Compression Ignition Conditions Research Articles

Towards Direct Numerical Simulations of Reactivity-Controlled Compression Ignition Engine Using n-Octanol/Ethanol Fuel Blends

AbstractThe results of a two-dimensional direct numerical simulation of a lean n-octanol-ethanol fuel blend under Reactivity Controlled Compression Ignition (RCCI) conditions are presented in this paper. Stratified temperature and high reactivity fuel fields of Gaussian, bi-modal, and log-normal distributions are studied for uncorrelated and correlated scenarios. The pimple loop is made to run twice to achieve compression heating. A chemical mechanism with 171 species and 734 reactions was developed to capture autoignition characteristics and flame propagation reasonably well. Diagnosing techniques published in the literature are used to determine whether the flame fronts are spontaneously propagating or not. As reported previously for other fuel blends under RCCI conditions, both deflagration and spontaneous ignition flame fronts are observed to co-exist. Gaussian, bi-modal, and log-normal fields respectively move towards a spontaneously igniting mode. Correlating temperature and high reactivity fuel fields not only makes combustion more spontaneously igniting but also more premixed. The analysis reveals the sensitivity of the DNS results with respect to the initial conditions which accordingly should be chosen with care.

Examining diesel-spray assisted ignition of ammonia under reactivity-controlled conditions using large-eddy simulations

Ammonia (NH3) has gained increasing attention as a promising carbon-free fuel for compression ignition engines. Nonetheless, its poor combustion characteristics and elevated nitrogen oxides (NOx) emissions present substantial obstacles. In the present study, we examine the utility of incorporating NH3 as a low-reactivity fuel (LRF) in diesel-assisted dual-fuel combustion under Reactivity Controlled Compression Ignition (RCCI) conditions. Three large-eddy simulations (LES) are performed to quantify the effect of varying concentrations of NH3 as LRF on the ignition characteristics and flame structure. The computational setup corresponds to the Engine Combustion Network (ECN) Spray A configuration, which provides the baseline for the present analysis. The ignition of the dodecane spray is found to be delayed by the presence of NH3, which increases with increasing NH3 content in the ambient. Local flamelets are extracted to examine the evolution of the flame structure starting from ignition at richer mixtures through low-temperature chemistry of dodecane, to finally stabilizing at the stoichiometric conditions. Near ignition, NH3 oxidation is observed to follow the autoignition behavior of the most reactive mixture fraction, whereas at post-ignition the behavior shifts towards canonical premixed flame propagation. This study shows that using NH3 as LRF under RCCI conditions offers an effective solution for NH3 operation in CI engines to reduce carbon emissions.

Effects of energy-share and ambient oxygen concentration on hydrogen-diesel dual-fuel direct-injection (H2DDI) combustion in compression-ignition conditions

This study investigates the ignition and combustion characteristics of interacting hydrogen (H2) and diesel surrogate jets under simulated compression-ignition engine conditions. The experimental setup includes two converging single-hole injectors in an optically accessible constant-volume combustion chamber (CVCC). The parameters varied in the study are fuel injection durations and ambient O2 concentrations (10 to 21 vol.%). The results show that a longer interaction between the diesel products and the H2 jet is required to achieve ignition of the H2 jet at lower O2 concentrations. Once ignited, the flame stabilises near or at the nozzle, except under the lowest ambient O2 condition of 10 vol.% where a lifted flame is observed. The lift-off response, however, is influenced by the relative injection duration of the fuels, with the interaction between the incoming H2 jet and the diesel combustion recession products possibly playing a role. The interaction between the jets also affects the recorded intensity and the distribution of the diesel fuel jet soot zone.

