Now both the reciprocating piston engine and the jet engine are internal combustion engines. They combust fuel within the confines of the engine. And they both do intake, compression, combustion and exhaust. in the reciprocating piston engine these events occur one after the other. The air and fuel comes in, we then compress this mixture, after that it’s combusted and then exhausted.
But in the jet engine all of these intake, compression, combustion and exhaust occur constantly and simultaneously with each other.
Although they seem modern, the jet engine and the piston engine actually rely on two of the most ancient mechanical devices known to man. The piston engine relies on a crankshaft and crankshafts have been used as early as the 2nd century AD in the Roman empire. Inside the piston engine the crankshaft converts the reciprocating motion of the piston into rotation which is then ultimately used to turn a propeller or the wheels of a vehicle.
But the jet engine at it’s core has something even more ancient. And that something is a turbine. Now turbines are present all around us and they’re powered by all sorts of fluids. Wind in the case of wind turbines. Steam in the case of thermoelectric power plants. Water in the case of hydroelectric power plants. Exhaust gasses in the case of turbochargers present in cars and trucks.
The big difference between ancient turbines like windmills and watermills and modern turbines like turbochargers or jet engines is that we have realized that we can create much more power if we increase the energy of the fluid that’s being fed into the turbine.
To better understand and appreciate the Jet Engine (turbojet in this case) we will split and analyze it in segments. We begin with the intake. The two main parts of the intake are the cone and the inlet guide. The cone serves the purpose of reducing drag and helping the air to smoothly enter the engine. The inlet guide directs and evens out the air entering the engine and also serves the purpose of protecting the engine from large foreign objects.
Once the air is in the engine the first thing it meets is the compressor section. As the name implies this section has the task of compressing the air. It consists of rotors and stators. Again, as the name implies the rotors rotate whereas the stators are stationary. The rotors suck in and push the air against the stators and force it into an ever smaller space. This compresses the air which increases its pressure and temperature. Or in other words it’s potential energy.
As you can see the compressor section consists of multiple rotors and stators with decreasing blade size. This tells us that the air inside a jet engine is compressed in stages. The air enters the compressor section at atmospheric pressure which is 14.7 psi, by the time it exists the compressor section that air will have a pressure of around 70 psi. This is a very significant pressure increase and if we tried to achieve this amount of compression with a single rotor and stator and a dramatic reduction in space we would likely encounter compressor stall which is a disruption in the airflow that can reduce power if it’s mild or even damage the compressor if a complete disruption disruption of the airflow occurs, at that point it is referred to as a compressor surge. Compressing the air in stages and incrementally increasing it’s pressure is more efficient and it helps prevent compressor stall. This compressed air now heads towards the combustor. Here fuel is administered and mixed with air. The fuel also carries its own potential energy. The air and fuel mixture is then ignited most often using some form of sparking device. The mixture combusts which causes it to rapidly expand greatly increasing the temperature and pressure. We now have an absolutely massive amount of energy heading towards the turbine. The turbine captures or harnesses this energy which causes it to rotate at an increasing speed.
Now the turbine is connected via a common shaft with the compressor wheel. This means that an increased turbine speed leads to an increased compressor speed. A faster spinning compressor sucks in ever more air which is then fed into the combustor increasing the strength of combustion which then increases the speed of the turbine.
As you can see the jet engine effectively feeds itself. The faster the turbine spins the more air is sucked in and the more powerful the combustion becomes. So does this mean that there is essentially no limit to the power that a jet engine can produce?
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