A numerical study was carried out to evaluate the influence of engine combustion chamber geometry and operating conditions on the performance and emissions of a homogeneous charge compression ignition (HCCI) engine. Combustion in an HCCI engine is a very complex phenomenon that is influenced by several factors that need to be controlled, such as gas temperature, heat transfer, turbulence and auto-ignition of the gas mixture. An eddy dissipation concept (EDC) combustion model was used to take into account the interaction between turbulence and chemistry. The model assumed that reactions occur in small turbulent structures called fine-scales, whose characteristic lengths and times depend mainly on the turbulence level. The model parameters were slightly modified with respect to the standard model proposed by Magnussen, to correctly simulate the characteristics of the HCCI combustion process. A reduced iso-octane chemical mechanism with 186 species and 914 chemical reactions was employed together with a sub-mechanism for NOx. The model was validated by comparing the results with available experimental data in terms of pressure and instantaneous heat release rate. Two engine chamber geometries with and without a cavity in the piston were considered, respectively. The two engines provided significant differences in terms of fluid-dynamic patterns and turbulence intensity levels in the combustion chamber. The results show that combustion started earlier and proceeded faster for the flat piston, leading to an increase in both the peak pressure and gross indicated mean effective pressure, as well as a reduction of CO and UHC emissions. An additional analysis was performed by considering a case without swirl for the flat-piston case. Such an analysis shows that the swirl motion reduces the time duration of combustion and slightly increases the gross indicated work per cycle.

As zero-carbon fuels, hydrogen and ammonia are of great interest in the transition toward a climate-neutral transportation system. In order to use these fuels and their blends in reciprocating engines, a characterization of the combustion of NH₃/H₂/air mixtures at high pressures and temperatures is needed. The aim of this work is to compute the Laminar Flame Speed (LFS) of NH₃/H₂/air mixtures by varying the thermochemical conditions of the reactants. For this purpose, several simulations have been carried out using different kinetic reaction mechanisms. The accuracy of the model has been assessed by comparing the results with experimental data available in the scientific literature. Finally, the influence of mixture composition and thermodynamic conditions of the reactants on LFS has been assessed by considering temperature and pressure values relevant to automotive applications and not yet explored in the literature. By adding H₂ to NH₃/air mixtures, LFS increases exponentially. By plotting the logarithm of LFS as a function of the H₂ mole fraction, the numerical results are well fitted by using a second-degree polynomial regression. However, a linear regression is accurate enough if the H₂ mole fraction does not exceed 0.6. Regarding the effect of pressure, the decrease in LFS with increasing pressure is less important as pressure increases. On the other hand, LFS increases with temperature, and this effect is more pronounced as the H₂ mole fraction decreases and pressure increases.

The aim of this work is to exploit the influence of using ozonized air to achieve stable and efficient combustion of lean mixtures in a gasoline-fuelled Spark-Ignition (SI) engine. The influence of ozone on the combustion of near-stoichiometric mixtures, which are typical of SI engines, has also been assessed. A Computational Fluid Dynamics (CFD) model has been employed to simulate compression, combustion, and expansion of a spark ignition, axisymmetric engine fuelled with iso-octane/air/ozone mixtures. The aim is to assess how ozone improves the engine performance under different engine speeds, ignition timings and equivalence ratios. The model has been validated against experimental data available in the literature. Parametric analyses have been carried out by considering three values of engine speeds (800, 1000 and 1200 rpm), three different ozone concentrations at Intake Valve Closure (IVC) (0, 100 and 200 ppm) and two equivalence ratios (0.9 and 0.7). The results show that ozone enables reactions in the Low Temperature Combustion (LTC) regime, modifies the mixture chemical composition and the auto-ignition tendency. Specifically, for all the cases under examination, the addition of ozone to the air/fuel mixture reduces the combustion duration, leading to an increase in terms of work output and a reduction of the specific fuel consumption. Moreover, the advantage of using ozone is greater for lean mixtures than for near-stoichiometric mixtures. Finally, for the near-stoichiometric cases, when the residence time of the mixture is high enough, auto-ignition occurs in the end gases.