METHOD AND DEVICE FOR MONITORING A SECONDARY AIR SUPPLY IN AN INTERNAL COMBUSTION ENGINE

Abstract

A method for monitoring a secondary air supply in an internal combustion engine. The method includes: a) determining, with the aid of a throttle equation and according to a pressure in the secondary air supply, a first secondary air mass flow in relation to the effective throttle area; b) determining a second secondary air mass flow based on a measured exhaust gas lambda value and taking into account a primary air mass flow of the internal combustion engine and a supplied fuel mass flow; c) deriving an effective throttle area from the first secondary air mass flow determined in step a) and the second secondary air mass flow determined in step b); d) monitoring the effective throttle area obtained in this way using at least one specified threshold value.

Claims

1. A method for monitoring a secondary air supply in an internal combustion engine, wherein the internal combustion engine includes the secondary air supply, a device configured to determine a pressure in the secondary air supply, and an exhaust gas lambda probe configured to determine a current exhaust gas lambda value, wherein the method comprises the following steps: a) determining, using a throttle equation and according to a pressure in the secondary air supply, a first secondary air mass flow in relation to an effective throttle area; b) determining a second secondary air mass flow based on a measured exhaust gas lambda value and taking into account a primary air mass flow of the internal combustion engine and a supplied fuel mass flow; c) deriving the effective throttle area from the first secondary air mass flow determined in step a) and the second secondary air mass flow determined in step b); and d) monitoring the effective throttle area derived in step c) using at least one specified threshold value.

2. The method according to claim 1, wherein the first secondary air mass flow is determined in step a) using the throttle equation, which relates the first secondary air mass flow to the pressure in the secondary air supply and the effective throttle area.

3. The method according to claim 1, wherein the derivation of the effective throttle area in step c) is effected using a mass flow balance according to an exhaust gas lambda value.

4. The method according to claim 1, wherein the derivation of the effective throttle area in step c) is effected taking into account temporal dynamics of the exhaust gas lambda probe.

5. The method according to claim 1, wherein the derivation of the effective throttle area in step c) is effected using a recursive least mean square (LMS) method or a recursive normalized least mean square (NLMS) method.

6. The method according to claim 5, wherein a difference between the secondary air mass flow derived from the pressure and the secondary air mass flow derived from the exhaust gas lambda value is used as an error term of the recursive least mean square (LMS) method or the recursive normalized least mean square (NLMS) method.

7. The method according to claim 1, wherein a leak is recognized in the event that the effective throttle area is greater than a specified first threshold value.

8. The method according to claim 1, wherein a jamming or clogging of the secondary air valve is recognized in the event that the effective throttle area is smaller than a specified second threshold value.

9. A diagnostic system for a secondary air supply in an internal combustion engine, wherein the internal combustion engine comprises: the secondary air supply configured to supply secondary air; a device configured to determine a pressure in the secondary air supply; an exhaust gas lambda probe configured to determine a current exhaust gas lambda value; and an evaluation device configured to: a) determine a first secondary air mass flow in relation to an effective throttle area using a throttle equation and according to the pressure in the secondary air supply; b) determine a second secondary air mass flow based on a measured exhaust gas lambda value and taking into account a primary air mass flow of the internal combustion engine and a supplied fuel mass flow; c) derive the effective throttle area from the first secondary air mass flow determined in step a) and the second secondary air mass flow determined in step b); and d) monitor the effective throttle area derived in step c) using at least one specified threshold value.

10. An exhaust system of an internal combustion engine, comprising: a diagnostic system for a secondary air supply in an internal combustion engine, wherein the internal combustion engine includes: the secondary air supply configured to supply secondary air; a device configured to determine a pressure in the secondary air supply; an exhaust gas lambda probe configured to determine a current exhaust gas lambda value; and an evaluation device configured to: a) determine a first secondary air mass flow in relation to an effective throttle area using a throttle equation and according to the pressure in the secondary air supply; b) determine a second secondary air mass flow based on a measured exhaust gas lambda value and taking into account a primary air mass flow of the internal combustion engine and a supplied fuel mass flow; c) derive the effective throttle area from the first secondary air mass flow determined in step a) and the second secondary air mass flow determined in step b); and d) monitor the effective throttle area derived in step c) using at least one specified threshold value.

