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Internal Combustion Engines Fundamentals Episode 2 part 2

Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 492 INTERNAL COMBUSTION ENGINE FUNDAMENTALS It will be clear from this summary that the compression-ignition combustion process is extremely complex. The details of the process depend on the characteristics of the fuel on the design of the engine s combustion chamber and fuel-injection system and on the engine s operating conditions. It is an unsteady heterogeneous three-dimensional combustion process. While an adequate conceptual understanding of diesel engine combustion has been developed to date an ability to describe many of the critical individual processes in a quantitative manner is lacking. Some important consequences of this combustion process on engine operation are the following 1. Since injection commences just before combustion starts there is no knock limit as in the spark-ignition engine resulting from spontaneous ignition of the premixed fuel and air in the end-gas. Hence a higher engine compression ratio can be used in the compression-ignition engine improving its fuel conversion efficiency relative to the SI engine. 2. Since injection timing is used to control combustion timing the delay period between the start of injection and start of combustion must be kept short and reproducible . A short delay is also needed to hold the maximum cylinder gas pressure below the maximum the engine can tolerate. Thus the spontaneous ignition characteristics of the fuel-air mixture must be held within a specified range. This is done by requiring that diesel fuel have a cetane number a measure of the ease of ignition of that fuel in a typical diesel environment see Sec. 10.6.2 above a certain value. 3. Since engine torque is varied by varying the amount of fuel injected per cycle with the engine air flow essentially unchanged the engine can be operated unthrottled. Thus pumping work requirements are low improving part-load mechanical efficiency relative to the SI engine. 4. As the amount of fuel injected per cycle is increased problems with air utilization during