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In this study, experiments were carried out in an optical single-cylinder Direct Injection Spark Ignition engine fuelled with n-butanol and gasoline, alternatively. The engine is equipped with the head of a commercial turbocharged engine with similar geometrical specifications (bore, stroke, compression ratio). The head has four valves and a centrally located spark device with surface charge ignition. A conventional elongated hollow Bowditch piston is used and an optical crown, accommodating fused-silica window, is screwed onto it. The injector is side mounted and features 6 holes oriented to guide the jets towards the piston crown. During the experimental activity, the injection pressure was maintained at 100 bar for all conditions; the injection timing and the number of injections were adjusted to investigate their influence on combustion and emissions.

A plasma ignition system was tested in a GDI engine with the target of combustion efficiency improvement without modifying engine configuration. The plasma was generated by spark discharge and successively sustained to enhance its duration up to 4 ms. The innovative ignition system was tested in an optically accessible single-cylinder DISI engine to investigate the effects of plasma on kernel stability and flame front propagation under low loads and lean mixture (λ≅1.3). The engine was equipped with the head of a commercial turbocharged engine with similar geometrical specifications (bore, stroke, compression ratio). All experiments were performed at 2000 rpm and 100 bar injection pressure. UV-visible 2D chemiluminescence was applied in order to study the flame front inception and propagation with particular interest in the early combustion stages. A bandpass filter allowed selecting luminous signal due to OH radicals.

The increasing limitations in engine emissions and fuel consumption have led researchers to the need to accurately predict combustion and related events in gasoline engines. In particular, knock is one of the most limiting factors for modern SI units, severely hindering thermal efficiency improvements. Modern CFD simulations are becoming an affordable instrument to support experimental practice from the early design to the detailed calibration stage. To this aim, combustion and knock models in RANS formalism provide good time-to-solution trade-off allowing to simulate mean flame front propagation and flame brush geometry, as well as “ensemble average” knock tendency in end-gases. Still, the level of confidence in the use of CFD tools strongly relies on the possibility to validate models and methodologies against experimental measurements.

To meet the future stringent emission standards, innovative diesel engine technology, exhaust gas after-treatment, and clean alternative fuels are required. Oxygenated fuels have showed a tendency to decrease internal combustion engine emissions. In the same time, advanced fuel injection modes can promote a further reduction of the pollutants at the exhaust without penalty for the combustion efficiency. One of the more interesting solutions is provided by the premixed low temperature combustion (LTC) mechanism jointly to lower-cetane, higher-volatility fuels. In this paper, to understand the role played by these factors on soot formation, cycle resolved visualization, UV-visible optical imaging and visible chemiluminescence were applied in an optically accessed high swirl multi-jets compression ignition engine. Combustion tests were carried out using three fuels: commercial diesel, a blend of 80% diesel with 20% gasoline (G20) and a blend of 80% diesel with 20% n-butanol (BU20).

The detailed study of part-load conditions is essential to characterize engine-out emissions in key operating conditions. The relevance of part-load operations is further emphasized by the recent regulations such as the new WLTP standard. Combustion development at part-load operations depends on a complex interplay between moderate turbulence levels (low engine speed and tumble ratio), low in-cylinder pressure and temperature, and stoichiometric-to-lean mixture quality (to maximize fuel efficiency). From a modelling standpoint, the reduced turbulence intensity compared to full-load operations complicates the interaction between different sub-models (e.g., reconsideration of the flamelet hypothesis adopted by common combustion models). In this article, the authors focus on chemistry-based simulations for laminar flame speed of gasoline surrogates at conditions typical of part-load operations. The analysis is an extension of a previous study focused on full-load operations.

A satisfactory answer to the future severe normative on emissions and to the market request for spark ignition engines seems to be the use of downsized engines for passenger cars. Downsizing permits the increase in engines power and torque without the increase in cylinder capacity. The downsizing benefits are evident at part loads; on the other hand, more work should be done to optimize boosted engines at higher and full load. To this goal, a detailed knowledge of the thermo-fluid dynamic processes that occur in the combustion chamber is fundamental. The aim of this paper is the experimental investigation of the effect of the fuel injection in the intake manifold on the combustion process and pollutant formation in a boosted spark ignition (SI) engine. The experiments were performed on a partially transparent single-cylinder port fuel injection (PFI) SI engine, equipped with a four-valve head and boost device.

