Extending the planar Particle Image Velocimetry (PIV) technique to enable measurements on multiple planes simultaneously allows for some of the 3 dimensional nature of unsteady flow fields to be investigated. This requires less hardware and retains the typically higher spatial resolution of planar PIV compared to fully 3-dimensional PIV techniques. Performing multi-plane PIV measurements requires the light scattered from the different measurement planes to be distinguishable. This may be achieved by using different laser wavelengths which adds significantly to the expense and complexity of the system, by using different light sheet polarisations which is challenging for engine measurements through windows due to stress-induced birefringence, or by making alternating measurements of each plane which sacrifices the simultaneity of the flow measurement across multiple planes. In this work, simultaneous PIV measurements on two parallel offset planes are performed by modifying a standard two-dimensional two-component (2D2C) PIV system consisting of a single high-speed camera and a dual-cavity PIV laser. Image splitting optics with a variable optical path length were used to image two measurement planes onto the single camera sensor. The degree of defocus of the droplet images arising from the narrow depth of field was used to distinguish between the measurement planes. The robustness of the degree of defocus method for plane discrimination was evaluated using both representative test targets and experimentally obtained PIV images of the in-cylinder flow field of a spark ignition internal combustion engine (ICE). This depth of field based method enables simultaneous multi-plane PIV measurements of in-cylinder flows to be performed with only minor alterations to a standard 2D2C PIV system using off-the-shelf optics.

This paper presents a method for simultaneous particle image velocimetry (PIV) on parallel planes offset in depth. The method places images from two planes onto a different half of a camera sensor by using image splitting optics with variable optical path lengths. A shallow depth of field is achieved to ensure only one plane is in focus on each half of the sensor. Without needing additional lasers, the method is designed as an inexpensive means to increase the number of measurement plane(s) of single/multi-plane PIV setups and can be combined with existing plane discrimination approaches such as polarization and wavelength. The method is useful for studying instantaneous flow correlations on different planes while retaining high in-plane spatial resolution of typical planar PIV measurement. The measurement uncertainty caused by crosstalk from out-of-focus images is discussed. Experimental results from a laminar flow rig test indicate that the average measurement error of each velocity component is lower than 0.1 pixels per time step, with a 20 mm plane separation in depth and a 35 × 54 mm2 field of view. As an application with varying background scatter and out-of-plane flow motions, in-cylinder flow measurements in an optically accessible internal combustion engine were performed on two swirl planes simultaneously. Characteristics of the proposed method performing stereoscopic PIV measurements will be studied in future.

Computational fluid dynamics (CFD) simulations of the in-cylinder flow field are widely used in the design of internal combustion engines (ICEs) and must be validated against experimental measurements to enable a robust predictive capability. Such validation is complicated by the presence of both large-scale cycle-to-cycle variations and small-scale turbulent fluctuations in experimental measurements of in-cylinder flow fields. Reynolds averaged Navier-Stokes (RANS) simulations provide overall flow structures with acceptable accuracy and affordable computational cost for widespread industrial applications. Due to the nature of averaging physical parameters in RANS, its validation against experimental results obtained by particle image velocimetry (PIV) requires consideration of how best to average or filter the measured turbulent flows. In this paper, PIV measurements on the cross-tumble plane were recorded every five crank angle degrees for 300 cycles during the intake process of a motored, optically accessible spark ignition direct injection (SIDI) engine. Several methods including ensemble averaging, speed-based averaging and low-order proper orthogonal decomposition (POD) reconstruction were applied to remove the fluctuations from experimental PIV vector fields and thus enable comparison to RANS simulations. Quantitative comparison metrics were used to evaluate the performances of each method in representing the intake jet. Recommendations are made on how to provide a fair validation between measured data and simulation results in highly fluctuating flow fields such as the engine intake jet.

Variations in the in-cylinder flow before and during combustion influence the propagation of the flame in spark ignition direct injection (SIDI) engines, which leads to variations in combustion between engine cycles. Accommodating such cycle-to-cycle variation in production engines requires design compromises that may reduce efficiency. The in-cylinder flow and flame evolve rapidly during combustion, and due to the existence of cyclic variation, the flame propagation in individual cycles cannot be replicated by taking measurements at different timings over different cycles, hence crank-angle-resolved measurements in the same cycle are needed. However, such high-speed flow and flame measurements usually require combining multiple laser-based diagnostic techniques, which can be costly due to the need for multiple lasers with high repetition rates at different wavelengths. In this work, the crank-angle-resolved flow velocity and the flame propagation on a swirl plane in an optically accessible SIDI engine were simultaneously measured by a single laser-based diagnostic technique — high-speed planar particle image velocimetry (PIV). In-plane flame boundaries were inferred from the absence of PIV seeding oil droplets in the burned gas regions. Line-of-sight flame images from two orthogonal views were recorded synchronously with the PIV data in order to validate the in-plane flame boundaries derived from the PIV data. The impact of the neighbouring flow on the evolution of the flame boundaries over crank angles is examined.

