Diesel Engine Throttling—The Classical Tool: To Adapt Exhaust Gas Temperature for Emission Control by Catalysts and Filters: From Its Beginning to the State of the Art in Euro 6/VI

Abstract

Part 1 The temperature for lighting off the combustion of soot stored in particulate traps is lowered from 600 to 300 °C by using catalysts, either coatings or fuel-borne, In CRT, it drops to 250 °C. Nevertheless, in a city bus in dense traffic only 150–200 °C is attained and even the catalysts fail to trigger the reaction. Low load duty cycles, similarly exist in many other applications also. Here, passive regeneration becomes ineffective and active support is desired. This chapter describes one active method for any diesel engine to increase the temperature in the exhaust whenever necessary. Since the load control of a diesel engine is only by the flow rate of the fuel, with the constant airflow for a given speed, the air–fuel ratio increases from when the load drops from λ = 1.5 to λ = 8; and it is the main reason for the low temperature at light loads. If the airflow is simultaneous with the reduction in the fuel flow, the air–fuel ratio can be reduced and the exhaust temperature can be increased up to 300 °C. Both computer simulation and experiments show intake throttling is an active tool to increase the temperature needed for the regeneration of a particulate filter. It is concluded that throttling downstream of the turbocharger compressor is preferred with negligible influence on fuel economy because the operation is only for a short time. To take care of fast changes in load the actuators must respond quickly according to the strategy based on the engine map. Furthermore, simpler solutions are possible for retrofitting DPF in older engines fitted with mechanical fuel injection equipment. It is concluded that air intake throttling in conjunction with a catalyzed particle filters or fuel-borne catalysts covers a wide range of applications and running conditions. Part 2 High PM is emitted by the Transport Refrigeration Units (TRU) powered by small diesel engines. The attendant health risks demand efforts to devise a cost-effective treatment to abate the emissions. Diesel particulate filters (DPF) of ceramic honeycomb construction efficiently trap even ultrafine particles below 300 nm that penetrate the lungs. Fuel-borne catalysts (FBC) facilitate trap regeneration at lower exhaust temperatures but do not assure reliable regeneration in all conditions. A Swiss development team with industrial partners developed a fully automatic active regeneration system for the California Air Resources Board using the FBC strategy in conjunction with a fast-acting intake air throttle valve that increases the exhaust temperature by 250 °C when closed and again provides high oxygen content to the heated filter when opened, thus decoupling the availability of temperature and oxygen for a controlled regeneration of the DPF. The electronic control unit (ECU) monitors exhaust backpressure, temperature, oxygen, and regeneration time and includes self-adapting elements. Here, the development and testing of a prototype unit in a TRU powered by a 26 kW diesel engine. The particle number of solid soot particles below 300 nm is reduced by 99%, EC-mass by 97%, PM by 86%, HC, and NO₂ on average by about 60%. This system is cost-effective for retrofitting small engines and off-road vehicles. Part 3 Recently, PM in winter and ozone in summer have increased in the cities. Diesel engines in heavy-duty vehicles such as garbage trucks, city buses, and construction vehicles are partly responsible for the emissions. The possibility of equipping vehicles older than Euro 3 with a universal retrofit kit to reduce the NOx emissions by 50% and the PM emissions by 99% without creating any secondary emissions are discussed. The soot on the DPF is burned with the oxygen in the exhaust relying less on NO₂. The optimal arrangement for EGR and DPF regeneration is evaluated on a car engine at the test bench. The research indicated that low-pressure EGR (NOx reduction) and intake throttling (DPF regeneration) are the best answers for a retrofit. To increase the EGR at low loads, a second throttle is introduced on the low-pressure side ahead of the compressor. Bench results revealed that with EGR, it is conceivable to decrease the NOx by 50% and the efficiency of the DPF by 99%. A control algorithm for the EGR and the regeneration systems is developed. Part 4 The application of intake or exhaust throttling assumes importance in vehicles applied to slow city traffic. DPF regeneration or the SCR light-off can be augmented by late injection of fuel in the cylinder or HC dosing ahead of a DOC. Early breakthroughs and the current status of the technology are described here.