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Increasingly stringent pollutant and CO2 emission standards require engine manufacturers to investigate innovative solutions. Among these techniques, low-temperature combustion (LTC) concepts have a large potential to simultaneously reduce NOx emissions and fuel consumption. A promising manner to realize LTC consists of adopting ultra-lean mixtures, where the combustion evolution is controlled by a proper spatial distribution of fuels with different chemical reactivities. In this context, depending on the proportion and stratification of the fuels, the heat release can primarily depend on chemistry progression, leading to a Reactivity Controlled Compression Ignition (RCCI) mode, or on flame propagation, locally initiated by a high reactivity fuel. In this work, the combustion characteristics of a large-bore research engine are experimentally investigated. Natural gas is supplied into the intake port, while light fuel oil (LFO) is directly injected in the cylinder. An experimental campaign is carried out including sweeps of engine load, air/fuel proportion, LFO amount, valve timing, and intake air temperature. Global engine operating parameters as well as cylinder pressure traces are recorded and analyzed. Based on the available experimental data, a phenomenological model handling both chemistries of fuels with different reactivities and flame propagation is developed and validated. The model is based on a multi-zone approach, where auto-ignition chemistry is solved by a tabulated method to preserve the computational effort. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, using a fixed tuning constant set. A dedicated correlation is built to reproduce the expected in-cylinder distribution of the directly injected liquid fuel. Global performance and combustion parameters are predicted with an average error below 5%. The model demonstrates to correctly describe the behavior of the tested engine under different operating conditions and to capture the physics behind such advanced combustion concepts.

Combustion stability, engine efficiency and emissions in a multi-cylinder spark-ignition internal combustion engines can be improved through the advanced control and optimization of individual cylinder operation. In this work, experimental and numerical analyses were carried out on a twin-cylinder turbocharged port fuel injection (PFI) spark-ignition engine to evaluate the influence of cylinder-by-cylinder variation on performance and pollutant emissions. In a first stage, experimental tests are performed on the engine at different speed/load points and exhaust gas recirculation (EGR) rates, covering operating conditions typical of Worldwide harmonized Light-duty vehicles Test Cycle (WLTC). Measurements highlighted relevant differences in combustion evolution between cylinders, mainly due to non-uniform effective in-cylinder air/fuel ratio. Experimental data are utilized to validate a one-dimensional (1D) engine model, enhanced with user-defined sub-models of turbulence, combustion, heat transfer and noxious emissions. The model shows a satisfactory accuracy in reproducing the combustion evolution in each cylinder and the temperature of exhaust gases at turbine inlet. The pollutant species (HC, CO and NOx) predicted by the model show a good agreement with the ones measured at engine exhaust. Furthermore, the impact of cylinder-by-cylinder variation on gaseous emissions is also satisfactorily reproduced. The novel contribution of present work mainly consists in the extended numerical/experimental analysis on the effects of cylinder-by-cylinder variation on performance and emissions of spark-ignition engines. The proposed numerical methodology represents a valuable tool to support the engine design and calibration, with the aim to improve both performance and emissions.

Homogeneous Charge Compression Ignition (HCCI) is a promising advanced combustion mode, featured by both high thermal efficiency and low emissions. In this context, a 0D multi-zone model has been developed, where the thermal stratification in the combustion chamber has been taken into account. The model is based on a control mass Lagrangian multi-zone approach. In addition, a procedure based on a tabulated approach (Tabulated Kinetic of Ignition - TKI) has been developed, to perform an accurate and fast prediction of the air/fuel mixture auto-ignition. This methodology allows combining the accuracy of detailed chemistry with a negligible computational effort. The tabulated procedure has been preliminarily verified through the comparison with the results of a commercial software (GT-Power™). In this assessment, single zone simulations have been performed comparing the TKI strategy to a conventional chemical kinetics one, in four different cases at varying the intake temperature and the equivalence ratio. Then, the proposed 0D multi-zone model has been validated against experimental data available in the literature. The analyses are carried out with reference to an HCCI engine fuelled with pure hydrogen and working in a single operating point, namely 1500 rpm, 2.2 bar IMEP and with a fuel/air equivalence ratio of 0.24. Three different temperatures, i.e., 373, 383, and 393 K, have been considered for the intake air. The experimental/numerical comparisons of pressure cycles and burn rates proved that the proposed numerical approach can reproduce the experiments with good accuracy, without the need for case-by-case tuning.

In recent years, internal combustion engine (ICE) downsizing coupled with turbocharging was considered the most effective path to improve engine efficiency at low load, without penalizing rated power/torque performance at full load. On the other side, issues related to knocking combustion and excessive exhaust gas temperatures obliged adopting countermeasures that highly affect the efficiency, such as fuel enrichment and delayed combustion. Powertrain electrification allows operating the ICE mostly at medium/high loads, shifting design needs and constraints towards targeting high efficiency under those operating conditions. Conversely, engine efficiency at low loads becomes a less important issue. In this track, the aim of this work is the investigation of the potential of the oversizing of a small Variable Valve ActuationSpark Ignition gasoline engine towards efficiency increase and tailpipe emission reduction. To enhance the potential improvements of such an approach, a lean combustion concept is adopted, where the flame speed propagation is supported by doping gasoline with the addition of a percentage of hydrogen (10% by mass). The analysis is carried out by a 1D simulation tool, widely validated for the base engine supplied with pure gasoline and under stoichiometric/rich combustion. The combustion and knock models are here extended to handle the flame speed and auto-ignition characteristics of gasoline/H2 blends. The comparison between the base gasoline engine and the oversized gasoline/H2 variant highlights significant efficiency advantages at full load operations, which are due to the possibility to remove fuel enrichment and combustion delays. Exceptfor unburned hydrocarbons, pollutants and CO2 emissionsare reducedthanks to the synergic effects of H2addition and ultra-lean mixtures. A certain penalization of efficiency arises at very low loads, where engine oversizingdegrades the combustion process.

