WO2001059269A2

WO2001059269A2 - Fuel injecting method for regenerating a nitrogen oxide trap

Info

Publication number: WO2001059269A2
Application number: PCT/FR2001/000377
Authority: WO
Inventors: Pierre Blanche; Laurent Philippe
Original assignee: Renault
Priority date: 2000-02-11
Filing date: 2001-02-09
Publication date: 2001-08-16
Also published as: FR2804997B1; FR2804997A1; EP1254303A2; WO2001059269A3

Abstract

The invention concerns a method for injecting fuel into at least one of the cylinders of a combustion engine (12), in particular a diesel engine, for regenerating a nitrogen oxide trap (16), which consists in injecting a main amount of fuel depending on the engine (12) operating point. The invention is characterised in that it consists in increasing the main amount during a phase regenerating the nitrogen oxide trap (16).

Description

"Fuel injection process for the regeneration of a nitrogen oxide trap"

The invention relates to a fuel injection method for a combustion engine. The invention relates more particularly to a method of injecting fuel from a combustion engine, in particular from a diesel engine or from a lean mixture gasoline engine for the regeneration of an oxide trap. nitrogen.

Examples are known of methods for regenerating nitrogen oxide traps which make it possible to reduce the emissions of nitrogen oxides.

Diesel engines and certain petrol engines (or spark ignition) operate with lean carbide mixtures, that is to say with an excess of oxygen. The combustion of poor mixtures produces nitrogen oxides or NOx which must be treated by catalytic conversion.

To reduce nitrogen oxides when the exhaust gases are rich in oxygen, one method consists in storing the nitrogen oxides contained in the exhaust gases in a nitrogen oxide trap also called "NOx trap". "when the composition of the exhaust gases does not allow their reduction, then periodically regenerating the nitrogen oxide trap by massively injecting reducing substances such as fuel, to reduce the nitrogen oxides stored in the trap.

The regeneration of the nitrogen oxide trap requires a modification of the chemical composition of the exhaust gases. A sufficient exhaust gas temperature and oxygen depletion are required to regenerate the adsorbed nitrogen oxides. In practice, it is necessary that the oxygen concentration, during desorption, be less than To reduce air pollution, nitrogen dioxides, which are a toxic substance, must be reduced to N ₂ which is not toxic, before exhaust gases are released into the atmosphere. Thus, it is necessary for the exhaust gases to contain reducing agents such as HC hydrocarbons or carbon monoxide CO, in order to reduce the nitrogen oxides.

Known methods propose, during the regeneration phase, to increase the hydrocarbon content of the exhaust gases, by post-injecting fuel into the combustion chamber, or directly into the exhaust line. Thus, on the one hand the oxygen content of the exhaust gases is reduced, which makes it possible to desorb the nitrogen oxides absorbed by the trap and, on the other hand, the reducing species supplied in excess, such as HC hydrocarbons, reduce nitrogen oxides to N ₂ .

However, the postinjectioπ must be performed late after the passage of the piston in top dead center. The walls of the combustion chamber are then largely sprayed with fuel, which causes a dilution of the lubricating oil of the contact between the piston and the walls of the chamber. The dilution of oil in the fuel increases the consumption of oil, as well as the formation of black smoke in the exhaust gases released into the atmosphere.

Another method proposes to reduce the oxygen concentration by recirculating the exhaust gases in the combustion chambers of the engine. To achieve such recirculation, it is necessary to interpose in the intake circuit a valve called "butterfly" which tends to degrade the performance of the engine, see to produce fumes. The management of the valve opening angle as a function of the quantity of gases which must be recirculated is ensured for example by an electronic control system. In addition, the nitrogen oxide trap stores, concomitantly with nitrogen oxides, sulfur which it is therefore necessary to release periodically. One solution consists in carrying out a so-called "thermal" purge which requires high temperatures of the order of 700 ° C. The nitrogen oxide trap regeneration methods do not allow such temperature values to be reached.

It is then necessary to use additional systems for heating the nitrogen oxide trap such as an electric heating resistor which consume a lot of energy.

In order to remedy these drawbacks, the invention provides a method of injecting fuel into at least one of the cylinders of a combustion engine, in particular a diesel engine, for the regeneration of a nitrogen oxide trap, of the type q which involves the injection of a main quantity of fuel depending on the operating point of the engine, characterized in that, during a regeneration phase of the nitrogen oxide trap, the main quantity is increased.

