Diesel Particulate Filters - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

Download Diesel Particulate Filters - Engine Combustion - Lecture Notes and more Study notes Sustainability Management in PDF only on Docsity! Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture30/30_1.htm[6/15/2012 3:07:41 PM] Module 6:Emission Control for CI Engines Lecture 30:Diesel Particulate Filters The Lecture Contains: Diesel Particulate Filters Regeneration of DPF Active DPF Regeneration Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture30/30_2.htm[6/15/2012 3:07:41 PM] Module6:Emission Control for CI Engines Lecture 30:Diesel Particulate Filters DIESEL PARTICULATE FILTERS Diesel particulate filters (DPF) also called as ‘particulate traps’ have been developed to filter out PM from the diesel exhaust gases to meet very stringent emission limits. Alumina coated wire mesh, ceramic fiber, porous ceramic monoliths etc., have been studied as filtration media. Presently, ceramic monolith of honeycomb type structure is used to trap the particulate matter as the gas flows through its porous walls. These filters are also termed as ‘ceramic wall flow filters’. A ceramic honeycomb type particulate filter is shown in Fig. 6.14. In this cellular structure, alternate cells are plugged at one end and open at the opposite end. The exhaust gas enters the cells that are open at the upstream end and flows through the porous walls to the adjacent cells. The adjacent cells are open at the downstream end from where the filtered gas exits .to the atmosphere. Flow path of gas through walls of the filter is also shown on Fig 6.14. Figure 6.14 Ceramic wall flow filter for diesel particulate Some advantages of wall flow filters over other filtration media are; The wall flow filters have a large filtration surface area per unit volume The pore size of walls can be controlled to provide gas flow without excessive pressure drop. Very high filtration efficiencies close to 98% are possible with ascceptable pressure drop. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture30/30_5.htm[6/15/2012 3:07:42 PM] Module6:Emission Control for CI Engines Lecture 30:Diesel Particulate Filters Regeneration of DPF It is relatively easy to filter and collect the particulate matter in the trap but the soot is to be burned in-situ i.e., ‘regenerate’ the trap so that pressure drop across the filter is kept always at an acceptable level. Soot and other particulate get collected on the filter that although improves the filtration efficiency, but it results in an increased pressure drop across the filter. The pressure loss in the new and clean filter is about 70 mm H2O for the 0.1 m/s gas velocity in filter channels. Design considerations limit soot loading to about 10 g/l of filter volume before filter should be cleaned or regenerated. As the soot is collected upto the limit of 10g/l, pressure drop increases to 300-350 mm of . An increase of engine backpressure by 350 mm results in 1 % loss in fuel economy at 65-km/h vehicle speed. For engine out PM emissions of about 0.25 to 0.33 g/kW-h and engine power output of about 17 kW/ liter swept volume, 10g/l soot would get collected on DPF in about 2 hours. Hence, the DPF regeneration should take place approximately every 2 hours. The soot in the filter is to be burned in-situ. Burning of soot particles begins at about 540º C. Such high exhaust gas temperatures do not occur during engine operation for sufficiently long periods of .time. The diesel exhaust gas temperatures in the exhaust pipe typically reach to about 300ºC only. For regeneration of the DPF, therefore additional measures are to be adopted. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture30/30_6.htm[6/15/2012 3:07:42 PM] Module6:Emission Control for CI Engines Lecture 30:Diesel Particulate Filters Types of Regeneration Systems Regeneration of the DPF to its original clean state has been a challenging task. Two types of regeneration systems have been investigated and a few developed for employment on production vehicles Active regeneration Passive regeneration Active DPF Regeneration A positive source of heat is employed to raise temperature of DPF to about 540º C. When oxidation of soot starts, the exothermic reactions sustain the combustion of soot particles and the DPF is regenerated. In the active regeneration systems, sensors are used to monitor pressure drop across the trap. On receiving the signal from the sensor, the exhaust gas temperature is increased above 500º C by any one of the following techniques: Engine throttling Use of electric heater upstream of filter Use of burner upstream of filter Throttling of air reduces airflow that results in decrease of overall air-fuel ratio, which increases the combustion and exhaust temperatures. However, engine throttling increases the engine pumping losses which results in loss of fuel efficiency. Throttling also decreases oxygen concentration in the exhaust and for oxidation of soot 2-5% oxygen in the exhaust is necessary. Hence, the extent of throttling is to be controlled. Under normal cruising conditions, throttling is unable to increase the exhaust temperature to the levels needed for regeneration. Hence, throttling can work only at high loads, which is deliberately applied to raise the gas temperatures. It also increases HC, CO and smoke emissions. Engine throttling as a means of trap regeneration therefore, has had only a limited success. For electric filter regeneration, power to the electric heater is supplied by the engine alternator. A typical truck DPF regeneration system may require a 3 kW heater. The electric regeneration hence, needs a high battery back-up or is to be done while the vehicle is in garage, which may not be accepted by the operators as regeneration is required quite frequently. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture30/30_7.htm[6/15/2012 3:07:42 PM] Module6:Emission Control for CI Engines Lecture 30:Diesel Particulate Filters Burner Regeneration A diesel fuel burner is placed in the exhaust in front of the filter to regenerate the diesel particulate filter. This system can perform at all engine speeds and loads. Two types of system have been used; 1. burner full flow system and 2. burner bypass system In the full flow system the total exhaust gas is to be heated to about 540º C. A large air pump for the burner and high burner fuel consumption are needed. Complex electronic controls to regulate burner fuel flow to maintain safe levels of gas temperature at inlet of filter are necessary. In the bypass system, only a small part of exhaust is allowed to flow through the filter when regeneration is carried out. A smaller air pump is required. A bypass burner regeneration system is shown on Fig 6.16. Regeneration process is independent of engine operating conditions, as the filter during regeneration is isolated from the engine exhaust. The fuel consumption by the burner to heat the inlet face of the filter to 540º C is an order of magnitude lower compared to the full flow system. The relative magnitudes of energy required to raise the exhaust temperature to 540º C in the full flow and bypass type burner regeneration system (10 % of total flow) are presented in Fig 6.17 for a 5.7 liter diesel engine. Figure 6.16 Schematic of diesel-fuelled Burner Bypass Regeneration system for Diesel Particulate Traps The inlet face when heated to 540º C, soot oxidation begins. Increase in temperature of the gas accelerates combustion of soot further. The burning process progresses from the front and oxidizes soot in the remainder of the filter.