Mayank Mittal

Cycle-to-cycle flow variations significantly influence the combustion variations from one cycle to the next, particularly at low operating loads in small spark-ignition engines. Hence in the present work, cycle-to-cycle flow variations are analyzed at low throttle opening of 25% in a small spark-ignition engine using particle image velocimetry (PIV) technique. Experiments are conducted in an optically accessible single-cylinder, port-fuel-injection engine (volume: 110 cm3) at 1200 rpm engine speed. Images are captured at different crank angle positions during both intake and compression strokes over a tumble measurement plane bisecting the intake and exhaust valves, and processed using cross-correlation method to obtain the instantaneous velocity fields considering 200 image pairs at each crank angle position considered. Turbulent kinetic energy and probability density functions of vorticity are then calculated from measured instantaneous velocity fields at different crank angle positions to quantify the cycle-to-cycle flow variations. It is found that cycle-to-cycle variations were increased from 84 CAD to 114 CAD during intake, and then were reduced during intake (from 114 CAD to BDC of intake) and compression, except a slight increase during late compression. CFD simulations are also performed using CONVERGE software, and showed good agreement with measured flow fields obtained using PIV.

In recent years, the permissible limits of engine exhaust emissions are reduced considerably. Hence a quick warm-up and high conversion efficiency of the catalyst system is essential to meet upcoming stringent emission regulations. In the present work, the transient thermal behavior of an oxidation catalyst is studied using a one-dimension mathematical modeling approach with the focus on CO oxidation for dual-fuel engine application. At first, the heat generation due to chemical reactions is considered negligible for studying the warm-up behavior. Upon obtaining a good agreement between predicted warm-up temperature profiles with available literature data, the effect of an electrical heater on the warm-up behavior is investigated. The model is then extended by incorporating heat generation due to CO oxidation. A simplified reaction rate model is considered in order to reduce the computational complexity. It is observed that the simplified model agrees well with the experimental data for both low and high levels of CO concentration at the inlet, typical in dual-fuel technology when an engine is operated under diesel and dual-fuel modes, respectively.

In-cylinder fluid flow has been studied intensively due to its well-known impacts on combustion and pollutant formation. The variation of the in-cylinder flow is a major impediment in achieving higher performance characteristics, particularly at low engine speeds and low to moderate loads. A comprehensive study with primary emphasis on flow evolution and its variations at different throttle openings has not been undertaken in the context of small dual-valve spark-ignition (SI) engines. Therefore, a detailed study of tumble motion in a small port-fuel-injection spark-ignition engine with displacement volume of 110 cm3 is carried out in the present work at three different throttle openings (viz. 25%, 50%, and wide-open throttle) using cross-correlation particle image velocimetry (PIV) measurement technique and proper orthogonal decomposition (POD). The flow was recorded over a tumble measurement plane at various crank angle positions during both intake and compression strokes at an engine speed of 1,200 rpm. Flow variations are quantified with the histogram of averaged vorticity from all instantaneous cycles using PIV data. Turbulent kinetic energy (TKE) is calculated at every measured crank angle position for all three throttle conditions. Triple decomposition of the velocity fields is performed to separate coherent structures and quantify the variations of large-scale structures. The flow variation is related to the turbulent kinetic energy and the variations of domi-nant POD modes.

In-cylinder fluid motion has a substantial impact on air-fuel mixture formation, combustion process and emission formation. In the present paper, a simulation study of the in-cylinder fluid flow is performed using a computational fluid dynamic (CFD) model of a port-fuel-injection (PFI) engine (volume: 110 cc). First, a 1-D model is developed, and validated with the cylinder pressure traces acquired in an optical engine experimentally. The model provided the boundary conditions for multi-dimensional numerical simulations. The predicted velocity fields from CFD are then compared with the measured data obtained using particle image velocimetry (PIV) at various crank angle positions with low throttle opening condition. A good relevance is observed on comparing numerically estimated results with experimental results.

Particle image velocimetry (PIV) has been widely used to investigate the flow fields in many areas. Images captured using PIV, however, are aberrated when viewed through a transparent cylinder such as in engines, and therefore, need to be compensated for distortions. The calibration procedure is an important step in flow diagnostics for the reconstruction of displacement vectors from image plane to physical space coordinates, also incorporating the distortion compensation. In engine flow diagnostics, the calibration procedure based on global pixel size, however, is commonly used; hence local pixel size variations are ignored, even with significant distortions. In the present work, an analysis is performed to quantify the error levels in the calibration procedure by acquiring the calibration images with and without the cylindrical liner at different measurement planes. Additionally, calibration is also performed utilizing the non-linear mapping functions to account for local pixel size variations, along with error determination. It is found that the error in the calibration procedure based on global pixel size is significant, hence highlights the importance of calibration based on mapping functions in engine flow diagnostics.

Application of direct injection (DI) technology in small-bore engines, the type used in two- and three-wheelers, could improve their performance significantly. It is recognised that the use of high fuel injection pressure is beneficial in large-bore engines for a good mixture preparation. However, simple systems incorporated with low-pressure DI are desirable in small-engine segment of automobiles. Further, high fuel pressures will result in excessive wall wetting when cylinder dimensions are small. Extensive studies were carried out to investigate the minimum fuel injection pressure required for homogeneous and lean modes of operation in such small bore DI engine. The effect of spark plug protrusion in the combustion chamber was also investigated under the spray-guided configuration. Comprehensive experiments and CFD simulations were performed for estimating the engine efficiency, emissions, mixture preparation characteristics, fuel spray and fuel impingement on combustion chamber walls. Results have demonstrated that engine performance and emissions did not deteriorate when fuel injection pressure was reduced from 150 to 50 bar at full load. However, at very low pressures, like 20–30 bar, THC, CO and smoke emissions increased. Fuel injection pressure did not influence the lean limit, that is, equivalence ratio of about 0.77, but influenced the thermal efficiency at lean conditions. In order to attain high efficiency, under lean conditions, a minimum fuel pressure of 80 bar was required. The spark plug protrusion that resulted in a gap of 0.75 mm with respect to the incoming fuel spray cone has given the best engine performance, while higher protrusions affected the tumble flow and led to the stratification of charge near the spark plug, which resulted in elevated CO and smoke emissions. Hence, this work highlights that relatively lower direct injection pressures are suitable in small bore engines, which will impact the development of cost effective components for such applications.

