US20090133386A1

US20090133386A1 - Procedure and control unit for heating up a catalyst arranged in the exhaust gas system of a supercharged combustion engine

Info

Publication number: US20090133386A1
Application number: US12/272,405
Authority: US
Inventors: Guido Porten; Juergen Raimann; Peter Schenk
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2007-11-22
Filing date: 2008-11-17
Publication date: 2009-05-28
Also published as: DE102007056217A1; JP2009133309A

Abstract

Suggested is a procedure for heating up a catalyst in the exhaust gas system of a charged combustion engine by adding air to the exhaust gas system, whereby the added air is taken from a suction system of the combustion engine in the direction of the air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system. The procedure distinguishes itself thereby that the combustion engine is driven with a direct injection of fuel in its combustion chambers, whereby it is driven after a start-up with an apportionment of a fuel amount, which has to be injected before the beginning of a combustion, into at least two partial injections per ignition, and with a suboptimal ignition angle efficiency. A second independent claim concerns a control unit, which is customized for controlling the course of the procedure.

Description

TECHNICAL FIELD

The present invention concerns a procedure for heating up a catalyst in the exhaust gas system of a charged combustion engine by adding air to the exhaust gas system, whereby the added air is taken from a suction system of the combustion engine in the direction of the air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system. The invention furthermore concerns a control unit of the combustion engine that is customized for controlling the course of the procedure.

BACKGROUND

Such a procedure and such a control unit are both known from DE 100 62 377 A1. The heating up of a catalyst by injecting secondary air into a rich exhaust gas atmosphere is also already known from the publication of DE 100 62 377 A1. The secondary air is usually injected behind the outlet valves of the combustion engine and promptly reacts exothermically with a rich exhaust gas atmosphere, which results from combustions of rich (air lambda value lower than 1) combustion chamber fillings of the combustion engine. A separate secondary air pump that is electrically driven is usually used for injecting the secondary air.
The procedure that works with secondary air injection can be associated with a group of heating procedures, which have in common that the heating-up takes place by the reaction heat of chemical reactions that take place in the exhaust gas. Furthermore interventions into the combustion engine control for heating-up the catalyst are known, which cause an increase of the exhaust gas temperature and/or the exhaust gas mass flow.
It is known for example to produce an extremely high amount of heat in the exhaust gas in an after-start phase of the combustion engine, without changing the engine power that has been raised during idling of the combustion engine nor the idle speed of about 1.200 min-1 that has been raised in the after-start phase. This is achieved at a combustion engine with direct fuel injection by injecting a first amount of fuel in the suction stroke and a second amount of fuel in the compression stroke. This causes a layered fuel apportionment in the combustion chamber with a zone, which results from the injection of the second amount with a comparably rich and therefore well ingnitable fuel/air mixture around the ignition plug. This operation of the combustion engine is also called homogeneous split mode, whereby ‘split’ refers to the apportionment of the injections.
The above mentioned DE 100 62 377 A1 is based on a two-stage concept for supercharging. The two-stage concept thereby provides an exhaust gas turbo charger in one embodiment, whose shaft is driven by an electromotor. By this drive (1. stage) the so-called turbo ‘lag’ shall be minimized at operating point changes. As it is generally known the turbo lag develops, because the turbine initially has to be accelerated during a sudden torque demand from an operating point with a low exhaust gas mass flow, in order to establish the necessary boost pressure on the compressor side. The resulting delay is reduced by the supporting electric drive. The second stage is equivalent to the traditional drive of the turbine by a sufficient big exhaust gas enthalpy.
This two-stage supercharging concept, which has nothing to do with a catalyst heating process, is used in DE 100 62 377 A1 in order to replace the separate secondary air pump. Therefore the turbo charger is electrically driven when the catalyst has to be heated. Thereby it already produces a certain boost pressure also in operating points with a low exhaust gas enthalpy, which is sufficient in order to let air flow out of the suction system over a pipe connection past the combustion chambers of the combustion engine into the exhaust gas system. Thereby a separate secondary air pump can be waived at two-stage supercharging concepts with turbo chargers that are supported by an electrical drive. But the injecting of the secondary air requires an electric drive even at such two-stage supercharging concepts.

