by Christopher Kalebjian
Chief Engineer and Program Manager, Achates Power, Inc.
I recently attended the Emissions 2013 Conference at Eastern Michigan University in Ypsilanti. It was a pleasure to be there and a treat to listen to some great talks. I was also there to present our opposed-piston, two-stroke engine’s capability of a low emissions and rapid light-off strategy.
With more stringent emissions and fuel efficiency requirements—not to mention the increased customer demand for fuel economy—some of the technologies discussed at the conference need to come sooner rather than later, with careful thought as to the cost incurred to the end product.
I was the third person to present in the automotive propulsion, fuels and advanced technologies panel and the two preceding speakers provided a very good lead in. The first, Dr. Scott Curran of Oak Ridge National Laboratory, discussed low oxides of nitrogen (NOx) and soot emissions, along with high carbon monoxide (CO) and hydrocarbon (HC) emissions, and the struggles to keep the aftertreatment above its light-off temperature in the extremely interesting world of Reactivity Controlled Compression Ignition (RCCI). The second, Cary Henry at Southwest Research Institute, focused on the benefits of a dedicated exhaust gas recirculation (EGR) cylinder and one of the key points was to improve the ratio of specific heats in order to increase efficiency.
My presentation followed nicely and covered the opposed-piston, two-stroke engine’s benefits to efficiency through leaner combustion (higher ratio of specific heats), lower heat transfer due to a lower surface area-to-volume ratio, and more optimally phased combustion.
After highlighting the efficiency benefits of our engine, I shared its capability for generating heat for the aftertreatment catalysts. The design inherently allows for the flexibility to calibrate the engine in a way to generate more than 400° C at the turbine outlet while producing 0.9mg/s of NOx at an elevated idle speed of 1000 rpm. Today’s diesel engines produce the majority (more than two-thirds) of the tailpipe emissions on an FTP-75 in the first 250 seconds. Therefore, low emissions and fast catalyst light-off is a must with the latest stringent emissions requirements.
As the engine runs with high boost and high back pressure, we end up with a low delivery ratio and, as a result, high trapped temperature. This low delivery ratio can be used in conjunction with a late multiple injection strategy to produce significant exhaust temperatures with low emissions, which is a highly desirable outcome for a cold-start emissions cycle.
There are several other situations where this type of strategy can be beneficial, such as at low loads, low ambient temperatures, real-world cold starts, active diesel particulate filter (DPF) regenerations, in-service conformity and performance ratio diagnostic compliance. Different levels of this strategy can provide the desired results for each of these conditions.
The flexibility of the Achates Power opposed-piston, two-stroke engine is an exciting feature for delivering the desired results required to meet and, more importantly, exceed future emissions and fuel efficiency requirements.