The oxidation of CH4 and CH4/C2H6 mixtures were studied at pressures relevant to knocking in large bore natural gas engines. The experiments were carried out in a rapid compression machine (RCM) at end of compression (EOC) temperatures ranging between 885-940 K at compressed gas pressures of 105, 125, 150, and 160 bar at varying equivalence ratios (0.417, 0.526, and 1.0) and dilution percentages (0, 10, and 30% Exhaust Gas Recirculation-EGR) that were defined in a test matrix. This study describes the method and limitations of performing high-pressure experiments of this magnitude in an RCM, modeling, and validation of the kinetic mechanism against experimental data. While the recently published AramcoMech 2.0 could well predict the ignition delay times (IDTs) for CH4 within the uncertainty ranges at comparatively higher pressures and lower temperatures (885-940 K), the predicted reactivity is, in general, lower than that of AramcoMech 1.3 as shown in our previous screening study. Based on the comparison between both mechanisms as well as sensitivity analysis on the predicted IDTs, the reaction rate constant for Ḣ-atom abstraction from CH4 by HȮ2 radical was optimized in order to achieve better agreement with the new data while maintaining the agreement to the previous data sets. The modified mechanism predicts well the IDTs and the trend of their variation caused by the change in pressure, equivalence ratio, dilution percentage, and mixture variation with C2H6.

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Downsizing of engines is a major area of interest in the combustion engines sector due to a variety of reasons, chief among which is the CO2 emission reduction due to increased power to weight ratio. Furthermore, the introduction of various auxiliary devices into an automotive product, as well as increased acoustic insulation, necessitate continuous trimming of the engine packaging space. In this paper, the potential and limitations of downsizing diesel engines to very small displacements is studied. The goal of the article is to determine the minimum displacement a diesel engine can achieve, given the limitations posed by state-of-the-art technology. At the same time, the objective is the maximization of power density with acceptable levels of fuel consumption. While the investigations focused on the thermodynamic behavior of downsizing, structural aspects were also considered. On the basis of a literature study, the article illustrates the benchmarking of existing small gasoline and diesel engines for different applications. Thereafter, a matrix of engine configurations, which were relevant to the investigations, was generated. This included, among others, various bore / stroke combinations, compression ratios, piston and nozzle geometries, as well as valve diameters. Further, the influence of injection pressure, swirl and air-fuel ratio were included in the study. With the aid of the 1D simulation software GT-Power and the 3D CFD code Kiva-3V, a detailed thermodynamic analysis was performed on the chosen variants. In the results detailed in this article, a promising downsizing potential for a cylinder displacement well below 200cm3/cylinder has been established. Further, best-in-class power densities at acceptable fuel consumption levels could be achieved. This opens up the possibility for the application of such small diesel engines in a new range of applications. The challenges on the thermodynamic and structural fronts, which need to be met in order to achieve targets, are also highlighted.

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