SUMMARY

With this background the task of the invention is to provide a procedure and a control unit, which allow a heating of catalyst in the exhaust gas of a combustion engine that is charged with a exhaust gas turbo charger, which uses secondary air without a separate secondary air pump and without an electrical drive of the turbo charger or a compressor that is arranged in the suction pipe.
This task is solved with the features of the independent claims.
The operation of the combustion engine with a direct injection of fuel in its combustion chambers and with an apportionment of a fuel amount that has to be injected before the beginning of a combustion, into at least two partial injections per ignition and combustion chamber, which takes place after a start-up, provides very stabile combustions, which allow the very late ignition angle. Late ignition angles up to 25 degrees after top dead center can be adjusted at air- and wall-formed combustion procedures, and at jet-formed combustion procedures even later ignition angles between 25 up to ca. 40 degrees after top dead center can be adjusted at a stabile engine speed behavior and at controllable raw emissions during idle. Thereby the ignition angle efficiency, which can be understood as the quotient between the torque at a delayed ignition angle in the numerator and the torque at an optimal ignition angle for a maximum torque development, sinks.
The efficiency loss causes a higher exhaust gas temperature and therefore a higher exhaust gas enthalpy due to thermodynamic regularity. Furthermore the combustion engine has to be operated with higher combustion chamber fillings at a delayed ignition, in order to compensate the torque loss that goes along with the efficiency failure. At the given ignition angles increases of the combustion chamber fillings occur up to values of over 75% of the maximum volume that is possible at normal conditions. This causes an increased exhaust gas mass flow, which also increases the exhaust gas enthalpy. With an increasing exhaust gas enthalpy the driver input that is transferred on to the turbine of the exhaust gas turbo charger increases. Altogether this results in a comparably high exhaust gas amount, whose temperature is comparably high due to the bad ignition angle degree, so that a maximum heat flow (enthalpy flow) adjusts in the exhaust system.
The achieved increase of the exhaust gas enthalpy causes already considered on its own a fast heating of the exhaust system. Furthermore the increase causes without a supporting electrical drive within a few seconds after a cold start that the turbo charger establishes a boost pressure and therefore a pressure drop or a scavenging loss to the exhaust gas, which is also sufficient big enough at low engine speeds in order to let air stream out of the exhaust gas system over a pipe connection past the combustion chambers of the combustion engine into the exhaust gas. Thereby a separate secondary air pump can be waived even at one-stage supercharging concepts, which work without electrically supporting turbo chargers and without an additional compressor (for example roots-injector, compressor) that is electrically or mechanically driven by the combustion engine. The invention therefore takes advantage of the already known homogeneous split mode at a supercharged combustion engine for a boost pressure increase, in order to achieve a scavenging loss (pressure drop) between the suction system and the exhaust gas system that is sufficient for a secondary air injection.
Further advantages accrue from the dependent claims, the description and the attached figures.
It shall be understood that the previously mentioned and the following features that still have to be explained cannot only be used in the stated combination, but also in other combinations or alone without leaving the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the drawings and further explained in the following description. It is schematically shown in:

FIG. 1 is a combustion engine with a gasoline direct injection and a control unit;

FIG. 2 is an injection model, which is used in the embodiment of the procedure according to the invention;

FIG. 3 is a flow diagram for illustrating the procedure aspects of the invention; and

FIG. 4 is timely correlating courses of operating parameters of the combustion engine during the implementation of an embodiment according to the procedure.

