Diesel Emission Test Stand for Advanced SCR Control

The combination of increasingly challenging emissions regulations and impending Corporate Average Fuel Economy (CAFE) standards of 54.5 mpg by 2025 presents auto makers with a challenge over the next 10 years. The most promising technologies currently available for meeting high fuel economy and low emissions regulations are increased hybridization, turbo downsizing, and increased Diesel engine implementation. Combining these into a hybrid turbo Diesel is an ideal transition technology for the very near future as battery and other alternative fuels become viable for widespread automotive use.

This paper presents a Diesel emission test stand to improve Selective Catalytic Reduction (SCR) systems for light duty Diesel vehicles, particularly hybrid power systems that experience many start-stop events. Advanced modeling and control systems for SCR systems will further reduce tailpipe emissions below existing Tier structures and will prepare manufacturers to meet increasingly stringent Tier 3 standards beginning in 2017. SCR reduces oxides of Nitrogen, NO, and NO2, from otherwise untreated Diesel emissions. Scientific study has proved that inhaling this harmful exhaust gas is directly responsible for some forms of lung cancer and a variety of other respiratory diseases. In addition to EPA Tier emissions levels and CAFÉ standards, the On-Board Diagnostics (OBD) regulations require every vehicle’s emission control systems to actively report their status during all engine-on vehicle operation. Testing and development with production NOx sensors and production SCR components is critical to improving NOx reduction and for OEMs to meeting strict Tier 3 light duty emission standards.

The test stand was designed for straightforward access to the NO x  sensors, injector, pump and all exhaust components. A Diesel Particulate Filter (DPF) followed by a Diesel Oxidizing Catalyst (DOC) precedes the Selective Catalytic Reducer (SCR) injector, mixing pipe and catalyst. An upstream NO x  sensor reads engine-out NO x  and the downstream NO x  sensor reports the post catalyst NO x  levels. Custom fabrication work was required to integrate the SCR mechanical components into a simple system with exhaust components easily accessible in a repeatable, controlled laboratory environment.

A Diesel generator was used in combination with a custom designed resistive load bank to provide variable NO x  emissions according to the EPA drive cycles. A production exhaust temperature sensor was calibrated and integrated into the software test manager. Production automotive NO x  sensors and SCR injector, pump and heaters were mounted on a production light duty vehicle exhaust system. The normalized nature of NO x  concentration in parts per million (ppm) allows the small Diesel generator to adequately represent larger Diesels for controls development purposes. Both signal level and power electronics were designed and tested to operate the SCR pump, injector, and three Diesel Exhaust Fluid (DEF) heating elements. An Arduino-based Controller Area Network (CAN) communications network read the NO x  Diesel emissions messages from the upstream and downstream sensors. The pump, injector, solenoid, and line heaters all functioned properly during DEF fluid injection. CAN and standard serial communications were used for Arduino and Matlab/Simulink based control and data logging software. Initial testing demonstrated partial and full NO x  reduction. Overspray saturated the catalyst and demonstrated the production NO x  sensor’s cross-sensitivity to ammonia. The ammonia was indistinguishable from NO x  during saturation and motivates incorporation of a separate ammonia sensor.