How cfm 56 -3 turbofan engine start ?

[ram18]

I know all procees of cfm 56 -3 . how its work and everything but i am not understanding starting system of cfm56 - 3.

cfm 56 -3 has two shaft n1 and n2 . N1 is coneected with low pressure compressor , lp turbine and connected with fan . N2 shaft is coneected with hp turbine and hp compressor.
In starting we have to give some external power to rotate engine until fuel fire.so first we have to rotate n2 HP turbine and high pressure compressor . but how n2 start to rotate? n1 and n2 are not mechanically connected. Gas stream passing across the turbine is only link between them.

if i want to design hydraulics starting system for cfm 56 -3 . how can we calculated how much horsepower required to rotate high spool ? how can we calculate moment of inertia of turbine ? can anybody explain me how starting system work for cfm 56 -3 ?

 

[JaredJames]

Well one way to start a jet engine is to simply blow air through it. Get enough air mass moving through it and the engine will start.

However, on larger engines (well most modern ones anyway) I've always been under the impression they use a motor of some sort - usually electric - to turn the turbines to generate airflow. They work via a gearing system attached to the shaft.

In the latter case, you either need the APU running or you need a GPU in order to power the motor.

 

[35 AoA]

Haven't flown the heavy metal high bypass motors yet, but on the F404's in my jet, we fire up the APU, and then the air from that is diverted through the L and R bleed air circuits to spin the compressor section. Once spun up through about 15-20% N1, you have enough air flow to introduce fuel (ie put the throttle up into idle), and it will spool up and light off from there. Regardless, you start turbine engines by cranking the compressor section in some way, which pushes compressed air through the combustion chamber area, allowing for fuel and spark to get the cycle going. Not sure exactly what you are wondering here, but if you are thinking that the turbines are cranked in order to drive the compressor (as they are in steady state operation), then you have it backwards in terms of starting.

Previous poster is correct, that in older generation aircraft, normally a GPU or "huffer" is used to do the same thing. Normally some sort of a gas combustion engine on a start cart, that pushes airflow into a hose that is connected to the motor housing and feeds this air through the compressor to get it turning. Most modern aircraft have a self contained start capability for ease of ground handling and allowing for the ability to operate out of fields that don't have such equipment available.

 

[mugaliens]

but how n2 start to rotate? n1 and n2 are not mechanically connected. Gas stream passing across the turbine is only link between them.

That's it. Everything's in windmill until pressures build beyond certain levels. Start is done at that level, and things stabilize almost immediately.

n1 and n2 are coaxial, but not mechanically connected, as it's simply more efficient that way.

Very diminishing returns (as below optimum) to continue to an n3 stage using this approach or beyond.

 

[alphy]

I will try to provide a general explanation of how the starting system of a CFM56-3 turbofan engine works. However, the specific details of the system may vary depending on the design and manufacturer of the engine.

First, it is important to understand that the CFM56-3 engine has two shafts, N1 and N2, which are connected to different parts of the engine. N1 is connected to the low pressure compressor, low pressure turbine, and the fan, while N2 is connected to the high pressure turbine and high pressure compressor.

To start the engine, an external power source is used to rotate the engine until fuel is ignited. This external power source can be an APU (auxiliary power unit) or an external air source. Once the engine is rotating, air is drawn into the engine and compressed by the low pressure compressor. This compressed air is then mixed with fuel and ignited, creating a combustion reaction that produces hot gas. This hot gas is then directed through the high pressure turbine, which drives the high pressure compressor, and the low pressure turbine, which drives the fan. This continuous cycle of air intake, compression, and combustion allows the engine to self-sustain and continue running.

Now, to address your question about how N2 starts to rotate. As you correctly mentioned, N1 and N2 are not mechanically connected. However, the gas stream passing through the high pressure turbine is the link between the two shafts. The expanding hot gas from the combustion reaction drives the high pressure turbine, which in turn drives the high pressure compressor, and eventually the low pressure turbine and fan. This creates a chain reaction that allows the engine to continue running without any external power source.

In terms of designing a hydraulic starting system for the CFM56-3, the amount of horsepower required to rotate the high spool (N2) would depend on the specific design and operating conditions of the engine. The moment of inertia of the turbine can also be calculated based on the design and materials used. However, these calculations would require detailed knowledge of the engine's specifications and design, and it is best left to the engineers and designers who have access to this information.

In summary, the starting system of the CFM56-3 engine relies on the continuous cycle of air intake, compression, and combustion, driven by the expansion of hot gas through the high pressure turbine. While the specific details of the starting system may vary