How to Start a Jet Engine

Question

I know how the basics of how jet engines work. I know that air goes into the compressor as the first step of the journey. However, where does the air come from that goes into the jet engine at such high speeds it turns the rotors. I don't think its coming from the air because, the plane wouldn't be moving and no air would be flowing at a high rate. Also, once the jet engine is started up, how does air keep coming into the jet engine in flight. From air in the sky?

Is there a difference in the processes for fighter jets and regular commercial planes?

Answer 1

Inside some jet engines there is a powerful electric motor that starts the engine by spinning the compressor up to speed, at which point the fuel flow and the ignition spark are turned on, and the resulting combustion then forces the turbine to spin. The turbine's power output is sent to the compressor stage to spin it, and the electric start motor can then be turned off.

On other jet engines, a small gas turbine is used to blow hot combustion exhaust through the main turbine, which spins the main compressor up to speed, then the fuel and spark come on, etc. The start turbine is then shut down once the main engine starts.

Note that whenever the engine is running, the compressor stage at the entrance to the engine is actively sucking in tremendous quantities of outside air and jamming that air into the combustors in the center of the engine, where it is burned with fuel. The power of the exhaust from the combustors then spins the turbine at the outlet end of the engine, and that power is fed back to the compressor on a spinning shaft to keep it sucking in more air.

Some military jet engines can be started by connecting a huge blower to the engine inlet and forcing compressed air through the engine, to spin the turbine and get the compressor going. This is called an air start. Some military jets can also be started by burning a solid-fuel cartridge made of black powder and shooting the hot exhaust into the turbine stage of the engine to spin it, and thereby spin the compressor stage up to speed and get things going.

Other military jets are started by a start cart which contains a powerful gasoline engine that spins a shaft geared onto the main shaft of the jet engine. Once the jet is up and running, the start shaft is disconnected from the jet and the start cart is rolled out of the way.

Answer 2

High pressure air is used to turn an air starter inside the engine. That starter is connected to the inner shaft of the engine via gearbox.

The source of the high pressure air can be several things:

• The auxiliary power unit (APU), a small gas-turbine engine in the tail of the airplane which itself is started with an electric motor
• A ground air starter, a compressor either on a small vehicle or directly connected to the air-bridge
• Another engine that is already started via cross-bleed.

Smaller jet-engines are also started with an electric motor connected to the inner shaft with a gear box, similar to an air starter. The electric starter is often used as a generator once the engine is running. This makes this concept very light and efficient, few moving parts, no high temperature/high pressure air ducts that can leak and cause issues, etc.

Answer 3

I know that air goes into the compressor as the first step of the journey

From the way you mention this phrase it sounds like you have a slight misunderstanding of how a jet engine works.

As your confusion illustrates. Air does not just "flow" unless there is a pressure difference. A pressure difference can be created by several means: atmospheric heating and cooling (which is how we get wind), airfoil moving through air (fans), impellers accelerating air around a circular path (blowers) etc.

Air does not go into the compressor. The compressor pushes air through the engine.

In a modern jet engine the compressor is designed like a fan. Thus the action that causes the air to flow through the compressor is the same as a table or ceiling fan: the airfoil of the compressor blade moving through air. Just like a regular fan you find in most homes, this causes the airfoil to create a high pressure region behind it causing the air to be blown backwards and a low pressure region in front of it causing air to be sucked into the engine.

In older jet engines from the 1940s and 1950s the compressor works kind of like a leaf blower or a car turbocharger compressor with impellers that push air sideways due to centrifugal force. Again, this causes a high pressure zone behind the impeller and the exit of the air inside the impeller causes a low pressure zone in front of it sucking air inside the compressor.

As in any application using fans, blowers or compressors such as a table fan or hair dryer or a turbocharger it is the motion of the compressor that causes air to move in a jet engine.

But unlike a table fan or hair dryer it is the hot jet exhaust that powers the compressor which is the same as how turbochargers work in cars. In fact both aircraft jet engines and car turbochargers are called the same thing: turbines.

How a jet engine is started depends on the design of the engine. Some use an electric motor, some use hydraulics, some use a piston engine and some use a separate "starter" blower to initially blow air into the engine to start the compressor spinning. There are even jet engines designed to start spinning the compressor by firing a shotgun shell inside the engine: these are called catridge start (google "catridge start jet engine" to find interesting youtube videos). You can read more about the various ways to start a jet engine (technically called "gas turbines") on wikipedia: https://en.wikipedia.org/wiki/Aircraft_engine_starting#Gas_turbine_engines