The desired "stratified charge effect" is obtained in this way: the cloud of air containing sufficient fuel to form an ignitable mixture is kept to a confined volume and surrounds the spark plug at the moment of ignition. 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.
After combustion, a layer of insulating air remains between the ignited mixture and the cylinder wall. This cuts the amount of heat lost to the engine block and increases the engine's operating efficiency.
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.
At full load, the fuel in injected synchronously with the air intake phase. This fills the combustion chamber homogeneously. Here again, this produces a definite reduction in fuel consumption together with higher power-output and torque figures than would be possible indirect fuel injection. This was demonstrated on the race-winning Le Mans engine, which runs permanently in the homogeneous mixture mode.
This has the advantages of reducing the tendency to knock as a result of direct fuel injection into the combustion chamber and the resulting internal cooling effect. In addition, the engine is capable of operating at a higher compression ratio.
The first of these new-generation engines is a two-litre four-cylinder unit. The engine block and the main dimensions are identical with the 96 kW (130 bhp) engine already familiar from the A4 and A6. It has a common rail injection system and a single-piston injection pump. Unlike Audi engines with fuel injection into the intake port, the cylinder head has four instead of five valves per cylinder. This is essential in order to provide space for the injector in the combustion chamber.
A two-stage intake pipe is used, with two length settings for use at higher and lower engine speeds. A continuous adjuster on the inlet camshaft varies the inlet valve opening times as required.
On the exhaust side of the engine we can see one of the fundamental elements needed for efficient exhaust emission control, the exhaust gas recirculation system. This operates more efficiently than previous systems, and diverts up to 30 percent of the exhaust gas back to the engine's combustion chambers. Two catalytic converters are provided for exhaust emission control: an underbonnet multi-stage three-way converter, and a 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 a NOx sensor installed at the discharge end. It is a well-known 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.
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 enriched for a short time. This raises the temperature of the exhaust gas, so that the barium molecules in the converter release the oxides of nitrogen, which are then converted into nitrogen. The frequency of this enrichment depends of course on the engine's operating conditions, but normally averages only a few seconds in each minute of operation.
The maximum power output of this two-litre, four-cylinder engine is 150 bhp at 6000 rpm. Its peak torque of 200 Newton-metres is available between 3500 and 4000 rpm. Compared with an engine with the same output and indirect injection, a fuel saving of up to 15 percent is thus possible.
15 percent less fuel consumption: this is what progress is all about. And rest assured: development work is still continuing.
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