11. A non-transitory machine-readable storage medium on which store stored commands for monitoring a secondary air supply in an internal combustion engine, wherein the internal combustion engine includes the secondary air supply, a device configured to determine a pressure in the secondary air supply, and an exhaust gas lambda probe configured to determine a current exhaust gas lambda value, the commands, when executed by at least one data processor, causing the at least one data processor to perform the following steps: a) determining, using a throttle equation and according to a pressure in the secondary air supply, a first secondary air mass flow in relation to an effective throttle area; b) determining a second secondary air mass flow based on a measured exhaust gas lambda value and taking into account a primary air mass flow of the internal combustion engine and a supplied fuel mass flow; c) deriving the effective throttle area from the first secondary air mass flow determined in step a) and the second secondary air mass flow determined in step b); and d) monitoring the effective throttle area derived in step c) using at least one specified threshold value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Example embodiments of the present invention are explained in more detail below with reference to the figures.

[0040] FIG. 1 shows an overview representation of an exhaust system.

[0041] FIG. 2 shows a physical model of the secondary air supply.

[0042] FIG. 3 shows a schematic representation of an estimation algorithm for ascertaining the effective throttle area, according to an example embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0043] One measure to increase the temperature in the exhaust gas aftertreatment system is secondary air injection. An external air mass flow is introduced into the exhaust manifold at the exhaust valves of the engine, which reacts exothermically in conjunction with a rich combustion chamber lambda on the hot surfaces of the manifold and turbocharger.

[0044] The diagnostic method and diagnostic system for a secondary air system described below is based on a physical model that describes the secondary air supply. This model is illustrated below based on FIG. 1. FIG. 1 shows an internal combustion engine 1 together with an exhaust gas duct 2 and a secondary air supply 3. In the combustion chambers of the internal combustion engine 1, the supplied fuel mass flow {dot over (m)}.sub.inj is partially reacted with the supplied primary air mass flow {dot over (m)}.sub.air. A secondary air mass flow {dot over (m)}.sub.secAir is supplied to the exhaust gas mass flow in the exhaust gas duct 2 via the secondary air supply 3, wherein the unburned fuel in the exhaust gas duct 2 reacts exothermically with the supplied secondary air. An exhaust gas lambda sensor 4 is arranged in the exhaust gas duct 2, which exhaust gas lambda sensor is designed to determine the exhaust gas lambda value .sub.sens.

[0045] The secondary air supply 3 can be regarded as a throttle. A pressure sensor 5 is arranged in the secondary air supply 3, which pressure sensor is designed to determine the pressure p.sub.s in the secondary air supply 3. The secondary air mass flow {dot over (m)}.sub.secAir is set based on the pressure difference between the modeled pressure p.sub.3 in the exhaust gas duct 2 and the measured pressure p.sub.s in the secondary air supply 3 and can be described by an effective throttle area A.sub.eff (referred to as effective opening cross-section), which is also drawn in FIG. 1.

[0046] The physical model mentioned above describes the relationship between the pressure p.sub.s in the secondary air supply 3 and the measured exhaust gas lambda value .sub.sens. This physical model is shown schematically in FIG. 2. Since the secondary air supply 3 can be represented as a throttle, in step 6, based on the pressure p.sub.s in the secondary air supply 3, the temperature T.sub.s in the secondary air supply 3, the pressure p.sub.3 in the exhaust gas duct 2 and the effective throttle area A.sub.eff, the secondary air mass flow {dot over (m)}.sub.secAir can be determined by means of a throttle equation. Based on the secondary air mass flow {dot over (m)}.sub.secAir determined in this way, the primary air mass flow {dot over (m)}.sub.air and the fuel mass flow {dot over (m)}.sub.inj, a lambda calculation 7 can subsequently be carried out in order to obtain a calculated lambda value .sub.sum. This calculated lambda value .sub.sum is then converted into a lambda value by applying a lambda dynamic 8, i.e. by taking into account the dynamics and the dead time of the exhaust gas lambda sensor 4, which are represented by the parameters .sub.sens and .sub.sens, into the actual exhaust gas lambda value .sub.sens(t) measured by the exhaust gas lambda sensor 4.

[0047] The calculations on which the physical model shown in FIG. 2 is based are shown below. The secondary air mass flow {dot over (m)}.sub.secAir through the secondary air supply 3, which can be described as a throttle, can be represented by means of the throttle equation

[00001]${\stackrel{}{m}}_{\mathrm{secAir}}=A_{\mathrm{eff}}.Math.p_S.Math.\sqrt{\frac{2}{R.Math.T_S}}.Math.\left({}_S\right)$

where

[00002]${}_S=\frac{p_3}{p_S}$

denotes the pressure ratio between the pressure p.sub.3 in the exhaust gas duct 2 and the pressure p.sub.s in the secondary air supply 3. Here, A.sub.eff is the effective throttle area, p.sub.s is the pressure in the secondary air supply 3 and T.sub.s is the temperature of the secondary air. (.sub.s) denotes a flow function according to the pressure ratio .sub.s

[00003]$\left(\right)=\{\begin{array}{c}{}_{\mathrm{crit}}.Math.\sqrt{2.Math.\frac{1-}{1-{}_{\mathrm{crit}}}-{\left(\frac{1-}{1-{}_{\mathrm{crit}}}\right)}^2},\mathrm{falls}{}_{\mathrm{crit}}1\\ {}_{\mathrm{crit}},\mathrm{falls}0{}_{\mathrm{crit}}\end{array}$

where .sub.crit=0.528 and .sub.crit=0.484.