International audience; Recently, the car manufacturers are moving towards innovative Spark Ignition (SI) engine architectures with unconventional combustion concepts, aiming to comply with the stringent regulation imposed by EU and other legislators. The introduction of burdensome cycles for vehicle homologation, indeed, requires an engine characterized by a high efficiency in the most of its operating conditions, for which a conventional SI engine results to be ineffective. Combustion systems which work with very lean air/fuel mixture have demonstrated to be a promising solution to this concern. Higher specific heat ratio, minor heat losses and increased knock resistance indeed allow improving fuel consumption. Additionally, the lower combustion temperatures enable to reduce NOX production.Since conventional SI engines can work with a limited amount of excess air, alternative solutions are being developed to overcome this constraint and reach the above benefit. Among all these solutions, replacing the spark-plug with a Pre-Chamber (PC) ignition system is gaining increasing interest. For this architecture, the combustion process starts in the PC and propagates in the main-chamber in the form of multiple turbulent jets of hot gas, with high-turbulence level. This ensures stable flame propagation even under extremely lean mixtures.In this research activity, an ultra-lean PC SI engine is numerically and experimentally investigated to assess the potential improvement of the thermal efficiency for ultra-lean operations. To this aim, a research single cylinder engine, fuelled with gasoline, is tested at fixed load and speed, realizing an air / fuel ratio sweep. A 1D/0D model of the examined engine is implemented in a commercial modelling framework (GT-Power™), where “in-house developed” sub-models are embedded, simulating in-cylinder phenomena, such as combustion, turbulence, heat transfer and pollutant emissions.The numerical approach, preliminarily tuned against 3D simulations and experimental outcomes, demonstrated to accurately reproduce the engine behaviour, without requiring any case-dependent tuning of the model constants. Both numerical and experimental results proved that working in ultra-lean condition allows to significantly improve the indicated thermal efficiency, abating the NOx emissions, while penalizing the HC production.

The complexity of modern hybrid powertrains poses new challenges for the optimal control concerning, on one hand, the thermal engine to maximize its efficiency, and, on the other hand, the vehicle to minimize the noxious emissions and CO2. In this context, the engine calibration has to be conducted by considering simultaneously the powertrain management, the vehicle characteristics, and the driving mission. In this work, a calibration methodology for a two-stage boosted ultra-lean pre-chamber spark ignition (SI) engine is proposed, aiming at minimizing its CO2 and pollutant emissions. The engine features a flexible variable valve timing (VVT) control of the valves and an E-compressor, coupled in series to a turbocharger, to guarantee an adequate boost level needed for ultra-lean operation. The engine is simulated in a refined 1D model. A simplified methodology, based on a network of proportional integral derivative (PID) controllers, is presented for the calibration over the whole operating domain. Two calibration variants are proposed and compared, characterized by different fuel and electric consumptions: the first one aims to exclusively maximize the brake thermal efficiency, and the second one additionally considers the electric energy absorbed by the E-compressor and drained from the battery. After a verification against the outcomes of an automatic optimizer, the calibration strategies are assessed based on pollutant and CO2 emissions along representative driving cycles by vehicle simulations. The results highlight slightly lower CO2 emissions with the calibration approach that minimizes the E-compressor consumption, thus revealing the importance of considering the engine calibration phase, the powertrain management, the vehicle characteristics, and its mission.

Cooled exhaust gas recirculation (EGR) is a viable technique to mitigate the knock occurrence, to improve the fuel consumption and to reduce the nitrogen oxides (NOx) emissions of spark-ignition engines. This work aims at investigating the effects of a low-pressure cooled EGR system on the performance and exhaust emissions of a small-size turbocharged SI engine through numerical and experimental analyses. First, the experiments are carried out at a speed of 3000 rpm and different engine loads. The standard engine calibration is applied at the reference test conditions. Then, the EGR system is activated and the load is controlled by adjusting the plenum pressure and the spark timing. The experimental results are used to validate a 1D engine model, developed in GT-Power™ software. The latter is integrated with “user-defined” sub-models for an accurate description of the in-cylinder processes, namely turbulence, combustion and heat transfer. Maximum EGR benefits over fuel consumption are achieved at low load, thanks to the reduction of the pumping losses. At high load, minor fuel consumption improvements are obtained, mainly arising from a slight increased knock resistance. Furthermore, increasing EGR rate results in a sensible NOx reduction at each engine load, with a slight penalty on the unburned hydrocarbon emission.

The description of knock phenomenon is a critical issue in a combustion model for Spark-Ignition (SI) engines. The most known theory to explain this phenomenon is based on the Auto-Ignition (AI) of the end-gas, ahead the flame front. The accurate description of this process requires the handling of various aspects, such as the impact of the fuel composition, the presence of residual gas or water in the burning mixture, the influence of cool flame heat release, etc. This concern can be faced by the solution of proper chemistry schemes for gasoline blends. Whichever is the modeling environment, either 3D or 0D, the on-line solution of a chemical kinetic scheme drastically affects the computational time. In this paper, a procedure for an accurate and fast prediction of the hydrocarbons auto-ignition, applied to phenomenological SI engine combustion models, is proposed. It is based on a tabulated approach, operated on both ignition delay times and reaction rates. This technique, widely used in 3D calculations, is extended to 0D models to overcome the inaccuracies typical of the most common ignition delay approaches, based on the Livengood-Wu integral solution. The aim is to combine the predictability of a detailed chemistry with an acceptable computational effort. First, the tabulated technique is verified through comparisons with a chemical solver for a semi-detailed kinetic scheme in constant-pressure and constant-volume configurations. Then a phenomenological model, based on the end-gas AI computation, is utilized to predict the knock occurrence in different SI engines, including both naturally-aspirated and turbocharged architectures. 0D/1D simulations are performed both with an online solution of the chemistry and employing the tabulated approach. Assessment with reference KLSA values shows that the knock model, based on the tabulated chemistry, is able to well reproduce the essential features of the auto-ignition process in the analyzed engines, with a limited impact on the computational time.

In this work, the effects of alumina nanoparticle contaminated coolant on the performance of a small Spark Ignition engine are investigated by 1D and 3D models. An analysis regarding the alumina nanofluid properties is carried out, and, in particular, a reliable correlation for the thermal conductivity ratio between nanofluid and base coolants is selected. Firstly, a single-cylinder 3D-CFD model of a similar engine cooling system is developed in Star-CCM+ and it is employed to derive the relative increase of the convective in-cylinder heat transfer coefficient. The latter takes into account the geometrical effects of the cooling system, the flow conditions and the nanofluid characteristics. Secondly, a 1D engine model is developed and validated against the available experimental findings with standard coolant fluid. The model is then employed, in a predictive way, to perform full and part load analyses, where the 3D-predicted heat transfer coefficient of the nanofluid contaminated coolant is imposed as an input. The outcomes reveal the potential of the considered nanofluid to achieve fuel consumption improvements (up to about 5.4%) at full load, mainly due to decreased knock tendency and reduced mixture over-fuelling, while minor fuel consumption penalizations (about 1.0 %) are observed at low loads.

