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Detailed experimental information on the early stages of spark ignition process represent a substantial part for guiding the development of engines with higher efficiencies and reduced pollutant emissions. Flame kernel formation influences strongly combustion development inside the cylinder, especially for a direct injection spark ignition engine. This study presents the analysis of the evolution of spark-ignited flame kernels with detailed view upon cycle-to-cycle variations. Experiments are performed in a SI optical engine equipped with the cylinder head and injection system of a commercial turbocharged engine. Blend of commercial gasoline and butanol (40% by volume) is tested at stoichiometric and lean mixture conditions. Experiments are carried out at 2000 rpm through conventional tests (based on in-cylinder pressure measurements and exhaust emission analysis) and through optical diagnostics. In particular, UV-visible digital imaging and natural emission spectroscopy are applied.

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.

The paper reports the results of an experimental investigation carried out in a prototype optically accessible compression ignition engine fuelled with different blends of commercial diesel and n-butanol. Thermodynamic analysis and exhaust gas measurements were supported by optical investigations performed through a wide optical access to the combustion chamber. UV-visible digital imaging and 2D chemiluminescence were applied to characterize the combustion process in terms of spatial and temporal occurrence of auto-ignition, flame propagation, soot and OH evolution. The paper illustrates the results of the spray combustion for diesel and n-butanol-diesel blends at 20% and 40% volume fraction, exploring a single and double injection strategy (pilot+main) from a common rail multi-jet injection system. Tests were performed setting a pilot+main strategy with a fixed dwell time and different starts of injection.

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.

In this work, a promising technique, consisting of a liquid Water Injection (WI) at the intake ports, is investigated to overcome over-fueling and delayed combustions typical of downsized boosted engines, operating at high loads. In a first stage, experimental tests are carried out in a spark-ignition twin-cylinder turbocharged engine at a fixed rotational speed and medium-high loads. In particular, a spark timing and a water-to-fuel ratio sweep are both specified, to analyze the WI capability in increasing the knock-limited spark advance. In a second stage, the considered engine is schematized in a 1D framework. The model, developed in the GT-Power™ environment, includes user defined procedures for the description of combustion and knock phenomena. Computed results are compared with collected data for all the considered operating conditions, in terms of average performance parameters, in-cylinder pressure cycles, burn rate profiles, and knock propensity, as well.

The potential benefits of water injection on performance and emissions were investigated on a downsized PFI twin-cylinder turbocharged spark ignition engine. Experiments were carried out at high load condition (~15.5 bar IMEP) within the engine speed range from 3500 to 4500rpm with a step of 500 rpm. For each test case the effect of the injected water quantity on combustion and exhaust emissions was investigated by sweeping from 10%w to 30%w the water to gasoline ratio. The water was injected at the same timing as the gasoline by a low pressure injection system external controlled. Tests were performed at WOT conditions exploring, for each operating condition, a spark sweep from knock-free up to knock-limited operation. Compared to the full gasoline reference case, the water injection allowed to advance extensively the spark timing without knock occurrence. The 20% water to gasoline mass fraction gave the best improvements in terms of IMEP.

In this paper, the results of an extensive experimental analysis regarding a twin-cylinder spark-ignition turbocharged engine are employed to build up an advanced 1D model, which includes the effects of cycle-by-cycle variations (CCVs) on the combustion process. Objective of the activity is to numerically estimate the CCV impact primarily on fuel consumption and knock behavior. To this aim, the engine is experimentally characterized in terms of average performance parameters and CCVs at high and low load operation. In particular, both a spark advance and an air-to-fuel ratio (α) sweep are actuated. Acquired pressure signals are processed to estimate the rate of heat release and the main combustion events. Moreover, the Coefficient of Variation of IMEP (CoVIMEP) and of in-cylinder peak pressure (CoVpmax) are evaluated to quantify the cyclic dispersion and identify its dependency on peak pressure position.

