A significant reduction of CO2 emission in the road transport is a major challenge for next years. In the combination with efficient powertrain technologies, the potential of natural gas is excellent for comparably light and cost effective reduction of CO2 and toxic emission in the future [1-2]. As the CNG fuelling stations in Poland are not so far widespread enough to make a dedicated natural gas vehicle practical, it results necessary to start with proposing alternatively CNG powered engines. Such a bi-fuel automotive engines are necessary to bridge the gap between petrol and natural gas. As a sample of modern engine design an Opel 1.6 litre 4-cylinder engine has been selected as a base of powertrain for development of bi-fuelpassenger car engine. The influence of ignition timing on CNG combustion process has been presented in this paper. A 1.6 litre SI engine has been tested in the special program. For selected engine operating conditions, following data were acquired: in cylinder pressure, crank angle, fuel mass consumption and exhaust gases temperatures. For the settings of 6, 9, 12 and 15 deg crank angle timing advance correction, the internal temperature of combustion chamber has been estimated, as well as the charge combustion ratio and ratio of heat release were tested. With the help of the mathematical model, emissions of NO, CO and CO2 were additionally estimated. Obtained results made it possible to compare the influence of ignition timing advance on natural gas combustion in SI engine.