Staff Profiles

Modules taught

Currently, Module Leader for:

• Thermo-fluids (MEng/BEng)
• Motorsport Engine Technology (BSc)

Also teaches on:

• Automotive Engines (MEng/BEng)

Dr Bonatesta’s research work has steadily focussed on experimental and numerical modelling activities for combustion optimisation and emissions reduction in both gasoline and diesel engines. His agenda at OBU concentrates mostly on Particulate Matter mechanisms and engine-out emissions in modern GDI engines. He has acquired extensive expertise in reactive flow modelling of engine cycles using both commercial and open-source CFD software.

At OBU, Dr Bonatesta has established a prospering research group, which works in collaboration with Ford Motor Company and CD-Adapco/Siemens for the optimisation of combustion and soot modelling capabilities. Other active research collaborations include the Soot Diagnostics Suite at the University of Nottingham, UK and the Department of Aeronautical and Automotive Engineering at Loughborough University. Since December 2015, he is a Visiting Research Fellow at NUMC (Nottingham University Malaysia Campus).

Dr Bonatesta is also supporting the successful development of pan-disciplinary research activities in the area of sustainable, energy-efficient heating systems for buildings, in collaboration with OISD, the Architectural Engineering Research Group, School of Architecture, OBU. Main focus of interest has been 3D CFD modelling and optimisation of Transpired Solar Collector systems.

Research grants and awards

• APC6 DynAMO – Dynamic Analysis Modelling and Optimisation of GDI engines. R&D project co-sponsored by the Advanced Propulsion Centre – APC6 Call. In collaboration with Ford Motor Company (Project Lead), Loughborough University, Bath University, Siemens CDA, Hartree Centre, Cambustion, DE&TC. Total project value: £22M; Project value for Oxford Brookes: £1.35M.

• Dr Bonatesta has also been the recipient of two Central Research Funds Awards (2013-14 and
  2014-15) and two Research Excellence Awards (2015-16 and 2016-17).

Research projects

• CFD modelling of mixture preparation, combustion and soot formation in GDI Engines, in collaboration with Ford Motor Company.

• Development and optimisation of chemical kinetics sub-models for Particulate Matter mechanisms relevant to partially-premixed spark-ignition gasoline combustion; in collaboration with NUMC (Nottingham University Malaysia Campus) and CFD software developers, Siemens CDA.

• Experimental investigation of PM characterisation and morphology in GDI engines, in collaboration with the Engine Research Group, Nottingham University.

Journal articles

• Bokor C, Rohani B, Humphries C, Morrey D, Bonatesta F, 'Investigating the Impact of Gasoline Composition on PN in GDI Engines Using an Improved Measurement Method'
  International Journal of Engine Research 22 (11) (2020) pp.3391-3406
  ISSN: 1468-0874 eISSN: 2041-3149
  Abstract

  An experimental investigation was carried out to investigate Particulate Number (PN) emissions from a modern, small-capacity Gasoline Direct Injection (GDI) engine. The first part of the study focused on improving measurement repeatability using the Cambustion DMS-500 device. Results showed that sampling near the exhaust valve – while dampening the pressure oscillations in the sampling line – can significantly improve the repeatability. It was also found that uncontrolled phenomena such as deposition in the exhaust system from earlier engine operation can undermine the accuracy of measurements taken at tailpipe level. The second part of the work investigated PN emissions from three types of gasoline fuel, Pump-grade, Performance and Reference. Fuel chemical composition was found to have an appreciable impact on PN, but the magnitude of this effect differs in various operating points, being more pronounced at higher engine load. The Reference fuel was found to have the lowest PN emission tendency, conceivably because of its lower aromatics, olefins and heavy hydrocarbons content. A sweep of operating parameters showed that higher injection pressure reduces PN, but the extent of the reduction depends on fuel physical properties such as volatility.
  

