crankcasescavenged

Crankcase scavenged refers to a method of scavenging used primarily in two-stroke internal combustion engines, where the crankcase itself serves as the reservoir and pump for the fresh air–fuel charge. In this scheme, the incoming mixture is drawn into the crankcase during the piston’s downstroke, often through a reed valve or similar intake device. As the piston rises, the decreasing crankcase volume pressurizes the trapped charge and forces it through transfer ports into the combustion chamber, aiding the expulsion of exhaust gases and filling the cylinder with a fresh charge.

In a crankcase-scavenged engine, the timing and geometry of transfer ports, the crankcase volume, and any intake devices determine how effectively the charge is moved from the crankcase to the cylinder. Modern implementations may use reed valves to regulate flow and prevent backflow, while older designs relied on port timing and piston motion alone. The approach is characteristic of many small two-stroke engines, such as certain motorcycle, chainsaw, and outboard marine engines, particularly in historical contexts.

Advantages of crankcase scavenging include simplicity, light weight, and low manufacturing cost, since the crankcase serves a dual role as a pump and reservoir. Disadvantages include less efficient scavenging in some designs, higher emissions from unburnt fuel, and potential lubrication challenges due to fuel mixing with crankcase oil. As a result, many modern two-stroke designs favor loop or uniflow scavenging methods to improve volumetric efficiency and emissions, but crankcase scavenging remains an important concept in the study of two-stroke engine technology.