Q15Basic Mechanical Engineering

1. Four-Stroke Engine Working: Completes one full thermodynamic cycle in four distinct strokes of the piston, which requires two complete revolutions ($720^{\circ }$) of the crankshaft. It utilizes poppet valves (intake and exhaust) driven by a camshaft. - Suction: Piston moves down, intake valve open, drawing in the fresh charge. - Compression: Both valves closed, piston moves up, highly compressing the charge. - Power/Expansion: Spark ignites the compressed charge. The explosion forces the piston down violently. - Exhaust: Exhaust valve opens, piston moves up, physically pushing out burnt gases. Advantage: Excellent fuel efficiency, clear exhaust, cleaner combustion. Disadvantage: Heavier, more complex, only one power stroke per two revolutions.

2. Two-Stroke Engine Working: Completes the entire cycle in just two strokes of the piston, requiring only one revolution ($360^{\circ }$) of the crankshaft. Instead of complex valves, it uses ports (holes) cut into the cylinder wall, covered and uncovered by the piston itself. - Upward Stroke: Piston moves up, compressing the charge in the cylinder while simultaneously drawing a new charge into the sealed crankcase below. - Downward Stroke (Power): Ignition occurs, forcing the piston down. As it descends, it uncovers the exhaust port to release burnt gases, and slightly later uncovers the transfer port, allowing the pressurized fresh charge in the crankcase to rush into the cylinder, helping sweep out exhaust (scavenging). Advantage: Very simple, lightweight, fires every revolution providing high power-to-weight ratio. Disadvantage: Poor fuel efficiency (unburnt fuel escapes out exhaust port), high pollution (oil must be mixed with fuel).