Crankshaft & Piston Ring MCQs with Detailed Explanation

Correct Answer: C. Lubricating connecting rod bearing

• The crankshaft has drilled oil passages inside it.
• Oil flows from the main bearing journal through this drilled passage to the crank pin.
• The crank pin supports the big end of the connecting rod through the connecting rod bearing.
• So, this hole is mainly provided to supply lubricating oil to the connecting rod bearing.
• It is not primarily provided for balancing, reducing vibration or reducing weight.

Correct Answer: B. Crank pin

• The main journal is supported by the main bearing.
• The crank pin is the offset journal where the connecting rod big end is fitted.
• The drilled passage connects these two zones so that pressurised oil can reach the crank pin.
• From the crank pin area, the connecting rod bearing receives lubrication.

Correct Answer: C. Metal-to-metal contact at the connecting rod bearing

• Connecting rod bearing works under heavy alternating load.
• Lubricating oil forms a thin film between the journal and bearing surface.
• This oil film reduces friction, wear and heating.
• Without proper lubrication, metal-to-metal contact can occur and bearing damage may happen.

Correct Answer: B. Two

• Compression rings are fitted near the top portion of the piston.
• Their main duty is to seal the combustion chamber.
• The pressure inside the combustion chamber is very high.
• One compression ring alone may not hold this pressure effectively.
• Therefore, automotive engines generally use a minimum of two compression rings.

Correct Answer: A. Seal the combustion chamber

• Compression rings prevent leakage of combustion gases from the combustion chamber to the crankcase.
• They also help prevent leakage of air-fuel mixture during the compression stroke.
• Good sealing helps maintain compression pressure and engine power.
• Oil control is mainly associated with oil rings, not compression rings.

Correct Answer: D. Cast iron

• Compression ring is a type of piston ring.
• It should resist wear and should not easily scuff the cylinder wall.
• Cast iron is commonly used because it has good wearing properties.
• Cast iron contains graphite, and graphite gives a self-lubricating effect.
• This makes cast iron suitable for piston rings working against the cylinder wall.

Correct Answer: C. Compression rings prevent leakage during compression and power strokes

• During the compression stroke, air-fuel mixture or air is compressed and leakage must be prevented.
• During the power stroke, high pressure combustion gases try to escape past the piston.
• Compression rings seal the clearance between piston and cylinder wall.
• Oil rings mainly scrape and control lubricating oil on the cylinder wall.
• Expander is associated with oil ring assembly support, not crankshaft bearing replacement.