DETAILED DESCRIPTION

In particular FIG. 1 shows a combustion engine 10 with at least one combustion chamber 12, which is sealed up with a piston 14. Fillings of the combustion chamber 12 with a mixture of fuel and air are ignited by an ignition plug 16 and afterwards combusted. In a preferred embodiment the combustion engine 10 is optimized for a jet-formed combustion procedure. Referred to as a combustion procedure is the way of the mixture formation and energy transformation in the combustion chamber 12. The jet-formed combustion procedure distinguishes itself thereby that the fuel in immediate proximity to the ignition plug is injected and evaporates there. This requires an exact positioning of the ignition plug 16 and fuel injector and a precise jet direction, in order to be able to ignite the mixture at the proper point of time.
An exchange of the filling of the combustion chamber 12 is controlled with gas change valves 18 and 20, which are opened and closed phase-synchronically with the movement of the piston 14. The different possibilities for operating the gas exchange valves 18 and 20 are known to the technician and are not shown in detail in FIG. 1 due to clarity. When the inlet valve 18 is open and the piston 14 is running downwards, thus in the suction stroke, air flows from a suction system 22 into the combustion chamber 12. By using an injector 24 fuel is dosed to the air in the combustion chamber 12. Exhaust gas that results from the combustion of the combustion chamber fillings is ejected into an exhaust gas system 28, which has at least one three-way catalyst 30, when the outlet valve 20 is opened. Generally the exhaust gas system 28 contains several catalysts, for example a pre-catalyst 30 that is build-in near the engine and a main catalyst 32 that is build-in far from the engine and that can be a three-way catalyst or a NOx-storage catalyst.
The combustion engine 10 provides a turbo charger 34 with a turbine 36 and a compressor 38. The turbine 36 is arranged between a manifold 40 and the pre-catalyst 30 in the flow path of the exhaust gases. By using a waste gate valve 42 the pressure drop over the turbine 36 can be limited. A secondary air duct 44 with a secondary air valve 46 lies between the suction system 22 and the exhaust gas system 28. When the secondary air valve 46 is opened and when there is a sufficient pressure drop from the suction system 22 (before the inlet valve 18) to the outlet of the secondary air duct 44 into the exhaust gas system 28, air flows from the suction stroke 22 past the combustion chambers 12 of the combustion engine 10 into the exhaust gas system 28 before the catalyst.
The combustion engine 10 is controlled by a control unit 48, which therefore processes signals of different sensors, which illustrate operating parameters of the combustion engine 10. These are in the incomplete illustration of FIG. 1 a rotation angle sensor 50, which determines an angle position °KW of a crankshaft of the combustion engine 10 and therefore a position of the piston 14, an air mass sensor 52, which determines an air mass mL that flows into the combustion engine 10, a pressure sensor 54, which determines the pressure p in the suction stroke 22 before the inlet valve 18, and, optional, one or several exhaust gas sensors 56, 58, which determine a concentration of an exhaust gas component and/or a temperature of the exhaust gas.
In the embodiment of FIG. 1 the exhaust gas sensor 56 is a lambda sensor, which determines an oxygen concentration in the exhaust gas as a measure of an air ratio L (L=lambda), while the sensor 58 determines an exhaust gas temperature T at the inlet of the pre-catalyst 30. The air ratio lambda is known to be defined as the quotient of an actually available air mass in the numerator and an air mass that is required for a stoichiometric combustion of a certain fuel mass in the denominator. Air ratios lambda higher 1 represent therefore an air surplus, while air ratios lambda smaller 1 represent a fuel surplus. As long as the exhaust gas system 28 provides an exhaust gas temperature sensor 58, it can be also arranged in a different position of the exhaust gas system 28, for example at the inlet of the main catalyst 32. This especially applies when the main catalyst 32 is a NOx-storage catalyst.
The control unit 48 creates corrective signals from the signals of this and if necessary further sensors in order to control actuators for controlling the combustion engine 10. In the embodiment of FIG. 1 these are especially a corrective signal S_L for controlling a throttle valve position sensor 60, which adjusts the angle position of a throttle valve 62 in the suction system 22, a signal S_K, with which the control unit 48 controls the injector 24, a corrective signal S_Z, with which the control unit 48 controls the ignition plug 16 or the ignition system 16, which also provides inductors and/or condensers for producing the ignition voltage, and a corrective signal S_SLE, with which the control unit 48 controls the inlet profile of the secondary air valve 46, as well as a signal S_WG for controlling the waste-gate-valve 42. Analogously to the illustration of the sensors it also applies to the depicted actuators, that the illustration of FIG. 1 is not complete and that modern combustion engines 10 can provide further actuators as exhaust gas recirculation valves, tank ventilation valves, actuators for variable controls of the gas exchange valves 18, 20 etc.
