Cycle-by-cycle variations of a natural gas direct-injection spark ignition engine at different compression ratios were investigated. The results show that the lean burn limit of the natural-gas direct injection engine can be extended to a larger overall excess air ratio compared with that of the homogeneous charge natural gas engine. The coefficient of variations (CoV) of indicated mean effective pressure decreases with the increase of compression ratio. However, CoV of indicated mean effective pressure is increased at high engine load when compression ratio is larger than 12. The cycle-by-cycle variations are more clearly demonstrated in CoV of indicated mean effective pressure rather than in CoV of cylinder peak pressure. Average values of flame development duration, main combustion duration, and total combustion duration are decreased and combustion is improved with increasing compression ratio. This is the reason for decreasing cycle-by-cycle variations in the natural gas direct-injection engine. Better interdependence exists between the indicated mean effective pressure and the flame development duration, as well as between the indicated mean effective pressure and late combustion duration. Cycle-by-cycle variations of the natural gas direct-injection engine are resulted from cycle-by-cycle variations in flame development duration and late combustion duration. This shows some difference to that of homogeneous charge natural gas engine, where cycle-by-cycle variations are mainly influenced by the variations in early flame development stage.

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Although the transient NOx emissions produced from a typical range-extended hybrid vehicle can meet the requirements of the China VI emissions regulations, it is rather difficult to comply with Beijing VI emissions regulations. This is due to the fact that the sharp fluctuation in air-fuel ratio control during engine start-up period resulting in lower conversion efficiency for the three-way catalyst (TWC). In response to these issues, the potentials of different loading strategies in meeting Beijing VI were explored on a light-duty natural gas (NG) engine for range-extended hybrid application. The experiments were performed on the basis of typical Chinese City Bus Cycle (CCBC). This paper analyzed the operation characteristics of the typical range-extended hybrid NG vehicle and investigated the impacts of engine loading strategy on the vehicle’s performance and emissions during the start-up phase. Results showed that the transient NOx emissions of 30 s-averaged decreased from 786 to 504 ppm when extending the loading time from 3 to 40 s in a typical loading path (i.e. engine speed and load increased linearly). The transient NOx emissions could meet the Beijing VI NOx limit but with little margin. Research also investigated the use of a new designed engine loading strategy (i.e. engine speed and load with time-separated control manner). Results revealed that this method effectively curbed the transient NOx emissions, reducing the maximum transient NOx emissions of 30 s-averaged from 504 to 271 ppm. In comparison, this low NOx level was better to meet the Beijing VI with sufficient margin.