engines
Application
The lawmakers are continually increasing the severity of legislation governing exhaust-gas emission limits for cars powered by diesel engines. Apart from the measures taken to optimize the engine’s internal combustion, the open and closed-loop control of func-tions related to exhaust-gas emissions are continuing to gain in importance. Introduc-tion of lambda closed-loop control offers major potential for reducing emission-value spread in diesel engines.
A broadband lambda oxygen sensor in the exhaust pipe (Fig. 1, 7) measures the resid-ual oxygen content in the exhaust gas.
This is an indicator of the A/F ratio (excess-air-factor lambda λ). The lambda oxygen-sensor signal is adapted while the engine is running. This ensures a high level of signal accuracy throughout the sensor’s service life.
The lambda oxygen-sensor signal is used as the basis for a number of lambda functions, which will be described in more detail in the following.
Lambda closed-loop control circuits are used to regenerate NOX accumulator-type catalytic converters.
76 Electronic diesel control Lambda closed-loop control
Fig. 1
1 Diesel engine 2 Diesel injection
component (here, common-rail injector) 3 C ontrol flap 4 Hot-film air-mass
meter 5 Exhaust-gas
turbocharger (here, VTG version) 6 Engine EC U for
EDC
7 Broadband lambda oxygen sensor 8 EGR valve
2
3
4
5
7 8
6 λ control
1
System overview of lambda closed-loop control for passenger-car diesel engines (example)
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Lambda closed-loop control is designed for all passenger-car fuel-injection systems with engine control units dating dating from the EDC16 generation.
Basic functions Pressure compensation
The unprocessed lambda oxygen-sensor signal is dependent on the oxygen concen-tration in the exhaust gas and the exhaust-gas pressure at the sensor installation point.
The influence of pressure on the sensor signal must, therefore, be compensated.
The pressure-compensation function incor-porates two program maps, one for exhaust-gas pressure, and one for pressure depen-dence of the lambda oxygen-sensor output signal. These two maps are used to correct the sensor output signal with reference to the particular operating point.
Adaption
In overrun mode (trailing throttle), lambda oxygen-sensor adaption takes into account the deviation of the measured oxygen centration from the fresh-air oxygen con-centration (approx. 21%). As a result, the system “learns” a correction value which is used at every engine operating point to correct the measured oxygen concentration.
This leads to a precise, drift-compensated lambda output signal for the service life of the lambda oxygen sensor.
Lambda-based EGR control
Compared with air-mass-based exhaust-gas recirculation, detecting oxygen concentra-tion in the exhaust gas allows tighter emis-sion tolerance bands for an automotive manufacturer’s entire vehicle fleet. For future limits, an emission advantage of approx. 10 to20% can be gained in this way for the exhaust-gas test.
Electronic diesel control Lambda closed-loop control 77
Engine-speed sensor Desired air mass Lambda oxygen
sensor
Hot-film air-mass meter
EGR valve
Turbocharger Injection system
+ +
+
-+ Start-of-injection
control
Desired injected fuel quantity
Adaption program map Engine ECU
Air-mass controller Calculating the injected fuel quantity from lambda
Program map for EGR desired-value
Boost-pressure control
Operating concept of average delivery adaption in “indirect control” mode
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Average delivery adaption
Average delivery adaption supplies a precise injection quantity signal to form the set-point for the exhaust-gas-related closed control loop. Correction of exhaust-gas recirculation plays a major role in emissions here. Average delivery adaption operates in the lower part-load range and determines the average deviation in the injected fuel quantity of all cylinders.
Fig. 2 (previous page) shows the basic struc-ture of average delivery adaption and its in-fluence on the exhaust-gas-related closed control loops.
The lambda oxygen-sensor signal and the air-mass signal are used to calculate the actually injected fuel mass, which is then compared to the desired injected fuel mass.
Differences are stored in an adaption map in defined “learning points”. This procedure ensures that, when the operating point
requires an injected fuel quantity correction, it can be implemented without delay even during dynamic changes of state.
These correction quantities are stored in the EEPROM of the ECU and are available immediately the engine is started.
Basically speaking, there are two average-delivery adaption operating modes. They differ in the way they apply detected devia-tions in injected fuel quantity:
Operating mode: Indirect Control
In Indirect Control mode (Fig. 2), a precise injection quantity setpoint is used as the in-put variable in various exhaust-gas-related reference program maps. The injected fuel quantity is not corrected during the fuel-metering process.