  Website 
• Tan JY, Bonatesta F, Ng HK, Gan S, 'Numerical Investigation of Particulate Matter Processes in Gasoline Direct Injection Engines through Integrated Computational Fluid Dynamics−Chemical Kinetic Modeling'
  Energy and Fuels 34 (4) (2020) pp.4909-4924
  ISSN: 0887-0624 eISSN: 1520-5029
  Abstract

  Despite improvements in thermal efficiency and fuel economy, gasoline direct injection (GDI) engines have been identified as a prominent source of ultrafine particulate matter (PM) in the atmosphere. Adverse impacts caused by PM on the environment and public health motivate the need to deepen the understanding of PM emissions from GDI engines. Hence, an integrated modeling approach is formulated to investigate PM processes in a wall-guided GDI engine by bridging the gap between computational fluid dynamics (CFD) and chemical kinetics. Serving as the gasoline surrogate, a reduced and validated toluene reference mechanism is selected. Spray, turbulence, fuel impingement, liquid film, spark ignition, combustion, and PM emissions are modeled by a complete set of CFD submodels. The dynamic multizone partitioning is introduced within the CFD framework for computational expenditure while soot modeling is addressed through the sectional method. In-cylinder pressures, number density, and mass density of PM are reproduced across engine speeds of 1600–3000 rpm and loads with torques of 60–120 N m. Under a homogeneous stoichiometric mode, dominant formation mechanisms of PM are highlighted as the emergence of fuel-rich regions and the presence of residual liquid fuel droplets at the spark timing. The former is attributed to film stripping and evaporation due to spray-wall interactions while the latter stems from poor droplet vaporization from fuel injected, rebounded, splashed, and/or stripped from the liquid film. Optimized control strategies for GDI engine operations should target to minimize these sources for effective PM abatement.

  Website 
• Tajdaran S, Kendrick C, Hopkins E, Bonatesta F, 'Geometrical optimisation of Transpired Solar Collectors using design of experiments and computational fluid dynamics'
  Solar Energy 197 (2020) pp.527-537
  ISSN: 0038-092X
  Abstract

  Transpired Solar Collectors (TSCs) are simple low maintenance air heating systems which have been widely used for agricultural and industrial applications. In spite of their potential, these systems have not been yet widely employed in residential buildings as they are unable to generate high grade heat for moderate and low ventilation demands. Hence there is an opportunity for optimisation studies in order to enhance the thermal performance of these systems.

  Optimisation and parametric studies can be costly and time consuming if carried out by physical experiments. CFD models however offer a more flexible and less expensive tool to carry out such studies. This research has aimed to optimise the geometry of the solar absorber plate using a validated CFD model which accounts for a wide range of the key factors affecting TSC performance.

  A 2nd order polynomial predictive model was developed based on the CFD results with Root Mean Squared Error (RMSE) of 3.8%. The predictive model was used to identify an optimal geometry which delivers a Heat Exchange Effectiveness (HEE) of 0.739. The optimised geometry demonstrated 43% increase in HEE whilst using 28% less material compared to the baseline geometry under the same operating conditions. This geometry can be integrated with other performance enhancement techniques to further improve the thermal performance of TSCs.

  Website 
• Adil H, Gerguri S, Durodola J, Fellows N, Bonatesta F, Audebert F, 'Comparative Study and Evaluation of Two Different Finite Element Models for Piston Design'
  International Journal of Engineering Research and Applications 9 (3) (2019) pp.23-37
  ISSN: 2248-9622
  Abstract

  The exposure of pistons to extreme mechanical and thermal loads in modern combustion engines has necessitated the use of efficient and detailed analysis methods to facilitate their design. The finite element analysis has become a standard design optimisation tool for this purpose. In literature two different approaches have been suggested for reducing the geometry of the cylinder and crank slider mechanism, to idealise piston finite element analysis load models,whilst trying to maintain realistic boundaries to obtain accurate results. The most widely used geometry is the combination of piston and gudgeon pin while the second geometry includes some portion of the connecting rod’s small end and cylinder in addition to the piston and gudgeon pin.No clear analyses have been made in literature about the relative effectiveness of the two approaches in terms of model accuracy. In this work both approaches have been carried out and analysed with respect to a racing piston. The results suggest that the latter approach is more representative of the load conditions that the piston is subjected to in reality.

