Editorial Special Issue “Combustion and Propulsion”

The increasing demand for socially and environmentally sustainable development requires appropriate use of energy resources, particularly in the transportation of people and goods. The main source of energy for propulsion is based on the use of fuels, mainly of fossil origin but increasingly produced from renewable sources. Therefore, combustion processes play a key role in the efficiency of energy use and for the environmental impact of transportation systems. The most relevant topic of recent scientific and technological research is the control of the fundamental processes, down to the molecular level, affecting combustion. Therefore, research focuses on the production of fuels from renewable sources, on fuel injection and vaporization, by means of new concepts of atomizers in gas turbines, or by means of injection systems with electronic control and high pressure in internal combustion engines, on air–fuel mixing. Moreover, research focuses on the control of oxidation processes by lean combustion or by new combustion concepts, combustion instabilities, micro-scale combustion, control of turbulence and the boundary layer by means of active systems, and the development of new combustion concepts such as the homogeneous charge compression ignition. Control and diagnostics are influenced by the peculiar characteristics of combustion in propulsion systems, with respect to stationary applications, because of the significant degree of unsteadiness of the operating conditions and the extremely high power density. The present Special Issue focuses on experimental, theoretical, and computational investigations on the fundamentals of combustion in propulsion systems, and on the resulting lines of technological development. The articles cover a wide range of topics, such as combustion in gas turbines and internal combustion engines for aeronautical, automotive, naval and railway engines, chemical space propulsion, control and diagnostics of combustion, new and renewable fuels, alternative combustion-based propulsion systems, new combustion concepts, pollutants formation and control in propulsive systems. Eleven papers have been approved for publication; most of them are related to the reduction and control of pollutants. The formation of pollutants from combustion systems is a topic that attracts considerable interest because of the negative impact on human health, air quality, and climate change. Numerous experimental and numerical efforts have been made in order to achieve a better understanding of the chemistry and physics involved in the pollutant formation process, with the aim of achieving low-emission combustion technologies. In this context, the work of [1] was aimed at obtaining both quantitative and qualitative information of soot particles generated with a set of laminar partially-premixed co-flow flames characterized by different equivalence ratios by an improved version of the thermophoretic particle densitometry (TPD) method. Results also show that increasing the remixing has the initial effect of increasing particle concentration because of the increase of the radical pool promoted by the oxygen addition, while the competitive effect of increasing dilution prevails when the primary air is further increased so that the reduction of the particle volume fraction is measured Aimed at achieving low exhaust emissions, lean-burn combustion systems, operating with a very lean air/fuel ratio are of great interest. However, in [2], it was shown that at very lean conditions the