The use of natural gas as a transportation fuel reduces operating costs and emissions and contributes to security of supply. Natural gas for vehicles (NGV) can be used in rail, road, marine and off-road applications as a replacement for diesel fuel either stored as liquefied natural gas (LNG) or compressed natural gas (CNG). These applications have large energy consumptions per engine which makes them attractive target markets. When NGV replaces diesel as a fuel, either engines are converted to spark ignition (Otto Cycle) or a small amount of diesel fuel is used as for pilot ignition (Dual fuel). Dual fueling has traditionally been hampered in its market acceptance due to poor low speed, low load efficiency combined with lower-than-expected levels of diesel substitution. The Canadian Gas Association NGV Research and Development Fund was created to improve the technology of using NGV for transportation applications. This paper will concentrate on one area of this R and D Program - recent advances made in dual-fueling, 4-stroke engines.