Tham khảo tài liệu 'internal combustion engines fundamentals episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 332 INTERNAL COMBUSTION ENGINE FUNDAMENTALS FIGURE 8-5 - . Schematic of velocity variation with crank angle at a fixed location in the cylinder during two con- g. secutive cycles of an engine. Dots indicate measurements of instantaneous velocity at the same crank Ề angle. Ensemble- or phase-averaged velocity obtained by averaging over a large number of such measurements shown as solid smooth line. Top graph low cycle-to-cycle flow variations. Here the i individual-cycle mean velocity and ensemble-averaged velocity are closely comparable. Bottom graph large cycle-to-cycle variations. Here the individual-cycle mean velocity dotted line is different from the ensemble-averaged mean by Ữ. The turbulent fluctuation u is then defined in relation to the individual-cycle process at many crank angle locations the ensemble-averaged velocity profile over the complete cycle is obtained. The ensemble-averaged mean velocity is only a function of crank angle since the cyclic variation has been averaged out. The difference between the mean velocity in a particular cycle and the ensemble-averaged mean velocity over many cycles is defined as the cycle-by-cycle variation in mean velocity ữ 0 i ữ ỡ 0 - ỮEA 0 M Thus the instantaneous velocity given by Eq. can be split into three conn ponents 7 U ff i ỮEA 0 ứ ớ i u ớ i i- CHARGE MOTION WITHIN THE CYLINDER 333 FIGURE 8-6 Schematic of jet created by flow through the intake valve indicating its turbulent 6 Figure 8-5 illustrates this breakdown of the instantaneous velocity into an ensemble-averaged component an individual-cycle mean velocity and a component which randomly fluctuates in time at a particular point in space in a single cycle. This last component is the conventional definition of the turbulent velocity fluctuation. Whether this differs significantly from the fluctuations about the ensemble-averaged velocity depends on whether the cycle-to-cycle fluctua- ions are small or large. The figure indicates these