Future emissions standards for passenger cars require a reduction of NOx (nitrogen oxide) and CO₂ (carbon dioxide) emissions of diesel engines. One of the ways to reach this challenge while keeping other emissions under control (CO: carbon monoxide, HC: unburned hydrocarbons and particulates) is to reduce the volumetric compression ratio (CR). Nevertheless complications appear with this CR reduction, notably during very cold operation: start and idle. These complications justify intensifying the work in this area.Investigations were led on a real 4-cylinder diesel 13.7:1 CR engine, using complementary tools: experimental tests, in-cylinder visualizations and CFD (Computational Fluid Dynamics) calculations. In previous papers, the way the Main combustion takes place according to Pilot combustion behavior was highlighted.This paper, presents an in-depth study of mixture preparation and the subsequent combustion process.Using laser illumination techniques, in-cylinder visualizations highlight - even at low engine speed during cranking operation - the existence of swirl motion moving the fuel being vaporized inside the chamber. In these conditions, visualizations and computations show that the vicinity of the glow plug and one injector spray leads to a combination of adequate temperature and air/fuel ratio and so to the combustion initiation. CFD calculations also point out that the "stair shape" evolution of heat release can come from a progressive propagation of combustion from one spray to another.Using the available tools, it appears also that some troubles encountered in cold idle can be attributed to inadequate engine control.The tools developed (experimental tests, in-cylinder visualizations, and calculations) have proven quite effective and complementary capabilities in order to investigate and to understand the physical phenomena encountered during cold operation (start and idle). Even if correlations are not always easy to establish between these phenomena and engine performance: speed stability or smoke opacity, these tools will allow to progress in this area and to further improve engine behavior and emissions.