[0048] In the following lambda calculation 7 of the model shown in FIG. 2, based on the total air supplied ({dot over (m)}.sub.secAir+{dot over (m)}.sub.air), the calculated lambda .sub.sum is calculated as

[00004]${}_{\mathrm{sum}}=\frac{{\stackrel{.}{m}}_{\mathrm{air}}+{\stackrel{.}{m}}_{\mathrm{secAir}}}{{}_{\mathrm{sto}}.Math.{\stackrel{.}{m}}_{\mathrm{inj}}}$

where .sub.sto=14.7 denotes the stoichiometric ratio upon the reaction of fuel with air. In the following lambda dynamics 8, the dynamic behavior of the lambda path is modeled by means of the following differential equation, taking into account the dynamics and dead time of the exhaust gas lambda sensor 4:

[00005]${}_{\mathrm{sens}}\frac{d{}_{\mathrm{sens}}\left(t\right)}{\mathrm{dt}}+{}_{\mathrm{sens}}\left(t\right)={}_{\mathrm{sum}}\left(t-{}_{\mathrm{sens}}\right)$

where .sub.sum denotes the lambda ascertained by calculation, .sub.sens and .sub.sens denote the parameters used for modeling the lambda dynamics and .sub.sens(t) denotes the exhaust gas lambda value detected by the exhaust gas lambda sensor 4.

[0049] On the basis of the model equations for the physical model shown in FIG. 2, an estimation algorithm for the effective throttle area A.sub.eff is now constructed. The procedure for ascertaining the effective throttle area A.sub.eff is illustrated schematically in FIG. 3. The aim is to ascertain the effective throttle area A.sub.eff and to monitor the secondary air supply using the ascertained effective throttle area A.sub.eff in order to be able to recognize and diagnose any problems with the secondary air supply as early as possible.

[0050] Initially, the lower path of the diagram shown in FIG. 3 for estimating the effective throttle area A.sub.eff is described. In the lower path, the associated secondary air mass flow {dot over (m)}.sub.secAirLambda is calculated based on the exhaust gas lambda value .sub.sens(t), which is measured by the exhaust gas lambda sensor 4, by means of an inverted lambda calculation 9 and using the primary air mass flow {dot over (m)}.sub.air and the fuel mass flow {dot over (m)}.sub.inj. For determining {dot over (m)}.sub.secAirLambda, the formula

[00006]${\stackrel{}{m}}_{\mathrm{secAirLambda}}={}_{\mathrm{sto}}.Math.{\stackrel{}{m}}_{\mathrm{inj}}.Math.{}_{\mathrm{sens}}-{\stackrel{}{m}}_{\mathrm{air}}$

is used, which results from the definition of lambda, where .sub.sto denotes the stoichiometric ratio of air to fuel, {dot over (m)}.sub.inj denotes the supplied fuel mass flow, .sub.sens denotes the measured exhaust gas lambda value and {dot over (m)}.sub.air denotes the primary air mass flow.

[0051] In the upper path of the diagram shown in FIG. 3, the secondary air mass flow {dot over (m)}.sub.secAirNoAeff, in relation to the effective throttle area A.sub.eff, is determined in the first step 10 by means of the throttle equation (without effective throttle area)

[00007]${\stackrel{}{m}}_{\mathrm{secAirNoAeff}}=p_S.Math.\sqrt{\frac{2}{R.Math.T_S}}.Math.\left({}_S\right).$

Here, .sub.s denotes the pressure ratio

[00008]${}_S=\frac{p_3}{p_S},$

p.sub.s denotes the pressure in the secondary air supply 3, T.sub.s denotes the temperature in the secondary air supply 3 and p.sub.3 denotes the pressure in the exhaust gas duct 2.

