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A heavy-duty, naturally aspirated diesel engine was converted into a turbocharged, aftercooled, compressed natural gas engine. Engine test results show that excess air ratio and ignition timing strongly affect NOx and THC emissions. Leaning the air-fuel mixture reduces NOx emission, but it increases THC emission and combustion becomes unstable above a certain excess air ratio. Retarding the ignition timing reduces both the NOx and THC emissions. Dual-plug ignition improves brake thermal efficiency. The NOx emission level can be reduced to meet the Japanese long-term emission regulation limit for heavy-duty gasoline engines with a sufficient safety margin by appropriately selecting the air-fuel ratio and ignition timing so as to keep the THC emission level below the regulation limit without using any after-treatment. The engine full torque characteristics were almost the same as the base engine throughout the engine speed range, while the maximum exhaust gas temperature was lower.

The injection nozzle inclination angle affects the flow characteristics in nozzle holes. Stroboscopic photographs of instantaneous spray plumes show that the length of each spray plume is different. Test results show that the fuel quantities injected from the holes are remarkably less when the nozzle hole spray angle relative to the injection nozzle axis is smaller compared with others with the same hole diameter. Hence, the authors analyzed the flow characteristics in injection nozzles using a computational fluid dynamic technique. Calculation results show good qualitative agreement with experimental results. INJECTION NOZZLES are normally installed in a two-valve cylinder head with an inclination angle. As shown in Fig. 1, the spray angle of each nozzle hole is different in order to maintain the same impingement height against the piston cavity for each spray.