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Technical Paper

Toward Predictive Combustion Modeling of CNG SI Engines in 1D Simulation Tools

2020-09-15
2020-01-2079
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 fuel 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.
Technical Paper

Efficient Thermal Electric Skipping Strategy Applied to the Control of Series/Parallel Hybrid Powertrain

2020-04-14
2020-01-1193
The optimal control of hybrid powertrains represents one of the most challenging tasks for the compliance with the legislation concerning CO2 and pollutant emission of vehicles. Most common off-line optimization strategies (Pontryagin minimum principle - PMP - or dynamic programming) allow to identify the optimal control along a predefined driving mission at the expense of a quite relevant computational effort. On-line strategies, suitable for on-vehicle implementation, involve a certain performance degradation depending on their degree of simplification and computational effort. In this work, a simplified control strategy is presented, where the conventional power-split logics, typical of the above-mentioned strategies, is here replaced with an alternative utilization of the thermal and electric units for the vehicle driving (Efficient Thermal Electric Skipping Strategy - ETESS).
Technical Paper

Experimental and 1D Numerical Investigations on the Exhaust Emissions of a Small Spark Ignition Engine Considering the Cylinder-by-Cylinder Variability

2020-04-14
2020-01-0578
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.
Technical Paper

Performance and Emissions of an Advanced Multi-Cylinder SI Engine Operating in Ultra-Lean Conditions

2019-09-09
2019-24-0075
In this work the performance and noxious emissions of a prototype Spark Ignition (SI) engine, working in ultra-lean conditions, are investigated. It is a four-cylinder engine, having a very high compression ratio, and an active pre-chamber. The required amount of air is provided by a low-pressure variable geometry turbocharger, coupled to a high-pressure E-compressor. The engine is equipped with a variable valve timing device on the intake camshaft. The goal of this activity is to support the development and the calibration of the described engine, and to exploit the full potential of the ultra-lean concept. To this aim, a combustion model for a pre-chamber engine, set up and validated in a previous paper for a similar single-cylinder unit, is utilized. It is coupled to additional in-house developed sub-models, employed for the prediction of the in-cylinder turbulence, heat transfer, knock and pollutant emissions.
Technical Paper

Impact of Cooled EGR on Performance and Emissions of a Turbocharged Spark-Ignition Engine under Low-Full Load Conditions

2019-09-09
2019-24-0021
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.
Technical Paper

A Tabulated-Chemistry Approach Applied to a Quasi-Dimensional Combustion Model for a Fast and Accurate Knock Prediction in Spark-Ignition Engines

2019-04-02
2019-01-0471
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.
Technical Paper

A Quasi-Dimensional Model of Pre-Chamber Spark-Ignition Engines

2019-04-02
2019-01-0470
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.
Technical Paper

Techniques for CO2 Emission Reduction over a WLTC. A Numerical Comparison of Increased Compression Ratio, Cooled EGR and Water Injection

2018-05-30
2018-37-0008
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.
Journal Article

Combined Effects of Valve Strategies, Compression Ratio, Water Injection, and Cooled EGR on the Fuel Consumption of a Small Turbocharged VVA Spark-Ignition Engine

2018-04-03
2018-01-0854
In this work, various techniques are numerically investigated to assess and quantify their relative effectiveness in reducing the Brake Specific Fuel Consumption (BSFC) of a downsized turbocharged spark-ignition Variable Valve Actuation (VVA) engine. The analyzed solutions include the Variable Compression Ratio (VCR), the port Water Injection (WI), and the external cooled Exhaust Gas Recirculation (EGR). The numerical analysis is developed in a 1D modeling framework. The engine is schematized in GT-Power™ environment, employing refined sub-models of the in-cylinder processes, such as the turbulence, combustion, knock, and heat transfer. The combustion and knock models have been extensively validated in previous papers, at different speed/load points and intake valve strategies, including operations with a relevant internal EGR rate and with liquid WI.
Technical Paper

Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part I: 3D Analyses

2018-04-03
2018-01-0850
Recently, a growing interest in the development of more accurate phenomenological turbulence models is observed, since this is a key pre-requisite to properly describe the burn rate in quasi-dimensional combustion models. The latter are increasingly utilized to predict engine performance in very different operating conditions, also including unconventional valve control strategies, such as EIVC or LIVC. Therefore, a reliable phenomenological turbulence model should be able to physically relate the actuated valve strategy to turbulence level during the engine cycle, with particular care in the angular phase when the combustion takes place.
Technical Paper

Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part II: Model Concept, Validation and Discussion

2018-04-03
2018-01-0856
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.
Technical Paper

Impact of Ethanol-Gasoline Port Injected on Performance and Exhaust Emissions of a Turbocharged SI Engine

