The different physical-chemical properties of HVO and OMEX make the study of their behavior in compression ignition engines essential. Quantitative parameters describing the evolution of diesel-like sprays such as liquid length, spray penetration, ignition delay, lift-off length, and flame penetration, as well as soot formation, were assessed in a constant high-pressure and high-temperature installation using optical diagnostics. Combustion stability, engine efficiency, and emissions in a multi-cylinder spark-ignition internal combustion engine can be improved through the advanced control and optimization of individual cylinder operation. In [5], experimental and numerical analyses were carried out on a twin-cylinder turbocharged port fuel injection spark-ignition engine to evaluate the influence of cylinder-by-cylinder variation on performance and pollutant emissions. The proposed numerical methodology represents a valuable tool to support engine design and calibration, with the aim to improve both performance and emissions. With respect to the optimization of compression ignition engines, two contributions to the Special Issue [6; 7] deal with the optimization of the injection process since fuel mixing is critical in determining combustion efficiency and emissions levels. In [6], simulation optimization methods and methods based on evolutionary algorithms were elaborated on for the optimization of the design and technology of injector nozzles in terms of minimizing energy losses on friction in compression ignition engines. The technical state of the injector apparatus significantly affected the engine performance, fuel consumption, toxicity, and smoke opacity of outlet gases. Alternatively, in [7], an analysis was carried out on the effects of the fuel injection rate shape on the diesel spray mixing process using a numerical simulation. The results show that a changing injection rate shape can enhance mixing during injection and achieve better mixing after the end of injection. In [8], an investigation was carried out to improve energy conversion for a four-stroke free-piston engine. Unlike crankshaft-based engines, free-piston engines allow for a higher degree of freedom in shaping the piston trajectory, including adaptive compression ratios, which enables optimal operation with alternative fuels. The possibility of adapting the stroke course results in new degrees of freedom with which the combustion process can be optimized. In this work, four-stroke trajectories with different amplitudes and piston dynamics were proposed and analyzed regarding efficiency. The trajectories were described analytically so that they can be used for a prototype in future work. Paper [9] deals with the use of a pre-turbine small-sized oxidation catalyst. The earlier activation of catalytic converters in internal combustion engines is becoming highly challenging due to the reduction in exhaust gas temperature caused by the application of CO2 reduction technologies. In this context, the use of pre-turbine catalysts arises as a potential way to increase the conversion efficiency of the exhaust aftertreatment system. In this work, a small-sized oxidation catalyst consisting of a honeycomb thin-wall metallic substrate was placed upstream of the turbine to benefit from the higher temperature and pressure before the turbine expansion. The effects of this device on engine performance and emissions under driving conditions were shown. The last two papers published in this Special Issue [10; 11] refer to the use of numerical methods to aid the engine development process. In [10], a computationally efficient tabulated chemistry solver for internal combustion engine optimization using stochastic reactor models was developed. The use of chemical kinetic mechanisms in computer-aided engineering tools for internal combustion engine simulations is of high importance for studying and predicting pollutant formation of conventional and alternative fuels.