Premixed Combustion Mode Research Articles

Experimental investigation on non-premixed CH4/air combustion in a novel miniature Swiss-roll combustor

In the past decades, combustion in miniature Swiss-roll combustors has widely investigated for applications to micro power generation systems. However, all the existing Swiss-roll combustors are designed for premixed combustion mode, in which the flame is prone to flash back or become extinguished at low flow rates. In the present work, a miniature Swiss-roll combustor was designed specifically for non-premixed combustion, which has two inlets for counterflows of methane and air, and two outlets for exhaust gas. Preliminary experimental investigation was conducted on a model combustor with a transparent quartz cover, which demonstrated that this new configuration brings about several merits. First, the flame structure and outer wall temperature distribution under most conditions are almost symmetric. Second, this combustor can run at a very low equivalence ratio (<0.3) and a very small fuel flow rate (˜0.029 L/min). As a result, the wall temperature level is relatively low, which makes this combustor a potential heat source for miniature thermoelectric devices using low-temperature materials such as Bi2Te3. Furthermore, the exhaust gas can be controlled at a very low temperature level (˜390–450 K), which means this micro combustor has a good heat recirculation performance.

Flame–cooling air interaction in an effusion-cooled model gas turbine combustor at elevated pressure

The focus of this work is the investigation of flame–cooling air interaction by means of wall temperature (2D themographic phosphor thermometry), gas phase temperature (coherent anti-Stokes Raman spectroscopy), flame structure (planar laser-induced fluorescence of OH) and flow field (particle image velocimetry) measurements in an effusion-cooled single sector model gas turbine combustor. The rig is operated under close-to-reality boundary conditions, i.e., elevated combustor inlet temperature and increased system pressure. Parametric variations of important parameters, namely swirl, staging and effusion cooling air mass flow are conducted to investigate their effect on total film cooling effectiveness. Furthermore, we derive a dimensionless flame–cooling air interaction intensity parameter on the basis of OH-PLIF data. The latter revealed a higher interaction intensity for fully premixed operation at a low swirl number—a trend that is reversed for higher swirl numbers, where a partially premixed combustion mode leads to increased intensities. This trend is reflected in wall temperature as well as resulting film cooling effectiveness. Peak wall temperature varies up to $$150\,{\text {K}}$$ across all parametric variations and up to $$200\,{\text {K}}$$ in axial direction on the effusion-cooled liner for a given condition. This corresponds to a variation of total film cooling effectiveness between 0.45–0.57 and 0.45–0.62, respectively.

Enhancing flame flashback resistance against Combustion Induced Vortex Breakdown and Boundary Layer Flashback in swirl burners

Swirl combustors have proven to be effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, their employment for lean premixed combustion modes while introducing alternative fuels such as high hydrogenated blends results in many combustion instabilities. Under these conditions, flame flashback is considered one of the major instability problems that have the potential of causing considerable damage to combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown is considered a very particular mode of flashback instability in swirling flows as this type of flashback occurs even when the fresh mixture velocity is higher than the flame speed, a consequence of the interaction between swirl structures and swirl burner geometries. Improvements in burner geometries and manipulation of swirling flows can increase resistance against this type of flashback. However, increasing resistance against Combustion Induced Vortex Breakdown can lead to augmentation in the propensity of another flashback mechanism, Boundary Layer Flashback. Thus, this paper presents an experimental approach of a combination of techniques that increase Combustion Induced Vortex Breakdown resistance, i.e. by repositioning a central injector and using central air injection, while simultaneously avoiding Boundary Layer Flashback, i.e. by changing the wall boundary layer characteristics using microsurfaces on the nozzle wall. Results show that using these techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a broader variety of fuel blends safely, while informing developers of the improvements obtained with the combined techniques.