Besides the control unit 48 is customized especially programmed to implement the suggested procedure and/or one of its embodiments and/or to control a corresponding course of procedure.
In a preferred embodiment the control unit 48 converts performance requirements of the combustion engine 10 into a nominal value for the torque that has to be produced altogether by the combustion engine 10, and apportions these torques into torque rates, which are influenced by the corrective signals S_L for the filling control, S_K for the fuel metering, S_Z for the ignition control and S_WG for the boost pressure control. The filling rate is adjusted with the corrective signal S_L by a corresponding setting of the throttle valve 62 or a variable controlling of inlet valves 18. The fuel rate is adjusted with the corrective signal S_K basically by the injected fuel mass and the way of the apportionment of the fuel mass that has to be injected into one or several partial injections as well as the relative status of the partial injections to each other and to the movement of the piston 14, thus by an injection timing. The maximal torque that is possible at the present air filling results from optimal air ratio lambda, optimal injection timing and optimal ignition angle.
FIG. 2 shows an injection model, which is used at the embodiment of the procedure according to the invention. Thereby the injector pulse widths ti_1 and ti_2 are each put in as high level over the crankshaft angle °KW of a working cycle from a suction stroke stroke_1, a compressor stroke stroke_2, a working stroke stroke_3 and an outlet stroke stroke_4. Upper top dead centers are labeled as OT.
In particular FIG. 2 shows an injection model M_1 for a homogeneous split operation for maximized exhaust enthalpy with a first partial injection ti_1, which takes place in the suction stroke stroke_1 and a second partial injection ti_2, which takes place later. The second partial injection ti_2 takes definitely place before the ignition, which is caused at the crankshaft angle KW_Z. As already mentioned KW_Z is possibly very late in the range of 10° to 35° KW after the ignition-ot, so that the second partial injection ti_2 can also be completely or partially in the working stroke stroke_3. But it is definitely before the ignition. Instead of an apportionment into two partial injections the fuel amount that is injected with the first injection model M_1 can also be apportioned into more than two partial injections. The possibility of apportioning is limited by the dosing ability of small quantities of the injector 24. The apportionment into at least two partial injections, of which the earlier preferably takes place in the suction stroke stroke_1 and the latter definitely in the same working stroke for the ignition, is significant for the model M_1, whereby the air ratio lambda in the combustion chamber (thus without secondary air) is smaller than 1 and an air ratio lambda in the exhaust gas (thus with secondary air) is higher than 1.
FIG. 3 shows a flow diagram of procedure aspects of the invention. After a start-up of the combustion engine 10 in step 64 initially its engine speed n is determined in step 66 and compared to a threshold value n_SE in step 68. An exceeding of the threshold value n_SE branches the procedure to step 70, in which the described homogeneous split mode HSP with retarded ignition and increased filling is activated. In a preferred embodiment the combustion engine 10 is thereby operated almost completely de-throttled, whereby an almost complete de-throttling means an operation with at least 75% of the maximal filling that is possible under the same conditions.
Simultaneously or quickly afterwards the secondary air valve 46 is opened in step 72 at a sufficient boost pressure. The opening can for example take place with a fixed time delay of the order of a few seconds towards the activating of the homogeneous split mode or depending on the exceeding of a boost pressure threshold value. Subsequently in step 74 a parameter A is established and determined, which shows the effect of the secondary air injection. A time meter reading or a constant that characterizes the temperature of the turbo charger 34, the manifold 40 or of a catalyst 30, 32 are preferred as a parameter. Combinations of such constants are also possible. The parameter A is compared to a threshold value S_A as a termination criteria in step 76. When exceeding S_A the homogeneous split mode is terminated in step 78, the secondary air valve 46 is closed and branched in step 80 in a normal operation of the combustion engine 10, in which no special measures for increasing the exhaust gas enthalpy are activated. The transfer can also take place step-by-step by closing the secondary air valve 46 first and then terminating the homogeneous split mode. The order can also be reversed.
