Inside It: Jet Engine
Jet Engine
Sources: Thermodynamics by Cengel and Boles.
Sources: Thermodynamics by Cengel and Boles.
Jet - Propulsion Cycle
A Jet engine works on the principle of Newton's Third Law of Motion, which states "Each action has an equal and opposite reaction"
Gas turbine cycles are used to power aircrafts because:
- They are light
- Compact
- Have a high power to weight ratio
The ideal jet propulsion cycle differs from an ideal Brayton Cylce (more about Brayton Cycle here #-Link-Snipped-#).
In the turbine the gases are not expanded to ambient pressure.
They are expanded to a pressure such that the power generated by turbine is sufficient to drive compressor and some auxiliary equipment. The net work output is almost zero. The exhaust gases are at a relatively high pressure and these are accelerated in the Nozzle and thus provide the thrust to propel the craft.
Air is compressed by a compressor. Mixed with fuel in combustion chamber and the mixture is burnt at a constant pressure. High pressure and temperature gases expand partially in the turbine. Gases then expand to ambient pressure in the nozzle and leave at a high velocity.
Thrust developed is the unbalanced force due to the difference in the momentum of low velocity air at the inlet and high velocity air at the exit.
This is determined by Newton's Second Law:
F = (mV)exit - (mV)inlet = m(Vexit -Vinlet)
Power developed by the thrust is called Propulsion Power.
W = FV
Propulsion efficiency is the ratio of Propulsion Power with Energy Input.
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