  Website 
• Sciortino DD, Bonatesta F, Hopkins E, Yang C, Morrey D, 'A combined experimental and computational fluid dynamics investigation of particulate matter emissions from a wall-guided gasoline direct injection engine'
  Energies 10 (9) (2017)
  ISSN: 1996-1073 eISSN: 1996-1073
  Abstract The latest generation of high-efficiency gasoline direct injection (GDI) engines continues to be a significant source of dangerous ultra-fine particulate matter (PM) emissions. The forthcoming advent in the 2017–2020 timeframe of the real driving emission (RDE) standards affords little time for the identification of viable solutions. The present research work aims to contribute towards a much-needed improved understanding of the process of PM formation in theoretically-homogeneous stoichiometric spark-ignition combustion. Experimental measurements of engine-out PM have been taken from a wall-guided GDI engine operated at part-load; through parallel computational fluid dynamics (CFD) simulations of the test-engine, the process of mixture preparation was investigated. About 80% of the total particle number is emitted on average in the 5–50 nm range, with the vast majority being below the regulated lower limit of 23 nm. The results suggest that both improved charge homogeneity and lower peak combustion temperature contribute to lower particle number density (PNDen) and larger particle size, as engine speed and load increase. The effect of engine load is stronger and results from greater injection pressure through better fuel droplet atomisation. Increases in pre-combustion homogeneity of 6% are associated with one order of magnitude reductions of PNDen. A simplified two-equation functional model was developed, which returns satisfactory qualitative predictions of PNDen as a function of basic engine control variables.Website 
• Tan J Y, Ng H K, Gan S, Bonatesta F, 'Numerical Simulations of Constant-Volume Spray Combustion of n-Heptane with Chemical Kinetics'
  Indian Journal of Science and Technology 10 (7) (2017)
  ISSN: 0974-6846 eISSN: 0974-5645
  Abstract Objectives: A reduced toluene reference fuel (TRF) mechanism of multi-component nature from the literature is utilized to simulate constant-volume spray combustion of n-heptane. The approach allows a preliminary assessment of fuel kinetic model and computational fluid dynamics (CFD) formulations in a simplified computational domain before integrating them in complex engine simulations. Methods: The operating conditions vary in ambient densities between 14.8 kg/m3 and 30 kg/m3 with initial oxygen concentrations ranging from 10% to 21%. The CFD models are first calibrated to replicate spray penetration lengths of the non-reacting condition. The tuned numerical models are then applied to simulate the combustion and soot formation events of reacting sprays. The soot model employed is the multi-step Moss-Brookes model with updated oxidation models. Findings: The relative errors for ignition delay and lift-off length predictions are within 35% and 22% respectively. Furthermore, simulated soot volume fraction contours agree qualitatively with the experimental soot clouds. Computed peak soot locations, however, are found to be further downstream axially as compared to the experimental results across all test cases. Application: Good agreement with experimental spatial soot distributions allows the incorporation of both fuel and soot models in engine configurations.
   
