Chapter 6: Lubrication and Cooling Systems

Principles of Engine Lubrication

• Primary Purpose: Lubricant
  • Reduce friction between moving parts
  • Oils are circulated in aircraft engines
  • Prevents metal to metal contact by maintaining an oil film
  • Reduces energy consumption and unwanted engine heat

Types of Friction

• Sliding Friction: Occurs with plain bearings, surfaces aren’t completely smooth.
• Rolling Friction: Caused by rollers or spheres such as ball or roller bearings; less friction than sliding.
• Wiping Friction: Between gear teeth, requires strong lubricants.

Functions of Engine Oil

• Cushioning: Oil acts as a cushion between metal parts.
• Cooling: Oil absorbs heat from pistons and cylinder walls.
• Sealing: Assists in forming a seal between piston and cylinder wall.
• Cleaning: Removes foreign particles carried to a filter.
• Corrosion Prevention: Leaves a coating on parts to prevent rust.

Requirements and Characteristics of Reciprocating Engine Lubricants

• Viscosity: Resistance of oil flow, affected by temperature.
• Grades: SAE ratings indicate viscosity, not quality.
• Viscosity Index: Measures change in viscosity with temperature.
• Flash/Fire Point: Temperatures at which oil ignites.
• Cloud/Pour Point: Temperatures affecting oil solidification and flow.
• Specific Gravity: Comparison of oil weight to water.

Types of Engine Oils

• Mineral Oils: Used during break-in periods.
• Ashless Dispersant Oils: Contain additives for better temperature range and cleaning properties.
• Multigrade Oils: Blend of synthetic and mineral oils, offers broader temperature use.

Reciprocating Engine Lubrication Systems

• Wet Sump Systems: Oil stored in engine reservoir.
• Dry Sump Systems: Oil pumped to an external storage tank.
• Pressure and Splash Lubrication: Pressure is principal, splash may be secondary.

Lubrication System Components

• Oil Tanks: Need expansion space, air-tight filler caps.
• Oil Pumps: Gear type, positive displacement, circulate oil.
• Oil Filters: Screen, Kuno, canister, spin-on types; remove impurities.
• Oil Pressure Regulators: Limit oil pressure to prevent system damage.

Maintenance Practices

• Oil Changes: Regular intervals to remove contaminants.
• Filter and Screen Cleaning: Regular checks for blockages or wear particles.
• System Checks: Ensure correct oil pressure and temperature.

Turbine Engine Lubrication

• Requirements: Low viscosity, high temperature stability, anti-foaming.
• Synthetic Oils: Preferred for turbine engines due to high temperature performance.
• System Components: Similar to reciprocating engines; includes oil tanks, pumps, filters.

Cooling Systems

• Reciprocating Engines: Air-cooled fins; cowling directs airflow.
• Turbine Engines: Internal air flow; uses excess air from compressors for cooling.

Maintenance and Cooling System Checks

• Inspection: Regular checks on cooling fins, cowlings, and deflectors.
• Temperature Monitoring: Use of thermocouples and temperature gauges to avoid overheating.

This chapter provides a thorough overview of the importance, operation, and maintenance of lubrication and cooling systems in both reciprocating and turbine aircraft engines. Proper understanding ensures optimal engine performance and longevity.