1984-10-01

Control of Fuel During Starting of a Gas Turbine 841511

Starting a gas turbine requires the correct amount of fuel to be delivered, by the fuel control, to achieve ignition then to successfully accelerate the engine to idle without compressor surge or turbine over-heating. This has been accomplished over the last twenty-five years at Lycoming by several different methods. These are reviewed and their advantages, disadvantages, necessary automatic safeguards, and field experience discussed. The more modern, non-scheduling systems, result in gradual controlled starts with lower turbine temperatures.
To start a gas turbine engine, the starter accelerates the compressor to about 15% speed. Sometime during this acceleration, the igniter plugs are switched on (sparking at a fixed rate of around five times a second) and fuel is introduced. Ignition occurs and the resulting torque on the turbine helps the starter to accelerate the engine. The thermodynamics enable the gas turbine to be self accelerating around 30% but the starter is usually left on to help the machine to idle, usually around 50%.
Fuel has to be delivered at the correct time and in the correct quantity to achieve the initial combustion and then to control the resultant temperatures and torques as the engine accelerates. These two requirements are usually, but not necessarily compatible. They are discussed separately in this paper.

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