Chapter 1 - Engine Starting and Ignition Systems Flashcards
What do gas turbine engines require to achieve their self-sustaining RPM, as compared to a piston engine?
A higher rate of turning
What is self-sustaining RPM?
The engine speed from which the engine will accelerate to idle RPM, under its own power, smoothly and without overheating
What is the balancing act of starting an engine?
Short a start time as possible to prevent stall, but without exceeding the turbine temperature limitation which could lead to ‘thermal shock’
What is a ‘hung start’?
The combined torque of the starter and the turbine isn’t enough to keep the engine accelerating to idle RPM
Why should an engine be shut down if a ‘hung start’ has resulted?
If fuel is burning but the engine fails to accelerate, a very high turbine temperature will be apparent
What might cause a ‘hung start’?
HP shut-off valve has not been opened
What three things does a start require?
- Rotation (for air)
- Fuel
- Ignition
What is meant by the fact that Isopropyl Nitrate (IPN) is a monofuel?
Provides its own oxygen
What is the health and safety hazard of IPN?
Easily passes into the bloodstream and can give headaches
Describe a gas turbine starter?
Free turbine connected to the main shaft of the engine(s) to be started
Gas turbine is itself started by a small electric motor and is then used to start the main engine
Once the main engine is started, the gas turbine starter is shutdown
What are the sources of air for the low pressure air starter unit?
- APU
- Ground supply
- An engine that is already running
What are the three types of valves used in air starter systems?
- Non-return valves
- ON/OFF valves
- Pressure regulating valves
What nominal value will the pressure regulating valve hold the pressure to?
28 ±2 psi
Where will the igniter plugs in the combustion chambers ignite the fuel?
As it is sprayed from the burners in to the toroidal vortex
What time must elapse before handling the various types of ignition system?
HT lead or ignite plug - at least 1 minute
Air gap plugs - at least 3 minutes
What is the charge delivered by a HEIU?
Anywhere between 18,000 to 28,000 volts
When will ignition be automatically switched on?
Continuously during a stall warning, and for 1 minute after the stall warning has cancelled
When will ignition be manually switched on (not including starting)?
When landing (or taking-off) on very wet or slushy runways
In what form is the energy dissipated in a HEIU?
High-voltage, high current discharge
What is the purpose of the choke coil in a HEIU?
Flatten out the jagged bursts of charge and smooth them out to make for a longer duration spark at the plug
What is the purpose of the safety resistor in the HEIU?
Protect the circuit in the event of it firing with no plug installed
What is the purpose of the discharge resistor in the HEIU?
Discharge the contents of the reservoir (storage) capacitor to earth to make the unit safe to handle
What is the purpose of the HT lead?
Protect other electrical and electronic services from interference from the very strong magnetic pulse which will occur every second or so
Describe the discharge of a surface discharge plug?
High-energy flashover from the pellet to the body which projects the spark an inch or more into the combustion chamber
What is the central electrode in an air gap type igniter plug, as in cathode or anode?
Cathode
What is required when starting an engine from a running engine?
The RPM of the supplying engine must be raised to the specified point to give the correct air pressure and to prevent the supply engine from exceeding its turbine temperature limitations
What are the types of engine starting?
- Isopropyl Nitrate
- Gas turbine
- Windmill
- Electric
- Low pressure air turbine
Where might a sprag clutch be found?
Inside a low pressure air turbine starter
What type of ignition system is used on all jet engines?
High-energy ignition with a dual system, with each ignition unit connected to its own igniter plug
How does a typical DC ignition unit work?
An induction coil, operated by a trembler mechanism, passes an AC voltage into the secondary circuit
AC is half-wave rectified by a diode and the remaining energy is stored in a large capacitor
When the energy stored in the capacitor creates sufficient ionisation in a sealed glass chamber, the charge will cross the discharge gap, through a choke coil to the plug