Effect of the engine calibration parameters on gasoline partially premixed combustion performance and emissions compared to conventional diesel combustion in a light-duty Euro 6 engine

A design of experiments analysis was performed to investigate the effects of pilot quantity, combustion phasing and exhaust gas recirculation on performance and emissions in a gasoline partially premixed combustion to find out the optimal combination of the all varied parameters.The experimental activities were performed on a light-duty Volvo Euro 6 diesel engine. The test was performed under steady-state operating conditions, nine test points were chosen inside the operating area of the New European Driving Cycle and the Worldwide Harmonized Light vehicles Test Cycles. A fractional factorial analysis in partially premixed combustion on the single and combined effect of the main engine calibration parameters and a global comparison between partially premixed combustion and conventional diesel combustion on the engine performance and emissions adopting the optimal calibration parameters obtained from design of experiments analysis for both combustion modes analysed were presented. The purpose was to obtain the calibration parameters setting that permits to achieve high efficiency and low emissions as well.The partially premixed combustion results show the highest efficiency and lowest NOx emissions adopting a high exhaust gas recirculation rate combined with advanced combustion phasing and lower pilot quantity. Higher efficiency, up to 2.0% units, was obtained in partially premixed combustion with respect to the conventional diesel combustion due to the lower heat transfer loss. Lower soot (about two times) and NOx (about −0.5 g/kWh) levels with partially premixed combustion were obtained and compared to conventional diesel combustion at the same exhaust gas recirculation level.A reduction of about 5% of CO2 and fuel consumption with a 50% of reduction on NOx and soot simultaneously were obtained for partially premixed combustion on the New European Driving Cycle estimation results with respect to the diesel combustion. The information derived from this work are useful to develop and calibrate a light-duty engine that operate in gasoline partially premixed combustion mode achieving NOx close to the Euro 6 limit without adopting any after treatment system.

Online Cross-Sectional Monitoring of a Swirling Flame Using TDLAS Tomography

The purpose of this paper is to monitor the dynamic cross-sectional behavior of swirling flames using the tunable diode laser absorption spectroscopy (TDLAS) tomographic system. The newly developed online and highly spatially resolved imaging system based on TDLAS tomography was employed to monitor and reveal directly the reaction process in the swirling flame by reconstructing the 2-D distributions of temperature and H2O concentration over a cross section of the flame. The system was demonstrated to be capable of capturing the temperature distribution accurately and inferring the thermal expansion over the cross section of interest in the swirling flame generated by a model swirl injector operating in partially premixed combustion mode. As the equivalence ratio was decreased, thermal oscillations extracted from the real-time 2-D reconstructions were used to infer the instability of the swirling flame. Furthermore, the developed system was applied to capture dynamically the process of blowout of the swirling flame, illustrating that the system can provide firsthand and reliable visual data to help prevent the flame from lean blowout (LBO). This paper reports the first experimental observation of the dynamic cross-sectional behavior of the swirling flame, enabled by the high spatial and temporal resolutions provided by the TDLAS tomographic imaging system. The developed system can help better understand the LBO mechanism so as to improve the performance of low-emission gas turbine combustors.

Enhancement flame flashback resistance against CIVB and BLF in swirl burners

Swirl combustors have proven as effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, employment of swirl combustors to work on lean premixed combustion modes while introducing alternative fuels such as high hydrogen blends result in many combustion instabilities. Under these conditions, flame flashback has been considered as one of the major instability problems that have the potential of causing considerable damages of the combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown (CIVB) is considered a very particular mode of flashback mechanism in swirling flows as this type of flashback occurs even when the fresh mixture’s velocity is higher than the flame speed, consequence of the interaction between swirl structures and swirl burner geometries. Improvements of burner geometries and manipulation of swirl flows can produce good resistance against this type of flashback. However, increase flame flashback resistance against CIVB can lead to an increase in the propensity of another flashback mechanism, Boundary Layer Flashback (BLF). Thus this paper presents an experimental and numerical approach that allows the increase in CIVB resistance by using diffusive air injection and simultaneously avoid BLF by changing the wall boundary layer characteristics using microsurface grids as a liner for the nozzle wall. Results show that using those two techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a great variety of fuel blends safely.

