Exhaust Emission Control System

Exhaust Emission Control System
The fuel injection system provides accurately metered quantities of fuel to the combustion chambers to ensure the most efficient air to fuel ratio under all operating conditions. A further improvement to combustion is made by measuring the oxygen content of the exhaust gases to enable the quantity of fuel injected to be varied in accordance with the prevailing engine operation and ambient conditions; any unsatisfactory composition of the exhaust gas is then corrected by adjustments made to the fuelling by the ECM.

The main components of the exhaust emission system are two catalytic converters which are an integral part of the front exhaust pipe assembly. The catalytic converters are included in the system to reduce the emission to atmosphere of carbon monoxide (CO), oxides of nitrogen (NOx) and hydrocarbons (HC). The active constituents of the catalytic converters are platinum (Pt), palladium (PD) and rhodium (Rh). Catalytic converters for NAS low emission vehicles (LEVs) from 2000MY have active constituents of palladium and rhodium only. The correct functioning of the converters is dependent upon close control of the oxygen concentration in the exhaust gas entering the catalyst.

The two catalytic converters are shaped differently to allow sufficient clearance between the body and transmission, but they remain functionally identical since they have the same volume and use the same active constituents.

The basic control loop comprises the engine (controlled system), the heated oxygen sensors (measuring elements), the engine management ECM (control) and the injectors and ignition (actuators). Other factors also influence the calculations of the ECM, such as air flow, air intake temperature and throttle position. Additionally, special driving conditions are compensated for, such as starting, acceleration, deceleration, overrun and full load.

The reliability of the ignition system is critical for efficient catalytic converter operation, since misfiring will lead to irreparable damage of the catalytic converter due to the overheating that occurs when unburned combustion gases are burnt inside it.

CAUTION:
^ If the engine is misfiring, it should be shut down immediately and the cause rectified. Failure to do so will result in irreparable damage to the catalytic converter.
^ Ensure the exhaust system is free from leaks. Exhaust gas leaks upstream of the catalytic converter could cause internal damage to the catalytic converter.
^ Serious damage to the engine may occur if a lower octane number fuel than recommended is used. Serious damage to the catalytic converter and oxygen sensors will occur if leaded fuel is used.

Air : fuel ratio
The theoretical ideal air:fuel ratio to ensure complete combustion and minimise emissions in a spark-ignition engine is 14.7:1 and is referred to as the stoichiometric ratio.

The excess air factor is denoted by the Lambda symbol (lambda), and is used to indicate how far the air:fuel mixture ratio deviates from the theoretical optimum during any particular operating condition.
^ When (lambda) = 1, the air to fuel ratio corresponds to the theoretical optimum of 14.7:1 and is the desired condition for minimising emissions.
^ When (lambda) > 1, (i.e. (lambda) = 1.05 to (lambda) = 1.3) there is excess air available (lean mixture) and lower fuel consumption can be attained at the cost of reduced performance. For mixtures above (lambda) = 1.3, the mixture ceases to be ignitable.
^ When (lambda) < 1, (i.e. (lambda) = 0.85 to (lambda) = 0.95) there is an air deficiency (rich mixture) and maximum output is available, but fuel economy is impaired.

The engine management system used with V8 engines operates in a narrower control range about the stoichiometric ideal between (lambda) = 0.97 to 1.03 using closed-loop control techniques. When the engine is warmed up and operating under normal conditions, it is essential to maintain (lambda) close to the ideal ((lambda) = 1) to ensure the effective treatment of exhaust gases by the three-way catalytic converters installed in the downpipes from each exhaust manifold.

Changes in the oxygen content has subsequent effects on the levels of exhaust emissions experienced. The levels of hydrocarbons and carbon monoxide produced around the stoichiometric ideal control range are minimised, but peak emission of oxides of nitrogen are experienced around the same range.

Fuel Metering
For a satisfactory combustion process, precise fuel injection quantity, timing and dispersion must be ensured. If the air:fuel mixture in the combustion chamber is not thoroughly atomized and dispersed during the combustion stroke, some of the fuel may remain unburnt which will lead to high HC emissions.

Ignition Timing
The ignition timing can be changed to minimise exhaust emissions and fuel consumption in response to changes due to the excess air factor. As the excess air factor increases, the optimum ignition angle is advanced to compensate for delays in flame propagation.