The Scavenging Flow Field in a Crankcase-Compression Two-Stroke Engine - A Three-Dimensional Laser-Velocimetry Survey 920417

Transfer-port and in-cylinder flow fields have been mapped in a crankcase-compression, loop-scavenged two-stroke engine under motored conditions (1600 r/min; delivery ratio: 0.5). The impulsive, high-velocity flow (initially ≳2200 m/s) issuing from the transfer ports is fairly uniform and symmetric in space. The resulting in-cylinder flow field displays a classic scavenging loop pattern, but is complex and asymmetric. The data also characterize backflow from the cylinder into the transfer ports and the spin-up and breakdown of the scavenging-loop vortex during compression. The detailed LDV results provide some quantitative support for the widely used Jante scavenging test.

FOR THE GREATER PART OF A CENTURY, the scavenging process has been recognized as critical to the performance of two-stroke-cycle engines. Much of our understanding of the scavenging process (see Blair's book [1] for a comprehensive treatment) has been obtained through physical reasoning, thermodynamic and gas-dynamic analysis, and global measurements (e.g., scavenging efficiency and trapping efficiency as a function of delivery ratio). Flow visualization with both air and water as the working fluids (e.g., [2, 3]) has also contributed substantially. For loop-scavenged engines, the Jante test [4], which maps the axial-air-velocity distribution at the top of the cylinder (with the head removed), provides both a relative indicator of scavenging performance and some physical insight into the spatial character of the scavenging flow field. This semi-quantitative, empirical procedure is used in the development of virtually all such engines.

Despite the acknowledged importance of the scavenging process, there exists little quantitative experimental information on the flow fields in practical loop-scavenged two-stroke engines. In contrast, there have been many experimental studies of in-cylinder flow fields in four-stroke engines (cf. the reviews of Refs. [5], [6] and [7]). For the most part, previous velocity measurements in two-stroke engines either have examined the heavy-duty uniflow configuration [8, 9] or have been confined to the cylinder's TDC clearance volume [10], [11], [12], [13], [14] and [15]. These studies therefore offer limited insight into the velocity fields involved in the gas-exchange processes of interest here. The flow field in the single-cylinder research engine developed at Princeton University has been characterized extensively (e.g., [16], [17], [18] and [19]), but the engine's porting is atypical, and it does not scavenge well [20]. Detailed LDV measurements of the port-efflux velocity field in a two-port, loop-scavenged model engine have recently been carried out under both steady flow [21] and motored (blown, 200 r/min) conditions [22] at The Queen's University of Belfast; little in-cylinder velocity data have been presented, however [21].

An automotive two-stroke engine must operate efficiently over a broad speed-load range. In this context, the importance of detailed, quantitative knowledge of the flow fields is underscored by recent three-dimensional computational-fluid-dynamics (CFD) studies of scavenging and combustion in loop-scavenged two-stroke engines [23], [24], [25] and [26]. These studies predict that the scavenging effectiveness, mixture formation, and combustion can be sensitive to details of both the port-inflow velocities and the in-cylinder velocity field. In a crankcase-compression engine, moreover, these scavenging flow fields can vary with speed and load.

In view of the paucity of pertinent experimental data and the potential importance of the information, we undertook systematic, three-dimensional measurements of the spatial structure and temporal evolution of the scavenging flows in a crankcase-compression, loop-scavenged two-stroke-cycle engine, examining both the in-cylinder flow field and the flow entering the cylinder through the faces of the transfer ports. A principal objective of this study was to obtain physical insight and engineering guidance. The quantitative results should also prove helpful both in confirming the three-dimensional CFD codes used to model such engines and in providing boundary-condition information for these computations.¹

The organization of the paper follows the usual order. §1 describes the experimental apparatus. The results are presented and discussed in the next two sections, which cover the formation (§2) and subsequent evolution and destruction (§3) of the scavenging loop, respectively. In §4, LDV and Jante-test results are compared, after which the conclusions of the study are summarized (§6). The appendices provide additional details of the LDV and engine-airflow systems.