The combustion of homogeneous lean or diluted mixtures would significantly increase the efficiency of SI engines, but common spark ignitions systems are incapable to ignite these mixtures. Pre-chamber ignition systems burn a small portion of the charge in a separated chamber, which is connected to the main chamber by multiple small orifices. The combustion in the pre-chamber generates hot gases, which penetrate the main chamber, increase the turbulence and ignite the mixture on multiple sites. This leads to an increased turbulent flame speed and an extended lean and dilution limit, if the mixture in the prechamber is kept stoichiometric. Pre-chamber ignition systems have been investigated since the 1970s for passenger cars and are today commonly used in large gas engines. The adaption of the prechamber fuelling system to passenger car engines is not trivial, due to the problematic mixture preparation in the pre-chamber. Injection of a gaseous fuel in the pre-chamber would require a second fuel system with high pressure storage tank. Liquid gasoline direct injection in the pre-chamber is difficult due to the small space available for mixture preparation and the high surface to volume ratio, resulting in insufficient evaporation especially during cold start conditions. ... mehrTo overcome this problem, we developed a pre-chamber ignition system with active fuelling by volatile fuel components, which facilitates the integration in passenger cars. The system uses a mixture of air saturated with gasoline vapour for the pre-chamber fuelling. This gaseous mixture is typically found in the fuel tank above the liquid level and hence available in passenger cars. Former publications by the authors already proved the ability to enrich the pre-chamber and stabilize the combustion at homogeneous lean operation. Recent work focused on the optimization of the pre-chamber fuelling system and the pre-chamber geometry. To simulate the fuel tank atmosphere under different environment settings, a system was built, which saturates air with volatile gasoline components. This mixture gets compressed and dosed to the pre-chamber by a solenoid valve. Multiple prototypes of the pre-chamber with different volumes and geometry were investigated in a full engine at characteristic operating points regarding thermal efficiency, combustion process and emissions. These prototypes incorporate a spark plug, fuelling valve, thermocouple and pressure transducer. The results show the ability to ignite homogeneous lean mixtures with λ ≈ 2.0. Optimum operation was achieved with λ = 1.85 at 4.5 bar IMEP and 1500 rpm. This operating point showed an efficiency gain of 15 % compared to stoichiometric spark plug operation and NOx emissions below 20 ppm. The technology enables the usage of actively fuelled pre-chambers in passenger cars. The volatile fuel components for the pre-chamber fuelling are available in the fuel tanks atmosphere and thus allow a single fuel solution with inexpensive components.