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.

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 (Δ_IGN) and combustion (Δ_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 Δ_IGN and Δ_COM had a strong influence on performance and emissions at 60° BTDC. At advanced BOI for both half and quarter-load operation, the best performance and hydrocarbon (HC) emissions could be obtained by optimally advancing Δ_IGN relative to TDC and minimizing the cyclic variability in the Δ_IGN. Furthermore, a clear dependence of Δ_COM on Δ_IGN was observed with the optimally advanced and the least-variable Δ_IGN producing the least Δ_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.

Diesel engines may be converted readily to operate primarily on natural gas using the injection of a diesel pilot to achieve ignition. Advanced low-pilot-ignited natural gas (ALPING) engines show significant potential to match diesel engines in their part-load and full-load efficiencies. Experiments were performed to study the effects of pilot injection timing (- 15 to - 60° ATDC), pilot quantity, intake manifold pressure and intake charge temperature on the performance and emissions from an ALPING engine under half-load (21 kW at 1700 r/min) and full-load (42 kW at 1700 r/min) conditions. Low NO_x emissions (below 0.03 g/kW h at - 60° ATDC) with satisfactory fuel conversion efficiency (31 per cent) for half-load and NO_x emissions of 0.2 g/kW h at -60° ATDC with fuel conversion efficiency of 40 per cent could be obtained for full-load engine operation. High HC emissions, 96 g/kW h at - 20° ATDC for half-load and 21 g/kW h at -60° ATDC for full-load operation, were recorded. The NO_x emissions showed an interesting trend for varied injection timings with maximum NO_x emissions occurring at -35° ATDC and minimum at - 60° ATDC injection timing. Increased pilot quantity, intake charge temperature and lower intake manifold pressures resulted in increased NO_x emissions and fuel conversion efficiency, and decreased HC emissions.

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.

Earlier efforts demonstrated the low NOx(40%) benefits of the low temperature, advanced low pilot injection natural gas (ALPING) combustion concept that utilized advanced injection (about 60° BTDC) of small diesel pilots (2–3% on an energy basis) to compression-ignite a premixed natural gas-air mixture. At these injection timings, combustion was accompanied by increased unburned hydrocarbons (HC) (mostly methane) and variations in torque fluctuations. In this article, hot exhaust gas recirculation (EGR) is proposed as a potential strategy to reduce HC emissions and torque fluctuations at low (quarter and half) loads. It is shown that the addition of hot EGR leads to a combination of one or more of the following effects on the intake mixture entering the cylinder: oxygen depletion, increased temperatures due to mixing with exhaust gases, dilution due to introduction of high specific heat species, and active recycling of unburned hydrocarbons to effect reburn in subsequent cycles. In particular, hot EGR addition extends the ALPING operation regime from 50°–60° BTDC to 60°–70° BTDC, increases low-load efficiencies by more than 5 percentage points, substantially improves combustion stability, and drastically reduces HC emissions (by more than 70%) with little associated penalty in NOx emissions.