jet engine : How does the process of starting a jet engine work?

• An intake, compressor stage, combustor and turbine stage make up a jet engine. Thrust is created by compressing air, adding fuel and allowing it to expand.
• The engine must be started by cranking the N2 compressor. An air starting motor or electricity can be used to accomplish this.
• Hot starts (inadequate airflow) and hanging starts (low RPM) are examples of startup faults. If startup does not remove excess fuel, a blowout cycle is required. In-flight restarts depend on the support of a windmill or an additional engine or APU.

Jet engines are complex and expensive machines that require safe handling, proper care, and routine maintenance. While aircraft engines may serve a similar purpose as an automobile machine – providing power, starting an engine is not as simple as turning on the car ignition. Pilots must perform a series of steps required to perform the jet engine start up process.

Pilots have a few checklists when starting an engine. The onboard auxiliary power unit (APU), the ground power unit (GPU) from the ramp, or cross bleed—air from another engine that is running—are the three main sources of air used to start the engine. This article explores the basic ideas behind jet engines, how they work, and the process of starting an engine.

The components of a jet engine that are involved in the startup

In summary, the components of a standard jet engine are the intake, compressor stage, combustor and turbine stage. It works much like an automobile engine. Air is first drawn into the intake and then compressed. Air is sent from the compressor stage to the combustion chamber, where it is ignited and fueled, burning the air.

After that, the air is directed into the turbine and given room to expand. This expansion gives the flow more kinetic energy and the aircraft experiences an equal and opposite force as it exits the engine. We refer to this as thrust.

The compressor stage must initially be rotated at a certain speed to draw in air to begin with. This is the first condition for starting a jet engine. A high bypass ratio jet engine in its most basic configuration has two compressor stages. N1 refers to the low compressor stage and N2 to the high-pressure stage. The N2 compressor is the one that most engine equipment turns on when the engine is started (oil and hydraulic pumps, etc) are connected to this compressor.

The N2 compressor can be rotated using one of two techniques. One way is through the use of electricity. Most often, turboprops and other small jet engines are started using this technique. In this example, one of the engine’s electrical generators acts as the starting motor. When powered it rotates and it is primed for the N2 compressor, resulting in the N2 compressor also rotating.

Larger jet engines use another method. Here, the N2 compressor is turned on by a separate starter motor. The motor runs on air only and is referred to as an air starter. An auxiliary power unit (APU) or a ground start unit can supply this air.

The start procedure

As mentioned earlier, the N2 compressor must be rotated to start the jet engine. For this to happen, air must be sent to the air starting motor. This air, often referred to as bleed air, can be supplied by the APU if it is installed in the aircraft. Air conditioning units receive their air supply from the APU during ground operations. However, the air to these devices is cut off at start-up to free up air for engine starting.

When the pilot contacts the starter, the start valve opens, allowing bleed air to enter the turbine of the starter motor. N2 is then turned by the starting motor. The pilot monitors this via cockpit instruments as N2 is being generated.The pilot uses the fuel switch to fuel the combustion chamber at about 20% N2 rotation. The mixture of gasoline and air is then ignited by igniters, which increases the temperature of the engine. This temperature, known as exhaust gas temperature (EGT), occurs at the turbine stage or exhaust in most jet engines.

The presence of excess fuel in the combustion chamber relative to air causes a rapid increase in EGT when fuel is initially injected. The cooling decreases with less air. EGT increases gradually as engine speed increases and more gasoline is added. Eventually, the engine reaches a self-sustaining speed so that it accelerates on its own without starting the motor.

The igniters are turned off and when this point is reached the starting motor automatically disengages the clutch from the N2 compressor. Then, as the fuel and air balance in the combustion chamber, the EGT reaches a value before falling. This ends the setup process.

Peak EGT is an important number. A high peak EGT may indicate engine failure. A problematic engine starting motor may be the cause. In any case, extremely high peak EGTs at startup should be reviewed with monitoring to avoid escalating into more serious problems. Turbine temperature is indicated by the EGT itself, and since it is heat-sensitive, there is an initial EGT limit that should never be exceeded. If this is exceeded, the aircraft should be diverted for maintenance and the engine should be shut down immediately.

In the event that the APU is unavailable or unserviceable, the aircraft may be connected to a ground start unit. The adapter receives a long hose from the start unit that supplies air to the engine from the unit. When using this procedure, pilots use a start unit to start one of their engines at the gate. The engine is separated from the aircraft as soon as it starts. The rest of the engine can then be started by diverting air from the ignited engine through the cross-bleed valve. We refer to this type of start as a cross-bleed start.

Startup malfunctions

In a jet engine, there are two primary starting failures. A hot start is one thing and a hang start is another. When fuel is supplied during a hot start, the EGT increases predictably, but the temperature quickly climbs to the EGT limit. If this occurs, pilots should shut off fuel and ignition immediately. If this is delayed, the engine may become inoperable within seconds if the EGT limit is exceeded.

There is a simple explanation for hot start. Inadequate ventilation. This could be due to a bad starter motor, a problem with the engine’s electronic control unit, or insufficient air supply to the APU or ground start unit. starting engine with a strong tailwind can also cause a hot start as the wind opposes the engine’s rotation.

In a stalled start, the engine compressor RPM is unable to reach the self-sustaining speed or desired value. EGT is higher than predicted due to low rpm, yet it “hangs” at an unacceptably low number. In the event of a hang start, the pilot must close the fuel valves to shut down the engine. A hang start is mainly caused by a bad starting motor.

If the engine doesn’t start, pilots have to do a “blowout” cycle before trying again. This is because during most failed starts the unburned fuel sinks into the combustion chamber. This fuel can ignite flooded fuel and cause a flame if an attempt is made to start in the chamber can come out of the engine exhaust. This is called a tailpipe fire or torching.

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