Both types of engines have advantages and disadvantages. The individual application governs the selection of a 2-stroke cycle or 4-stroke cycle unit.
The primary difference between the engines is cylinder design, and:
• The 2-stroke cycle requires the piston to make only two strokes through the cylinder (one revolution of the crankshaft) for each complete combustion cycle.
• The 4-stroke cycle requires the piston to make four strokes through the cylinder (two revolutions of the crankshaft) for each complete combustion cycle.
• The 2-stroke cycle engine is the less complicated of the two, since it has no CAM-actuated intake valves and most have no CAM-actuated exhaust valves.
Some 2-stroke cycle units do, however, have CAM-actuated exhaust valves.
• The 2-stroke cycle engine requires a positive scavenging air pressure (reciprocating scavenging air cylinder, mechanically-driven blower or turbocharger).
The 4-stroke cycle engine can be naturally aspirated. More horsepower can be derived from the 4-stroke cycle engine by utilizing a mechanically-driven blower (supercharger or turbocharger).
• The 4-stroke cycle engine has a longer functional stroke than the 2-stroke cycle engine, since the 4-stroke cycle maintains a positive pressure (due to combustion) on the piston for more degrees of crankshaft rotation and opens the exhaust valve near the bottom dead center. Therefore, if a 2-stroke cycle and a 4-stroke cycle cylinder had the same displaced volume and were operating at the same crankshaft speed with the same average cylinder pressures, the 2-stroke cycle would be developing 65 to 75% more horsepower.
• The 2-stroke cycle engine develops more power per cubic inch of displaced volume.
• The 2-stroke cycle performs efficiently 100 to 110% of its rated load. The efficiency drops rapidly as the load is reduced. Modern 2-stroke cycle engines do operate more efficiently at lighter loads than the earlier generations did.
• The 4-stroke cycle engine is more efficient over a wider load range and responds faster to drastic load changes, since the flow of gases is better controlled by the intake and exhaust valves.