Understanding the Two-Stroke Engine Cycle

The question asks about the number of crankshaft revolutions required to complete the working cycle of a two-stroke engine. Let's break down how a two-stroke engine operates to understand this.

How a Two-Stroke Engine Works

A two-stroke engine differs significantly from a four-stroke engine in how it completes its operational cycle. Instead of separate strokes for intake, compression, power, and exhaust, a two-stroke engine combines these events into just two movements (strokes) of the piston, corresponding to a single revolution of the crankshaft.

Here are the two main strokes that occur during one full cycle:

1. Upward Stroke: As the piston moves upwards from the bottom of the cylinder, it first covers the exhaust and transfer ports. This movement compresses the fuel-air mixture that was previously drawn into the cylinder above the piston. Simultaneously, a fresh charge of fuel-air mixture is drawn into the crankcase below the piston through the intake port (or reed valve), which is uncovered by the piston's movement.
2. Downward Stroke: At or near the top of the stroke, the spark plug ignites the compressed fuel-air mixture. This causes an explosion, driving the piston forcefully downwards (the power stroke). As the piston moves down, it first uncovers the exhaust port, allowing the burnt gases to escape. Shortly after, it uncovers the transfer port, and the compressed fuel-air mixture from the crankcase is pushed into the cylinder (called scavenging), helping to push out the remaining exhaust gases and filling the cylinder with a fresh charge for the next cycle. The downward movement of the piston also compresses the mixture in the crankcase.

Crankshaft Revolutions per Cycle

Observe that the entire process – intake, compression, ignition, power, and exhaust (including scavenging) – happens within just one upward and one downward stroke of the piston. Since one complete upward and downward movement of the piston corresponds exactly to one full revolution of the crankshaft, the working cycle of a two-stroke engine is completed in a single crankshaft revolution.

In contrast, a four-stroke engine requires four piston strokes (intake, compression, power, exhaust) to complete one cycle, which takes two full revolutions of the crankshaft.

Therefore, the working cycle of a two-stroke engine is completed in one revolution of the crankshaft.

Revision Table: Two-Stroke Engine Key Points

Additional Information: Applications of Two-Stroke Engines

Two-stroke engines are often used in applications where simplicity, light weight, and high power-to-weight ratio are important. Some common examples include:

• Small motorcycles and scooters
• Chainsaws and leaf blowers
• Outboard motors for boats
• Snowmobiles
• Older or smaller diesel engines (like some large marine engines)

Their simplicity and fewer moving parts contribute to their lighter weight and lower manufacturing cost compared to four-stroke engines of similar power output. However, they are often less fuel-efficient and produce more emissions than modern four-stroke engines, particularly due to the scavenging process where some fresh fuel-air mixture can escape through the exhaust port.