Abstract
In compliance with new European and American legislation automotive manufacturers are obliged to ensure the continuous monitoring of the components involved in emission control and reduction during the whole vehicles life. The system must determine whether a failure in any of these components would cause tailpipe emissions to exceed a set threshold level imposed on CO, HC and NOx emissions for a given driving cycle.
The research deals with the Three-Way Catalyst (TWC) converter used to decrease pollutants emitted by gasoline powered vehicles. Indeed, gasoline is burned with an amount of air in proportions leading to complete combustion products (CO2 and H2O) as well as incomplete combustion products (CO and unburned hydrocarbons HC). During combustion process high temperatures are produced, inducing the thermal fixation of nitrogen to form NOx. The ideal solution to control the catalyst would consist of the direct measurement of the concentrations in CO, HC and NOx on the vehicle. But, at the present moment there is no light and accurate enough on-board low cost device.
One of the suitable methods for on-board vehicle applications is based on the basic hypothesis that the efficiency of a catalytic converter depends directly on its Oxygen Storage Capacity. The OSC determination is based on a comparison between two lambda sensor signals (a pre-catalyst and a post-catalyst sensor) while the engine air/fuel ratio is oscillating at regulated frequencies around the stoichiometric ratio.
The purpose of this paper is to present a research dealing with Oxygen Storage Capacity (OSC) determination of the catalysts. The research aims to answer the expectations of automotive manufacturers and suppliers, who are trying to satisfy the E-OBD (European On-Board Diagnostic) requirements in terms of emission control.
The OSC can be calculated from the delay between the pre- and post-catalyst sensor signals when the air/fuel ratio of the gases is oscillating in steps around the stoichiometric ratio. This method is labelled the step method. With these laboratory results, in addition to the measurement of CO, HC and NOx concentrations on a gas analyser, the OSC=f(pollutants concentrations) curve can be plotted and so the threshold level of the pollutants concentrations in accomodation with the legislation can be converted on the OSC threshold. This OSC threshold value is then implemented in the engine ECU and compared with the OSC measured by the on-board diagnostic system during vehicles life. When the measured value exceeds the previously defined threshold level, the calculator turns on a Malfunction Indicator Light (MIL) on the dashboard.
Tests for the OSC calculation using the step method were performed on a regulated gasoline burner in place of a standard gasoline engine to create the gases needed for the experiments. The burner was developped in our laboratory in reference to the most widespread type of high-pressure oil burner. The gasoline injection device was taken from a mechanically operated indirect mutlipoint injection system for vehicle applications. The valve adjusting the mass flow of gasoline was controlled by a stepper motor monitored by a Labview software application.
The research included laboratory tests on a dedicated test rig to calculate the OSC and also an airflow simulation using Fluent CFD software. These simulations were intended to validate the burner architecture. The laboratory tests were conducted on catalysts of different ages and aimed to validate the gasoline burner as a laboratory tool to determine the OSC of Three-Way Catalysts.