Couldn’t the compressed air by itself make a plane fly? Or does it need that extra energy from the combustion chamber to make a plane fly?
In: Engineering
A compressed air engine is basically like a steam engine; it works, but the fuel storage requirements are high. Airplanes wouldn’t be viable; you wouldn’t have enough compressed air to go very far.
Gasoline engines, and similarly jet engines, release and use the huge amounts of chemical energy “trapped” in gasoline / jet fuel. To understand the amount of extra power, let’s use the gasoline engine as an example: with 4 cylinders, at any time, one is sucking in air and fuel, one is pushed to compress that air/fuel to an extremely high pressure, one is releasing the burnt smoke and carbon dioxide, and one cylinder is in the process of conflagrating (it’s not quite a military-grade explosion) the air/fuel *and creating power*.
That one cylinder produces enough power to not just compress air and otherwise “overpower” the other 3 cylinders, but also run the oil and water pumps attached to the motor, the alternator (generator) for recharging the battery, and your air conditioning system, with enough power left to peel tires or go 0-60 in a few seconds.
There are a number of jet engine designs, but in all of them the combustion overpowers air compression and other “ancillary” systems attached to the engine, so basically without the energy released by combustion it just wouldn’t work.
I’m assuming you’re talking about like on a commercial airliner, which uses a turbofan jet engine.
The combustion chamber is what is generating the energy to drive the fan to begin with, so it is quite necessary. It needs to be turned somehow! With a regular old engine that would just be a propeller plane, or using the turbine separate from the fan would be a turboprop.
What it comes down to is this: propeller planes are good for lower speeds, we’ve had them for a century now. The jet engine comes along (in the form of the turbojet) and revolutionizes airplanes as now they can go fast — *really* fast. But turbojets (where all the air goes through the gas turbine engine itself) were not very good at lower speeds. So a turbofan combines the “propeller low speeds” and the “turbojet high speeds” into a medium speed engine perfect for commercial airliners that want to go fast but not *too* fast.
So why not use just the air from the fan itself? Because of that medium speed they want, and because if the jet exhaust isn’t also being used to go forward, that’s wasted energy, Just like how the heat in the exhaust or the radiator of your car is energy that got wasted by not going to the wheels. Turbofan engines put 100% of energy produced into propulsion (minus frictional losses and any power sipped off to run the airplane electric system and cabin pressurization).
It’s efficient and it’s right in the perfect performance zone for a 747.
What energy do you use to compress the air? Compression of air requires energy and it cant come out of nowhere.
A jet engine has a compressor in the front, the combustion, and then a turbine that is used to drive the compressor. If the compressor drives the turbine the best you could evet get is the same amount of energy and you put into the compressor so no energy to propel the aircraft. In practice, none of them are 100% efficient so it would just stop.
You can compare it to if you use an electric motor to drive the generator that power it. There is losses and it would just stop.
The combustion increases the temperature and the speed of the gas when it passes over the turbine so you have a net increase in energy that can drive the compressor and push out the exhaust to propel the aircraft.
Without a combustion chamber it’s just a ducted propeller plane. So yes it can fly*, but not if you want to be able to travel high, fast, far, and carrying a lot of stuff.
*They could build planes around such engines. Current jets can not fly without fuel.
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