How does a jet engine start
A jet engine can be started in a variety of ways. Gunpowder cartridges were employed to start them in the early phases of their history, causing the explosive gases to activate the compressor’s movement. External compressors, starting motors, and even small jet engines have been employed in the past, as is the case with the Mirage V’s turbines.
It’s not like your car’s engine, but it does have a starter motor (which can run on either compressed air or electricity, depending on the type of aircraft). Smaller jets and turboprops typically use electric starters to start the shaft rotation. Air turbine starters that utilize compressed air are commonly used in transport category airplanes.
The compressor’s shaft (and, of course, the turbomachinery coupled to that compressor by that shaft) is rotated to a rotational speed of roughly 41-62 percent of maximum RPM during the starting function (varies for each engine). Because it stays engaged for a far longer time than the duty cycle on your car’s starting, this speed necessitates a very “tight” starter. The Fuel Control Unit (FCU) starts feeding fuel into the burner (or combustion chamber) cans once the shaft reaches the correct RPM (referred to as a “percentage of N1” in most jet engines), where high-energy “igniters” (similar to spark plugs but considerably more powerful) light it. The initial fuel flow is relatively low, but it gradually increases as the shaft RPM is monitored by a pre-programmed flow program included within the FCU.
The fascinating thing is that if the starter is disengaged when the light comes on, the engine’s RPMs will cease growing and the engine will “cool down.” The starter remains engaged to protect the motor from this phenomena and continues to assist the rotating shaft in increasing its RPM. This is known as “bootstrapping,” and it continues until the rotational speed hits “Ground Idle,” the power setting at which the engine can run without the assistance of the starter, usually about 70% N1 (70 percent of the maximum speed of this shaft in normal operation). The Ignitors can also be stopped at this time (the pilot has the option to continue using them if freezing conditions enter). This operation can take 15-25 seconds on smaller motors, but it can take 40-45 seconds or more on large high bypass fans.
Once you grasp how a jet engine works, you’ll know why starting it is the most difficult part of its life. To put it another way, if we never shut them down and let them idle, they’d last a lot longer before needing to be overhauled.
Most big commercial aircraft now use air-driven starters to ignite their jet engines. This is accomplished by diverting pressurized air from the APU (auxiliary power unit) to the engine just during start-up. The fuel is injected and ignited by the spark plugs as soon as the engine reaches the requisite RPMs. The engine then achieves idle RPM, and the engine is deemed started at this point.
Air-driven starters are not found on the most recent aircraft, such as the B-787. Instead, electric ones are already in use. In fact, because the new aircraft’s engine does not have an air leak, it maintains its efficiency and burns less fuel.
How do planes’ jet engines get started?
Gas turbine engines are available in a variety of shapes and sizes. A typical “jet” engine on an airplane is one type of turbine engine detailed in How turbine engines function. The hot gases emitted by the fuel burning convert vanes in the same way that wind does in a windmill. The vanes are connected to a rotor that rotates the compressor of the turbine. Another form of gas turbine engine, which is common in tanks and helicopters, has one set of vanes for driving the compressor and the other set for controlling the output shaft. To start either of these types of engines, you will need to get the main shaft rotating.
The main turbine shaft is generally spun by an electric motor during this starting operation. The motor is mounted to the outside of the engine and connects to the main shaft via a shaft and gears. The main shaft is spun by the electric motor until enough air is blown through the compressor and combustion chamber to light the engine. Fuel begins to flow, and an igniter like a spark plug ignites it. The engine’s fuel flow is then boosted to bring it up to operational speed. If you’ve ever been to an airport and watched a large jet engine start up, you know how slowly the blades begin to rotate. This is done via the electric starter motor. Then you (sometimes) hear a pop and see smoke billowing from the engine’s back. The engine then starts spinning and produces thrust.
Another approach to start a smaller turbine engine (particularly home-built ones) is to simply blow air through the air intake with a hair drier or leaf blower. This method achieves the same result as an attached starter motor in terms of getting air moving through the combustion chamber, but without the complexity or weight.
Aside from the starter shaft, most large jet engines have an output shaft for operating things like electrical generators, air conditioning compressors, and other components required to operate and maintain the plane. This shaft might join the main turbine shaft at the same place as the starter or at a different location. Some jet jets include a separate turbine that only generates auxiliary power (often in the tail cone of the plane). When the plane is parked on the tarmac, it is more efficient to use this smaller turbine.