The effect of the procedure according to the invention is illustrated by the time course of the engine speed n, the boost pressure p and a control bit SB that are shown in FIG. 4. Before the point of time t=0 the combustion engine 10 stands still. Therefore its engine speed n that is shown in FIG. 4 a initially equals zero and the boost pressure p that is shown in FIG. 4 b corresponds with the surrounding pressure of about 1000 mbar. The value of the control bit SB that is shown in FIG. 4 c is still low.
A starter accelerates the combustion engine 10 at the point of time t0 onto a starter engine speed of a little over 200 min-1. With constituting combustions in the combustion chambers 12 the engine speed n of the combustion engine 10 increases more and exceeds a starting engine speed threshold of about 400 min-1 at the point of time t1. Subsequently it quickly levels out at an increased idle engine speed of about 1.200 min-1. Due to the suction of the first combustion chamber fillings from the suction system 22 at a turbine 36 that is still not rotating or still not rotating fast the boost pressure p before the inlet valves 18 sinks initially. When exceeding the starting engine speed threshold at the point of time t1 the after-starting phase begins. The control bit SB from FIG. 4 c is set on its high level. The procedure according to the invention or one of its embodiments is implemented at a high level.
In order to provide a high enthalpy flow in the exhaust gas during this after-starting phase, the control unit 48 provides suboptimal ignition angles over the corrective variable S_Z, which cause a torque loss over the therefore reduced ignition angle efficiency, which is compensated by an increased filling of the combustion chambers 12 that is produced by corrective signals S_L. The turbine 36 of the exhaust gas turbo charger 34 is quickly accelerated by the enthalpy flow in the exhaust gas that is high due to the almost complete de-throttling, so that the boost pressure p increases quickly up to values of over 1200 mbar. During such boost pressures the pressure difference between the boost pressure on the fresh air side of the secondary air duct 44 and the exhaust gas side of the secondary air duct 44 is big enough in order to let fresh air from the suction system 22 flow into the exhaust gas system 28 at an opened secondary air valve 46.
Therefore the control unit 48 opens the secondary air valve 46 by releasing an opening corrective signal S_SLE. By an additional influence of the fuel corrective signals S_K an air ratio lambda is altogether adjusted in the exhaust gas in the over-stoichiometric operation, for example an air ratio lambda=1,1. Depending on the amount of the fresh air that has been injected into the exhaust gas, the air ratio lambda in the combustion chamber 12 is adjusted on to correspondingly lower values, which can also lie in the under-stoichiometric operation (lambda<1, fuel surplus). Thereby a good ignition ability and a stabile combustion of the fuel/air mixture that is comprised in the combustion chambers are achieved. Simultaneously the over-stoichiometric air ratio in the exhaust gas is very important especially in the first phase after a start finish, because the still cold pre-catalyst 30 can not reduce hydrocarbons yet. Therefore the only possibility to limit the hydrocarbon emissions that are stored in the environment is to limit the raw emissions of the combustion engine 10. This limitation is a desired result of the operation with an air ratio lambda bigger than 1 in the exhaust gas.
A high exhaust gas amount is produced by the increased filling, which has furthermore a comparably high temperature due to the suboptimal ignition angle efficiency and which provides a oxygen surplus. Altogether a high heat flow or enthalpy flow is therefore produced. As soon as a termination criteria is fulfilled at the point of time t2, the increase of the exhaust gas enthalpy is terminated. The engine speed n of the combustion engine 10 falls then back on its normal idle engine speed, which lies typically between 500 and 100 min-1. The de-throttling that exceeds the necessary scope during normal operation is terminated. Thereby the pressure p between the throttle valve 62 that is than less opened and the inlet valves 18 drops a lot. In the drawing of FIG. 4 the pressure sinks up to about 400 mbar, whereby the actual value can vary form combustion engine to combustion engine and also depending on other conditions. The low pressure is then not sufficient for a secondary air injection, so that the secondary air valve 46 is closed in time.
The pressure difference dp represents the extent of the pressure change, which is produced between the points of time t1 and t2 and which is used for a secondary air injection. Without the idea for using the pressure change for a secondary air injection the increased exhaust gas enthalpy, which results from the homogeneous split mode, would be rather terminated by opening the waste gate valve 42.

