Final answer:
In a 2-stroke engine, four events take place over two strokes: intake and compression occur during the upward stroke, while combustion and exhaust happen during the downward stroke. Intake and compression of the new fuel-air mixture occur simultaneously with the power and exhaust events of the previous cycle.
Step-by-step explanation:
The Events of a 2-Stroke Engine
In a 2-stroke engine, there are essentially four events that take place over the course of two strokes: intake and compression during the upward stroke, followed by combustion (power) and exhaust during the downward stroke. Unlike a four-stroke engine that performs these steps in four separate strokes, a 2-stroke engine accomplishes this in two strokes, making it more compact and lightweight, often resulting in higher power-to-weight ratios.
The intake event occurs as the piston moves down, drawing a fresh fuel-air mixture into the crankcase. The compression event happens simultaneously but in a different space: above the piston where a previously-introduced fuel-air mixture is being compressed. When the piston reaches the top of its cycle, the mixture is ignited, causing an explosion that drives the piston downward—this is the combustion or power event. In a very coordinated process, as the piston moves down once more, it uncovers the exhaust port, allowing the burnt gases to escape—an event known as the exhaust.
Two of these events, specifically the intake and compression of a new fuel-air mixture, occur concurrently with the power and exhaust events of the cycle just completed. This overlapping of functions allows the 2-stroke engine its efficiency in completing a power cycle in just two strokes.