HCCI and CAI Engines - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture33/33_1.htm[6/15/2012 3:08:52 PM] Module 7:Advanced Combustion Systems and Alternative Powerplants Lecture 33:HCCI and CAI Engines The Lecture Contains: HCCI/CAI ENGINES HCCI v/s CAI Basics of HCCUI/CAI Process CAI GASOLINE ENGINES Methods to Obtain CAI Regime of CAI Operation Emissions with CAI and SI Engine Operation Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture33/33_2.htm[6/15/2012 3:08:52 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 33:HCCI and CAI Engines HCCI/CAI ENGINES Stringent emission standards and the need to reduce greenhouse gas, CO2 emissions from vehicles has led to intensive research on new combustion systems namely, the homogeneous charge compression ignition (HCCI) or controlled autoignition (CAI) engines. These combustion concepts have the following features; HCCI/CAI involves autoignition of very lean homogeneous mixtures of fuel and air so that the combustion temperatures are low. Due to low combustion temperatures NOx formation is negligibly small. NOx formation is two orders of magnitude lower than those from the current SI and CI engines Very little soot is formed as the homogeneous charge is burnt. High fuel efficiencies similar to DI diesel engines can be obtained as very lean mixtures are burned. The first attempts to utilize HCCI/CAI combustion were made to control irregular and misfiring combustion in 2-stroke SI engines at light loads by Japanese researchers during late 1970s. Autoignition of the homogeneous charge was obtained by retaining large amounts of hot residual gas containing partially oxidized hydrocarbons and active chemical species in the cylinder. Honda motors applied this form of combustion on a motorcycle engine prototype during mid-1990s, which was termed as ‘Active Radical Combustion (ARC)’. Fuel economy improvements of about 30% and HC reduction of 50% were obtained compared to normal 2-stroke engine operation. The autoignition of lean homogeneous charge has been called by a variety of names such as Active Thermo Atmosphere Combustion (ATAC), Premixed Charge Compression Ignition (PCCI), Premixed Lean Diesel Combustion (PREDIC), Active Radical Combustion (ARC), Controlled Autoignition (CAI), Homogeneous Charge Compression Ignition (HCCI) etc. HCCI v/s CAI Application of autoignition of lean homogeneous charge has been studied in the conventional gasoline as well as diesel engines. The processes adopted to auto-ignite homogeneous charge and the objectives of its application to SI and CI engines are some what different In the gasoline engines, external heating of intake charge or use of hot residual gas has been employed to cause controlled autoignition of high octane gasoline or natural gas –air mixtures. Therefore, the auto-ignited combustion process when applied to gasoline engines has been termed as controlled autoignition (CAI). The main objective of CAI application to the gasoline engines is reduction in fuel consumption and NOx emissions. In the conventional diesel engines, the fuel air mixture is heterogeneous and compression of air to high temperature is used to auto-ignite diesel fuel. The diesel fuel has low self-ignition temperature. In application of this concept to diesel engines, the main approach is to premix as much fuel as possible before autoignition without encountering negative effects of auto ignition on combustion parameters and emissions. Autoignition may be caused by other forms of heating of fuel-air mixture in addition to compression heating. This process when applied to diesel engines is usually called as homogeneous charge compression ignition (HCCI). The main objective of HCCI application in the diesel engines is to reduce NOx and particulate emissions. It may be noted that fundamentally the HCCI and CAI processes are the same. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture33/33_4.htm[6/15/2012 3:08:53 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 33:HCCI and CAI Engines Figure 7.5 The Fuel –air equivalence ratio –Temperature diagram for hydrocarbon - air mixture conditions for formation of soot and NOx. The conditions of normal SI and CI engine operation and target region for HCCI/CAI for low NO and soot formation are shown. The HCCI/CAI combustion process has two main steps: (i) Preparation of lean premixed, homogeneous fuel-air mixture, and (ii) Autoignition leading to combustion of lean premixed charge. Different approaches have been investigated to accomplish the above steps leading to HCCI/CAI combustion CAI GASOLINE ENGINES The objective of CAI combustion in gasoline engines is to obtain high fuel efficiency and low NOx emissions, the PM emissions already being insignificant. Fig. 7.6 compares P-V diagrams for a SI and CAI engine operation. It may be noted that the combustion in CAI engine is much faster and it approaches close to constant volume combustion. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture33/33_4.htm[6/15/2012 3:08:53 PM] Figure 7.6 Comparison of P-V diagrams for SI and CAIcombustion. Controlled autoignition (CAI) of homogeneous charge requires: Charge is kept at a high temperature for sufficient duration Composition of charge to ensure acceptable rate of combustion and low NOx formation - Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture33/33_5.htm[6/15/2012 3:08:53 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 33:HCCI and CAI Engines Methods to Obtain CAI The following parameters have been used to obtain the required temperature, pressure and composition of charge for CAI operation: Residual gas content or EGR Compression ratio Intake mixture temperature Intake mixture pressure Fuel-air ratio Coolant temperature Injection timing in gasoline direct injection engines The practical engines have to operate in CAI/SI dual mode. The SI engine mode at high loads is required to have good specific power output. The autoignition of gasoline or natural gas requires CR > 20:1.Such high CR would be unacceptable at high loads when fuel-air ratio is to be increased as severe engine knock would result. The engine CR is to be limited to around 10:1. Hence, increase of CR to obtain CAI operation has not been a practical option. Intake mixture heating by using the waste heat of the exhaust gas although helpful but gives a further loss in volumetric efficiency as already a highly diluted charge is used to control the heat release rates subsequent to autoignition. The most practical approach for CAI operation is use of large amounts of hot residual gases. Trapping of the residual gases inside the cylinder also termed as ‘internal EGR’ has been found a more acceptable approach. A negative valve overlap period is used to retain high amount of residual gases in the cylinder. The exhaust valve is closed before the piston reaches top dead centre in the exhaust stroke and to prevent backflow of the burned gases in the intake system, the intake valves open well after TDC. The relative valve timings for a standard SI engine and for CAI operation are compared in Fig. 7.7. To get maximum expansion work ideally the exhaust valves are to be opened at the usual time in the cycle but are to be closed earlier. The intake valve is to open late while they should close as in the normal SI engine operation. To obtain this flexibility in SAI-SI dual mode operation fully flexible, variable valve actuation systems are necessary which are very complex in construction and presently are not in production. In the practical engines, use of low valve lift cam profile provides substantial reduction in valve opening period. This approach along with variable cam timing devices has been used in prototype engines. By varying the exhaust valve closing time load can be varied. As the exhaust valve closes earlier to trap more residual gases, less fresh charge is admitted resulting in lower engine output. The valve timings with lower valve lift are also shown on Fig. 7.7. The trapping of residual gas in the cylinder results in high charge temperatures.. At high loads, the auto ignition however, may occur too early and high rates of pressure rise would result.