Principle of a Gas Turbine Engine
Gas Turbine, Subsystems IV
In WW-II and shortly thereafter, piston powered aircraft peaked in power, performance and complexity wise. Power went up to over 4000 bhp for large multi-row radial engines. Only to be defeated by the jet, which was developed by (among others) Germany's Dr. Hans von Ohain and separately in the UK by Sir Frank Whittle. Its principles are based on the "Aeolipile" of the ancient Greek scientist Hero and other great thinkers like Leonardo da Vinci and the laws of Isaac Newton.
Compared to a piston, the gas turbine has less parts and the moving parts rotate in only one direction without stopping and accelerating as the pistons normally do in a engine. Thus, a running gas turbine is basically free of the vibrations normally found in piston models, which translates in much longer service life (TBO) and higher reliability.
There are a number of engine subsystems which assist in keeping the engine running. Lubrication, engine cooling and bleed air. Without going into great depth we will uncover how they work.
Turbines use a recirculating oil system where oil is collected in tanks after it has been pumped around by pressure and scavenge pumps. Oil coolers and filters are also used to remove any excess heat and any particulate matter picked up by the oil which could damage bearings and gears reducing the life of the engine.
Oil in a turbine does not get anywhere near the combustion process as in a piston engine where it lubricates the pistons and is exposed to these high temperatures, reducing service life and increasing the need to change oil on a very regular basis.
AeroShell has a document about turbine engine oils.
There are basically two types of oil systems used in turbines: the full flow system and the other is with a pressure relief valve. The major difference is in the way how the oil flow is being controlled. In the cockpit there are indicators for pressure and temperature for each engine on both systems.
A pressure relief valve sets the oil pressure to a predetermined value, much as in a piston model. There are variations where the valve settings varies depending on RPM, so that flow rate is constant. A full flow system runs without a pressure relief valve, thereby providing adequate oil flow even at maximum allowable RPM.
An expendable oil system is sometimes used as this saves weight, there is no need for an oil cooler, filters or scavenge pumps. But oil usage is considerable and the system is not very common in normal day to day engines. They are found in vertical lift or booster engines, which are in use only for a short period of time.
Cooling of engine oil is done by either ambient air or through a fuel heater / oil cooler core. Giving the advantage of heating the fuel going to the engine and reducing possible blockage due to ice in the fuel system.
Bleed air, cooling and sealing
Bleed air is internal air from the compressor that is used for other things than engine thrust. For example: cabin pressurization and air conditioning, airframe and engine ice protection, cooling of internal parts, fuel heating and windscreen rain removal. During takeoff and other high power situations bleed air valves on the engine can be closed so that maximum thrust is available.
To minimize losses in engine thrust output, bleed air is taken after the first couple of stages in the compressor.
Secondary air (not bleed or bypass air) is used to cool the internal parts of the GTE. After starting the temperature will rise and due to thermal growth metals will expand and minimum clearances of blade tips and seals must be maintained by applying sufficient cooling air. Care must be taken that for maximum efficiency the temperature limits of the engine are not exceeded.
In burner cans secondary cooling air is used, after combustion is complete, to lower the very high gas temperature below the limits of the turbine blades and the static nozzle guide vanes. During a hot start, these parts suffer the most.
Seals are used to prevent oil leakage from the bearings, control air flow and as barrier to prevent combustion gases leaking to turbine disc cavities. For this purpose different type of seals are developed: labyrinth, ring, hydraulic, carbon and brush seals. Each with their own specific application in the engine.