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.