Performance and sensitivity analysis of raw biogas combustion under homogenous charge compression ignition conditions

This study aims to investigate the feasibility of using raw biogas as the main fuel in homogeneous charge compression ignition (HCCI) engines and to identify the effects of different trace elements found in biogas on engine performance and emissions. This study was conducted using many models that simulate the operation of HCCI process. These models include the chemical kinetics, combustion model, and the heat transfer models. Raw biogas composition was selected based on the most available traces. The traces that were studied are Hydrogen Sulfide (H2S), Ammonia (NH3), Carbon monoxide (CO), Nitrogen (N2), Oxygen (O2), and Water vapor (H2O). Other parameters like the equivalence ratio, intake temperature, and pressure were also studied. It was found that raw biogas mixtures could produce higher power outputs in the presence of H2S and NH3. However, H2S and NH3 were found to increase the SOx and NOx percentages owing to their higher in-cylinder temperatures. Other traces such as CO and H2O can reduce the power output, but at the same time, they can reduce the engine NOx emissions but not the CO emissions. Engine performance was positively affected by increasing the intake pressure and equivalence ratio within the studied ranges. However, higher intake temperatures and reduce the engine efficiency and increase the engine emissions. On the other hand increasing the compression ratio (CR) increase the system efficiency but up to a certain limit. Considering the exhaust gas residual (EGR), this parameter reduce system performance but at the same time lower the NOx emissions significantly. A sensitivity analysis for the mentioned parameters indicates that EGR is the most effective parameter while CR are the least significant one.

The contribution of intermediate-temperature heat release to octane sensitivity

Carbon footprint reduction can be achieved by increasing the efficiency of combustion engines. For spark-ignition (SI) internal combustion engines (ICE), fuel octane sensitivity (OS)—defined as the difference between a fuel’s research octane number (RON) and motor octane number (MON)—can help identify ideal fuels for avoiding unwanted knocking conditions. Moreover, it will help to extend the range of operating conditions that can be used in next-generation high-efficiency engines. Presently, there is a controversy over whether high or low OS fuels are beneficial for increased efficiency of next-generation engines depending on application and technology, e.g. high OS fuels are advantageous for down-sized and turbocharged spark-ignition (SI) engines, whereas low OS fuels are superior in novel systems such as pre-chamber engines. This work aims to improve the understanding of the impact of OS on the combustion performance of engines by analyzing the contribution of intermediate-temperature heat release (ITHR) to OS. The heat release of several mixtures of toluene, iso-octane, and n-heptane with similar RON but varying OS were studied under homogeneous charge compression ignition (HCCI) conditions in a cooperative fuel research (CFR) engine. This method gives insight into the heat release of the auto-igniting fuel without the obfuscation of the spark-ignited flame. At RON conditions, ITHR was a marker of OS. Furthermore, a new parameter called ITHR sensitivity was defined as the difference between the ITHR measured at RON conditions and the ITHR measured at MON conditions. ITHR sensitivity decreased with increasing octane sensitivity. This shows that non-sensitive fuels experience a greater change in the amount of ITHR as compared to sensitive fuels when changing from MON conditions to RON conditions. Thus, for engines requiring high OS fuels, it is recommended that components with high ITHR sensitivity are used and vice-versa for engines requiring low OS fuels.

Experimental investigation on reactivity-controlled compression ignition (RCCI) combustion characteristic of n-heptane/ammonia based on an optical engine