Nowadays, due to stringent emission regulations, it is imperative to incorporate modeling efforts with experiments. This paper presents the development of a phenomenological model to investigate the effects of various in-cylinder strategies on combustion and emission characteristics of a common-rail direct-injection (CRDI) diesel engine. Experiments were conducted on a single-cylinder, supercharged engine with displacement volume of 0.55 l at different operating conditions with various combinations of injection pressure, number of injections involving single injection and multiple injections with two injection pulses, and EGR. Data obtained from experiments was also used for model validation. The model incorporated detailed phenomenological aspects of spray growth, air entrainment, droplet evaporation, wall impingement, ignition delay, premixed and mixing-controlled combustion rates, and emissions of nitrogen oxides (NOx) and diesel soot. The detailed spray configuration provided an edge to the present model in predicting engine combustion and emission characteristics accurately. Results showed that a simultaneous reduction of NOx and soot is possible with an optimized combination of EGR and dwell period between multiple injections at high injection pressure particularly for low operating loads.

Cycle-to-cycle combustion variations not only degrade the performance and combustion characteristics of an engine, it also has a considerable influence on engine durability. Thus, a deeper understanding of cycle-to-cycle combustion variations, particularly at low loads, is required. In the present study, both statistical and wavelet transform methods have been used to investigate the cycle-to-cycle combustion variations at different operating loads. A single-cylinder spark-ignited engine was used, and cylinder pressure data was recorded for 1200 consecutive engine cycles at each operating point for the analysis of cycle-to-cycle variations. As expected, it was found from the statistical analysis that cycle-to-cycle variations in indicated mean effective pressure (IMEP) were decreased with the increase in load. Wavelet analysis revealed that cycle-to-cycle variations occur at multiple time scales for both loads. However, wavelet power spectrum at the lower load was characterized by intermittent high frequency oscillations, and with the increase in load, these high frequency oscillations were shifted to lower frequency range.

Biogas is a renewable gaseous fuel and has the potential to replace fossil fuels for sparkignition engines; however, a higher volumetric proportion of CO2in biogas degrades the engine characteristics significantly. Biogas upgradation techniques are limited by higher fuel costs, and strenuous modifications would be required for improving engine physical parameters. In this study, experimental investigations were performed with hydrogenenriched biogas to enhance low operating load limit and engine characteristics, and to the best of authors' knowledge, studies related to operating range and low load enhancement by hydrogen addition in biogas fueled engines are not reported in literature. Gaseous-fuels blending setup was developed to fabricate the gaseous fuel mixtures in desired proportions and moderate amounts of hydrogen (5, 10, 20, and 30% by vol.) were blended with biogas. The experiments were conducted on a single-cylinder SI engine operated at the compression ratio of 10:1 and 1500 rpm for stationary applications. It was found that the coefficient of variation (COV) of indicated mean effective pressure decreased from 10% in case of biogas to 8.69, 6, 3.05, and 1.66%, respectively, for 5, 10, 20, and 30% hydrogen cases at 6 N·m loading condition. Low operating load limit enhanced from 6 N·m in case of biogas to 5.3, 2.2, 1.5, and 0.8N·m, respectively, for 5, 10, 20, and 30% of hydrogen share in the fuel mixture and brake thermal efficiency also improved with hydrogen enrichment. Carbon-based emissions decreased with hydrogen addition, whereas oxides of nitrogen increased but it was well below the baseline case with pure methane. Overall results indicated that hydrogen enrichment enhances the low load limit and engine characteristics of biogas-fueled SI engines for stationary power generation applications in rural areas.

Laser-based diagnostic techniques like particle image velocimetry (PIV) and molecular tagging velocimetry (MTV) are used to measure flow fields at a high spatial resolution. Post-processing of the obtained flow fields is essential for outlier correction as the datasets may be skewed by local flow vectors with a disproportionate disparity in magnitude or directions from neighborhood vectors. The rationale behind this work is to propose an algorithm using proper orthogonal decomposition (POD), namely, POD-OROC (POD-based outlier removal and outlier correction), which can correct outliers in an ensemble of flow fields. The proposed algorithm is first validated on synthetic flows with a known percentage of outlier rate and then applied to engine in-cylinder flow fields. The algorithm ran for a few iterations for both flow datasets and rejected frames with high outlier rates (above 15%) and then post-processed the remaining ones to detect and correct local spurious vectors. It was found that outlier vectors with larger deviation from neighboring vectors are detected in earlier iterations. An error analysis was performed to quantify the total error in an ensemble and, in using it, two types of errors - over-detection and under-detection - were identified. With this insight, several parameters of the model for synthetic flows were optimized for best performance, and then the model was modified for application to in-cylinder flows. The impact of POD-OROC was studied through changes in the POD energy spectra where the energy share of the first mode increased to 99.9% for synthetic flows and to 82.5% and 68.9% for the two in-cylinder flow sets. Finally, POD-OROC is now matured enough to be applied to in-cylinder flow datasets and can detect and correct both single and cluster outliers.