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Engine Start

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Engine Start

Old 14th Apr 2008, 02:38
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Engine Start

Hi Everyone,
Could someone please explain the process undertaken for an engine start. In particular where the bleed air enters the engine and the role of the starter valve during this process.

Is there any major differences when conducting a x-bleed start?

Thanks in advance
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Old 14th Apr 2008, 04:55
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Engines generally have pneumatic starters, an air source will power the starter after the start valve has been opened by several different methods, a cross bleed start is simply using the already running engine for an air source.
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Old 14th Apr 2008, 06:24
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Newpic,

Turbine engines are started with both electrical starters and with larger engines, air starters (to save weight). The start valve is nothing more than an air valve, which admits air to the starter; a relatively lightweight motor which is powered by air. The starter is normally attached to the engine via a sparague clutch and worm gear. The starter motor itself is a little like a small turbine engine, with pneumatic air blowing over the blades to move the starter.

The start concept for the turbine engine is to get the engine spinning as fast as possible using the starter. The engine speed is measured in a percent of it's operating RPM, rather than the actual engine speed. That is, rather than looking for 6,000 rpm, for example, we see 15%...something like that. The 100% operating speed of the engine is typically 18,000-35,000 rpm, and we see some percentage of that, to make it easier to deal with.

When the engine has reached the highest percent RPM the starter can spin it, fuel is introduced, along with ignition. At some point, the fuel lights off, and the exhaust gases from the burning fuel act on the turbine wheels downstream to turn the engine faster. At some point, typically about 50% RPM, the starter is no longer providing any useful assistance, and is shut down by closing the air valve to the starter motor. (If the starter is allowed to go to higher speeds, it may "burst" or suffer a failure which damages not only itself, but surrounding components).

Pneumatic air from a ground "huffer" cart, the auxilliary power unit, or even from other engines is used to power the starter motor. Typically the APU or ground cart can produce a greater air volume than the engines do at idle, for "bleed" air...the air which is bled off the engines to provide air conditioning, pressurization, move leading edge devices, etc. To do a cross bleed start, using the air from one engine to start another, the operating engine or engines must be operated at a higher value than idle in order to boost the airflow and pressure in the aircraft pneumatic manifold high enough to start an engine.

If the air pressure used to move the starter motor isn't high enough, it won't turn the engine up to a fast enough speed during the start process. If this happens and the engine lights off at a low speed, it may not accelerate properly; it's temperature will increase faster than the engine speed increases, and a hung start or a hot start occurs. In a jet engine, most of the airflow through the actual engine portion (the core) is used for cooling...not for burning. If not enough airflow is passing through the engine then there's not enough cooling air, and not enough air to prevent the flame in the burner cans inside the engine from contacting the burner can walls. The result can be an unsatisfactory start and even engine damage.

Most of the functions that go on in the engine during the start are automatic. Various valves open, such as acceleration bleed valves, which manage the airflow through the engine by bleeding off excess pressure at some places while it builds in others. These are self-contained within the engine, and the pilot and/or flight engineer is scarcely aware of their function. Some engines do everything automatically,and some require the pilot or FE to move certain levers at certain times and conduct each part of the start process manually. Some engines automatically regulate the temperature, others put it all on the flight crew.

The engine start isn't over once the engine lights off. Monitoring temperatures is very critical to the life of the engine, as is the speed at which the engine operates, at which it accelerates, and where it idles. Time limits are imposed which must be watched during the start; total starter time, time to light off, time to on-speed, etc. Some powerplants have events which must be noted at different points in the start process, such as starter cutout, or the automatic opening of secondary fuel valves, etc. Typically a jet engine will take about 45 seconds from start to on-speed, but may be as long as a minute and a half. Even after the engine is started and on-speed, some engines must be closely monitored as a tailwind or other factors could cause the engine temps to take of and require a rapid shutdown to save the engine.

