Emission Control Systems: Description and Operation
Engine Emission Control
COMPONENT LOCATION
Engine Emission Control:
INTRODUCTION
The engine emission control system comprises:
- EGR system
- Crankcase ventilation system
- Secondary Air Injection (SAI) system
The EGR system is controlled by a PWM signal from the ECM. The system comprises an EGR valve and a gas transfer pipe.
The crankcase ventilation system is ventilated through a part load and a full load breather. The part load breather is connected through a PCV valve, which prevents air flow back into the engine.
The engine emission control system also features a Secondary Air Injection (SAI) system, comprising a pump, solenoid control valve and associated pipes. The pumps operation is controlled by the engine management system.
EGR SYSTEM
The EGR (exhaust gas recirculation) system is controlled by a PWM signal from the ECM (engine control module). The system comprises an EGR (exhaust gas recirculation) valve and a gas transfer pipe.
The EGR (exhaust gas recirculation) valve is a 4 pole 60 point stepper motor with an integral cooler unit which is mounted on the side of the induction elbow. The EGR (exhaust gas recirculation) valve receive four 12 Volt signals from the ECM (engine control module). The EGR (exhaust gas recirculation) valve is cooled by the return coolant flow from the electric throttle.
The EGR (exhaust gas recirculation) valve is connected to the LH exhaust manifold via the gas transfer pipe.
CRANKCASE VENTILATION
The engine is ventilated through a part load and a full load breather. The part load breather is a flexible composite hose connected between the PCV (positive crankcase ventilation) valve mounted above the oil separator in the bank A camshaft cover and the induction elbow. The PCV (positive crankcase ventilation) valve prevents reverse flow into the crankcase.
The full load breather is a flexible hose connected between the oil separator in the bank B camshaft cover and the air intake duct.
The ends of the breather hoses incorporate quick release connectors.
The oil separators consist of wire gauze packed into an open ended enclosure below the breather outlet in each camshaft cover.
SECONDARY AIR INJECTION
Secondary Air Injection Component Location
Secondary Air Injection:
Secondary Air Injection (SAI) is fitted to the vehicle to reduce exhaust emissions to meet European Union (EU) Stage 4 and Federal Petrol Emission standards. The SAI system comprises:
- SAI pump
- SAI valve
- Associated pipes
- SAI Absolute Pressure (MAP) sensor (NAS only)
The SAI pump is used to provide a supply of air into the exhaust manifolds during the cold start period of the engine. The SAI cycle lasts for up to 65 seconds. The hot unburnt fuel particles leaving the combustion chamber mix with the air injected into the exhaust manifolds and immediately combust.
This subsequent combustion of the unburnt and partially burnt carbon monoxide (CO) and hydrocarbon (HC) particles help to reduce the emission of these pollutants from the exhaust system. The additional heat generated in the exhaust manifold also provides rapid heating of the exhaust system catalytic converters. The additional oxygen which is delivered to the catalytic converters also generates an exothermic reaction which causes the catalytic converters to reach their optimum operating temperature and 'light off' quickly.
The catalytic converters only start to provide effective treatment of emission pollutants when they reach an operating temperature of approximately 250 Degrees C (482 Degrees F) and need to be between temperatures of 400 Degrees C (752 Degrees F) and 800 Degrees C (1472 Degrees F) for optimum efficiency. Consequently, the heat produced by the SAI 'afterburning' reduces the time delay before the catalysts reach an efficient operating temperature.
SAI Pump
The sai pump is located behind the right-hand side of the front bumper cover. The pump is fitted on rubber mountings to help prevent noise which is generated by SAI pump operation. The SAI pump is powered from the vehicle battery by a dedicated relay and supplies approximately 10 to 15 kg/hr (22 to 33 lb/hr) of air when the engine is at idle speed and the ambient temperature is below 20 Degrees C (68 Degrees F).
Air is drawn into the SAI pump through vents in its front cover and is then passed through a non serviceable paper element filter. The air is delivered to the exhaust manifold on each side of the engine through a combination of plastic pipes and stainless steel tubes.
One second after the SAI pump is energized, the ECM switches on the SAI switching valve, which opens to allow vacuum from the SAI vacuum reservoir to be applied to the vacuum operated SAI control valve. When the vacuum is applied to the SAI control valve, it opens to allow the air from the SAI pump through to the exhaust manifolds.
When the ECM switches off the SAI switching valve, the vacuum supply to the SAI control valve is cut-off and the valve closes to prevent further air being injected into the exhaust manifolds. With an approximate five second delay after as the SAI switching valve is closed, the ECM removes power from the SAI pump relay, and this in turn stops the SAI pump from operating.
SAI Switching Valve
The ECM switches on the SAI switching valve with a one second delay after initiating SAI pump operation. When the SAI switching valve is open, a steady vacuum supply is allowed through to open the vacuum operated SAI control valve. When the ECM switches off the SAI switching valve, the valve closes and immediately shuts off the vacuum supply to the SAI control valve. The pump continues to operate for a further five seconds for system diagnostic purposes.
When the SAI switching valve is switched off, the vacuum supply line opens to atmosphere, and this causes the SAI switching valve to close automatically to prevent any further injection of air.
SAI control Valve
The injected air from the SAI pump is controlled by the SAI control valve. This allows the correct amount of air to be injected directly into the exhaust manifolds via tuned restrictors on each bank of cylinders. The SAI control valve prevents exhaust gasses from blowing back into the SAI pump.
The SAI control valve is assisted in operation by a vacuum source from the SAI vacuum reservoir located in the left hand side at the front of the engine. This assistance allows the actuation of the SAI control valve independently from the intake manifold vacuum levels available.
When the pressure in the exhaust system is higher than in the SAI system, the SAI control valve closes the circuit, and this protects the SAI system from exhaust gasses blowing back into the SAI system.
Vacuum to the SAI switching valve is provided from the intake manifold vacuum by the SAI vacuum reservoir. A small bore vacuum hose provides the vacuum route between the SAI vacuum reservoir and SAI switching valve. A further small bore vacuum hose is used to connect the SAI switching valve to the SAI control valve.
SAI Pressure Sensor - NAS only
The SAI system is monitored by measuring the system pressure by using the SAI pressure sensor at several instances during its cycle of operation.
The SAI system pressure is measured before operation of the SAI pump. The SAI pump is then switched on and with a one second delay, the SAI switching valve is opened. After a stabilizing period, the system pressure is measured again, this time by taking the average of a one second duration of readings, and normalizing for variations in battery voltage and atmospheric pressure. If the system pressure measured at this time has not risen enough with respect to the initial SAI pressure reading then a failure will be flagged.
A second pressure measurement is made after the requirement for SAI into the exhaust system has expired, but continuing on from the same period of SAI pump operation, i.e. the pump is left running, against a closed SAI switching valve. Again this pressure measurement is the average of a one second duration of readings normalized for variations in battery voltage and atmospheric pressure. If the system pressure measured at this time has not risen enough or has risen too much with respect to the system pressure during normal operation of SAI then a failure will be flagged.
A final pressure reading is taken after the SAI system has been switched off to ensure the system shuts down.
SAI Vacuum Reservoir
The SAI vacuum reservoir is located on the LH side of the engine at the front.
The SAI vacuum reservoir is included in the vacuum supply line between the intake manifold and the SAI switching valve.
The SAI vacuum reservoir contains a one-way valve to stop vacuum leaking back towards the intake manifold side. The SAI vacuum reservoir holds a constant vacuum so that the SAI control valve opens as soon as the SAI switching valve is switched on.