Fuel Conversion Efficiency Research Articles

A 3D CFD-FEA co-simulation study of low thermal effusivity TBCs applied to the piston and valves of an SI engine

When applied on combustion chamber walls, thermal barrier coatings (TBCs) with low thermal effusivity provide a pathway for reducing heat transfer and improving SI engine efficiency. A 3D CFD-1D FEA co-simulation routine was employed to study the effects of a proprietary TBC on SI engine performance under different permutations of coating the piston, exhaust valves, and intake valves. Marginal reductions (<0.1% points) in total heat transfer and improvements to efficiency were observed when all the three components were coated with the proprietary TBC. Two hypothetical TBC materials with ideally low thermal effusivities were formed by modifying the current material properties and their effect on engine performance was similarly studied at three engine loads at the same engine speed. It was found that coating all the three components with the lowest thermal effusivity TBC offers the largest improvements (∼0.5% points) in net fuel conversion efficiency accompanied by largest reduction (∼1.1% points) in total heat transfer, thus establishing expectations from future TBC materials.

Effect on polytropic index, performance and emission of diesel engine using hydrogen as gaseous fuel with additive di-tert butyl peroxide

Diesel engines are commonly used due to its higher reliability and better fuel conversion efficiency. However, it produces exhaust emissions like carbon dioxide (CO2) and particulate matter (PM). Hydrogen gas is regarded as a clean fuel as it is free from carbon. The addition of hydrogen fuel enhances the burning rates and extends the flammability limits of fossil fuels, and therefore has the potential of reduced exhaust emission in diesel engines operating on dual fuel mode. This paper has investigated the effect of hydrogen addition to the performance and emissions of a single-cylinder diesel engine, working in dual fuel mode with additive di-tert butyl peroxide (DTBP). About 25% hydrogen was introduced into the cylinder on a mass basis via the engine intake manifold with the addition of additive di-tert butyl peroxide up to 5% in diesel fuel. In this experimental study, mean gas temperature, indicated mean effective pressure (IMEP), brake mean effective pressure (BMEP), polytropic index, and exhaust emissions were studied at medium (53%) and high (69%) load conditions. The indicated mean effective and brake mean effective pressure was 9.79 bar with a 3% addition of additive and 4.15 bar with the 5% addition of additive as compared with 8.71 bar and 4.11 bar diesel fuel operation. 16% hydrogen addition with 1% di-tert butyl peroxide, IMEP, and BMEP are 10.92 bar, and 4.14 bar respectively.

COMPREHENSIVE REVIEW OF INNOVATION IN PISTON ENGINE AND LOW TEMPERATURE COMBUSTION TECHNOLOGIES

Global transport today is mainly powered by the Internal Combustion Engine (ICE) and throughout its century and a half of development it has become considerably more efficient and cleaner. Future prospects of the ICE rely on the scientific work conducted today to keep this trend of higher efficiency and cleaner emissions in new engines going. The aim of this article is to give a comprehensive review of development directions in novel piston engine designs, which seek to overcome the drawbacks of the ubiquitous 4-stroke piston engine. One of the directions of development is devoted to improving the mechanisms and the general layout of the piston engine to reduce losses within the engine. Research teams working with alternative engine work cycles like the 5- and 6-stroke engine and technologies for extracting waste heat seek to reduce thermal losses while novel layouts of valve trains and crank assemblies claim to significantly improve the mechanical and Volumetric Efficiency (VE) of piston engines. These novel ideas include camless or Variable Valve Action (VVA) and engines with Variable Compression Ratio (VCR) or opposed pistons. One alternative approach could also be to totally redesign the reciprocating mechanism by replacing the piston with some other device or mechanism. Additional scientific work is investigating Low Temperature Combustion (LTC) technologies such as Turbulent Jet Ignition (TJI) and Homogeneous Charge Compression Ignition (HCCI) and its derivatives like Premixed Charge Compression Ignition (PCCI) and Reactivity Controlled Compression Ignition (RCCI) that have shown improvements in thermal and fuel conversion efficiency while also significantly reducing harmful emissions. These combustion strategies also open the path to alternative fuels. The contemporary work in the combustion engine fields of research entail technical solutions from the past that have received a modern approach or are a completely novel idea. Nonetheless, all research teams work with the common goal to make the piston engine a highly efficient and environmentally friendly device that will continue to power our transport and industry for years to come. For this, solutions must be found to overcome the mechanical limitations of the traditional layout of the piston engine. Similarly various improvements in combustion technology are needed that implement state of the art technology to improve combustion characteristics and reduce harmful emissions.