Claims

1. A method of heating up a catalyst in an exhaust gas system of a charged combustion engine, the method comprising:

adding air to the exhaust gas system, wherein the added air is taken from a suction system of the combustion engine in a direction of air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system;

driving the combustion engine with a direct injection of a fuel into a plurality of combustion chambers, wherein the combustion engine is driven:

after a start-up with an apportionment of a fuel amount injected before a beginning of a combustion, and wherein the fuel amount is apportioned into at least two partial injections per ignition per combustion chamber; and

with a suboptimal ignition angle efficiency.

2. A method according to claim 1, further comprising injecting the fuel into each of the plurality of combustion chambers such that a rich fuel/air mixture develops in each of the plurality of combustion chambers.

3. A method according to claim 2, further comprising injecting the fuel into each of the plurality of combustion chambers such that an exhaust gas lambda value is greater than or equal to 1 after adding air.

4. A method according to claim 1, further comprising operating the combustion engine in an almost completely throttled state.

5. A method according claim 1, further comprising operating the combustion engine after a start-up with an increased idle engine speed.

6. A method according to claim 1, further comprising detecting an engine speed of the combustion engine at a start-up; and activating the heating-up of the catalyst when the engine speed exceeds a start-up engine speed threshold value.

7. A method according to claim 1, further comprising using a combustion engine that is compatible with a jet-formed gasoline direct injection.

8. A method according to claim 7, further comprising operating the combustion engine during an idle with an ignition angle between 25 degrees and 40 degrees after a top dead center.

9. A control unit, especially a control unit of a supercharged combustion engine, configured to implement steps of a method of heating up a catalyst in an exhaust gas system of a charged combustion engine, the method comprising: adding air to the exhaust gas system, wherein the added air is taken from a suction system of the combustion engine in a direction of air current behind a compressor of an exhaust gas turbo charger that is arranged in the suction system; driving the combustion engine with a direct injection of a fuel into a plurality of combustion chambers, wherein the combustion engine is driven: after a start-up with an apportionment of a fuel amount injected before a beginning of a combustion, and wherein the fuel amount is apportioned into at least two partial injections per ignition per combustion chamber; and with a suboptimal ignition angle efficiency.

10. The control unit of claim 9, further configured to implement steps comprising at least one of:

injecting the fuel into the combustion chamber such that a rich fuel/air mixture develops in the plurality of combustion chambers;

injecting the fuel into the plurality of combustion chambers such that an exhaust gas lambda value is greater than or equal to 1 after adding air;

operating the combustion engine in an almost completely throttled state;

operating the combustion engine after a start-up with an increased idle engine speed;

detecting an engine speed of the combustion engine at a start-up; and activating the heating-up of the catalyst when the engine speed exceeds a start-up engine speed threshold value; and

operating the combustion engine during an idle with an ignition angle between 25 degrees and 40 degrees after a top dead center.