Careful study has been made of the aspects of unstable combustion due to heavy E.G.R. (Exhaust Gas Recirculation) while attempting to achieve low NOx emission levels. The authors have concluded that the fast burn engine allowed a wider E.G.R. tolerance, with stable combustion, at very low NOx emission levels in comparison with the conventional SI engine. This was true even under heavy E.G.R. Better fuel economy was achieved through a combination of improved cycle efficiency (using the fast burn concept) and reduced pumping loss (applying heavy E.G.R.). The Nissan NAPS-Z Fast Burn Engine was developed as a consequence of and through the use of the results mentioned above. The characteristic data of the engine (4 cyl. 1.8 ltr.) described in the paper showed a method of achieving a low NOx emission level with better fuel economy and driveability.