LRSHK 9.20 (Continuous Post-Catalyst Lambda Control)

See the funktionsrahmen for the following diagrams:

lrshk-lrshk: function overview

lrshk-lrhkini: initialization of the post-catalyst lambda control

lrshk-lrhkebg: general switch conditions post-catalyst lambda control

lrshk-lrhkla: determination of the error signal to lambda level

lrshk-dlahksm: selection of fr-synchronous lambda averaging/filtering by average value/linearizing lrshk-lambda directly

lrshk-lrhkebp: cylinder bank-specific readiness switch

lrshk-lrhkb1: PI controller post-catalyst with activation condition, cylinder bank 1

lrshk-lrhkb2: PI controller post-catalyst with activation condition, cylinder bank 2

lrshk-lrhkeb: cylinder bank-specific enable of proportional and integral components, cylinder bank 1

lrshk-lrhkeb2: cylinder bank-specific enable of proportional and integral components, cylinder bank 2

lrshk-lrhkip: PI controller, cylinder bank 1

lrshk-lrhkip2: PI controller, cylinder bank 2

lrshk-lahkma: fr-synchronous averaging

Function Description

Control with the post-catalyst probe is superimposed on the pre-cat lambda control.

Control action on the pre-catalyst control is via the delta-lambda-correction variables dlahi_w and dlahp_w.

Post-catalyst Control:

This is switched off by setting bit 0 in word CLRSHK code to 1 (FALSE).

PI Control Action

Post-catalyst lambda control is achieved with a PI controller. Control action via the proportional component dlahp_w will be immediate because it has no "memory" of the correct sign with respect to the control position after a change of lambda probe voltage due to enrichment or enleanment by the delta-lambda intervention.

Via the integral component, post-catalyst control LRSHK is able to compensate, to a large extent, for exhaust gas deterioration, caused by a shift of the steady-state probe characteristic.

The LRSHK calculation is carried out continuously on the lambda level. This requires that the probe voltage ushk_w is linearized via the characteristic LALIUSH (lamsonh_w). A similar linearization is performed with the voltage target value USRHK (lamsolh_w). The pseudo-value lamsonh_w can continue to work via the project-specific codeword CLRSHK

(a) directly (--> default in continuous pre-catalyst control, intervention is possible every 10 ms)

(b) via a PT1 filter (--> project-specific)

(c) fr-synchronous averaged (--> default for two-point control, as the ratio can be added only before the fr-jump)

because lamhm_w will supply the control error dlashkm_w.

By assessing the characteristic curves KDLASHKP and KDLASHKI, the control error dlashkm_w can be corrected separately according to the catalyst properties before the calculation of the P and I components.

The resulting skewed control errors dlashkp_w or dlashki_w are now weighting with KPLRHML = f (ml) of the proportional component dlahp_w, or by weighting with KILRHML = f (ml) of the integral component dlahi_w.

In the case of aged catalysts, control oscillation of the pre-catalyst control imprinting itself on the post-catalyst probe voltage behaviour which, if proportional intervention is left unchanged, can lead to post-catalyst control oscillations. Moreover, catalyst ageing, which is associated with a decrease in the oxygen storage capacity, the need for the proportional control action in post-catalyst control is less important. Therefore, in a further multiplication by the weighting factor from the characteristic PLRHAV = f(avkatf), the proportional component of the post-catalyst control is revoked for aged catalysts.

Effect on LRSHK of the Lambda Probe Diagnostics

Post-catalyst control takes over the additional delta Lambda offsets (dlahki_w --> pre-catalyst actual value offset, dlahkp_w --> pre-catalyst target value offset) from the former control in LRS 15.40. The magnitude of the intervention dlahi_w is a measure of probe ageing and is used in the diagnosis of lambda probe aging. A symmetric increase in the probe response time cannot be detected by dlahi_w.

