Exhaust Gas Thermal Management

Hannu Jääskeläinen

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• Introduction
• Engine Based Measures
• Exhaust System Based Measures
• Passive Measures

Introduction

There are numerous reasons to manage diesel engine exhaust gas thermal conditions including respecting the thermal limits of engine components exposed to exhaust gas, ensuring safe operation and maximizing engine performance. However, enabling low emissions is arguably the most challenging reason to do so. Modern engines are equipped with catalytic exhaust aftertreatment systems that require temperatures within a specified range to ensure adequate catalytic activity, for periodic regeneration and for the prevention of excessive physical and/or chemical fouling. Managing engine exhaust temperature is a critical task of the modern engine and aftertreatment control systems. Without this function, low emissions over the lifetime of the engine would not be possible.

To maximize catalytic activity, the exhaust catalysts should reach the required minimum activation temperature as quickly as possible after a cold start and remain above a minimum threshold throughout the entire time that the engine is operational. The measures required to achieve these objectives depend on engine type, duty cycle and the level of exhaust emission control required. For engines with little need to meet strict exhaust emission control requirements and with very simple catalytic aftertreatment systems, few measures may be required to manage exhaust temperature. For engines meeting strict emission requirements, the required measures will depend on engine type and application; engines with high exhaust temperature and that operate for long periods of time at high load may require relatively few measures for managing exhaust temperature while those with low exhaust temperature operating at light loads, even for relatively short periods of time, will require the most comprehensive set of measures for managing exhaust temperature.

The minimum temperatures requirements for urea SCR systems and DOCs are examples of what is required before some aftertreatment system components can function effectively. For engines with urea SCR catalysts, exhaust temperatures above 185-200°C or higher are required before urea dosing is enabled. Dosing at lower temperatures can lead to the formation of urea based deposits and poor NOx conversion. For engines that use a DOC to generate additional heat for downstream catalysts, the DOC catalyst temperature should exceed 200-250°C before the catalyst will oxidize hydrocarbons and produce the required amount of heat. In addition to normal functionality, aftertreatment systems can be subject to fouling and may need periodic exposure to specific temperatures for regeneration. Some examples of the later are listed in Table 1 [4865].

A variety of different measures are available to manage exhaust thermal conditions. Not all work equally well for a given objective. There are different penalties as well. Some require close integration with other vehicle systems. The technologies can be broadly categorized as:

• Those that increase exhaust enthalpy. Exhaust enthalpy depends on both exhaust temperature and flow. A high exhaust enthalpy is critical for achieving a rapid warm-up of aftertreatment system components to minimize cold start emissions.
• Those that increase exhaust temperature. While numerous measures can generate high exhaust temperature, in some cases they can decrease or have little impact on exhaust enthalpy. While this may exclude them from being useful for achieving rapid catalyst warm-up, they may still be useful after warm-up to maintain the catalyst temperatures required for high catalyst activity or for catalyst regeneration.
• Those that reduce heat losses from exhaust gas. These can be passive measures such as reduction of the mass of material being heated or the use of various forms of thermal insulation. They can also be active measures such as bypassing coolers and high mass parts of the exhaust system during some parts of the drive cycle. Some, such as those that reduce the mass of material being heated, can be useful during cold starts while others, such as the use of insulation, are more effective after exhaust temperature has exceeded a minimum value.

Some developments that prompted further refinement of exhaust thermal management include the CARB 2024/EPA 2027 emission standards that have been adopted to further reduce NOx emissions from on-road heavy-duty engines and vehicles. Meeting these emission standards requires significant reductions in cold start NOx emissions through more rapid SCR catalyst temperature rise, earlier supply of ammonia to the SCR catalyst and the prevention of SCR catalyst cooling during idle and low load operation.

Traditionally, thermal management has been more important for light-duty vehicles where emissions are typically measured entirely on a cold start cycle (NEDC and WLTC) or a cold start followed by a hot start (light-duty FTP). For heavy-duty engines, cold start emissions have been less important as demonstrated by the 1/7 weighting factor applied to cold start emissions for engines certified over the US heavy-duty FTP cycle. However, for modern heavy-duty engines, NOx emissions over the hot heavy-duty FTP cycle are so low that the only way to achieve the CARB 2024/EPA 2027 standards is to reduce cold start emissions despite the 1/7 weighting factor.

Implementation Options. Thermal management of engine exhaust can be achieved via numerous implementation options. In addition to the three categories listed above, the options can be classified in other ways including active/passive measures and engine-based and exhaust system based measures. Active engine based measures can be further broken down into those that increase exhaust losses, reduce air-fuel ratio (AFR), increase the fuel flow rate to the engine and redistribute heat flows. Table 2 summarizes these as well as a variety passive and exhaust-based options.

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