Combustion Diagnosis in a Spark-Ignition Engine Fueled with Syngas at Different CO/H2 and Diluent Ratios

The gasification of residues into syngas offers a versatile gaseous fuel that can be used to produce heat and power in various applications. However, the application of syngas in engines presents several challenges due to the changes in its composition. Such variations can significantly alter the optimal operational conditions of the engines that are fueled with syngas, resulting in combustion instability, high engine variability, and misfires. In this context, this work presents an experimental investigation conducted on a port-fuel injection spark-ignition optical research engine using three different syngas mixtures, with a particular focus on the effects of CO/H2 and diluent ratios. A comparative analysis is made against methane, considered as the baseline fuel. The in-cylinder pressure and related parameters are examined as indicators of combustion behavior. Additionally, 2D cycle-resolved digital visualization is employed to trace flame front propagation. Custom image processing techniques are applied to estimate flame speed, displacement, and morphological parameters. The engine runs at a constant speed (900 rpm) and with full throttle like stationary engine applications. The excess air–fuel ratios vary from 1.0 to 1.4 by adjusting the injection time and the spark timing according to the maximum brake torque of the baseline fuel. A thermodynamic analysis revealed notable trends in in-cylinder pressure traces, indicative of differences in combustion evolution and peak pressures among the syngas mixtures and methane. Moreover, the study quantified parameters such as the mass fraction burned, combustion stability (COVIMEP), and fuel conversion efficiency. The analysis provided insights into flame morphology, propagation speed, and distortion under varying conditions, shedding light on the influence of fuel composition and air dilution. Overall, the results contribute to advancing the understanding of syngas combustion behavior in SI engines and hold implications for optimizing engine performance and developing numerical models.

Greenhouse gas reduction in a medium-duty compression ignition engine with optimization for B20

Soy-based biodiesel can reduce well-to-wheels greenhouse gas (GHG) emissions per unit energy (i.e., gCO2e/MJ) by 66%–72% as compared to the petroleum-based diesel fuel with currently adopted agricultural and industrial practices. Biodiesel can reduce particulate matter and carbon monoxide emissions with a manageable degree of increase in NOx emissions. From the perspective of GHG emissions reduction per unit travelling distance (i.e., gCO2e/mile), the application of B20 in compression ignition engines without the adjustment in engine control unit (ECU) settings will not extract the best carbon emissions reduction that B20 could achieve. Optimizing the engine control settings permits re-calibration to achieve the maximum brake fuel conversion efficiency (BFE) based on comprehensive understanding on the impact of both “fuel” and “ECU calibration” on BFE and other criteria pollutant emissions. The maximum GHG emissions reduction with B20 application is experimentally measured with the optimized ECU calibration, thus providing the understanding of the combined impact of biodiesel fuel and calibrations on engine performance and emissions. Six steady operating modes were considered, that can be combined to estimate the US federal test procedure BFE and emissions over the Federal Test Protocol (FTP) 75 cycle. Combined with the weight factors to simulate the EPA FTP 75 cycle from these 6 “mini-map” test points, 0.53% improvement in the energy requirement per unit traveling distance (i.e., MJ/mile) is achieved for B20 with the final ECU calibration, in addition to the degree of GHG emissions reduction on a “gCO2e/MJ” basis from the use of B20 blend of soy biodiesel of ∼12.5% reduction in gCO2e/MJ, for a total GHG emissions reduction of 13%.

Numerical and experimental analysis of the combustion in a Single-Cylinder research engine with passive TJI pre-chamber operating with hydrated ethanol

Advanced ignition systems, such as turbulent jet ignition, have the capacity to improve the combustion process of internal combustion engines, increasing fuel conversion efficiency and reducing emissions, especially when applied with ethanol biofuel. Technologies that take advantage of the local energy potential, taking into account the geographic, demographic and social characteristics of each country, increase the diversity of solutions for the sustainable future of the transport sector. The present work aims to develop a system with a passive pre-chamber operating in turbulent jet ignition mode, applied to a single-cylinder research engine with a stoichiometric air–fuel mixture of ethanol. Numerical simulations were carried out and experimentally validated applying the commercial software CONVERGE CFD. The results showed that the optimized configuration presented higher fuel conversion efficiency, when compared to conventional spark ignition, thus producing greater savings per kilometer driven and lessing the dependence on fossil fuels. The pre-chambers with both lateral holes and a central hole showed a maximum NOx reduction of 5.9 g/kWh, reducing the second half of combustion by 6.2 crank angle degrees and the first half of combustion by up to 1.8 crank angle degrees compared to SI. This indicates a potential for increasing the compression ratio for pre-chamber configurations without the occurrence of knock. Furthermore, there is a direct relationship between the quenching of the jets after exiting the pre-chamber holes and turbulence, with quenching occurring when turbulent kinetic energy was greater than 15000 m2/s2.

