|August 4, 2003
Audi is adding a thrifty sprinter to the range of versions of its compact sports model, the A3: inside its engine compartment, the A3 1.6 FSI has a four-cylinder engine with FSI petrol direct injection, developing 85 kW (115 bhp) and 155 Nm of torque.
It propels the dynamic three-door model up to a top speed of 196 km/h; the speedometer needle hits 100 km/h after 10.9 seconds. But this four-cylinder engine has more than just sports talents to recommend it.
The 1.6 litre FSI engine also demonstrates potential for fuel economy thanks to its petrol direct injection technology. The A3 1.6 FSI consumes just 6.5 litres of Super Plus fuel per 100 kilometres, pushing the figure down to a remarkable 5.3 litres per 100 km in extra-urban conditions.
This is undoubtedly an unbeatably low figure for a petrol engine, considering its performance. The 55-litre fuel tank means that a range of over 850 kilometres is possible without pauses for refuelling.
And the FSI engine treats its drivers to yet another welcome benefit of engineering finesse: as the vehicle is classified in emissions category EU4, cars registered as new before January 2005 will qualify for exemption from motor-vehicle tax in Germany.
This new version, which is going into production in August 2003, extends the Audi A3 range to three 4-cylinder petrol engines – as well as the 1.6, there is a further petrol direct injection model in the guise of the 2.0 FSI. The A3 1.6 FSI with 6-speed manual gearbox as standard will cost 20,100 in Germany.
Motor sport expertise
In technical terms, the new engine for the A3 is closely related to the racing engine that powered the Audi R8 racing cars to victory at the Le Mans 24 Hours in 2001 and 2002.
Here again it was a combination of more power from less fuel that demonstrated Audi’s technological lead in a very literal sense. With lower fuel consumption than conventional racing engines, the cars were able to put in an extra lap between refuelling stops.
Fuel injection under high pressure
The term “petrol direct injection” highlights the most telling feature which distinguishes these engines from conventional petrol engines. In contrast to the conventional indirect injection principle, the fuel is injected directly into the combustion chamber.
The injector, located on the admission side in the cylinder head, is served by a high-pressure pump driven by the camshaft and a pressure reservoir shared by all cylinders – the common rail system.
The injector regulates fuel delivery with millisecond precision, at injection pressures of up to 110 bar. By way of comparison, an indirect injection system operates at a maximum of four bar.
There is a further special feature on the admission side – the tumble flap. This can be adjusted to two different positions on the 1.6 FSI engine, thus specifically influencing the movement of the incoming air.
This paves the way for two different operating modes as the very basis of the FSI principle’s versatility: homogeneous-charge and stratified-charge operation. Depending on the engine speed, load status and accelerator pedal position, the engine electronics always select the optimum mode – without the driver even noticing.
Homogeneous-charge operation at full load
A conventional engine with indirect fuel injection establishes an ignitable air-fuel mixture of a ratio of 14.7:1 (lambda = 1) throughout the entire combustion chamber. An FSI engine likewise operates in this “homogeneous” mode whenever it is called upon to deliver a high power output.
At full load, the fuel is injected synchronously with the air intake phase. This fills the combustion chamber evenly. By virtue of the precision injection process, the extremely fine atomisation and the internal cooling effect when the fuel vaporises directly inside the combustion chamber, the FSI engine can run at a higher compression ratio than an engine with an indirect injection system. This permits greater efficiency.
Stratified charge: maximum fuel saving
The crucial feature of the new engine that paves the way for fuel-saving, however, is stratified-charge operation at partial loads.
In this operating mode, fuel is not injected until the compression phase. It is now injected directly into the air in the combustion chamber, in which a tumbling movement is induced by the diagonal position of the tumble flap and the special shape of the piston crown.
This specific tumbling movement makes it possible to establish the required stratification: precisely at the point of ignition, the cloud of fuel and air which fills only part of the combustion chamber reaches the spark plug, permitting efficient combustion in spite of the excess air in the remainder of the combustion chamber.