Although the propeller-driven airplane is not nearly as efficient  as the jet, particularly at the higher altitudes and cruising  speeds required in modern aviation, one of the few advantages  the propeller-driven airplane has over the jet is that maximum  thrust is available almost at the start of the takeoff roll. Initial  thrust output of the jet engine on takeoff is relatively lower  and does not reach peak efficiency until the higher speeds. The  fanjet or turbofan engine was developed to help compensate  for this problem and is, in effect, a compromise between the  pure jet engine (turbojet) and the propeller engine. A jet engine is a gas turbine engine. A jet engine develops  thrust by accelerating a relatively small mass of air to very  high velocity, as opposed to a propeller, which develops  thrust by accelerating a much larger mass of air to a much  slower velocity. Piston and gas turbine engines are internal combustion  engines and have a similar basic cycle of operation; that  is, induction, compression, combustion, expansion, and  exhaust. Air is taken in and compressed, and fuel is injected  and burned. The hot gases then expand and supply a surplus  of power over that required for compression and are finally  exhausted. In both piston and jet engines, the efficiency of  the cycle is improved by increasing the volume of air taken  in and the compression ratio. Like other gas turbine engines, the heart of the turbofan  engine is the gas generator—the part of the engine that  produces the hot, high-velocity gases. Similar to turboprops,  turbofans have a low-pressure turbine section that uses most  of the energy produced by the gas generator. The low pressure  turbine is mounted on a concentric shaft that passes through  the hollow shaft of the gas generator, connecting it to a ducted  fan at the front of the engine.  [Figure 15-2] Part of the expansion of the burned gases takes place in the  turbine section of the jet engine providing the necessary  power to drive the compressor, while the remainder of the  expansion takes place in the nozzle of the tail pipe in order  to accelerate the gas to a high velocity jet thereby producing  thrust.  [Figure 15-1] Air enters the engine, passes through the fan, and splits into  two separate paths. Some of it flows around—bypasses the  engine core, hence its name, bypass air. The air drawn into the  engine for the gas generator is the core airflow. The amount  of air that bypasses the core compared to the amount drawn  into the gas generator determines a turbofan’s bypass ratio.  Turbofans efficiently convert fuel into thrust because they  produce low-pressure energy spread over a large fan disk area.  While a turbojet engine uses the entire gas generator’s output  to produce thrust in the form of a high-velocity exhaust gas  jet, cool, low-velocity bypass air produces between 30 percent  and 70 percent of the thrust produced by a turbofan engine. In theory, the jet engine is simpler and more directly converts  thermal energy (the burning and expansion of gases) into  mechanical energy (thrust). The piston or reciprocating  engine, with all of its moving parts, must convert the thermal  energy into mechanical energy and then finally into thrust  by rotating a propeller. One of the advantages of the jet engine over the piston engine  is the jet engine’s capability of producing much greater  amounts of thrust horsepower at the high altitudes and high  speeds. In fact, turbojet engine efficiency increases with  altitude and speed. The fan-jet concept increases the total thrust of the jet engine,  particularly at the lower speeds and altitudes. Although  efficiency at the higher altitudes is lost (turbofan engines are  subject to a large lapse in thrust with increasing altitude), the  Jet Engine Basics Figure 15-1.  Basic turbojet engine. Direction of flight Ducted fan Gas generator Concentric shaft Low-pressure turbine 15-2