🚗 Why Do Diesel Engines Have Heavier Flywheels Than Petrol Engines?
When comparing diesel engines and petrol (gasoline) engines, one noticeable difference lies in the flywheel design. Diesel engines typically use heavier flywheels, and the reason is rooted in their operating principles, compression ratios, and vibration characteristics.
🔹 Role of a Flywheel in Engines
A flywheel is a rotating mechanical device attached to the crankshaft. Its primary functions are:
- Storing rotational energy (momentum).
- Reducing speed variations in the crankshaft caused by the cyclic nature of combustion.
- Smoothing out vibrations and ensuring steady power delivery.
- In simple terms, the flywheel absorbs energy during the power stroke and releases it during non‑power strokes, keeping the engine rotation smooth.
🔹 Why Diesel Engines Need Heavier Flywheels
1. Higher Compression Ratio
- Diesel engines operate at much higher compression ratios compared to petrol engines.
- This results in very high cylinder pressures, which cause stronger impulses and greater vibration.
- A heavier flywheel helps counter these vibrations by stabilizing crankshaft speed.
2. Engine Size and Mass
- Diesel engines are generally larger and heavier than petrol engines.
- The bigger size means more vibration absorption is required, which is achieved by a flywheel with higher inertia.
3. Cyclic Speed Variations
- All internal combustion engines experience speed fluctuations due to the four strokes: induction, compression, combustion, and exhaust.
- Diesel engines, with their higher compression and torque pulses, show greater crankshaft speed variation.
- A heavier flywheel reduces these fluctuations by storing and releasing energy as needed.
🔹 Flywheel Principle in Action
- In a single‑cylinder engine, speed variations are most significant, making the flywheel essential.
- In multi‑cylinder engines, variations are less pronounced, but diesel engines still require heavier flywheels due to their operating pressures.
- The moment of inertia of the flywheel is the key factor—it determines how effectively the flywheel can dampen impulsive loads.
🔹 Key Takeaways
- Diesel engines have heavier flywheels because of their higher compression ratios and greater cylinder pressures.
- The heavier flywheel provides stability, vibration damping, and smoother crankshaft rotation.
- Petrol engines, with lower compression and lighter construction, can operate with lighter flywheels.