Honda 1991 Accord Sedan, Owner'S Manual, Page: 135 / 146

Emission Controls (cont'd)
Engine Exhaust Controls
The engine exhaust emission control systems are designed to
control combustion during idle, acceleration, cruise, and deceleration.
These systems are entirely separate from the crankcase and
evaporative emission control systems described previously.
PGM Fl System
The PGM-FI system consists of three independent sub-systems;
Air Intake, Electronic Control and Fuel Control, thus allowing more
accurate control of air/fuel ratios under all operating conditions.
The Electronic Control Unit (ECU) detects the amount of air
drawn into the cylinders and determines the amount of fuel to be
injected to provide the optimum air/fuel ratio for all engine needs.
Ignition Timing Control System
This system automatically controls the ignition timing to reduce
the amount of HC and NOx.
Catalytic Converter
The catalyst is used to convert hydrocarbons (HC), carbon
monoxide (CO) and oxides of nitrogen (NOx) in the exhaust gas,
to carbon dioxide (CO
2
), dinitrogen (N
2
) and water vapor.
Exhaust Gas Recirculation (EGR)
The EGR system is designed to control the formation of oxides of
nitrogen (NOx) caused when fuel mixture burns at high
temperature. It works by recirculating exhaust gas through the
EGR valve and intake manifold into the combustion chambers
where it reduces peak temperature by diluting the air/fuel mixture.