Gas Turbine
Engine systems
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 engine, 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 engine, 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 engines, which
translates in much longer engine
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.
Lubrication
Turbine engines 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.
Oil in a turbine engine 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 turbine
engines: the full flow system and the other is with a pressure
relief valve. 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 engine. There are variations where the valve settings varies
depending on engine 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 oil
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 blockage due
to ice.
Bleed air, cooling and sealing
Bleed a ir 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 engine
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, bleed air
is taken after the first couple of stages in the compressor.
Cooling
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.
Sealing
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.
Sistem
pelumasan
Sistem ini
memiliki dua fungsi penting, selain melumasi onderdil yang bergerak juga
mendinginkan dan membuang panas mesin.
Sebelum terbang Anda
harus yakin jumlah pelumas cukup dan jenis pelumas harus sesuai dengan
rekomendasi pabrik. Jenis yang digunakan harus disesuaikan dengan kondisi
sekitar daerah operasional pesawat.
Pada panel ada
dua petunjuk pelumas yaitu suhu pelumas dan tekanan pelumas untuk setiap mesin.
Jika tekanan pelumas di bawah garis normal berarti pelumas tidak dipompa cukup
untuk bersirkulasi ke seluruh mesin, sedangkan jika tekanan melewati batas
berarti ada saluran yang tersumbat. Harus diwaspadai semua kondisi abnormal
diatas karena akan banyak onderdil vital yang tidak terlumasi. Jika mengalami
hal ini segera ikuti POH pesawat Anda. Umumnya POH merekomendasikan mematikan
mesin segera apabila tekanan pelumas tidak cukup setelah 30 detik mesin hidup.
matikan mesin jika
tekanan oli tidak naik dalam 30 detik
Sedangkan
petunjuk suhu pelumas biasanya berdampingan dengan tekanan pelumas untuk
memudahkan mengawasinya secara bersamaan. Petunjuk suhu berubah gradual seiring
naiknya suhu, sedang petunjuk tekanan pelumas akan langsung naik. Anda harus
memeriksa suhu pelumas secara periodik, terutama jika Anda memakai high
power setting. Petunjukan yang abnormal dapat berarti penyumbatan atau berkurangnya
jumlah pelumas atau kesalahan pengukuran.
SISTEM PELUMASAN
Pengertian
•
Gesekan antara logam yang bersinaggungan tidak
dapat dihilangkan sama sekali
tetapi dapat dikurangi dengan cara melapisi
bagian yang bergesekan.
Tujuan
•
Untuk menurunkan gesekan, pelumasan itu sendiri
berarti memisahkan dua
permukaan yang bergerak dengan memberikan selaput
pelumas diantara dua permukaan
tersebut.
Fungsi
•
Untuk mengatasi terjadinya gesekan
•
Menyerap panas komponen
•
Membersihkan kotoran
•
Merapatkan kelonggaran
•
Meredam getaran dan kejutan pada sambungan karena
gerakan tenaga
yang selalu berubah,seperti terjadi pada bantalan
jalandan duduk,pin
piston,dan bushing poros nok
ANNISA DYAN ANGESTI
XII AP 1
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