Diesel engines are known to be reliable and economical. In recent years, they have also significantly reduced the particulate and nitrogen oxide emissions through advanced emissions control systems. An unfortunate side effect of cleaning up diesel exhaust, however, can be a drop in fuel efficiency and a need to do diagnostics of whether the systems is operating in its design state.
Now, a partnership led by researchers at Michigan Technological University is addressing the problem. The three-year, $2.8 million project is being funded largely by a $1.7 million grant from the US Department of Energy’s National Energy Technology Laboratory. Additional support and in-kind goods, services and expertise is provided by the partners from the diesel engine companies Cummins, John Deere, and Navistar; sensor manufacturer Watlow; and Johnson Matthey, a producer of diesel catalysts and pollution-control systems. Scientists at Oak Ridge and Pacific Northwest National Laboratories are also collaborating.
The overall goal for this project is energy efficient emission control for heavy-duty diesel engines and the development of accurate methods for On Board Diagnostics (OBD). Energy efficiency impacts associated with emission control can be classified as direct or indirect. Diesel particulate filters (DPFs) have two direct fuel penalty effects. First, active regeneration, to oxidize accumulated particulate matter (PM), can result in a fuel penalty. Second, inefficient regeneration control strategies can cause unnecessary exhaust backpressure, and potentially fuel penalties due to inefficient engine operation. The inability to identify and adapt control strategies to aging or compromised components is a secondary fuel penalty scenario. Taken to the extreme, this falls into the category of the need for on-board diagnostics (OBD).
Selective catalytic reduction devices (SCRs) are used to reduce exhaust NOx levels through reaction with ammonia. The typical ammonia delivery mechanism is through injection of liquid urea. The fuel penalty associated with inefficient SCR operation arises from over injection of urea. This requires more frequent resupply and wastes the energy required for urea production and distribution. Excessive ammonia slippage, an unregulated toxic emission, is a side outcome of this scenario and should be minimized through proper control and OBD strategy implementation.
Each has its own set of problems. Diesel Particulate Filters (DPF’s) fill up with particulate matter. “To clear that out, you inject diesel fuel,” says John Johnson, a presidential professor of mechanical engineering at Michigan Tech. “It ignites and burns out the particulate.” The process can affect engine efficiency in two ways, by using excess fuel to clean the filter and, when the filter is clogged, by causing backpressure on the engine. Selective catalytic reduction systems use urea to chemically scrub NOx out of diesel exhaust. “Sometimes, however, the system gets overloaded with urea, which is not only wasteful but also can cause toxic emissions of ammonia gas.”
Exploration of biodiesel effects on DPF and SCR functionality, with particular attention to control system impact, is another aspect of this study. The results of this research could have significant indirect fuel efficiency impact not only in proper functioning of DPFs and SCRs on engines using biodiesel, but also in reducing the U.S. reliance on diesel fuel which comes from foreign crude. “Biodiesel blends are becoming more common and many manufactures are certifying to 20% biodiesel similar to the use of ethanol in gasoline engines,” says Jeffrey Naber, an associate professor of mechanical engineering at Michigan Tech. “However the engine and emission systems can react very differently to even these low biodiesel blends.” With proper technology, biodiesel could provide even lower emissions than traditional fuel without sacrificing fuel economy.
The researchers will create models and methods to improve the performance of both systems. One focus of their models will be for on-board diagnostics, so the driver can tell quickly and easily when an emissions control system needs attention. “It sounds pretty straightforward, but it can be tricky when you are dealing with complicated emissions controls,” says Gordon Parker, professor of mechanical engineering.
The benefits of this research are numerous, both direct and indirect. The diesel engine emission community will be able to apply and mold the estimation strategies developed by this project to fit their unique control and OBD development plans. Due to the wide-spectrum of NOx/PM ratios considered, the performance for existing and low-NOx engines can be assessed. Since the sensor type and combination is intimately tied to the state estimation activity, it’s very likely that this research will motivate sensor suppliers to develop new technologies, both passive and active, with the promise of being able to achieve emission control performance. Biodiesel effects control performance. The studies with biodiesel will determine changes that are needed in the control models due to engine out particulate matter changes in the reaction rate both passive and active in the DPF. Also, any effects of the biodiesel fuel on the DOC and SCR will also be determined so that appropriate changes in the control models could be implemented.
Finally, the organizational structure, bringing together engine OEMs, sensor and catalyst developers into a single focused research program will foster a level of system-level research focus not typically seen in user-specific research programs that focus only on the sensor, or the catalyst or the engine.
The principal investigator on the project is John Johnson, presidential professor of mechanical engineering-engineering mechanics. Co-investigators in the Department of Mechanical Engineering-Engineering Mechanics include Professors Gordon Parker, and Song-Lin Yang, and associate professor Jeff Naber. Jason Keith, an associate professor of chemical engineering, is also a co-investigator.