A natural gas and diesel dual-fuel turbocharged compression ignition (CI) engine is developed to reduce emissions compared with a heavy-duty diesel engine. The compressed natural gas (CNG) pressure regulator is specially designed to feed back the boost pressure to simplify the fuel metering system. The natural gas bypass improves the engine response to acceleration. The modes of diesel injection are set according to the engine operating conditions. The application of honeycomb mixers changes the flowrate shape of natural gas and reduces hydrocarbon (HC) emission under low-load and low-speed conditions. The cylinder pressures of a CI engine fuelled with diesel and dual fuel are analysed. The introduction of natural gas makes the ignition delay change with engine load. Under the same operating conditions, the emissions of smoke and NOx from the dual-fuel engine are both reduced. The HC and CO emissions for the dual-fuel engine remain within the range of regulation.