Heat-Up Catalyst Systems

W. Addy Majewski

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Abstract: A heat-up catalyst is commonly used to increase exhaust gas temperature in diesel engines to ensure adequate performance of the emission aftertreatment system. The heat-up catalyst is a specialized diesel oxidation catalyst to which fuel is supplied via the in-cylinder fuel injectors or a separate dedicated injector mounted in the exhaust system. An electronic controller maintains the fuel injection rate to achieve the desired catalyst outlet temperature.

Introduction

A heat-up catalyst is a very common option for raising diesel engine exhaust temperature and enthalpy by catalytic combustion of fuel. The heat-up catalyst is typically a diesel oxidation catalyst (DOC) to which fuel is supplied via the in-cylinder fuel injectors or a separate dedicated injector mounted in the exhaust system. Heat-up catalyst systems were first adopted and remain to be widely used for the regeneration of diesel particulate filters.

Compared to engine based measures such as intake throttling, retarded injection timing and post injections for thermally managing diesel exhaust, using a heat-up catalyst results in a smaller fuel economy penalty, as the heat is released over the heat-up catalyst rather than in-cylinder, thus eliminating heat losses in the engine and in the section of the exhaust system between the engine exhaust manifold and the DOC. According to some estimates, the fuel economy penalty can be up to 50% less [1257]. The efficiency of engine based measures has been estimated to be as low as 17% [1485]. However, despite the low efficiency of engine based measures, they are still typically required with a heat-up catalyst system to raise and maintain the temperature of the DOC above the light-off temperature at light-load conditions.

Heat-up catalyst systems for heavy-duty OEM applications—such as those launched in US 2007 truck engines—typically use a dedicated exhaust system mounted fuel injector to supply fuel to the catalyst. In light- and medium-duty applications—where low system cost is more important and less concern exists about the impact of oil dilution on engine durability—the fuel is typically supplied by after-injections of fuel in one or more engine cylinders. However, exhaust injectors have been used in some systems for passenger cars as well (e.g., in the 1.5 dCi diesel engine in Renault Clio and Modus models in 2006).

Retrofit DPF systems utilizing heat-up catalysts for filter regeneration have also been developed by a number of manufacturers, targeting mainly heavy-duty onroad and nonroad engines.

System Schematic

The main components of a heat-up catalyst system are shown in the schematic in Figure 1. In this case, a fuel injector is installed in the exhaust pipe between the engine exhaust manifold and the oxidation (heat-up) catalyst. The injector is controlled by an electronic control unit, which can be integrated with the engine control unit (ECU). In light- and medium-duty applications, after-injections supplied by the in-cylinder fuel injector are commonly used instead. In Figure 1, fuel is supplied to the injector by a dedicated fuel pump (which may also include a fuel filter to protect the injector from impurities). Alternatively, the fuel can be metered by an ECU-controlled dosing pump.

Figure 1. Heat-up catalyst system for managing exhaust temperature and enthalpy

Depending on the design, the return fuel line from the injector may be redundant. The static mixer is optional, but good mixing is required to ensure catalyst durability.

When exhaust temperature and/or enthalpy needs to be increased, the control unit initiates fuel dosing to the heat-up catalyst. However, catalytic combustion of fuel is only possible if the exhaust gas temperature is sufficiently high to ensure catalyst activity. Thus, under light engine load conditions, the heat-up catalyst activation includes two stages:

  1. The exhaust gas temperature is increased to reach the catalyst light-off—usually about 250°C—using engine management methods, such as intake throttle and injection system strategies [1256]. If such methods are not available (e.g., in retrofit DPF applications), fuel dosing has to be delayed until the engine is operated at a higher load.
  2. Fuel dosing over the heat-up catalyst is initiated to reach the target temperature and maintained as long as needed.

Once injected into hot exhaust gas, the fuel evaporates. Fast and complete evaporation is facilitated by good atomization of fuel at the injection nozzle. In order to avoid hot spots and/or coking at the catalyst inlet face that could lead to potential damage to the catalyst, the fuel must be completely evaporated and the vapors must be thoroughly mixed with the exhaust gas. Non-uniform fuel supply across the catalyst inlet face may also cause non-uniform and incomplete fuel oxidation over the DOC and lead to non-uniform DOC outlet temperature and durability challenges. In cases where inadequate mixing length is available between the exhaust mounted fuel delivery and the DOC inlet to achieve adequate fuel distribution, a static mixer may be used [4908][4909].

The hydrocarbon-rich exhaust gas then enters the oxidation catalyst that promotes HC oxidation by oxygen present in the exhaust. Heat released during this exothermal reaction increases the exhaust temperature to the required level. The amount of injected fuel must be precisely controlled to maintain the desired temperature at any given engine-out exhaust temperature and flow rate conditions.

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