A turbojet engine is a heat engine that turns the chemical energy of fuel into the kinetic energy of a fast exhaust jet. It matters because the thrust that moves an aircraft forward comes from changing the momentum of air passing through the engine. The turbojet follows the Brayton cycle, the same ideal thermodynamic cycle used to model gas turbines.
Understanding this cycle helps engineers predict thrust, fuel use, temperature limits, and engine efficiency.
Air enters the intake, is compressed to high pressure, receives heat from burning fuel in the combustor, expands through the turbine, and then accelerates through the nozzle. The compressor raises the air pressure and temperature, while the combustor adds energy mostly at nearly constant pressure. The turbine extracts enough work from the hot gas to drive the compressor, leaving the remaining energy to form a high-speed exhaust.
The nozzle converts thermal and pressure energy into jet velocity, producing thrust by Newton's third law and momentum conservation.
Understanding Engineering: Jet Engine Thermodynamics
The Brayton cycle is easiest to understand by tracking energy per kilogram of air. Compression needs work, so the air leaves the compressor hotter than it entered. A higher compressor exit pressure divided by inlet pressure usually improves the ideal thermal efficiency.
The reason is that more of the added heat can be turned into useful expansion work. However, raising pressure too far is not free. The compressor requires more shaft work, produces more heating, and may become less stable.
If airflow separates from compressor blades, the compressor can surge or stall. This causes a sharp loss of pressure rise and can damage the engine. Blade shape, blade angle, and variable guide vanes help keep the airflow stable over different speeds and altitudes.
The turbine is connected to the compressor by a shaft, so its main job is not to create thrust directly. It must remove enough energy from the hot gas to keep the compressor turning. Engineers call the difference between turbine work and compressor work the net work available from the core.
A turbine cannot take all the gas energy because the nozzle still needs high pressure and temperature to accelerate the exhaust. This creates a design balance. Taking more turbine energy can support a larger compressor, yet it can leave less energy for the jet.
The maximum temperature after combustion is especially important. Higher temperature can improve performance, but turbine blades face extreme thermal stress, oxidation, and creep.
Creep is slow permanent deformation caused by high temperature over long periods. Modern turbines use heat resistant alloys, internal cooling passages, and protective ceramic coatings to survive.
The nozzle shows why pressure alone is not the final goal. A nozzle changes random thermal motion into directed motion. As gas expands, its temperature falls while its speed rises.
Under some conditions, the gas reaches the speed of sound at the narrowest part of the nozzle. This is called choking. Once choked, changing the pressure farther downstream does not immediately increase the mass flow through the narrow point.
A converging nozzle works well for many subsonic aircraft engines. Supersonic exhaust needs a converging diverging nozzle, with a narrow throat followed by an expanding section. The pressure at the nozzle exit matters too.
If it differs from outside air pressure, there is an extra pressure contribution to thrust. Engines are designed for particular flight conditions, so their nozzle performance changes with altitude and aircraft speed.
Real engines differ from the ideal cycle because every component has losses. Compression and expansion are not perfectly reversible. Air friction causes pressure drops in ducts and the combustor.
Fuel does not burn with perfect uniformity. Bearings and gears lose some shaft power. The intake must slow incoming air before it reaches the compressor, which becomes difficult at high flight speed.
Students should separate ideal models from real measurements. On a temperature versus entropy diagram, ideal compression and expansion have no entropy increase, while real processes move toward higher entropy.
When solving problems, state the control volume, use consistent units, and identify whether a value refers to total conditions or static conditions. Total temperature includes the effect of flow speed, while static temperature describes the moving gas itself.
Key Facts
- Ideal Brayton cycle steps: isentropic compression, constant-pressure heat addition, isentropic expansion, constant-pressure heat rejection.
- Compressor pressure ratio: r_p = P2 / P1, where P2 is compressor exit pressure and P1 is inlet pressure.
- Ideal Brayton efficiency: η = 1 - 1 / r_p^((γ - 1)/γ), for an ideal gas with constant γ.
- Thrust from momentum change: F = ṁ(V_exit - V_inlet) + (P_exit - P_ambient)A_exit.
- Compressor work per unit mass is approximately w_c = c_p(T2 - T1).
- Turbine work per unit mass is approximately w_t = c_p(T3 - T4), and in a turbojet much of it powers the compressor.
Vocabulary
- Brayton cycle
- The ideal thermodynamic cycle for gas turbines, made of compression, heat addition, expansion, and heat rejection processes.
- Compressor
- The rotating engine section that raises the pressure and temperature of incoming air before combustion.
- Combustor
- The chamber where fuel mixes with compressed air and burns to add thermal energy to the flow.
- Turbine
- The rotating section that extracts energy from hot gas to drive the compressor through a shaft.
- Nozzle
- The duct at the engine exit that accelerates gas to high speed to create thrust.
Common Mistakes to Avoid
- Treating the turbine as the part that directly pushes the airplane forward is wrong because the turbine mainly powers the compressor, while thrust mostly comes from the exhaust jet leaving the nozzle.
- Assuming pressure stays high through the nozzle is wrong because the nozzle converts pressure and thermal energy into velocity, so static pressure usually drops as speed rises.
- Ignoring the inlet air speed is wrong because thrust depends on the change in momentum, so F is related to V_exit - V_inlet, not just exhaust speed alone.
- Using the ideal Brayton efficiency without checking assumptions is wrong because real engines have compressor losses, turbine losses, pressure drops, heat limits, and nonideal combustion.
Practice Questions
- 1 A turbojet ingests air at 250 m/s and exhausts it at 650 m/s. If the air mass flow rate is 40 kg/s and the pressure thrust term is zero, calculate the thrust.
- 2 For an ideal Brayton cycle with γ = 1.4 and compressor pressure ratio r_p = 9, calculate the ideal thermal efficiency using η = 1 - 1 / r_p^((γ - 1)/γ).
- 3 Explain why increasing turbine inlet temperature can increase turbojet performance, and also explain one engineering limit that prevents it from being increased without bound.