AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Dumb jet engine 101 question, the combustion chamber          AirTalk.org Forum Index -> Aviation (Students) View previous topic :: View next topic   Author Message P S Guest Posted: Sun May 06, 2007 4:40 am    Post subject: Dumb jet engine 101 question, the combustion chamber I came across a number of tutorials, including the FAA's airplane flying handbook, with a section describing how a jet engine works. There would be a cross-section picture, showing the Turbojet, or Turbofan engine. The air comes in through the inlet, going through a multi-stage compressor, then into the combustion chamber, where the fuel is injected and burned. The physics of the last part is not intuitive. In a piston engine, when the spark plug fires, both the inlet and exhaust valves are closed, so the burning gas can not escape but only expand by pushing the piston out. In a jet engine's combustion chamber, there is no valves to close for the intake air/fuel. The burning gas would push out the rear end, but wouldn't it also go back up to ward the combustion chamber ? Effective this stops the air from coming into the combustion chamber. Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? P S Back to top Mxsmanic Guest Posted: Sun May 06, 2007 5:03 am    Post subject: Re: Dumb jet engine 101 question, the combustion chamber P S writes: Quote: The physics of the last part is not intuitive. In a piston engine, when the spark plug fires, both the inlet and exhaust valves are closed, so the burning gas can not escape but only expand by pushing the piston out. In a jet engine's combustion chamber, there is no valves to close for the intake air/fuel. The burning gas would push out the rear end, but wouldn't it also go back up to ward the combustion chamber ? Effective this stops the air from coming into the combustion chamber. The dynamic movement of air towards the rear of the engine prevents combustion products from moving forward in normal operation. Quote: Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? There's no ignition at all under normal conditions; the heat of existing combustion and compression are sufficient to ignite fuel as it is sprayed into the combustion chamber. Igniters are used only to start the engine and under conditions where there is a risk of the self-sustaining combustion stopping (if the combustion stops by itself, the igniter will restart it immediately). Jet engines usually have a "continuous" igniter position among the controls for this latter situation, and it is used during critical flight phases when flameouts are more of a possibility, such as take-offs and descents at idle. -- Transpose mxsmanic and gmail to reach me by e-mail. Back to top Morgans Guest Posted: Sun May 06, 2007 5:29 am    Post subject: Re: Dumb jet engine 101 question, the combustion chamber The air pushing into the combustion section is pushing harder than the restriction of the gasses going out the back. This is not always the case, if operated incorrectly, or in the case of mechanical problems. That is then called a compressor stall. There is a loud bang, then usually, things get going back in the right direction. Combustion is a constant process, except for the old (not used for commercial aviation, or pretty much anywhere else) pulse jet engine, such as was found in the V-1 buzz bomb of WW II. -- Jim in NC Back to top Robert M. Gary Guest Posted: Sun May 06, 2007 6:13 am    Post subject: Re: Dumb jet engine 101 question, the combustion chamber On May 5, 4:40 pm, P S <mister...@gmail.com> wrote: Quote: Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? Others have answered this. However, when teaching the Aviation Merit Badge for BSA I find that a lot of people believe the old "you can't have fire w/o a spark". That's a good rule of thumb in the woods but not true. The correct way to say it is that "you can't have fire without reaching the flash point temperature of whatever material (air/ fuel mixture in this case) you are working with". In a jet engine the combustion is continuous so it is always at a temperature that air/ fuel will ignite. -Robert Back to top P S Guest Posted: Thu May 10, 2007 6:13 am    Post subject: Re: Dumb jet engine 101 question, the combustion chamber On May 7, 7:23 am, C J Campbell <christophercampb...@hotmail.com> wrote: Quote: On 2007-05-05 16:40:52 -0700, P S <mister...@gmail.com> said: I came across a number of tutorials, including the FAA's airplane flying handbook, with a section describing how a jet engine works. There would be a cross-section picture, showing the Turbojet, or Turbofan engine. The air comes in through the inlet, going through a multi-stage compressor, then into the combustion chamber, where the fuel is injected and burned. The physics of the last part is not intuitive. In a piston engine, when the spark plug fires, both the inlet and exhaust valves are closed, so the burning gas can not escape but only expand by pushing the piston out. In a jet engine's combustion chamber, there is no valves to close for the intake air/fuel. The burning gas would push out the rear end, but wouldn't it also go back up to ward the combustion chamber ? Effective this stops the air from coming into the combustion chamber. Not a dumb question at all. It was a big problem that had to be overcome with early jet engines. It was possible for the engine to run backwards! Hot gases will follow the path of least resistance. The compressors are bringing in a constant supply of air, so the gases are forced out the back. Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? Modern jet engines fire continuously. A glow plug is used for starting the engine, but once the engine gets going the glow plug is turned off. After that it is simply a matter of dribbling fuel into the fire in the combustion chamber. Of course, any interruption of fuel flow is likely to induce a flameout and the engine could cool to the point that it will not re-ignite on its own. -- Waddling Eagle World Famous Flight Instructor Indeed. I admire the first people who came up with the jet engine idea. That is before there was anything in existence to suggest that such a design would ever work. I feel lucky to be here after those genius. P S Back to top Peter Dohm Guest Posted: Thu May 10, 2007 9:35 pm    Post subject: Re: Dumb jet engine 101 question, the combustion chamber Quote: Hot gases will follow the path of least resistance. The compressors are bringing in a constant supply of air, so the gases are forced out the back. Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? Modern jet engines fire continuously. A glow plug is used for starting the engine, but once the engine gets going the glow plug is turned off. After that it is simply a matter of dribbling fuel into the fire in the combustion chamber. Of course, any interruption of fuel flow is likely to induce a flameout and the engine could cool to the point that it will not re-ignite on its own. -- Waddling Eagle World Famous Flight Instructor Indeed. I admire the first people who came up with the jet engine idea. That is before there was anything in existence to suggest that such a design would ever work. I feel lucky to be here after those genius. P S We, who stand upon the shoulders of giants, are fortunate indeed. Peter Back to top C J Campbell Guest Posted: Fri May 11, 2007 6:01 am    Post subject: Re: Dumb jet engine 101 question, the combustion chamber On 2007-05-09 20:43:20 -0700, P S <mistersky (AT) gmail (DOT) com> said: Quote: On May 7, 7:23 am, C J Campbell <christophercampb...@hotmail.com wrote: On 2007-05-05 16:40:52 -0700, P S <mister...@gmail.com> said: I came across a number of tutorials, including the FAA's airplane flying handbook, with a section describing how a jet engine works. There would be a cross-section picture, showing the Turbojet, or Turbofan engine. The air comes in through the inlet, going through a multi-stage compressor, then into the combustion chamber, where the fuel is injected and burned. The physics of the last part is not intuitive. In a piston engine, when the spark plug fires, both the inlet and exhaust valves are closed, so the burning gas can not escape but only expand by pushing the piston out. In a jet engine's combustion chamber, there is no valves to close for the intake air/fuel. The burning gas would push out the rear end, but wouldn't it also go back up to ward the combustion chamber ? Effective this stops the air from coming into the combustion chamber. Not a dumb question at all. It was a big problem that had to be overcome with early jet engines. It was possible for the engine to run backwards! Hot gases will follow the path of least resistance. The compressors are bringing in a constant supply of air, so the gases are forced out the back. Also, does the ignition in a jet engine happen periodically, or is it continuous, and requiring no cyclic firing, like a spark plug does in a piston engine ? Modern jet engines fire continuously. A glow plug is used for starting the engine, but once the engine gets going the glow plug is turned off. After that it is simply a matter of dribbling fuel into the fire in the combustion chamber. Of course, any interruption of fuel flow is likely to induce a flameout and the engine could cool to the point that it will not re-ignite on its own. -- Waddling Eagle World Famous Flight Instructor Indeed. I admire the first people who came up with the jet engine idea. That is before there was anything in existence to suggest that such a design would ever work. I feel lucky to be here after those genius. P S The first jet engine was built by Alexandro Ciurcu, who suggested that it be used to power a dirigible. It was tested on a small ship on the Seine River in 1886 and on a rail car in 1888. However, it was never used on an aircraft. Fellow Romanian Henri Coanda built and flew the first jet airplane called the Coanda-1910 in 1910. He did not actually intend to fly it; it accidentally took off while he was at the controls running an engine test. It darned near killed him and he never flew again. We remember him best in aviation for his discovery of the Coanda effect. -- Waddling Eagle World Famous Flight Instructor Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         AirTalk.org Forum Index -> Aviation (Students) All times are GMT Page 1 of 1   Jump to: Select a forum Aviation----------------alt.aviationAviation safety EnhancementPlanes, gliders and flying in AustraliaAviation et pilotage (Français)rec.aviationAviation (Aerobatics)Hot air, gas balloons and airshipsHang-gliding, paragliding, foot-launched flightAircraft Design & BuildingInstrument Flight RulesAviation (Classifieds)Military AircraftNaval AviationAviation (Miscellaneous Topics)Owning AirplanesAviators (General Discussions)Aviation related productsAircraft RestorationHelicopters and rotary wing aircraftsFlight SimulationSailplanesAviation (Students)Light aircraftFlying related activities in the UK Airports----------------alt.airportsEdinburgh Airport Glasgow Airport Humberside Airport Luton Airport Aberdeen Airport Glasgow Prestwick Airport  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.