Abstract
The use of natural gas (NG) in dual-fuel heavy-duty engines has the potential to reduce pollutant and greenhouse gas (GHG) emissions from the transport sector when compared to the conventional diesel engines. However, NG composition and methane slip are of interest because both can adversely affect the benefits of NG as an alternative fuel, especially when considering GHG emissions. Therefore, this study experimentally investigated the effects of NG fuel properties on the performance and emissions of both conventional dual-fuel and reactivity-controlled compression ignition (RCCI) engine operations. Three different gas mixtures were selected to simulate typical NG compositions available in the world market, with methane numbers (MN) of 80.9, 87.6 and 94.1. These fuels were tested in a single-cylinder compression ignition engine operating at 0.6, 1.2 and 1.8 MPa net indicated mean effective pressure (IMEP). A high-pressure common rail system allowed for the use of various diesel injection strategies while a variable valve actuation system enabled the effective compression ratio to be adjusted via late intake valve closing (LIVC). The RCCI combustion was found to be more sensitive to changes in MN than the conventional NG-diesel dual-fuel operation. The gas mixture with the lowest MN reduced both total unburned hydrocarbons emissions and methane slip at the expense of higher nitrogen oxides (NOx) emissions. The effects of MN on the net indicated efficiency were more significant at 0.6 MPa IMEP, yielding differences of up to 4.9% between the RCCI operations with the lowest and highest MN fuels. Overall, this work revealed that the combination of the RCCI combustion and LIVC can achieve up to 80% lower methane slip and NOx emissions and relatively higher net indicated efficiency than the conventional dual-fuel regime, independent of the NG composition.
Highlights
Natural gas (NG), either compressed or liquefied, is an attractive substitute for gasoline and diesel in the transportation sector due to its relatively lower carbon footprint and competitive cost per unit of energy.[1]
This behaviour was associated with a shorter ignition delay measured at elevated engine loads, which was induced by the increased charge reactivity and the higher incylinder gas pressures and temperatures at such conditions.[31,55]
The aforementioned effect can be demonstrated by the ignition delay of 21.75 crank angle degrees (CAD) recorded at 1.8 MPa, where all three NG mixtures had the onset of the low-temperature heat release (LTHR) prior to the diesel injection at a calculated in-cylinder gas temperature of approximately 950°C
Summary
Natural gas (NG), either compressed or liquefied, is an attractive substitute for gasoline and diesel in the transportation sector due to its relatively lower carbon footprint and competitive cost per unit of energy.[1] the use of NG in naturally aspirated spark ignition (SI) engines can decrease the peak torque and power due to a reduction in volumetric efficiency when compared to a counterpart gasoline SI engine.[2] Turbocharged SI engines could potentially minimise these effects and take advantage of the high knock. The use of NG in a lean combustion system is challenging due to reduced mixture flammability, resulting in poor fuel conversion efficiency This results in the need for a relatively expensive and complex aftertreatment system, which can no longer rely exclusively on a three-way catalyst.[3] lean NG-diesel dual-fuel compression ignition combustion have been demonstrated as an effective means of utilising NG and potentially lowering the total cost of ownership of heavy-duty vehicles.[4]
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