Doosan G643, Руководство по обслуживанию

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G643(E) Service Manual Chapter 5. Engine Management System (EMS) 161
The MI-07 system is capable of operating with either
a distributor based ignition system or a
distributorless ignition system. The current
application uses a distributor based ignition system.
The distributor will have no internal advance
mechanisms giving the SECM consistent authority
over ignition timing. The spark is sent to the
appropriate cylinder in the conventional way via the
rotor arm and spark plug wires. The SECM uses the
signal from the GM (General Motors) Delco Ignition
Module to determine the engine position and RPM
at any time. It uses this information together with the
information from the TPS sensor and TMAP to
calculate the appropriate ignition timing settings.
The General Motors (GM) distributor (Figure 14)
used in the Delco EST ignition system, incorporates
a Variable Reluctance (VR) sensor, which transmits
a reference signal to the GM ignition module (Figure
15) located on the distributor. A variable reluctance
sensor is an electromagnetic device consisting of a
permanent magnet surrounded by a winding of wire.
The sensor is used in conjunction with a ferrous
signal rotor on the distributor shaft. The signal rotor
has six lobes, one for each cylinder. Rotation of the
signal rotor near the tip of the sensor changes the
magnetic flux, creating an analog voltage signal in
the sensor coil.
(1)
lgnition coil (
2
) Ignition module
Figure 15. GM Ignition Module
The rising edge of the VR signal is converted to a
rising 5-volt signal by the ignition module. As the VR
signal passes back through zero volts, a falling edge
is created producing a square wave or digital signal,
similar to the signal produced by a Hall effect sensor.
This falling edge signal provides a stable engine
position reference at all engine speeds for the
SECM.
Exhaust System
Heated Exhaust Gas Oxygen Sensors (HEGO)
The MI-07 system utilizes two HEGO (O2) sensors.
One sensor is a pre-catalyst sensor that detects the
amount of oxygen in the exhaust stream and is
considered the primary control point. Based upon
the O2 sensor feedback, the MI-07 system supplies
a stoichiometric air-fuel ratio to the catalytic
converter. The catalytic converter then reduces
emissions to the required levels. The second sensor
is a post-catalyst sensor that detects the amount of
oxygen after the catalyst. This sensor is used as a
secondary control point to adjust the pre-catalyst
setpoint to ensure proper catalyst conversion
efficiency.
Figure 18. HEGO (O2) Sensor
Once a HEGO sensor reaches approximately 600°F
(316°C), it becomes electrically active. The
concentration of oxygen in the exhaust stream
determines the voltage produced. If the engine is
running rich, little oxygen will be present in the
exhaust and voltage output will be relatively high.
Conversely, in a lean situation, more oxygen will be
present and a smaller electrical potential will be
noticed.
In order for the sensor to become active and create
an electrical signal below 600°F (316°C) a heated
element is added to the sensor housing. Two wires
provide the necessary 12 Vdc and ground signal for
the heater element. A fourth wire provides an
independent ground for the sensor. The pre-catalyst
sensor heater is powered by the main power relay
and is always powered. The post-catalyst sensor
heater is powered from an additional relay that is
controlled by the SECM. This relay is only energized
when the SECM calculates that water condensation
in the exhaust system and catalytic muffler prior to
the sensor should be evaporated. This is to avoid
thermal shock of the sensor that could prematurely
fail the sensor.