Two-stroke engines have certain advantages. They produce more power and more compact than the four-stroke engine, and they are lightweight and less costly. On the other hand, some fuel gets wasted in a two-stroke engine, decreasing its efficiency.
In the case of two-stroke engines, for every two strokes of the piston inside the cylinder, one power stroke is produced. In four-stroke engines, power is produced once during four strokes of the piston. For the same size engine, the power produced by the two-stroke engine is more that the four-stroke engine. Ideally the power produced by the two-stroke engine is double that of the four-stroke engine, but in actual practice it is only about 30% more than four-stroke engine. Also, since the power produced by the two-stroke engine is higher, these engines are small and compact in size.
Since there are no valves in the two-stroke engine and only ports, they are cheaper and require less maintenance. With two-stroke engines, the torque produced on the crankshaft is more uniform because the power is produced during every alternate stroke of the piston.
1) When the inlet valve of the engine is opened for intake of the air-fuel mixture, the exhaust valve is also open. Although there is deflector between the inlet and exhaust areas of the engine, some fresh air-fuel mixture always escapes through the exhaust area. Thus part of the fuel that would have produced power goes wasted. This increases the fuel consumption and reduces the engine's overall efficiency.
2) The two-stroke engine does not operate with the same efficiency at different speeds. When the carburetor's throttle valve is partly opened, the air-fuel mixture taken inside the cylinder is not sufficient to drive out all the exhaust gases, leaving some of the exhaust gases inside the cylinder even during the combustion stroke. This causes non-uniform burning of the fuel and inconsistent efficiency at different speeds.
One disadvantage that applies to both diesel and petrol two-stroke engines is the extensive cooling and lubricating requirements of the two-stroke engines. Since in two-stroke engines power stroke is produced after every stroke, a large amount of heat is generated within them. To reduce the temperature of the engine and keep the moving parts well-lubricated, good lubrication and cooling systems for the engine are required.