Write short notes on the following: (a) Friction losses and fuel consumption in auxiliary engines (b) Importance of stroke/ bore ratio in engine performance (c) Noise reduction used in main engines to improve noise level conditions for personnel in the engine room

(a) Friction Losses and Fuel Consumption in Auxiliary Engines

The power developed by combustion inside an auxiliary engine is called indicated power (IP). The power available at the output shaft is called brake power (BP).

The difference between indicated power and brake power is mainly due to friction and mechanical losses.

[
BP = IP - \text{Mechanical losses}
]

The ratio of brake power to indicated power is called mechanical efficiency:

[
\eta_m=\frac{BP}{IP}\times100
]

Mechanical efficiency of an engine is generally about 95%, meaning approximately 5% of the developed power is lost. However, not all of this loss is due to friction.

Sources of friction losses

Friction occurs mainly:

• Between piston rings and cylinder liner
• In main and connecting-rod bearings
• In camshaft and other bearings
• In gears and other mechanical drives
• In other moving engine components

Friction losses are unavoidable, but they can be reduced by:

• Proper engine design
• Correct lubrication
• Maintaining correct clearances
• Proper maintenance

Reduction in friction results in lower fuel consumption and improved engine efficiency.

⸻

(b) Importance of Stroke/Bore Ratio in Engine Performance

The stroke/bore ratio is:

[
\text{Stroke/Bore ratio}=\frac{\text{Stroke}}{\text{Bore}}
]

It is an important parameter in engine design because it affects engine speed, piston speed, power and propeller requirements.

For efficient ship propulsion, a large-diameter, slow-turning propeller is generally preferred. Large ships such as bulk carriers and tankers normally use large, slow-turning propellers and therefore require long-stroke, low-speed engines.

The mean piston speed is:

[
C_m=\frac{2\times L\times RPM}{60}
]

Where:

• (L) = Stroke in metres
• RPM = Engine speed

For large marine diesel engines, piston speed is generally maintained around 6–9 m/s. Even in small high-speed engines, it generally does not exceed about 12 m/s.

If piston speed is too low, it may result in:

• Poor sealing at piston rings
• Loss of compression
• Greater heat loss through the liner
• Lower compression temperature
• Ignition problems

If piston speed is too high, it causes:

• Higher inertia forces
• Increased friction
• Higher liner temperature
• Breakdown of cylinder lubrication
• Increased piston-ring and liner wear

Since:

[
C_m \propto L\times RPM
]

when engine RPM is reduced, the stroke must be increased to maintain the required piston speed. Therefore, low-speed marine engines generally have a higher stroke/bore ratio.

Examples

Application	Bore	Stroke	Stroke/Bore
Large bulk carrier engine	700 mm	2800 mm	4.00
Post-Panamax container ship engine	900 mm	2550 mm	2.83

Thus, stroke/bore ratio is selected according to the required engine speed, power, propeller characteristics and ship type.

⸻

(c) Noise Reduction Techniques Used in Main Engines

Noise from a main engine can be classified mainly into:

1. Exhaust gas noise
2. Airborne noise
3. Structure-borne noise/vibration

1. Exhaust gas noise

Exhaust gas produces noise due to gas pulsations.

Noise is reduced by:

• Providing a large exhaust gas receiver between the cylinders and turbocharger.
• The exhaust gas receiver reduces low-frequency gas pulsations.
• Using an absorption-type exhaust silencer.
• Mineral wool or glass wool is used as sound-absorbing material.

2. Airborne noise

Airborne noise is produced by vibration of engine components and surfaces, causing the surrounding air to pulsate.

Noise reduction methods include:

• Absorption material inside the scavenge-air pipe
• Ring diffuser absorption plate at the top of the scavenge-air cooler
• External insulation of the scavenge-air cooler
• Absorption material around the engine
• Sound-absorbing material on engine-room bulkheads
• Turbocharger intake silencer

3. Structure-borne noise and vibration

Vibrations produced by combustion forces and reciprocating piston movement are transmitted through the engine structure to the engine feet and then to the ship’s structure.

Noise and vibration can be reduced by:

• Using resilient mountings/vibration isolators, particularly on some four-stroke engines.
• Using suitable structural isolation.
• In accommodation spaces, a floating floor construction may be used to reduce transmitted noise and vibration.

Conclusion

Noise reduction improves the working conditions and safety of engine-room personnel and reduces the transmission of noise and vibration to other parts of the ship.