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Eric,
Q--I'll respect any differing opinions, I'm just curious since the engine
companies don't seem to agree with what I thought were accepted standards.
Eric Ahlstrom
A--Please review the excerpt below for a more detailed description of the
TCM FADEC system. This is for the Continental IOF-240-B but applies to the
IOF-550 in terms of redundancy.
This is out of the IOF-240-B Operation Manual.
Fuel and Ignition Control
The IOF-240-B ignition spark and fuel mixture is controlled by a FADEC (Full
Authority Digital Engine Control) system. The FADEC system automatically
sets the fuel mixture and ignition timing for optimum performance given
current operating conditions. The fuel system is of the sequential port
injection variety and the ignition system is waste-spark. The system
consists of two electronic control units, an engine harness, various sensors
(referred to as the sensor set), four injector control coils, a cabin
harness, and a cockpit mounted Health Status Annunciator (HSA) (see Figure
1-1 below).
The two electronic control units are referred to as MPC unit 1 and MPC unit
2. The lower portion of each unit contains an electronic circuit board and
the upper portion houses the ignition coils. Each unit contains two
independent microprocessor controllers and the discrete components necessary
for each. A microprocessor and its associated components are referred to as
a control channel. Each control channel is assigned to a single engine
cylinder; thus, a single MPC unit can operate two engine cylinders. There
are no shared electronic components between the two control channels.
The FADEC system is fully redundant. A control channel is capable of
operating its assigned cylinder and the cylinder assigned to the second
control channel within the same MPC unit. All critical sensors are
redundant with one sensor from each type pair connected to channels in
different MPC units. Synthetic software default values are also used should
both sensors of a redundant pair fail.
The MPC units receive information from the sensor set via the engine
harness. The harness interfaces with the MPC unit via a 50-pin DIN
connector (19). The engine harness connects to the cabin harness (not
shown) via two firewall mounted bulkhead connectors (20). Input from the
pilot and primary/secondary power is supplied to the MPC units via the cabin
harness through the bulkhead connectors. Information from the MPC units is
also conveyed to the HSA and the cockpit mounted data port through the same
cabin harness/bulkhead connector assembly.
Sensor input to each channel includes engine speed and crank position (5),
fuel pressure (9), manifold pressure (8), manifold air temperature (11) and
(12) and wide open throttle position (10). In addition, each channel also
receives exclusive signals for measuring its cylinder’s head temperature
(14) and exhaust gas temperature (13). The control channels use the signals
from the sensor set to determine the required fuel mixture and ignition
timing for its cylinder’s next combustion event. The required fuel quantity
is injected into the cylinder intake port at the appropriate time, with
respect to crank position, via a solenoid style fuel injector (one per
cylinder). The injector’s control coil (15) is driven directly by the
associated control channel.
Ignition spark is timed to the engine’s crank position. The timing is
variable from cranking speed up to 2,000 rpm depending on engine load
conditions. The spark energy is also varied with respect to engine load.
Above 2,000 rpm the timing is fixed at 26° BTDC unless the overspeed control
function is activated. Because the ignition system is a waste-spark type,
each cylinder’s spark plugs are fired twice per engine cycle, once on the
compression stroke and again on the exhaust stroke. The polarity of the
ignition spark changes with each firing to prevent magnetization of the
spark plugs.
Each channel controls its assigned cylinder in a manner that will yield
optimum performance for the current operating conditions to the extent that
normal operating parameters are not exceeded. When this occurs, fuel
mixture may be enriched or leaned and ignition timing may be retarded in an
effort to minimize the extent of limit excursion for the given parameter.
The FADEC system is electrically powered and not self-excited. As such, the
system requires two power supply sources. Typically, one source will be the
aircraft’s primary electrical buss, referred to as PWR A. The second
source, referred to as PWR B, may be a second aircraft buss, an engine
driven generating device, or a battery. Details relating to the power
output and reliability of the power sources are provided in Chapter 3.
Electrical power to the FADEC system is controlled from the cockpit by two
switches used for interrupt PWR A and PWR B. Using a conventional
aircraft-style ignition switch the pilot controls the enabling, starting and
disabling of the FADEC system and thereby controls the operation of the
engine. A complete description of the operation of these controls is
provided in Chapter 6 of this manual.
The fuel conducting portion of the IOF-240-B is similar to other
conventional aircraft engine fuel systems (see Figure 1-2). An engine-driven
positive displacement vane pump (2) supplies the fuel to the injectors. Fuel
leaving the pump passes through a 20 micron filter (36) (airframe supplied)
before entering the fuel distribution block (15). From this point, fuel
travels to each solenoid injector (30).
For optimum operation, the fuel system must be pressurized to at least 25
psig. For this reason, an electric boost pump is required for starting and
at times during low rpm operation. The boost pump is controlled using the
cockpit mounted Boost Pump Mode Switch (BPMS). The BPMS has three operating
positions, OFF, ON and AUTO. With the BPMS in the AUTO position, the FADEC
system can activate and deactivate the electric boost pump as required. In
the ON and OFF positions the pilot has authority over the state of the
electric boost pump.
The health status of the FADEC system is conveyed to the pilot via the HSA.
Discrete lamps in the HSA will illuminate upon detection of system faults
and some normal control actions. A complete description of the HSA and its
operation is provided in Chapters 6 and 7 of this manual.
…
Questions???
ERic--
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