Internal combustion engine exhaust aftertreatment systems employ oxidation catalysts (OC), selective catalytic reduction (SCR) catalysts, particulate filters (PF) and other exhaust aftertreatment devices. In these systems, the OC devices frequently employ upstream located hydrocarbon (HC) injectors to inject HC, generally fuel, into the exhaust gas flow for oxidation in the OC to raise the temperature of the exhaust gas flow; such as when regeneration of the PF is desired. The efficient use of the HC in the OC is of critical importance, since it directly affects the efficiency (e.g., fuel economy) of the engine, as well as the emission performance of the engine and exhaust aftertreatment system, since the emission of unburned HC (HC slip) is regulated by law. In order to ensure efficient HC utilization in systems that employ HC injection, these systems frequently employ mixers downstream of the injectors, also referred to as evaporators or vaporizers, to ensure that the liquid fuel injected into the system is completely vaporized and dispersed into the exhaust gas flow so that it can be oxidized to the greatest extent possible in the OC. These mixers are designed to promote turbulence in the exhaust gas flow to provide mixing and dispersion of the HC. While effective for this purpose, mixers also create backpressure in the exhaust gas flow that is associated with the partial obstruction of the flow passage and the creation of the intended turbulence. Since the mixers are permanently installed in these systems, they create backpressure and affect flow even when HC is not being injected and their use is not needed.
Other mixers are also employed in conjunction with the use of other exhaust aftertreatment devices. For example, the SCR catalyst devices employed frequently include urea SCR (U-SCR) catalysts that require upstream injection of urea, such as a urea-water solution, into the exhaust gas flow. The performance, durability and operating cost of the U-SCR catalyst devices and other downstream aftertreatment devices depend strongly on the mixing and dispersion (e.g., evaporation) of the injected fluid into the exhaust gas flow. Mixers are also used upstream of these devices to increase the dispersion of the injected fluid into the exhaust gas flow and the production of ammonia for catalysis. These mixers may also produce undesirable system backpressure and affect flow even when urea is not being injected and their use is not needed.
Accordingly, it is desirable to provide mixers and exhaust aftertreatment systems having mixers, and employing mixing methods, that provide the desired mixing functions and also reduce system backpressure.