How Do Jet Engines Work?
Pilots must have a strong understanding of every aspect of an airplane to ensure a safe and efficient flight. This guide explores the question: how do jet engines work?
For those born into the jet age, it is easy to take this technology for granted. Even when on a jet, long journeys such as from Florida to Hawaii can still take hours, but imagine undertaking the flight without the powerful speeds jet engines can provide. Pilots who fly jets require a type rating and other certifications which are beyond the private certificate, and those who are authorized to act as pilot in command (PIC) dedicate study time to learning about how jet engines work. Having a strong grasp of this enables pilots to aviate safely, more efficiently, and with a greater understanding of how the engine is working with aerodynamic forces to land, cruise, and take off again.
Development of the Jet Engine
To fully appreciate the importance of the jet engine and its place in aviation, it is best to know how they came to be and what they have for the most part replaced. Early aviation dreamers sketched prototypes of jet engines before even balloons and gliders became possible. Before jets, airplanes were (and many still are) powered by piston engine, propeller-driven engines. While the development of turboprop engines helped to increase the speed, thrust, and power of airplanes, aeronautical engineers still struggled to harness jet power.
As with most aircraft innovation, jet engine innovation was propelled by war. A handful of the first aviation pioneers, among them Samuel Langley, were funded by the U.S. War Department to achieve powered human flight so that it could be deployed as a weapon. Although the first Wright Brothers flight took place only a few years before the outbreak of World War I, aviation technology quickly advanced during the war to the point where airplane-to-airplane dogfighting took place in open cockpit aircraft.
World War II prompted scientists and engineers racing to develop not only rocket and missile technology, but also jet engines. As early as 1939, jet engines were in existence but mostly in laboratories. German physicist Hans van Ohain developed a workable jet engine which could be used in a fighter. The airplane itself was constructed by Messerschmitt and called the Me 262. Like all jets, the airplane consumed an enormous amount of fuel, and engineers struggled with this early version as it was difficult to keep it in the air when consumables were in high demand. It did not fly much, but it was a strong first step. At the same time, British innovator Frank Whittle developed his own jet engine, which was used in the Gloster Meteor. It was occasionally deployed as a defensive measure, but its relative lack of speed made it impractical for overseas combat.
After the war, the application of jet engines turned to passenger airlines. Once this was achievable, jet travel became much cheaper and more accessible. The jet age is generally accepted as beginning in 1958, when the now-defunct Pan American Airlines began overseas jet service on Boeing 707s.
Principles and Mechanics of Jet Engines
The tremendous speed of a jet engine works on the Third Law of Physics (“For every action is an equal and opposite reaction.”) The Third Law is set into motion by thrust generated by the gas turbines within. At the front of a jet engine, a fan sucks in air. (If you look head-on at a jet engine on a passenger jet aircraft, you can see the blades of this fan.) The air is then retained inside the engine, where a compressor places it under pressure. The compressor contains more fans, all of which are fitted with blades and fastened to a shaft.
After these fans have done their work of compressing the air, fuel is introduced. A spark is then ignited, causing the mixture of fuel and air to burn. This combination then rapidly expands and is directed through a nozzle, which is placed at the back of the engine. This concentrated energy is the thrust which propels the airplane. The reaction takes place with extreme speed and the turbines in most modern jet engines spin over 10,000 times a minute. Colloquially, many flight instructors describe this process to their students as “suck, squeeze, bang, blow.”
What is in fuel mixture which causes this powerful reaction? Jet fuel is technically known as aviation turbine fuel, or ATF. While initial jet engine experiments used steam power and early piston engines ran on gasoline. Modern jet engines fly on kerosene-based fuel, and have done so since the end of World War II and the aviation world, it is commonly abbreviated as “avtur.”
ATF is usually clear or light yellow. It consists of a mixture of hydrocarbons, and for safety reasons is processed with international specifications and standards. In commercial aviation, most jet engines use fuels known as Jet A and Jet A-1. The difference between Jet A and Jet A-1 is that Jet A freezes at 40 degrees below zero, and Jet A-1 does so at -53 degrees. Most general aviation airplanes using turbine engines use a compound called Jet B, a performance type which is specifically designed for cold weather.
What is the Difference Between Jet Engines and Turboprop Engines?
Jet engines do not use propellers; the “propellers,” so to speak, are inside the airplane engine in the function of the fan. However, they are inefficient, and jet fuel is expensive. Turboprop airplanes are a combination of modern engineering and innovative use of lightweight materials.
If the turboprop is considered a transition aircraft between piston-driven airplanes and jets, why are airplanes containing them still flown? Turboprops are becoming rarer, but they are still seen on regional airlines and general aviation aircraft. They are favored by many pilots because they are generally less automated and much more efficient on shorter journeys. For example, it makes sense to fire up a jet engine to fly from Maine to Nevada, but a shorter hop from Colorado to New Mexico is more efficiently served on less fuel. Turboprops are more typically the airplane of choice in these circumstances.