tailieunhanh - Internal Combustion Engines Fundamentals Episode 2 part 11

Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 850 INTERNAL COMBUSTION ENGINE FUNDAMENTALS More rapid initial flame growth results in less variation in flame center motion during the critical flame-development phase. The resulting geometric variations in the flame front chamber wall interaction are therefore reduced this decreases the variations in burn rate that result from these geometric variations. Also the faster burning process ends earlier in the expansion stroke. Thus the problem of occasional slow burning cycles partial burning cycles and eventually misfire which occurs with dilute mixtures under normal burning conditions due to quenching of the combustion process as gas temperatures fall during expansion is largely avoided see Sec. . Factors That Control Performance VOLUMETRIC EFFICIENCY. Combustion chamber shape affects volumetric efficiency through its constraints on maximum valve size and through the degree of swirl if any that the chamber and port designs produce to achieve the desired combustion characteristics. To obtain maximum performance and to reduce pumping losses the size of the valve heads should be as large as practical the valve sizes that can be accommodated depend on cylinder head layout. Table lists the typical maximum valve sizes that can be accommodated into several common chamber shapes see Fig. 15-15 . The approximate mean piston speed at maximum power is a measure of the maximum air flow that each engine design can pump. Note that of the two-valve configurations the designs with inclined valve stems permit substantially greater maximum air flow. The four-valve pentroof design which also has inclined valve stems is the best of those listed since it accommodates the largest valve and port areas there are other four-valve head designs which are comparable . Swirl can be generated during the intake process through suitable port valve and head design. It requires either that the flow through the intake valve be dữected tangentially into the cylinder so that gas flows