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Wide Band Sensors: interesting reading
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parrishabc Offline

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Wide Band Sensors: interesting reading

How does the wide-band sensor work?

The Bosch LSU 4 wide-band sensor integrates a conventional (narrow-band) heated oxygen sensing Nernst cell with an oxygen pump cell. With an appropriate electronic interface such as the WEGO, this integrated sensor element is capable of measuring the air/fuel ratio (AFR) of hydrocarbon fuels over a very wide range. In our case, we are primarily interested in gasoline fuel.

It is important to understand that the wide-band sensor is measuring the apparent AFR based on the composition of the exhaust gas. The actual AFR is the mass of air divided by the mass of fuel inducted into the engine. For any given AFR, the concentrations of the various exhaust gas constituents can be measured under experimental conditions or calculated using computer programs based on chemical kinetics. Under rich conditions, excess hydrocarbons and carbon monoxide remain in the exhaust gas. Under lean conditions, excess oxygen remains in the exhaust gas. Earlier generations of exhaust gas analyzers were based on measuring the carbon monoxide level using infrared absorption techniques.

Bosch LSU 4 Sensor Element

(Protective Shroud Removed)

Diagram of Bosch LSU 4 Sensor and WEGO Circuitry

The Bosch LSU 4 wide-band sensor element consists of a heater cell, conventional (narrow-band) oxygen sensing Nernst cell (with associated reference cell exposed to ambient air), and an oxygen pump cell. The three bottom cells (heater, reference, and Nernst) are identical to a conventional heated narrow-band oxygen sensor (4-wire type) widely used in automotive applications since the 1980s. As shown in the graph below, the VSENSE output of the Nernst cell is exactly 0.45V at the stoichiometric AFR (14.67 for gasoline). In the Bosch LSU 4, the Nernst cell compares the partial pressure of oxygen within the pump cell cavity to ambient air (outside the sensor). The sensing range of the Nernst cell is relatively narrow - the output is linear from about 14.5-14.9 AFR.

Bosch LSU 4 Nernst Cell Output Versus AFR

Exhaust gas continually diffuses into the pump cell cavity through a small diffusion gap. The pump cell can also pump oxygen into or out of the cavity depending on the direction of current for the IPUMP terminal (the fifth wire for a 5-wire wide-band sensor). When IPUMP is negative, oxygen is pumped into the cavity. When IPUMP is positive, oxygen is pumped out of the cavity. The pump control loop (shown as summing junction and operational amplifier) maintains the pump cell cavity at stoichiometric conditions (VSENSE=0.45V).

If the pump cell cavity becomes slightly rich, VSENSE increases and the pump control loop makes IPUMP negative to pump oxygen in. Under rich conditions, this oxygen is generated by electrochemical decomposition of water and carbon monoxide in the exhaust gas at the surface of the pump cell. Chemical reactions between the excess hydrocarbons, carbon monoxide, and pumped oxygen then restore stoichiometric conditions within the cavity.

If the pump cell cavity becomes slightly lean, VSENSE decreases and the pump control loop makes IPUMP positive to pump excess oxygen out. The pump control loop is a feedback and control system that maintains stoichiometric conditions in the pump cell cavity as the exhaust gas AFR changes. The relationship between pump current and exhaust AFR is shown in the graph below. If the exhaust gas is already at stoichiometric AFR, no oxygen pumping is required to maintain the cavity at the stoichiometric point and IPUMP=0.

Bosch LSU 4 Oxygen Pump Current Versus AFR

The digital signal processing (DSP) block changes the non-linear relationship between oxygen pump current and AFR into a linear 0-5V output as shown in the graph below. The DSP block also filters the oxygen pump current signal to remove noise. The WEGO implements both the pump control loop and DSP functions in firmware that runs on an Atmel microcontroller.

The DSP block also includes a control loop that maintains the heater cell at 750 deg C. Pulse width modulation (PWM) turns the heater current on and off at a 30 Hz rate. The PWM duty cycle (percent of time that current is on) determines the average heater current. The resistance of the Nernst cell is inversely proportional to temperature. Additional circuitry (not shown) measures the Nernst cell resistance. The resistance value is used as feedback for the heater temperature control loop.

WEGO Output Versus AFR

The Bosch LSU 4 wide-band sensor is affected by exhaust pressure as shown on the graph below. The error (%) applies to the oxygen pump cell current. Note that 1 bar corresponds to normal sea level atmospheric pressure. For most performance applications, excessive exhaust back pressure is not a concern and the resulting small error can be disregarded. At high elevations, the error is also relatively small. At 10,000 feet elevation (about .68 bar), AFR values near 13.0 will be shifted up approximately +0.15 AFR.

Bosch LSU 4 Pressure Dependency

What are the limitations of the wide-band sensor?

The sensor will be quickly degraded if leaded racing gasoline is used. Under these conditions, expected sensor life will be less than 10 hours. As the sensor degrades, free air calibration will become impossible.

Oil or other hydrocarbon residues in the exhaust will affect the sensor readings. Likewise, gasoline containing ethanol will result is slight air/fuel reading errors.

The sensor responds to the partial pressure of oxygen. Excessive exhaust back pressure will affect sensor readings. This should not be a problem with any performance exhaust system. When used with a turbo, make sure the sensor is located downstream of the turbo.

Make sure that power is on to the WEGO whenever the engine is run. Without power to the internal heating element, the sensor will clog with hydrocarbon residues and may be permanently degraded. If you want to remove the sensor, we sell an 18 x 1.5mm hex plug.
06-04-2008 09:25 AM
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