Ammonia as a carbon-free fuel used in IC engines always suffers from large cyclic variations and high NOx emissions. Ammonia blending with high-reactivity fuels is recognized as an effective attempt to overcome these shortcomings. In this work, for the first time, the combustion and emission characteristics of n-heptane/ammonia were investigated under reactivity-controlled compression ignition (RCCI) conditions in an optical engine. An intake adapter was introduced to modify turbulent flow motion and n-heptane was directly injected to regulate mixture reactivity based on energy ratios. Particle image velocimetry (PIV) was employed for in-cylinder flow measurement, high-speed photography and instantaneous pressure acquisition were used for combustion evolutions, and Fourier transform infrared (FTIR) spectroscopy was adopted for NOx and NH3 emissions. The results show both mixture reactivity and turbulence have significant impacts on ammonia combustion. Specifically, n-heptane addition manifesting mixture reactivity dominates combustion processes at early-injection conditions, while the turbulence with increasing intensity becomes significant at late-injection conditions. Visualization images show that the initiation of initial flame kernels is controlled by the local distribution of n-heptane fraction, and that high n-heptane fraction at early injection or low n-heptane fraction at late injection conditions tends to induce homogeneous combustion. Flame stretch analysis confirms the promotion of mixture reactivity and turbulence motion on turbulent flame propagation and identifies that a high n-heptane fraction with swirl-tumble motion exhibits lower flame stretch sensitivity, especially for late-injection scenarios. Besides, low mixture reactivity with early injection and high mixture reactivity with late injection strategies can achieve high-efficiency combustion. However, when considering NOX/NH3 emissions, a high n-heptane fraction at late injection with swirl-tumble motion has the most potential to achieve the compromise between thermal efficiency and emissions. The present work demonstrates that with optimized fuel injection and flow arrangement, ammonia blending with high-reactivity fuels can achieve high-efficient combustion while maintaining NOx/NH3 emissions within reasonable levels.

Effects of biohydrogen on premixed charge compression ignition engine operating at various inlet air temperatures

The usage of the diesel engine has been increasing every year and simultaneously the emissions by the fossil fuels are mounting up and damaging the environment. Hence, it is of profound interest to seek the solution to reduce the emissions released by fossil fuels. In this proposed study, the biohydrogen influence on the diesel engine has been examined. Since biohydrogen is a green gas, it reduces the emissions of certain pollutants significantly. Hence, here the diesel engine is tested with 10% and 30% of biohydrogen flow through air intake. Two different combinations such as DH10 (10% Biohydrogen and 90% diesel) and DH30 (30% biohydrogen and 70% diesel) were tested at different mean effective pressure (MEP). The MEP has been varied from 1.05 bars to 4.2 bars. Since the engine is operating on premixed charge compression ignition conditions, inclusion of the biohydrogen is challenging. To avoid biohydrogen backfire flame arrestor has been attached between the cylinder and flowmeter. The crucial parameters such as brake thermal efficiency, brake specific fuel consumption, heat release rate, cylinder pressure, combustion timing, and emissions data were collected and analyzed to understand the sustainability of the biohydrogen gas running on the diesel engine. Based on findings it is clear, adding the biohydrogen gas to the engine increases the BTE and HRR. Further the cylinder pressure values were also increased due to the usage of compressed gas. Nevertheless, the brake specific fuel consumption has been at higher levels due to the energy discrepancy. With regards to the emission, CO, CO2and NOx concentration in the exhaust tailpipe has decreased marginally, which is pleasing. Comparing DH10 and DH30, DH30 outperforms with regard to emission data. On the other hand, the performance values are superior for DH10 than DH30.

Understanding the effects of doping a regular E10 gasoline with EHN in an HCCI engine: Experimental and numerical study

In this study, the effects of doping a regular E10 gasoline with 2-ethlyhexyl nitrate (EHN) are investigated under homogeneous charge compression ignition conditions. Experiments are performed in a 1-liter single-cylinder engine fueled with both straight and EHN-doped E10 gasoline. Numerical studies are performed with an internal combustion engine single-zone reactor utilizing detailed chemical kinetic mechanism with EHN and NOx chemistry and a surrogate fuel. The kinetic model reproduces the experimental data well for the straight fuel at high initial temperature (TBDC) conditions, whereas the low temperature heat release (LTHR) is under-predicted. Adding EHN reduces the required TBDC, while EHN thermal decomposition rate has to be significantly reduced to accurately reproduce the experimental result, by preventing over-estimation of EHN effect and LTHR. EHN decomposition generates NO2 and 3-heptyl radicals. Using the well-matched mechanism, the numerical results indicate that among the product of EHN decomposition, NO2 decreases the autoignition reactivity whereas the production of 3-heptyl radical is the main source for enhancing the low-to-intermediate chemistry by which OH production is accelerated. The production of 3-heptyl radical is highly sensitive to the EHN decomposition reactions. Despite the reactivity enhancement, increase in NOx emission is observed when the fuel-doping increases.