The occurrence of knock is the most limiting hindrance for modern Spark-Ignition (SI) engines. In order to understand its origin and move the operating condition as close as possible to onset of this potentially harmful phenomenon, a joint experimental and numerical investigation is the most recommended approach. A preliminary experimental activity was carried out at IM-CNR on a 0.4 liter GDI unit, equipped with a flat transparent piston. The analysis of flame front morphology allowed to correlate high levels of flame front wrinkling and negative curvature to knock prone operating conditions, such as increased spark timings or high levels of exhaust back-pressure. In this study a detailed CFD analysis is carried out for the same engine and operating point as the experiments. The aim of this activity is to deeper investigate the reasons behind the main outcomes of the experimental campaign.

Knock occurrence and fuel enrichment, which is required at high engine speed and load to limit the turbine inlet temperature, are the major obstacles to further increase performance and efficiency of down-sized turbocharged spark ignited engines. A technique that has the potential to overcome these restrictions is based on the injection of a precise amount of water within the mixture charge that can allow to achieve important benefits on knock mitigation, engine efficiency, gaseous and noise emissions. One of the main objectives of this investigation is to demonstrate that water injection (WI) could be a reliable solution to advance the spark timing and make the engine run at leaner mixture ratios with strong benefits on knock tendency and important improvement on fuel efficiency.

This paper reports a numerical and experimental analysis on a twin-cylinder turbocharged Spark Ignition engine carried out to investigate the cylinder-to-cylinder variability in terms of performance, combustion evolution and exhaust emissions. The engine was tested at 3000 rpm in 20 different steady-state operating conditions, selected with the purpose of observing the influence of cylinder-by-cylinder A/F ratio variations and the EGR effects on the combustion process and exhaust emissions for low to medium/high loads. The experimental outcomes showed relevant differences in the combustion evolution (characteristic combustion angles) between cylinders and not negligible variations in the emissions of the single cylinder exhaust and the overall engine one. This misalignment resulted to be due to differences in the injected fuel amount by the port injectors in the two cylinders, mainly deriving from the specific fuel rail geometry.

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 effect of ammoniac deoxidizing agent (Urea) on the reduction of NOx produced in the Diesel engine was investigated numerically. Urea desolved in water was directly injected into the engine cylinder during the expansion stroke. The NOx deoxidizing process was described using a simplified chemical kinetic model coupled with the comprehensive kinetics of Diesel oil surrogate combustion. If the technology of DWI (Direct Water Injection) with the later injection timing is supposed to be used, the deoxidizing reactants could be delivered in a controlled amount directly into the flame plume zones, where NOx are forming. Numerical simulations for the Isotta Fraschini DI Diesel engine are carried out using the KIVA-3V code, modified to account for the “co-fuel” injection and reaction with combustion products. The results showed that the amount of NOx could be substantially reduced up to 80% with the injection timing and the fraction of Urea in the solution optimized.

In this paper, a Low Temperature Premixed Combustion (LTPC) was investigated employing a four cylinder D.I. common rail Diesel engine, used for passenger cars on the European market. Experiments were carried out setting the engine speed at 2500 rpm with a fuel amount of 26 mg/str to realize an operating condition close to the point of NEDC at 0.8 MPa of BMEP. The experimental approach was the management of the start of injection, injection pressure and EGR rates as a method to control NOx and soot production. The investigation was first carried out testing engine performances and emissions as set from the commercial engine map. Afterward, engine tests were carried out exploring performances, gaseous and smoke emissions at late start of combustion [10 to 17.5 cad ATDC], injection pressures from 80 to 120 MPa and EGR rates up to 50%.

A PC-programmable electronic control unit (PECU), able to manage both conventional and future electronic injection systems to make a fixed number of consecutive injections (1 to 5 or more) controlling the injection pressure and the injection pulses duration as well as the separation time or dwell in between was used to study the behaviour of a Bosch common rail injection system both on dynamic spray bench and on engine test bench. The PECU allowed a reduction in the dwell time between consecutive injection pulses from the current value of 1800 μs to 500 μs. Photographic sequences of a five holes mini-sac nozzle making five consecutive injections at 400 - 800 and 1200 bar respectively were taken at ambient pressure and temperature. They showed that both spray penetration and cone angle at all operative conditions are very uniform and stable.