Control Threshold from Map KFUSHK

If the post-catalyst probe reports that the mixture is, for example, too lean, dlahp_w will be negative according to the selected control direction and dlahi_w will become smaller. Thus, there is an enrichment until ushk goes back up to the control threshold usrhk. In contrast to the pre-cat control, a map is provided for the post-catalyst control threshold. Via the choice of threshold, a slight load or speed-dependent lambda offset can be achieved.

If catalyst diagnostics are required in the short test B_fakat = TRUE is switched to the threshold USRHKFA.

LRSHK Control Dynamics

The superimposed control is significantly slower than the control applied before the catalyst. Since at low air mass flow rates (low load or engine speed point), the post-catalyst probe voltage as a general rule can exhibit more erratic behaviour and oscillations, following low probe voltages it should not be evaluated so strongly here. The time constant of the post-catalyst control depends on the air mass flow rate ml (--> characteristic KILRHML). At high air mass flow rates, the integration rate should be selected higher as a general rule.

Activation Conditions

If post-catalyst control LRSHK is disabled, the learned integrator value dlahi_w up to that point is the output of the post-catalyst controller. Also, when stopping the engine over the value of the continuous RAM.

The activation conditions for the proportional and integral components are defined differently and are indicated by the bits B_lrhkp and B_lrhk.

The following conditions apply for the proportional component:

When pre-catalyst control readiness (B_lr = 1) is detected, LRSHK is enabled after the delay time TBLRH. This is only useful for lambda target values (lamsons_w = 1) of the pre-catalyst control.

Post-catalyst regulation is only activated above a certain catalyst temperature threshold (tkatm > TKATMLRH) and the operational readiness of the post-catalyst probe (B_sbbhk) is activated.

The following additional conditions apply for the integral component:

Thus, the integrator is only disabled when nmot or rl is in the ranges (NLRHU =< nmot =< NLRHO and RLRHUN(nmot) =< rL =< RLRHON(nmot)). The characteristic curves RLRHUN and RLRHON make it possible to select engine speed-dependent rL-limits on the control range. This allows the control range to be defined so that the operational ranges which give rise to incorrect adaptation of post-catalyst control are delineated. This can happen at operating points where, for example, air mass flow rates are too low.

After the overrun fuel cut-off, the catalyst is saturated with oxygen. The post-catalyst probe voltage will retain small, lean values​​ for a certain time. In this phase, the system deactivates the section LRSKA of the post-catalyst control via bit B_lrka.

After the end of catalyst clear out, post-catalyst control is prohibited until the air mass MLNKAX has passed through the catalytic converter.

If the bit B_tehb corresponding to “tank venting high loading” is set, the integral component of LRSHK is deactivated because the integrator would learn wrong values in this case. The proportional component remains active in this case since it helps to reduce exhaust problems.

In addition, a series of diagnostic errors deactivates post-catalyst control.

Dynamic Overshoot of the Control Threshold after Catalyst Clear Out

After the end of catalyst clear out, the post-catalyst probe voltage oscillates significantly higher than the nominal value of 600 mV for typically 5 to 30 s. The probe voltage attains values ​​of 750-800 mV. The overshoot depends on the catalytic properties. With catalyst types that do not exhibit this behavior, the excesscan be applied away.

SCHEMATIC

The probe voltage characteristic ushk and the status bits B_sa (boost cut-off) and B_lrka (catalyst clear out) are illustrated schematically in the diagram above.

Thus the "time" (air mass MLNKAX) during which the post-catalyst control is prohibited can be kept as short as possible, the probe voltage behaviour after catalyst clear over time is described by a dynamic increase in the target value. The input of a quick PT1 filter is populated with LASHKAB and governed by the time constant ZLASHKAB to 0. The time constant is derived from the adopted course of the probe voltage.

Through this function it is possible, in cases in which the catalyst clear out function has not been successful, or a situation in which the pre-catalyst control condition gives rise to a lean post-catalyst probe voltage, the probe voltage can be raised via LRSHK.

