The four strokes of jet propulsion
The jet engine cycle can be remembered as suck, squeeze, bang, blow. Air is drawn into the intake, compressed by fan and compressor stages, ignited in combustor cans with fuel, and exhausted at high velocity through the turbine and nozzle. The pressure increase from compression raises the air temperature dramatically, which amplifies the energy release from combustion. Most energy exits as the high-velocity exhaust jet that creates thrust.
Unlike a piston engine that creates power strokes intermittently, a jet engine operates continuously: air flows through all stages at once, with the turbine extracting just enough energy to drive the compressor, allowing the remaining energy to accelerate the exhaust. This continuous flow design is thermodynamically efficient and scales well to the high pressures and temperatures modern turbines can withstand.
Compression ratio and thermodynamic efficiency
Modern turbofan engines achieve compression ratios of 40:1 or higher: air entering at sea-level pressure is compressed to 40 times that pressure. This high compression raises temperature to 400-500 degrees Fahrenheit, heating the air and enabling more efficient combustion. The turbine must withstand these extreme conditions, requiring exotic superalloy blades that operate near their melting points.
The bypass ratio of turbofan engines (the fraction of air that bypasses the core and is accelerated by the fan) has steadily increased because bypassed air contributes to thrust at high efficiency. Modern engines achieve 10:1 bypass ratios, meaning nine parts of the thrust comes from bypassing air and one from the hot exhaust core. This shift has made jets progressively quieter and more fuel-efficient, as thrust from low-speed fan air is more efficient than thrust from hot exhaust.