For check up of exhaust gases oxygen sensors are used. Long time ago is was zirconium narrow band Lambda sensors (in the beginning – without heating, after – with additional heating, which allows probes to be ready more quicker and also ensures more accurate data), beginning with BMW N series engine they are replaced by zirconium wide-band (for fuel mixture adjustment) sensors.
Unlike narrow-band probes, which linear range is 0.99 .. 1.01, wide-band sensors can measure coefficient from 0.65 to atmospheric air composition.
Basics regarding performance of wide-band zirconium probes you can find in internet, in this post I will put more accent to some specific nuances.
First generation of Bosch probes, known by name LSU 4.2, were distinctive with need to re-calibrate them, because atmospheric air was used as reference current source. With next generation – LSU 4.9 – this problem was solved: semiconductor transition is used as reference current source.
LSU 4.2
LSU 4.9
Basic technical information:
LSU 4.9 provides more accurate Lambda measurements: reference data are defined in 30 points on Lambda/Ipump table (LSU 4.2 had only defined 10 points).
Together with probes, Bosch OEM offered also control chipsets for probes: CJ110, CJ120, CJ125. CJ110 and CJ120 were intended to work with LSU 4.2 probes, CJ125 – also with type LSU 4.9 oxygen sensor.
Differently from CJ110, CJ120 includes also dynamic resistance control of Nernst cell, which was used to control the temperature of oxygen sensor. Optimum resistance of Nernst cell for LSU 4.2, measured at 1..4 kHz frequency: 80 Ohm.
CJ125 is supplemented with some specific nuances to work with LSU 4.9 oxygen sensor. The dynamic resistance of Nernst cell for LSU 4.9: 300 Ohm (when the optimum working temperature is reached).
Later CJ125 chipset was changed to CJ135 controller with built-in ADC, LSU 4.9 oxygen sensor was replaced with LSU 5.2.
Common flaws for CJ110, CJ120, CJ125 was increased energy consumption (which was higher than 30 mA/150 mW and chipset was forced to perform in hard thermal conditions), large offset voltage for pump cell current measuring amplifier (CJ110, CJ120, CJ125): even up to +/-10 mV, although for exact measurements offset voltage not more than several hundred uV would be necessary. Same shortage is actual for temperature measurement module, used in CJ120, CJ125. To solve these problems, all mentioned before chipsets use chopper process to compensate offset voltage and to compare measured values with benchmark. Unfortunately, mosfet keys used for choppers (commutation) has increased leakage current, which has quite great impression to measurement accuracy and also increases amount of parasitic interference. Functional management for CJ120 and CJ125 is intended via SPI serial interface, management of heating – external.
N52, N53 and analogous engines uses LSU 4.2 type wide-band oxygen sensors for control of fuel mixture. To calibrate the reference point (Lambda = 1.00), the narrow-band oxygen sensors are used. This nuance has to be taken in to the account, when one of the banks shows balanced (fuel trim integrator is stable and within proper value range) Lambda value, different form 1.00.
Technical parameters, common to CJ110, CJ120 and CJ125:
voltage of Nernst cell: 450 mV
reference voltage, Ipump: 1.500 V
Ipump shunt resistor value: 62 Ohm
coefficient of Ipump amplifier: 8/17 (rich/lean mode)
Note: N series engines have voltage reference value: 2.00 V (voltage of Nernst cell’s pin seems to be reported) and different amplifier coefficient from CJ series control chipsets.
P.S.: Using CJ120, CJ125 probe management controllers, keep in mind, that Bosch offers (not legally) several controller releases, which have some differences in SPI management (SPI management registers and necessary data DON’T COMPLY with datasheet), it means, that, for example, when you have to replace the controller, you can meet some undeterminable troubles, which will result in worsening of Lambda measurements – chopper solutions will not perform, etc.
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