Aircraft Engine Lubrication and Cooling Systems

The primary purpose of a lubricant is to reduce friction between moving parts. An engine lubricant also functions in engine cooling, sealing and cushioning moving parts, cleaning the engine interior, and protecting against corrosion. Because engines require a lubricant that can circulate freely, liquid lubricants (such as oils) are most widely used in aircraft engines.

The primary purpose of a reciprocating engine lubrication system is to deliver oil to the internal engine components. The most common ways of distributing oil are pressure, splash, and spray lubrication. To ensure adequate lubrication, reciprocating engines rely on a combination of pressure and splash lubrication. However, on larger engines, adequate oil circulation can be accomplished only through the use of pressure, splash, and spray lubrication.

The lubrication system of a turbine engine supplies oil to the internal moving parts to reduce friction and heating. In most cases, pressure is used to lubricate all the necessary components within a turbine engine. Unlike reciprocating engines, whose moving parts can splash oil around the engine, the moving parts of a turbine engine simply rotate on bearings. Because turbine engines operate at much higher temperatures than their reciprocating counterparts, the lubrication system must carry a greater amount of heat away from the components that it lubricates. To do this, oil typically circulates through a turbine engine at a high flow rate.

Aircraft engines convert heat energy into mechanical energy. However, in doing this , only about one-third of the heat produced is used for thrust. The remaining two-thirds is wasted and must be removed from an engine. Therefore, cooling systems are designed to remove the unused heat energy produced by combustion to enable an engine to operate at peak efficiency.

Of the heat that is generated in an internal combustion engine, approximately 30 percent is converted to useful work while 40 to 45 percent is expelled through the exhaust. The remaining 25 to 30 percent is absorbed by the oil and metal mass of the engine. It is this heat that is removed by an aircraft's cooling system. Without cooling, engine performance suffers as volumetric efficiency decreases. Additionally, excessive heat shortens the life of engine parts and reduces the lubricating properties of the oil. The two most common methods for cooling an engine are direct air cooling and liquid cooling.

Like reciprocating engines, turbine engines convert heat energy into work. However, the continuous combustion process in a turbine engine produces more heat, so most of the cooling air passes through the inside of the turbine engine. Otherwise, internal engine temperatures could rise above 4,000 degrees Fahrenheit. In practice, a typical turbine engine uses approximately 25 percent of the total inlet air flow to support combustion. This airflow is referred to as the engine's primary airflow. The remaining 75 percent is used for cooling, and is referred to as secondary airflow.

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