I was thinking (sometimes a rare occurance!), and was pondering how one could make an external-combustion engine on an aircraft that would operate at relatively low speeds rather than at hypersonic speeds. I'm an Aero major, so these sort of questions get me going. I wondered what would happen if there wasn't enough compression to trigger the fuel combustion, and instead an alternate method of triggering the fuel were used, like an ultra-hot metal strip or an electric arc or something were placed in the flow of fuel-air mix. But that's when the question hit me... Why is fuel, in any practical engine, compressed before it is burnt? I know there are fuel-air mixtures that burn without initial compression, and the resulting expansion could be used to perform work. So then why is the fuel-air mixture in almost any engine compressed before it is burnt? The idea is that if you could have such an engine that needed no compression, one could make a very simple engine that needed no method of compression. Naturally this can't be true otherwise it would be done. Any explenations? Thanks!
I think that compression is required for optimum efficiency in internal combustion engine (or a diesel). I think a low efficiency engine could be designed to operate using a fuel/oxygen mixture at atmospheric pressure. BTW: Some (if not all) diesel engines function without a spark plug. I think that jet engines function without compressing fuel. Steam engines function without compressed fuel and are very powerful (perhaps more powerful than any internal combustion engine).
My hypothesis is that the compressed fuel once released allows the combustion to occur at specified area to maximize the force exertion.
The main reason for compression is to accelerate the burn rate, otherwise unburned fuel will be in the exhaust wasting energy and reducing performance.
Without compression the fuel wouldn't explode, I don't think. If you lose compression on a car engine, the power is reduced greatly. Diesel is ok once the engine's running, but to start it off you need glowplugs if it's cold.
THE ENGINE IS DESIGNED THAT WAY,,,, if the piston is all the way out..... its all the way out... it doesnt go out farther,,,,, we need the exploding burning gases to move the piston the greatest distance and with the greatest force... if the piston was all the way out... the exploion would do no work. if the piston is all the way in.... and the gas fully compressed... then when it explodes it can reach a higher pressure and do alot of work, by pushing the piston OUT..... thats why. but thats only for internal combustion piston engines. -MT