  Website 
• Tan JY, Bonatesta F, Ng HK, Gan S, 'Developments in computational fluid dynamics modelling of gasoline direct injection engine combustion and soot emission with chemical kinetic modelling'
  Applied Thermal Engineering 107 (2016) pp.939-959
  ISSN: 1359-4311
  Abstract Designed to inject gasoline fuel directly into the combustion chamber, gasoline direct injection
  (GDI) combustion systems are gaining popularity among the automotive industry. This is
  because GDI engines offer less pumping and heat losses, enhanced fuel economy and improved transient response. Nonetheless, the technology is often associated with the emission of ultra-fine particulate matter (PM) to the atmosphere. With the increasingly stringent emission regulations, detailed understanding of PM formation within GDI engine configurations is very crucial. To complement the findings based on experimental and optical techniques, computational fluid dynamics (CFD) modeling has been widely utilized to study the in-cylinder physical and chemical events. The success of CFD simulations also requires an accurate representation of gasoline fuel kinetics. Set against the background, the present review reports on the recent developments in chemical kinetic modeling of gasoline fuels and CFD numerical studies for GDI engines emphasizing the combustion and emission stages. Regarding fuel kinetics, the use of primary reference fuel (PRF) and ternary reference fuel (TRF) mechanisms is evaluated. In addition, the current trend portrays a progression towards multi-component surrogate models to account for the complex mixture of practical fuels. It is however observed that many reaction mechanisms proposed in the literature are validated under homogeneous charge compression ignition (HCCI) engine conditions rather than GDI-related ones. CFD modeling of GDI engines typically covers the simulations of spray, mixture formation and combustion processes. Progress in combustion modeling for both homogeneous and stratified charge modes is discussed thoroughly. Still in its infancy, soot modeling studies for GDI engines are reviewed in which several soot models adapted are appraised. The majority of soot models have been previously applied in diesel combustion systems and flame configurations. Significant efforts are currently carried out to improve the model predictions of soot emission from GDI engines.
  Website 
• Tajdaran S, Bonatesta F, Ogden RG, Kendrick CC, 'CFD modeling of transpired solar collectors and characterisation of multi-scale airflow and heat transfer mechanisms'
  Solar Energy 131 (2016) pp.149-164
  ISSN: 0038-092X
  Abstract

  Transpired Solar Collectors (TSCs) are building-integrated air-heating systems that are able to fully or partially meet the heating demands of buildings. They convert solar radiation into warm air that can either be used for ventilation, or to heat thermal storage media. TSCs are becoming an increasingly viable alternative to conventional fossil fuel-based heating systems or, more commonly, can be used in a way that is complementary to these systems such that reliance on fossil fuels is reduced. As a consequence TSCs have a potentially important role in meeting future carbon reduction goals.

  This research has produced a comprehensive numerical model for TSCs based on Computational Fluid Dynamic (CFD) analyses. The model allows parametric studies of key variables and is differentiated from previous models in that it takes full account of factors such as: wind speed and direction, non-uniform flow, turbulent flow, solar radiation intensity, sun position and flow suction rates. It comprises a full size section of cassette-panel TSC that can be easily morphed to reflect a wide range of geometries. A multi-block meshing approach has been employed to reduce grid size and to also resolve jet flows and boundary layers taking place in the plenum and around the absorber plate. Accuracy of the CFD model has been validated against experimental data.

  Modeling demonstrated that factors such as wind angle have unexpectedly significant adverse effects on system thermal performance. The studies also furthered understanding of key performance attributes including the effects of suction ratio in terms of optimising performance, and the relationship between sun angle and system operating temperature  (important for  effective operation of heat storage systems). Consideration of these factors is essential if the future performance of TSCs is to be optimised and the technology developed to its fullest potential.

  Website 
• Tajdaran S, Bonatesta F, Ogden R, Kendrick C, 'Use of CFD Modelling for Transpired Solar Collectors and Associated Characterization of Multi-Scale Airflow and Heat Transfer Mechanisms'
  Energy Procedia 78 (2015) pp.2238-2243
  ISSN: 1876-6102
  Abstract Transpired Solar Collectors (TSCs) are façade-integrated solar air-heating systems which comprise perforated wall-mounted cladding or over-cladding panels. The thermal performance of TSCs can be modeled, however current approaches tend to rely on non-realistic assumptions and simplifications, casting doubts over the resulting accuracy. The aim of this research has been to provide a comprehensive numerical model for TSCs using Computational Fluid Dynamics (CFD) able to take full account of factors such as: solar radiation, wind direction, non-uniform flows (particularly around the perforated plate), and the various types of heat transfer that occur. Many of these are not easily modeled using conventional CFD based approaches used for smaller or more easily predictable technologies.
   
