Ford Workshop Service and Repair Manuals

In contrast to the 1.8L Duratec-HE engine, the 1.8L Duratec SCi engine uses an external EGR system.

The reason for this modification is the increased amount of exhaust gas which is recirculated. This causes an increase of the temperature in the EGR system.

The exhaust gases consist of fully burned air/fuel mixture (inert gas) and, in stratified mode, of air as well.

On the 1.8L Duratec SCi engine the EGR system plays a particularly important role, as due to the high exhaust gas recirculation rate the NOx emissions are reduced in such a way that the other measures used for the post-treatment of the exhaust gas can be reduced. This improves fuel consumption.

There needs to be a pressure difference between the intake manifold and the exhaust tract to make it possible at all for exhaust gas to be sucked in via the EGR valve. However, direct petrol injection engines are operated in stratified mode with an almost fully opened throttle valve (operation with no throttle restriction).

In addition there is a large quantity of oxygen in the recirculated exhaust gas when the engine is in stratified mode (this is due to the large surplus of air in the combustion chamber).

The operation with almost no throttle restriction and the high oxygen content of the recirculated exhaust gas therefore necessitate a control strategy which co-ordinates both the throttle valve and the EGR valve. This results in great demands for the EGR system.

The stepper motor controlled EGR system is extremely reliable and precise in its operation. In addition, the system is relatively unsusceptible to deposits which could form as a result of the high exhaust gas rates.

Due to the high exhaust gas rate in stratified mode and the associated high temperatures which develop in the EGR system, appropriate cooling is required for the stepper motor.

This cooling function is performed via the coolant circuit of the engine. For this purpose the EGR valve is flanged via the coolant inlet onto the cylinder head.

Exhaust system - overview of individual components

Catalytic converters

On this engine, three catalytic converters are used in total to reduce the exhaust gas emissions.

In homogeneous mode all of the catalytic converters operate as 3-way catalytic converters. This means that in the region of lambda = 1 these catalytic converters convert the three harmful components hydrocarbon (HC), carbon monoxide (CO) and the nitrogen oxides (NOx) to water vapour (H2O), carbon dioxide (CO2) and nitrogen (N2).

The upstream catalytic converter and the main catalytic converter serve in stratified mode as oxidation-type catalytic converters. This means that HC and CO are burned to H2O and CO2.

The nitrogen oxides which are generated to a highly increased level in stratified mode (because of the large surplus of air in the combustion chamber) are therefore passed on almost entirely untreated.

However, a further catalytic converter is positioned downstream of the main catalytic converter in order to eliminate these emissions, the so-called NOx catalytic converter.

In terms of its fundamentals the NOx shares the same layout as a conventional 3-way catalytic converter, and during homogeneous mode it also operates as one.

NOx catalytic converter

However, in contrast to a 3-way catalytic converter the NOx catalytic converter also has an additional coating of barium oxide (BaO) in addition to the noble metal coatings of platinum, rhodium and palladium.

The increased levels of nitrogen oxides (NOx) generated in stratified mode as a result of the surplus of air are trapped by the barium oxide (trapped in the NOx catalytic converter).

With increasing amounts of trapped NOx, the capability of the catalytic converter to trap further NOx is reduced. This means that the NOx catalytic converter needs to be regenerated at certain intervals and therefore the trapped NOx needs to be released back out again .

During the regeneration process (release of the trapped nitrogen oxides), a surplus of fuel (lambda < 0.8) is used to achieve a reduction of these compounds, which are then converted in a conventional 3-way catalytic converter to nitrogen (N2), carbon dioxide (CO2) and water vapour (H2O).

The end of the regeneration phase is detected by the HO2S for NOx release when a high hydrocarbon (HC) value is registered as a result of a rise in the probe voltage.

The storage capability of the NOx catalytic converter depends greatly on the temperature. The maximum storage capability is achieved in the range between 300 ... 400 °C.

This means that the advantageous temperature range is very much lower than on a conventional 3-way catalytic converter (approximately 400 ... 800 °C).

For this reason three separate catalytic converters are used for catalytic cleaning of the exhaust gas:

• one 3-way pre-catalytic converter located near the engine and another 3-way underfloor catalytic converter, and
• one NOx catalytic converter far away from the engine.

Desulphurisation

The sulphur content of the petrol represents a problem for the NOx catalytic converter. Because of this it is necessary to desulphurise the NOx catalytic converter at regular intervals.

In the first phase of the desulphurisation process the NOx catalytic converter is heated to between 600 ... 650 °C. This is done by enriching the air/fuel mixture.

In the second phase the NOx catalytic converter is supplied for a few minutes with exhaust gas alternating between a rich mix (lambda = 0.95) and a lean mix (lambda = 1.05).

The time intervals at which the NOx catalytic converter needs to be desulphurised are modelled by the Powertrain Control Module (PCM).

Depending on the market-specific sulphur content of the fuel, the desulphurisation process is started every 2 to 50 tank fillings.