Effect of additives on laser ignition and compression ignition of methane and hydrocarbons in a rapid compression machine

Abstract

Despite recent efforts to develop new energy systems that do not rely on combustion of fossil fuels, internal combustion (IC) engines powered on fossil fuels (i.e. gasoline, diesel or natural gas) will remain as an integral component of the global energy portfolio for years to come and increasing the efficiency of IC engines will be a necessary means to reduce fossil fuel consumption and greenhouse gas emissions. In this study, the effect of fuel additives on natural gas and gasoline spark ignited engines were investigated using laser ignition and compression ignition experiments performed in a rapid compression machine (RCM). The goal of the laser ignition study was to examine the effect of additives to extend the lean limit of natural gas engines, while the goal of the compression ignition experiments were to examine the ability of fuel additives to decrease knock propensity of gasoline fuels. For the laser ignition study, methane/air mixtures containing various fuel additives at temperatures and pressures representative of the compressed conditions inside an internal combustion engine were ignited in the RCM. An Nd:YAG laser operating at a wavelength of 1064 nm was used to ignite methane/air mixtures ranging in equivalence ratio from stoichiometric down to 0.4 using a rapid compression machine (RCM). Experiments were conducted to determine the lean limit, minimum spark energy (MSE), and minimum ignition energy (MIE). Three different fuel additives at varying concentrations were tested. The results show that laser ignition exhibits a stochastic behavior which must be interpreted statistically. A 90% probability of occurrence is used to evaluate the MSE and MIE which resulted in MSE90=2.3 mJ and MIE90=7.2 mJ for methane/air mixtures of equivalence ratio equal to 0.4. The lean limit, defined as greater than 90% of the theoretically possible heat release, was found as equivalence ratio of 0.47 for methane/air mixtures. All three fuel additives resulted in a reduction of the baseline methane/air MIE, while only DTBP and NM resulted in a reduction of the lean limit. For the compression ignition study, the effects of various fuel additives on the auto-ignition characteristics of gasoline reference fuels were studied in the RCM. Fuel additives were added to stoichiometric fuel/air mixtures of liquid gasoline surrogate fuels and were auto-ignited in a RCM. Experiments were conducted to determine the ignition delay, heat release rate, and net heat release of the gasoline surrogate/air mixtures with and without fuel additives. Five different gasoline fuel additives were tested in an Iso-Octane and Toluene Reference base fuel. The results show that the majority of the additives increased the reactivity and decreased the ignition delays of the base fuels. However, a select few of the tested additives decreased the reactivity and increased the ignition delays of the base fuel at select conditions, which could be beneficial to increasing the efficiency of internal combustion engines.