General information

Emission control and exhaust systems / General information

Emission control systems 1 All petrol engine models have the ability to use unleaded petrol and are controlled by engine management systems that are “tuned “ to give the best compromise between driveability, fuel consumption and exhaust emission production. In addition, a number of systems are fitted that help to minimise other harmful emissions: a crankcase emissioncontrol system that reduces the release of pollutants from the engines lubrication system is fitted to all models, catalytic converters that reduce exhaust gas pollutants are fitted to most models and an evaporative loss emission control system that reduces the release of gaseous hydrocarbons from the fuel tank is fitted to all models.

Crankcase emission control 2 To reduce the emission of unburned hydrocarbons from the crankcase into the atmosphere, the engine is sealed and the blow-by gases and oil vapour are drawn from inside the crankcase, through a wire mesh oil separator, into the inlet tract to be burned by the engine during normal combustion.

3 Under conditions of high manifold depression (idling, deceleration) the gases will be sucked out of the crankcase. Under conditions of low manifold depression (acceleration, full-throttle running) the gases are forced out of the crankcase by the (relatively) higher crankcase pressure. If the engine is worn, the raised crankcase pressure (due to increased blow-by) will cause some of the flow to return under all manifold conditions. On certain engines, a pressure regulating valve (mounted on the side of the cylinder head) controls the flow of gases from the crankcase.

4 On engine code PY, the excess inlet air generated by the G-charger, when the engine is on part throttle or overrun, is directed through the camshaft cover by the bypass valve.

Exhaust emission control 5 To minimise the amount of pollutants which escape into the atmosphere, most models are fitted with a catalytic converter in the exhaust system. On all models where a catalytic converter is fitted, the fuelling system is of the closed-loop type, in which a Lambda sensor in the exhaust system provides the engine management system ECU with constant feedback, enabling the ECU to adjust the air/fuel mixture to optimise combustion.

6 The Lambda sensor has a heating element built-in that is controlled by the ECU through the Lambda sensor relay to quickly bring the sensor’s tip to its optimum operating temperature. The sensor’s tip is sensitive to oxygen and relays a voltage signal to the ECU that varies according on the amount of oxygen in the exhaust gas. If the inlet air/fuel mixture is too rich, the exhaust gases are low in oxygen so the sensor sends a low-voltage signal, the voltage rising as the mixture weakens and the amount of oxygen rises in the exhaust gases. Peak conversion efficiency of all major pollutants occurs if the inlet air/fuel mixture is maintained at the chemically-correct r a t i o f o r c o m p l e t e combustion. This equates to 14.7 parts (by weight) of air to 1 part of fuel (the ‘stoichiometric’ ratio). The sensor output voltage alters in a large step at this point, the ECU using the signal change as a reference point and correcting the inlet air/fuel mixture accordingly by altering the fuel injector pulse width. Details of the Lambda sensor removal and refitting are given in Chapter 4A or B as applicable .

Evaporative emission control 7 To minimise the escape of unburned hydrocarbons into the atmosphere, an evaporative loss emission control system is fitted to all petrol models. The fuel tank filler cap is sealed and a charcoal canister is mounted underneath the front right-hand wing to absorb the vapour released from the fuel contained in the fuel tank. It stores them until they can be drawn from the canister via the purge valve into the inlet tract, where they are then burned by the engine during normal combustion. On single-point injection models, the purge valve is controlled electronically by the engine management system ECU. On models with multi-point injection, the purge valve is controlled by vacuum supplied from the inlet manifold.

8 To ensure that the engine runs correctly when it is cold and/or idling and to protect the catalytic converter from the effects of overfuelling, the purge control valve(s) are not opened by the ECU until the engine has warmed up, and the engine is under load; the valve solenoid is then modulated on and off to allow the stored vapour to pass into the inlet tract.

Exhaust systems
9 The exhaust system comprises the exhaust manifold, a number of silencer units (depending on model and specification), a catalytic converter (where fitted), a number of mounting brackets and a series of connecting pipes.