A New, Highly Improved Two-Cycle Engine
- Created: Monday, 01 September 2008
Performance is improved while mechanical complexity is reduced.
The figure presents a cross-sectional view of a supercharged, variable-compression, two-cycle, internal-combustion engine that offers significant advantages over prior such engines. The improvements are embodied in a combination of design changes that contribute synergistically to improvements in performance and economy. Although the combination of design changes and the principles underlying them are complex, one of the main effects of the changes on the overall engine design is reduced (relative to prior two-cycle designs) mechanical complexity, which translates directly to reduced manufacturing cost and increased reliability. Other benefits include increases in the efficiency of both scavenging and supercharging. The improvements retain the simplicity and other advantages of two-cycle engines while affording increases in volumetric efficiency and performance across a wide range of operating conditions that, heretofore have been accessible to four-cycle engines but not to conventionally scavenged two-cycle ones, thereby increasing the range of usefulness of the two-cycle engine into all areas now dominated by the four-cycle engine.
The design changes and benefits are too numerous to describe here in detail, but it is possible to summarize the major improvements:
•Reciprocating Shuttle Inlet Valve
The entire reciprocating shuttle inlet valve and its operating gear is constructed as a single member. The shuttle valve is actuated in a lost-motion arrangement in which, at the ends of its stroke, projections on the shuttle valve come to rest against abutments at the ends of grooves in a piston skirt. This shuttle-valve design obviates the customary complex valve mechanism, actuated from an engine crankshaft or camshaft, yet it is effective with every type of two-cycle engine, from small high-speed single cylinder model engines, to large low-speed multiple cylinder engines.
•Variable Compression Ratio
The piston has a stepped configuration: It includes a narrower power section (the upper portion in the figure) and a wider compressor/ supercharger section (the lower portion in the figure). The variable-compression-ratio mechanism includes a high-pressure oil lubrication circuit acting in unison with the pulsating flow and pressure of the air caused by the reciprocation of the compressor/ supercharger section of the piston. In terms that are necessarily oversimplified for the sake of brevity, the operation of this mechanism involves interactions among pressures and flows of air, oil, and combustion gases, to vary the axial position of a floating combustion bowl in the power section of the piston and thereby vary the compression ratio. The design of the mechanism is such that when the throttle opening is suddenly changed, the compression ratio becomes adjusted relatively quickly to the value at which the engine operates most efficiently.
The stepped-piston arrangement obviates the complication and high cost of “add-on” supercharging mechanisms like those used on prior engines. During the compression stroke, the motion of the compressor/ supercharger section of the piston gives rise to a flow of air at high pressure from the compressor cylinder through one-way transfer valves, through a plenum, into the power cylinder. This flow contributes to scavenging and cooling of the power cylinder. The highly compressed air continues to enter the plenum and power cylinder after the exhaust ports are closed and the supercharging of the cylinder has been completed. The compressed air that continues to enter the plenum after the inlet ports are covered by the rising power piston is retained in the plenum under pressure until the end of the expansion stroke, when the lowering power piston opens the exhaust ports. Soon after this, the abutments in the piston skirt make contact with the projections on the reciprocating shuttle inlet valve, forcing the valve to the open position, in which the compressed air rushes from the plenum into the power cylinder, thereby effecting the initial scavenging. An additional benefit of the stepped-piston arrangement is that the blow-by gases and particulate matter that escape past the power-piston rings are isolated from the crankcase and returned to the power cylinder on the following stroke.
This work was done by Bernard Wiesen of Wiesen Engine for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18043-1.