Crankcase scavenged two-stroke engines

Abstract

A crankcase scavenged two-stroke engine includes a piston reciprocably mounted in a cylinder. The cylinder wall has an exhaust port and a rear transfer port opposed thereto formed in it. The rear transfer port communicates with the interior of the crankcase via a rear transfer passage and is arranged to open before the exhaust port closes, whereby, in use, the cylinder is scavenged. An inlet duct is arranged to supply combustion air to the crankcase and a throttling valve is arranged to throttle the flow of air through the inlet duct. A carburettor is arranged to supply fuel into the inlet duct. The interior of the crankcase is divided into at least two separate crankcase volumes, a rich volume (V1,V2) and a lean volume (V3). Each crankcase volume communicates with the cylinder via a respective hole in the crankcase wall. The cylinder wall also has at least one lateral transfer port formed in it at a position between the rear transfer port and the exhaust port. The lateral transfer port is arranged to open before the exhaust port closes. The lateral transfer port communicates with the lean volume (V3) via a lateral transfer passage. The rear transfer port communicates with the rich volume (V1,V2). The inlet duct is divided over at least part of its length into at least two inlet passages, a rich passage and a lean passage, which communicate with the rich volume (V1,V2) and the lean volume (V3), respectively. The carburettor and/or the throttle valve are so constructed and arranged that, under high load operation, substantially all the fuel supplied by the carburettor is introduced into the rich passage and, under low load operation, the fuel supplied by the carburettor is introduced into both the rich and lean passages.