The engine start system of the present invention generally relates to mechanical actuation of components used to start a turbojet engine in a more economical fashion than currently known systems.
A one-shot start system for a turbojet engine is typically comprised of a pyrotechnic device called an ignitor activated by an initiator. Initiators (often called squibs) are generally electroexplosive devices utilizing an electrically heated bridgewire to ignite an explosive charge. The bridgewire is designed to ignite the charge only when a specific level of current is applied across it by a firing circuit. Since the initiator is electrically activated, the bridgewire and associated firing circuit must be carefully designed to prevent accidental activation from exposure to external electromagnetic signals produced by equipment such as conventional radars. Manufacturing an initiator wherein the bridgewire is electrically heated at a very precise firing current requires strict manufacturing and quality control processes that tend to increase the cost of the device.
A bullet operates in an analogous fashion to the aforementioned ignitor. A sharp blow from the hammer of a gun mechanically fires an initiator, usually termed a primer. The primer then ignites a gunpowder charge which propels the bullet down a gun's barrel. The primer serves the same actuation function for a bullet as the bridgewire initiator does for an ignitor. However, unlike the initiator, the primer is a percussion-activated device. As such, it is much cheaper to produce than the initiator since it has no critical electronic components. Additionally, the primer used in a bullet is not susceptible to accidental activation from electromagnetic radiation as evidenced by the longstanding practice of using conventional bullets in combat aircraft applications. The use of a percussion primer initiator for turbojet engine start system will substantially reduce overall system cost.
Most known turbojet engines utilize an electric fuel pump to meter fuel into an engine combustor. Engines using through-shaft fuel induction may utilize a low pressure fuel delivery system in conjunction with a pressurized fuel tank. This obviates the need for a high pressure fuel delivery device such as a pump. A low pressure fuel injector, for example an automotive injector, may be used in place of a metering fuel pump.
Conventional automotive injectors are typically operated by a solenoid actuated valve. These valves may be opened and closed by a pulse width modulated signal (PWM) which varies the valve actuation time by varying the signal pulse duration. (See for example U.S. Pat. No. 5,052,174, assigned to assignee of the present invention.) Fuel delivery systems using automotive type injectors have proven acceptable for use in turbojet engines when operating at pulse frequencies of 60 to 140 hertz. Automotive injectors are readily available, durable, and very cost effective when compared to metering pumps.