Is it possible to make a jet engine that doesn’t use a turbine (like a simple furnace or piston) and what other fuels could be used in jet engines?
You may have heard of jet engines called “turbojets” or “turbofans.” How necessary is this “turbo” really? Could we use something else? And are there alternatives to kerosene?
Coincidentally, we already don’t use turbines on some planes! But simple furnaces and flasks won’t do the trick in these spots — and you probably won’t be seeing them on commercial airliners any time soon.
dr Maciej Mazur, an associate professor at RMIT University’s School of Engineering, says that turbojet and turbofan engines are the two most common types of things we refer to as “jet engines.”
“As the name suggests, these engines are equipped with turbines,” says Mazur.
In both turbojet and turbofan engines, the turbine’s primary job is to extract energy from the high-velocity stream of hot gases and convert it into rotary motion. This drives a compressor: air in a jet engine must be compressed so that it is hot and explosive enough to produce the right thrust when combusted with fuel. You could write a whole extra article on how these engines work in more detail (and we have that), but in short: the hot, burned gases create thrust, or “thrust” when pushed out of the end of the jet engine.
“With the turbojet engine, all the air flows through the engine itself, and the thrust is generated only by the exhaust flow,” says Mazur.
“In a turbofan engine, not only does the turbine drive the compressor, it also drives a fan at the front of the engine that directs some air through the engine and slightly outside the engine. The thrust is generated by a combination of the engine’s exhaust gases and the air that is ducted around the engine by the fan.”
This addition of redirected air makes the turbofan engine more efficient and increases its thrust.
“For both turbofan and turbojet engines, compression of the air entering the engine prior to mixing with fuel and combustion is a key factor that enables high performance and efficiency,” says Mazur.
So we don’t necessarily need a turbine – we just need something that compresses air. Could we use something else? Mazur says other means are possible.
“In fact, several different designs have been developed over the years, including ramjet and scramjet engines. Rather than using a turbine-driven supercharger system, these engines rely on the engine’s rapid forward motion to compress the incoming air with a ram effect,” says Mazur.
“The air is then directed into the engine’s combustion chamber, where it is mixed with fuel and burned to create hot gases that create thrust as they exit the engine. Due to the high airspeed required to produce sufficient air compression, ramjet and scramjet engines can only be operated at high speeds, typically supersonic or hypersonic.”
Because they can only operate at very high speeds, ramjet and scramjet engines cannot take off or accelerate on their own—they need a rocket or other means of getting fast enough to start work.
“They have the advantage that they have a simpler mechanical structure due to fewer moving parts,” says Mazur.
“However, they present other challenges associated with controlling supersonic airflow, effectively mixing with fuel, achieving fast fuel burn, and dealing with the high temperatures in the engine and aircraft.”
Other jet engines can completely eliminate the compression problem but are not as efficient.
“There are also jet propulsion engines that do not require intake air compression and can be operated at static or low speeds like pulsejet engines,” says Mazur.
Pulsejet engines “operate on the principle that an air-fuel mixture in the combustion chamber is periodically ignited, producing a pulsating jet of exhaust gas that intermittently produces thrust”.
“These motors are also simple in design, but typically have a low level of efficiency,” says Mazur.
And there are other types of jet engines in the works. One, called a rotating detonation engine, works by detonating fuel rather than burning it (deflagrating) — that is, burning it with a flame that expands so quickly that it breaks the sound barrier and becomes supersonic.
RDEs inject fuel and oxidizer into an annular (annular) chamber, causing a rotating detonation wave.
“RDEs are mechanically simple, but present many technical challenges in terms of maintaining sustained operation, stable combustion and acceptable sustainable operating temperatures, and are the subject of ongoing research,” says Mazur.
Incidentally, pulsejet engines could also be made more efficient by detonation, but that poses a number of other engineering challenges, so there is much less research interest in them.
So yes, turbine-less jet engines are possible – but you probably won’t see them on the Sydney to Melbourne route any time soon.
What about fuels? Currently, turbojet and turbofan engines use kerosene, gasoline or naphtha as fuel – all chemically very similar to kerosene. In theory, they could easily use other fuels – although Mazur points out that kerosene is also a useful lubricant, helping the engine run smoother.
dr Alan Finkel, a former chief scientist and current government special adviser on low-emission technology, says kerosene-derived fuel is popular because it is a “fuel with high energy density and excellent properties such as ease of handling and operability at very low and high ambient temperatures.” “.
However, high-emission kerosene is not always the best option.
“There is a constant search for carbon-free alternatives,” says Finkel.
“Biofuels are the most tried, but they are not zero emissions and they are costly. There is talk of synthetic aviation fuels made from zero-emission hydrogen and carbon dioxide from industrial processes. That would be zero-emission fuel, but very expensive.
“For short-haul aircraft, some prototypes use batteries to power electric motors to spin the turbine.”
Finkel also points out that hydrogen is being researched in both propeller engines and jet turbines. Last month, Airbus announced they were testing hydrogen to fly an A380, with plans to launch hydrogen flights by 2035.
But for now, unless you’re traveling at the speed of sound, turbines and kerosene remain paramount.
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