Direct Digital, a new Method of Wideband O2 Signal Processing 2009-01-1680
The mass ratio of air to fuel (air-fuel ratio) of an operating internal combustion engine is a very important metric for pollution control. Typically the air-fuel ratio is not directly measured, but instead the excess air factor Lambda (λ) is used. Lambda is the ratio of actual air-fuel ratio to the stoichiometric air-fuel ratio.
Commonly switching type sensors are used. Those can detect 3 states: λ =1, λ >1 and λ< 1, and are used under low and medium load conditions to keep λ in the optimum operating range for a catalytic converter.
Wideband O2 sensors are exhaust analysis devices that are used to measure air-fuel mixtures over a very large range up to air. These sensors are used in more and more engines today for closed loop fueling control under all operation conditions. They are especially important for new lean-burn technologies, clean diesel applications and for alternative fuel engines. However, todays typical control methodology for these sensors has drawbacks regarding response times, accuracy and drift over time. For example, individual cylinder control for injector balancing requires sensors with very fast response times, if a single sensor is to be used for a group of cylinders. The alternative is to use individual sensors for each cylinder, an approach that is cost-prohibitive.
The commonly used control method for wideband sensors today typically limits the 3dB measurement bandwidth to less than 10 Hz for the fastest implementations. This bandwidth is too low to be usable for individual cylinder control with a single sensor per cylinder group. It also severely limits their application in closed loop control of transitional engine states.
This paper describes a new approach for wideband O2 sensor control that possibly enables:
Individual Cylinder Balancing
Automatic calibration to compensate for sensor drift
Simplified, low cost implementation
Cost reduced single-cell wideband sensors
Detailed sensor diagnostics
The method described treats a pump-cell wideband not as a feedback controlled system, but as the “analog” front-end of a modified Delta-Sigma Analog-to-Digital Converter. This results in fast, highly linear measurement response and simplified implementation of the control electronic for wideband O2 sensing. This new method is possible now due to the advances in integration of digital systems and the advances in digital signal processing.