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Diesel fuel is a convenient source of energy for the regeneration of diesel particulate filters. Due to the utilization of chemically bound energy, regeneration systems based on fuel combustion are not limited by energy availability considerations, as it is the case with electric filters. Although in general any fuel can be used for DPF regeneration, for example propane, diesel fuel is the most obvious choice, due to its availability on the vehicle.
During active filter regeneration, the exhaust gas temperature can be increased by combusting an additional quantity of fuel either (1) in the engine cylinder, or (2) in the exhaust system, upstream of the filter. In the first approach, the fuel is introduced through late cycle injection (post-injection) using the fuel injection system of the diesel engine. In the second method, fuel is introduced to and combusted in the exhaust system using specialized hardware.
In the exhaust system, the fuel can be combusted to increase the exhaust gas temperature using the following methods:
The burner system offers more flexibility for regeneration at the expense of more complex hardware. The burner system can be designed for regeneration at any engine operating condition. The catalytic combustion system on the other hand, requires a certain minimum catalyst temperature. Thus with the catalytic system regeneration might not be possible at idle or at light engine load conditions, when the exhaust gas temperature is below the catalyst light-off temperature (unless additional strategies are implemented to increase the temperature). In the combined burner-catalyst approach, a small burner installed upstream of the catalyst facilitates catalyst light-off regardless of the engine operating conditions.
While exhaust methods are the focus of this paper, certain aspects of the regeneration processes based on in-cylinder and exhaust injection with catalytic combustion are similar. Not all of the post-injected fuel can be combusted in the cylinder, resulting in increased HC concentrations in the exhaust. Therefore, in-cylinder strategies typically also include a catalyst to oxidize the remaining HCs and to further increase exhaust temperature. Some DPF systems combine both strategies—in-cylinder and exhaust injection of fuel—to ensure filter regeneration.
In comparison to in-cylinder injection, exhaust injection results in a smaller fuel economy penalty, as the heat is released immediately upstream of the filter, eliminating heat losses in the engine and in the section of the exhaust system between the engine exhaust manifold and the DPF. According to some authors, the fuel economy penalty when using exhaust injection can be up to 50% less than that associated with in-cylinder injection . Poor efficiency of in-cylinder injection has also been demonstrated by others . An example heat balance comparison when using two methods—in-cylinder injection and a burner, both in combination with a catalyst—is shown in Figure 1  (the thickness of a line is proportional to the heat flux; upward pointing arrows represent heat losses). The burner approach in Figure 1 represents a 35% fuel economy advantage. The results depend on the engine load and speed point used as basis for the calculation.
Figure 1. Heat Balance Comparison for Different Fuel Combustion Methods
A: Engine post-injection + catalyst; B: Burner + catalyst
Historically, flame combustion systems were developed earlier than those based on catalytic combustion. The first commercial fuel burner filters were introduced in the early 1990s. Fuel burner DPFs have been used for retrofitting of in-use diesel engines—primarily in Germany, Switzerland, and other European countries—in a variety of applications ranging from diesel forklifts through construction machinery and urban buses to railroad locomotives. The first wide-scale OEM application of burner regenerated filters were US 2007 heavy-duty truck engines. Caterpillar introduced burner regenerated filters across its 2007 ACERT engine line-up, while other manufacturers introduced burners in selected cold duty cycle applications (e.g., Mack refuse trucks with a DPF burner system by Emcon).
Catalytic combustion systems, on the other hand, have been developed mainly for OEM applications, including most US 2007 truck engines, as well as heavy-duty engines in Japan. In medium- and especially in light-duty applications—where low system cost is more important and less concern exists about the impact of oil dilution on engine durability—in-cylinder injection has been the predominant method. However, exhaust injection was also introduced in some filter systems for passenger cars (for instance, in the 1.5 dCi diesel engine in Renault Clio and Modus models in 2006). Retrofit DPFs with catalytic combustion of fuel have been under development by a number of manufacturers, targeting heavy-duty highway and nonroad engines, but are still in the initial phase of commercialization.