Improving low load combustion, stability, and emissions in pilot-ignited natural gas engines

Abstract

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.

Theoretical study of the effects of engine parameters on performance and emissions of a pilot ignited natural gas diesel engine☆

“…In the present work only an outline of the model is given, on the one hand because of the lack of space and on the other because this is based on an existing model that has been presented in detail in previous publications by the authors [22][23][24][25][26][27]. As stated above, the main purpose of this work is the theoretical study, which is to be conducted via the simulation model, concerning the effect of pilot diesel fuel quantity and injection timing on both engine performance and emissions.…”

“…The boundary of this jet defines the second zone, the ''burning zone'' as shown in Fig. 1 [22][23][24][25][26][27][28][29]. According to the model, inside the burning zone takes place the process of combustion, and for this reason the main constituents of the zone are combustion products, unburned evaporated diesel fuel, unburned gaseous fuel and air that has not yet participated in combustion.…”

“…Many research studies [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] have reported that, the main drawback of pilot ignited natural gas/diesel engine operation in contrast with conventional diesel operation, is the negative effect on engine efficiency, carbon monoxide (CO) and unburned hydrocarbon (HC) emissions, especially at low and intermediate load operating points. At high load, the improvement of gaseous fuel utilization leads to a relevant improvement of both engine performance and carbon monoxide emissions, but their values continue to be inferior to the respective values observed under normal diesel operation.…”

“…According to prior studies [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], alterations in various engine operating parameters, such as pilot fuel quantity, pilot diesel fuel injection timing and air inlet preheating, may be used to improve the engine efficiency and to restrain the increase of CO and HC emitted from a compression ignition engine running under pilot ignited natural gas diesel operating mode.…”

“…Theoretical results are validated against experimental values, which were obtained at various speeds and loads from a single cylinder DI diesel engine operating under dual fuel mode at fixed pilot fuel quantity and normal injection advance without air inlet preheating (normal pilot ignited dual fuel operation -NPDFO). Moreover, model predictions are contrasted with additional experimental results obtained from the literature [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]28,29] to ensure that the two-zone model predicts with reasonable accuracy the effect of pilot fuel quantity and air inlet preheating on engine performance characteristics, soot, NO and CO emissions. This comparison revealed that the developed model captures more in a qualitative rather than in quantitative manner the influence of pilot fuel quantity and intake air preheating on engine performance and exhaust emissions.…”