To start my car (powered by a reciprocating-piston internal combustion engine), I turn the key:
-an electric starter motor engages the flywheel and begins spinning the engine at a couple hundred RPM.
-The computer injects fuel at the appropriate time, and fires the spark plugs at the appropriate time;
-After a few revolutions, a viable mixture gets ignited in one or more cylinders.
-The engine accelerates up to idle speed, and the starter disengages when I release the key. The engine is now running, ready to produce mechanical power.
To start my Boeing 747-400, what’s the process like? The plane is fueled up, full of passengers, and the tug has pushed the plane back from the gate. It’s time to start the engines. What are the steps the cockpit crew has to take, and what’s happening down in each of the engines? Unlike my car, the thermodynamic processes in a running jet engine (intake/compression/ignition/expansion) are all happening on a continuous basis - I can’t just pick one step and start there. How are all of those processes brought from “not running” to a self-sustaining idle condition, ready to produce thrust?
For an Avro RJ, Prior to push-back the fuel pumps are turned on (one for each of its four engines), the engine anti-ice is checked On, the Start Master is turned on, the Start Select is turned to engine 4 and the starting checklist is completed.
During push-back the Starter Switch is selected to Start. This engages the starter motor to turn the selected engine over and turns the igniters in the combustion chamber on. Once the high speed compressor stage (N2) is rotating at 10% the thrust lever for that engine is moved from Fuel Off to Idle. This introduces fuel to the combustion chamber. The fuel is automatically metered by the engine electronics to ensure the start does not result in an over-temp. Normally within five seconds the fuel/air mixture in the chamber ignites, indicated by a rise in the exhaust gas temperature gauge. The N2 continues to rise along with the EGT, N1, and oil pressure. At around 25PSI oil pressure warning light goes out and at 40% N2 the engine becomes self sustaining, the starter disengages and the igniters automatically turn off. At 50% N2 with the EGT decreasing to about 450°C the engine is stable at ground idle and the process is started for engine 3. It takes about 30-40 seconds to start each engine.
The start is aborted for the following:
The engine doesn’t rotate when start is selected.
The EGT doesn’t rise within 15 seconds of the fuel being introduced (failure to “light up”).
The EGT appears likely to exceed the start limits of 649-713°C for 15 seconds.
The engine fails to become self sustaining, ie, it stagnates at around 30% N2.
Most jet engines have only 2 ignitors (AKA spark plugs - sorta) even though they may have 8 or more combustion chambers or cans. There are tubes between the cans so that fire can move from one to the other. It’s something like lighting one side of a gas grill with the push button then just turning the other knob to ‘on’ to get the other side to light.
I worked on one aircraft that used an electric motor to start the jet engine instead of air pressure. Once started, the motor then became the generator.
Some aircraft, such as older fighter aircraft like the F-111, didn’t have an auxiliary power units and might have to land in places that didn’t have things like a Dash 60 air compressor. They could carry starter cartridges which would burn and cause enough pressure to start the engine instead.
I was a mechanic on the military versions of the Sabreliner (T-39A), DC-9-30 (C-9A), and the previously mentioned F-111.
Never got near the flight line myself, but my general impression was that ground starter equipment varied quite a lot. There was the Dash 60 that you mentioned: an air compressor (a smaller gas turbine engine?) that pumped air into the turbine system and got it turning so that the compression/ignition could fire. This seemed pretty common. But I read about the ground starters for the SR-71which appear to be a direct mechanical transmission linkage between the cart and an input shaft in the JP-58 engine. The cart apparently had a couple of large-block GM car engines to turn the starter transmission.
What are N2 and N1? I thought that for an ordinary jet or turbofan, the compressor and turbine sections were hard-coupled, i.e. there was one RPM for the entire engine (not so if we’re talking about a turboprop or a helicopter).
You speak of a starter motor. This is powered by a battery? How big a battery is this? I’m guessing there’s a lot of kinetic energy in a turbine (especially a large-diameter turbofan as on a 747) spinning at 10% of max RPM.
N2: primary turbine speed (the turbine which drives the compressor)
N1: secondary or “free” turbine speed which drives the fan or output shaft on a turbofan or turboshaft, respectively.
And whether the engine has a starter motor depends on the specific engine. AFAIK most large airliner turbofans start using compressed air from either the APU or a “start cart.” Smaller engines like turboprops and the turboshafts used in helicopters have starter motors. Here’s a Lycoming T55 turboshaft which has the starter motor sitting right on top of the thing. This engine was originally used in Chinook helicopters and was co-opted by hydroplane teams for use in race boats.
