Subaru impreza 2002 Service Manual page 22

Emission Control (Aux. Emission Control Devices)
1. System Overview
There are three emission control systems which are as follows:
D
Crankcase emission control system
D
Exhaust emission control system
D
Three-way catalyst system
D
Air/fuel (A/F) control system
D
Ignition control system
D
Evaporative emission control system
NOTE: The STi model is not provided with a precatalytic converter.
Item
Crankcase emission control
system
Exhaust Catalyst Pre
emission system (Ex-
control cept
system STi
model)
Front
Rear
A/F control
system
Ignition control
system
Evaporative emission control
system
SYSTEM OVERVIEW
Main components Function
Positive crankcase Draws blow-by gas into intake manifold from crankcase and burns it
ventilation (PCV) together with air-fuel mixture. Amount of blow-by gas to be drawn in
valve is controlled by intake manifold pressure.
Three-way catalyst Oxidizes HC and CO contained in exhaust gases as well as reducing
NOx.
Engine control mod- Receives input signals from various sensors, compares signals with
ule (ECM) stored data, and emits a signal for optimal control of air-fuel mixture
ratio.
Front oxygen (A/F) Detects quantity of oxygen contained exhaust gases.
sensor
Rear oxygen sensor Detects density of oxygen contained exhaust gases.
Throttle position Detects throttle position.
sensor
Mass air flow sensor Detects amount of intake air.
and intake air tem- and intake air tem-
Detects intake air temperature of air cleaner case.
perature sensor
ECM Receives various signals, compares signals with basic data stored in
memory, and emits a signal for optimal control of ignition timing.
Crankshaft position Detects engine speed (Revolution).
sensor
Camshaft position Detects reference signal for combustion cylinder discrimination.
sensor
Engine coolant tem- Detects coolant temperature.
perature sensor
Knock sensor Detects engine knocking.
Canister Absorbs evaporative gas which occurs in fuel tank when engine
stops, and releases it to combustion chambers for a complete burn
when engine is started. This prevents HC from being discharged into
atmosphere.
Purge control Receives a signal from ECM and controls purge of evaporative gas
solenoid valve absorbed by canister.
EC-2