JET ENGINE PROPULSION Chapter 7 Start Ignition System

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.

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

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 25.000 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.

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

STARTING AND IGNITION Any Questions?