how does an engine on a jet plane start?

Question

once a jet engine starts, how does it stay running?

Answer 1

Oddly enough there are several ways and there is only one way to start a brayton cycle, (turbine,) engine.

The engine works by using it's compressor section to compress air, the hot section burns fuel mixed with the compressed air to increase the pressure, which then blows through the tubine wheel. The turbine wheel powers the cold section, (compressor section,) so you have a continuous combustion.

The key to getting a turbine started is getting enough air moving through the compressor section before the igniters in the hot section are powered and fuel is introduced.

Smaller turboprop and jet aircraft use a starter/generator to spool the engine to a speed, (about 15% maximum RPM,) before the ignition engages and 20% before fuel is introduced into the combustion liners. After engine start, the swtich is pushed through the off detent to the other side and the same generator that spooled the engine becomes the generator providing electrical power to the aircraft.

Aircraft in the ranges of the Gulfstream IV and Challenger 604 and above can be started with "huffers." A huffer is basicaly a large compressor that puts enough air through the turbine wheel to bring the engine's inner stage to a sufficient speed to support combustion whe the spark and fuel are supplied. Starting internally, bleed air off the aircrafts' APU is used to spin the engine fast enough.

The fun one, however, is the blackpowder charges used to start the B-52B's #2 engine. These came in a coffee can sized charge, and produced enough expanded gas to spin the turbine, and subsequently compressor, up to come in speed and bleed air was used to start the other engines. Hittting the ignitor and selecting the wrong engine tended to fill the navigator's cubicle with smoke and carbon monoxide.

Turbines, even old turbojets, only burn about 15% of the air that the compressor produces. Bleed air refers to air high pressure air from the compressors that's used for everything from engine starting, to clearing the windscreen, to pressurizing the cabin.

And if you're thinking of building one of those toy turbine engines from an automotive turbocharger, a shotgun shell and a model rocket igniter is a bad idea.

Good Luck

Answer 2

I worked the Eng. Pratt Whitney--G>E. and Tf 41,You have 2 Half circle Manifolds,with cluster of five Nozzles-Eight Comb Chambers.The Ignition is the # 4 &# 5 Spark Igniters Last Mil Jet I ran up was B1 B ,Start sequence,First a Min of 100,000 Lbs of fuel.Start Sequence is to get Compresser speed R.P.M.22% then move Start Sw. This gives a spark to # 4 &5 also allows Fuel.To get Compressers to Speed you have three Options,you need Air Source,Use a Small Jet Eng.located between the two Engs.There are two more ways.You can do and Auto Start,by just moving the Eng. Start.If you have an air source,On start you have30 seconds
Spark to 4&5.There is also a back up ,alt Start in which 2 Min of continueous Spark,Now the Other part is how it keeps running.The only way to Kill engine Is St Sw sends Msg to Eng fuel Feed to close.It is similer to a Deisel truck,You cut Fuel off or Air,The F-105 with J75 19 W was similar,You Had to attach lg. cables to left and Right LDG. Struts to Hold A.C.(.The A 7 carried a Rolls Royce Eng ),later Allison was Prime , .You didn"t want Ground Man close to Intake,A Vacum So on Eng Run up on ground had to install Screen in front of Eng. Inlet,I never ran this one, But worked everything else. .

Answer 3

F-16s most certainly do NOT use hydrazine to start the main engine, as it's extremely toxic. H-70 Hydrazine is used to power the EPU during engine failures to maintain hydraulic and electric controls for a short time (to allow for successful glide landings). The F-16 starter is called the JFS and is a small centrifugal turbine (jet) engine which spins up the main engine through the accessory drive gearbox. The JFS itself is spun up by pressurized hydraulic cylinders activated by the start switch in the cockpit. Should the JFS fail to start the ground crew can recharge the hydraulic cylinders with a manual pump in the MLG wheel well (right side, if I remember correctly).

Also, just for fun, look up the start system on the SR-71. I remember somewhat that these giant jet engines were started by a pair of regular V-8 gasoline engines on a ground cart, but I could be mistaken...

Answer 4

I think jetteck answered the best and I can read his enthusiasm for the gas turbine. I was on a America West 737-200 in Phoenix ready for push back. The captain announced that the APU was not working so the ground crew was going to start the right engine with compressed air and then use the electrical power from the running engine to start the left engine. Needless to say this did not instill a lot of confidence in the passengers.

Answer 5

As you can see there a lot of ways to start a Jet engine. I will summarize for you and maybe it will be clearer.

1) Electrical starter - used on smaller engines

2) Air Starter - common on bigger engines and there are several potential sources of air
a) Aircrafts onboard APU can provide air for start
b) External air cart
c) Cross-start, using an already running engines bleed air

3) Cartridge start - military application using gunpowder shell to get engine started usually in a "scramble"

4) Flight start - uses in flight "windmilling" speed of engine to start.

All of these methods do one thing, start the high pressure section turning. As it spins, pressure increases to the point where if fuel and spark are introduced, the air will burn and rapidly expand.

There are certainly others that I am forgetting.

A jet engine continues to run because unlike a car engine, there is no interruption of the detonation. There are pipes which connect the burner cans in the combustion section of the engine called flame tubes. These allow a constant source of ignition for the incoming fuel. Although there are spark plugs (called ignitors) in a jet engine, they are only necessary for start.

Answer 6

^^ ours engine start kinda the same way. We have an APU (auxilary power unit) thats essentially an small turbine engine that provides electricity, and air. This air is routed through ducting to the engines. We arm our ignition, and turn on our fuel boost pumps. When we push the start switch on the panel, it opens a series of valves, that direct air to that engine. That air begins to crank the air turbine starter, and get it spinning, once it achieves a certain speed, we introduce fuel, and there is light off. Once the Inlet Turbine Temp is begins to roll back, its considered a good engine start. Once they are started, an electronic fuel computer works with thrust lever position, to keep the engines running.

Answer 7

There is really NO warm-up "per se" for jet engines - Oil temperature has to be in "green band" (40º C for a JT9D engine on 747) - That takes generally 2 minutes if the ambient temperature is very cold - We have also an other procedure before takeoff - The engine fuel feed configuration (tanks and boost pumps) must be set for at least 2 minutes before takeoff - So we start any remaining engine before takeoff and select tanks and boost pumps for takeoff feed configuration for at least 2 minutes - that in turn would "warm-up" the oil of the engine that was not operating during taxi-out for departure - With the 747, the airlines often taxi-out on 3 engines (with nº3, or sometimes nº2 to be started) and wait for the last to be started when they expect to be soon cleared for departure - Fuel savings procedure... .

Answer 8

Nowadays the "spooling up" is computer controlled - the pilot sellects the switch position and a motor spins the turbines within the engine untill pressures and rpm are correct then fuel is injected and ignited to start combustion and generate sufficient heat to maintain continuous combustion.

Answer 9

Typically, jet engines have an 'accessory drive', a shaft off of the turbine shaft. It drives hydraulic pumps, fuel pumps and generators and it is driven by an electric starter motor. When the shaft spins the rotor shaft (with the compressor and turbine rotors) up to sufficient speed to move sufficient air through the combustor, fuel and ignition are 'turned on' and the engine is ignited/started. More fuel, more speed, less fuel, less speed. Speed equalling thrust or propulsion.

Jet engines can stall and backfire! (not good)

Up, up, and away...

Answer 10

All the Above answer are correct.
But some Military aircraft notable the f-16 use and internal Hydrazine tank to spool up the turbines