In this article, different manifold reduction techniques are implemented for the post-processing of Particle Image Velocimetry (PIV) images from a Spark Ignition Direct Injection (SIDI) engine. The methods are proposed to help make a more objective comparison between Reynolds-averaged Navier-Stokes (RANS) simulations and PIV experiments when Cycle-to-Cycle Variations (CCV) are present in the flow field. The two different methods used here are based on Singular Value Decomposition (SVD) principles where Proper Orthogonal Decomposition (POD) and Kernel Principal Component Analysis (KPCA) are used for representing linear and non-linear manifold reduction techniques. To the authors' best knowledge, this is the first time a non-linear manifold reduction technique, such as KPCA, has ever been used in the study of in-cylinder flow fields. Both qualitative and quantitative studies are given to show the capability of each method in validating the simulation and incorporating CCV for each engine cycle. Traditional Relevance Index (RI) and two other previously developed novel indexes: the Weighted Relevance Index (WRI) and the Weighted Magnitude Index (WMI), are used for the quantitative study. The results indicate that both POD and KPCA show improvements in capturing the main flow field features compared to ensemble-averaged PIV experimental data and single cycle experimental flow fields while capturing CCV. Both methods present similar quantitative accuracy when using the three indexes. However, challenges were highlighted in the POD method for the selection of the number of POD modes needed for a representative reconstruction. When the flow field region presents a Gaussian distribution, the KPCA method is seen to provide a more objective numerical process as the reconstructed flow field will see convergence with an increasing number of modes due to its usage of Gaussian properties. No additional criterion is needed to determine how to reconstruct the main flow field feature. Using KPCA can, therefore, reduce the amount of analysis needed in the process of extracting the main flow field while incorporating CCV.

Stirling pulse tube cryocoolers (SPTCs) can operate by incorporating a displacer into the warm end of a pulse tube cryocooler. Previous studies have shown that SPTCs running with an active displacer demonstrate good performance and higher efficiencies than pulse tube cryocoolers that utilize inertance tubes. Having an actively driven displacer requires a second phase from the power electronics, and the extra motor and electrical feedthrough adds to the complexity of the design, so it is desirable to have a passively driven displacer. This study presents the analysis and design of a passive displacer driven by the pressure difference across the displacer piston. A harmonic analysis and design of the displacer unit was completed, in conjunction with a numerical model constructed in Sage. This permitted the correct characteristics for the displacer to be designed in order to operate with the existing compressor and cold head. This included testing and analysis of different spring assemblies, optimisation of the displacer dimensions, and the piston moving mass. The passive displacer will replace the active displacer in an existing 80 K coaxial SPTC.

直喷发动机燃油喷雾撞击壁面形成油膜,导致燃烧效率降低,颗粒物排放增加.伴随撞壁的动态传热过程对油膜蒸发具有重要影响.本文针对正戊烷、甲醇、甲醇-汽油混合燃料瞬态喷雾撞击壁面,研究了不同条件下蒸发性对燃油瞬态喷雾撞击壁面动态传热影响.结果表明,提高喷油温度可促进燃油雾化,增大喷油压力或降低喷油距离可提高液滴撞壁强度,缩短液膜存在时间.撞壁瞬态温度与热流密度动态变化特征受燃油蒸发性与喷雾条件联合影响.

