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News of  August 08, 2001

New generation of Audi engines - Greater power, lower consumption

FSI – direct injection petrol engine

This is a step forward in technology that justifies comparison with the introduction of the TDI diesel engine. That too, in its day, succeeded in combining high power output and lower fuel consumption to a previously unattainable extent. 

What can the FSI engine, in which petrol is injected directly into the cylinders, do better than a conventional engine with fuel injection into the intake ports?

The answers are: 

• it is distinctly more dynamic
• both its torque and power output are higher
• yet fuel consumption goes down by as much as 15 percent at the same time.

The main factor contributing to these improvements is the stratified charge principle at part load. In this operating mode, the engine only needs a fuel-air mixture capable of immediate ignition in the area around the spark plug. The remainder of the combustion chamber is filled with a leaner mixture, that is to say one with a considerable degree of excess air. As a result of this, the engine can be run without the incoming mixture flow being throttled. The direct-injection engine also benefits from reduced heat losses, because the layer of air around the ‘cloud’ of ignitable mixture isolates the latter from the cylinder and cylinder head.

One may well ask why this principle has not been fully exploited long ago if it has such fundamental advantages. After all, the first direct petrol-injection engines were built back in the nineteen-fifties, and other manufacturers have made use of similar techniques more recently. However, they were unable to reap the full benefit of this mixture supply technique because stratified-charge operation was only possible over a much smaller zone of the engine’s operating range.

What is new about the Audi FSI engine? The answer is: Audi engineers had to develop a large number of key components, including:

• a high-pressure ‘common rail’ fuel injection system with a single-piston injection pump specially developed for the purpose; this only supplies sufficient fuel to maintain the desired pressure in the system.
• a new cylinder head with four valves per cylinder and valve operation by roller cam followers
• a developed version of the air-guided combustion process with continuous control of charge movement
• an exhaust gas recirculation system
• a further development of the exhaust emission control system, with a NOx storage-type catalytic converter and NOx sensor

In the meantime, low-sulphur petrol has become generally available, so that the full fuel-saving potential of these engines will be attainable in day-to-day operation.

Critical to efficiency is the FSI engine’s combustion principle.

On this engine, fuel is not injected into the intake port but directly into the combustion chamber. The injector, which is supplied by a single-piston pump and common rail fuel line, is in the side of the cylinder head, and controls the injection time to within thousandths of a second, at injection pressures of up to 110 bar. 

In the stratified-charge operating mode, fuel is injected on the engine’s compression stroke and is picked up by the movement of the air that has been drawn into the combustion chamber. This movement is imparted to the air by a movable flap in the intake pipe and by the shape of the intake port and the piston crown. The resulting controlled movement is known as “tumble”.

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 side-mounted injector, the cloud of fuel makes scarcely any contact with the piston crown. This is why we speak of an “air-guided” combustion process. 

In addition 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, lambda values of up to 4.0 related to the combustion chamber as a whole are achieved. 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, we achieve a definite reduction in fuel consumption and higher power-output and torque figures than we would with indirect fuel injection. As demonstrated, by the way, on our 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 cylinders 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 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 in accordance with engine management signals. 

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: a multi-stage three-way converter at the outlet point from the exhaust manifold, in other words close to the engine, 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 side. 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 made richer 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 rich-mixture purification programme depends of course on the engine’s operating conditions, but normally averages only a few seconds in each minute of operation.

Fuel consumption going down by as much as 15 percent in comparison with conventional engines means engines 15 percent more economical. And that is what progress is all about. And rest assured: development work is still continuing.

(August 3, 2001)

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