I. Field of the Invention
The present invention relates to a turbine engine starting system.
II. Description of the Prior Art
The previously known gas turbine engines, and particularly smaller units commonly known as auxiliary power units, typically employ electric motors to both pump the fuel to the engine and to rotatably drive the turbine shaft in order to start the turbine engine. The starting motors must produce sufficient torque to rotatably drive the turbine shaft to a high enough speed so that sufficient compressed air and fuel is obtained for the initiation of efficient combustion in the turbine engine. Moreover, sufficient heat or live flame energy must be supplied to the main combustion chamber of the turbine engine in order to initiate self sustaining combustion in the main combustion chamber.
It has been the previous practice with such turbine engines to power the electric starting motors from batteries and to also couple the batteries through high tension coils and spark plugs in order to fire or ignite the combustion chamber. The batteries and electric motor used to start the turbine engine are not only expensive but are also heavy and bulky in construction.
These previously known gas turbine engine starting systems, while reasonably reliable at normal operating conditions, are unreliable in sub-zero temperatures and/or very low ambient pressures (for example, 2-10 psia). During such adverse operating conditions, not only is the electrical energy output from the batteries reduced but also more electrical energy is required to rotatably drive the gas turbine engine rotor to a point at which the turbine engine can begin self-sustaining combustion. At times, however, the electrical energy from the battery is depleted before the gas turbine engine can be started.
Still another previously known turbine engine starting system employs stored hydraulic pressure which rotatably drives an electric motor to produce the required electrical energy. Such systems, however, are expensive in construction and ineffective when the hydraulic pressure is depleted.
As the result of the above mentioned factors, the previously known turbine engine starting systems have been designed as a compromise between the desired reliability, cost, and multi-start capability for the turbine engine.