Heat-Up Catalyst Systems

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]. Despite the low efficiency of engine based measures, they are still often 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.

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:

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.

Controller

The control unit initiates and maintains the fuel injection rate to achieve the desired DOC outlet temperature. It also provides diagnostic functions, monitors and provides feedback on system status. In retrofit applications, a stand alone controller is used which may communicate with the engine ECU. In OEM applications, control is typically performed by the engine ECU. The system controller must maintain the desired DOC outlet temperature during transient conditions, as illustrated in Figure 2 [1111].

As seen in the chart, maintaining constant DOC outlet temperature at variable exhaust gas mass flow rate and variable DOC inlet temperature conditions is not a trivial task. Control issues have been reported to result in white smoke under certain transient conditions, presumably due to high HC emissions.

Effective control strategies combine a model based approach with feedback input. Reliance on feedback alone to predict the energy requirement (fueling rate) to achieve a desired DOC outlet temperature is not sufficient due to slow system response caused by thermal delay [1111].

Heat-Up Catalyst

Most DPF systems with heat-up catalysts utilize a dedicated heat-up catalyst that facilitates catalytic combustion of the injected fuel. In theory, the injected fuel could also be oxidized over a catalyst coated on the DPF, eliminating the need for the DOC. However, a dedicated DOC gives more flexibility in designing the DOC and DPF catalyst formulations and configurations, and may result in a lower noble metal load per vehicle. A dedicated DOC can also provide better CO light-off—an important function of the emission system in EU passenger cars. A DPF system configuration for Euro 5 passenger cars has also been proposed, which utilizes a single DPF substrate where the heat-up catalyst is zone-coated in the inlet section of the filter [1259].

The heat-up catalyst is a diesel oxidation catalyst optimized for low HC light-off temperature, high activity and thermal durability. In addition, NO₂ generation activity is often desired, as increased NO₂/NOx ratios promote passive DPF regeneration that can minimize the fuel economy penalty associated with DPF regeneration.

Low light-off temperature is a desired feature of the heat-up catalyst. Catalysts with lower light-off temperature allow active regeneration over a wider area on the engine map—including operation at lighter engine loads—without using engine management strategies to increase temperature. Commercial catalyst formulations are capable (in the aged state) of sustaining the combustion of fuel at catalyst inlet temperatures around 250°C under diesel exhaust conditions [1255][1252]. Light-off temperatures of below 150°C have been reported with blends of synthetic gases of low molecular mass, but such figures are not representative of diesel exhaust operation.

Due to the high temperatures, active DPF regeneration requires much better thermal durability of the catalyst than many other DOC applications. An additional source of activity loss of the heat-up catalyst is fouling by unburned fuel and/or material of high boiling point such as coke. Catalyst manufacturers have been developing more durable catalyst formulations to meet these demands. However, oxidation catalysts tend to gradually loose activity after extended exposure to temperatures above approximately 750°C. Fuel injection systems—including exhaust and in-cylinder enrichment—must be designed with these durability limits in mind. As already mentioned, good mixing of the injected fuel with the exhaust gas is also very important for catalyst durability. Local high HC concentrations from poor mixing may result in localized high temperatures that can damage the catalyst.

Fuel Delivery System

Exhaust Fuel Delivery

In most heavy-duty applications, diesel fuel is delivered to the heat-up catalyst via a hydrocarbon injection (HCI) system—a.k.a. diesel delivery unit (DDU)—that utilizes a dedicated exhaust system injector (Figure 1). The injector is typically served by a dedicated fuel pump. The main performance requirements of the fuel system include (1) fast transient response and (2) uniform temperature distribution over the face of the catalyst. The latter requirement is achieved through good fuel atomization and spray pattern, Figure 3 [1111].

The injector must provide good atomization of the fuel injected into the exhaust pipe, to ensure rapid and complete evaporation and thorough mixing with the exhaust gas. A better quality of injection can be achieved using air assisted atomization. However, an air compressor is required, which would increase the complexity and cost of the system. Some HCI systems utilize a static mixer between the injector and the DOC to further improve mixing. Adequate evaporation and mixing can also be ensured by a higher fuel injection pressure and by increasing the length of the exhaust pipe between the injection point and the DOC inlet.

A production-style fuel injector shown in Figure 4 is installed at a short distance downstream of the turbocharger. Fuel is supplied to the injector by a dedicated pump installed on the other side of the engine. The injector is cooled by the engine coolant to avoid such potential problems as coking of fuel which might lead to a blockage of the injection nozzle.

The following figures show two example injectors used in commercial diesel delivery units. The low-pressure injector in Figure 5 operates at a fuel pressure of 5 bar. The poppet valve opens and delivers fuel when the fuel pressure on the inner surface of the valve overcomes the valve spring preload. Due to the low injection pressure and other design characteristics, the injector produces a hollow spray cone of relatively high diameter droplets and is used together with a static mixer [6735].

The high-pressure injector in Figure 6—derived from a GDI injector design—operates at a much higher pressure of 200 bar. The amount of injected fuel is controlled electronically by pulsating the current supplied to the injector’s solenoid valve. The injector provides better fuel atomization and can be used without a mixer. A BSFC reduction of up to 1.5-2.0% was claimed with the high-pressure injector due to the elimination of the mixer’s pressure loss and better efficiency of DPF regeneration [6735].

Some of the potential issues with fuel atomization and mixing could be overcome by evaporating the fuel and injecting fuel vapor, rather than liquid, into the exhaust stream. A concept of a fuel vaporizer—an injector that incorporates a small electric heater—is shown in Figure 7. The fuel vaporizer includes a small reservoir fitted with a heater (similar to a diesel glow plug) where the fuel is evaporated [1257]. The vapor is introduced to the exhaust stream through a small pipe and a diffuser, which have been designed to avoid clogging by soot. The predominantly gaseous hydrocarbons mix more rapidly with exhaust gases than a liquid fuel spray.

In-Cylinder Fuel Delivery

In light- and medium-duty applications, fuel supply via the in-cylinder injectors is common. One advantage of this approach is reduced hardware costs. In order to provide fuel to the DOC, after-injections are typically used. These injections occur late in the expansion stroke in order to minimize combustion of this fuel in-cylinder. However, due to lower cylinder pressures, there is a risk of increased oil dilution due to fuel impinging on the cylinder liner. High performance fuel injectors allow the after injection to be split into a series of shorter injections that reduce fuel jet penetration and reduce the risk of oil dilution. In some cases, partial oxidation of this fuel can lower DOC light-off temperature compared to using a dedicated exhaust injector.

In engines using the in-cylinder injectors for dosing and also equipped with high pressure EGR, a significant amount of unburned fuel would be introduced into the EGR system if the EGR valve were to remain open during fuel dosing and EGR system fouling would be significant. In these applications, closing the EGR valve during dosing is common [4855]. However, this would lead to an increase in engine-out NOx emissions during dosing that would translate into increased tailpipe NOx emissions. In engines with a split exhaust manifold where EGR is taken from only some cylinders, fuel dosing could be carried out in the cylinders that do not supply EGR and EGR flow could still be maintained during dosing [4854]. Some US 2007 mid-range engines used an oxidation catalyst mounted in the EGR system in an effort to reduce EGR system fouling; unburned fuel passing through this catalyst during in-cylinder dosing potentially had the benefit of raising EGR temperature and thus burning off some of the deposits accumulated in the EGR system [1574].