Numerical analysis of soot emissions from gasoline-ethanol and gasoline-butanol blends under gasoline compression ignition conditions

In the present work, computational fluid dynamics (CFD) simulations of a single-cylinder gasoline compression ignition (GCI) engine were performed to investigate the impact of blending two biofuels, ethanol and n-butanol, with gasoline on the trade-off between combustion phasing and soot emissions under low load conditions. In order to represent market gasoline (RD5-87), a four-component toluene primary reference fuel (TPRF) + ethanol (ETPRF) surrogate (with 20% ethanol by mole; E20) was formulated using a neural network based octane predictor such that the surrogate had the same ethanol content, Research Octane Number (RON) and Octane Sensitivity (S). In addition, a novel skeletal kinetic mechanism for ETPRF and TPRF + n-butanol (BTPRF) blends, incorporating polycyclic aromatic hydrocarbon (PAH) chemistry, was developed. A three-dimensional (3D) engine CFD formulation employing the skeletal mechanism, adaptive mesh refinement (AMR), finite-rate chemistry approach, and hybrid method of moments (HMOM) was adopted to capture the in-cylinder combustion phenomena and soot emissions. The engine CFD model was validated against RD5-87 experimental data for a broad range of start-of-injection (SOI) timings (-21/-27/-36/-45 crank angle degrees (CAD) after top-dead-center (aTDC)), with respect to in-cylinder pressure, heat release rate, combustion phasing, and soot emissions. The closed-cycle simulation results were analyzed to elucidate the non-monotonic trend of soot emissions versus SOI timing: SOI-36 > SOI-45 > SOI-21 > SOI-27. Thereafter, the validated CFD model was employed to simulate the combustion of a gasoline-ethanol blend with 45% (by mole) ethanol (E45) and a gasoline-butanol blend with 45% (by mole) n-butanol (B45) under the same operating conditions to study the effects of fuel composition and SOI timing on combustion phasing and soot emissions. The sooting propensity followed the trend: B45 > E20 > E45 at all SOI timings. Overall, it was observed that the autoignition propensity was primarily related to fuel chemistry. On the other hand, sooting propensity showed strong coupling with both fuel chemistry and physical properties, with greater impact of fuel physical properties at advanced SOI timings.

Large-eddy simulation of split injection strategies in RCCI conditions

In this study, we investigate the effect of different split injection strategies on ignition delay time (IDT) and heat release rate (HRR) characteristics in Reactivity Controlled Compression Ignition conditions via large-eddy simulation and finite-rate chemistry. A diesel surrogate (n-dodecane) is injected into a domain with premixed methane and oxidiser in two separate injection pulses. Three different split injection strategies are investigated by fixing the amount of total fuel mass: varying the first injection timing, varying the second injection timing, and changing the fuel mass ratio between the two injections at a fixed injection timing. A compression heating mass source term approach is utilised to take compression heating into account. The main findings of the study are as follows: (1) In general, the IDT shifts towards the top-dead centre when the first injection is advanced or the second injection is retarded. The size and spatial pattern of the ignition kernels are shown to depend on the dwell time between the injections. (2) A precisely timed first injection offered the best control over ignition and HRR characteristics. However, advancing the first injection may lead to over-dilution downstream, preventing volumetric ignition and reducing the peak HRR value. (3) Approximately 21% decrease in the maximum HRR value, as well as a factor of 2.8 increase in combustion duration could be achieved by advancing the first injection timing. (4) As indicated by frozen-flow chemistry analysis, in the investigated configurations, the reactivity stratification is controlled by mixture stratification rather than temperature. The findings indicate that the first injection controls the downstream reactivity stratification, offering ignition and HRR control.

