Stratified Charge 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/lecture32/32_1.htm[6/15/2012 3:08:31 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 32:Stratified Charge Engines ADVANCED COMBUSTION SYSTEMS AND ALTERNATIVE POWERPLANTS The Lecture Contains: DIRECT INJECTION STRATIFIED CHARGE (DISC) ENGINES Historical Overview Potential Advantages of DISC Engines DISC Engine Combustion Requirements Methods of Charge Stratification and Combustion Modes of DISC Engine Operation DISC Engine Performance and Emissions Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture32/32_2.htm[6/15/2012 3:08:31 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 32:Stratified Charge Engines ADVANCED COMBUSTION SYSTEMS AND ALTERNATIVE POWERPLANTS DIRECT INJECTION STRATIFIED CHARGE (DISC) ENGINES Historical Overview For many decades the researchers have pursued development of direct injection stratified charge SI engines to have overall very lean engine operation for higher fuel efficiency. Charge stratification is a means of ensuring repeatable ignition without misfire and stable combustion while using overall very lean fuel-air ratios that is otherwise not possible with homogeneous mixtures. In the stratified charge engines, the mixture composition is varied within the combustion chamber such that stoichiometric or slightly richer mixture exits near spark plug to provide good ignition characteristics and the mixture gets progressively leaner away from the spark plug. Overall air-fuel ratio in the cylinder is significantly leaner than the stoichiometric. A typical configuration of DISC engine is shown on Fig 7.1. Liquid fuel is injected in the cylinder. The fuel spray is directed by air motion or by the geometry of piston crown or by combination of the both towards spark plug. By the time fuel spray reaches the spark plug electrodes some fuel gets vaporized and forms combustible mixture with air. The vaporized fuel in spray is then ignited by spark, combustion begins and the flame spreads in the combustion chamber. Figure 7.1 Schematic of a direct injection stratifiedcharge (DISC) engine combustion system Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture32/32_5.htm[6/15/2012 3:08:32 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 32:Stratified Charge Engines Methods of Charge Stratification and Combustion The methods of charge stratification and combustion that have been studied and some of these employed in production engines may be grouped in the following three types. These are also shown on Fig. 7.2. Figure 7.2 Spray, flow and wall controlled DISCengine combustion. Spray Controlled: The fuel spray characteristics primarily controls the charge stratification in this strategy. Ignitable mixture is formed at the boundaries of the fuel spray. The spark plug is placed close to the spray as it is there that the ignitable mixture is present at the time of ignition. Formation of good quality mixture becomes difficult at high engine loads. The combustion being highly sensitive to spray characteristics smoke formation is often observed at high loads. Wetting of the spark plug by liquid fuel causes frequent spark plug fouling. Wall Controlled: In the wall-controlled concept, fuel injection is directed towards a specially designed piston cavity. The piston cavity is off centre. The spark plug is located away from the fuel injector on the side of combustion chamber. Fuel impinges on the piston cavity walls where it evaporates and mixes with air. An intense reverse tumble charge motion transports the mixture to spark plug electrodes. Flow Controlled: Mixture is formed by interaction between fuel spray and suitably directed air motion like swirl or tumble. The spark plug and injector are generally widely spaced in these configurations. The air motion transports mixture to the spark plug such that the ignitable mixture is present at spark plug electrodes at the time of spark. When air motion is well organized, the combustion chamber walls do not get wetted by liquid fuel and a stable stratified charge operation is obtained over a wide range of engine operation. The characteristics of combustion process obtained with the three charge stratification and combustion methods are compared in Table 7.1 Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture32/32_6.htm[6/15/2012 3:08:32 PM] Module7:Advanced Combustion Systems and Alternative Powerplants Lecture 32:Stratified Charge Engines Table 7.1 Comparison of DISC engine combustion process parameters with Spray, Wall and Flow controlled approaches, 2000 rpm, imep =2.8 bar Combustion parameter Spraycontrolled Wall controlled Flow controlled 1 Injection timing, ºbtdc (approx.) 60 50 -55 40 2 Ignition timing after injection ends, º CA 3 to 5º 35 -40 10 -15 3 Combustion duration, º CA 0 -5% heat release 0- 50% 0- 85% 85-90 % 20 – 25 45 75-80 10 10 30 50-55 30 10 25 50-55 5 In the spray controlled method formation of mixture takes longer hence initial heat release rates are small and the combustion duration corresponding to 0 -5% heat release is much longer compared to the other two methods. . On the other hand in the wall controlled approach, although the fuel initially evaporates quite rapidly due to hot walls, but it takes longer to evaporate all the injected fuel Thus, the end of combustion is significantly delayed with the wall controlled method compared to other methods. The flow controlled method has the shortest combustion duration due to better mixture preparation and high intensity of fluid motion and turbulence. In practice, the production DISC engines use a combination of both the wall and air flow controlled strategies to obtain good ignition and combustion characteristics. Mitsubishi engine uses a specially designed piston cavity and tumble air motion while Toyota engine employs a specially designed piston cavity and air swirl. Modes of DISC Engine Operation The early DISC engines such as Ford PROCO and TCCS attempted to operate in stratified charge mode throughout the engine operation range but it proved to be unsuccessful and acceptable engine performance and emissions over the entire speed-load range could not be obtained. With mechanical injection system precise control of injection timing also could not be maintained. The current direct injection gasoline engines operate in the stratified charge mode at part loads and up to medium speeds while at high engine loads the engine are made to operate as stoichiometric homogeneous charge engines. This is controlled by fuel injection timing. Stratified Charge Operation: Fuel is injected late in the compression stroke There is not enough time for fuel to fully mix with air and a stable charge stratification at part loads at the time of ignition is obtained. Engine operates unthrottled and very lean overall air-fuel ratios reaching up to 50:1 are used. Homogeneous Stoichiometric Operation: Fuel is injected early in the intake stroke to allow adequate time before ignition for fuel evaporation, mixing and formation of homogeneous mixture At high loads, early fuel injection timing is used to operate the engine in stoichiometric homogeneous mode. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture32/32_6.htm[6/15/2012 3:08:32 PM] Homogeneous Lean Operation: Engine operates homogeneous lean in the mid-load range. This is the transition zone from stratified mode at low loads to stoichiometric operation at full engine load.