Increasingly stringent pollutant and CO2 emission standards require the car manufacturers to investigate innovative solutions to further improve the fuel economy of their fleets. Among these techniques, an extremely lean combustion has a large potential to simultaneously reduce the NOx raw emissions and the fuel consumption of spark-ignition engines. Application of pre-chamber ignition systems is a promising solution to realize a favorable air/fuel mixture ignitability and an adequate combustion speed, even with very lean mixtures. In this work, the combustion characteristics of an active pre-chamber system are experimentally investigated using a single-cylinder research engine. Conventional gasoline fuel is injected into the main chamber, while the pre-chamber is fed with compressed natural gas. In a first stage, an experimental campaign was carried out at various speeds, spark timings and air-fuel ratios. Global engine operating parameters as well as cylinder pressure traces, inside main combustion chamber and pre-chamber, were recorded and analyzed. Based on the available experimental data, a phenomenological model of this unconventional combustion system with divided combustion chambers was developed and validated. The model was then implemented in a 1D code. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, using a fixed tuning constant set. The model demonstrates to correctly describe the behavior of a pre-chamber combustion system under different operating conditions and to capture the physics behind such an innovative combustion system concept.

As known, reliable information about underlying turbulence intensity is a mandatory pre-requisite to predict the burning rate in quasi-dimensional combustion models. Based on 3D results reported in the companion part I paper, a quasi-dimensional turbulence model, embedded under the form of “user routine” in the GT-Power™ software, is here presented in detail. A deep discussion on the model concept is reported, compared to the alternative approaches available in the current literature. The model has the potential to estimate the impact of some geometrical parameters, such as the intake runner orientation, the compression ratio, or the bore-to-stroke ratio, thus opening the possibility to relate the burning rate to the engine architecture. Preliminarily, a well-assessed approach, embedded in GT-Power commercial software v.2016, is utilized to reproduce turbulence characteristics of a VVA engine. This test showed that the model fails to predict tumble intensity for particular valve strategies, such LIVC, thus justifying the need for additional refinements. The model proposed in this work is conceived to solve 3 balance equations, for mean flow kinetic energy, tumble vortex momentum, and turbulent kinetic energy (3-eq. concept). An extended formulation is also proposed, which includes a fourth equation for the dissipation rate, allowing to forecast the integral length scale (4-eq. concept). The impact of the model constants is parametrically analyzed in a first step, and a tuning procedure is advised. Then, a comparison between the 3- and the 4-eq. concepts is performed, highlighting the advantages of the 3-eq. version, in terms of prediction accuracy of turbulence speed-up at the end of the compression stroke. An extensive 3-eq. model validation is then realized according to different valve strategies and engine speeds. The user-model is then utilized to foresee the effects of main geometrical parameters analyzed in part I, namely the intake runner orientation, the compression ratio, and the bore-to-stroke ratio. A two-valve per cylinder engine is also considered. Temporal evolutions of 0D- and 3D-derived mean flow velocity, turbulent intensity, and tumble velocity present very good agreements for each investigated engine geometry and operating condition. The model, particularly, exhibits the capability to accurately predict the tumble trends by varying some geometrical parameter of the engine, which is helpful to estimate the related impact on the burning rate. Summarizing, the developed 0D model well estimates the in-cylinder turbulence characteristics, without requiring any tuning constants adjustment with engine speed and valve strategy. In addition, it demonstrates the capability to properly take into account the intake duct orientation and the compression ratio without tuning adjustments. Some minor tuning variation allows predicting the effects of bore-to-stroke ratio, as well. Finally, the model is verified to furnish good agreements also for a two-valve per cylinder engine, and with reference to two different high-performance engines.

In this work, various techniques are numerically applied to a base engine - vehicle system to estimate their potential CO2 emission reduction. The reference thermal unit is a downsized turbocharged spark-ignition Variable Valve Actuation (VVA) engine, with a Compression Ratio (CR) of 10. In order to improve its fuel consumption, preserving the original full-load torque, various technologies are considered, including an increased CR, an external low-pressure cooled EGR, and a ported Water Injection (WI). The analyses are carried out by a 1D commercial software (GT-Power™), enhanced by refined user-models for the description of in-cylinder processes, namely turbulence, combustion, heat transfer and knock. The latter were validated with reference to the base engine architecture in previous activities. To minimize the Brake Specific Fuel Consumption (BSFC) all over the engine operating plane, the control parameters of the base and modified engines are calibrated based on PID controllers. The calibration procedure is also verified with a direct fuel consumption minimization carried out by an external optimizer. The calibration provides the optimal Spark Advance (SA), Air-to-Fuel (A/F) ratio, Waste-Gate (WG) opening, and VVA setting, complying with limitations on knock intensity, turbine inlet temperature, boost level, turbocharger speed and in-cylinder pressure peak. The performance and calibration maps are computed for various combinations of the above technologies, including a two-stage CR system, and are compared to the ones related to the base architecture. The results show that EGR offers some BSFC benefits at low load, mainly thanks to the pumping work reduction, while it is practically ineffective for knock mitigation at high load. On the contrary, WI has the potential to substantially increase the knock resistance, improving the fuel consumption at high load. No substantial advantages are, indeed, detected with WI under knock-free operation. Computed BSFC maps are then embedded in a vehicle model with the aim of estimating the CO2 emission of a segment A vehicle over a WLTC. The proposed results give a clear outlook of the above technique potentials and offer a guideline to assess the trade-off between engine complexity and improved CO2 emission.

