Brayton Cycle & Jet Engine

March 18, 2013 by Jerry Jin 2 Comments

Recently in my ChE Thermodynamics class, we studied the Brayton Cycle. Brayton Cycle is a thermodynamics cycle that specializes in the production of work through combinations of constant pressure and constant heat supplies. It is commonly served as a jet engine because in this cycle the power is available as kinetic energy in the jet of exhaust gases leaving the nozzle. Therefore, a lot of aircraft and rockets use this as the their power source. A Brayton Cycle aka Jet Engine consists of three main components: a gas compressor, a combustion chamber, and an expansion turbine.

In general, the process goes as follows:

First, air is drawn into the compressor through the blades, a diffuser is sometimes used to reduce the work of compression in order to increase efficiency.

Second, after air is compressed and pressurized, it is sent to the combustion chamber where fuel is injected and combustion happens, creating really hot gases.

Then, hot gases pass through an turbine where it expands and gives up energy, the work extracted by the turbine is used to drive the compressor.

Next, the rest of the work created goes to the nozzle where the exit velocity is increased.

Finally, the difference in entering air and exiting air velocity creates a thrust that eventually propels your aircraft forward.

In order to better understand how Brayton Cycle works, you can look at the Temperature vs. Entropy diagram for this process. In an actual non-ideal cycle, the gas is first adiabatically (constant heat) compressed, and heat is added isobarically (constant pressure), followed with a adiabatic expansion and isobaric rejection of the gas. In this diagram to the left, the black lines are the ideal Brayton Cycle, where during the compression and expansion, gas is operated under constant heat and constant entropy. The blue lines are how the actual process is run. The reason for the difference between the two lines is that in real world, entropy tends to go in the direction of increase, stated by the ‘Second Law of Thermodynamics’. And due to thermal irreversibility, it’s hard to get it to work an ideal Brayton Cycle, but close enough! In general, the hotter the gas entering the turbine, the higher the efficiency of the engine. However, the strength of the metal turbine blades determines how hot the gas could be, which will the limiting factor for maximum efficiency.

Here is a video that explains in more detail about how the Brayton Cycle and Jet Engine works:

Sources:Frank A. Taylor (1939), “Catalog of the Mechanical Collections Of The Division Of Engineering”, United States National Museum Bulletin 173 (United States Government Printing Office)