[0052] By means of the following lambda dynamics 11, the dynamics and time delay of the exhaust gas lambda sensor 4, which are described by the parameters .sub.sens and .sub.sens, are applied to the secondary air mass flow {dot over (m)}.sub.secAirNoAeff, which is ascertained in this way in relation to the effective throttle area. In this way, a time-dependent secondary air mass flow {dot over (m)}.sub.secAirNoAeffDly(t) is obtained in relation to the effective throttle area, which is brought into phase with the secondary air mass flow {dot over (m)}.sub.secAirLambda determined on the basis of the measured exhaust gas lambda value .sub.sens(t). The following differential equation is used to apply lambda dynamics to the time-dependent secondary air mass flow {dot over (m)}.sub.secAirNoAeff(t) in relation to the effective throttle area

[00009]${}_{\mathrm{sens}}\frac{d{\stackrel{}{m}}_{\mathrm{secAirNoAeffDly}}\left(t\right)}{\mathrm{dt}}+{\stackrel{}{m}}_{\mathrm{secAirNoAeffDly}}\left(t\right)={\stackrel{}{m}}_{\mathrm{secAirNoAeff}}\left(t-{}_{\mathrm{sens}}\right)$

[0053] Next, the effective throttle area A.sub.eff is determined based on the secondary air mass flow {dot over (m)}.sub.secAirLambda determined in the lower path and the secondary air mass flow {dot over (m)}.sub.secAirNoAeff determined in the upper path and in relation to the effective throttle area. This step is drawn in FIG. 3 as step 12.

[0054] In order to determine the effective throttle area A.sub.eff, the variable {dot over (m)}.sub.secAirLambda (t) could be divided by the variable {dot over (m)}.sub.secAirNoAeff (t). However, it has been shown that calculating quotients in this way can lead to inaccurate results. In particular, if the secondary air mass flow becomes very small or even approaches zero, inaccurate values could be obtained for the effective throttle area A.sub.eff.

[0055] For this reason, it is advantageous to use a recursive least square algorithm to recursively determine the effective throttle area A.sub.eff, in the present case for example an LMS (least mean square) algorithm or further preferably an NLMS (normalized least mean square) algorithm. The starting point for the recursive determination of the effective throttle area is the equation

[00010]${\stackrel{}{m}}_{\mathrm{secAirMod},k}={\stackrel{}{m}}_{\mathrm{secAirNoAeffDly},k}.Math.A_{\mathrm{effEst},k}$

which establishes a relationship between the secondary air mass flow {dot over (m)}.sub.secAirMod,k determined by means of the throttle equation via the effective throttle area {dot over (m)}.sub.secAirNoAeffDly,k and the secondary air mass flow determined by means of the exhaust lambda sensor 4.

[0056] In the case of using an LMS (least mean square) algorithm as the adaptation algorithm, the derivation results in

[00011]$\frac{K.Math.w}{w^T.Math.w}.Math.\mathrm{error}$

as the adaptation factor and

[00012]$\frac{K}{w}.Math.\mathrm{error}$

error in the scalar case, where w denotes the regressor, K is a proportionality constant, and error denotes the error. In the present case, the regressor w is the secondary air mass flow obtained from the throttle equation without effective throttle area {dot over (m)}.sub.secAirNoAeffDly,k. However, if the secondary air mass flow approaches zero, the LMS algorithm, for example, may have the problem that the adaptation factor becomes infinite.

[0057] In order to avoid this problem, it is advantageous to use an NLMS (normalized least mean square) algorithm in the implementation. The adaptation factor for the NLSM algorithm is

[00013]$\frac{K.Math.w}{1+w^T.Math.w}.Math.\mathrm{error},$

which results in

[00014]$\frac{K.Math.w}{1+w^2}.Math.\mathrm{error}$

in the scalar case. For the recursive estimation of the effective throttle area, using a modified NLMS algorithm results in

[00015]$A_{\mathrm{effEst},k}=$$=A_{\mathrm{effEst},k-1}+\frac{K_{\mathrm{id}}.Math.{\stackrel{}{m}}_{\mathrm{secAirNoAeffDly},k}}{1+K_{\mathrm{id}}.Math.{\stackrel{}{m}}_{\mathrm{secAirNoAeffDly},k}^2}.Math.\left({\stackrel{}{m}}_{\mathrm{secAirLambda},k}-{\stackrel{}{m}}_{\mathrm{secAirMod},k}\right)$

[0058] K.sub.id is a setting parameter for the estimation speed. This difference equation is repeated at each time step. The estimated value A.sub.effEst,k obtained in this way is then compared with preset threshold values. This means that a leak can be recognized if A.sub.effEst,k becomes too large. In addition, jamming or clogging of the secondary air valve can be recognized if A.sub.effEst,k becomes too small.

[0059] The present invention can also be extended to other exhaust gas topologies. For example, if there are a plurality of valves in the secondary air supply, it would be possible to use a different reference variable for modeling the throttle instead of p.sub.s, such as the exhaust back pressure or the boost pressure. Modeled variables can also be used instead of pressure sensors. The features disclosed herein may be of importance both individually and in any combination for the realization of the present invention in its various embodiments.