Turning Moment in a 4-Stroke IC Engine - ESE 1996 Important Question

Understanding the forces and torques acting on the crankshaft of an internal combustion (IC) engine is crucial for mechanical engineering concepts. Let's analyze a common question related to the compression stroke.

Question:

In a 4-stroke IC engine, the turning moment during the compression stroke is:

(a) positive throughout
(b) negative throughout
(c) positive during major portion of the stroke
(d) negative during major portion of the stroke

Video Solution:Watch a detailed explanation with diagrams and animations to understand the concept of turning moment and engine strokes.

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Correct Answer:

(b) negative throughout

Explanation:

During the compression stroke, the piston moves from Bottom Dead Center (BDC) to Top Dead Center (TDC). For this to happen, the piston must compress the air-fuel mixture inside the cylinder. This compression requires external work to be done on the piston.

The kinetic energy stored in the engine's flywheel is used to drive the piston upward against the increasing pressure. Since the flywheel is doing work on the piston to overcome the resistance of compression, the force exerted on the crankshaft is in the opposite direction of the engine's rotation. This results in a negative turning moment (or torque) that works against the engine's rotation throughout the entire stroke.

Key Concepts:

• Positive Turning Moment: Occurs during the power stroke when the expanding gases push the piston down, producing torque to drive the crankshaft.

• Negative Turning Moment: Occurs during the compression and exhaust strokes when work is done on the piston by the flywheel's inertia.

Exam Tip:

Remember the key role of the flywheel. It stores energy during the power stroke (positive turning moment) and releases it during the other three strokes (negative turning moments) to ensure smooth and continuous rotation of the crankshaft.