Performance Evaluation of Mixed-Potential HC, NOx and NH3 Sensors in diesel and lean gasoline exhaust

Eric  Brosha, Los Alamos National Laboratory

Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of nitrogen oxide (NOx), hydrocarbon (HC), and ammonia sensing technology.  These devices are fundamentally simple, inexpensive, and robust owing to their close relationship to the well-established and ubiquitous automotive Lambda sensor. Our first engine experiments (2014) focused on testing a single device in diesel exhaust from a 1.9L GM turbo-diesel engine in both NOx and HC modes with gas sampling occurring both before and after the diesel oxidation catalyst (DOC).  In general, there was excellent qualitative agreement between the sensor response and the actual exhaust gas composition (determined by FTIR and FID). Recently, multiple mixed potential sensors were tested simultaneously in the exhaust of a lean-burn, 4 cylinder 2.0-liter naturally aspirated, direct injection gasoline engine (MY2008, E87 European model BMW 1-series 120i.)  A mixed potential NOx sensor and a new NH3 sensor were tested downstream of the three way catalyst (TWC) while a dedicated HC sensor was simultaneously tested upstream of the TWC. The sensor response and heater power applied to the sensors (e.g. sensor temperature) were continuously monitored while the high frequency impedance of a dedicated fourth sensor was tested for the purpose of independently monitoring sensor electrolyte temperature stability during engine operation. Data from the sensors were collected under rich, stoichiometric, lean homogeneous, and lean stratified conditions (lambda 0.98-1.8) and at various engine loads. To simulate the occurrence of ammonia slip in a selective catalytic reduction (SCR) emissions system, NH3 was injected upstream of the NH3 sensor. This system was used to evaluate the NH3 sensitivity in the presence of other exhaust gas constituents.  We will also present a brief introduction of the design and laboratory testing results of our new mixed-potential NH3 sensor fabricated by Electro-Science Laboratories (ESL, King of Prussia PA) using their proprietary High Temperature Co-Fired Ceramic (HTCC) techniques and the benefits of using a common platform for all three sensors.

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