Effects of stratification and charge cooling on combustion in a gasoline direct-injection compression ignition (GDCI) engine

With the development of low temperature engine combustion strategies, performance of gasoline-type fuels under compression ignition conditions has attracted extensive research interest. Meanwhile, for the sake of co-optimization of engines and fuels for future ground transportation, identification and evaluation of general fuel properties should be a core research priority instead of endless testing of specific fuels. In this study, the roles of fuel octane sensitivity in characterizing the ignition performance of gasoline surrogates have been systematically investigated under typical gasoline direct ignition compression ignition (GDCI) engine conditions using 3D combustion CFD simulation, especially considering the subsequent in-cylinder charge stratification and charge cooling. Two different operating conditions, high boost pressure low boost temperature (beyond-RON) case and low boost pressure high boost temperature (beyond-MON) case, were considered. By comparing with our previous zero-dimensional chemical kinetic study of gasoline surrogates in advanced compression ignition (ACI) engines, the effects of stratification and charge cooling on the combustion processes are investigated. It is found that different fuel octane sensitivities lead to slight difference in equivalence ratio stratification and charge cooling due to differences in volatility. However, fuel reactivity is still the more dominant factor than the stratification and charge cooling effects in determining combustion phasing. The present results help to justify the P-T domain framework for engine autoignition analysis of overlapping pressure-temperature trajectory with ignition delay iso-contour. The results also provide useful guidance to the understanding of GCI combustion process, and to the evaluation of controlling fuel properties and the selection of alternative fuels in GCI engines

Numerical investigation of glycerol/diesel emulsion combustion in compression ignition conditions using Stochastic Reactor Model

In this work, we numerically investigate the potential of glycerol as a diesel fuel additive to reduce soot emissions. Many different fuel additives offer environmental benefits, since there is a surplus supply of glycerol as a by-product of biodiesel production, it is considered as a promising available candidate. The combustion process of glycerol in a direct injection compression ignition engine has been previously investigated experimentally for a 10 % (vol) glycerol emulsion. Based on the experimental set-up, a zero-dimensional Stochastic Reactor Model (SRM) is constructed to conduct numerical analysis with a detailed chemical mechanism for the combustion of hydrocarbon and oxygenated fuels. Applied models are presented and discussed with a focus on the characteristic mixing time scale of micromixing closure which is calculated using a 0D k-ε. The model constants are optimized to the analysed engine with the aid of a genetic algorithm and the sensitivity analysis is presented. For both fuels, emissions of major species are reproduced well but NOx results are slightly overestimated. Emissions of carbon monoxide are predicted well for the reference diesel fuel, but the SRM does not capture the trend of increased CO with glycerol emulsion. The stochastic nature of the model is used to represent the mixture inhomogeneity, and the results are analysed using equivalence ratio–temperature plots to identify soot promoting conditions. The evolution of selected soot precursor and oxidizers is analysed, showing the capability of glycerol to reduce the former and increase the latter with a moderate effect. Soot formation and decomposition rates are presented, with very similar behaviour for both fuels. A shorter combustion phasing for the glycerol emulsion was observed and identified as the main contributory factor that led to the soot reduction while the chemical effect i.e oxygen content of the glycerol was present but not the dominant factor. The soot model results do not provide any evidence for the origin of experimentally observed small-sized particles, but it is discussed that the emissions results indicate the presence of low volatile compounds that could condense in the exhaust system.

