Two-stroke engines have a distinct "lopey" idle and a unique sound due to their design and combustion process. Unlike four-stroke engines, which have separate intake and exhaust strokes, two-stroke engines complete the entire combustion cycle in just two strokes: the compression stroke and the power stroke. This fundamental difference in operation leads to several characteristics that affect the engine's sound and performance.
One major factor is the timing of the exhaust and intake ports in a two-stroke engine. The exhaust and intake ports are typically opened and closed by the piston's movement and the engine's design. During the combustion cycle, the exhaust port opens to allow the burned gases to escape, and shortly after, the intake port opens to let in the air-fuel mixture. This timing creates a direct connection between the intake and exhaust systems, resulting in some of the fresh mixture escaping through the exhaust port. This loss of fresh mixture leads to incomplete combustion, which contributes to the distinct sound and lopey idle of a two-stroke engine.
Additionally, the intake and exhaust processes in a two-stroke engine occur simultaneously, which can cause some inefficiencies. When the exhaust port is open, some fresh mixture can escape, reducing the overall efficiency of the engine. This loss of fresh charge affects low-speed operation, resulting in a rougher idle and less smoothness at lower throttle positions.
As for diesel two-stroke engines, they operate differently from their gasoline counterparts. Diesel engines rely on the compression of air alone, without the need for a spark plug, to ignite the fuel. Diesel two-stroke engines, also known as opposed-piston engines, typically have a different design that mitigates some of the characteristics associated with the lopey idle and roughness.
Diesel two-stroke engines often use opposing pistons and multiple cylinders, which allows for better balance and smoother operation. The opposing piston design reduces the unbalanced forces that contribute to the lopey idle. Additionally, diesel fuel's higher energy density and combustion characteristics result in smoother and more controlled combustion, reducing the roughness typically associated with gasoline two-stroke engines.
In summary, the lopey idle and rough sound of two-stroke engines are primarily due to their design, intake-exhaust timing, and incomplete combustion characteristics. Diesel two-stroke engines, with their different design and combustion characteristics, tend to have smoother operation and do not exhibit the same issues as gasoline two-stroke engines.