Search Results

Abstract The preliminary gas turbine combustor design process uses a huge amount of empirical correlations to achieve more optimized designs. Combustion efficiency, in relation to the basic dimensions of the combustor, is one of the most critical performance parameters. In this study, semi-empirical correlations for combustion efficiencies are examined and correlation coefficients have been revised using an experimental air-blasted tubular combustor that uses JP8 kerosene aviation fuel. Besides, droplet diameter and effective evaporation constant parameters have been investigated for different operating conditions. In the study, it is observed that increased air velocity significantly improves the atomization process and decreases droplet diameters, while increasing the mass flow rate has a positive effect on the atomization—the relative air velocity in the air-blast atomizer increases and the fuel droplets become finer.

Abstract TOC

Abstract In this study low temperature plasma technology was applied to expand auto-ignition operation region and control auto-ignition phasing of the homogeneous charge compression ignition (HCCI) combustion. The low temperature plasma igniter of a barrier discharge model (barrier discharge igniter (BDI)) with high-frequency voltage (15 kHz) was provided at the top center of the combustion chamber, and the auto-ignition characteristics of the HCCI combustion by the low temperature plasma assistance was investigated by using a single-cylinder gasoline engine. HCCI combustion with compression ratio of 15:1 was achieved by increasing the intake air temperature. The lean air-fuel (A/F) ratio limit and visualized auto-ignition combustion process on baseline HCCI without discharge assistance, spark-assisted HCCI, and BDI-assisted HCCI were compared.

Abstract Insulating combustion chamber surfaces with thermal barrier coatings (TBCs) provides thermal efficiency improvement when done appropriately. This article reports on insulation heat transfer, engine performance characteristics, and damage modelling of “temperature swing” TBCs. “Temperature swing” insulation refers to the insulation material applied on surfaces of combustion chamber walls that enables selective manipulation of its surface temperature profile over the four strokes of an engine cycle. A combined GT Suite-ANSYS Fluent simulation methodology is developed to investigate the impact of thermal properties and insulation thickness for a variety of TBC materials for its “temperature swing” characteristics. This one-dimensional transient heat conduction analyses and engine cycle simulations are performed using scaled-down thermal properties of yttria-stabilized zirconia.

Abstract Reactivity-controlled compression ignition (RCCI) is a dual fuel low temperature combustion (LTC) strategy which results in a wider operating load range, near-zero oxides of nitrogen (NOx) and particulate matter (PM) emissions, and higher thermal efficiency. One of the major shortcomings in RCCI is a higher unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. Unlike conventional combustion, aftertreatment control of HC and CO emissions is difficult to achieve in RCCI owing to lower exhaust gas temperatures. In conventional RCCI, an early direct injection (DI) of low volatile diesel fuel into the premixed gasoline-air mixture in the combustion chamber results in charge stratification and fuel spray wall wetting leading to higher HC and CO emissions. To address this limitation, a homogeneous charge reactivity-controlled compression ignition (HCRCCI) strategy is proposed in the present work, wherein the DI of diesel fuel is eliminated.

Abstract Higher water evaporation and proper water vapor distribution in the cylinder are very vital for improving emission and performance characteristics of water-injected engines. The concentration of water vapor should be higher and uniform near the walls of the combustion chamber and nil at the spark plug location. In direct water-injected engines, water evaporation, vapor distribution, and spray impingement are highly dependent on injector parameters, viz., water injector orientation (WIO), location, and spray angle. Therefore, in this article, a computational fluid dynamics (CFD) investigation is conducted to study the effects of water injector spray angle (WISA), and WIO on the water evaporation, emission, and performance characteristics of a four-stroke, wall-guided gasoline direct injection (GDI) engine. The WISA is varied from 10° to 35°, whereas the WIO is varied from 15° to 35° in steps of 5°.

Abstract In recent years, lean-burn gasoline Spark-Ignition (SI) engines have been a major subject of investigations. With this solution, in fact, it is possible to simultaneously reduce NOx raw emissions and fuel consumption due to decreased heat losses, higher thermodynamic efficiency, and enhanced knock resistance. However, the real applicability of this technique is strongly limited by the increase in cyclic variation and the occurrence of misfire, which are typical for the combustion of homogeneous lean air/fuel mixtures. The employment of a Pre-Chamber (PC), in which the combustion begins before proceeding in the main combustion chamber, has already shown the capability of significantly extending the lean-burn limit. In this work, the potential of an ultralean PC SI engine for a decisive improvement of the thermal efficiency is presented by means of numerical and experimental analyses.

