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The effects of the late-intake-valve-closing strategy and the different types of cam profile were observed in a singlecylinder compression ignition research engine. Experiments were carried out with two engine loads in naturally aspirated conditions. The late-intake-valve-closing strategy exhibited an improvement in the conventional trade-off between the nitrogen oxide emissions and the smoke emissions, as stated in other relevant work. However, it was found to be effective only for the premise that a sufficiently high mass of oxygen is trapped inside the cylinder, which ensured that the smoke emissions did not deteriorate with exhaust gas recirculation. This improvement in the trade-off decreased when the global air excess ratio inside the cylinder reached close to unity. The major disadvantages of the late-intake-valveclosing strategy included deterioration in the indicated mean effective pressure and the reduced mass of oxygen trapped inside the cylinder. The decrease in the indicated mean effective pressure was attributed to the reduction in the effective compression ratio followed by the reduction in the thermal efficiency in terms of the thermodynamics. The volumetric efficiency decreased owing to the backflow of the in-cylinder charge into the intake manifold. This implied that intake boosting was necessary not only to recover the efficiency to the original level but also to extend the engine load with a sufficient amount of air. The manipulation of cam profiles yielded further improvement in the trade-off relationship between the nitrogen oxide emissions and the smoke emissions. The flow field measurements obtained using particle image velocimetry and direct imaging of the combustion of the fuel spray demonstrated that the asymmetric cam profile effectively increased the swirl ratio inside the cylinder. Further improvement in the trade-off relationship between the nitrogen oxide emissions and the smoke emissions was realized because of this increased swirl intensity, which provided a better environment for air utilization. The smoke emissions were suppressed without a significant increase in the nitrogen oxide emissions.

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