Dynamometer Testing of Planar Mixed-Potential Sensors

Recently strengthened federal regulations are driving the reduction of pollutants emitted from heavy-duty diesel engines. The use of selective catalyst reduction (SCR) and exhaust gas recirculation (EGR) systems combined with regenerative particulate traps have reduced NO_x and particulate matter (PM) tailpipe emissions from heavy-duty diesel vehicles by 95%.¹ Despite these recent advances, there is a lack of low cost exhaust gas sensor technology to monitor tailpipe emissions and to control and maintain optimized SCR and EGR system efficiencies². Derivatives of the hugely successful robust, zirconia-based electrochemical sensors currently used for gasoline engine control and OBD in conjunction with three-way catalytic converters, has attracted considerable interest in diesel applications. However, the implementation of the diesel analogue to OBD has proved to be more complicated.

Mixed-potential sensors fabricated via well-established commercial manufacturing methods present a promising avenue to enable the widespread utilization of NO_x sensing technology.  The Materials Synthesis and Integrated Devices group at LANL has worked in collaboration with Electro-Science Laboratories (ESL, King of Prussia, PA) to fabricate planar mixed-potential sensors via the readily scalable, cost-effective high temperature co-fired ceramic (HTCC) technology already employed in the manufacturing of planar O₂ λ-sensors.

The two sides of the planar, self-heated, tape cast sensor prototype are shown in Figure 1. A Pt-heater with independent leads is printed on the backside of the ceramic substrate. The sensing component consists of dense La_1-xSr_xCrO₃ (LSCrO)- working and Pt-counter electrodes coated with a porous YSZ electrolyte layer. The voltage response of a sensor operated at 490˚C to 100 ppm of varying analyte gases is shown in Figure 2. Operated at open circuit, this combination of electrode materials serves as a sensor for reducing gases, such as hydrocarbons and ammonia.  However, the sensor can be transformed to a total NO_x sensor by appropriate tuning of the current bias and operating temperature³. 

The performance of the planar sensors under engine-out conditions was recently evaluated at the Oak Ridge National Laboratory National Transportation Research Center.  The sensor was mounted in a quartz tube with the Pt heater resistance set to correspond to an operating temperature of 490˚C. The sensor was placed in the exhaust gas downstream of an orifice regulating the flow to 1000ccm and upstream of the FTIR used to analyze the exhaust gas components.  A California Instruments Flame-Ionized Detector (FID) was also used to analyze total hydrocarbon content (THC) immediately upstream of the orifice. 

Figure 3 shows the sensor response during varying engine operating conditions. The sensor response while operating in hydrocarbon sensing mode (0 bias) during engine start-up is shown in Figure 3a.  The sensor response closely tracks the THC measured by the FID, while the THC measured via FTIR lags the response of both the FID and the sensor. Figure 3b shows the sensor response while operating in NO_x sensing mode (+0.2 μA bias) while the engine was held at 1600 rpm and EGR was cycled on and off.  The sensor response qualitatively tracks the NO concentration in the exhaust stream, with the sensor response becoming more negative with increasing NO, in agreement with the biased response shown in Figure 2.  In this work, we will discuss the performance of the planar, mixed-potential sensors under engine-out conditions and compare the dynamometer performance with laboratory measurements. 

References

1.  G. Sandhu and H. C. Frey, Transportation Research Record (2012).

2.  P. K. Sekhar, E. L. Brosha, R. Mukundan, W. Li, M. A. Nelson, P. Palanisamy and F. H. Garzon, Sensors and Actuators B: Chemical 144 (1), 112-119 (2010).

3.  R. Mukundan, K. Teranishi, E. L. Brosha and F. H. Garzon, Electrochemical and Solid-State Letters 10 (2), J26 (2007).

 Acknowledgments

The research was funded by the US DOE, EERE, Vehicle Technologies Office and the authors wish to thank Technology Development Manager Roland Gravel. The sensors were fabricated by ESL ElectroScience, King of Prussia, PA.