Advancements in both combustion system design and emission control technologies have allowed more efficient diesel engines to meet stringent exhaust emissions standards. With very stringent emission regulations currently in place, combustion system design and combustion management are used in conjunction with available exhaust after treatment technologies for optimal reduction of emissions to meet regulatory targets. Current diesel vehicles in the US are equipped with a diesel oxidation catalyst (DOC) to control CO and HC emissions, a diesel particulate filter (DPF) to control particulate matter (PM) and selective catalyst reduction (SCR) or lean-NO_x trap (LNT) to control NO_X emissions. In SCR, NO_x is reduced to H₂O and N₂over a catalyst through the injection of an ammonia source provided by the onboard storage of diesel exhaust fluid (DEF), typically a mixture of urea and de-ionized water.  These emission control technologies add significant complexity and cost to a vehicle and some result in additional fuel penalty needed for their operation.

Meeting the EPA Tier 3 emissions reduction requirements while simultaneously increasing fuel economy to meet new CAFE standards will require optimization of advanced combustion strategies and emissions control technologies as well as their state of operation under transient vehicle operation. NH₃ sensors are needed to monitor DEF injection and prevent NH₃ slip—a condition under which unreacted ammonia from the SCR is emitted in the tailpipe exhaust. Ideally, both NH₃ and NO_xsensors would be incorporated into a closed-loop system to maintain ammonia close to stoichiometry in the SCR system, and could also serve to monitor engine out and tailpipe out emissions in order to ensure regulatory compliance.

Mixed-potential sensors are electrochemical devices that develop a non-Nernstian potential due to differences in the redox kinetics of various gas species at each electrode/electrolyte gas interface^1,2. The difference in the steady-state redox reaction rates between the “fast” and “slow” electrodes gives rise to the measured mixed potential response. A patented LANL sensor design incorporates dense electrodes and a porous electrolyte and reproducible and stable sensors are prepared using a high temperature ceramic co-fire approach³. The use of dense electrodes minimizes deleterious heterogeneous catalysis, and the increased morphological stability of dense electrodes yields a robust electrochemical interface, increasing lifetime durability⁴. The use of a dense gold-alloy working electrode (e.g. the slow electrode), imparts preferential selectivity to NH₃, and avoids the stability problems one finds when using thin film, porous Au electrodes⁵.

Previously, we reported on the response of an experimental, tape-cast version of a mixed potential ammonia sensor to simulated diesel exhaust in a high-flow bench reactor at the Oak Ridge National Laboratory (ORNL) National Transportation Research Center (NTRC)⁷. In this work, we present recent results from evaluating the performance of a pre-commercial form of a mixed potential-based NH₃ sensor in engine exhaust conditions at the NTRC using a GM 1.9L CIDI diesel engine.  A planar NH₃ sensor with a protective porous overcoat was tested in diesel engine exhaust gas (20 L/min) sampled downstream of the engine’s DOC.  In order to simulate NH₃ slip from a full SCR emissions control system, NH₃ was injected immediately upstream of the sensor using a calibrated mass flow controller. The sensor response quantitatively tracked levels of injected NH₃ and transients measured via Fourier transform infrared (FTIR) analyzer, using a calibration curve derived from an ammonia staircase response measured in the engine exhaust at low NO_x and HC concentrations (<20 ppm) and steady-state engine conditions. Exhaust gas recirculation (EGR) switching and EGR sweeps were used to evaluate the NH₃ sensor response under different amounts of total NO_x. This calibration curve was used to directly compare the [NH₃] calculated from sensor response to the gas phase composition measured via FTIR.

References

 

1. J. W. Fergus, Journal of Solid State Electrochemistry 15 (5), 971-984 (2010).
2. F. H. Garzon, R. Mukundan and E. L. Brosha, Solid State Ionics 136-137, 633-638 (2000).
3. P.K.Sekhar et. al.,  Sensors and Actuators B: Chemical 144, 112 (2010).
4. R. Mukundan, E. L. Brosha and F. H. Garzon, Journal of The Electrochemical Society 150 (12), H279 (2003).
5. J. Tsitron et. al., Sensors and Actuators B 192 (2014) 283-293.
6. E. L. Brosha et al., “Response Characteristics of a Stable Mixed Potential NH₃ Sensor in Simulated Diesel Exhaust,” 228^th Meeting of the ECS, Phoenix, AZ, October 2015.

Acknowledgments

 

The authors would like to thank Roland Gravel and the DOE Vehicle Technology Office for providing funding for this work. Engine-based studies were conducted at the National Transportation Research Center User Facility at Oak Ridge National Laboratory.