# How to make a jet engine pdf

## Jet engine

A Jet engine (coll .: Jet engine; engl .: Turbojet or. Turbofan) is an engine that works on the principle of the recoil drive. A jet engine consists of an impeller, compressor, combustion chamber, turbine and exhaust nozzle. The air sucked in by the impeller is compressed and heated in the combustion chamber. The hot gases then flow out through the turbine and thrust nozzle. Just as much drive power is generated in the turbine as the compressor needs. The remaining energy is used to accelerate the air jet.

### Momentum balance

If the system boundary is placed above the surface of the jet engine, five momentum flows can be identified: Force from the aircraft (F.), Front pressure (F.D1) and back (F.D2), convective impulse current in front (I.p1) and back (I.p2). The resulting force on the outer surface is small and is not taken into account here. Because the momentum content of the jet engine does not change in stationary operation, the following applies

[math] F + F_ {D1} + F_ {D1} + I_ {p1} + I_ {p2} = 0 [/ math]

If you add the mass balance (excluding fuel) for stationary operation

[math] I_ {m1} + I_ {m2} = 0 [/ math]

and neglects the sum of the two pressure forces, applies

[math] {-} F = (v - v_ {s1}) I_m - (v - v_ {s2}) I_m [/ math]

I.m describes the amount of the mass flow rate flowing through, vs1 and vs2 stand for the two backward-directed flow velocities measured relative to the engine. v is the speed of the engine relative to the surrounding air (reference system).

The reaction force too F., the force on the aircraft, is called the thrust F.Sch. One last transformation leads to

[math] F_ {Sch} = (v_ {s2} - v_ {s1}) I_m = \ rho (v_ {s2} - v_ {s1}) v_ {s1} A_1 [/ math],

in which A.1 describes the cross section of the inlet opening. The flow has been modeled here as homogeneous, i.e. it was assumed that the flow velocity is the same across the entire cross-section.

### Energy consideration

The gas flowing through the jet engine is additionally charged with energy in the combustion chamber. If you only consider the increase in mechanical energy, the following applies to the process performance

[math] P = \ frac {1} {2} (v_ {s2} ^ 2 - v_ {s1} ^ 2) I_m = \ frac {v_ {s2} + v_ {s1}} {2} (v_ {s2 } - v_ {s1}) I_m = \ frac {v_ {s2} + v_ {s1}} {2} F_ {Sch} [/ math]

The power charged to the gas jet by the engine is equal to the mean flow velocity times the thrust.

### Efficiency

The power of the thrust relative to the surrounding air can be measured with the process power in the engine. This efficiency thus compares the power mechanically converted in the engine with the power dissipated by the aircraft per engine against the air

[math] \ eta = \ frac {P (F_ {thrust})} {P} = \ frac {v F_ {Sch}} {\ frac {v_ {s2} + v_ {s1}} {2} F_ {Sch }} = \ frac {2 v} {v_ {s2} + v_ {s1}} [/ math]

Set the flow velocity at the entrance v1 Equal to the amount of the speed of the aircraft, the formula for the efficiency is simplified to

[math] \ eta = \ frac {2 v} {v_ {s2} + v} = \ frac {2 v} {\ Delta v + 2 v} [/ math]

This efficiency is zero when the aircraft is not moving and one when the exit velocity equals the entry velocity. In the first case, no thrust is generated and in the second, no power is implemented.

### thermodynamics

In cruise flight (steady state), the air accumulates when it enters the engine, is then compressed over several stages, continues to heat up through the combustion of the fuel, then drives the turbine and builds up speed at the rear end. As a first approximation, this process can be idealized under the term Joule cycle. To do this, consider a small air package that is reversibly transported through the engine:

1. isentropic compression (compression up to the combustion chamber)
2. isobaric heating (combustion process up to entry into the turbine)
3. isentropic expansion (decompression in the turbine and in the air flow)
4. isobaric cooling (in the surrounding air)