Carbon dioxide emissions, particulate matter and nitrogen oxides are the most important exhaust gas emission components from the compression ignition internal combustion engines. The diesel cycle shows a superior thermal efficiency compared to the spark ignition (SI) combustion. Unlike SI engines a complete (100%) replacement of liquid fuel with gaseous fuel, containing less carbon, such as natural gas, is not possible for compression ignition (CI) engines. Since resistance of methane to auto-ignition is high, it is necessary to use another fuel with higher reactivity for ignition. Therefore a partial substitution of diesel fuel with low-carbon fuel seems to be a meaningful approach to a step reduction of CO2 emissions. One option is a dual-fuel operation of the diesel engine. As of today, the majority of dual-fuel (DF) engines research and development activities are focused on large cylinder bore and heavy duty engines operated with lean mixtures, several previous works describe the evaluation of DF principle in the light duty engines operating in lean and stoichiometric region. The goal of this work is to identify the potentially interesting regions for practical use of the DF combustion in a broad area of loads, NG substitution ratios, boost level and speed, on a diesel engine of the cylinder size and structure compatible with engines for passenger cars and vans. The work was performed on a single cylinder engine and the tests were focused on engine performance, efficiency, gaseous emissions and particulate matter. The fouling of injector was identified and quantified on an injector test bed. (C) 2017 Elsevier Ltd. All rights reserved.