temperature (about 600-800° C.) which promotes a continuous oxidization of the accumulated soot.
However, the active regeneration is not the only regeneration process which the diesel particulate filter is subjected to. In fact, during the normal operation of the internal combustion engine, the diesel particulate filter may be subjected to a “passive” regeneration, also referred as CRT (Continuously Regenerating Trap), which causes part of the accumulated soot to be burned off.
A passive regeneration is a phenomenon that generally happens if the temperature of the exhaust gasses passing through the diesel particulate filter is comprised between 250° C. and 450° C., and the content of nitrogen oxides (NOx) at the inlet of the diesel particulate filter is large enough to promote the oxidation of the accumulated soot.
During a passive regeneration, the quantity of soot that actually oxidizes is generally smaller than the quantity of soot that, at the same time, is carried by the exhaust gasses and is trapped inside the diesel particulate filter, so that the net quantity of soot accumulated inside the diesel particulate filter is still increasing. Despite the increase of the accumulated soot quantity, the pressure drop across the diesel particulate filter during the passive regeneration generally decreases. The cause of this reduction of the pressure drop is that the passive regeneration oxidizes the soot which is trapped deeply in the porosity of the porous substrate structure of the diesel particulate filter, whereas the soot carried by the exhaust gasses is accumulated on the external surfaces.
The mathematical model used by the ECU to estimate the soot loading level in the diesel particulate filter generally correlates any reduction of the pressure drop to a correspondent reduction of the soot loading level. As a consequence, during a passive regeneration, this mathematical model may lead to a strong underestimation of the soot loading level inside the diesel particulate filter.
In order to solve this side effect, the ECU generally checks whether the diesel particulate filter is operating under the conditions for which a passive regeneration occurs, namely whether the temperature of the exhaust gasses is comprised in a predetermined range of values (typically 250-450° C.) and the nitrogen oxides content at the inlet of the diesel particulate filter exceeds an upper threshold. If these conditions are met, the ECU corrects the soot variation provided by the mathematical model with an additive correction.
At present, the additive correction is determined by means of three calibration maps, each of which is empirically determined for a different level of the environmental pressure and is designed to correlate the additive correction to the engine speed and the engine load. In use, the ECU measures the environmental pressure, the engine speed and the engine load, then uses the environmental pressure to select the proper calibration map, and the engine speed and the engine load to read in the selected calibration map the corresponding additive correction.
A drawback of this known strategy is that the estimation of the soot variation inside the diesel particulate filter during a passive regeneration phenomenon is still not sufficiently reliable, because of the too few parameters that are taken into account for determining the additive correction.
At least one object herein is that of solving the above mentioned drawback, in order to improve the estimation of the soot variation and consequently of the soot loading level in the diesel particulate filter.
At least another object is to attain this goal with a simple, rational and rather inexpensive solution. In addition, 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.