FSI, an acronym for fuel stratified injection, is an Audi and Volkswagen AG petrol engine family that uses direct injection technology. In this technology, fuel is injected into the combustion chamber, instead of being mixed in the intake like conventional engines. Audi claim that this increases power, torque and fuel-efficiency.
The amount of fuel injected into the combustion chamber is precisely regulated by the FSI system, and it is mixed with air from the intake duct. The ignitable fuel in the chamber is surrounded by this buffer of air, which reduces the overall temperature. FSI engines also have a higher mechanical compression ratio, resulting in improved thermodynamic performance. A feature of engines in the FSI range is that they have a two-stage variable intake manifold. There are two modes of operation, one for high and one for low revs. The first uses a short duct length for greater power. The second, longer duct provides increased torque at low revs.