Analysis of pulverized coal flame stabilized in a 3D laminar counterflow

In this paper, pulverized coal flames stabilized in a three-dimensional laminar counterflow configuration are simulated with detailed chemistry and the flame behaviors are analyzed in detail. Effects of radiation, coal particle mass flow rate and strain rate on the pulverized coal flame structure are investigated. The results show that the coal particles transported by the air stream tend to be ignited in a premixed combustion mode, which is followed by a non-premixed flame reaction zone, forming a typical double-flame type structure. The contribution of premixed combustion to the total heat release rate is sensitive to the studied operating conditions. Both volatile combustion and char off-gases combustion contribute to premixed combustion and their relative importance is influenced by the operating conditions. The pulverized coal combustion is significantly affected by radiative heat transfer. Without radiation, the reaction zone becomes thinner and the ignition is delayed. As the coal particle mass flow rate increases, the coal particles are ignited earlier and the combustion of char off-gases expands over a larger region. As the strain rate increases, both the premixed combustion share and the contribution of char off-gases combustion to the total heat release rate are decreased. For the extremely high strain rate case, the oxidizer can diffuse into the coal cloud from the oxidizer side to ignite the gas fuels at the fuel side (i.e. effect of oxidizer “leakage”). In order to properly consider the interphase heat transfers in gaseous flamelet models, a new tabulation method (compared to the conventional ones, e.g., Wen et al., (2017)) is proposed. The a priori analysis of the new tabulation method on different configurations shows that, compared to the conventional tabulation method, it better accounts for the heat transfers between the coal particle phase and gas phase, and can be applied to combustion systems with different oxidizer streams without introducing additional manifolds.

Carrier-phase DNS of pulverized coal particle ignition and volatile burning in a turbulent mixing layer

Direct numerical simulations (DNS) of a three-dimensional turbulent mixing layer are performed, where coal particles seeded in an air stream mix with hot lean combustion products in a second stream. This case mimicks conditions of a pulverized coal flame stabilized by hot products such as found in industrial furnaces. Particles are heated up by the hot gases and devolatilize, followed by volatile combustion in the gas phase. The carrier-phase DNS resolves all relevant scales of the fluid phase except the boundary layers around individual particles. The simulation results are assessed in terms of instantaneous contour plots of relevant quantities, spatially averaged statistics, scatter plots and (pseudo-)flamelets. The analysis provides insight into the mechanisms of solid particle ignition and burning stabilized by hot combustion products, as well as the flame structure and combustion mode. It is shown that ignition initially occurs at very lean conditions when particles are entrained in the hot gases. Subsequently volatile combustion proceeds in non-premixed as well as premixed combustion modes, characterized by means of the flame index, with an overall higher heat release in non-premixed zones. At late times two flames can be clearly distinguished, an upper flame burning into the air carrying the particles and a lower flame burning into the lean products. The latter flame shows a pure non-premixed behavior, while the former illustrates a complex flame structure with both premixed and non-premixed modes, as well as flame quenching. The DNS database and initial analysis lay the foundation for future systematic studies in similar configurations and support the development of models suitable for the combustion of solid fuel particles.

An experimental and numerical study of the effect of diesel injection timing on natural gas/diesel dual-fuel combustion at low load

