Lambda closed-loop control
Oxygen probes – also referred to as lambda sensors – measure the oxygen contained in automotive exhaust emissions. The air-fuel ratio, or lambda number (λ) determines the mass ratio of air and fuel in the combustion chamber, as it relates to the stoichiometric air-fuel ratio. When λ=1, ideally balanced combustion conditions result in neither oxygen starvation nor excess. A reading of λ < 1 indicates air starvation (rich mixture), whereas λ > 1 indicates an excess of air (lean mixture). The classic lambda closed-loop control provides a stoichiometric air-fuel mixture for combustion in spark ignition engines. The process enriches the exhaust gases in a proportion suitable for optimal processing by three-way catalytic converters. Diesel engines or gasoline engines with direct injection are operated with a lean combustion across a wide load range, with the nitrogen oxide (NOx) content of emissions steeply rising in conjunction with the increasing combustion temperature. Exhaust gas recirculation is a means of reducing the combustion temperature, along with the NOx proportion of emitted exhaust gases. The residual NOx in the stream of exhaust gas can be held in the NOx storage catalytic converter, until it can be reduced to nitrogen during phases of rich combustion. The lambda number represents the setpoint control value for both exhaust recirculation and catalytic exhaust gas scrubbing. On diesel engines, the lambda number can be used as the basic value for setting the smoke limit at full load.
Lambda oxygen sensors
The functional principle of lambda oxygen sensors is based on the oxygen ion conductance of zirconium oxide at high temperatures. Discrete-level (or bistable) sensors measure the conductive voltage of a measuring cell (Nernst cell), producing, within a narrow range at the stoichiometric point, highly sensitive responses to changes in oxygen concentration. Conventional lambda control places discrete-level sensors either upstream or downstream of a catalytic converter, serving as leading and control probes, respectively. The purpose of the control probe is to assist control optimization as well as to monitor catalytic converter function within as part of onboard diagnostics. In the case of wideband zirconia sensors, the oxygen concentration of the test gas inside the Nernst cell is set to λ=1. This is accomplished by pumping – i.e., injecting or extracting – oxygen ions into or from the test gas. The size and direction of the pumping current facilitate the accurate definition of lambda values even in the presence of nonstoichiometric combustion. When deployed as main sensors, wideband zirconia lambda sensors provide oxygen control in both rich and lean mixture ranges. Due to their stable measuring characteristics, these sensors enhance the dynamics of the classic lambda control loop.
The LSU wideband sensors from Bosch work reliably at temperatures above 600 °C (1112 °F). They are designed for a permanent duty cycle at exhaust gas temperatures of up to 930 °C (1706 °F) and short-term peaks of up to 1030 °C (1886 °F). As the sensors incorporate an internal heater, they are operational with cold exhaust flow and functional within a few seconds of starting a cold engine. The effect of the heater minimizes the influence of the exhaust temperature on the sensor signal.