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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.

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.

Engine knock is one of the most limiting factors for modern Spark-Ignition (SI) engines to achieve high efficiency targets. The stochastic nature of knock in SI units hinders the predictive capability of RANS knock models, which are based on ensemble averaged quantities. To this aim, a knock model grounded in statistics was recently developed in the RANS formalism. The model is able to infer a presumed log-normal distribution of knocking cycles from a single RANS simulation by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. As a main advantage, the model is able to estimate the earliest knock severity experienced when moving the operating condition into the knocking regime.

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.

Alternative combustion control in the form of lean operation offers significant advantages such as high efficiency and “clean” fuel oxidation. Maximum dilution rates are limited by increasing instability that can ultimately lead to partial burning or even misfires. A compromise needs to be reached between high tumble-turbulence levels that “speed-up” combustion and the inherent stochastic nature of this fluid motion. The present study is focused on gaining improved insight into combustion characteristics through thermodynamic analysis and flame imaging, in a wall-guided direct injection spark ignition engine with optical accessibility. Engine speed values were investigated in the range of 1000 to 2000 rpm, with commercial gasoline fueling, in wide open throttle conditions; mixture strength ranged from stoichiometric, down to the equivalence ratios that allowed acceptable cycle-by-cycle variations; and all cases featured spark timing close to the point of maximum brake torque.

Conventional fossil fuels are more and more regulated in terms of both engine-out emissions and fuel consumption. Moreover, oil price and political instabilities in oil-producer countries are pushing towards the use of alternative fuels compatible with the existing units. N-Butanol is an attractive candidate as conventional gasoline replacement, given its ease of production from bio-mass and key physico-chemical properties similar to their gasoline counterpart. A comparison in terms of combustion behavior of gasoline and n-Butanol is here presented by means of experiments and 3D-CFD simulations. The fuels are tested on a single-cylinder direct-injection spark-ignition (DISI) unit with an optically accessible flat piston. The analysis is carried out at stoichiometric undiluted condition and lean-diluted mixture for both pure fuels.

Aim of this work is to develop a general purpose model for combustion and knocking prediction in SI engines, by coupling a thermo-fluid dynamic model for engine simulation with a general detailed kinetic scheme, including the low-temperature oxidation mechanism, for the prediction of the auto-ignition behavior of hydrocarbons. A quasi-D approach is used to describe the in-cylinder thermodynamic processes, applying the conservation of mass and energy over the cylinder volume, modeled as a single open system. The complex chemistry model has been embedded into the code, by using the same integration algorithm for the conservation equations and the reacting species, and taking into account their mutual interaction in the energy balance. A flame area evolution predictive approach is used to evaluate the turbulent flame front propagation as function of the engine operating parameters.

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.

Multi-fuel operation is one of the main topics of investigative research in the field of internal combustion engines. Spark ignition (SI) power units are relatively easily adaptable to alternative liquid-as well as gaseous-fuels, with mixture preparation being the main modification required. Numerical simulations are used on an ever wider scale in engine research in order to reduce costs associated with experimental investigations. In this sense, quasi-dimensional models provide acceptable accuracy with reduced computational efforts. Within this context, the present study puts under scrutiny the assumption of spherical flame propagation and how calibration of a two-zone combustion simulation is affected when changing fuel type. A quasi-dimensional model was calibrated based on measured in-cylinder pressure, and numerical results related to the two-zone volumes were compared to recorded flame imaging.

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).

The recent interest in alternative non-fossil fuels has led researchers to evaluate several alcohol-based formulations. However, one of the main requirements for innovative fuels is to be compatible with existing units’ hardware, so that full replacement or smart flexible-fuel strategies can be smoothly adopted. n-Butanol is considered as a promising candidate to replace commercial gasoline, given its ease of production from bio-mass and its main physical and chemical properties similar to those of Gasoline. The compared behavior of n-butanol and gasoline was analyzed in an optically-accessible DISI engine in a previous paper [1]. CFD simulations explained the main outcomes of the experimental campaign in terms of combustion behavior for two operating conditions. In particular, the first-order role of the slower evaporation rate of n-butanol compared to gasoline was highlighted when the two fuels were operated under the same injection phasing.

The addition of alcohol to conventional hydrocarbon fuels for a spark-ignition engine can increase the fuel octane rating and the power for a given engine displacement and compression ratio. In this work, the influence of butanol addition to gasoline was investigated. The experiments were performed in an optical ported fuel injection single-cylinder SI engine with an external boosting device. The engine was equipped with the head of a commercial SI turbocharged engine having the same geometrical specifications (bore, stroke and compression ratio). The effect of a blend of 20% of n-butanol and 80% of gasoline (BU20) on in-cylinder combustion process was investigated by cycle-resolved visualization. The engine worked at low speed, medium boosting and wide open throttle. Changes in spark timing and fuel injection phasing were considered. Comparisons between the flame luminosity and the combustion pressure data were performed.

