A cutaway turbofan tells you what the cowling hides: a jet engine is not one machine but four, bolted end to end and turning at different speeds on shafts nested inside each other. (Link in bio to get one) Air enters through the fan. On a modern high-bypass engine, roughly nine tenths of that air never reaches the combustor at all. It is accelerated by the fan and dumped straight out of the bypass duct, and it generates around 80 percent of total thrust. The core exists mostly to turn the fan. The remaining tenth goes through the compressors. Alternating rows of rotating blades and stationary vanes squeeze the flow stage by stage, each one adding pressure and temperature. Overall pressure ratios on current engines exceed 50:1, and air leaving the final compressor stage sits around 600 to 700 degrees Celsius before a single drop of fuel is injected. Combustion then happens at roughly constant pressure. Fuel burns continuously in an annular chamber and gas leaves it beyond 1700 K. Th...