The modern internal combustion engines show complex architectures in order to improve their performance in terms of brake torque and fuel consumption. Among the different solutions, a compression ratio (CR) increase represents a well assessed path to achieve the above result. However, CR has to be limited in order to comply with the mechanical and thermal engine safety and to avoid knocking combustion. In the present work, a 10-cylinder naturally aspirated spark ignition engine is investigated to evaluate the effects of an increased CR on the performance. In a preliminary stage, the engine is experimentally tested under full load operation for a base CR of 12.6. The main performance parameters and the in-cylinder pressure cycles are measured. The engine is schematized in a one-dimensional model (GT-Power™), where “user routines” are implemented to simulate the turbulence, combustion, knock and heat transfer phenomena. The 1D model is validated against experimental data at full load, denoting a good accuracy. The model is then used to estimate the engine performance variations passing from the base CR up to an increased CR value of 13.3. The results underline a reduced improvement of the engine performance for the higher CR configuration, mainly deriving from a higher thermodynamic efficiency. The proposed methodology shows the capability to predict the effects of a partial engine re-design on a completely theoretical basis and presents the potential to be very helpful in reducing the related experimental costs and time-to-market.

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. Experiments were carried out on a downsized port fuel injection (PFI) twin-cylinder turbocharged spark ignition engine equipped with a variable valve actuation (VVA) system over the speed range from 2500 to 4500rpm (step of 500 rpm) under a mediumhigh load condition. The engine was equipped with a prototype low-pressure injection system including two solenoid injectors installed in the runners, upstream of the gasoline ones, able to inject, at phased timing, a controlled amount of water within the intake ports. Experiments were carried out at wide open throttle (WOT) conditions choosing a water to gasoline mass ratio of 0.2, as a result of a previous investigation carried out to optimize the water amount in terms of engine performance. At the different engine speeds and at full gasoline operations, the relative air-fuel ratio (? - measured by a lambda meter located in the exhaust, upstream of the three-way catalyst) was that set by the standard electronic control unit (ECU) map corresponding to the knock limited spark advance. As the WI was activated the amount of fuel was steadily reduced up to reach the stoichiometric condition and a sweep was performed up to the most advanced spark timing without knock occurrence. In-cylinder pressure data, acquired by pressure sensors flush-mounted within the combustion chamber of both cylinders, allowed estimating the main combustion parameters such as the rate of heat release and the combustion phasing. The effect of water injection on turbine inlet temperature and combustion efficiency, estimated from the exhaust gaseous emissions (HC, CO, CO2), are also presented. Further, combustion noise analysis carried out post-processing the in-cylinder pressure signal, decomposed into three sub-signals corresponding to the relevant physical phenomena: pseudo-motored operation (compression-expansion), combustion, and combustion chamber resonance. Such pressure signal decomposition was applied to find cause-effect relationships between the source signal generated by the engine during combustion process and both the objective and subjective parameters of the noise. The results coming from such analysis are discussed to determine the capability of water injection to reduce combustion noise level and improve sound quality index.

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(TM) 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. Finally, the validated model is applied to investigate the full potential of water injection in reducing the knock tendency and improving the fuel economy in a wide load range. The numerical results highlight that WI technique involves significant Brake Specific Fuel Consumption (BSFC) advantages, especially at the medium-high loads. These benefits are limited by the maximum allowable levels for the in-cylinder pressure, while additional advantages are obtained in terms of reduced turbine inlet temperature, turbocharger speed, and boost pressure. The developed numerical procedure, based on validated combustion and knock sub-models, is able to take into account the complex interactions among different parameters, which affect the engine behavior. It is hence believed to realistically forecast the WI-related BSFC advantages and constraints, induced by thermo-mechanical stresses. Simultaneously, it underlines the need of a partial engine redesign to fully exploit WI potential.

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. The model predictions are compared with the experimental data, in terms of in-cylinder pressure cycle, burn rate profile and knock propensity. The numerical model correctly reproduces the experimental findings of fuel consumption, turbine inlet temperature and in-cylinder peak pressure. The combustion process, both with and without water addition, is predicted quite well, except for some inaccuracies in the early stage of combustion. Both experimental and numerical data confirm that the WI technology is able to improve significantly the fuel consumption of the tested engine under high-load knocklimited operation. Main drivers of the above advantages are a reduced over-fueling and a better combustion phasing. In a second stage, the validated model is used to build-up a complete engine operating map aimed at investigating the potential of WI technique to improve the fuel economy of a segment A vehicle. Engine maps with and without WI are introduced in a vehicle model to estimate the grams of CO2 per kilometer over a WLTP driving cycle. A reduced impact of WI is observed, since a knock-free operation occurs along most of the WLTP cycle. Nevertheless, some limited benefits can be still appreciated.

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 (?) 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 parallel, the experimentally tested engine is fully schematized in a 1D framework. The 1D model, developed in the GT-Power(TM) environment, makes use of user defined sub-models for the description of combustion, turbulence and knock phenomena. 1D analyses are carried out for various engine speeds, load levels, ? ratios, and spark timings, without changing any tuning constant. In a first stage, the model is validated in terms of overall engine performance parameters, and ensemble-averaged pressure traces inside the cylinder, and within the intake and exhaust ducts, as well. A more detailed comparison is also performed with reference to the average rate of heat release in different operating conditions. In a subsequent step, the effects of CCVs are introduced in the model in terms of Gaussian distributed modifications of the burning rate, predicted with reference to the ensemble-average operation. Consistently with the experimental data, applied burning rate are modified to simulate a train of pressure cycles statistically equivalent to the measured ones. The influence of the CCVs on the instantaneous peak position, air flow rate, and Indicated Specific Fuel Consumption (ISFC) is consequently investigated on a cycle-bycycle basis, and compared to the average operation. Numerical analyses show that CCVs cause a reduced ISFC penalty, which can be considered significant only in case of delayed combustions and increased CoVs. Knock limited spark advance is also identified with and without CCV, highlighting some additional fuel economy penalties

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. The Coefficient of Variation of the net Indicated Mean Effective Pressure (CoVIMEP) and of in-cylinder peak pressure (CoVp,max) are utilized to quantify the cyclic dispersion and identify its dependency on combustion phasing and duration. Vibrational data are provided by a non-intrusive accelerometer sensor located on the head of cylinder #1. In particular, a proper processing of the accelerometer signal is applied to build correlations able to estimate with a relevant accuracy the cycle-by-cycle scattering of the crank angle position and amplitude of the in-cylinder pressure peak, as well as the related CoV. A maximum in-cylinder pressure error below 2 bar and a maximum crank angle error below 1 degree was obtained in most of data points

Downsizing is widely considered one of the main paths to reduce the fuel consumption of spark ignition internal combustion engines. Some drawbacks usually arise when the engine operates at full load and low speeds, where the compressor experiences close-to-surge operation. In the paper, both experimental and numerical approaches are followed to describe the unsteady behavior of a small in-series turbocharger compressor. The compressor is included in the in-series intake circuit and a pulsating flow is generated by a motor-driven cylinder head fitted with a variable valve actuation system. Different rotational speeds and various valve opening strategies, characterized by different opening durations, are considered. The unsteady compressor behavior is investigated in terms of instantaneous inlet and outlet static pressures, and mass flow rate. The experimental test-rig is schematized in a commercial 1D code. The compressor is described following an enhanced map-based approach, where proper capacities are placed upstream and downstream the compressor to consider the mass and energy storage effects. The model is validated for various rotational speeds and valve lift strategies. After being validated, the methodology is employed to analyze the effect of the equivalent engine speed and valve strategy on the surge resistance.

