The 2.0-liter direct-injection turbo-diesel found under the hood of the A3 we drove was significantly reengineered and upgraded earlier this year. It’s essentially the same unit used in the 50-state-compliant Volkswagen Jetta TDI, and here it makes the same 140 horsepower and 236 pound-feet of torque. In the process of the upgrade, Audi switched from the previously used pump injectors, one for each cylinder, to common-rail fuel injection, in which fuel travels through a single high-pressure rail. The common-rail system allows several injections per cylinder stroke, making it easier to meet emissions targets, and it has the added benefit of being quieter.
The shift to common-rail technology is clearly felt. Whereas the old diesel engine virtually jerked the A3 forward, the new engine is more linear in response. Power delivery is more smooth, and there is noticeably less noise and harshness.
Source: Car And Driver