2018-04-03
2018-01-0914
This paper presents results of an experimental investigation on a flexible port dual fuel injection using different ethanol to gasoline mass fractions. A four stroke, two cylinder turbocharged SI engine was used for the experiments. The engine speed was set at 3000 rpm, tests were carried out at medium-high load and two air-fuel-ratio. The initial reference conditions were set running the engine, fueled with full gasoline at the KLSA boundary, in accordance with the standard ECU engine map. This engine point was representative of a rich mixture (λ=0.9) in order to control the knock and the temperature at turbine inlet. The investigated fuels included different ethanol-gasoline mass fractions (E10, E20, E30 and E85), supplied by dual injection within the intake manifold. A spark timing sweep, both at stoichiometric and lean (λ=1.1) conditions, up to the most advanced one without knock was carried out.
Technical Paper

A Comparison Between Two Phenomenological Combustion Models Applied to Different SI Engines

2017-10-08
2017-01-2184
Nowadays, the development of a new engine is becoming more and more complex due to conflicting factors regarding technical, environmental and economic issues. The experimental activity has to comply with the above complexities, resulting in increasing cost and duration of engine development. For this reason, the simulation is becoming even more prominent, thanks to its lower financial burden, together with the need of an improved predictive capability. Among the other numerical approaches, the 1D models represent a proper compromise between reliability and computational effort, especially if the engine behavior has to be investigated over a number of operating conditions. The combustion model has a key role in this contest and the research of consistent approaches is still on going. In this paper, two well-assessed combustion models for Spark Ignition (SI) engines are described and compared: the eddy burn-up theory and the fractal approach.
Journal Article

Water Injection: a Technology to Improve Performance and Emissions of Downsized Turbocharged Spark Ignited Engines

2017-09-04
2017-24-0062
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.
Technical Paper

Numerical Study of the Potential of a Variable Compression Ratio Concept Applied to a Downsized Turbocharged VVA Spark Ignition Engine

2017-09-04
2017-24-0015
Nowadays different technical solutions have been proposed to improve the performance of internal combustion engines, especially in terms of Brake Specific Fuel Consumption (BSFC). Its reduction of course contributes to comply with the CO2 emissions legislation for vehicle homologation. Concerning the spark ignition engines, the downsizing coupled to turbocharging demonstrated a proper effectiveness to improve the BSFC at part load. On the other hand, at high load, the above solution highly penalizes the fuel consumption mainly because of knock onset, that obliges to degrade the combustion phasing and/or enrich the air/fuel mixture. A promising technique to cope with the above drawbacks consists in the Variable Compression Ratio (VCR) concept. An optimal Compression Ratio (CR) selection, in fact, allows for further improvements of the thermodynamic efficiency at part load, while at high load, it permits to mitigate knock propensity, resulting in more optimized combustions.
Journal Article

Extension and Validation of a 1D Model Applied to the Analysis of a Water Injected Turbocharged Spark Ignited Engine at High Loads and over a WLTP Driving Cycle

2017-09-04
2017-24-0014
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.
Journal Article

Experimental and Numerical Study of the Water Injection to Improve the Fuel Economy of a Small Size Turbocharged SI Engine

2017-03-28
2017-01-0540
In this work, a promising technique, consisting of a liquid Water Injection (WI) at the intake ports, is investigated to overcome over-fueling and delayed combustions typical of downsized boosted engines, operating at high loads. In a first stage, experimental tests are carried out in a spark-ignition twin-cylinder turbocharged engine at a fixed rotational speed and medium-high loads. In particular, a spark timing and a water-to-fuel ratio sweep are both specified, to analyze the WI capability in increasing the knock-limited spark advance. In a second stage, the considered engine is schematized in a 1D framework. The model, developed in the GT-Power™ environment, includes user defined procedures for the description of combustion and knock phenomena. Computed results are compared with collected data for all the considered operating conditions, in terms of average performance parameters, in-cylinder pressure cycles, burn rate profiles, and knock propensity, as well.
Journal Article

Water Injection to Enhance Performance and Emissions of a Turbocharged Gasoline Engine under High Load Condition

2017-03-28
2017-01-0660
The potential benefits of water injection on performance and emissions were investigated on a downsized PFI twin-cylinder turbocharged spark ignition engine. Experiments were carried out at high load condition (~15.5 bar IMEP) within the engine speed range from 3500 to 4500rpm with a step of 500 rpm. For each test case the effect of the injected water quantity on combustion and exhaust emissions was investigated by sweeping from 10%w to 30%w the water to gasoline ratio. The water was injected at the same timing as the gasoline by a low pressure injection system external controlled. Tests were performed at WOT conditions exploring, for each operating condition, a spark sweep from knock-free up to knock-limited operation. Compared to the full gasoline reference case, the water injection allowed to advance extensively the spark timing without knock occurrence. The 20% water to gasoline mass fraction gave the best improvements in terms of IMEP.
Journal Article

A Modeling Study of Cyclic Dispersion Impact on Fuel Economy for a Small Size Turbocharged SI Engine

2016-10-17
2016-01-2230
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.
Technical Paper

A Non-Linear Regression Technique to Estimate from Vibrational Engine Data the Instantaneous In-Cylinder Pressure Peak and Related Angular Position

2016-10-17
2016-01-2178
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.
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