A turbojet engine is a type of internal combustion engine primarily used to power aircraft. Air enters through an intake and is forced into a rotating compressor. The compressor increases the air’s pressure before it flows into the combustion chamber. Inside the combustion chamber, fuel is mixed with the compressed air and ignited by a flame within a flame holder.
This combustion process dramatically raises the gas temperature. The hot gases then expand through the turbine, which extracts power to drive the compressor. As the gases exit the turbine, their temperature and pressure are still significantly higher than the surrounding atmosphere. These gases are then forced through a propelling nozzle, accelerating them to create a high-velocity jet. If the jet’s velocity exceeds the aircraft’s speed, it generates forward thrust.
The compressor prevents backflow, ensuring continuous operation. The process can be compared to a four-stroke cycle, but with all stages occurring simultaneously. Engine efficiency relies heavily on the pressure ratio (combustion chamber entry pressure divided by intake pressure) and the turbine inlet temperature.
Compared to propeller engines, turbojets accelerate a small mass of air to a high velocity, while propellers accelerate a large mass of air to a lower velocity.
Turbojet exhaust is most efficient at high speeds, especially supersonic speeds, and high altitudes. Turboprops, gas turbine-powered propeller engines, are more common and efficient for slower, shorter-range flights. Small aircraft often use piston engines to drive propellers, but smaller turboprops are becoming increasingly prevalent.
The basic turbojet design involves a single shaft connecting the compressor and turbine. More advanced designs use two concentric shafts to improve compressor stability during throttle changes. The outer shaft connects the turbine to the high-pressure compressor, forming the engine’s core, or gas generator. The inner shaft connects the low-pressure compressor to the low-pressure turbine, forming the LP spool. Both spools can operate at their optimal speeds.