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A four-cylinder turbocharged diesel engine was operated with natural gas and pilot diesel fuel ignition over a wide range of load and speed. The influences of different diesel-gas substitution ratios and air restrictions were evaluated regarding the characteristic parameters of combustion, performance and emissions. Based on data from the pressure-crank angle diagram, it was possible to evaluate some combustion characteristics such as the start of combustion, the maximum rate of pressure rise and the peak pressure. The parameters of the engine performance and emissions were analyzed through the brake specific fuel consumption (BSFC), unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOX). The results show that the increase of substitution ratio increase the maximum rate of pressure rise and the peak pressure at higher loads, but decrease at lower loads. With the air restriction, decrease the peak pressure.