In this paper, a high temporal resolution optical technique, based on the multi-wavelength UV-visible-near IR extinction spectroscopy, was applied at the exhaust of an automotive diesel engine to investigate the post-injection strategy impact on the fuel vapor. Experimental investigations were carried out using three fuels: commercial diesel (B5), a blend of 80% diesel with 20% by vol. of gasoline (G20) and a blend of 80% diesel with 20% by vol. of n-butanol (BU20). Experiments were performed at the engine speed of 2500rpm and 0.8MPa of brake mean effective pressure exploring two post-injection timings and two EGR rates. The optical diagnostic allowed evaluating, during the post-injection activation, the evolution of the fuel vapor in the engine exhaust line. The investigation was focused on the impact of post-injection strategy and fuel properties on the aptitude to produce hydrocarbon rich gaseous exhaust for the regeneration of diesel particulate trap (DPF).

In-cylinder optical diagnostic was applied to study butanol-gasoline blend combustion in a SI engine. Spark timing and fuel injection mode were changed to work in normal and knocking conditions. The experiments were realized in a single-cylinder ported fuel injection SI engine with an external boosting device. The engine worked like-stoichiometric mixture at 2000 rpm, medium boosting and wide open throttle. UV-visible natural emission spectroscopy allowed to follow the formation and the evolution of the main compounds and radical species that characterize the combustion process from the spark ignition until the exhaust. Particular interest was devoted to OH and CO₂* evolution, and to the spectral evidence of soot precursors due to fuel deposits burning. OH resulted the best marker for combustion both in normal and abnormal conditions.

The new real driving emission cycles and the growing adoption of turbocharged GDI engines are directing the automotive technology towards the use of innovative solutions aimed at reducing environmental impact and increasing engine efficiency. Water injection is a solution that has received particular attention in recent years, because it allows to achieve fuel savings while meeting the most stringent emissions regulations. Water is able to reduce the temperature of the gases inside the cylinder, coupled with the beneficial effect of preventing knock occurrences. Moreover, water dilutes combustion, and varies the specific heat ratio of the working fluid; this allows the use of higher compression ratios, with more advanced and optimal spark timing, as well as eliminating the need of fuel enrichment at high load. Computational fluid dynamics simulations are a powerful tool to provide more in-depth details on the thermo-fluid dynamics involved in engine operations with water injection.

Alcohols are largely used in spark-ignition (SI) engines as alternative fuels to gasoline. Particularly, the use of butanol meets growing interest due to its properties that are similar to gasoline, if compared with other alcohols. This paper aims to make a comparative analysis on the atomization process of gasoline and n-butanol fuel injected by a multi-hole injector nozzle for spark ignition engines. Phase Doppler Anemometry technique was applied to investigate the behavior of a spray emerging from a six-hole nozzle for direct injection spark ignition engine applications. Commercial gasoline and pure n-butanol were investigated. The fuels were injected at two pressures: namely at 5 and 10 MPa, in a test vessel at quiescent air conditions, ambient temperature and backpressure. Droplets diameter and velocity were estimated along the axis and on the edge direction of a jet through Phase Doppler Anemometry in order to provide useful information on the atomization process.

Crank angle resolved imaging in the UV-visible spectral range was used to investigate flame front characteristics during normal combustion, surface ignition and light knock conditions. ‘Line of sight’ measurements provided information on local wrinkling: the evaluation was based on a statistical approach, with multiple frames taken at the same crank angle during consecutive cycles. This allowed the results during normal combustion to be representative for the specific operational conditions and to a good degree independent from the effects of cyclic variation. Abnormal combustion on the other hand, was investigated on a cycle-to-cycle basis, given the stochastic nature of such phenomena. The experimental trials were performed at fixed engine speed on an optically accessible direct injection spark ignition (DISI) engine equipped with the cylinder head of a four cylinder 16-valves commercial power unit.

The use of alcohols as alternative to gasoline for fuelling spark-ignition (SI) engines is widespread. Growing interest is paid for n-butanol because of its characteristics that are similar to gasoline. If compared with other alcohols, n-butanol has higher energy content and miscibility with gasoline, lower hygroscope and corrosive properties making it an attractive solution for gasoline replacement. Even if several studies have been conduced to characterize the n-butanol combustion within Spark Ignition engines, few data are available on atomization and spray behavior. This paper reports the results of an experimental investigation to characterize the velocity vector field of two fuel-sprays injected by a 6-hole nozzle for Direct Injection Spark Ignition (DISI) engine. 2D Mie-scattering and Particle Image Velocimetry (PIV) measurements were carried out in an optically accessible vessel at ambient temperature and pressure.