Nowadays the main part of investigations in controlled auto-ignition (CAI) engines are centered on performance or some engine processes simulation, leaving aside particle number (PN) emission. The present work is focused on this last topic: PN emission analysis using two different injectors in a 2-stroke CAI engine, and a global comparison of PN emission of this engine with its homonymous 4-stroke engines at two operating conditions. The study was performed in a single-cylinder gasoline engine with 0.3 l displacement, equipped with an air-assisted direct-injection (DI) fuel injection system. Concerning the injectors evaluated, significant differences in PN emission have been found. When the I160X injector (narrow spray angle) was used, PN emissions were reduced. The spray cone angle during the injection event appears to be a key factor for PN emission reduction. In order to further explore the origin of the PN emitted, the possible oil leakage from the piston oil injector to the combustion chamber was also investigated, confirming that PN is formed during the combustion process itself instead of nucleation-particles caused by the oil. Regarding the global PN comparison, PN emission results at each operating condition with injector I160X reveal to be two orders of magnitude lower than the raw PN emission of a DI diesel engine. However, an increase in PN of one order of magnitude compared to a homogeneous gasoline direct injection (GDI) engine has been found. Contrarily, PN emission in a stratified charge GDI engine, which operates with lean mixture, appears to be equivalent to those of the 2-stroke CAI engine.