Chapter 15: Engine Cooling Systems

15.1 The Need for Cooling

• The thermal efficiency of a piston engine is only about 25-28%66. Over 70% of the heat
• energy from combustion is wasted, with about 40% going out the exhaust and 32% being
• absorbed by the engine components and oil67. This excess heat must be removed to prevent:
•  Breakdown of the oil's lubricating properties.
•  Structural failure of engine components.
•  Pre-ignition and detonation68.
• However,
• ver-cooling is also detrimental, as it reduces the BHP available and can lead to poor fuel
• vaporization69.

15.2 Cooling System Types

•  Liquid Cooling: Uses a mixture of water and ethylene glycol pumped through passages in
• the engine and cooled by an air-cooled radiator70. While efficient, these systems are heavy.
•  Air Cooling: This is the preferred method for most aircraft piston engines due to its
• simplicity and lighter weight71. It relies on the flow of air over the engine.
• Cooling Fins: The cylinders are covered in thin metal fins to greatly increase the surface area
• available for heat dissipation72.

• Cowling and Baffles: The engine is enclosed in a streamlined cowling with internal baffles
• that are designed to direct the cooling air evenly over all cylinders73737373.
• Cowl Flaps: These are adjustable doors at the cooling air outlet. They are opened during
• high-power, low-airspeed conditions (like takeoff and climb) to maximize airflow and
• cooling. They are closed during cruise and descent to maintain optimal engine temperature
• and reduce drag74.

15.3 Monitoring Engine Temperature

•  Cylinder Head Temperature (CHT) Gauge: The pilot monitors engine temperature with a
• CHT gauge75. The sensor is a
• thermocouple, which produces a voltage proportional to its temperature76. If only one is
• fitted, it will be on the hottest-running cylinder, typically one of the rear ones77.