In this work, a boosted single-cylinder spark ignition port-fuel injection optical engine was used for the experimental activity. Firstly, it was equipped with a four-valve head of a commercial turbocharged multi-cylinder engine. Then a prototype engine head with flush installed intake valves was tested. The effect of the different head geometry was evaluated in closed intake valves fuel injection condition. High spatial resolution cycle-resolved digital imaging was used to characterise the flame propagation. Moreover, the presence of diffusion-controlled flames near the valves and on the cylinder walls was investigated. These flames induced the formation of unburned hydrocarbons and soot particles. The spatial distribution and temporal evolution of soot were evaluated by the two colour pyrometry. The prototype configuration showed higher combustion process efficiency than the standard one inducing a little increase in performance and a slight reduction in carbon oxides emissions.

High spatial and temporal resolution optical techniques were applied in a spark ignition (SI) engine in order to investigate the thermal and fluid dynamic phenomena occurring during the combustion process. The experiments were realized in the combustion chamber of an optically accessible single-cylinder port-fuel injection (PFI) SI engine. The engine was equipped with a four-valve head and with an external boost device. Two fuel injection strategies at closed-valve and open-valve occurring at wide open throttle were tested. Cycle-resolved digital imaging was used to follow the flame kernel growth and flame front propagation. Moreover, the effects of an abnormal combustion due to the firing of fuel deposition near the intake valves and on the piston surface were investigated. Natural emission spectroscopy in a wide wavelength range from ultraviolet to infrared was applied to detect the radical species that marked the combustion phenomena in the selected operating conditions.

Electronic engine controls based on non-intrusive diagnostics can significantly help in complying with the stricter and stricter regulations on pollutants emissions and fuel consumption. The aim of this paper is the use of a low-cost linear capacitive accelerometer placed on the engine block for non-intrusive diagnosis of combustion process in spark ignition engines. In particular, good correspondences between the engine block vibrations and the combustion pressure signal were obtained. The angular position of pressure peak evaluated by accelerometer data can be used in a closed-loop control system for real time control of spark advance.

Homogeneous Charge Compression Ignition (HCCI) combustion was applied to a transparent diesel engine equipped with high pressure Common Rail (CR) injection system. By means of CR system the quantity of fuel was split into five injections per cycle. Combined measurements, based on digital imaging and spectroscopic techniques, were applied to follow the evolution of HCCI combustion process with high temporal and spatial resolution. Digital imaging allowed to analyse injection and combustion phases. Broadband ultraviolet - visible extinction spectroscopy (BUVES) and flame emission measurements were carried out to evaluate the presence of radicals and species such as HCO, OH, CH, and CO. In particular, BUVES measurements were performed to follow fuel oxidation, and pollutant formation and oxidation. During injection and cool combustion, bands of aromatic compounds and alkyl peroxides, indicating fuel decomposition, and hydrogen peroxides were detected.

Nowadays HCCI combustion process is revealing the most useful technique for reducing pollutant emission from internal combustion engines. In the present paper, HCCI combustion was realized by means of single late injection at high pressure and heavy EGR, up to 50%. A transparent Direct Injection (DI) diesel engine equipped with high pressure Common Rail (CR) injection system was used. The engine was fed with commercial diesel fuel and ran in continuous mode. Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied to study the low temperature combustion process. Injection and combustion phases were analysed by digital imaging. Mixing process, autoignition and pollutants formation were investigated by Broadband Ultraviolet - Visible Extinction Spectroscopy (BUVES) and flame emission measurements. Radicals and species such as OH, CH and CO were detected in the combustion chamber.

The present study proposes the use of a MLP neural network to model the relationship between the engine crankshaft speed and parameters derived from the in-cylinder pressure cycle. This allows to have an indirect measure of cylinder pressure permitting a real time evaluation of combustion quality. The structure of the model and the training procedure is outlined in the paper. The application of the model is demonstrated on a single-cylinder engine with data from a wide range of speed and load. Results confirm that a good estimation of combustion pressure parameters can be obtained by means of a suitable processing of crankshaft speed signal.