  The model comprises a full size section of a typical TSC that can be easily morphed. A multi-block meshing approach was used to reduce grid size and to capture jet flows taking place in the plenum region through the perforations. When compared to experimental data over a wide range of climatic conditions, the modeled values of outlet temperatures at the absorber plate and plenum demonstrated a high level of accuracy, giving assurance regarding the validity of the approach. To the authors’ best knowledge, the model represents the most comprehensive TSC simulation tool so far developed.
  Website 
• Bonatesta F, Altamore G, Kalsi J, Cary M, 'Fuel economy analysis of part-load variable camshaft timing strategies in two modern small-capacity spark ignition engines'
  Applied Energy 164 (2015) pp.475-491
  ISSN: 0306-2619 eISSN: 1872-9118
  Abstract Variable Camshaft Timing strategies have been investigated at part-load operating conditions in two 3 cylinder, 1.0-litre, Spark Ignition engines. The two small-size engines are different variants of the same 4-valve/cylinder, pent-roof design platform. The first engine is naturally aspirated, port fuel injection and features high nominal compression ratio of 12:1. The second one is the turbo-charged, direct injection version, featuring lower compression ratio of 10:1. The aim of the investigation has been to identify optimal camshaft timing strategies which maximise engine thermal efficiency through improvements in brake specific fuel consumption at fixed engine load. The results of the investigation show that the two engines demonstrate consistent thermal efficiency response to valve timing changes in the low and mid part-load envelope, up to a load of 4 bar BMEP. At the lower engine loads investigated, reduced intake valve opening advance limits
  the hot burned gas internal recirculation, while increasingly retarded exhaust valve opening timing favours engine efficiency through greater effective expansion ratio. At mid load (4 bar BMEP), a degree of intake advance becomes beneficial, owing mostly to the associated intake de-throttling. In the upper part-load domain, for engine load of 5 bar
  BMEP and above, the differences between the two engines determine very different efficiency response to the valve timing setting. The lower compression ratio engine continues to benefit from advanced intake valve timing, with a moderate degree of exhaust timing retard, which minimises the exhaust blow-down losses. The higher compression ratio engine is knock-limited, forcing the valve timing strategy towards regions of lower intake advance and lower hot gas recirculation. The theoretical best valve timing strategy determined peak fuel economy improvements in excess of 8% for the port fuel injection engine; the peak improvement was 5% for the more efficient direct injection engine platform.Website 
• La Rocca A, Bonatesta F, Fay MW, Campanella F, 'Characterisation of soot in oil from a gasoline direct injection engine using transmission electron microscopy'
  Tribology International 86 (June 2015) (2015) pp.77-84
  ISSN: 0301-679X eISSN: 1879-2464
  Abstract In this work, an investigation of soot-in-oil samples drawn from the oil sump of a gasoline direct injection (GDI) engine was carried out. Soot particulate was characterised in terms of size, distribution and shape of the agglomerates, and internal structure of the primary particles. The test engine was a 1.6 l modern light-duty EURO IV engine operated at speed between 1600 and 3700 rev/min, and torque between 30 and 120 Nm. After a double oil-flushing procedure the engine was operated for 30 h. Oil samples were drawn from the sump and prepared for Transmission Electron Microscopy (TEM) and High resolution TEM analysis (HRTEM) by a combination of solvent extraction, centrifugation and diethyl ether bathing. Soot agglomerates were measured in terms of their skeleton length and width, and fractal dimension. The mean skeleton length and width were 153 nm and 59 nm respectively. The fractal dimension was calculated using an iterative method and the mean value was found to be 1.44. The primary particles were found to be spherical in shape with some irregularities and presented an average diameter of 36 nm with a mode of 32 nm and standard deviation of 13 nm. The majority of particles showed an inner core and outer shell similar to diesel soot, although an amorphous layer was also clearly visible.Website 