Natural gas/diesel dual-fuel combustion compression ignition engine has the potential to reduce NOx and soot emissions. However, this combustion mode still suffers from low thermal efficiency and high level of unburned methane and CO emissions at low load conditions. The present paper reports the results of an experimental and numerical study on the effect of diesel injection timings (ranging from 10 to 50 °BTDC) on the combustion performance and emissions of a heavy duty natural gas/diesel dual-fuel engine at 25% engine load. Both experimental and numerical results revealed that advancing the injection timing up to 30 °BTDC increases the maximum in-cylinder pressure. However, with further advancing the injection timing up to 50 °BTDC, the maximum in-cylinder pressure decreases which is mainly due to the lower in-cylinder temperature before SOC. Moreover, the analysis of OH spatial distribution shows that, at very advanced diesel injection timings, the non-reactive zones are much narrower than later injection timings during the last stages of combustion, indicating a more predominant premixed combustion mode. At retarded diesel injection timings, the consumption of premixed fuel in the outer part of the charge is likely to be a significant challenge for dual-fuel combustion engine at low engine load conditions. However, with advancing the diesel injection timing, the OH radical becomes more uniform throughout the combustion chamber, which confirms that high temperature combustion reactions can occur in the central part of the charge. Diesel injection timing of 30 °BTDC appears to be the conversion point of all conventional dual-fuel combustion modes. Further advancing diesel injection timing beyond this point (30 °BTDC) results in noticeable reduction in NOx and unburned methane emissions, while CO emissions exhibit only slight drop. However, at very advanced diesel injection timings of 46 and 50 °BTDC, NOx and unburned methane emissions continue to drop, whereas CO emissions tend to increase. The results showed also that the highest indicated thermal efficiency is achieved at these very advanced diesel injection timings of 46 and 50 °BTDC. Finally, the results revealed that, by advancing diesel injection timing from 10 °BTDC to 50 °BTDC, NOx, unburned methane, and CO emissions are reduced, respectively, by 65.8%, 83%, and 60% while thermal efficiency is increased by 7.5%.

An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, [Formula: see text] and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

Thermal analysis of a light-duty CI engine operating with diesel-gasoline dual-fuel combustion mode

Reactivity Controlled Compression Ignition (RCCI) is one of the most promising premixed combustion modes, aimed at reducing simultaneously engine emissions and fuel consumption. In this work, the global energy balance (GEB) of a single-cylinder engine operating with dual-fuel and RCCI modes is analysed. The methodology used allows determining the energy degradation from the energy release during the combustion to the final work output, thus evaluating the performance of such combustion mode. The weight of each GEB term is analysed using two references: the injected and the burned fuel energies, thus decoupling the combustion and thermal processes. The analysis is performed by evaluating the effect of some key parameters: low/high reactivity fuel ratio, injection timing and EGR rate. The results show that increasing the low reactivity fuel leads to a better shape of the heat release rate, with a reduction of heat transfer and exhaust losses, thus improving the thermal efficiency about 1% with respect to a conventional Diesel combustion. Injection timing swept shows that a suitable optimization leads to further thermal efficiency increase up to 4%, while EGR has a limited effect. It is also concluded that there is still room for further improvement by reducing incomplete combustion losses.

Towards Identifying Flame Patterns in Multiple, Late Injection Schemes on a Single-Cylinder Optical Diesel Engine

ABSTRACTThe work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.

LES of pulverized coal combustion with a multi-regime flamelet model

It is well known that premixed and non-premixed flamelets coexist in the combustion system, especially for multiphase combustion (Luo et al., 2011; Bai et al., 2016). In this work, a multi-regime combustion model for partially premixed multiphase combustion (2PMC) is developed in the framework of large eddy simulation (LES). In this model, the multi-regime combustion mode is decomposed into two pure combustion regimes (i.e. premixed and non-premixed) through combustion regime indicator. Different combustion models are chosen for different combustion regimes. For example, for premixed combustion regime, the flamelet generated manifold (FGM) tabulation method in combination with the artificially thickened flame (ATF) approach is used. For non-premixed combustion regime, on the other hand, the thermo-chemical quantities will be extracted from the non-premixed chemtable generated by the flamelet/progress variable (FPV) approach. The proposed multi-regime flamelet model is then extended to adapt to the pulverized coal combustion (PCC) and applied to a laboratory-scale pulverized coal jet flame. The simulation results show that the percentage of the volume-weighted premixed combustion regime of this studied flame is up to 18%. Quantitative comparisons between the experimental data and the numerical results with the present multi-regime flamelet model, the FPV model, and the Eddy Break Up (EBU) model shows that the proposed multi-regime flamelet model has several advantages. It performs better than the FPV model in the regions where premixed combustion mode prevails and also much better than the EBU model in species concentration prediction.