Abstract Interactions between fuel sprays and stepped-lip diesel piston bowls can produce turbulent flow structures that improve efficiency and emissions, but the underlying mechanisms are not well understood. Recent experimental and simulation efforts provide evidence that increased efficiency and reduced smoke emissions coincide with the formation of long-lived, energetic vortices during the mixing-controlled portion of the combustion event. These vortices are believed to promote fuel-air mixing, increase heat-release rates, and improve air utilization, but they become weaker as main injection timing is advanced nearer to the top dead center (TDC). Further efficiency and emissions benefits may be realized if vortex formation can be strengthened for near-TDC injections. This work presents a simulation-based analysis of turbulent flow evolution within a stepped-lip combustion chamber.

Abstract Cycle-by-cycle combustion prediction in real time during engine operation can serve as a vital input for operating at optimal performance conditions and for emission control. In this work, a real-time capable physics-based combustion model has been proposed for the prediction of the heat release rate in a direct injection diesel engine. The model extends the approaches proposed earlier in the literature by considering spray dynamics such as spray penetration and Sauter mean diameter in order to calculate the mass of evaporated fuel from the spray. Wall impingement of the liquid spray is predicted by considering the liquid length based on the prevailing in-cylinder conditions. These effects are considered even after the hydraulic end of injection till the last droplet of fuel impinges on the combustion chamber wall. The fuel evaporated from the wall film and its contribution to the kinetic energy of the charge are also considered.

Abstract In recent years, several innovative diesel combustion systems were developed and optimized in order to enhance the air and injected fuel mixing for engine efficiency improvements and to mitigate the formation of fuel-rich regions for soot emissions reduction. With these aims, a three-dimensional computational fluid dynamics (3D-CFD) numerical study was carried out in order to evaluate the impact of three different piston bowl geometries on a passenger car four-cylinder diesel engine, 1.6 liters. Once the numerical model was validated considering the baseline re-entrant bowl, two innovative bowl geometries were defined: one based on the stepped-lip bowl; the other including a number of radial bumps equal to the nozzle holes number. Firstly, the rated power engine operating condition was investigated under nonreacting conditions to evaluate the piston bowl effects on the in-cylinder mixing.

Abstract The proven impact of combustion chamber deposits, CCD, on advanced compression ignition, ACI, combustion strategies has spurred researchers to develop thermal barrier coatings, TBC, which can mimic CCD benefits on combustion efficiency and operational range expansion. However, application of TBCs within multi-mode engines exposes them to non-negligible soot radiation. In the present paper, the impact of radiation heat transfer on combustion chamber deposits is studied. The morphological construction of the combustion chamber deposit layer is shown to be partially transparent to radiation heat transfer, drawing corollaries with ceramic-based TBCs. Additional experimentation eliminates the optical transparency of CCD to reveal an “effective radiation penetration depth” facilitated by open surface porosity. The effective radiation penetration depth is then utilized to establish the relative communicating porosity of CCD and a magnesium zirconate TBC.

Abstract A numerical study using large-eddy simulations (LES) to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (SI) units, that is, a homogeneous-charge slow-speed single-cylinder research unit (the transparent combustion chamber (TCC)-III, Engine 1) and a stratified-charge high-revving speed gasoline direct injection (GDI) (Engine 2) one, are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV. A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (1% of mass fraction burnt (MFB1)) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics, and the crucial task to determine the dominant causes of its variability (if any) is undertaken.

Abstract The main objective of this study is to investigate the effect of pilot injection mass and timing on main combustion and engine emissions. The experiments have been conducted on a single-cylinder diesel engine at fixed engine speed with various loads. In the computational fluid dynamics (CFD) simulations of the combustion, only a segment of the cylinder was considered. A numerical multiphase simulation of the internal nozzle flow delivered the required initial conditions for the spray primary breakup model. For the combustion the ECFM-3Z model was employed with a two-stage autoignition model. The measurements show that a pilot injection can reduce the nitrogen oxides (NOx) emissions at low engine load. With higher engine loads an increase in the NOx emission values was observed. The numerical investigation exhibited that the thermal nitrogen monoxide (NO) formation is a mixing-controlled process. The NO is formed generally in two different zones.

Abstract Nowadays the role played by passenger vehicles on the greenhouse effect is of great value. To slow down both global warming and fossil fuel wasting, the design of high-efficiency engines is compulsory. Downsized Turbocharged Gasoline Direct Injection (TGDI) engines comply with both high-efficiency and power demand requirements. Nevertheless, Direct Injection (DI) inside downsized chambers may result in the fuel wall impingement, depending on the operating conditions. The impact of fuel on the cylinder liner leads to the mixing of the fuel and the lubricant oil in the cylinder wall. When the piston moves, the piston top ring scraps the non-evaporated fuel-oil mixture. Then the scraped fuel-oil mixture may be scattered into the combustion chamber, becoming a source of diffusive flames in all conditions and abnormal combustions known as Low-Speed Pre-Ignitions (LSPI) at the highest loads.