According to other characteristics of the invention: - during a regeneration phase of the nitrogen oxide trap, the start of the injection of the main quantity of increased fuel is out of phase with respect to the passage of the piston in neutral high ;

- The start of the injection of the main quantity of increased fuel is out of phase by at least fifteen degrees of angle of rotation of the crankshaft after the passage of the piston to top dead center;

- the main quantity of fuel increased and out of phase is such that the ratio between the quantity of fuel and gas admitted into the cylinder of the combustion engine is greater than 1;

- the injection of the main quantity of fuel is preceded by a step of injecting another quantity predetermined fuel dependent on the engine operating point;

- during a regeneration phase of the nitrogen oxide trap, said other predetermined quantity of fuel is increased;

- during the regeneration phase, part of the exhaust gas is recirculated in the gas admitted into the cylinder, in particular to promote the initiation of fuel combustion; during the regeneration phase of the nitrogen oxide trap, it makes it possible to increase the value of the temperature of the exhaust gases to a value greater than or equal to a predetermined temperature for purging, by combustion, the sulfur stored in the nitrogen oxide trap. Other characteristics and advantages of the invention will appear on reading the detailed description which follows for the understanding of which reference will be made to the appended drawings among which:

- Fig ure 1 is a schematic representation of an exhaust line of a combustion engine equipped with a nitrogen oxide trap;

- Figure 2 is a diagram which represents the quantities of fuel injected as a function of the angle of the crankshaft relative to the top dead center of the piston, according to the state of the art; - Figure 3 is a diag ramme which represents the amount of fuel injected, depending on the angle of the crankshaft relative to the top dead center of the piston, according to the invention;

FIG. 4 represents a diagram which makes it possible to determine the phase shift, relative to the top dead center of the piston, of the start of the injection of the quantity of fuel as a function of the engine speed and of the torque to be supplied;

- Figure 5 is a ram diagram similar to that of Figure 3 according to a variant of the invention. There is shown on fig ure 1, a processing system

10 of the exhaust gases G of a combustion engine 12. The engine 12 is a diesel engine or a gasoline engine operating in a lean mixture such as a gasoline engine with direct injection.

An exhaust line 14 allows the evacuation of the gases G from the engine to the atmosphere. A treatment system intended to purify the exhaust gases G is interposed in line 14. I t mainly consists of a nitrogen oxide trap 16 called "NOx trap" arranged in a chamber 18.

An oxidation catalyst 20, of the priming catalyst, is advantageously inserted into the exhaust line 14 upstream of a nitrogen oxide trap 16.

The injection of fuel into the engine 12 is controlled by an electronic control system 22 which makes it possible to manage the law of fuel injection into the combustion chambers.

The electronic control system 22 controls the injection of the quantities of fuel into the combustion chambers of the engine 12 by playing on two main parameters:

- the instants of the start of the injection of the fuel quantities and the opening time of the injectors; and

- the fuel pressure at the outlet of the injectors. In the case of a direct diesel injection of the "common rail" type, the electronic control system 22 controls the pressure inside the supply rail, in the case of a direct injection by of so-called "pump" injectors, the electronic control system 22 controls the pressure inside each injector. Thus, the opening time of the injectors being determined, the electronic control system controls the pressure of the fuel at the outlet of the injectors so that the entire quantity of determined fuel is injected before the injectors are closed. Measuring means 24, such as pressure sensors, temperature sensors and nitrogen oxide sensors are arranged in the exhaust line and are connected to the electronic control system 22. Advantageously, the means 24 provide the concentration of nitrogen oxides in the exhaust gas G downstream of the nitrogen oxide trap 16. The concentration of nitrogen oxides, provided by the measurement means 24 corresponds to the filling level of the nitrogen oxide trap 16. In fact, when the nitrogen oxide trap 16 is saturated, the nitrogen oxides which are produced by the combustion of the fuel in the engine 12 are no longer stored and they then pass through the trap 16 and are almost entirely evacuated to the atmosphere. It is also advantageous to introduce a bypass 26 into the line 14 upstream of the treatment system 1 0. The bypass 26 allows the connection of a line 28 which connects the lig ne 14 to the air intake 30 of the engine 12. A valve 32 located on the line 28 makes it possible to adjust the flow rate of the exhaust gases G in the line 28. A recirculation circuit 34 of the exhaust gases G known as RG E is thus produced.