In this work, experimental data, carried out on a twin-cylinder turbocharged engine at full load operations and referred to a spark advance of borderline knock, are used to characterize the effects of cyclic dispersion on knock phenomena. 200 consecutive in-cylinder pressure signals are processed through a refined Auto-Regressive Moving Average (ARMA) mathematical technique, adopted to define the percentage of knocking cycles, through a prefixed threshold level. The heuristic method used for the threshold selection is then verified by 1D and 3D analyses. In particular, a 1D model, properly accounting for cycle-by-cycle variations, and coupled to a reduced kinetic sub-model, is used to reproduce the measured cycles, in terms of statistical distribution of a theoretical knock index. In addition, few individual cycles, representative of the whole dataset, are selected in a single operating condition in order to perform a more detailed knock analysis by means of a 3D CFD approach, coupled to a tabulated chemistry technique for auto-ignition modeling. Outcomes of 1D and 3D models are compared to the ARMA results and a substantial coherence of the numerical and experimental results is demonstrated. The integrated 1D and 3D analyses can hence help in supporting the choice of the experimental threshold level for knock identification, following a more standardized theoretical approach.

It is widely recognized that air-fuel mixing, combustion and pollutant formation inside internal combustion engines are strongly influenced by the spatial and temporal evolution of both marco- and micro- turbulent scales. Particularly, in spark ignited engines, the generation of a proper level of turbulence intensity for the correct development of the flame front is traditionally based on the onset, during the intake and compression strokes, of a tumbling macro-structure. Recently, in order to both reduce pumping losses due to throttling and develop advanced and flexible engine control strategies, fully variable valve actuation systems have been introduced, capable of simultaneously governing both valve phasing and lift. Despite the relevant advantages in terms of intake system efficiency, this technology introduces uncertainties on the capability of the intake port/valve assembly to generate, at low loads, sufficiently coherent and stable structures, able therefore to promote adequate turbulence levels towards the end of the compression, with relevant effects on the flame front development. It is a common knowledge that 3D-CFD codes are able to describe the evolution of the in-cylinder flow field and of the subsequent combustion process with good accuracy; however, they require too high computational time to analyze the engine performance for the whole operating domain. On the contrary, this task is easily accomplished by 1D codes, where, however, the combustion process is usually derived from experimental measurements of the in-cylinder pressure trace (Wiebe correlation). This approach is poorly predictive for the simulation of operating conditions relevantly different from the experimental ones. To overcome the above described issues, enhanced physical models for the description of in-cylinder turbulence evolution and combustion to be included in a 1D modeling environment are mandatory. In the present paper (part I), a 0D (i.e. homogeneous and isotropic) phenomenological (i.e. sensitive to the variation of operative parameters such as valve phasing, valve lift, intake and exhaust pressure levels, etc.) turbulence model belonging to the K-k model family is presented in detail. The model is validated against in-cylinder results provided by 3D-CFD analyses carried out

As discussed in the part I of this paper, 3D models represent a useful tool for a detailed description of the mean and turbulent flow fields inside the engine cylinder. 3D results are utilized to develop and validate a 0D phenomenological turbulence model, sensitive to the variation of operative parameters such as valve phasing, valve lift, engine speed, etc. In part II of this paper, a 0D phenomenological combustion model is presented, as well. It is based on a fractal description of the flame front and is able to sense each of the fuel properties, the operating conditions (air-to-fuel ratio, spark advance, boost level) and the combustion chamber geometry. In addition, it is capable to properly handle different turbulence levels predicted by means of the turbulence model presented in the part I. The turbulence and combustion models are included, through user routines, in the commercial software GT-Power". With reference to a small twin-cylinder VVA turbocharged engine, the turbulence/combustion model, once properly tuned, is finally used to calculate in-cylinder pressure traces, rate of heat release and overall engine performance at full load operations and brake specific fuel consumption at part load, as well. An excellent agreement between numerical forecasts and experimental evidence is obtained. © 2013 The Authors.

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 work, an experimental activity is carried out to characterize the effects of cyclic dispersion on knock phenomena for different engine speeds, at full load operations and referring to a spark advance of borderline knock. In each case, a train of 200 consecutive in-cylinder pressure traces is processed and the knocking cycles are identified through a standard FFT analysis, compared to an auto-regressive (AR) technique. The latter, proved to be more sensitive, is utilized to define the percentage of knocking cycles occurring in each operating condition, through the assignment of a proper threshold level. Then, a 1D model is set up to reproduce the above experimental pressure traces in terms of average IMEP and cycle-by-cycle variation. A kinetic sub-model is used to compute the heat released in the end-gas zone to be related to the knock occurrence. A new knock index is defined for each simulated cycle and its distribution is compared with the AR model outcomes. The above comparison proves a substantial congruence between the AR model-based knock detection methodology and the numerical one.

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(TM). It is used to provide time-varying boundary conditions (BCs) for the 3D code, Star-CD(TM). Particularly, information between the two simulation tools are at first exchanged under motored conditions to tune an "in-house developed" turbulence sub-model included in the 1D software. 1D results are then validated against the experimental data under fired full load operations, by employing a further "in-house developed" combustion sub-model. BCs are finally passed back to the 3D code to carry out a detailed knock analysis for two full load points, namely 2100 and 4000 rpm. In particular, the knock intensity is predicted, for experimentally actuated and earlier spark advances, and the results are qualitatively compared to the AR model outcomes.

