Abstract
ABSTRACT:
In stoichiometric spark- ignited (SI) engine operation, emission levels heavily depends on how accurate the air-fuel ratio can be kept at lamda=l . Due to measurement and computational tolerances, sufficiently accurate stoichiometric operation requires a closed loop control.
In SI engines, the air-fuel ratio lambda is either very lean at part load or stochiometric at medium and high load. A stoichiometric ratio of lamda=l should lead to an ideal combustion. A change of the average would already double the emission rates. Therefore, it is important to have an accurate closed loop lambda control to guarantee an average air-fuel ratio within a window smaller than 0.1% around lamda=l. When engine speed and torque change, lambda deviation of 2-3% over a short period of time is allowed. If the average accuracy can be held, such deviations go into both directions. Within the volume of the catalytic converter excursions of the air-fuel ratio in one direction are compensated by those in the opposite direction. At the exhaust pipe tail, short time lambda deviations of a few percent do not deteriorate the emissions after the catalytic converter.
In the block-diagram of the lambda controlled SI engine the amount of injected fuel is controlled by the engine control unit which gets its feedback from the lambda sensor in the exhaust pipe as well as the mass air flow signal in the inlet pipe. Additional variables like engine speed and engine temperature are also used in the control scheme.
A lambda sensor is used to measure the concentration of oxygen in the exhaust pipe. The sensor is mounted in the collective exhaust pipe where the individual exhaust pipes from the cylinders end in. In engines with 6 or mor cylinders two lambda sensors are used, so the stochiometric point can be determined. The sensor consists of a solid ceramic electrolyte (zirconium dioxyde), which conducts oxygen ions at temperatures above 250C. The outer electrode is covered with platinum. The oxygen partial pressure on the surface of the ceramic material is thus identical with the one inside the catalytic converter. The inner electrode has a direct contact with the ambient air. The exhaust gases flow around the outer electrodes. Another sensor is based on strontium titanate. Stontium Titanate is a ceramic semiconductor material. Its conductivity in strontium titanate is less influenced by surface effects at high temperatures than in other materials.
The dependance of the probe resistance from the temperature decreases at higher temperatures leaving the dependance on lambda only. The strontium titanate sensor has a planar structure. An advance of the planar device is short response time of a few miliseconds after lambda deviations. Because of its operation at temperatures around 800C it can be fitted closer to the engine. The characteristic between the output voltage and the air-fuel ratio lamda is non-linear. After operation for many years this characteristic slightly ages. Therefore the most stable measuring range of the characteristic is taken for the control. Consequently the control loop reacts relatively slow to dynamic transitions between operating points.