The aim of this study is to describe the impact on particle matter and co 2 emissions of converting an existing heavy-duty diesel engine for on-highway truck applications to a dual-fuel engine (diesel + natural gas), especially in transient operation. A dual-fuel engine with homogeneous gas charge injection in the intake line before turbocharger was considered. The results showed the feasibility of this kind of technology for transient operation reaching a significant reduction of particle matter plus a decrement in co 2 emissions at the expense of a small decrement of brake fuel conversion efficiency and an increment of unburned hydrocarbons in the exhaust gases.