Turbocharging technique is more and more widely employed on compression ignition and spark ignition internal combustion engines, as well, to improve performance and reduce total displacement. Experimental studies, developed on dedicated test facilities, can supply a lot of information to optimize the engine-turbocharger matching, especially if tests can be extended to the typical engine operating conditions (unsteady flow). A specialized components test rig (particularly suited to study automotive turbochargers) has been operating since several years at the University of Genoa. The test facility allows to develop studies under steady or unsteady flow conditions both on single components and subassemblies of engine intake and exhaust circuit. In the paper the results of an experimental campaign developed on a turbocharger waste-gated turbine for gasoline engine application are presented. Preliminarily, the measurement of the turbine steady flow performance map is carried out. In a second step the same component is tested under unsteady flow conditions. Instantaneous inlet and outlet static pressure, mass flow rate and turbocharger rotational speed are measured, together with average inlet and outlet temperatures. A numerical procedure, recently developed at the University of Naples, is then utilized to predict the steady turbine performance map, following a 1D approach. The model geometrically schematizes the component basing on few linear and angular dimensions directly measured on the hardware. Then, the 1D steady flow equations are solved within the stationary and rotating channels constituting the device. All the main flow losses are properly taken into account in the model. The procedure is able to provide the sole “wheel-map” and the overall turbine map. After a tuning, the overall turbine map is compared with the experimental one, showing a very good agreement. Moreover, in order to improve the accuracy of a 1D engine simulation model, the classical map-based approach is suitably corrected with a sequence of pipes that schematizes each component of the device (inlet/outlet ducts, volute and wheel) included upstream and downstream the turbine to account for the wave propagation and accumulation phenomena inside the machine. In this case, the previously computed “wheel-map” is utilized. The turbine pipes dimensions, are automatically provided by the geometrical module of the proposed procedure to correctly reproduce the device volume and the flow path length.

In the paper the results of an experimental campaign regarding the steady characterization of a turbocharger waste-gated turbine (IHI-RHF3) for gasoline engine application are presented. The turbine behavior is analyzed in a specialized test rig operating at the University of Genoa, under different openings of the waste-gate valve. The test facility allows to measure inlet and outlet static pressures, mass flow rate and turbocharger rotational speed. The above data constitute the basis for the tuning and validation of a numerical procedure, recently developed at the University of Naples, following a 1D approach (1D turbine model - 1DTM). The model geometrically schematizes the entire turbine based on few linear and angular dimensions directly measured on the hardware. The 1D steady flow equations are then solved within the stationary and rotating channels constituting the device. All the main flow losses are properly taken into account in the model. A preliminary tuning is carried out based on the characteristic map measured for a completely closed waste-gate valve. A 1D schematization of the experimental test rig is implemented within the commercial GT-Power™ software, including the turbine, the waste-gate and the upstream and downstream circuits. In particular, the turbine model itself consists of a sequence of pipes that schematize each component of the device (inlet/outlet ducts, volute and wheel). The pipes dimensions are automatically provided by the geometrical module of the 1DTM. In the turbine schematization, the “wheel-map”, estimated by means of the above mentioned 1DTM, is utilized. The latter does not take into account phenomena and losses occurring in the volute and in the intake/outlet ducts, as well. A refined modeling of the waste-gate system reveals to be necessary to correctly reproduce the swallowing capacity of the by-pass port. The waste-gate model is tuned against the experimental findings for a completely closed turbine wheel and various settings of the by-pass port. Finally, once the 1D models of the turbine and of the waste-gate system are independently tuned, their behavior under parallel flow is analyzed. The proposed numerical and experimental results demonstrate the mutual interaction of the two components. In addition, it is put into evidence that turbine and waste-gate circuit cannot be considered as two systems working in parallel under the same pressure ratio, as usually assumed in 1D codes.

The present paper reports 1D and 3D CFD analyses of the air-filter box of a turbocharged VVA engine, aiming to predict and improve the gas-dynamic noise emissions through a partial re-design of the device. First of all, the gas-dynamic noise at the intake mouth is measured during a dedicated experimental campaign. The developed 1D and 3D models are then validated at full load operation, based on experimental data. In particular, 1D model provides a preliminary evaluation of the radiated noise and simultaneously gives reliable boundary conditions for the unsteady 3D CFD simulations. The latter indeed allow to better take into account the geometrical details of the air-filter and guarantee a more accurate gas-dynamic noise prediction. 3D CFD analyses put in evidence that sound emission mainly occur within a frequency range of 350 to 450 Hz. Starting from the above result, the original air-box design is modified through the installation of a single Helmholtz resonator, taking into account layout constraints and the influence on engine performance, as well. First, the isolated air-box is studied through a 3D FEM approach to compute the Transmission Loss (TL) parameter both for the original and the modified design. This analysis is employed to verify the noise abatement in the frequency range of interest, for a zero-mean flow. Then, the new intake system configuration is investigated under actual engine operation, through unsteady 3D CFD simulations. The resulting reduction in the gas-dynamic noise is verified both in terms of overall radiated SPL and frequency spectrum at full and part load operating conditions, as well. The proposed integrated 1D-3D approach demonstrated to be a useful tool for the design and the analysis of new geometrical intake system configurations, aiming to improve the engine acoustic performance without significant penalties on the delivered power and fuel consumption.

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. The latter is based on the solution of a kinetic scheme in the end-gas zone, whose thermodynamic conditions are reconstructed by means of a two-zone inverse heat release analysis. Trains of consecutive experimental pressure cycles are acquired on a "downsized" turbocharged SI engine at full load and for different engine speeds. The above data are processed by the two techniques, and knock occurrence and intensity is estimated through suitably defined indices. The presented results demonstrate that the proposed methods give similar, although not coincident, results. While the kinetic procedure is able to furnish a more detailed insight of the thermo-kinetic conditions inside the cylinder, the ARMA technique is indeed characterized by a lower computational effort. In addition it may be applied to vibrational signals acquired by low-cost accelerometers, too. For this reason, it can be more easily implemented within a on board real time control system, aiming to adjust the spark advance and avoid abnormal combustion phenomena.

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. Different settings of the VVA device at part-load are moreover considered to quantify the trade-off existing between fuel consumption and radiated noise reduction. The analyses put into evidence that the optimal control strategy for fuel consumption improvement does not allow a satisfactory abatement of the radiated noise, as well. Presented results show that, at low engine speed, the VVA setting realizing the lowest fuel consumption also produces an acceptable overall noise level. On the contrary, a small fuel consumption penalty must be paid at medium speed to realize a sufficiently low radiated noise. The proposed 1D-3D procedure hence constitutes a very useful tool to realize a fully-numerical pre-calibration of the engine under development.