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.

In-cylinder combustion phenomena, performance and emissions of a new single cylinder (225 cm3) naturally aspirated DI Diesel engine, with an advanced low cost common rail system for multiple injections, were investigated. The main objective of the present work was the study of the combustion system in terms of combustion chamber geometry, spray angle and number, injection pressure as well as injections number per engine cycle to find the best compromise between smoke and NOx emissions. CFD simulations were made to optimise the combustion chamber shape and the spray angle of a 6 holes nozzle to control the in-cylinder soot formation. The common rail (CR) system consisted of an in-house modified low cost PF Bosch injection unit for pumping the fuel up to 60MPa, a high pressure accumulator (rail) equipped with a pressure regulator valve and sensor as well as improved fast electronic drivers to drive both the pressure regulator valve and a commercial solenoid injector.

Comfort requirements, government regulations as well as consumer action groups are pressing the automotive industry to produce less noisy vehicles than in the past. These circumstances become more and more important for off-road and human operating machines forcing engine developers to investigate new and more effective control strategies of noise emissions. This paper concerns with the experimental vibro-acoustic analysis of a small (224 cc) single-cylinder direct-injection diesel engine used for agricultural and industrial applications as well as off road small vehicles. In order to evaluate the engine acoustic behaviour, experimental identification and localization of noise sources were performed at different speed and load engine conditions by several investigating tools. Within them, the intensity technique was chosen because of its peculiarities to be performed “in situ” without a specific anechoic test environment.

In this paper, a downsized twin-cylinder turbocharged spark-ignition engine is experimentally investigated at test-bench in order to verify the potential to estimate the peak pressure value and the related crank angle position, based on vibrational data acquired by an accelerometer sensor. Purpose of the activity is to provide the ECU of additional information to establish a closed-loop control of the spark timing, on a cycle-by-cycle basis. In this way, an optimal combustion phasing can be more properly accomplished in each engine operating condition. Engine behavior is firstly characterized in terms of average thermodynamic and performance parameters and cycle-by-cycle variations (CCVs) at high-load operation. In particular, both a spark advance and an A/F ratio sweep are actuated. In-cylinder pressure data are acquired by pressure sensors flush-mounted within the combustion chamber of both cylinders.

Considering the generalized diversification of the energy mix, the use of alcohols as gasoline replacement is proposed as a viable option. Also, alternative control strategies for spark ignition engines (SI) such as lean operation and exhaust gas recirculation (EGR) are used on an ever wider scale for improving fuel economy and reducing the environmental impact of automotive engines. In order to increase the stability of these operating points, alternative ignition systems are currently investigated. Within this context, the present work deals about the use of plasma assisted ignition (PAI) in a direct injection (DI) SI engine under lean conditions and cooled EGR, with gasoline and n-butanol fueling. The PAI system was tested in an optically accessible single-cylinder DISI engine equipped with the head of a commercial turbocharged power unit with similar geometrical specifications (bore, stroke, compression ratio).

UV-visible digital imaging and 2D chemiluminescence were applied on a single cylinder optically accessible compression ignition engine to investigate the effect of different alcohol/diesel fuel blends on the combustion mechanism. The growing request for greenhouse gas emission reduction imposes to consider the use of alternative fuels with the aim of both partially replacing the diesel fuel and reducing the fossil fuel consumption. To this purpose, the use of ABE (Acetone-Butanol-Ethanol) fermentation could represent an effective solution. Even if the different properties of alcohols compared to Diesel fuel limit the maximum blend concentration, low blend volume fractions can be used for improving combustion efficiency and exhaust emissions. The main objective of this study was to investigate the effects of the different fuel properties on the combustion evolution within the combustion chamber of a prototype optically accessible compression ignition engine.