Filters with Fuel Burners

W. Addy Majewski

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• Introduction
• Fuel Burners
• Combined Burner-Catalyst Systems
• Commercial DPF Systems

Introduction

Definitions and Classifications

Diesel fuel is a convenient source of energy for the regeneration of diesel particulate filters. In the exhaust system, the fuel can be combusted to increase the exhaust gas temperature using the following methods:

• Catalytic combustion—in catalytic diesel filters, fuel-derived hydrocarbons, introduced to the exhaust gas via in-cylinder fuel injection strategy or through a dedicated exhaust injector, are oxidized over an oxidation catalyst.
• Flame combustion—the fuel is combusted in a fuel burner, usually with a dedicated supply of combustion air, with the flame entering the exhaust system.
• Combined flame and catalytic combustion—a combination of the above methods, where a fuel burner is followed by a catalytic combustion system.

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, while the catalytic combustion system requires a certain minimum catalyst temperature. Even if additional thermal management strategies are implemented, regeneration might not be possible in the catalytic system at low engine load conditions, when the exhaust gas temperature is below the catalyst light-off temperature. In the combined burner-catalyst approach, a small burner (mini-burner) installed upstream of the catalyst facilitates catalyst light-off regardless of the engine operating conditions.

Fuel burner systems can be designed to perform the regeneration at any engine operating conditions, or else may require that the engine be operated at certain conditions—e.g., at low idle speed—for the time of regeneration. Accordingly, burner systems can be divided into two categories:

1. Single point burner systems, and
2. Full flow burner systems.

Single point filter systems require an intervention by the vehicle operator to perform the regeneration, which is carried out at steady-state conditions. When the filter control system detects that the maximum soot load is reached, it alerts the operator that filter regeneration should be initiated. Single point filters require that the engine is either operated at idle during the regeneration cycle (typically 15-20 minutes) or else they supply the gas flow using a blower and require that the engine be shut down for regeneration. The burner operates at a constant power, selected to increase the exhaust gas temperature to a desired level.

The use of single point systems has been limited to certain retrofit applications, such as construction machinery or diesel forklifts operating in enclosed spaces. They are not suitable for on-road use due to the need for operator’s intervention and the required idling or shut down period during regeneration.

Full flow burner systems feature fully automated regeneration at the actual exhaust gas flow during the regular duty cycle of the vehicle. When a predetermined soot load is accumulated, the filter controller triggers regeneration without the intervention or knowledge of the vehicle operator. The filter control unit starts the burner and controls its operation to maintain the required filter temperature. After the filter is cleaned, the controller stops the burner and the filter enters the next loading cycle.

Full flow fuel burner filters require electronic control units to maintain a thermally balanced regeneration which minimizes thermal stress to the filter monolith. Here the term “thermal balance” means that the thermal energy produced by the burner corresponds to the soot load in the filter and to the actual exhaust gas flow conditions. In order to avoid filter overheating, the filter controller has to watch the actual soot load in the filter, as well as the actual engine speed, in order to effectively control the exhaust gas temperature. That control is realized by varying the amount of thermal energy produced by the burner. In addition, the regeneration is performed by cycling through a sequence of temperatures, rather than maintaining the filter face at a constant elevated temperature.

Many fuel burner systems utilize uncatalyzed filter substrates. In the absence of the catalyst, the regeneration process depends on the oxidation of soot by oxygen. To initiate regeneration, the exhaust temperature at the filter inlet is increased to approximately 650-700°C. This results in somewhat higher fuel consumption (and fuel economy penalty) compared to the catalytic combustion systems. Therefore, in the combined flame and catalytic combustion approach, catalysts are used to lower the regeneration temperature and shorten the duration of regeneration. Catalysts can also be necessary whenever control of gaseous emissions and/or diesel odor is required.

Commercial Status

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 for retrofit applications. 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 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. However, the burner DPF system was troubled with technical problems, contributing to the withdrawal of the company from the onroad truck engine market.

While most new (OEM) diesel engines eventually adopted catalytic filter systems, fuel burners continued to be used in selected cold applications of low engine load factor. In the US market, examples include Mack refuse trucks and medium-duty trucks by Hino Motors [3247], both employing a DPF burner system by Emcon (now Faurecia). In the Hino system, the function of the burner was not only DPF regeneration, but also ‘de-crystallization’ of the SCR catalyst and components.

Acknowledgements

We would like to acknowledge the help and assistance from several individuals in completing this paper: Karl-Heinz Breuer of Deutz AG contributed the section on the DPFS filter system; Gerd Gaiser of Eberspächer supplied information and figures on the Fuel Processor burner; Don Newburry of Miratech Corporation and Christoph Kopp of Hug Engineering provided technical information and pictures of the MobiClean filter system.

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