September 17, 2001

IAA 2001: FSI - The Direct-Injection Petrol Engine
Text and pictures courtesy of Audi AG

To develop a new motor-vehicle engine is, generally speaking, to be faced with a difficult decision. In a nutshell, this is 'more power or less fuel' - a conflict of objectives as old as the spark-ignition engine itself and one that has lost almost none of its complexity, despite ongoing technical progress.

Audi is now presenting a new generation of engines that makes a quantum leap in terms of operating efficiency: the FSI principle is opening up a new dimension for the spark-ignition engine. This is a step forward in technology that justifies comparison with the introduction of TDI technology for diesel engines. That too, in its day, succeeded in combining high power output and an effective reduction in 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 answer is:

  • It is distinctly more dynamic,
  • Both its torque and power output are higher,
  • Yet fuel consumption is simultaneously as much as 15 percent lower.

    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 had not been fully exploited long ago if it brings such fundamental advantages. After all, the first direct injection petrol 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? Audi engineers have had to develop a large number of new components and assemblies themselves, including:

  • A high-pressure common rail fuel injection system with a demand-controlled 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 further-developed version of the air-guided combustion process with continuous control of charge movement
  • An external exhaust gas recirculation system
  • A further development of the exhaust emission treatment 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.

    The FSI engine's special combustion principle is critical to its efficiency. 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 port and by the special shape of the piston crown. This effect is known as "tumble".

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