There are 3 major reasons for a four-stroke engine of a two-wheeler being more eco-friendly than a two-stroke engine: In a two stroke engine, a part of the incoming air and fuel is short-circuited directly through the exhaust port of the cylinder and is carried away with the exhaust gases. This pollutes the atmosphere. But, the design of four-stroke engine is different there are valves (inlet and exhaust).
Scavenging is effectively done in a four-stroke engine than in a two-stroke. (‘Scavenging’ is a process of removal of burnt gases from the engine cylinder.)
If scavenging is not done properly, the residual burnt gas that inside the cylinder will be combusted again, this leads to smoky emission in two-stroke engines.
The lubrication system of a two-stroke engine is different from that of a four-stroke engine. In two-stroke engines, oil is mixed with the fuel (to provide lubrication for moving surfaces such as piston skirt and roller bearings). It is subsequently carried into the combustion chamber by the air stream, where it is partially burnt and partially pushed through the exhaust port, along with the exhaust gases. This results in smoky emission.
The suction, compression, expansion and exhaust strokes takes place in four strokes of the piston or two revolution of the crank shaft, in a four stroke engine
In a two stroke engine compression and suction takes place in one stroke of piston and expansion and exhaust strokes also takes place in one stroke of piston. Hence in two strokes of piston all the four processes are completed which makes scavenging less efficient.
Scavenging is the process in which the products of combustion (i.e. burnt exhaust gases) are cleared from the cylinder and filling it with more or less completely with a fresh mixture of air and fuel. The incoming fresh mixture entering into the engine cylinder at certain velocity sweeps out the products of combustion from the cylinder.
The fresh mixture entering acts as a scavenging or sweeping agent. But instead of acting as a sweeping agent, the fresh mixture also escapes through the exhaust port, as unburn hydrocarbons which are a pollutant.