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Ignition delay in diesel engine combustion comprehends both a chemical and a physical amount, the first depending on fuel composition and charge temperature and pressure, the last resulting of time needed for the fuel to atomize, vaporize and mix with air. Control of this parameter, which is mandatory to weight the relative amount of premixed to diffusive stage of the hydrocarbon combustion, is here considered. Experimental measurements of flame intensity spectra obtained by in situ measurements on an optically accessible test device show the presence of peaks corresponding to radicals as OH and CH appearing at the pressure start of combustion. Since OH radicals result from chain branching reactions, a numerical simulation is performed based on a reduced kinetic scheme which allows to measure the branching agent concentration, and whose approximate nature is adequate to the proportion chemical aspects contribute to the overall delay.

Polychromatic extinction and chemiluminescence techniques, from ultraviolet to visible, were applied in an optical diesel engine, in order to analyze the temporal and spatial evolution of a high pressure fuel jet interacting with a swirling air motion. A fully flexible Common Rail fuel injection system equipped with a single hole nozzle was used. The experiments were performed at fixed engine speed and air/fuel ratio for three injection strategies. The first one consisted of a main injection to compare with those operating at low pressure injection. The other ones were based on a pilot and main injections, typical of current direct injection diesel engines, with different dwell time. A detailed investigation of the mixture formation process inside the combustion chamber during the ignition delay time was performed. The liquid and vapor fuel distribution in the combustion chamber was obtained analyzing the polychromatic extinction spectra.

In the last years, there has been an increasing concern on the emission of ultrafine particles in the atmosphere. A detailed study of formation and oxidation of these particles in the environment of the diesel cylinder presents many experimental difficulties due to the high temperatures, pressures and extremely reactive intermediate species. In this paper, in order to follow the different phases of diesel combustion process, high temporal and spatial resolution optical techniques were applied in the optically accessible chamber of diesel engine, at 2000 rpm and A/F=80:1 and 60:1. Simultaneous extinction, scattering and flame chemiluminescence measurements from UV to visible were carried out, in order to study the diesel combustion process from the droplet ignition to the formation of soot, through the growth of its precursors.

The optimization of diesel engine performance and emissions can be achieved through a better understanding of the in-cylinder combustion process. Advanced non-intrusive optical techniques are providing new tools for investigating the thermo-fluid dynamics processes as well as they are contributing to develop predictive models for DI diesel combustion. High-speed images of spray and flame evolution as well as UV-visible chemiluminescence measurements were carried out in an optical 0.5-liter, single-cylinder, four-stroke, direct- injection diesel engine equipped with a prototype four valves cylinder head and a fully flexible CR injection system. In order to evaluate the effect of different injection strategies on the combustion process, measurements were performed varying injection parameters. The ignition location and time were individuated by combustion visualization and detection of radical species, obtained by chemiluminescence measurements.

Conventional methods to measure gas concentrations and, in particular, NO are typically based on sampling by valve, sample treatment and subsequent analysis. These methods suffer low spatial and temporal resolution. The introduction of high energy lasers in combination with fast detection systems allowed to detect the NO distribution inside optically accessible Diesel engines. In this paper, a high spatial and temporal resolution in-situ technique based on ultraviolet - visible absorption spectroscopy is proposed. The characterization of the combustion process by the detection of gaseous compounds from the start of combustion until the exhaust phase was performed. In particular, this technique allows the simultaneous detection of NO and OH absolute concentrations inside an optically accessible Diesel combustion chamber.