• Zuber MA, Wan Mahmood WMF, Harun Z, Abidin ZZ, La Rocca A, Shayler P, Bonatesta F, 'Modeling of In-Cylinder Soot Particle Size Evolution and Distribution in a Direct Injection Diesel Engine'
  SAE Technical Papers 2015-01-1075 (2015)
  ISSN: 0148-7191
  Abstract The focus of this study is to analyse changes in soot particle size along the predicted pathlines as they pass through different in-cylinder combustion histories obtained from Kiva-3v CFD simulation with a series of Matlab routines. 3500 locations representing soot particles were selected inside the cylinder at 8° CA ATDC as soot was formed in high concentration at this CA. The dominant soot particle size was recorded within the size range of 20-50 nm at earlier CA and shifted to 10-20 nm after 20° CA ATDC. Soot particle quantities reduce sharply until 20° CA ATDC after which they remain steady at around 1500 particles. Soot particles inside the bowl region tend to stick to the bowl walls and those remaining in the bowl experience an increase in size. Soot particles that move to the upper bowl and squish regions were observed to experience a decrease in size. The decrease in size and number of soot particle was predominantly due to higher rate of soot oxidation compared to surface growth at later crank angle. However, soot particles inside the bowl region experience higher surface growth than oxidation rates hence slightly increase in size.Website 
• Bonatesta F, Chiappetta E, La Rocca A, 'Part-load particulate matter from a GDI engine and the connection with combustion characteristics'
  Applied Energy 124 (July 2014) (2014) pp.366-376
  ISSN: 0306-2619 eISSN: 1872-9118
  Abstract The Gasoline Direct Injection engines are an important source of ultra-fine particulate matter. Significant research effort is still required as improved understanding of soot formation is critical in considering further development or adoption of new technologies. Experimental measurements of engine-out soot emissions have been taken from a modern Euro IV GDI engine at part-load operating conditions. The engine speed and torque were varied in the range 1600–3700 rev/min, and 30–120 Nm, respectively. The engine was invariably operated in stoichiometric and homogeneous combustion mode, with fuel injection early in the intake stroke. The results indicate that for engine load in excess of 3 bar Brake Mean Effective Pressure, due to incomplete gas-phase mixture preparation, a consistent linear correlation establishes between combustion duration and soot particle number. On average, a sixfold increase in number concentration between 1.0 and 6.0 × 106 particle per cc, arises from shortening the rapid duration of 4 crank angle degrees. For engine speed in excess of 3000 rev/min and load in excess of 7 bar BMEP, this correlation appears to be superseded by the effects of spray-to-piston impingement and consequent pool-fire. Three main areas of concern have been identified within the part-load running envelope: (1) the higher load-lower speed range and (2) the mid load-mid speed range, where high nucleation rates induce copious increases of engine-out soot mass; (3) the upper part-load range where, most likely as a result of spray impingement, high levels of soot concentration (up to 10 million particles per cc) are emitted with very small size (23–40 nm).Website 
• Bonatesta F, Waters B, Shayler P, 'Burn angles and form factors for Wiebe function fits to mass fraction burned curves of a spark ignition engine with variable valve timing'
  International Journal of Engine Research 11 (2) (2010) pp.177-186
  ISSN: 1468-0874
  Abstract