Application Notes

LRSHK Application Procedure:

Codeword CLRSHK

The codeword CLRSHK was introduced in order influence the treatment of the adaptation value dlahi_w within the application. The importance of the individual control bits in CLRSHK are described under the block comments.

Sensible combinations, in decimal, are listed below:

CLRSHK = odd: LRSHK is deactivated

CLRSHK = 16: dlahi_w will erase memory errors when reset with the value DLAHIINI, otherwise default status for LRSHK

CLRSHK = 24: dlahi_w is reset with the value DLAHIINI when the engine starts

Parameter LRSHK

The application of LRS must be completed.

4 x 4 grid points are provided for map KFLASOHK:

Suggestion nmot sample points: 1000, 1800, 2400 & 3000 rpm

rL: 14, 42, 56 & 70%

- Lower control limit e.g. NLRHU = 1200 rpm

Characteristic curve RLRHUN is dependent on n

- Upper control limit e.g. NLRHO = 3000 rpm

Characteristic curve RLRHON is dependent on n

The characteristic curves RLRHUN and RLRHON are strongly project-dependent. However, a characteristic with four sample points, which lie between NLRHU and NLRHO should be sufficient.

- TKATMLRH is chosen so as to control catalyst temperatures >300°C. There is a catalyst temperature model (module ATM) which yields catalyst temperatures, tkatm.

- TBLRH is dependent on the catalytic properties and should be at least 1 second to be selected. Via this label, the time that elapses after switching on the lambda control until the post-catalyst probe signal is correlated against the pre-catalyst control scheme is defined.

- KILRHML curve describes the rate of integration of the air mass in %/s.

Reference points for example engine with ml load: 450 kg/hr

ml: 8, 28, 88, 200, 400 kg/hr

KILRHML: 0.0015, 0.003, 0.0045, 0.006 and 0.0075 /s

Characteristic Curves KDLASHKI and KDLASHKP

The control error corresponding to project-specific lambda probes and catalytic converter properties can be defined via the characteristic curves KDLASHKI and KDLASHKP. So firstly, inaccuracies of the probe voltage linearization (LALIUSH) are corrected and secondly, the emissions characteristics of catalytic converters are considered.

Application of the Proportional Component in the LRSHK PI-Control Scheme:

The effective action of the proportional component of the post-catalyst control system is calculated as follows:

dlahp_w = dlashkl x KPLRHML (ml) x PLRHAV (avkatf)

The influence of catalyst ageing is included as a multiplier in the calculation (RAM cell dlahp_w) using a factor from the characteristic curve PLRHAV, as described above. For a new catalytic converter (avkatf at 0.0), PLRHAV is populated with the value 1.0. With increasing amplitude ratio (as the catalyst ages), PLRHAV is returned to 0.0.

The choice of parameters is determined mainly by the properties of the catalyst. When we ask questions in the application development function, please contact us.

Application of the Parameter MLNKAX:

The overshoot voltage of the lambda probe after the end of the catalyst clear out function is a project-specific phenomenon, which disrupts the LRSHK. Therefore, LRSHK should be blocked until the air mass MLNKAX has been enforced. Since there is no experience (especially with the new catalyst types), the definition of the parameters should be consulted in the responsible function for LRSKA.

Application of the Parameter KILRHML:

During application of the map KFLASO in module LRS, the post-catalyst control integration rate will be set by means of the curve KILRHML so that one sample point of the integrator control stroke dlahi_w of +/-0.03 to +/-0.04 is measured. During measurement, the air mass at the respective operating point is noted. After completion of the application of map KFLASO, the set values ​​from KILRHML are plotted against air mass. The air mass is obtained from a scatter plot. The actual curve KILRHML in LRSHK is obtained by averaging the point cloud.

For more detailed information, please refer to the general application note in the module covering Continuous Lambda Control.

Abbreviations