The flows in-cylinder have a profound effect on the mixture preparation and subsequent combustion in all engines. These flows are highly three-dimensional in nature and information from multiple planes is required to characterise the flow dynamics. The flow measurements reported here are from three orthogonal planes in an optical access engine that is based on the Jaguar Land Rover AJ200 Gasoline Direct Injection (GDI) engine. Particle Image Velocimetry (PIV) measurements have been taken every 5°CA from the start of induction to the end of compression. Data have been obtained from 300 cycles for separate experiments measuring flows in the tumble plane, the swirl plane and the cross-tumble plane. Vector comparison metrics are used to quantitatively compare ensemble averaged PIV flow fields to Computational Fluid Dynamics (CFD) simulations across each plane in terms of both the velocity magnitude and direction. These quantitative metrics enable identification of regions of interest and differences in flow characteristics across a range of engine operating conditions. Combination of the PIV vector fields from all three planes into composite flow fields enables the interaction between the flows in the three planes to be visualised. During induction the intake jet is visible in all three planes, while the swirl plane measurements show a pair of counter-rotating vortices either side of the intake jet and the tumble plane shows the development of the tumble flow. During compression the center of the tumble vortex traverses the cylinder leading to flow reversal in the swirl and cross tumble measurement planes.

Coaxial pulse tube cryocoolers are the configuration of choice as they allow better access to the cold head. Hence, a previously built and tested in-line pulse tube cryocooler which uses an active displacer for phase control has been modified into a coaxial configuration. The active displacer allows the mass flow and the pressure pulse at the cold end of the pulse tube to be easily adjusted for optimum performance. The displacer also allows the expansion power at the warm end of the pulse tube to be recovered in order to operate more efficiently. A numerical Sage model is used to demonstrate this by examining the work flows throughout the cryocooler and it is shown that more than 6% of the power required to drive the cryocooler comes from the warm end of the pulse tube via the displacer. When using an inertance tube or orifice, this expansion power is dissipated as heat which is why using a displacer can lead to a more efficient cryocooler. Moreover, the effect of changing the displacer phase and stroke on cryocooler performance and pressure characteristics is examined both experimentally and numerically.

Currently 99.8% of global transport is powered by internal combustion engines (ICEs) and 95% of transport energy comes from liquid fuels made from petroleum. Many alternatives including battery electric vehicles (BEVs) and other fuels like biofuels and hydrogen are being considered. However, all these alternatives start from a very low base and face very significant barriers to unlimited expansion so that 85–90% of transport energy is expected to come from conventional liquid fuels powering combustion engines even by 2040. Hence it is imperative that ICEs are improved in order to reduce the local and global environmental impact of transport. This paper considers the scope for such improvement after discussing the basic principles that govern engine efficiency and the technologies to control exhaust pollution. The great scope for such improvement is illustrated by considering various practical approaches already in the market. For instance, the best in class SI engines in the U.S. have 14% lower fuel consumption compared to the average. Engine and conventional powertrain developments alone could reduce the fuel consumption by over 30% for light duty vehicles (LDVs). Implementing other technologies such as hybridisation and light-weighting could reduce fuel consumption by 50% compared to the current average for LDVs. Current after-treatment technology can ensure that the exhaust pollutant levels meet the most stringent current emissions requirements. Indeed, with the most modern diesel vehicles, the exhaust can be cleaner than the intake air in urban centres. The implications for transport policy, particularly where there are plans to ban ICEs, are considered in the final discussion. All available technologies need to be deployed to mitigate the environmental impact of transport and it would be extremely short-sighted to discourage further development of ICEs by limiting their sales.

The in-cylinder flow field plays a key role in determining the combustion performance of internal combustion engines (ICEs) and it is critically important to validate numerical simulations of the flow field by comparison to experimental measurements using techniques such as particle image velocimetry (PIV). With the current trend for high-speed diagnostics, methods for quantitative comparison of vector fields are required which can provide robust spatially averaged results, without inspection of individual flow fields. The quality of match between vector fields, when quantified using current metrics such as the relevance index (RI), can be overly sensitive to the alignment of regions of low velocity such as the tumble vortex centre. This work presents complementary metrics, weighted using a function of the local velocity, for robust quantification of the alignment and magnitude differences between vector fields, the weighted relevance index (WRI) and the weighted magnitude index (WMI). These metrics are also normalized and combined in the combined magnitude and relevance index (CMRI). PIV measurements taken up to every 2 crank angle degrees within the tumble plane of a motored, optically accessible ICE are used to demonstrate the motivation for development and the application of the WRI, WMI, and CMRI metrics. The metrics are used to determine the number of cycles required to provide a representative mean flow field and to identify single cycles of interest. Variability of the flow field is quantified using the metrics and shows high variability in the region of the spark plug near typical ignition timings.Graphic abstract

