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Jet engines are usually a lot easier to start than a prop. And the process is pretty simple. It comes down to lots of air under pressure, some fuel, and boom, you're lit. The tough part is getting enough compressed air.
To understand how a turbine engine starts, you should know how it works. They run off a suck, squeeze, burn, blow principle. Take this turbofan engine diagram, like the one that powers an ERJ.
It sucks air through the intake, and then sends the air through a fan, compressing it. In the ERJ, the fan is a single stage, which means is has one row of blades. Then, most of the air goes around the engine. It's called bypass air, and it's used for most of the engine's thrust, as well as cooling. The rest of the air goes through the "hot section" of the engine. That's the part that burns.
Air traveling through the hot section get compressed even further, by passing through fourteen rows, or stages, of compressor blades. At the end of the compressor section, the highly compressed air, which by now is extremely hot from all of the compression, passes into a diffuser, where it slows down. Then it moves into the combustion chamber where it mixes with fuel and burns.
Normally, the fuel air mixture in a jet engine burns constantly, unlike in a reciprocating engine. Some people call this the "bang" section, but we like burn better, because it makes it easier to understand that the fuel air mixture is always burning. It's not a series of individual explosions.
When the fuel burns, the air heats up, but this time it expands, and it's forced out through the turbine blades. In the ERJ, there are five rows, called stages. The first two are high pressure turbines, and the last three are low pressure turbines. The passing air spins the turbines, which then drive a shaft connected to the fan and compressor blades, spinning them. After passing through the turbines, the hot air leaves the engine, generating some more thrust.
So to start a turbofan engine, you need to spin the compressor fast enough to start pushing compressed air into the diffuser and combustion chamber. Simple, right? Not really.
During a start sequence, the engine core needs to spin at 14% of its maximum speed before the igniters start. The engine core speed is called N2, and it's expressed as a percentage of maximum RPM. On an ERJ, 100% N2 is roughly 16,000 RPM, so the engine needs to reach 2,200 RPM before the igniters start firing. You're not gonna hand prop that.
Then, the core needs to speed up to 28.5% of N2, which is over 4,500 RPM, before the engine can introduce fuel and light the mixture. And when it adds fuel, it does so at about 200 pounds per hour. That's roughly 1/2 gallon each minute. So you need a lot of compressed air coming into the combustion chamber to handle all of that fuel.
So where does the compressed air come from? For most civilian turbines, it comes from one of three sources. The most common is the APU, or auxiliary power unit.
This is a small turbine engine, usually located near the tail, that provides compressed air and electricity. It powers the aircraft's electrical systems on the ground, supplements compressed air while flying, and acts as an all-around backup for electrical and pneumatic needs. And, it can send compressed air to both engines' air turbine starters. It's small enough to start with a battery. So when you normally start the jet, you first start the APU.
But what if it's broken? (This happens more often than you think.) In that case, you'll need an external source of compressed air. And the most common is the huffer cart.
It's an air compressor that hooks up to the side of an aircraft, and provides compressed air for engine starting. It's also called an air start unit, but no one really calls it that.
The third source of air is from a running engine. Turbine engines bleed off bypass air for aircraft systems, like the heated anti-ice systems and the cabin pressurization system. And, that compressed air can be routed from one engine to the other. You simply open the bleeds on one engine, open the crossbleed to the other engine, you're ready to start.
So there you have it. Starting a jet engine is pretty straightforward. You need a lot of air, some fuel, an ignition, and you're up and running.