Aircraft jet engines need to be cranked externally to reach a critical self-sustaining speed, beyond which they are capable of accelerating to full speed and power. On the aircraft itself, this cranking is done through a smaller gas turbine started through a small DC motor powered with a battery.
This is fine for occasional starts but is unsuitable for frequent starting when testing engines on the ground. An economical and reliable means of starting these engines frequently is required on the test bed.
The System Concept:
A variable displacement hydraulic motor was connected to the gas turbine rotor shaft through a step up gearbox. A variable displacement axial piston pump provides the necessary flow in response to control signals under the required pressures to accelerate the turbine shaft.
The starter system supplied by us had the following requirements:
Maximum Starter speed: 4700rpm
Speed - Torque characteristics: 0rpm @ 52 Nm, 2000rpm @ 520Nm, 4700rpm @ 140Nm
Time for start up: 40 to 60 secs.
A combination of hydroquips engineering expertise and Kawasaki Technology resulted in the successful application of the K3VG.
The system had a maximum flow requirement of 240lpm and a minimum flow requirement of 150lpm, hence a K3VG 180cc/rev pump was selected and driven with a standard electric motor at 1440rpm.
A positive displacement control which permits the pump displacement to be varied through a remote pressure signal was found to suit the requirement ideally. The system needed to operate at a maximum pressure of 330bar and a minimum pressure of 270bar, well within the capability of the K3VG.
A remote pressure signal modulated through a proportional control valve was used to vary the displacement of the pump in line with the system requirements. An external proportional relief valve was also incorporated on the pump outlet line to limit the maximum pressure of the system and thereby control the acceleration.
The complete system has been fully commissioned and found to perform to exceed customer expectation. The starter characteristics as required can be achieved repeatedly and consistently. The strength of partnership between Hydroquip and Kawasaki has been actively demonstrated by the customer's repeat orders.