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This paper reports 1D and 3D CFD analyses aiming to improve the gas-dynamic noise emission of a downsized turbocharged VVA engine through the re-design of the intake air-box device, consisting in the introduction of external or internal resonators. Nowadays, modern spark-ignition (SI) engines show more and more complex architectures that, while improving the brake specific fuel consumption (BSFC), may be responsible for the increased noise radiation at the engine intake mouth. In particular VVA systems allow for the actuation of advanced valve strategies that provide a reduction in the BSFC at part load operations thanks to the intake line de-throttling. In these conditions, due to a less effective attenuation of the pressure waves that travel along the intake system, VVA engines produce higher gas-dynamic noise levels.

Control of knock phenomenon is becoming more and more important in modern SI engine, due to the tendency to develop high boosted turbocharged engines (downsizing). To this aim, improved modeling and experimental techniques are required to precisely define the maximum allowable spark advance. On the experimental side, the knock limit is identified based on some indices derived by the analysis of the in-cylinder pressure traces or of the cylinder block vibrations. The threshold levels of the knock indices are usually defined following an heuristic approach. On the modeling side, in the 1D codes, the knock is usually described by simple correlation of the auto-ignition time of the unburned gas zone within the cylinders. In addition, the latter methodology commonly refers to ensemble-averaged pressure cycles and, for this reason, does not take into account the cycle-by-cycle variations.

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.

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.

In the recent years, the interest in heavy-duty engines fueled with Compressed Natural Gas (CNG) is increasing due to the necessity to comply with the stringent CO2 limitation imposed by national and international regulations. Indeed, the reduced number of carbon atoms of the NG molecule allows to reduce the CO2 emissions compared to a conventional fuel. The possibility to produce synthetic methane from renewable energy sources, or bio-methane from agricultural biomass and/or animal waste, contributes to support the switch from conventional liquid fuels to CNG. To drive the engine development and reduce the time-to-market, the employment of numerical analysis is mandatory. This requires a continuous improvement of the simulation models toward real predictive analyses able to reduce the experimental R&D efforts. In this framework, 1D numerical codes are fundamental tools for system design, energy management optimization, and so on.

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.

In the present paper, two different methodologies are adopted and critically integrated to analyze the knock behavior of a last generation small size spark ignition (SI) turbocharged VVA engine. Particularly, two full load operating points are selected, exhibiting relevant differences in terms of knock proximity. On one side, a knock investigation is carried out by means of an Auto-Regressive technique (AR model) to process experimental in-cylinder pressure signals. This mathematical procedure is used to estimate the statistical distribution of knocking cycles and provide a validation of the following 1D-3D knock investigations. On the other side, an integrated numerical approach is set up, based on the synergic use of 1D and 3D simulation tools. The 1D engine model is developed within the commercial software GT-Power™. It is used to provide time-varying boundary conditions (BCs) for the 3D code, Star-CD™.

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.

Modern VVA systems offer new potentialities in improving fuel consumption for spark-ignition engines at low and medium load, meanwhile they grant a higher volumetric efficiency and performance at high load. Recently introduced systems enhance this concept through the possibility of modifying the intake valve opening, closing and lift, leading to the development of almost ‘throttle-less’ engines. However, at low loads, the absence of throttling, while improving the fuel consumption, also produces an increased gas-dynamic noise at the intake mouth. Wave propagation inside the intake system is in fact no longer absorbed by the throttle valve and directly impact the radiated noise. In the paper, 1D and 3D simulations of the gas-dynamic noise radiated by a production VVA engine are performed at full load and in two part-load conditions. Both models are firstly validated at full load, through comparisons with experimental data.

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.

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.

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.

The technique of liquid Water Injection (WI) at the intake port of downsized boosted SI engines is a promising solution to improve the knock resistance at high loads. In this work, an existing 1D engine model has been extended to improve its ability to simulate the effects of the water injection on the flame propagation speed and knock onset. The new features of the 1D model include an improved treatment of the heat subtracted by the water evaporation, a newly developed correlation for the laminar flame speed, explicitly considering the amount of water in the unburned mixture, and a more detailed kinetic mechanism to predict the auto-ignition characteristics of fuel/air/water mixture. The extended 1D model is validated against experimental data collected at different engine speeds and loads, including knock-limited operation, for a twin-cylinder turbocharged SI engine.