In the paper a coupled Multi-Body and FEM-BEM methodology used to predict the noise radiated by a turbocharged 4-cylinder diesel engine prototype is described. A Multi-Body Dynamic Simulation (MBDS) of the engine has been carried out, simulating an engine speed sweep from 1500 to 4000 rpm, in order to determine the excitation force of the powertrain, and in particular to estimate the forces acting on the cylinder block. Thanks to the Multi-Body approach, the dynamics of the engine powertrain have been described taking into account both the effects of the burnt gas pressure during the combustion process and the inertia forces of the moving parts. Moreover to assess the real engine operating behaviour, both the crank and the block have been considered as flexible bodies. Afterwards, the cylinder block excitations, in terms of Modal Participation Factors, have been used to evaluate the engine radiated noise with the wellknow MATV methodology, at a distance of one meter from the engine according to the ISO 3744 directives. The dynamics of the engine powertrain and its vibro-acoustic behaviour have been described using LMS Virtual.Lab tools.

The paper presents a combined experimental and numerical investigation of a small unit displacement two-stroke SI engine operated with gasoline and Natural Gas (CNG). A detailed multi-cycle 3D-CFD analysis of the scavenging process is at first performed in order to accurately characterize the engine behavior in terms of scavenging patterns and efficiency. Detailed CFD analyses are used to accurately model the complex set of physical and chemical processes and to properly estimate the fluid-dynamic behavior of the engine, where boundary conditions are provided by a in-house developed 1D model of the whole engine. It is in fact widely recognized that for two-stroke crankcase scavenged, carbureted engines the scavenging patterns (fuel short-circuiting, residual gas distribution, pointwise lambda field, etc.) plays a fundamental role on both of engine performance and tailpipe emissions. In order to assess the accuracy of the adopted numerical approach, comparisons between numerical forecasts and experimental measurements of instantaneous in-cylinder pressure history for steady-state operations of the engine are at first performed and shown in the paper. Subsequently, results from 3D simulations are used to improve the scavenging characterization within the 1D model, where particular emphasis is now devoted to the investigation of the knock occurrence. In order to limit the computational cost of the simulations, the activity is at first carried out within the experimental and 1D modeling frameworks, where a quasi-dimensional combustion and knock model is used. The 1D model is used to compute a numerical knock index which can be useful to address the tuning of the spark advance, given a prescribed and controlled percentage of knock released heat. At the end of the simulation process, the 1D knock index is qualitatively compared to results from full 3D knocking analyses for different in-cylinder compositions and spark timings. The intrinsic knock-resistance of the CNG fuel is finally numerically exploited, through variations of both compression ratio and spark advance.

Despite the recent efforts devoted to develop alternative technologies, it is likely that the internal combustion engine will remain the dominant propulsion system for the next 30 years and beyond. Also as a consequence of more and more stringent emissions regulations established in the main industrialized countries, strongly demanded are methods and technologies able to enhance the internal combustion engines performance in terms of both efficiency and environmental impact. Present work focuses on the development of a numerical method for the optimization of the control strategy of a diesel engine equipped with a high pressure injection system, a variable geometry turbocharger and an EGR circuit. A preliminary experimental analysis is presented to characterize the considered six-cylinder engine under various speeds, loads and EGR ratios. The fuel injection system is separately tested on a dedicated test bench, to determine the instantaneous fuel injection rate for different injection strategies. The collected data are employed for tuning proper numerical models, able to reproduce the engine behaviour in terms of performances (in-cylinder pressure, boost pressure, air-flow rate, fuel consumption), noxious emissions (soot, NO) and radiated noise. In particular, a 1D tool is developed with the aim of characterizing the flow in the intake and exhaust systems and predicting the engine-turbocharger matching conditions, by including a short-route EGR circuit; a 3D model (AVL Fire™) is assessed to reproduce into detail the in-cylinder thermo-fluidynamic processes, including mixture formation, combustion, and main pollutants production; an in-house routine, also validated against available data, is finally developed for the prediction of the combustion noise, starting from in-cylinder pressure cycles. Obviously, data exchange between the codes is previewed. The overall numerical procedure is firstly checked with reference to the experimentally analysed operating points. The 1D, 3D and combustion noise models are then coupled to an external optimizer (ModeFRONTIER™) in order to select the optimal combination of the engine control parameters to improve the engine performance and to contemporary minimize noise, emissions and fuel consumption. Under the hypothesis of a pilot-main injection strategy, a multiobjective optimization problem is solved through the employment of a genetic algorithm. Eight degrees of freedom are defined, namely start of injection, dwell time, energizing time of pilot and main pulses, EGR valve opening, throttle valve opening, swirl level, and turbine opening ratio. It is shown that non-negligible improvements can be gained, also depending on the importance given to the various objectives.

In the present paper a combined procedure for the quasi-dimensional modelling of heat transfer, combustion and knock phenomena in a “downsized” Spark Ignition two-cylinder turbocharged engine is presented. The procedure is extended to also include the effects consequent the Cyclic Variability. Heat transfer is modelled by means of a Finite Elements model. Combustion simulation is based on a fractal description of the flame front area. Cyclic Variability (CV) is characterized through the introduction of a random variation on a number of parameters controlling the rate of heat release (air/fuel ratio, initial flame kernel duration and radius, laminar flame speed, turbulence intensity). The intensity of the random variation is specified in order to realize a Coefficient Of Variation (COV) of the Indicated Mean Effective Pressure (IMEP) similar to the one measured during an experimental campaign. Moreover, the relative importance of the various concurring effects is established on the overall COV. A kinetic scheme is then solved within the unburned gas zone, characterized by different thermodynamic conditions occurring cycle-by-cycle. In this way, an optimal choice of the “knock-limited” spark advance is effected and compared with experimental data. Finally, the CV effects on the occurrence of individual knocking cycles are assessed and discussed.

