Lean burning in piston engines gives opportunity to achieve important enviromental and fuel economy objectives. Unfortunately, lean mixtures are not readily ignited and are characterized by poor flame propagation. Thus, techniques which enhance ignition, flame formation and its propagation must be applied, if lean combustion is to be reliably sustained. To overcome difficulties related to the lean burning, it has been proposed to equip the engine cylinders with prechambers, containing catalytic inserts, shaped in form of an open coil. The coils not only facilitate ignition but also generate turbulence, to intensify the combustion process in prechamber.A four-cylinder Diesel engine has been adapted for experiments aimed at confirmation of the effectiveness of the catalytic action on operation of the system.The modification of Diesel engine to the Otto cycle was performed in a simple way, i.e. by replacing fuel injectors with prechambers equipped with ignition plugs and catalytic inserts. In this configuration the engine compression ratio decreases to ε=16. Thus, the engine with a catalytic insert in prechamber, can operate in a stable way within the range of lean mixture, i.e. λ up to 1.55.Comparison of the load characteristics, for a constant speed n = 3000 r.p.m., shows that for an engine with a catalytic insert the specific fuel consumption has undergone a distinct decrease (about 15%) for operation within the range of partial load torque (50 to 75% full load). The concentration of unburned hydrocarbons is reduced by 30 to 50%.Considerable decreases in misfires were observed. The concentration of NOx was relatively low - particularly for higher air excess coefficients.