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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

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

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

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.
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.
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

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

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

Improving Acoustic Performance of an Air Filter Box. TL Analysis and Device Optimization

2016-06-15
2016-01-1813
The characteristics of the intake system affect both engine power output and gas-dynamic noise emissions. The latter is particularly true in downsized VVA engines, where a less effective attenuation of the pressure waves is realized, due to the intake line de-throttling at part-load. For this engine architecture, a refined air-box design is hence requested. In this work, the Transmission Loss (TL) of the intake air-box of a commercial VVA engine is numerically computed through a 3D FEM approach. Results are compared with experimental data, showing a very good correlation. The validated model is then coupled to an external optimizer (ModeFRONTIERTM) to increase the TL parameter in a prefixed frequency range. The improvement of the acoustic attenuation is attained through a shape deformation of the inner structure of the base device, taking into account constraints related to the device installation inside the engine bay.
Journal Article

Knock and Cycle by Cycle Analysis of a High Performance V12 Spark Ignition Engine. Part 2: 1D Combustion and Knock Modeling

2015-09-06
2015-24-2393
The results of the experimental analyses, described in Part 1, are here employed to build up an innovative numerical approach for the 1D modeling of combustion, cycle-by-cycle variations and knock of a high performance 12-cylinder spark-ignition engine. The whole engine is schematized in detail in a 1D framework simulation, developed in the GT-Power™ environment. Proper “in-house developed” sub-models are used to describe the combustion process, turbulence phenomenon, cycle-by-cycle variations (CCV) and knock occurrence. In particular, the knock onset is evaluated by a chemical kinetic scheme for a toluene reference fuel, able to detect the presence of auto-ignition reactions in the end-gas zone. In a first stage, the engine model is validated in terms of overall performance parameter and ensemble averaged pressure cycles, for various full and part load operating points and spark timings.
Journal Article

Fuel Economy Improvement and Knock Tendency Reduction of a Downsized Turbocharged Engine at Full Load Operations through a Low-Pressure EGR System

2015-04-14
2015-01-1244
It is well known that the downsizing philosophy allows the improvement of Brake Specific Fuel Consumption (BSFC) at part load operation for spark ignition engines. On the other hand, the BSFC is penalized at high/full load operation because of the knock occurrence and of further limitations on the Turbine Inlet Temperature (TIT). Knock control forces the adoption of a late combustion phasing, causing a deterioration of the thermodynamic efficiency, while TIT control requires enrichment of the Air-to-Fuel (A/F) ratio, with additional BSFC drawbacks. In this work, a promising technique, consisting of the introduction of a low-pressure cooled exhaust gas recirculation (EGR) system, is analyzed by means of a 1D numerical approach with reference to a downsized turbocharged SI engine. Proper “in-house developed” sub-models are used to describe the combustion process, turbulence phenomenon and the knock occurrence.
Journal Article

Experimental Investigation and 1D Simulation of a Turbocharger Compressor Close to Surge Operation

2015-04-14
2015-01-1720
Downsizing is widely considered one of the main path to reduce the fuel consumption of spark ignition internal combustion engines. As known, despite the reduced size, the required torque and power targets can be attained thanks to an adequate boost level provided by a turbocharger. However, some drawbacks usually arise when the engine operates at full load and low speeds. In fact, in the above conditions, the boost pressure and the engine performance is limited since the compressor experiences close-to-surge operation. This occurrence is even greater in case of extremely downsized engines with a reduced number of cylinders and a small intake circuit volume, where the compressor works under strongly unsteady flow conditions and its instantaneous operating point most likely overcomes the steady surge margin. In the paper, both experimental and numerical approaches are followed to describe the unsteady behavior of a small in-series turbocharger compressor.
Technical Paper

The Use of Vibrational Signals for On-Board Knock Diagnostics Supported by In-Cylinder Pressure Analyses

2014-11-11
2014-32-0063
In the present work, an Auto Regressive Moving Average (ARMA) model and a Discrete Wavelet Transform (DWT) are applied on vibrational signals, acquired by an accelerometer placed on the cylinder block of a Spark Ignition (SI) engine, for knock detection purposes. To the aim of tuning such procedures, the same analysis has been carried out by using the traditional MAPO (Maximum Amplitude of Pressure Oscillations) index and an Inverse Kinetic Model (IKM), both applied on the in-cylinder pressure signals. Vibrational and in-cylinder pressure signals have been collected on a four cylinder, four stroke engine, for different engine speeds, load conditions and spark advances. The results of the two vibrational based methods are compared and in depth discussed to the aim of highlighting the pros and cons of each methodology.
Journal Article

A Comparison Between External and Internal Resonators Employment to Reduce the Gas-Dynamic Noise of a SI Engine

2014-10-13
2014-01-2864
This paper reports 1D and 3D CFD analyses aiming to improve the gas-dynamic noise emission of a downsized turbocharged VVA engine through the re-design of the intake air-box device, consisting in the introduction of external or internal resonators. Nowadays, modern spark-ignition (SI) engines show more and more complex architectures that, while improving the brake specific fuel consumption (BSFC), may be responsible for the increased noise radiation at the engine intake mouth. In particular VVA systems allow for the actuation of advanced valve strategies that provide a reduction in the BSFC at part load operations thanks to the intake line de-throttling. In these conditions, due to a less effective attenuation of the pressure waves that travel along the intake system, VVA engines produce higher gas-dynamic noise levels.
Technical Paper

A Knock Model for 1D Simulations Accounting for Cyclic Dispersion Phenomena

2014-10-13
2014-01-2554
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.
Journal Article

Analysis of Knock Tendency in a Small VVA Turbocharged Engine Based on Integrated 1D-3D Simulations and Auto-Regressive Technique

2014-04-01
2014-01-1065
In the present paper, two different methodologies are adopted and critically integrated to analyze the knock behavior of a last generation small size spark ignition (SI) turbocharged VVA engine. Particularly, two full load operating points are selected, exhibiting relevant differences in terms of knock proximity. On one side, a knock investigation is carried out by means of an Auto-Regressive technique (AR model) to process experimental in-cylinder pressure signals. This mathematical procedure is used to estimate the statistical distribution of knocking cycles and provide a validation of the following 1D-3D knock investigations. On the other side, an integrated numerical approach is set up, based on the synergic use of 1D and 3D simulation tools. The 1D engine model is developed within the commercial software GT-Power™. It is used to provide time-varying boundary conditions (BCs) for the 3D code, Star-CD™.
Journal Article

Strategies for Improving Fuel Consumption at Part-Load in a Downsized Turbocharged SI Engine: a Comparative Study

2014-04-01
2014-01-1064
It is commonly recognized that the paths for improving fuel consumption (BSFC) in a spark-ignition engine at part-load require more advanced valve actuation strategies, which largely affect the pumping work. Since several years, many different solutions have been proposed, characterized by different levels of complexity, effectiveness, and cost. Valve systems currently available on the market allow for variable phasing (VVT - Variable Valve Timing), and/or lift (VVA - Variable Valve Actuation). Usually VVT devices are applied on intake and exhaust camshafts, in the “phased” or “unphased” configuration, as well. VVA devices are instead commonly mounted on the intake camshaft. More recent VVA systems also allow for a double intake valve lift during a single engine cycle (multi-lift), or may include a small intake pre-lift during the exhaust stroke. The latter solutions may determine further BSFC reductions. Alternatively, an external-EGR circuit can be considered, as well.
Journal Article

Advanced Numerical/Experimental Methods for the Analysis of a Waste-Gated Turbocharger Turbine

2014-04-01
2014-01-1079
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.
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