With the continuous growth of energy demand for the commercial transport sector, the market share of diesel vehicles is rising in several areas worldwide. Global demand for transport energy is therefore believed to be strongly skewed towards heavier fuels; particularly diesel. The strengthen emissions legislation is another issue facing the current generation of diesel engines; where there is an increasing concern with their high NOx and PM emissions. Dual-fuelling of diesel engines with natural gas (NG) stands as an attractive solution to reduce the dependence on diesel fuel and mitigate the harmful effects of diesel engines emissions. The main attractions of NG as a contributor to a more sustainable fuel market include its lower carbon content and relatively higher natural reserves, in addition to the renewable aspect of methane production from biogas.
In dual-fuelling strategy, most of the engine power output is provided by the NG, while a pilot amount of diesel fuel is used as an ignition source for the NG-air mixture. While the concept is not new and it has been deliberated lengthily in the past two decades, several uncertainties still exist as relative to engine combustion, exhaust emissions, and practicality. The present contribution aims at critically reviewing part of the prevalent literature about NG-diesel dual-fuel engines; highlighting the concepts and challenges. Throughout this review, several topics are explored and evaluated based on research importance and maturity. The overview of these works indicates that research effort in this field could be broadly categorized into fuel delivery researches or charge composition studies; where each category is directly linked to either the NG or the pilot fuel. Following this, a roadmap for future research directions in the field is presented, to spot some potential topics for proceedings and continuation.