A numerical model for a coaxial Stirling pulse tube cryocooler with an active displacer has been developed. An active displacer, in place of an inertance tube, has already demonstrated good efficiency in an in-line pulse tube, but incorporating this design into a coaxial configuration permits better access to the cold head. A model of the coaxial cold head was developed to include the radial flow and cooling in this region. The sub-assembly models were validated with flow testing of the physical sub-units of the cryocooler. The performance has been predicted for the cryocooler as a function of: fill pressure, operating frequency, and phase angle between the position of the linear compressor and displacer. The projected difference in performance and efficiency of the coaxial configuration was compared to the in-line design. The coaxial cryocooler numerically simulates 6 W of cooling at 80 K with an input power of 85 W, at a fill pressure of 28 bar, an operating frequency of 60 Hz, and a compressor-displacer phase angle of 41 degrees. Overall, the coaxial cryocooler outperforms the in-line design in terms of cooling power, but not in terms of efficiency.

Successful operation of Stirling pulse tube cryocoolers relies on minimising flow mixing within the pulse tube. Hence, the pulse tube and the flow straighteners at either end must be designed with great care. In this study, the flow within the pulse tube of an existing Stirling pulse tube cryocooler is numerically analysed and alternative flow straightener designs are suggested. The numerical simulation have been carried out using CONVERGE CFD, a finite volume Navier-Stokes solver. The standar k-epsilon RANS turbulence model has been used to account for the effects of turbulence within the pulse tube.

In-cylinder flow motion has a significant effect on mixture preparation and combustion. Therefore, it is vital that CFD engine simulations are capable of accurately predicting the in-cylinder velocity fields. Highspeed planar Particle Image Velocimetry (PIV) experiments have been performed on a single-cylinder GDI optical engine in order to validate CFD simulations for a range of engine conditions. Novel metrics have been developed to quantify the differences between experimental and simulated velocity fields in both alignment and magnitude. The Weighted Relevance Index (WRI) is a variation of the standard Relevance Index that accounts for the local velocity magnitudes to provide a robust comparison of the alignment between two vector fields. Similarly, the Weighted Magnitude Index (WMI) quantifies the differences in the local magnitudes of the two velocity fields. The WRI and WMI are normalised and combined to produce a combined metric, the Combined Magnitude and Relevance Index (CMRI), that quantifies the differences between two flow fields in both magnitude and alignment simultaneously. PIV measurements were made every 5°ca in the central tumble plane during the induction and compression strokes. The WRI, WMI and CMRI metrics are used to validate numerical simulations of the motored in-cylinder flow measured with PIV for a range of valve lift profiles and engine speeds.

In-cylinder temperatures and their cyclic variations strongly influence many aspects of internal combustion engine operation, from chemical reaction rates determining the production of NOx and particulate matter to the tendency for auto-ignition leading to knock in spark ignition engines. Spatially resolved measurements of temperature can provide insights into such processes and enable validation of Computational Fluid Dynamics simulations used to model engine performance and guide engine design.This work uses a combination of Two-Colour Planar Laser Induced Fluorescence (TC-PLIF) and Laser Induced Grating Spectroscopy (LIGS) to measure the in-cylinder temperature distributions of a firing optically accessible spark ignition engine. TC-PLIF performs 2-D temperature measurements using fluorescence emission in two different wavelength bands but requires calibration under conditions of known temperature, pressure and composition. Here the TC-PLIF technique is calibrated in-situ using high precision (<1%) LIGS point measurements.Temperature distributions were recorded during the compression stroke for fired operation with Direct Injection and with Plenum Fuel Injection of three two-component fuels containing toluene and iso-octane. Temperature inhomogeneity was observed for all fuels and injection strategies, with mm-scale regions having temperatures up to 10% higher than the local environment. Charge cooling of 3% due to direct injection was resolved.Proper Orthogonal Decomposition (POD) was used to quantify the cycle-to-cycle variation of the temperature data. Low-order POD modes featured most of the cyclic variation in temperature and the corresponding mode coefficients were used to investigate correlations with combustion analysis, fuel injection strategies and toluene content of the fuel. Additionally, the low-order POD mode coefficients provided an opportunity to identify cycles containing local hotspots or outlier measurements.