The match among the increasing performance demands and the stringent requirements of emissions and the fuel consumption reduction needs a strong evolution in the two-wheel vehicle technology. In particular, many steps forward should be taken for the optimization of modern small motorcycles and scooters at low engine speeds and high loads. To this aim, detailed understanding of thermo-fluid dynamic phenomena that occur in the combustion chamber is fundamental. In this work, low-cost solutions are proposed to optimize ported fuel injection spark ignition (PFI SI) engines for two-wheel vehicles. The solutions are based on the change of phasing and on the splitting of the fuel injection in the intake manifold. The experimental activities were carried out in the combustion chamber of a single-cylinder 4-stroke optical engine fuelled with European commercial gasoline. The engine was equipped with a four-valve head of a commercial scooter engine.

The small engine for two-wheel vehicles has generally high possibility to be optimized at low speeds and high loads. In these conditions fuel consumption and pollutants emission should be reduced maintaining the performance levels. This optimization can be realized only improving the basic knowledge of the thermo-fluid dynamic phenomena occurring during the combustion process. It is known that, during the fuel injection phase in PFI SI engines, thin films of liquid fuel can form on the valves surface and on the cylinder walls. Successively the fuel films interact with the intake manifold and the combustion chamber gas flow. During the normal combustion process, it is possible to achieve gas temperature and mixture strength conditions that lead to fuel film ignition. This phenomenon can create diffusion-controlled flames that can persist well after the normal combustion event. These flames induce the emission of soot and unburned hydrocarbons.

Nowadays an elevated number of two, three and four wheels vehicles circulating in the world-wide urban areas is equipped with Port Fuel Injection Spark Ignition (PFI SI) engines. Their technological level is high, but a further optimization is still possible, especially at low engine speed and high load. To this purpose, the scientific community is now focused on deepening the understanding of thermo fluid dynamic phenomena that takes place in this kind of engine: the final purpose is to find key points for the reduction in engine specific fuel consumption and exhaust emissions without a decrease in performance. In this work, the combustion process was investigated in an optically accessible single cylinder PFI SI engine. It was equipped with the head, injection device and exhaust line of a commercial small engine for two-wheel vehicles, it had the same geometrical characteristics in terms of bore, stroke and compression ratio.

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.

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.

Multidimensional simulation of the soot formation process in a diesel engine is realised exploiting quantitative measurements of the soot volume fraction and diameter obtained by optical techniques. Broadband extinction and scattering measurements are performed on an optically accessible 4-stroke engine where a forced air motion allows a strong prevalence of the premixed stage of combustion with respect to the non-premixed one. Two semi-empirical models for soot formation are tested in the numerical simulation, which is performed using a customized version of the KIVA-3 code. The need of furnishing coherent values of the soot particles density and mean diameter to the one of the two models requiring this kind of information, is highlighted and demonstrated to be crucial in avoiding over-prediction of the soot concentration.

Non-intrusive diagnostic techniques based on broadband (190-550 nm) extinction and scattering spectroscopy were applied to undiluted exhaust Common Rail (CR) diesel engine. The influence of engine speed and load on soot mass concentration, size distribution of emitted particles and NOx concentration was analysed. NOx concentration was evaluated by “in situ” ultraviolet-visible absorption measurements and compared with those obtained by conventional analyser. The extinction and scattering spectra were compared with those evaluated by the Lorenz-Mie model for spherical particles in order to retrieve the size, the number concentration of the emitted particles and particulate mass.

Spectroscopic measurement and high speed visualization were used in single cylinder, four-stroke DI diesel engine, optically accessible. It was equipped with a four valves head and fully flexible electronic controlled ‘Common Rail’ injection system. The effect of pilot and main injection on combustion process was evaluated. Mixing formation, autoignition and soot formation process were analyzed by broadband ultraviolet-visible flame emission spectroscopy and high-speed digital imaging. The autoignition phase occurred near the tip of the jet and was characterized by strong presence of OH radicals for both investigated conditions The presence of C2 and OH radicals strongly characterized CR diesel combustion process during soot formation and evolution. In particular, high presence of OH concentration for the whole process from the autoignition to the soot formation and successive phases contributes to lower soot levels.