(a)Methods used to increase power-to-weight ratio

1. Increase mean effective pressure (MEP)

• By improving combustion and fuel injection, the indicated and brake mean effective pressure are increased.
• Higher MEP gives more power from the same cylinder size.

2. Supercharging / Turbocharging

• More air is supplied to the cylinders by turbochargers.
• Allows more fuel to be burnt efficiently.
• Hence power output increases without greatly increasing engine size or weight.

3. Increase engine speed (rpm)

• Power is proportional to mean effective pressure × speed.
• Higher rpm gives greater power output for the same engine dimensions.
• Common in medium and high-speed engines.

4. Improve scavenging and charging efficiency

• Better air flow, port timing, and exhaust gas exchange improve cylinder filling.
• More fresh air leads to better combustion and higher output.

5. Use lighter and stronger materials

• Use of alloy steels, aluminium alloys, and improved cast materials reduces component weight.
• Stronger materials allow thinner sections with adequate strength.

6. Improved cooling and lubrication

• Better cooling of pistons, liners, and cylinder heads allows operation at higher thermal and mechanical loading.
• Improved lubrication reduces wear and permits higher speeds and pressures safely.

7. Better fuel injection and combustion design

• High-pressure fuel injection, improved atomization, and optimized combustion chamber design increase combustion efficiency.
• This produces more power with less engine size increase.

8. Reduction in structural weight

• Use of welded fabricated bedplates, compact design, fewer heavy castings, and improved structural design.
• This reduces overall engine weight while maintaining rigidity.

Limiting Factors:

1. Thermal and Mechanical Stresses:
   • Materials used in engine construction have limits to the pressures and temperatures they can withstand.
   • Excessive increase in cylinder pressure or temperature leads to fatigue, cracking, and reduced component life.
2. Vibration and Torsional Stresses:
   • Higher power output or speed can increase torsional vibration levels.
   • Excessive vibrations may cause mechanical damage and reduce operational reliability.
3. Lubrication Limits:
   • Increased load and higher operating temperatures can cause breakdown of the lubricating oil film.
   • This results in metal-to-metal contact, leading to wear and potential seizure.
4. Combustion Limitations:
   • Increasing pressure and temperature may cause incomplete combustion or excessive NOx emissions.
   • Stringent environmental regulations restrict further increases in combustion intensity.
5. Cooling Limitations:
   • As power density increases, effective removal of heat from cylinder liners, pistons, and valves becomes more difficult.
   • Inadequate cooling leads to thermal deformation and reduced efficiency.
6. Structural Strength:
   • Weight reduction is limited by the need to maintain structural rigidity and resistance to fatigue.
   • The engine must be robust enough to handle fluctuating loads and stresses during operation.
7. Propeller Speed Limitations:
   • Slow-speed engines must operate within the efficient range of the propeller.
   • Increasing engine RPM beyond the optimal range reduces propulsive efficiency and overall performance.

Typical Power-to-Weight Ratio:

• For modern slow-speed marine diesel engines, the power-to-weight ratio typically ranges from 5 to 10 kW per tonne.