Abstract The search for new solutions for engine ignition systems and combustion of lean air-gas mixtures has led to the application of a two-stage combustion system, previously also known as turbulent jet ignition (TJI). In this article, the authors proposed using a pre-chamber filled with a rich ignition mixture, which is turned into an outgoing burning jet that facilitates the combustion of lean mixtures in the main combustion chamber. Various two-stage combustion systems were tested in a high-speed natural gas-powered combustion engine as a part of the Horizon 2020-GasOn (Gas-Only Internal Combustion Engines) research program. The results of testing the effectiveness of using various pre-chambers are discussed in this article. These tests were carried out in a selected engine’s operating range using three pre-chambers with different geometry. The first phase of the research concerned the selection of a gas supply system (injection on the intake valve or Heinzmann mixer).

Abstract Ducted fuel injection (DFI) has been shown to attenuate engine-out soot emissions from diesel engines. The concept is to inject fuel through a small tube within the combustion chamber to enable lower equivalence ratios at the autoignition zone, relative to conventional diesel combustion. Previous experiments have demonstrated that DFI enables significant soot attenuation relative to conventional diesel combustion for a small set of operating conditions at relatively low engine loads. This is the first study to compare DFI to conventional diesel combustion over a wide range of operating conditions and at higher loads (up to 8.5 bar gross indicated mean effective pressure) with a four-orifice fuel injector. This study compares DFI to conventional diesel combustion through sweeps of intake-oxygen mole fraction (XO2), injection duration, intake pressure, start of combustion (SOC) timing, fuel-injection pressure, and intake temperature.

Abstract Gasoline direct-injection (GDI) engines are finding increasing applications due to their potential benefits such as low fuel consumption and superior performance. However, their use in small engines poses challenges in locating the injector and avoiding the fuel impingement on the combustion chamber walls. The selection of suitable injection timing(s) and injection of fuel in multiple pulses can overcome these difficulties. In the present work, a small-bore spray-guided GDI engine was developed by modifying a base carbureted engine. Detailed experiments and supporting CFD simulations were conducted to study the influence of the number of injections per cycle and injection timings on the performance, emissions, and combustion characteristics including cycle-to-cycle variations. It was found that the homogeneity of the fuel-air mixture was improved and fuel impingement was reduced with multiple injections.

Abstract This article investigates the effect of ambient oxygen (O2) levels and ambient density on the primary soot size under diesel engine-like conditions via the Lagrangian soot tracking (LST) method. The numerical studies and soot analysis are carried out for an n-heptane spray flame in the Sandia constant volume combustion chamber. Numerical studies are carried out at two O2 levels of 15% and 12%, as well as two ambient densities of 14.8 kg/m3 and 30 kg/m3. The LST model involves treating the soot particles formed in the spray flame as Lagrangian particles, and their individual soot information is stored. Based on the primary soot size distribution for soot particles in the core of the spray jet, an increase in ambient density from 14.8 kg/m3 to 30 kg/m3 is shown to increase the peak and mean soot size by a factor of 1.5. Furthermore, the peak and mean primary soot size decreases with decreasing O2 levels from 15% to 12%.

Abstract The addition of a spark plug in place of the original fuel injector and fumigating natural gas (NG) inside the intake manifold is an economical way to convert heavy-duty diesel engines to NG spark-ignition (SI) operation. The literature shows that, when compared to a conventional SI engine combustion chamber, the different in-cylinder flow motion, turbulence intensity distribution, and interaction of propagating flame with chamber boundaries in these converted engines produce distinctive combustion stages. As the current understanding of how these combustion stages affect the engine performance is limited, this study used a triple-Wiebe combustion model to determine the effect of spark timing (ST) on the phasing and mass fraction of each combustion stage, at lean operation (ϕ = 0.73) and low engine speed (N = 900 rpm).

Abstract Increasing thermal and fuel efficiency in Internal Combustion Engines (ICEs) requires thorough investigations on the combustion process and its thermodynamics. The first law of thermodynamics expresses the balance of the energy, while the second law specifies the maximum achievable work. In this article, Low-Temperature Combustion (LTC) strategies, including Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI), Partially Premixed Combustion (PPC), and Direct Dual-Fuel Stratification (DDFS) were analyzed by the first and second law approaches, and they were compared with ideal-diesel cycle and Conventional Diesel Combustion (CDC). HCCI and RCCI had the highest exergy efficiency of 50.8% and 49.2%, respectively, compared to other cases, and exergy destruction in these cases was the lowest (25.3% and 27.5%, respectively).