Turbine engines are very reliable and put a lot of thrust out for their weight. A small mistake during the start process can do millions of dollars in damage, however, so it's a process to take quite seriously.
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Old 15th Apr 2008, 14:01
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Guppy, great explanation!
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Old 15th Apr 2008, 15:18
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SNS3Guppy

One of the best posts i'v read on here! spot on!
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Old 16th Apr 2008, 00:48
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yeah, um, what guppy said!
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Old 16th Apr 2008, 01:09
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newpic, I don't frequently give out "me toos", but you could pay hundreds of (insert local currency) and get no finer training than SNS3Guppy has delivered.

The only thing I'll add is that crossbleed may supply more air pressure than an APU or GPU, meaning that after the first engine is started, the remaining one(s) may start faster and cooler. The corollary to this is that if you know one engine habitually starts fast & cool, start it first (SOP permitting, of course) then use it as a crossbleed source.

Last edited by barit1; 16th Apr 2008 at 21:50.
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Old 17th Apr 2008, 02:47
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That was fantastic description SNS3Guppy, thanks for taking the time to give such a detailed answer. I do have one further question however:

I understand the bleed air is directed to the starter valve, however what does the starter do to turn the engine and which part of the engine turns first? (N1,compressor blade?)

Would appreciate any further input, thanks again Newpic
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Old 17th Apr 2008, 03:24
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I understand the bleed air is directed to the starter valve, however what does the starter do to turn the engine and which part of the engine turns first? (N1,compressor blade?)

Would appreciate any further input, thanks again Newpic
The starter air spins a small turbine in the starter which typically drives the N2/N3 through a gearbox. The idea is to spin up the last compressor rotor spool just fast enough that it can pressurize the burner and still have enough inertia in the rotor to overcome the back pressure from the burner lighting off. Too little speed and the lightoff goes very badly and the rotor just hangs, never accelerating as the back-end temperature climbs.
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Old 17th Apr 2008, 11:28
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Back Pressure

The idea is to spin up the last compressor rotor spool just fast enough that it can pressurize the burner and still have enough inertia in the rotor to overcome the back pressure????
lomapaseo. Could you please explain the back pressure that needs to be overcome.
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Old 17th Apr 2008, 11:53
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Originally Posted by newpic
what does the starter do to turn the engine and which part of the engine turns first?
Some pictures were posted here.
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Old 17th Apr 2008, 11:53
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N1% for crossbleed

It must also be noted that at ground idle (N1 20%) the bleed air from an engine may be only approx 15 - 20psi. To obtain the required 30+ psi required to prevent a 'hung start' it is necessary to bring the N1% past approx 60% That can be very noisy and are avoided where possible. Usually, in our outfit, a cross bleed is only carried out iof the APU is inop...
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Old 17th Apr 2008, 12:09
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Turbine engines come in a variety of designs that range from a single shaft to which most parts are connected, to complex designs involving multiple shafts and stages that work independently from one another. How the starter turns them depends on the particular engine. In all cases, however, the part of the engine in which the burning takes place and the parts attached to it, may be thought of as the "core," sometimes referred to as the gas turbine generator, or just simply as the gas turbine. It's this that the starter turns.

The names N1, N2, N3, etc, become somewhat confusing. The easiest thing to remember is that the heart of the engine is the part with the fire, where the first stage turbines are located (the first turbine wheels downstream from the burner cans), and which drive the highest stage compressors...the ones closest to the burner cans and the last stage or stages through which the air passes before arriving at the burner cans. This "core" or "gas generator" can be thought of as the engine that drives the engine...everything else is being driven by the core.

In the high-bypass turbofans, with a very large frontal section and big fan blades (as you see on most airline equipment), the actual core to the engine, the N2 section in a two-stage engine and the N3 in a three stage engine, is much smaller than the engine you see hanging on the airpalne. Most of what you see is what's being driven by the small core. I like to think of the core as the poodle, and what you see on the wing is the poodle before it gets wet or gets groomed. It's just a very expensive, very powerful poodle.

The starter is connected to the core through various means of gearing. The starter motor is actually attached outside the engine, usually to a shaft in the accessory section. that shaft works through the accessory section gearbox (or in some cases directly) to turn the core gas generator. It's this action that starts sucking air in at the front of the engine, and provides just enough airflow to allow the engine to light off at the right time and accelerate. The other stages of the engine don't really contribute much at this point in time...they're available to do a lot of work at higher power settings, but aren't doing a lot for you in the starting process.

