1. JET ENGINE PROPULSION
Chapter 7
Start Ignition System

2. The basic sequence of the start cycle
STARTING AND IGNITION
The basic sequence of the start cycle
Starter Motor
External Gearbox
Starter motor drives
the external gearbox.
Airflow is produced in engine.
Engine now drives the external gearbox
which then drives all the accessories.
At a design RPM fuel is released
to the burners.
External gearbox drives
the engine shaft, turning
the compressor and turbines.
The igniters are powered and
produce a spark to ignite the fuel.
Once the engine attains self sustaining
RPM the igniters are switched off.

3. Sprag Clutch Mechanism
STARTING AND IGNITION
Rolls-Royce AVON - (Graphic Representation)
Starter Motor
Sprag Clutch Mechanism
Drive to Engine Shaft
Air In
Air Out
Starter Motor
Output Shaft
Reduction Gear
External Gearbox
Turbine

4. IGNITION There are two types of igniter plugs Air Gap
Surface Discharge
The air gap type has a larger gap
between the body of the igniter
and the electrode.
The surface discharge type works
the same as the air gap, but as the
gap is smaller, the electrode only
requires a voltage difference of
approximately 2000 Volts in order
to operate. 
This means that a voltage difference
of approximately Volts is
required to ionize the gap before a
spark will occur.
As was seen in previous lessons there are usually only two igniters as all the
flame tubes are interconnected.
The only time you know that an igniter has failed is when the second one fails.

5. STARTING AND IGNITION Igniter Box – (Not to Scale) Surface Discharge
High Tension Output
Low Tension Input
Control Input
Surface Discharge
Air Gap
Connector Thread
Engine Cross Section
Igniter Plugs
Igniter Boxes
HT Leads
Spanner Flats
Igniter Mounting Thread
Cooling Air Inlet Holes
Insulator
Centre Electrode
Semi Conductor Pellet
Cooling Air Outlet Holes
Igniter Tip

6. STARTING AND IGNITION
Any Questions?