Since aircraft are mission-oriented and designed according to use, there are a wide-range of engine types. Commercial aircraft in particular have long relied on gas turbine engines as their preferred means of propulsion. Turbofan and turboprop engines fall into the gas turbine category of engine types. Through a process of combustion, they produce a large amount of energy. A fan sprays air into the engine from the front, while a compressor increases the air pressure.
Turbofan and Turboprop Engines
With a modern turbofan, the fan sucks in air via an inlet, 80-90% of which is exhausted through the fan nozzle to provide a majority of the thrust generated by the engine. The remaining air is pressurized in the compressor and is either used for cooling or mixed with fuel and burned in the combustor. Meanwhile, exhaust gasses from the combustor travel through the turbine, producing the necessary mechanical energy to turn the shaft that drives the fan and compressor. The gasses exiting the turbine pass through the exhaust nozzle at high speed, which provides additional thrust. Turboprop engines are similar in design to turbofans, but the primary difference is that turboprops utilize a propeller in free air to generate thrust rather than taking advantage of a fan in a nacelle.
Efficiency of Gas Turbine Engines
Gas turbine engines are designed with overall efficiency, weight, additional drag, and reliability in mind. Overall efficiency refers to the efficiency with which the engine produces propulsive power. The most efficient commercial aircraft gas turbines being devised today have takeoff thrusts of 20,000 pounds or more. They usually operate at cruise speeds, with motor thermodynamic efficiencies of up to 55% and propulsive efficiencies of over 70%.
Cost effectiveness is a major factor in the popularity of gas turbines, alongside efficiency and weight. This cost includes manufacturing costs, maintenance costs, and fuel costs. Generally, operators often sacrifice weight for efficiency and reduced maintenance costs.
Gas Turbine Characteristics
Gas turbine engines have many characteristics that set them apart from other power plants like internal combustion engines and electric drives. All engines produce waste heat that must be expelled. With gas turbine engines, the heat from the spent fuel travels out as exhaust, producing additional positive thrust. At constant throttle settings, a modern turbofan engine’s thrust varies with speed and altitude. Furthermore, most gas turbine engines are optimized to burn little fuel during cruise.
Large commercial engines are designed for long-range aircraft, wherein fuel consumption is not a consideration. This is because there is a trade-off between engine weight and fuel consumption when an aircraft has a longer range. Smaller engines, on the other hand, are designed for short-range commercial aircraft and will have many more daily flight cycles, necessitating frequent refueling and maintenance. While larger engines are optimized for higher efficiency, smaller engines provide advantageous lightweight characteristics and more flight cycles. Put more simply, small engines are not designed with the same efficiency as large engines.
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