The compressor sections of the other stages are turned by their own turbine wheels, located downstream from the core turbine wheels. The core takes energy out of the gas flow coming from the burner cans and uses it to turn it's own compressor discs or wheels. The other stages get what's left of that energy to turn their own wheels...and it's slow going at first. Often when starting, the N1 stage may not even be seen to rotate for 15 or 20 seconds...the core needs to build up some speed first and until light off, it's only the air being drawn into the engine by the core compressor that provide any motive force to turn the other engine stages.

The first parts of the engine to turn, then are the accessory section gears (or tower shaft, which is the part some starters attach to, to drive the engine), the core turbine, and the core compressor. You can picture the core section like an axle on your car, with a wheel at each end. Start turning the shaft with the starter motor, and you have the wheels turning at each end. One wheel, aft of the burner can, is the turbine wheel that gets turned by exhaust gasses, the other wheel is the compressor, ahead of the burner can.

The question was asked regarding back pressure. As the compressor begins to build up pressure, it dumps it into a small chamber expands the volume, and slows the airflow down somewhat. This is called the diffuser, and where the airflow decreases in velocity, it experiences a pressure rise. The goal of the compressor is two-fold; draw in as much air as possible, and boost it to the highest pressure possible. That's all good and well as long as the air can be kept moving. But at slow engine speeds, the air isn't moving very fast at all...and suddenly it gets backed up at the diffuser.

Ever get off a flight and find that half a gazillion of you want to fit through the door at baggage claim? Moving toward the exit works great so long as you've got a lot of room, but when you get to that door, the line slows up. There's pushing and shoving, and the pressure builds...much like in the engine. If only many people can fit through the door, the ripple effect moves aft, and people begin to slow down behind. You see the same thing as cars get backed up on a highway. Airflow is getting sucked in as fast as the engine can make it go, but can only move through the engine so fast.

The pressure rise in the middle of the engine means that the pressure at the front of the engine is less...and just like people who don't like to get crushed in the rush for the exit, that airflow desires to move to the area of lowest pressure. It doesn't move forward so well, seeks somewhere to go, and that seeking action is called back pressure.

To relieve that back pressure, "bleed valves" are installed which allow a certain amount of pressure to automatically leak overboard, out the sidesof the engine, until the engine is turning fast enough. Some bleeds open at certain speeds, some due to demans inside the engine, but the effect is the same. These bleeds are often referred to as acceleration bleeds, and what they do is bleed off excess pressure to allow the engine to accelerate, and keep airflow going through the engine.

Another way to think about it is an "unloading" process. The inability to move air through the engine and the reduction in velocity as it approaches the diffuser result in increasing resistance to airflow (back pressure), and an additional load on the compressor (and in turn the turbine wheel, and as we're talking starts here, the starter motor, too. Bleeding off some of that pressure and excess air unloads that resistance, allowing the engine to accelerate more efficiently. Sort of like spillgates in a dam getting rid of extra water.

The blades in the compressor work much like propeller blades or any other airfoil, but inside a very small room, or duct. To be most efficient, the airflow meeting those blades has to be at a narrow range of angles. When the airflow starts backing up, the "angle of attack" or angle at which the airflow through the compressor meets the blades can quickly become an inefficient angle, or even a stalled angle...this is where the term you've probably heard comes from...a "compressor stall." Acceleration bleeds are placed at various points in the engine to prevent this from happening, especially during start.

As the engine turns faster and faster and can process more of that airflow, the acceleration bleeds close and allow all the air to pass through the engine.
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Old 17th Apr 2008, 16:25
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Post #3 is now 2nd best to post #13!

As a 737 FO, I’ve learned a lot about what actually happens when you turn that switch to “GND”

Thanks!
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Old 17th Apr 2008, 17:39
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Cool

To obtain the required 30+ psi required to prevent a 'hung start' it is necessary to bring the N1% past approx 60%
What engine is that on?
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Old 17th Apr 2008, 19:30
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You may find this link of help too

http://www.b737.org.uk/aircraftsystems.htm
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