Direct numerical simulation of spray evaporation and autoignition in a temporally-evolving jet

This work investigates spray evaporation and autoignition in a temporally-evolving jet by means of Direct Numerical Simulation (DNS). The impact of shear on evaporation and spray autoignition mechanisms is quantified by comparing droplets evolving in a high-speed jet flow or in a nearly quiescent environment. Comparisons are based on the topology of the reaction fronts, as characterized by mixture fraction, scalar dissipation rate, flame index, as well as fields of temperature and heat release. The impact of local equivalence ratio, droplet diameter and jet velocity have been investigated by varying these parameters. The results reveal that the temporally-evolving jet is a promising numerical configuration to study spray-turbulence interaction, evaporation, mixing, and auto-ignition mechanisms. It is observed that the autoignition delay time strongly depends on the droplet diameter and jet velocity, while it is far less sensitive onto the equivalence ratio. With high shear, ignition occurs on the lean side, peak heat release being found near stoichiometry in a premixed combustion mode. In the absence of shear, ignition occurs simultaneously over a broad range of mixture fractions, and peak heat release is found for a rich mixture involving both premixed and nonpremixed flames. It is also noticed that the conditional mean of the scalar dissipation rate shows non-monotonic behavior when varying jet velocity.

Numerical Study of the Effect of Nozzle Configurations on Characteristics of MILD Combustion for Gas Turbine Application

The MILD (moderate or intense low-oxygen dilution) combustion is characterized by low emission, stable combustion, and low noise for various kinds of fuel. This paper reports a numerical investigation of the effect of different nozzle configurations, such as nozzle number N, reactants jet velocity V, premixed and nonpremixed modes, on the characteristics of MILD combustion applied to one F class gas turbine combustor. An operating point is selected considering the pressure p = 1.63 MPa, heat intensity Pintensity = 20.5 MW/m3 atm, air preheated temperature Ta = 723 K, equivalence ratio φ = 0.625. Methane (CH4) is adopted as the fuel for combustion. Results show that low-temperature zone shrinks while the peak temperature rises as the nozzle number increases. Higher jet velocity will lead to larger recirculation ratio and the reaction time will be prolonged consequently. It is helpful to keep high combustion efficiency but can increase the NO emission obviously. It is also found that N = 12 and V = 110 m/s may be the best combination of configuration and operating point. The premixed combustion mode will achieve more uniform reaction zone, lower peak temperature, and pollutant emissions compared with the nonpremixed mode.

An analysis of the structure of an n-dodecane spray flame using TPDF modelling

With a view to understanding ignition and combustion behaviours in diesel engines, this study investigates several aspects of ignition and combustion of an n-dodecane spray in a high pressure, high temperature chamber, known as Spray A, using data resulting from modelling using the transported probability density function (TPDF) method. The model has been validated comprehensively with good to excellent agreement in our previous work against all available experimental data including for mixture-fraction and velocity fields in non-reacting cases, and flame lift-off length and ignition delay in reacting cases. This good agreement encourages further investigation of the numerical model results to help understand the structure of this flame, which serves to complement the experimental information that is available, which is very limited due to the difficult experimental conditions in which this flame exists. For example, quantitative experimental measurements of local mixture-fraction, temperature, velocity gradients, etc. are not yet possible in reacting cases. Analysis of the model results shows that two-stage ignition is found to occur across the ambient temperature conditions considered: the first stage is rapidly initiated on the lean side where temperatures are high and sequentially moves to richer, cooler conditions. The first stage is extremely resilient to turbulence, occurring in a region of very low Damköhler number. The second stage of ignition occurs first in rich mixtures in a region behind the head of the fuel jet where mixture gradients are low, and appears to be influenced strongly by turbulence. Relative to a homogeneous reactor, it is delayed on the lean side but advanced on the rich side, suggesting entrainment and mixing from the early igniting lean regions into richer mixtures is an important moderator of the ignition process. The second-stage ignition front propagates at very high velocities initially, suggesting it is a sequential ignition moving according to gradients of ignition delay and/or residence time. The flame stabilises however on the lean side in a region of much lower velocity, where turbulent velocity fluctuations are sufficiently high such that turbulent transport influences the propagation. It stabilises in a region of low Damköhler number which implies that a competition of chemistry versus micro-mixing might also be involved in stabilisation. The stabilisation mechanism is investigated by an analysis of the transport budgets, showing the flame is stabilised by autoignition but moderated by turbulent diffusion. Further analysis of the flame index supports this stabilisation mechanism, and demonstrates the simultaneous existence of non-premixed and premixed combustion modes in the same flame. Analysis of the flow fields also reveals that local entrainment and dilatation are important flow features near the flame base.