  The charge burn characteristics of a port-injected spark ignition engine with a pent-roof combustion chamber and variable valve timing have been investigated experimentally. The engine was run under stoichiometric mixture operating conditions over ranges of intake and exhaust valve timings, engine speed, and engine load. Empirical functions have been developed for the 0-90 per cent mass fraction burned angle and the form factor, which define Wiebe function fits to the mass fraction burned variation in the crank angle domain. The burn angle and form factor have been related to the level of charge dilution by burned gas, engine speed, ignition timing, and charge density at spark timing. The dilution level has the strongest influence on the burn rate and profile. The dilution level varied with intake and exhaust valve timings, external exhaust gas recirculation, and engine load. The results indicate that intake and exhaust valve timings influence combustion primarily by modifying the charge dilution with burned gas and charge density.

  Website 
• Bonatesta F, Shayler P, 'Factors influencing the burn rate characteristics of a spark ignition engine with variable valve timing'
  Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 222 (-) (2008) pp.2147-2158
  ISSN: 0954-4070
  Abstract The charge burn characteristics of a four-cylinder port-fuel-injected spark ignition engine fitted with a dual independent variable-valve-timing system have been investigated experimentally. The influence of valve timings on the flame development angle and the rapid burn angle is primarily associated with valve overlap values and internal gas recirculation. Conditions examined cover light to medium loads and engine speeds up to 3500 r/min. As engine loads and speeds exceeded about 6 bar net indicated mean effective pressure and 3000 r/min respectively, combustion duration was virtually independent of the valve timing setting. At lower speeds and work output conditions, valve timing influenced burn angles through changes in dilution mass fraction, charge density, and charge temperature. Of these, changes in dilution mass fraction had the greatest influence. Increasing the dilution by increasing the valve overlap produced an increase in both burn angles. The effects of mean piston speed and spark timing have also been examined, and empirical expressions for the flame development and the rapid burn angles are presented.Website 

Book chapters

• Bonatesta F, 'Premixed combustion in spark ignition engines and the influence of operating variables' in Advances in Internal Combustion Engines and Fuel Technologies, In-Tech Open Access Publishing (2013)
  ISBN: 978-953-51-1048-4
  Abstract Chapter 1Website 

Conference papers

• Bonatesta F, Hopkins E, Francavilla C, Bell D, La Rocca A, 'Combustion and particulate matter formation in modern GDI engines: a modelling study using CFD'
  (2016)
  eISBN: 978-0-9572076-9-1
  Abstract Modern GDI engines are efficient power platforms, but produce large quantities of ultra-fine soot particles. Fuel mal-distribution and, in some cases, liquid fuel film are commonly addressed as the primary causes of particulate matter formation. Multi-dimensional engine modelling can be used effectively to gain an improved understanding of the in-cylinder processes leading to particulate matter. The work presented here investigates soot mechanisms in a modern wall-guided GDI engine using commercial CFD software Star-CD. Two part-load operating conditions are investigated, 2300 rev/min - 60 Nm, and 2300 rev/min - 120 Nm. The multi-stage semi-empirical Soot Sectional Method is used to simulate the physical and chemical in-cylinder mechanisms leading to soot emissions.
   
  The results of the simulations show better mixture preparation in the high load case, mostly on account of enhanced fuel atomisation and stronger mixing. The lower load case features wider mixture stratification, with a more confined, lower temperature burning zone. In both cases, a strong temperature drop establishes between the hot core and the cylinder walls. Higher levels of oxygen correspond to regions of lower temperature near the walls and vice-versa. This unfavourable arrangement, compounded to the lack of mixture homogeneity, leads to high levels of EVO soot in the lower engine load case.
  Website 
• Bonatesta F, La Rocca S, Hopkins E, Bell D, 'Application of Computational Fluid Dynamics to Explore the Sources of Soot Formation in a Gasoline Direct Injection Engine'
  (2014)
  Abstract Gasoline Direct Injection engines are efficient devices which are rivaling diesel engines with thermal efficiency approaching the 40% threshold at part load. Nevertheless, the GDI engine is an important source of dangerous ultra-fine particulate matter. The long-term sustainability of this technology strongly depends on further improvement of engine design and combustion process.
   
