Stratified charge: maximum fuel reduction
The basis for the new engine's economy is the so-called stratified-charge mode at part load. This involves a combustible mixture being located only in the area around the spark plug - in other words, some 14.7 parts air to 1 part fuel whilst a clear surplus of air exists in the rest of the combustion chamber.
In the stratified-charge mode, the fuel is not injected until the compression phase. It thus passes directly into the air in the combustion chamber, which, due to the position of the flap in the intake port on the one hand, and the special shape of the piston crown on the other, has been set in a motion that is known as "tumble".
Since the fuel is delivered at a shallow angle by the injector, the cloud of fuel makes scarcely any contact with the piston crown: a so-called "air-guided" process.
This controlled tumble motion also enables the required stratified charge to be achieved: the cloud of fuel which has become swirled with sufficient air to produce an ignitable mixture is kept to a confined volume and reaches the spark plug at the moment of ignition.
In stratified charge operation, incidentally, we achieve lambda values of up to 4.0 related to the combustion chamber as a whole. This is essential if fuel consumption is to be reduced at low and medium engine speeds.
An additional benefit: during combustion, a layer of air insulates the hot, ignited mixture from the cylinder wall. This cuts the amount of heat lost to the engine block and increases the engine's operating efficiency.
Highly effective exhaust emission control
In the past the subject of NOx emissions posed a major obstacle for the developers of petrol direct injection engines based on the stratified-charge principle. Due to the excess of air, it was not possible to reduce the resulting oxides of nitrogen completely to nitrogen gas using a conventional catalytic converter.
Audi has tackled this problem on its 2.0 FSI engine with a whole series of measures.
On the exhaust side of the engine there is one of the fundamental elements needed for efficient exhaust emission control, the exhaust gas recirculation system. Controlled by the engine electronics and regulated by the exhaust gas recirculation valve and variable intake camshaft, it diverts up to 30 percent of the total exhaust gas back into the combustion chamber. This helps to reduce the combustion temperature and thus lowers the raw emission of oxides of nitrogen by up to 70 percent in stratified lean-burn operation.
With NOx storage converter
The FSI engine is fitted with two catalytic converters for exhaust emission control: the underbonnet three-way converter behind the manifold, and an NOx storage-type converter under the floor pan.
The NOx storage converter has been specially designed to suit the needs of a direct injection engine, and has an NOx sensor installed at the discharge end. It is an established fact that the conventional three-way catalytic converter is unable to break down oxides of nitrogen sufficiently in the engine's lean-burn phase; for this, the composition of the exhaust gas must be stoichiometric (14.7 parts air to one part fuel).
The higher levels of oxides of nitrogen that remain therefore have to be reduced to harmless nitrogen gas. This task is performed efficiently in the storage-type catalytic converter, which has a barium coating with which the oxides of nitrogen combine.
The storage-type converter is controlled by a mapped operating characteristic and by temperature. When the converter is saturated, the engine's mixture is made richer for a short time. This raises the temperature of the slightly rich exhaust gas so that the barium molecules in the converter release the oxides of nitrogen, which are then reduced to nitrogen. This process goes unnoticed by the driver.
An additional, beneficial result of the efficient exhaust emission control does not go unnoticed though by the Audi A4 2.0 FSI owner: both saloon and Avant comply with the strict EU IV emissions standard and thus save up to € 307 in road tax in Germany.