Machine Learning-Based Modeling and Predictive Control of Combustion Phasing and Load in a Dual-Fuel Low-Temperature Combustion Engine

<div>Reactivity-controlled compression ignition (RCCI) engine is an innovative dual-fuel strategy, which uses two fuels with different reactivity and physical properties to achieve low-temperature combustion, resulting in reduced emissions of oxides of nitrogen (NO<sub>x</sub>), particulate matter, and improved fuel efficiency at part-load engine operating conditions compared to conventional diesel engines. However, RCCI operation at high loads poses challenges due to the premixed nature of RCCI combustion. Furthermore, precise controls of indicated mean effective pressure (IMEP) and CA50 combustion phasing (crank angle corresponding to 50% of cumulative heat release) are crucial for drivability, fuel conversion efficiency, and combustion stability of an RCCI engine. Real-time manipulation of fuel injection timing and premix ratio (PR) can maintain optimal combustion conditions to track the desired load and combustion phasing while keeping maximum pressure rise rate (MPRR) within acceptable limits.</div> <div>In this study, a model-based controller was developed to track CA50 and IMEP accurately while limiting MPRR below a specified threshold in an RCCI engine. The research workflow involved development of an imitative dynamic RCCI engine model using a data-driven approach, which provided reliable measured state feedback during closed-loop simulations. The model exhibited high prediction accuracy, with an <i>R</i><sup>2</sup> score exceeding 0.91 for all the features of interest. A linear parameter-varying state space (LPV-SS) model based on least squares support vector machines (LS-SVM) was developed and integrated into the model predictive controller (MPC). The controller parameters were optimized using genetic algorithm and closed-loop simulations were performed to assess the MPC’s performance. The results demonstrated the controller’s effectiveness in tracking CA50 and IMEP, with mean average errors (MAE) of 0.89 crank angle degree (CAD) and 46 kPa and Mean absolute percentage error (MAPE) of 9.7% and 7.1%, respectively, while effectively limiting MPRR below of 10 bar/CAD. This comprehensive evaluation showcased the efficacy of the model-based control approach in tracking CA50 and IMEP while constraining MPRR in the dual-fuel engine.</div>

Study of the influence of adding Al2O3 nanoparticles to biodiesel on the performance and emissions characteristics of a diesel engine

The use of biodiesel in compression ignition (CI) engines has great potential to reduce emissions and the dependence on fossil fuels. However, the complete adoption of biodiesel in CI engines is limited by several drawbacks. A promising solution to these problems is the addition of nanoparticles to biodiesel or diesel-biodiesel blends. Hence, this paper studies the impact of adding aluminum oxide (Al2O3) nanoparticles on the performance and emission characteristics of a diesel engine running on biodiesel and a mixture of diesel and biodiesel. Al2O3 nanoparticles were added at concentrations of 50, 100 and 150 ppm to biodiesel (B100) and a mixture of 50 % biodiesel and 50 % commercial diesel (B50). The results indicated an increase in fuel conversion efficiency and a reduction in specific fuel consumption. The highest fuel conversion efficiency was 26.5 % for B50 with 100 ppm of Al2O3, indicating an improvement of around 28.6 % compared to diesel fuel (B0). In general, CO emissions were reduced with the addition of nanoparticles, with a maximum reduction of 42.8 % for B100 with 50 ppm Al2O3 compared to B0. The highest reduction in NOX emissions was 30.3 % for B50 with 100 ppm Al2O3 compared to pure diesel B0.

Impact of bioenergy feedstock carbon farming on sustainable aviation fuel viability in the United States.

Biomass-derived sustainable aviation fuel holds significant potential for decarbonizing the aviation sector. Its long-term viability depends on crop choice, longevity of soil organic carbon (SOC) sequestration, and the biomass-to-biojet fuel conversion efficiency. We explored the impact of fuel price and SOC value on viable biojet fuel production scale by integrating an agroecosystem model with a field-to-biojet fuel production process model for 1,4-dimethylcyclooctane (DMCO), a representative high-performance biojet fuel molecule, from Miscanthus, sorghum, and switchgrass. Assigning monetary value to SOC sequestration results in substantially different outcomes than an increased fuel selling price. If SOC accumulation is valued at $185/ton CO2, planting Miscanthus for conversion to DMCO would be economically cost-competitive across 66% of croplands across the continental United States (US) by 2050 if conventional jet fuel remains at $0.74/L (in 2020 US dollars). Cutting the SOC sequestration value in half reduces the viable area to 54% of cropland, and eliminating any payment for SOC shrinks the viable area to 16%. If future biojet fuel prices increase to $1.24/L-Jet A-equivalent, 48 to 58% of the total cultivated land in the United States could support a more diverse set of feedstocks including Miscanthus, sorghum, or switchgrass. Among these options, only 8-14% of the area would be suitable exclusively for Miscanthus cultivation. These findings highlight the intersection of natural solutions for carbon removal and the use of deep-rooted feedstocks for biofuels and biomanufacturing. The results underscore the need to establish clear and consistent values for SOC sequestration to enable the future bioeconomy.

