Most alternative fuels for internal combustion engines have low cetane numbers. This is particularly true of natural gas (NG) which has excellent clean burning characteristics and low greenhouse gas production. Compression ignition (CI) engines supply an increasing amount of the world's power because of their very high efficiency and suitability for large units. However, low cetane number fuels such as NG are not suited to compression ignition because their long ignition delay causes severe engine knock. Nevertheless, they can be used in CI engines by adopting a mixed combustion process called dual-fuelling (DF). Here, ignition of the alternative fuel is stimulated by a pilot distillate spray. When compared with NG spark ignition engines, the DF process has the added advantage of fuel flexibility during the development of an alternative fuel distribution system. Engine tests have highlighted many DF combustion problems. In this research program, a constant volume combustion bomb and an associated 3D numerical model have been used to examine the DF combustion of NG/air mixtures. The numerical model gives pressure traces which compare well with experiment. Using it. parametric studies illustrate the importance of the injection process on the subsequent NG combustion.