In-Cylinder Gas Velocity Measurements Comparing Crankcase and Blower Scavenging in a Fired Two-Stroke Cycle Engine 940401

The in-cylinder flow field of a Schnürle (loop) scavenged two-stroke engine has been examined under conditions simulating both blower and crankcase driven scavenging. Measurements of the radial component of velocity were obtained along the cylinder centerline during fired operation at delivery ratios of 0.4, 0.6, and 0.8. Both mean velocity profiles and root mean square velocity fluctuations near top center show a strong dependence on the scavenging method. Complementary in-cylinder pressure measurements indicate that combustion performance is better under blower driven scavenging for the engine geometry studied.
IN THE PAST TEN YEARS the engine research and development community has demonstrated a renewed interest in two-stroke engine technology. Many manufacturers have new engine designs operating on test stands and in prototype vehicles being road tested. This recent development activity has resulted in the adoption of both crankcase and external blower scavenged designs as the baseline engine configurations. Both design concepts have their advantages and disadvantages, and there seem to be situations where each is best used.
Crankcase scavenging with piston-controlled porting and dry-sump lubrication is certainly the simplest engine configuration from the size/weight/complexity point-of-view. The General Motors CDS2 design and one of the Orbital Engine Company designs are examples of current crankcase scavenged configurations (Wyczalek [1]*). This design offers the potential of improved fuel economy at light load operation due to lower pumping and friction losses. Some disadvantages, however, include the need to either mix or inject lubricating oil into the intake charge and a crankcase design requiring individual cylinder sealing along with the use of roller bearings. In addition, the crankcase pumping by the reciprocating piston leaves little flexibility in controlling the scavenging flow, leading under some conditions to undesirably low scavenging efficiency. The scavenging efficiency can be somewhat improved using crankcase compression with uni-flow scavenging, which requires the use of valves and valve control mechanisms with the associated increase in complexity.
External blower scavenging can be implemented in different configurations. For example, Subaru uses piston-controlled porting and a screw-type compressor; Toyota uses camshaft-driven valves and a roots-type blower; and Orbital employs flywheel mounted centrifugal blowers with piston-controlled porting in two of their current designs (Wyczalek [1]). These blower scavenged engines use wet-sump lubrication systems. The use of an external blower itself adds to the mechanical complexity of the engine, and the additional use of valves further increases the complexity. On the other hand, the use of an external blower and valves can lead to an improvement in the control of the scavenging process and, in turn, an improvement in engine performance. Furthermore, wet-sump lubrication allows the use of common, inexpensive journal bearings and a less-complex crankcase arrangement along with a reduced concern over exhaust catalyst poisoning due to use of improper lubrication oil.
Previous investigations of the scavenging process have been focused on one or the other scavenging mode and not on a comparison of the two. In addition to a global understanding of the good and bad features of the two scavenging methods, a direct comparison of the two could provide engine designers with additional insight as to which method is more suitable for their application. The intent of the work reported in this paper is to characterize and compare the in-cylinder flow fields associated with both crankcase and external blower scavenging configurations, in an engine that can be operated in either mode. In-cylinder pressure measurements are also reported to provide a relative indication of the combustion performance associated with the two scavenging methods. A comparison of this sort provides
information on the general features associated with each flow field, and the possible suitability of each configuration to specific engine geometries.


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