The one-dimensional (1D) modeling of a turbocharged engine requires the availability of the turbine and compressor characteristic maps. This leads to two main problems: • performance maps of the turbocharger device are usually limited to a reduced number of rotational speeds, pressure ratios and mass flow rates. Extrapolation of maps’ data is commonly required; • performance maps are experimentally derived on stationary test benches, while the turbocharger usually operates under unsteady conditions, when coupled to an internal combustion engine (ICE). To overcome the above problems, in the present paper the flow inside a rotating pipe of a centrifugal compressor is simulated within a 1D modeling approach, with the aim of predicting its characteristic map. The main improvement with respect to the employment of a steady experimental map consists in the absence of data extrapolation and in the possibility of fully characterizing the unsteady operation of the component. In this way it is also possible to handle on a physical point of view surge phenomena (backflows) which may arise particularly at low engine speed and high load. To this aim, the actual evolution of the blade-to-blade duct profile is specified through the assignment of proper data which define the duct orientation in the space along its curvilinear abscissa. Flow equations are next generalized to include additional terms arising as a consequence of pipe rotation. The whole compressor is schematized as a number of rotating channels in parallel, linked at the impeller inlet and outlet by a constant pressure boundary condition. The velocity triangles are employed to handle the transition between the absolute and the relative motion occurring at the impeller inlet and outlet. The presence of a vaneless diffuser is also considered at the exit section of the rotor blades. The procedure includes some correlations to keep into account slip effects, incidence and distributed losses. The above methodology is utilized to directly compute the stationary map of the device, starting from the specification of its geometry and rotating speed. A comparison with experimental data shows a good agreement with the predicted performance curves. The same procedure can be easily extended to the simulation of a radial turbine, too. A one-dimensional model of a centrifugal compressor was described in detail under both isentropic and real flow conditions. Classical correlations for the various flow losses and slip effects were introduced in the model, and their effect on the computed results was discussed with reference to a test compressor. The model was then applied to the simulation of a backswept vanes compressor constituting the turbocharger of a small displacement SI engine. The numerical procedure, after a limited adjustment of few tuning constants, was proven to furnish the steady performance map of the device all over its operating region. Both the pressure ratio and the outlet temperature showed a very good agreement with the test data. The stable operating limit of the compressor was also estimated with good accuracy. The proposed methodology was proven to overcome some of the limitations related to the employment of the steady performance map, to compute the engine-turbocharger matching conditions. The direct calculation of the performance map in fact allows avoiding the need of map’s extrapolation and can be realized with the present model basing on few geometrical data of the device. The intrinsic unsteadiness of the methodology also gives the possibility to fully characterize the transient operation of the component. A first example of the model capabilities in describing the unsteady operation of the compressor was included in the paper. The presented results authorize to further develop the model with the objective to describe the unstable operating regime of the compressor. In this way, it will be also possible to handle on a physical point of view surge phenomena (backflows) which may arise particularly at low engine speed and high load.

The paper presents a 1D-3D numerical model to simulate the scavenging and combustion processes in a small-size spark-ignition two-stroke engine. The engine is crankcase scavenged and can be operated with both gasoline and Natural Gas (NG). The analysis is performed with a modified version of the KIVA3V code, coupled to an in-house developed 1D model. A time-step based, two-way coupled procedure is fully described and validated against a reference test. Then, a 1D-3D simulation of the whole two-stroke engine is carried out in different operating conditions, for both gasoline and NG fuelling. Results are compared with experimental data including instantaneous pressure signals in the crankcase, in the cylinder and in the exhaust pipe. The procedure allows to characterize the scavenging process and quantify the fresh mixture short-circuiting, as well as to analyze the development of the NG combustion process for a diluted mixture, typically occurring in a two-stroke engine. Results in terms of performance and emission characteristics of this engine are presented and discussed.

CFD techniques are more and more utilized in the development of new solutions for performance improvement of internal combustion engines. Three dimensional models, in general, are able to provide detailed and sound information on engine phenomena, but often they are time consuming and hard to be implemented. On the other hand, one-dimensional models can reproduce the entire engine cycle with acceptable computational times; however they need semi-empirical correlations in order to model the flow field details and the burning speed within each cylinder. In this paper, an example of hierarchical structure of 3-D and 1-D models has been proposed. The main performances of a small turbocharged spark-ignition engine have been calculated. Variable-speed and full load operating points have been analyzed. The 3-D model provided the details of the in-cylinder flow field and turbulent indices. These results have been utilized as reference data in tuning a quasi-dimensional turbulent combustion model, included within a 1-D model of a “downsized” turbocharged spark-ignition (SI) engine. Combustion modeling utilizes a deeply validated approach based on a fractal schematization of the flame front surface. Knock occurrence has been predicted by the solution of a kinetic-scheme inside the “end-gas” unburned zone. Once tuned, the one-dimensional code has been finally applied to find the knock-limited spark-advance at wide-open-throttle conditions, for different engine speeds. Numerical results have been compared to the experimentally determined values and a good agreement has been found. This work demonstrates as matching models of different hierarchical levels could be a useful tool in the industrial development of high efficiency engines.

An experimental and numerical work has been performed to characterize the performance of a medium-size spark-ignition engine and the related gas-dynamic noise emitted at the intake mouth. The noise attenuation of the main component of the intake system, namely the air flow box, has been experimentally measured and compared to the numerical results obtained using a tri-dimensional code. Then, the 3D-CFD code has been used to improve the noise attenuation of the above component through the introduction of a Helmholtz and a column resonator along the inflow pipe. Both the base and the modified air box have been coupled to the engine, installed inside a vehicle. An experimental analysis has been carried out to measure the engine performance and the gasdynamic noise at the intake. Some comparisons have been then reported with the numerical results derived from a one-dimensional analysis of the whole engine. Finally, a time-step based, fully integrated 1D-3D analysis has been carried out to improve the accuracy of the numerical results.

Numerical simulations of reactive flows are among the most computational demanding applications in the scientific computing world. KIVA-3V, a widely used computer program for CFD, specifically tailored to engine applications, had been deeply modified in order to improve accuracy and stability, while reducing computational time. The original methods included in KIVA to solve equations of fluid dynamics had been fully replaced by new solvers, with the aim of both improving performance and writing a fully parallel code. Almost every feature of original KIVA-3V has been partially or entirely rewritten, a full 1D code has been included and a strategy to link directly 3D zones with zero dimensional models has been developed. The result is a reliable program, noticeably faster than the original KIVA-3V in serial mode and obviously even more in parallel, capable of treating more complex cases and bigger grids, with the desired level of details where required.