2002 Renault Megane Emmisions & rough idle

This is typical of a misfire related to worn spark plugs, wires, or ignition coil. By driving it like that, the excessive unburned fuel has seriously overheated the catalytic converter and destroyed it. That is what is causing the horrible smell now. The first thing to do is address the misfire, otherwise a new catalytic converter will also be damaged. The Check Engine light should have been flashing to indicate too much fuel was going into the exhaust system.