Combustion Mode Design with High Efficiency and Low Emissions Controlled by Mixtures Stratification and Fuel Reactivity

This paper presents a review on the combustion mode design with high efficiency and low emissions controlled by fuel reactivity and mixture stratification that have been conducted in the authors’ group, including the charge reactivity controlled homogeneous charge compression ignition (HCCI) combustion, stratification controlled premixed charge compression ignition (PCCI) combustion, and dual-fuel combustion concepts controlled by both fuel reactivity and mixture stratification. The review starts with the charge reactivity controlled HCCI combustion, and the works on HCCI fuelled with both high cetane number fuels, such as DME and n-heptane, and high octane number fuels, such as methanol, natural gas, gasoline and mixtures of gasoline/alcohols, are reviewed and discussed. Since single fuel cannot meet the reactivity requirements under different loads to control the combustion process, the studies related to concentration stratification and dual-fuel charge reactivity controlled HCCI combustion are then presented, which have been shown to have the potential to achieve effective combustion control. The efforts of using both mixture and thermal stratifications to achieve the auto-ignition and combustion control are also discussed. Thereafter, both charge reactivity and mixture stratification are then applied to control the combustion process. The potential and capability of thermal-atmosphere controlled compound combustion mode and dual-fuel reactivity controlled compression ignition (RCCI)/highly premixed charge combustion (HPCC) mode to achieve clean and high efficiency combustion are then presented and discussed. Based on these results and discussions, combustion mode design with high efficiency and low emissions controlled by fuel reactivity and mixtures stratification in the whole operating range is proposed.

Fuel-efficient exhaust thermal management using cylinder throttling via intake valve closing timing modulation

Most diesel engines meet today’s strict NOx and particulate matter emission regulations using after-treatment systems. A major drawback of these after-treatment systems is that they are efficient in reducing emissions only when their catalyst temperature is within a certain range (typically between 250 °C and 450 °C). At lower engine loads this is a major problem as the exhaust temperatures are usually below 250 °C. The primary objective of this study was to analyze “cylinder throttling” via both delayed and advanced intake valve closure timing. The effect of cylinder throttling on exhaust gas temperatures, fuel consumption, in-cylinder combustion and emissions is outlined. A significant increase in turbine outlet temperature accompanied by a decrease in fuel consumption, NOx, and particulate matter emissions was observed. Both delayed and advanced intake valve closure timings were equally effective. The increase in exhaust gas temperatures was attributed to a drop in air flow through the engine, which resulted from a reduction in the volumetric efficiency via cylinder throttling. The increase in fuel efficiency resulted from a decrease in the pumping work through a reduction in air flow through the engine. Reductions in NOx are attributed to the combined effect of a lower in-cylinder temperature due to a reduction in piston-motion-induced compression and a shift to a more premixed combustion mode. Particulate matter emissions were also reduced as a result of additional premixing. At the 1200 RPM and 2.5 bar brake mean effective pressure (BMEP) operating point, both delayed and advanced intake valve closure timings resulted in a turbine outlet temperature increase from 195 °C to 255 °C accompanied by an increase in brake thermal efficiency of 1.5% (absolute) and a reduction in brake-specific NOx and particulate matter emissions by 40% and 30%, respectively.

