Cyclic Combustion Variations in Dual Fuel Partially Premixed Pilot-Ignited Natural Gas Engines

Abstract

Dual fuel pilot-ignited natural gas engines are identified as an efficient and viable alternative to conventional diesel engines. This paper examines cyclic combustion fluctuations in conventional dual fuel and in dual fuel partially premi.xed combustion (PPC). Conventional dual fueling with 95% (energy basis) natural gas (NG) substitution reduces NO., emissions by almost 90% relative to neat diesel operation; however, this is accompanied by 98% increase in HC emissions, 10 percentage points reduction in fuel conversion efficiency (FCE) and 12 percentage points increase in COVimep. Dual fuel PPC is achieved by appropriately timed injection of a small amount of diesel fuel (2-3% on an energy basis) to ignite a premixed natural gas-air mixture to attain very low NO^ emissions (less than 0.2 glkWh). Cyclic variations in both combustion modes were analyzed by obsemng the cyclic fluctuations in start of combustion (SOC), peak cylinder pressures (Pmax), combustion phasing (Ca50), and the separation between the diesel injection event and CaSO (termed "relative combustion phasing" ). For conventional dual fueling, as NG substitution increases, Pmax decreases, SOC and Ca50 are delayed, and cyclic variations increase. For dual fuel PPC, as diesel injection timing is advanced from 20 deg to 60 deg BTDC, Pmax is observed to increase and reach a maximum at 40 deg BTDC and then decrease with further pilot injection advance to 60 deg BTDC, the Ca50 is progressively phased closer to TDC with injection advance from 20 deg to 40 deg BTDC, and is then retarded away from TDC with further injection advance to 60 deg BTDC. For both combustion modes, cyclic variations were characterized by alternating slow and fast burn cycles, especially at high NG substitutions and advanced injection timings. Finally, heat release return maps were analyzed to demonstrate thermal management strategies as an effective tool to mitigate cyclic combustion variations, especially in dual fuel PPC.

Experimental investigation of the equivalence ratio influence on combustion, performance and exhaust emissions of a dual fuel diesel engine operating on synthetic biogas fuel

“…Srinivasan et al [24] found that DF (natural gas/diesel) engine could participate to minimize carbon dioxide emission by reason of the smallest carbon-hydrogen ratio. The natural gas may decrease greatly both NOx emission and soot [25,26], which is complicated to attain in conventional mode.…”

Potential and Limitations of Dual Fuel Operation of High Speed Large Engines

“…Note that it has already been shown that very low NOx emissions can also be achieved with larger diesel fractions, cf., Refs. [3,5,[11][12][13][14][15]. However, it must be taken into account that these investigations used boundary conditions that differ significantly from those of the investigations in this paper.…”

“…Effects of variations in diesel fraction and injection timing have already been investigated in Refs. [3,5,11,[13][14][15][16]. For example, Eichmeier et al [11] studied the influence of the injection timing on combustion phasing, efficiency, and emissions at a comparatively high IMEP of 19bar while employing an EGR rate of 55%.…”

Impact of methane energy fraction on emissions, performance and cyclic variability in low-load dual fuel combustion at early injection timings

“…Dual fuel combustion where a highly reactive fuel (usually with higher propensity for autoignition) is used to ignite a predominantly lean, premixed mixture of air and a fuel with lower reactivity (usually with a lower propensity to auto ignite) such as RCCI, 11 and dual fuel low temperature combustion (DFLTC) with natural gas modes 21 exhibits significant cyclic combustion variations, particularly at high natural gas substitutions. 30 Cyclic variations have been extensively characterized in spark-ignition (SI) engines. Young 31 presents a detailed literature review on cyclic variations in SI engines, which lists many studies as early as the 1960s.…”

“…The ultimate impact of cyclic combustion variability is to produce variations in indicated mean effective pressure (IMEP) so large as to affect drivability. Srinivasan et al 30 developed Matekunas’s idea further in the context of dual fuel engine combustion. They categorized fast-burn and slow-burn cycles based on crank angle corresponding to maximum firing pressure (CAPmax), maximum firing pressure (Pmax) and crank angle corresponding to 50% cumulative heat release (CA50) or combustion phasing.…”