发动机起动系统

Overview

The engine starting system is the mechanism that accelerates a jet engine's core from rest to a rotational speed at which combustion can be initiated and sustained without external assistance. Modern commercial aircraft rely on one of two primary starting methods: pneumatic starting, which uses compressed bleed air to spin an air turbine starter mounted on the engine accessory gearbox, and electric starting, which drives starter-generators to motor the engine electrically. Most conventional narrowbody and widebody jets use pneumatic starting, while next-generation aircraft such as the Boeing 787 Dreamliner introduced a fully electric starting architecture that eliminates the bleed-air requirement entirely.

The starting sequence is a carefully choreographed progression of events managed by the Full Authority Digital Engine Control (FADEC) or, on older aircraft, by the engine start controller in conjunction with the flight crew. A successful start must deliver sufficient airflow and ignition energy to establish a stable combustion kernel before the ignition system is switched off and the engine reaches idle power under its own thrust.

How It Works

In a pneumatic starting sequence, high-pressure air from the Auxiliary Power Unit (APU), a ground air cart, or a running engine is directed through the aircraft's bleed air ducting to the air turbine starter. The starter converts the pressure energy into rotational torque on the accessory gearbox, which is mechanically coupled to the high-pressure compressor shaft. As spool speed increases toward the light-off RPM threshold (typically 15–25 percent N2), the ignition system energises and fuel flow begins. Combustion is established, and the engine accelerates under its own power toward idle. The starter decouples automatically via a sprag clutch once engine speed exceeds starter speed.

On aircraft using electric starting, such as the Boeing 787-9, large starter-generator units — each rated at 250 kVA — motor the engine to light-off RPM using electrical power from either the APU generator or ground power. These units then transition from motor mode to generator mode once the engine is running, supplying the aircraft's electrical loads. This bleed-free architecture reduces thermodynamic losses in the engine core and is a key contributor to the 787's fuel efficiency advantage.

Key Components

• Air Turbine Starter (ATS): A small turbine that converts bleed-air pressure into shaft rotation. Mounted on the accessory gearbox and decoupled by a sprag clutch after start.
• Starter-Generator (electric architectures): Dual-mode electrical machine that motors the engine during start and generates power during normal operation.
• Bleed Air Duct and Valve: Delivers high-pressure air from the APU or crossbleed source to the starter. A start valve (pneumatically operated butterfly valve) modulates flow.
• Ignition System: Typically dual-channel with two igniter plugs per engine. High-energy capacitor-discharge units generate spark energy sufficient to ignite the fuel-air mixture at altitude relight conditions.
• Engine Start Controller / FADEC: Sequences the opening of fuel metering valves, ignition timing, starter engagement, and starter cutout based on N1/N2 speed and exhaust gas temperature (EGT) limits.

Aircraft Applications

• Boeing 737-800 — pneumatic start via APU or crossbleed from engine 2
• Airbus A320-200 — pneumatic start; APU or ground air cart standard for line operations
• Boeing 787-9 — fully electric start using 250 kVA starter-generators; no bleed air required

Advantages and Limitations

Pneumatic starting is mature, mechanically simple, and requires no heavy electrical infrastructure on the aircraft. Its primary limitations are the thermodynamic cost of extracting high-pressure compressor air (which reduces engine efficiency when bleed air is used in flight) and dependence on a working APU or external air source at the gate.

Electric starting eliminates bleed-air extraction from the engine core, preserving thermodynamic efficiency and allowing a cleaner engine aerodynamic cycle. The trade-off is the need for large, heavy starter-generator units and a high-capacity electrical distribution system capable of handling the motoring loads. Weight penalties and system complexity are partially offset by the elimination of the bleed-air ducting and associated valves throughout the airframe. As next-generation narrowbodies and widebodies mature, electric starting is expected to become increasingly prevalent.