Doosan G20P-5, Руководство по обслуживанию

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G424F(FE) Service Manual Chapter 5. Engine Management System (EMS) 145
The HEGO stoichiometric air-fuel ratio voltage target
is approximately 500 mV and changes slightly as a
function of speed and load. When the pre-catalyst
HEGO sensor sends a voltage signal less than 450
mV the SECM interprets the air-fuel mixture as lean.
The SECM then decreases the PWM duty cycle sent
to the fuel trim valves in order to increase the fuel
pressure to the mixer inlet; thus richening air-fuel
mixture. The opposite is true if the SECM receives a
voltage signal above 450 mV from the HEGO. The
air-fuel mixture would then be interpreted as being
too rich and the SECM would increase the duty
cycle of the trim valves.
CAUTION
The HEGO sensors are calibrated to work with
the MI-07 control system. Use of alternate
sensors may impact performance and the ability
of the system to diagnose rich and lean
conditions.
Catalytic Muffler
In order to meet 2007 emission requirements a 3-
way catalyst is necessary.
All exhaust gases pass through a catalyst that is
mounted in the catalytic muffler. It filters the harmful
gases through a dense honeycomb structure coated
with precious metals such as platinum, palladium,
and rhodium. Chemical reactions occur on these
surfaces to convert the pollutants into less harmful
gases. Catalysts store oxygen on lean mixtures
(less than optimal amount of fuel) and release
oxygen on rich mixtures (more than optimal amount
of fuel). The primary pollutant produced on the lean
swing is nitrous oxide. Oxygen is removed from
nitrous oxide by the converter, resulting in nitrogen
gas, a harmless emission. On the rich cycle, the
primary pollutant is carbon monoxide. By adding the
oxygen that was stored on the lean cycle to the
carbon monoxide, carbon dioxide is produced.
Inside the catalytic muffler is a three-way catalyst as
well as sound dampening and spark arresting
features. The three-way catalyst section consists of
a honeycomb coated with a mixture of platinum,
palladium and rhodium. As engine exhaust gases
flow through the converter passageways, they
contact the coated surface, which initiate the
catalytic process. The reduction catalyst is the first
stage of the catalytic converter. It uses platinum and
rhodium to help reduce the NOx emissions. The
oxidation catalyst is the second stage of the catalytic
converter. It reduces the unburned hydrocarbons
and carbon monoxide by burning (oxidizing) them
over a platinum and palladium catalyst. Cerium is
also used to promote oxygen storage and improve
oxidation efficiency.
As exhaust and catalyst temperatures rise the
following reaction occurs:
•
Oxides of nitrogen (NOx) are reduced into simple
nitrogen (N2) and carbon dioxide (CO2).
•
Hydrocarbons (HC) and carbon monoxide (CO)
are oxidized to create water (H2O) and carbon
dioxide (CO2).
The MI-07 control system monitors the exhaust
stream pre and post catalyst and uses this
information to control the air-fuel mixture. By using
the signals from the HEGOs, the SECM can
increase or decrease the amount of oxygen in the
exhaust by modulating the FTVs and adjusting the
air-fuel ratio. This control scheme allows the SECM
to make sure that the engine is running at the
correct air to fuel ratio so that the catalyst can
perform as required to meet the emissions
certification.