On the combustion of premixed gasoline - natural gas dual fuel blends in an optical SI engine

Natural Gas (NG) is a promising alternative fuel. Historically, the slow burning velocity of NG poses significant challenges for its utilisation in energy efficient Spark Ignited (SI) engines. It has been experimentally observed that a binary blend of NG and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even in comparison to that of the base fuels. The mechanism of such effects remains unclear. In this work, an optical diagnosis has been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of NG to gasoline in a binary Dual Fuel (DF) blend. Experiments are performed under a sweep of engine load, quantified by the engine intake Manifold Air Pressure (MAP) (0.44, 0.51. 0.61 Bar), and equivalence air to fuel ratio (Φ = 0.8, 0.83, 1, 1.25). NG was added to a gasoline fuelled engine in three different energy ratios 25%, 50% and 75%. The results showed that within the flamelet combustion regime, the effect of Markstein length is dominating the lean burn combustion process both from a stability and velocity prospective. The effect of the laminar burning velocity on the combustion process gradually increases as the air fuel ratio shifts from stoichiometric to fuel rich values

On the combustion of premixed gasoline—Natural gas dual fuel blends in an optical SI engine

Natural Gas (NG) is a promising alternative fuel. Historically, the slow burning velocity of NG poses significant challenges for its utilisation in energy efficient Spark Ignited (SI) engines. It has been experimentally observed that a binary blend of NG and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even in comparison to that of the base fuels. The mechanism of such effects remains unclear. In this work, an optical diagnosis has been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of NG to gasoline in a binary Dual Fuel (DF) blend. Experiments were performed under a sweep of engine load, quantified by the engine intake Manifold Air Pressure (MAP) (0.44, 0.51. 0.61 bar) and equivalence air to fuel ratio (Φ = 0.8, 0.83, 1, 1.25). NG was added to a gasoline fuelled engine in three different energy ratios 25%, 50% and 75%. The results showed that within the flamelet combustion regime, the effect of Markstein length dominates the lean burn combustion process both from a stability and velocity prospective. The effect of the laminar burning velocity on the combustion process gradually increases as the air fuel ratio shifts from stoichiometric to fuel rich values

Experimental investigation on the Laminar burning velocities and Markstein lengths of methane and PRF95 dual fuels

© 2016 American Chemical Society.Natural gas is a promising alternative fuel. The main constituent of natural gas is methane. The slow burning velocity of methane poses significant challenges for its utilization in future energy efficient combustion applications. The effects of methane addition to PRF95 on the fundamental combustion parameters, laminar burning velocity (Su0) and Markstein length (Lb), were experimentally investigated in a cylindrical combustion vessel at equivalence ratios of 0.8, 1, and 1.2, initial pressures of 2.5, 5, and 10 bar, and a constant temperature of 373 K. Methane was added to PRF95 in three different energy ratios 25%, 50%, and 75%. Spherically expanding flames were used to derive the flow-corrected flame velocities, from which the corresponding Lb and Su0 were obtained. The flame velocities were corrected for the motion of burned gas induced by the cylindrical confinement. It has been found that at stoichiometric conditions there is a linear decrease in Lb and Su0 with the dual fuel (DF) ratio in all investigated pressures. At rich conditions, all DFs resulted in having lower Su0 as compared to methane and to a larger extent PRF95. The values of Lb for all DFs were lower than methane and comparable to those of PRF95. At lean conditions, the values of Lb for all DFs are biased toward those of methane whereas the values of Su0 are found to be higher than those of PRF95 at pressures of 2.5 and 5 bar. At 10 bar both Lb and Su0 reduce with DF ratio although Su0 of all DFs converge to that of PRF95. The findings of the current study indicate a distinct synergy in the utilization of dual fueling in future lean burn energy efficient combustion applications

Experimental study on laminar flame characteristics of methane-PRF95 dual fuel under lean burn conditions