78 Electronic diesel control Lambda closed-loop control
Engine ECU
Engine-speed sensor
+
-+ +
Lambda oxygen sensor
λ desired
λ actual
Hot-film air-mass meter
Injection system
Smoke-limitation quantity Calculation of the preliminary quantity
Desired-value map for smoke limitation
Lowest value
Lambda controller
Calculation of the injected fuel quantity Full-load smoke limitation using the lambda closed-loop control: Principle of operation
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Operating mode: Direct Control
In Direct Control mode, the quantity devia-tion is used in the metering process to cor-rect the injected fuel quantity so that the actual fuel quantity injected coincides more precisely with the reference injected fuel quantity. In this case, this is (more or less) a closed quantity control loop.
Full-load smoke limitation
Fig. 3 shows the block diagram of the con-trol structure for full-load smoke limitation using a lambda oxygen sensor. The objective here is to determine the maximum fuel quantity which may be injected without exceeding a given smoke-emission value.
The signals from the air-mass meter and the engine-speed sensor are applied together with a smoke-limitation map to determine the desired air/fuel ratio value λdesired. This, in turn, is applied together with the air mass to calculate the precontrol value for the max-imum permissible injected fuel quantity.
This form of control is already in serial pro-duction, and has a lambda closed-loop con-trol imposed on it. The lambda concon-troller calculates a correction fuel quantity from the difference between the desired air/fuel ratio
λdesiredand the actual air/fuel ratio value
λactual. The maximum full-load injected fuel
quantity is the total of the pilot-control quantity and the correction quantity.
This control architecture permits a high level of dynamic response due to pilot control, and improved precision due to the superimposed lambda control loop.
Detection of undesirable combustion The lambda oxygen sensor signal helps to detect the occurrence of undesirable com-bustion in overrun mode. It is detected if the lambda oxygen-sensor signal drops below a calculated threshold. In this case, the engine can be switched off by closing a control flap and the EGR valve. The detection of unde-sirable combustion represents an additional engine safeguard function.
Summary
A lambda-based exhaust-gas recirculation system can substantially reduce emission-value spread over a manufacturer’s vehicle fleet due to production tolerances or aging drift. This is achieved by using average deliv-ery adaption.
Average delivery adaption supplies a precise injection quantity signal to form the setpoint for the exhaust-gas-related closed control loop. The precision of these control loops is increased as a result. Correction of exhaust-gas recirculation plays the major role on emissions here.
In addition, the application of lambda closed-loop control permits the precise metering of the full-load smoke quantity and detection of undesirable combustion in overrun (trailing throttle) mode.
Furthermore, the lambda oxygen sensor’s high-precision signal can be used in a lambda closed control loop to regenerate NOX catalytic converters.
Electronic diesel control Lambda closed-loop control 79
80 Closed-loop and open-loop control
Fig. 1
a Closed control loop
b Open control loop c Block diagram of
a digital closed-control loop
w Reference variable x Controlled variable
(closed loop) x A Controlled variable
(open loop) y Manipulated
variable
z 1,z 2Disturbance values T Sampling time
* Digital signal values A Analog D Digital
Closed-loop and open-loop control
Application
The closed-loop and open-loop control appli-cations are of vital importance for various on-board systems.
The term (open-loop) control is used in many cases, not only for the process of con-trolling, but also for the entire system in which control takes place (for this reason, the gen-eral term “control unit” is used, although it may perform a closed-loop control function).
Accordingly, arithmetic processes run in con-trol units to calculate both closed-loop and open-loop functions.
Closed-loop control
Closed-loop control is a process in which a parameter (controlled variablex ) is detected continuously, compared to another parameter (reference variablew 1), and adapted to the reference variable in an adjustment process depending on the result of the comparison.
The resulting action takes place in a closed circuit (closed control loop).
Closed-loop control has the function of adjusting the value of controlled variables to a value specified by a reference variable, despite any disturbance influences that may occur.
The closed control loop (Fig. 1a) is a closed-loop control circuit with a discrete ac-tion. Controlled variablex acts within a loop configuration in a form of negative feedback.
Contrary to open-loop control, closed-loop control considers the impact of all disturbance
values (z 1,z 2) occurring within the control loop. Examples of closed-loop systems in a vehicle:
Lambda closed-loop control
Idle-speed control
ABS/TSC/ESP control
Air conditioning (interior temperature)
Open-loop control
Open-loop control is the process within a sys-tem in which one or several parameters act as input variables affecting other parameters due to intrinsic laws governing the system. A fea-ture of open-loop control is the open action sequence across an individual transfer ele-ment or the open control loop.
An open control loop (Fig. 1b) is an
arrangement of elements that interact on each other in a loop structure. It may interact in any possible way with other systems as an entity within a higher-level system. The open control loop can only counter the impact of a distur-bance value measured by the control unit (e.g.z 1); other disturbance values (e.g.z 2) may act unimpaired. Examples of open-loop systems in a vehicle:
ElectronicTransmissionControl (ETC)
Injector delivery compensation and pres-sure-wave correction for calculating injected fuel quantity
Closed-loop and open-loop control applications
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