Validation of Kinetic Mechanisms against Various Ignition Delay Data and the Development of Ignition Delay Correlations for Ethanol, Natural Gas, and Primary Reference Fuel Blends under Homogeneous Charge Compression Ignition Conditions

Validation of Kinetic Mechanisms against Various Ignition Delay Data and the Development of Ignition Delay Correlations for Ethanol, Natural Gas, and Primary Reference Fuel Blends under Homogeneous Charge Compression Ignition Conditions

Development and validation of a n‐butanol reduced chemical kinetic mechanism under engine relevant conditions

AbstractA n‐butanol reduced mechanism (60 species and 306 reactions) was developed using the direct relation graph with error propagation and isomer lumping methods. Sensitivity analysis (SA) was then performed to identify reactions that were sensitive to the ignition delay (ID) for the adjustments of the pre‐exponential factor to replicate the experimental ID times. As compared with the experimental measurements at initial temperatures of 343–1350 K, initial pressures of 0.02–80 bar, and equivalence ratios of 0.5–2.0, the maximum deviation for the predictions by the n‐butanol reduced mechanism was at 37.75% for the ID times, 8 cm/s for the laminar flame speed, and a factor of three for both the species concentration profiles under jet‐stirred reactor and premixed laminar flame. Furthermore, a maximum deviation of 2.5 crank angle degrees in combustion phasing was obtained when the reduced mechanism was benchmarked against the detailed mechanism under homogeneous charge compression ignition (HCCI) conditions at initial temperature of 413 K, initial pressure of 1 bar, and equivalence ratios of 0.5–2.0. Despite these deviations, the low temperature IDs and the laminar flame speed predicted by the present reduced mechanism were in better agreement to the experimental measurements than the existing n‐butanol reduced mechanisms of comparable sizes. Hence, this collectively indicates that improvements were made to the present reduced mechanism, particularly for the low‐temperature reactions. The n‐butanol reduced mechanism was also compared with gasoline and diesel surrogates under 0D HCCI simulations. The results indicate that the combustion characteristics of n‐butanol is closer to gasoline than to diesel. This suggests that n‐butanol could safely replace gasoline fuel in neat form while it is more suitable to be blended with diesel fuel to maintain engine performance and prevent any adverse effects on the engine.

High Temperature HCCI Critical Compression Ratio of the C1-C4 Alcohol Fuels

<div class="section abstract"><div class="htmlview paragraph">In this work, a high temperature (HT) homogeneous charge compression ignition (HCCI) critical compression ratio (cCR) was defined as the compression ratio which resulted in HCCI combustion with a crank angle location of 50% fuel burned (CA50) of 3.0 degrees after top dead center (aTDC) while operating at an equivalence ratio of 0.33 (λ = 3), an intake pressure of 1.0 bar (naturally aspirated), an intake temperature of 473 K (200°C), and an engine speed of 600 rpm. Using a Cooperative Fuel Research engine, the HT HCCI cCR of seven alcohol fuels were experimentally determined and found to be ordered as follows (ordered from least reactive to most reactive): isopropanol > sec-butanol > methanol ≈ ethanol ≈ n-propanol ≈ isobutanol > n-butanol. The HT HCCI cCR for the alcohol fuels correlated well with experimental HCCI data from a modern gasoline direct injection (GDI) engine architecture with a pent-roof head and a rebreathe valvetrain. The HT HCCI cCR was also correlated with available cetane number (CN), motor octane number (MON), and research octane number (RON) values found in the literature. The coefficient of determination (R<sup>2</sup>) for these correlations was 0.02, 0.95, and 0.69, respectively. When the RON values were modified to account for the high cooling potential of the alcohol fuels, the R<sup>2</sup> increased from 0.69 to 0.96. When a wider array of other fuels were considered, including pure components of different fuel classes, pure-component fuel blends, and gasoline fuel blends, the critical compression ratio no longer correlated well with MON or HOV-modified RON. It was concluded that when fuels of a similar fuel class are considered, such as neat C1-C4 alcohols, MON or an HOV-modified RON may be sufficient to compare the HCCI reactivity of the fuels at MON-like cylinder conditions. However, due to the complex nature of the octane number tests, the HCCI ignition propensity is not well predicted by the standard octane numbers when fuels of varying chemical characteristics and combustion chemistry attributes are considered. Therefore, a metric for characterizing gasoline-like fuels under compression ignition conditions is required, such as an HCCI test method on the CFR octane rating engine.</div></div>

Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study

The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, ELchemrel, correlates strongly with the heat release fraction by deflagration. The maximum ELchemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H2O2 loop reactions, the reactions of the H2–O2 mechanism, and the conversion reaction of CO to CO2, CO+OH=CO2+H, are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases (∼1400 K), resulting in a net reduction in exergy loss.