Homemade jet engines are often started with leaf blowers. The leaf blower is used to spool up the turbine (usually that of an automotive turbocharger) and then the ignitors and fuel are turned on.
To add on to what everybody else has already posted:
On small turbine-powered airplanes, there is a (typically DC) electric starter/generator mounted to the accessory gearbox of each engine. This is basically just a beefier version of the starter on your car, except it doesn’t disconnect from the accessory gearbox the way the ones on piston engines disconnect from the flywheel. Instead, at a preset RPM it switches from operating as a motor to operating as a generator - this saves weight by using the same component to perform two jobs. The power to turn the starter can come from several sources, the most basic of which are the ship’s batteries. On corporate and regional airplanes, there are usually two 24v batteries which are each about 2-3 times the size of a 12v automotive battery. However, in practice these aren’t usually used to start the main engines. Because it takes 10-30 minutes to get the cockpit set up, the airplane’s electrical system is typically already powered via a ground power unit (an aviation-specific generator) or an on-board auxiliary power unit (APU). An APU is a small turbine engine usually mounted in the tail that provides electrical power and an air source while the main engines aren’t running. This brings us to the other method of turning the engines: a turbine air starter. On larger jet engines, the large mass of the engine core makes a sufficiently powerful electric starter weight prohibitive. Instead, high pressure air is bled off the APU and routed to a turbine air starter. This is basically a gigantic version of a dentist’s drill, but instead of turning a drill bit it turns the accessory gearbox which turns the engine core.
N1/N2:
Small turbine engines have one shaft, with a compressor and turbine affixed to it. They’re basically just a turbocharger with a fuel nozzle and an igniter to get things started. Larger engines have two concentric shafts - think of a 10 cm long skewer inside of a 9 cm straw. Now put a fan on one end of the skewer and a pinwheel on the other. This is your N1, or LP (low pressure), spool, complete with a compressor and a turbine. Now, starting at the front of the straw and working your way toward the middle, attach a series of smaller and smaller fans. Do the same with pinwheels, but starting with the largest at the rear. This is your N2, or HP, spool. The starter/generator or turbine air starter is attached to the N2 via a series of gears. As the N2 is spun up by the starter, compressed air flows out of the N2 turbine section and over the N1 turbine, which gets the N1 spool rotating. Now add fuel, hit the igniters, and watch the magic happen - but be sure to keep a close eye on the turbine temps during the start, or you’ll do this!
ETA - There are a handful of triple spool engines. I’ve never operated one, so I can’t tell you much about them.
I once flew a Fouga Magister, which is a relatively old twin-jet. The engine start process was around six steps involving monitoring temps and RPM. At one point you had to move a lever while pressing the igniter button on top, and I was warned to do that very carefully. If you moved the lever and let go of the button you could dump fuel on the tarmac, which could in turn ignite.
I was surprised the instructor let me do it. Was very relieved when I got both engines lit without incident.
N1 is a compressor/turbine stage* couple together. It is the first compressor stage and the final turbine stage. The second compressor stage is coupled to the first turbine stage and makes up N2. N2 is the high speed / high pressure stage, N1 is low speed / low pressure.
On the PW123 and similar turboprop engines, they are referred to as NL and NH instead of N1 and N2. That model turboprop has a third turbine stage that is not coupled to any compressor stages and instead drives the propellor via a reduction gearbox.
As mentioned by others, electric starters tend not to be used on large airliners. But the Avro RJ is a small 100 seat regional jet with four relatively small engines. Starting off the battery is possible with a modification that some of our aircraft have, but I’ve never done it. Instead the APU generator is used to power the start. If the APU is not available we can use a ground power cart. If there is no ground power and the battery start mod is fitted we can use the battery.
The battery is a 28V NiCad, about the size of two car batteries together.
Not necessarily. The free-turbine on a turbo-prop such as the PW123 I mentioned earlier isn’t called N anything, it is simply the power turbine. It does have the equivalent of N1 and N2 though, but neither of those sections are part of the power output. On the other hand, a fixed-shaft turbo-prop, like a garret, doesn’t have a free turbine section at all.
*Edit: “Stage” isn’t really correct, it refers to the spool which could consist of several compressor stages.
