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Stringent environmental policies and the ever-increasing demand for energy have triggered interest in novel combustion technologies that use alternative fuels as energy sources. Of these, pilot-ignited natural gas engines that employ small diesel pilots (~1-5 per cent on an energy basis) to compression ignite a premixed natural gas-air mixture have received considerable attention. This paper discusses the effect of intake charge temperature and pilot injected quantity on the onset of ignition (D IGN ) and combustion (D COM ) in a pilot-ignited natural gas engine with specific focus on early diesel pilot injection [begining of injection (BOI) at about 60°before top dead centre (BTDC)] for low-load operation. Both D IGN and D COM had a strong influence on performance and emissions at 60°BTDC. At advanced BOI for both halfand quarter-load operation, the best performance and hydrocarbon (HC) emissions could be obtained by optimally advancing D IGN relative to TDC and minimizing the cyclic variability in the D IGN . Furthermore, a clear dependence of D COM on D IGN was observed with the optimally advanced and the least-variable D IGN producing the least D COM variations. Engine performance, stability, and emissions were more sensitive to intake charge temperatures in comparison with pilot injected quantities. The best improvement in performance and emissions was obtained with increasing intake temperature at half load, where fuel conversion efficiency (FCE) increased from approximately 31 per cent to 38 per cent, coefficient of variation of indicated mean effective pressure (COV IMEP ) decreased from about 11 per cent to 4 per cent, and HC emissions decreased from 72 to 23 g/kW h, while oxides of nitrogen (NO x ) emissions increased from 16 to 142 mg/kW h. Performance and emissions trends at quarter load were similar to those observed at half load.high-octane fuels (e.g. methane); particularly for extended-use applications. Spontaneous combustion AL 35487-0276, USA.

The performance and emissions of a single-cylinder natural gas fueled engine using a pilot ignition strategy have been investigated. Small diesel pilots (2–3% on an energy basis), when used to ignite homogeneous natural gas-air mixtures, are shown to possess the potential for reduced NOx emissions while maintaining good engine performance. The effects of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling were studied. With appropriate control of the above variables, it was shown that full-load engine-out brake specific NOx emissions could be reduced to the range of 0.07–0.10 g/kWh from the baseline diesel (with mechanical fuel injection) value of 10.5 g/kWh. For this NOx reduction, the decrease in fuel conversion efficiency from the baseline diesel value was approximately one to two percentage points. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. The nature of combustion under these conditions was analyzed using heat release schedules predicted from measured cylinder pressure data. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.

The Advanced (injection) Low Pilot Ignited Natural Gas (ALPING) engine is proposed as an alternative to diesel and conventional dual fuel engines. Experimental results from full load operation at a constant speed of 1700rev∕min are presented in this paper. The potential of the ALPING engine is realized in reduced NOx emissions (to less than 0.2g∕kWh) accompanied by fuel conversion efficiencies comparable to straight diesel operation. Some problems at advanced injection timings are recognized in high unburned hydrocarbon (HC) emissions (25g∕kWh) and poor engine stability reflected by high COVIMEP (about 6%). This paper focuses on the combustion aspects of low pilot ignited natural gas engines with particular emphasis on advanced injection timings (45°–60° BTDC). Ignition phasing at advanced injection timings (∼60° BTDC), and combustion phasing at retarded injection timings (∼15° BTDC) are recognized as important combustion parameters that profoundly impact the combustion process, HC emissions, and the stability of engine operation.

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