The operation of the processing system 10 is as follows.

The exhaust gases G produced by the engine 12 are evacuated in line 14.

When passing through chamber 18, the nitrogen oxides in the gases G are stopped by the nitrogen oxide trap 16.

Periodically, the nitrogen oxides thus trapped in the nitrogen oxide trap 16 are desorbed and reduced during a regeneration phase.

The measuring means 24 transmit to the electronic control system 22 information representative of the loading level of the nitrogen oxide trap 16. The electronic control system 22 takes into account the level of loading of the nitrogen oxide trap 16, as well as the operating point of the engine, in order to trigger the regeneration phase. Regeneration of the nitrogen oxide trap 16 requires an oxygen concentration of the exhaust gases G of less than 1% and an excess of reducing substances.

Several solutions can be implemented independently or concomitantly. Among these, in accordance with FIG. 2, an additional quantity Q2 of fuel or post-injection, is injected into the combustion chamber of the engine after the injection, in the vicinity of the top dead center TDC, of a main quantity of fuel Q 1 intended to supply the engine torque. The end of the injection of the additional quantity Q2 is carried out before a point located at 60 ° angle of rotation of the crankshaft after the piston has passed to top dead center.

The additional quantity Q2 undergoes partial combustion which reduces the concentration of the exhaust gases G in oxygen, which releases a large quantity of unburnt hydrocarbons HC and which makes it possible to increase the temperature of the exhaust gases G.

The invention consists in injecting an additional quantity Q2 'of fuel into the combustion chamber of the engine 12, the quantity Q2' being grouped with the injection of the main quantity Q 1 as shown diagrammatically in FIG. ure 3.

The injection of the additional quantity of fuel Q2 'can, for example, be merged with the injection of the main quantity Q 1, which results in a single injection of a quantity of fuel Q 1 + Q2', or although the injection of the additional quantity of fuel Q2 ′ can be "joined" to the main quantity of fuel Q 1 in the form of a second injection immediately following the injection of the main quantity of fuel Q 1. In the following description, the main quantity of fuel increased Q1 + Q2 'will be called the sum of the main quantity Q1 and the additional quantity Q2'.

The start of the injection of the main quantity Q1 is phase shifted by at least 15 ° of angle of rotation of the crankshaft after the passage of the piston at top dead center. In addition, the sum of the main quantities Q1 and additional Q2 'is such that the richness of the fuel mixture, that is to say the ratio between the quantity of fuel and gas admitted into the cylinder of the engine 12, is greater than 1. It is preferably of the order of 1.1 to 1.2.

Thus, the oxygen concentration is sufficiently low and the temperature of the exhaust gases G is sufficiently high to allow the reduction of the nitrogen oxides of the trap 16, with unburnt HC hydrocarbons and carbon monoxide CO present in sufficient quantity to reduce them.

The oxidation catalyst 20 makes it possible to obtain a further reduction in the oxygen concentration by oxidizing part of the reducing species such as HC hydrocarbons or carbon monoxide CO, present in the exhaust gases G.

The priming of the oxidation catalyst 20 is favored by the injection of the increased main quantity Q1 + Q2 ′ which causes an increase in the temperature of the exhaust gases G at the outlet of the cylinders of the engine 12.

The diagram shown in FIG. 4 makes it possible to determine the phase shift of the start of the injection of the quantity of fuel Q1 + Q2 ′ as a function of the speed N of the engine 12 and of the torque C to be supplied. For example, when engine 12 rotates at 2400 rpm and a torque of 50 Nm is required, the start of injection is out of phase by -15 ° crankshaft (° V) after the piston has passed the top dead center. The injection method according to the invention thus allows i, during the regeneration phases of the nitrogen oxide trap 16, to reduce a significant part of the nitrogen oxides.

In addition, when the processing system 10 comprises a recirculation circuit 34, a low flow rate of the exhaust gases G recirculated is necessary. This low recirculation promotes fuel combustion. The unburnt hydrocarbons HC q ui are reintroduced into the combustion chambers of the engine 12, participate very early in the combustion and therefore favor its initialization. This makes it possible to compensate for the degradation of the yield due to the phase shift of the main injection in the cycle.