The effects of methane addition to PRF95 (primary reference fuel with 95% volume of iso-octane and 5% volume of n-heptane) on the fundamental combustion parameters are experimentally investigated in a cylindrical combustion vessel using classical schlieren technique. In this study, methane is added with three energy fractions of 25%, 50% and 75% to PRF95. The laminar flame propagation, Markstein length and flame instability of dual fuels under different initial pressures and with different equivalence ratios, especially under lean burn condition, are well studied. Spherical flames are experimentally investigated at the initial temperature of 373 K and under the pressures of 2.5 bar, 5 bar and 10 bar. The equivalence ratios vary with 0.8, 1.0 and 1.2. The stretched flame speeds are determined by outwardly spherical flame method. The results show that at low initial pressures, the addition of methane to PRF95 increases the stretched flame speeds with lean equivalence ratios while decreases it in rich region. Laminar flame of methane-PRF95 mixtures burn faster than those of pure methane and PRF95 with equivalence ratio of 0.8 over the whole range of the initial pressures investigated, and this trend is more obvious at low pressure. Comparing the data of 25% methane dual fuel (DF25) with that of base fuels with the equivalence ratio of 0.8 and under the initial pressure of 2.5 bar, it can be seen that the flame speed of DF25 is 57% faster than that of methane and 22% faster than that of PRF95. These results provide important theoretical references to lean burn SI engine with methane-gasoline dual fuels under lean burn conditions

On the combustion of premixed natural gas/gasoline dual fuel blends in SI engines

The continuous update of challenging emission legislations has renewed the interest for the use of alternative fuels. The low carbon content, the knocking resistance, and the abundance reserves, have classified natural gas as one of the most promising alternative fuels. The major constituent of natural gas is methane. Historically, the slow burning velocity of methane has been a major concern for its utilisation in energy efficient combustion applications. As emphasized in a limited body of experimental literature, a binary blend of methane and gasoline has the potential to accelerate the combustion process in an SI engine, resulting in a faster combustion even to that of gasoline. The mechanism of such effects remains unclear. This is partially owned to the inadequate prior scientific understanding of the fundamental combustion parameters, laminar burning velocity (Su0) and Markstein length (Lb), of a gasoline-natural gas Dual Fuel (DF) blend. The value of Lb characterises the sensitivity of the flame to stretch. The flame stretch is induced by aerodynamic straining and/or flame curvature. The current research study has therefore being concerned on understanding the combustion mechanism of premixed gasoline - natural gas DF blends both on a fundamental as well as practical SI engine level. The understanding on the contribution of Su0 and Lb to the velocity of a stretched laminar propagating flame has been extended through numerical analysis. A conceptual analysis of the laminar as compared to the SI engine combustion allowed further insights on the effect of turbulence to the mass burning rate of the base fuels. On a fundamental level, the research contribution is made through the quantification of the response of Su0 and Lb with the ratio of methane to PRF95 (95%volliq iso-octane and 5%volliq n-heptane) in a DF blend. Methane has been used as a surrogate for natural gas and PRF95 as a surrogate for gasoline. Constant volume laminar combustion experiments have been conducted in a cylindrical vessel at equivalence ratios of 0.8, 1, 1.2, initial pressures of 2.5, 5, 10 Bar, and a constant temperature of 373 K. Methane was added to PRF95 in three different energy ratios 25%, 50% and 75%. Spherically expanding flames visualised through schlieren photography were used to derive the values of Lb and Su0. It has been concluded that for pressures relevant to SI engine operation (>5bar) and stoichiometric to lean Air Fuel Ratios (AFRs), there is a positive synergy for blending methane to PRF95 due to the convergence of Lb of the blended fuel towards that of pure gas and Su0 towards that of pure liquid. In an SI engine environment, the research contribution is made through the characterisation and scientific understanding of the mechanism of DF combustion, and the importance of flame-stretch interactions at various engine operating conditions. Optical diagnostics have been integrated with in-cylinder pressure analysis to investigate the mechanism of flame velocity and stability with the addition of natural gas to gasoline in a DF blend, under a sweep of engine load (Manifold Absolute Pressure = 0.44, 0.51. 0.61 Bar), speed (1250, 2000, 2750 RPM) and equivalence ratio (0.8, 0.83, 1, 1.25). Consisted with the constant volume experiments, natural gas was added to gasoline in energy ratios of 25%, 50% and 75%. It has been concluded that within the flamelet combustion regime the effect of Lb is dominating the lean burn combustion process both from a flame stability and velocity prospective. The effect of Su0 on the combustion process gradually increases as the AFR shifts from stoichiometric to fuel rich values. For stoichiometric to fuel lean mixtures, the effect of turbulence on the increase of the mass burning rate is on average 13% higher for natural gas as compared to gasoline. The higher turbulence sensitivity of natural gas is attributed to its lower Lb value