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.

Modern VVA systems offer new potentialities in improving the fuel consumption for spark-ignition engines at low and medium load, meanwhile they grant a higher volumetric efficiency and performance at high load. Recently introduced systems enhance this concept through the possibility of concurrently modifying the intake valve opening, closing and lift leading to the development of almost "throttle-less" engines. However, at very low loads, the control of the air-flow motion and the turbulence intensity inside the cylinder may require to select a proper combination of the butterfly throttling and the intake valve control, to get the highest BSFC (Brake Specific Fuel Consumption) reduction. Moreover, a low throttling, while improving the fuel consumption, may also produce an increased gas-dynamic noise at the intake mouth. In highly "downsized" engines, the intake valve control is also linked to the turbocharger operating point, which may be changed by acting on the waste-gate valve.

The stringent worldwide exhaust emission legislations for CO2 and pollutants require significant efforts to increase both the combustion efficiency and the emission quality of internal combustion engines. With this aim, several solutions are continuously developed to improve the combustion efficiency of spark ignition engines. Among the various solutions, EGR represents a well-established technology to improve the gasoline engine performance and the nitrogen-oxides emissions. This work presents the results of an experimental investigation on the effects of the EGR technique on combustion evolution, knock tendency, performance and emissions of a small-size turbocharged PFI SI engine, equipped with an external cooled EGR system. Measurements are carried out at different engine speeds, on a wide range of loads and EGR levels. The standard engine calibration is applied at the reference test conditions.

The characteristics of the intake system affect both engine power output and gas-dynamic noise emissions. The latter is particularly true in downsized VVA engines, where a less effective attenuation of the pressure waves is realized, due to the intake line de-throttling at part-load. For this engine architecture, a refined air-box design is hence requested. In this work, the Transmission Loss (TL) of the intake air-box of a commercial VVA engine is numerically computed through a 3D FEM approach. Results are compared with experimental data, showing a very good correlation. The validated model is then coupled to an external optimizer (ModeFRONTIERTM) to increase the TL parameter in a prefixed frequency range. The improvement of the acoustic attenuation is attained through a shape deformation of the inner structure of the base device, taking into account constraints related to the device installation inside the engine bay.

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.

During the last years, a number of techniques aimed at the experimental identification of the knocking onset in Spark-Ignition (SI) Internal Combustion Engines have been proposed. Besides the traditional procedures based on the processing of in-cylinder pressure data in the frequency domain, in the present paper two innovative methods are developed and compared. The first one is based on the use of statistical analysis by applying an Auto Regressive Moving Average (ARMA) technique, coupled to a prediction algorithm. It is shown that such parametric model, applied to the instantaneous in-cylinder pressure measurements, is highly sensitive to knock occurrence and is able to identify soft or heavy knock presence in different engine operating conditions. An alternative, more expensive procedure is developed and compared to the previous one.

Optical diagnostic was applied to undiluted engine exhaust to supply a low cost and real time evaluation of the oil dilution tendency of selected fuels. Specifically, UV-visible-near IR extinction spectroscopy was applied in the exhaust line of a Euro 5 turbocharged, water cooled, DI diesel engine, equipped with a common rail injection system. The engine was fuelled with commercial B5 fuel and a B30 v/v blend of RME and ultra low sulfur diesel. The proposed experimental methodology allowed to identify the contribution to the multi-wavelength extinction of soot, fuel vapor, hydrocarbons and nitrogen oxide. Further, the evolution of each species for different post-injection interval settings was followed. On-line optical results were correlated with off-line liquid fuel absorption values. Moreover, spectroscopic measurements were linked to in-cylinder pressure related data and with HC and smoke exhaust emissions.