Fuel economy analysis of part-load variable camshaft timing strategies in two modern small-capacity spark ignition engines

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

Part-load particulate matter from a GDI engine and the connection with combustion characteristics

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)

CFD modeling of transpired solar collectors and characterisation of multi-scale airflow and heat transfer mechanisms

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

Use of CFD Modelling for Transpired Solar Collectors and Associated Characterization of Multi-Scale Airflow and Heat Transfer Mechanisms

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

Developments in computational fluid dynamics modelling of gasoline direct injection engine combustion and soot emission with chemical kinetic modelling

Designed to inject gasoline fuel directly into the combustion chamber, gasoline direct injection (GDI) combustion systems are gaining popularity within 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 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) modelling 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 this background, the present review reports on the recent developments in chemical kinetic modelling 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 toluene 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 modelling of GDI engines typically covers the simulations of spray, mixture formation and combustion processes. Progress in combustion modelling for both homogeneous and stratified charge modes is discussed thoroughly. Still in its infancy, soot modelling 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 being carried out to improve the model predictions of soot emission from GDI engines

Numerical Simulations of Constant-Volume Spray Combustion of n-Heptane with Chemical Kinetics

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

Characterisation of soot in oil from a gasoline direct injection engine using Transmission Electron Microscopy

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 1.6l modern light-duty EURO IV engine operated at speed between 1600 and 3700rev/min, and torque between 30and120Nm.After a double oil-flushing procedure the engine was operated for 30h. 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 153nm and 59nm 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 36nm with a mode of 32nm and standard deviation of 13nm.The majority of particles showed an inner core and outer shell similar to diesel soot, although an amorphous layer was also clearly visible

Lipid Metabolism and Molecular Changes in Normal andAtherosclerotic Vessels

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Combustion and particulate matter formation in modern GDI engines: a modelling study using CFD

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