Thus, it is possible to eliminate the valve 32 and its electronic control system, the diameter of the passage of the exhaust gases G from the conduit 28 being determined so as to ensure the gas flow necessary for recirculation.

Since the fuel injection is unique, the electronic control system 22 is simplified since the injection law comprises a single injection, instead of a main injection Q 1 and an additional injection Q2.

Finally, by modifying the additional quantity Q2 ′ of fuel, it is possible to increase the temperature of the exhaust gases G sufficiently to reach the predetermined value, of the order of 700 ° C., which allows the so-called "purge" thermal "of the sulfur stored in the nitrogen oxide trap 16.

Preferably, the modification of the additional quantity Q2 \ which causes the temperature to increase up to the predetermined value, is carried out concomitantly with the regeneration phase of the nitrogen oxide trap 1 6. The frequency of the thermal purge of the sulfur stored in the nitrogen oxide trap 16 can be a multiple of the frequency of the regeneration phase of the trap 16.

According to a variant, it is known to inject a predetermined quantity or q uantity quantity, before the injection of the quantity Q1. The pilot quantity is used in particular to reduce the noise linked to the combustion of the main driving quantity Q1.

The invention proposes performing the phase shifted injection of the quantity Q1 + Q2 ', and injecting a predetermined additional quantity Q3. Determination of the predetermined quantity

Q3 advantageously depends on the operating point of the engine 12.

The additional quantity Q3 then makes it possible to provide an additional torque which is linked to the quantity of fuel injected during the pre-injection.

The additional quantity Q3 can be increased, for example to decrease the concentration of oxygen and to increase the quantity of reducing agents in the exhaust gases G. The engine torque is then supplied on the one hand by the additional quantity Q3 and, on the other hand, by the offset injection of Q1 + Q2 '.

Advantageously, the start of the injection of the additional quantity Q3 is carried out between 20 and 25 degrees of angle of rotation of the crankshaft before the start of the injection of the quantity

Q1 + Q2. The injection of the additional quantity Q3 "follows" therefore the injection of the quantity Q1 + Q2 '.

This variant makes it possible to improve the performance of the process for regenerating the nitrogen oxide trap 16. In fact, for an overall quantity of fuel injected according to the invention, the composition of the exhaust gases G is more favorable for regeneration of the nitrogen oxide trap 16 only if this same overall amount of fuel is injected according to the state of the art.

Claims

CLAIMS 1. Method for injecting fuel into at least one of the cylinders of a combustion engine (12), in particular a diesel engine, for the regeneration of a nitrogen oxide trap (16), of the type which comprises injecting a main quantity of fuel (Q1) depending on the operating point of the engine (12), characterized in that, during a regeneration phase of the nitrogen oxide trap (16), said main quantity is increased (Q1 + Q2 ').

2. Injection method according to the preceding claim, characterized in that, during a regeneration phase of the nitrogen oxide trap (16), the start of injection of the increased main quantity of fuel (Q1 + Q2 ') is out of phase with respect to the passage of the piston at top dead center.

3. Injection method according to the preceding claim, characterized in that the start of the injection of the increased main quantity of fuel (Q1 + Q2 ') is phase shifted by at least fifteen degrees of angle of rotation of the crankshaft after the passage of the piston in top dead center.

4. Injection method according to one of claims 2 to 3, characterized in that the main quantity of fuel increased and out of phase is such that the ratio between the quantity of fuel and gas admitted into the cylinder of the engine (12) combustion is greater than 1.

5. An injection method according to any one of the preceding claims, characterized in that the injection of the main quantity of fuel (Q1) is preceded by a step of injection of another predetermined quantity of fuel (Q3 ) depending on the engine operating point (12).

6. An injection method according to the preceding claim, characterized in that, during a regeneration phase of the nitrogen oxide trap (16), said other predetermined quantity of fuel (Q3) is increased.

7. Injection method according to any one of the preceding claims, characterized in that during the regeneration phase, part of the exhaust gases (G) is recirculated in the gases admitted into the cylinder, in particular to promote l initiation of fuel combustion.

8. Injection method according to any one of the preceding claims, characterized in that during the regeneration phase of the nitrogen oxide trap (16), it makes it possible to increase the value of the temperature of the gases d exhaust (G) up to a value greater than or equal to a predetermined temperature for purging, by combustion, the sulfur stored in the nitrogen oxide trap.