  This work presents the initial development of a full-cycle CFD model of a modern wall-guided GDI engine operated in homogeneous and stoichiometric mode. The investigation was carried out at part-load operating conditions, with early injections during the intake stroke. It included three engine speeds at fixed engine-equivalent load. The spray model was calibrated using test-bed and imaging data from the 7-point high-pressure fuel injectors used in the test engine. Experimental data on combustion were also used for calibration purposes, whereas measurements of engine-out soot number density from a Differential Mobility Spectrometer formed the basis and motive of the investigation.
   
  Following the ECU controller, as the speed is increased at fixed engine load, the fuel injection is advanced to enable longer real-time for fuel-air mixing. In spite of stronger in-cylinder motion, this causes extended liquid spray impingement, potentially leading to the formation of liquid film, a source of soot formation during combustion. At increasing engine speed the mixture appears better prepared at spark timing, and the Air Fuel Ratio approaches correct stoichiometry in the vicinity of spark-plug. While the process of mixing continues after combustion commences, leading to new charge stratification, the higher engine speed case shows greater peak temperature during combustion. These mechanisms are used to explain the increase in soot number density measured at higher engine speed.
  Website 
• Wan Mahmood W, La Rocca A, Shayler P, Bonatesta F, 'Predicted paths of soot particles in the cylinders of a direct injection diesel engine'
  (2012)
  Abstract

  Soot formation and distribution inside the cylinder of a light duty direct injection diesel engine, have been predicted using Kiva-3v CFD software. Pathlines of soot particles traced from specific in-cylinder locations and crank angle instants have been explored using the results for cylinder charge motion predicted by the Kiva-3v code. Pathlines are determined assuming soot particles are massless and follow charge motion. Coagulation and agglomeration have not been taken into account. High rates of soot formation dominate during and just after the injection. Oxidation becomes dominant after the injection has terminated and throughout the power stroke. Computed soot pathlines show that soot particles formed just below the fuel spray axis during the early injection period are more likely to travel to the cylinder wall boundary layer. Soot particles above the fuel spray have lesser tendency to be conveyed to the cylinder wall. The upper part of the cylinder liner seems to be the most vulnerable to soot transfer to the wall layer. This soot is formed at the early crank angles after the start of injection, e.g. 8° ATDC. Soot generated at later crank angles, e.g. 18° ATDC, swirls around the combustion chamber following the piston in the expansion stroke and does not come near the cylinder wall. If nucleation was retarded soot transfer to the cylinder liner boundary layer could be reduced.

  Website 
• Bonatesta F, La Rocca A, Shayler PJ, Wahab E, 'The Influence of Swirl Ratio on Soot Quantity and Distribution in the Cylinder of a Diesel Engine'
  (2007)
  Abstract The effects of swirl ratio on soot production and in-cylinder distribution in a HPCR diesel engine have been investigated using Kiva-3v CFD code. Model validation was based on comparisons with experimental test-bed records, which covered wide ranges of engine speed and load. When variable levels of air-swirl are used in conjunction with large amounts of fuel injected, the in-cylinder fuel distribution and the location of combustion are the most effective parameters. As the increasing level of air-swirl tends to demote combustion, the total amount of soot formed steadily increases, while the oxidation mechanism is firstly promoted and then depressed by swirl ratios greater than 1.8/2.1.

Other publications

• Hopkins E, Bonatesta F, Francavilla C, Bell D, 'Soot Modelling in a Modern GDI Engine'
  (2016)
• Hopkins E, Bonatesta F, Bell D, 'Soot Modelling in Modern GDI Engines: Some Preliminary Results'
  (2016)
• La Rocca A, Bonatesta F, Fay M, Campanella F, 'Nanoparticle characteristics of exhaust and soot-in-oil from gasoline direct injection automotive engines'
  (2014)
• Bonatesta F, La Rocca A, Chiappetta E, 'Exploring the correlation between soot number density and combustion duration in a GDI engine at part-load conditions'
  (2014)