Red Seal Motorcycle Technician Practice Exam

What is the difference between a two-stroke and a four-stroke engine?

1. The number of cylinders used

2. The number of power strokes per crankshaft revolution

3. The type of fuel required

4. The design of the exhaust system

The correct answer is: The number of power strokes per crankshaft revolution

The distinction between a two-stroke and a four-stroke engine primarily lies in the number of power strokes generated for each complete crankshaft revolution. In a two-stroke engine, there is one power stroke for every revolution of the crankshaft, resulting in a power pulse with each turn. This is achieved through a more simplified cycle that combines the intake and compression phases into one stroke, and the power and exhaust phases into another. Conversely, a four-stroke engine executes a more complex series of operations to complete one full cycle of power production. It requires two complete revolutions of the crankshaft to achieve four distinct strokes: intake, compression, power, and exhaust. This means that in a four-stroke engine, there is one power stroke only for every two revolutions of the crankshaft. This fundamental difference in the operation and efficiency of these engines affects factors such as power output, torque characteristics, and emissions, which are critical for understanding engine performance in motorcycles and other vehicles. The other options—while they describe certain attributes of engines—do not address the core difference in the power delivery mechanics between two-stroke and four-stroke engines.