Gasoline Direct Injection (GDI) engines equipped with a turbocharger or supercharger (known as boosted engines) have achieved significant market penetration due to their advantages in reducing CO2 emissions compared to Port Fuel Injection (PFI) engines. These engines are known to emit particulate matter and recent studies have characterized these emissions. Fuel composition is known to have an impact on Particle Number (PN) emissions from GDI engines, however its effect on emissions from highly boosted engines is not known. In this work, 10 different fuels have been tested on an extremely highly boosted engine (at up to 35 bar BMEP) at several different operating conditions both unboosted and boosted and the PN emissions and size distributions have been measured using a Cambustion DMS500. The applicability of three previously developed indices linking fuel composition to PN emissions is tested with the results from these fuels. The results show that the PM index is a good predictor of PN emissions from this engine for all fuels at all operating points (unboosted and boosted). The Moriya and PN indices predict the PN emissions using fuels that are market representative, but are poor predictors of PN emissions over all of the fuels tested. Very small accumulation mode particle sizes of around 30 nm have been noted from all fuels at boosted conditions, which has relevance to future legislation and after treatment.

Pulse tube cryocoolers have been widely used for cooling infrared sensors, superconducting and gas liquefaction. An active phase shifter allows the phase angle between the pressure and the mass flow rate to be varied, thus high efficiency and better performance can be achieved. In this work, a CFD technique using dynamic meshing was adopted to investigate the oscillating flow and performance sensitivity of an in-line pulse tube cryocooler with an active phase shifter. A high performance computer (HPC) was used for the 2D simulation. The effect of phase angle, operating frequency, pressure drop, and fill pressure were studied. The pulse tube cryocooler appears to have an optimal operating frequency (55 Hz) and displacement phase angle (40 deg) between compressor and displacer. A fill pressure of 28 bar leads to better performance than 24 bar and 20 bar. The CFD results are validated by the experimental results of a prototype in-line pulse tube cryocooler.

Laminar burning velocity measurements have been made in a constant volume vessel using both flame front imaging and the pressure rise methods. Results from the two different methods are shown to be the same, so long as appropriate techniques are used for analysing the data. Comparisons are presented for the laminar burning velocity of mixtures with air of methane, ethanol and biogas (60% methane, 40% carbon dioxide) for a wide range of flammable mixtures at pressures of 2 and 4 bar and temperatures of 380 and 450 K. Methods for measuring the laminar burning velocity are still the subject of controversy, with different researchers favouring different approaches. Open flame techniques are very popular and the so-called heat flux method is now well established. The alternative technique of using a constant volume combustion vessel is also in common use, and has two distinct methods of use: either the imaging of flame front propagation at conditions of constant pressure, or the measurement of the pressure rise combined with a constant volume combustion model. The pressure rise method requires a more complex analysis, but has the advantage that a single experiment generates data across a range of linked temperatures and pressures, and the pressure and temperature rise also mean that data can be obtained for engine-like conditions.

Fuel spray impingement on piston surfaces is a concern because it can cause particulate exhaust emissions from gasoline direct injection (GDI) engine. Transient heat transfer plays an important role that directly influences liquid film evaporation and its lifetime. In this paper, the effects of injection temperature, injection pressure, piston temperature and impact distance on n-pentane spray impingement heat transfer were fully examined. Results showed that increasing the piston temperature could increase the rate of heat transfer with a larger surface temperature reduction and a higher heat flux, which led to a shorter liquid film lifetime on the piston surface. Increasing the fuel injection temperature helped to improve atomization of the fuel spray, reduce the penetration distance and mitigate impact, which in turn led to reduced surface cooling and less liquid film on the piston surface. A decrease in impact distance and an increase in injection pressure both caused an increase in surface temperature reduction and heat flux but a decrease in the liquid film residence time. The dimensionless heat flux in terms of Biot and Fourier numbers presented a high similarity during the rapid cooling stage. A dimensionless correlation was formed to quantify this fast time-varying heat transfer behaviour.

This paper reviews the fundamental requirements of liquid hydrocarbon fuels for spark ignition engines, namely that the fuel should vaporise satisfactorily and burn in a controlled manner. The phenomenon of knock and the development of the octane scale are discussed. The variation in the pressure–time histories for different engines is discussed, together with the reason why this leads to different fuel requirements. The difference in the octane rating tests and the way in which engine downsizing exacerbates these differences in the pressure–time histories are discussed. The applicability of the research octane number and the motor octane number to modern engines is reviewed, together with the phenomena of low-speed pre-ignition and superknock. The effects of the hydrocarbon fuel distillation characteristics on the driveability and the emissions are reviewed and discussed with respect to the historical context and the current legislative requirements. Brief mention is made of other fuel requirements such as the density, the gum content and the aromatic content.