Flywheel sizing comparison Compared with similar petrol (spark-ignition) engines, Diesel (compression-ignition) engines generally require what size of flywheel to smooth speed fluctuations?

Introduction / Context:
The flywheel stores rotational energy and smooths crankshaft speed fluctuations between firing strokes. Differences in combustion characteristics and operating speeds between Diesel and petrol engines influence flywheel mass selection.

Given Data / Assumptions:

• Diesel engines operate at higher compression ratios and often lower rated speeds.
• Combustion pressure rise and torque pulsations can be larger in Diesels.
• Objective: limit cyclic speed variation to acceptable values for drivability and NVH.

Concept / Approach:
Required flywheel inertia J depends on torque irregularity and target speed fluctuation (delta omega). For the same power, a slower-turning Diesel with stronger pulse amplitudes benefits from a larger inertia to equalize angular velocity. Hence, Diesels typically use heavier (bigger) flywheels than comparable petrol engines.

Step-by-Step Solution:
Identify torque ripple: Diesel > petrol (generally).Relate inertia to smoothing: greater J reduces speed fluctuation.Conclusion: choose a bigger flywheel for the Diesel to meet NVH criteria.

Verification / Alternative check:
Compare parts catalogs: Diesel variants often list heavier dual-mass flywheels versus petrol single-mass options in the same vehicle platform.

Why Other Options Are Wrong:
Smaller or lighter flywheels worsen fluctuations.

“Same size” ignores typical differences in torque ripple and speed.

All piston engines require some flywheel effect; “no flywheel” is incorrect for road engines.

Common Pitfalls:
Overlooking modern dual-mass flywheels that separate torsional damping from inertia; confusing crankshaft balance weights with flywheel inertia.

Final Answer:
bigger flywheel