Question

Consider the following statements regarding C.I. engine and S.I. engine

(A) C.I. engines are more bulky than S.I. engines

(B) C.I. engines are more efficient than S.I. engines

(C) Lighter flywheels are required in C.I. engines

Choose the correct answer from the options given below:

• A. (A) and (C) only
• B. (B) and (C) only
• C. (A) and (B) only
• D. (A), (B) and (C)

Hint:

Think about compression ratio, structural strength and turning moment variation for each engine type.

Answer 1

The Correct Option is C

We can also look at this from the point of view of design requirements instead of general statements.

Because a CI engine ignites fuel by compression alone, it must be built for compression ratios of about 16 to 22, compared to about 8 to 12 for an SI engine. Higher compression means higher peak cylinder pressure, and the components must be thicker and heavier to survive this, which is why CI engines end up bulkier. This supports (A).

The higher compression ratio in a CI engine also raises the ideal thermal efficiency, because a bigger expansion ratio lets the engine draw more useful work from the same fuel charge before exhaust. This is the direct reason CI engines are known to be more fuel efficient than SI engines, supporting (B).

Flywheel size depends on how uneven the turning moment is over one cycle. A CI engine has sharper pressure peaks and a torque curve that swings more strongly, so it actually needs a heavier flywheel to store enough energy and smooth out the crankshaft speed. This means statement (C), which claims a lighter flywheel, is wrong.

So only (A) and (B) hold true, confirming option 3.

\[\boxed{\text{(A) and (B) only}}\]