Performance Evaluation of Mixed-Potential HC, NOx and NH3 Sensors in Diesel and Lean Gasoline Engine Exhaust

Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of NO_x, hydrocarbon, and ammonia sensing technology.  These devices are fundamentally simple, inexpensive, and robust devices owing to their close relationship to the well-established and ubiquitous automotive Lambda sensor. Our first dynamometer experiments (2014) focused on testing a single device in lean-burn, diesel exhaust from a 1.9L GM turbo-diesel engine in both NO_x and HC modes with gas sampling occurring both before and after the DOC.  In general, there was excellent qualitative agreement between actual exhaust gas composition (determined by FTIR and FID) however the inability to reproduce engine exhaust gas conditions exactly made precisely correlating sensor data to specific engine events while testing in the respective sensor operating modes difficult.  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 (model year 2008, E87 European model BMW 1-series 120i.)  A mixed potential NO_x sensor and a new NH₃ sensor were tested downstream of the three way catalyst 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 dynamometer operation. Data from the sensors were collected under rich, stoichiometric, lean homogeneous, and lean stratified conditions (0.98<λ<1.8) and at various engine loads. To simulate the occurrence of ammonia slip in a selective catalytic reduction (SCR) emissions system, ammonia was injected upstream of the NH₃ sensor to evaluate the ammonia sensitivity in the presence other exhaust gas constituents. The sensor response in the exhaust gas environment was compared with laboratory calibrations to assess the relative importance of the varying interferents species on the sensor response to the target species and to develop quantitative calibration curves relevant to the exhaust gas environment.