Overview
The ignition system operates automatically for the start sequence on the ground or in the air. In some conditions (automatic ignition), the ignition system can operate continuously. An ENGINE control panel in the flight compartment manually controls the ignition system. The ignition system gets its electrical power from the aircraft electrical system through the electronic control units (ECU). The ECUs transmit this power to energize the related channel of the ignition unit.
The dual-channel ignition unit receives its 28 VDC power from the aircraft through the ECU. The ignition unit changes the 28 VDC input voltage into an 8 to 14 kVDC output voltage that has a pulse. A power transformer in the ignition unit increases this voltage and supplies it to the unit’s discharge section. In the discharge section, a capacitor collects this voltage until it gets to the capacitor’s voltage limit. The capacitor then releases this energy at a rate of three pulses for each second, through the two ignition leads. Two ignition plugs receive these pulses to make the electrical sparks.
01/07/16
Igniter Plug
There are two ignition plugs in the ignition system. One ignition plug is on the lower left of the combustion chamber. The other ignition plug is on the lower right of the combustion chamber. The ignition plugs supply three sparks a second to start the combustion of the fuel and the air mixture. The ignition plug is a surface discharge type that has five iridium electrodes, one center and four outer. Each iridium electrode is isolated by a ceramic insulation.
01/07/16
Ignition Lead
The two ignition leads connect the output connectors on the ignition unit to the two ignition plugs. The ignition leads go from the ignition unit, through the bottom service strut to the ignition plugs at the engine core. The ignition leads transmit the high energy and high voltage electrical current from the ignition unit to the ignition plugs.
03/30/22
System Operation
Each ECU controls the channel A or channel B of the dual channel ignition system. The ECU supplies the electrical power to the ignition system during an engine start. It automatically turns the ignition system off when the engine runs at idle speed.
For automatic ground starts, the ECU uses one ignition channel, and will use the other channel for the subsequent start. For all other engine starts, two ignition channels are used at the same time. During a ground start, if an engine ignition fails in eight seconds of ignition start, the ECU will change from one ignition channel to the other channel.
The L (R) ENGINE switches, the L (R) STARTER switches, and the IGNITION push button annunciator (PBA) control the ignition system. The L (R) ENGINE switches are found on the throttle quadrant, aft of the throttle levers. The L (R) STARTER switches and the IGNITION PBA are found on the ENGINE control panel.
When the ignition system is in operation, a green IGN indication is shown on the engine indication and crew alerting system (EICAS) display. If the ignition system fails, an amber IGN indication is shown. As soon as the L (R) STARTER switch is set to STARTER position, a cyan START indication will show on the EICAS display. When an engine start is successful, or when an engine start is cancelled, the cyan START indication goes off. In a failed engine start, an amber STARTER indication is shown.
There are five different sequences to start an engine with ignition. They are: usual engine ground start, engine IGNITION PBA start, maximum motoring ground start, windmill start, and the airborne starter– assisted start.
In a usual engine ground start, the L (R) ENGINE switch is first set to RUN. This sends a signal to the ECU, which starts the fuel DC boost pumps, to supply the fuel. The L (R) STARTER switch is then set to START and held for one second, then released. This opens the fuel– feed shutoff valve (SOV) and the starter control valve to supply fuel and bleed air to the engine. This sequence also sends 28 VDC power from the DC ESSENTIAL BUS to energize the ignition unit. The ignition unit changes the 28 VDC input into a high output voltage of 8 to 14 K V. This high output voltage is sent to the ignition plugs through the ignition leads. The ignition plugs generate a rate of three sparks a second, at 0.5 joule a spark. With the fuel and bleed air available, engine ignition starts.
The IGNITION PBA can be used for a manual engine ignition with the aircraft airborne or on the ground. While airborne or on ground with the engines in operation, the L and R ENGINE switches are already set to RUN. The fuel– feed SOV and the starter control valve are already opened by the ECU to supply fuel and bleed air.
When the IGNITION PBA is pushed, the two channels of the ignition system will operate. The ignition plugs will supply the sparks continuously for engine ignition. With the aircraft on ground before an engine start, the L and R ENGINE switches are first set to RUN. When the IGNITION PBA is pushed in, only one STARTER switch, L or R, is necessary to start the two ignition channels for a continuous ignition.
The ignition will stop only when the IGNITION PBA is pushed again, or when the L and R ENGINE switches are set to OFF.
The maximum motoring ground start is used after a cancelled start caused by engine overtemperature. The L (R) ENGINE switch is first set to OFF. The L (R) STARTER switch is set to START and held. The N2 speed increases with the bleed air flow, and the ECU sends 28 V dc power to the ignition system. When N2 gets to a maximum motor speed, the L (R) ENGINE switch is set to RUN, and the START switch is released. The ECU sends a signal to open the fuel– feed SOV and the starter control valve to supply fuel and bleed air. With the fuel and bleed air available, and with the high voltage sparks from the ignition plugs, engine ignition occurs.
When the aircraft is airborne and with engine flameout, the windmill start is automatic. In this start, the L and (R) ENGINE switches are already set to RUN. An engine flameout condition signals the ECU to automatically turn on the DC boost pump to supply fuel, and the ignition plugs to start engine ignition. The ECU turns off the ignition system when the engines are lit and operates at idle speed. When the N2 speed is more than 4%, the ECU will send a signal to open the fuel– feed SOV to supply fuel. When N2 speed is less than 18%, the hydraulic pump depressurizes to support the windmill start.
The windmill start will stop when N2 speed is less than 4%, or when the L (R) ENGINE switch is set to OFF.
The airborne starter– assisted start is used when a windmill start is not successful. In this sequence, the L (R) ENGINE switch is already in the RUN position and fuel is available. The L (R) STARTER switch is then set to START to open the starter control valve to supply bleed air. And electrical power is supplied to the ignition system through the ECU. With the fuel and bleed air available, and with the sparks from the ignitor plugs, engine ignition occurs.
The ignition system also automatically operates when the stall protection computer (SPC) senses the aircraft in near stall condition. When the aircraft is near a stall condition, the SPC sends a ARINC 429 discreet signal to the ECU to start continuous engine ignition. The ignition system is turned off when the SPC senses the aircraft is out of the stall condition.
01/07/16
System Interface
The ignition system interfaces with the following systems/components:
- Ignition Unit
- Electronic Control Unit (ECU)