Spray and Combustion Characteristics of Gasoline-like Fuel under Compression-Ignition Conditions

An experimental study was performed to assess the combustion characteristics of iso-octane (a gasoline surrogate) at compression-ignition (CI) conditions. The fuel was injected into a quiescent-ste...

Engine efficiency enhancement and operation range extension by argon power cycle using natural gas

Argon power cycle engine has been proved as a promising technology to increase engine efficiency and to reduce engine out emissions. In this research work, experiments were conducted in a port injection single-cylinder engine with air and argon/oxygen as the working fluids under spark ignition, homogeneous charge compression ignition, and spark assisted compression ignition conditions. Natural gas was used as fuel. Experimental results indicate that argon power cycle increases thermal efficiency, extends lean burn limit, suppresses oxides of nitrogen and unburned hydrocarbon emissions, compared to conventional air cycle. The maximum thermal efficiency of argon power cycle reaches 41.5% under low equivalence ratios and high compression ratio conditions. Given a fixed oxygen content, the thermal efficiency increases with decreasing equivalence ratio until the lean burn limit. With the decrease of oxygen content from 21% to 7.5%, the thermal efficiency of argon power cycle first increases and then drops with the peak occurring at ~15%. In practical applications, the engine load of argon power cycle can be controlled by changing equivalence ratio, oxygen content, or both. With argon power cycle, the use of spark assisted compression ignition can further increase the combustion stability, thermal efficiency, and decrease unburned hydrocarbon emission. Combined with an optimal spark timing, the engine cycle’s coefficient of variation of the indicated mean effective pressure decreases up to 84% and the thermal efficiency increases up to 2%, compared to homogeneous charge compression ignition.

Numerical Investigation of the Free and Ducted Fuel Injections under Compression Ignition Conditions

A ducted fuel injection (DFI) strategy has been proposed as an efficient approach to reduce the soot emission in direct-injection compression ignition engines. By injecting the fuel through a small...

An experimental and numerical investigation to characterize the low-temperature heat release in stoichiometric and lean combustion

This work reports on an experimental and modeling study on the low-temperature heat release (LTHR) characteristics for three RON 90 binary blends (n-heptane blended with isooctane, toluene and ethanol) in a Cooperative Fuel Research (CFR) engine at lean and stoichiometric conditions that are representative of homogeneous charge compression ignition (HCCI) and spark-ignition (SI) end-gas combustion conditions, respectively. An analysis of the end-gas temperature-pressure (T-P) trajectories was performed to identify the intake conditions leading to similar T-P trajectories between the two lambdas for each fuel blend. A heat release analysis was then conducted for the identified cases, where fuel-to-fuel differences in LTHR were identified and found to be sensitive to the operating condition. Simulations were conducted for these cases using a recently updated chemical kinetic model and a 0-D engine model, where good qualitative and reasonable quantitative agreements in LTHR were obtained. Sensitivity analysis was also performed directly on the rates of LTHR, to understand the controlling chemical reactions of LTHR, providing further insights into the fuel-to-fuel differences. The results demonstrate the significant promoting effect of n-heptane on LTHR rates, while inhibiting effects were seen for ethanol and toluene. Also highlighted was the importance of H-atom abstraction reactions from the chemistry of each fuel component, which could lead to contradictory fuel behavior depending on the locations of the H site of the abstraction reaction due to the different ensuing pathways for the primary fuel radicals.