Numerical investigation to the dual-fuel spray combustion process in an ethanol direct injection plus gasoline port injection (EDI + GPI) engine

Ethanol direct injection plus gasoline port injection (EDI+GPI) is a new technology to make the use of ethanol fuel more effective and efficient in spark ignition engines. Multi-dimensional computational fluid dynamics modelling was conducted on an EDI+GPI engine in both single and dual fuelled conditions. The in-cylinder flow field was solved in the realizable k−ε turbulence model with detailed engine geometry. The temporal and spatial distributions of the liquid and vapour fuels were simulated with the spray breakup and evaporation models. The combustion process was modelled with the partially premixed combustion concept in which both mixture fraction and progress variable were solved. The three-dimensional and five-dimensional presumed Probability Density Function (PDF) look-up tables were used to model the single-fraction-mixture and two-fraction-mixture turbulence–chemistry interactions respectively. The model was verified by comparing the numerical and experimental results of spray pattern and cylinder pressure. The simulation results showed that the combustion process of EDI+GPI dual-fuelled condition was partially premixed combustion because of the low evaporation rate of ethanol spray in low temperature environment before combustion. Compared with GPI only, the higher flame speed of ethanol fuel contributed to the greater pressure rise rate and maximum cylinder pressure in EDI+GPI condition, which consequently resulted in higher power output and thermal efficiency. The lower adiabatic flame temperature of ethanol, partially premixed combustion mode and stronger cooling effect of ethanol direct injection in EDI+GPI led to the reduced combustion temperature which contributed to the decrease of NO emission. Among these three factors, the lower adiabatic flame temperature and partially premixed combustion mode were the dominating factors that resulted in the low combustion temperature of EDI+GPI. On the other hand, CO and HC emissions increased because of the ethanol’s low evaporation rate in low temperature environment before combustion, which caused incomplete combustion.

Effect of Premixed Fuel Preparation for Partially Premixed Combustion With a Low Octane Gasoline on a Light-Duty Multicylinder Compression Ignition Engine

Gasoline compression ignition (GCI) concepts with the majority of the fuel being introduced early in the cycle are known as partially premixed combustion (PPC). Previous research on single- and multicylinder engines has shown that PPC has the potential for high thermal efficiency with low NOx and soot emissions. A variety of fuel injection strategies have been proposed in the literature. These injection strategies aim to create a partially stratified charge to simultaneously reduce NOx and soot emissions while maintaining some level of control over the combustion process through the fuel delivery system. The impact of the direct injection (DI) strategy to create a premixed charge of fuel and air has not previously been explored, and its impact on engine efficiency and emissions is not well understood. This paper explores the effect of sweeping the direct injected pilot timing from −91 deg to −324 deg ATDC, which is just after the exhaust valve closes (EVCs) for the engine used in this study. During the sweep, the pilot injection consistently contained 65% of the total fuel (based on command duration ratio), and the main injection timing was adjusted slightly to maintain combustion phasing near top dead center. A modern four cylinder, 1.9 l diesel engine with a variable geometry turbocharger (VGT), high pressure common rail injection system, wide included angle injectors, and variable swirl actuations was used in this study. The pistons were modified to an open bowl configuration suitable for highly premixed combustion modes. The stock diesel injection system was unmodified, and the gasoline fuel was doped with a lubricity additive to protect the high pressure fuel pump and the injectors. The study was conducted at a fixed speed/load condition of 2000 rpm and 4.0 bar brake mean effective pressure (BMEP). The pilot injection timing sweep was conducted at different intake manifold pressures, swirl levels, and fuel injection pressures. The gasoline used in this study has relatively high fuel reactivity with a research octane number of 68. The results of this experimental campaign indicate that the highest brake thermal efficiency (BTE) and lowest emissions are achieved simultaneously with the earliest pilot injection timings (i.e., during the intake stroke).