The 2.0TFSI EA888 3b as fitted to the A4 35 and 40 TFSI and Polo GTI (among others) can actually be quite lumpy at low speeds and idling on a cold start (almost a mild kangarooing - experienced it both on our 2 Polos - 1 with and one without GPF and a hired Arteon) e.g. when crawling out of your 10mph limit works car park or decelerating to a roundabout for the first mile or so after the cold start. That's attributable to cold start use of port injection. When the car has switched to direct injection it's a lot smoother.
It is a great engine, and makes short steps to bridging the gap between petrol and diesel, but at the end of the day, chemistry and thermodynamics are always going to favour diesel. The Budack cycle that this engine utilises is about 30% more efficient than standard otto/miller cycle petrol ignition. That's great, but it can only do it under extremely low loads (under 20kW output). At best, the A4 will only be able to run under 20kW for a quarter of its time, to give an 8% improvement overall unless you are driving everywhere at a constant 50mph.
Diesel is only 8% more energy dense than petrol, yet yields 30-50% more mpg because more of the heat generated is used to expand the gases of combustion (and all that air in excess) further for converting into kinetic energy utilised by the drivetrain for motion. Petrol chucks so much of its energy generated as heat straight out of the exhaust. That will always be the case unless they get the octane number so high that you can compress petrol so much before ignition that it might as well be a diesel.