Characterization of Hydrotreated Vegetable Oil (HVO) in a Euro 6 Diesel Engine as a Drop-In Fuel and With a Dedicated Calibration

Renewable fuels can play an important role in achieving future goals of energy sustainability and CO2 reduction. In particular, hydrotreated vegetable oil (HVO) represents one of the most promising alternatives to petroleum-derived diesel fuels. Several studies have shown that conventional diesel engines can run on 100% HVO without significant modifications to the hardware and control strategies. The current activity has experimentally evaluated the potential of HVO as a “drop-in” fuel, i.e., without changes to the original baseline calibration, comparing it to conventional diesel fuel on a 2.3-litre Euro 6 compression ignition engine.Tests revealed that HVO can significantly reduce engine-out soot (by more than 60%), HC and CO emissions (by about 40%), compared to diesel, while NOx levels and fuel conversion efficiency remain relatively unchanged under steady-state warmed-up conditions. The advantages of HVO proved to be further enhanced when the engine has not yet warmed up.Using statistical techniques of design of experiments (DoE) at three warmed-up steady-state operating points, the main engine control parameters were recalibrated to demonstrate that engine-out emissions can be further optimized with a dedicated calibration.

Step-by-step tuning of morphology and band gap of ZnS/MoS2 photocatalyst for enhanced visible light to hydrogen fuel conversion

To address the inherent limitations of ZnS and MoS2 as visible light photocatalysts, herein, we report a step-by-step tuning of mixed sulfide composite system (ZnS/MoS2) to achieve enhanced visible light to hydrogen fuel conversion efficiency in the absence of any sacrificial agents. In the first stage, ZnS/MoS2 composite was synthesized by using a single step hydrothermal route and the photocatalytic activity of the synthesized ZnS/MoS2 composite was tuned by varying the ratio of Zn and Mo in the composite. In the second stage, the photocatalytic efficiency of the composite was further enhanced by tuning the band gap and visible light absorption characteristics of the material through cobalt doping (Co-doping) in the MoS2 co-catalyst. The hydrogen generation activity of the tuned composition (1:1.3 ratio of Zn and Mo) of the ZnS/MoS2 composite with 3% Co-doping is obtained as the best (1256 μmol of H2 in 1 h) as compared to the other compositions of the doped composites (1%, 3% and 5% Co doping). The enhanced hydrogen generation through visible light induced water splitting of the developed mixed sulfide composite systems (ZnS/MoS2 and ZnS/Co-doped MoS2) even in the absence of any sacrificial agents is ascribed to their tuned band gap and low electron-hole recombination rate.

Hydrogen addition to ethanol-fuelled engine in lean operation to improve fuel conversion efficiency and emissions

This work applies computer simulation to investigate the combined approach of hydrogen (H2) addition with lean operation to enhance the potential of ethanol as a clean engine fuel for low-carbon transportation. To this end, this research evaluates how this operation regime impacts brake power, fuel conversion efficiency, and exhaust emissions. A dedicated software was used to perform a parametric study with varying amount of hydrogen addition and excess air ratio. The software combustion and performance parameters were calibrated against available experimental data and optimised for engine operation at the tested conditions. The results show that hydrogen addition does not produce noticeable effects on brake power and fuel conversion efficiency for lean operation with up to 20% air excess. However, for operation with 40% air excess, 6% H2 addition is necessary to keep the power at the same level of the engine baseline condition while simultaneously improving fuel conversion efficiency. Operation in the range of air excess between 10% and 30% together with hydrogen addition up to 2% can simultaneously produce noticeable improvements of fuel conversion efficiency, carbon monoxide (CO) and hydrocarbon (HC) emissions, without compromising brake power and oxides of nitrogen (NOX) emissions.

A two-material thermal barrier coating spatially tailored for high-efficiency GCI combustion

Continued research into thermal barrier coatings (TBCs) for internal combustion engines has generated insights into the design of TBCs that can increase fuel conversion efficiency. In this work, two low thermal inertia TBCs (a commercially available Gen 1 material, Gadolinium Zirconate, and a proprietary Gen 2 material) were experimentally tested in gasoline compression ignition (GCI) on a light-duty single cylinder research engine. Compared to a metal baseline, a 250-micron thick Gen 1 coated piston produced an efficiency benefit of 0.4 percentage points (pp) at 6 and 10 bar net indicated mean effective pressure (IMEPn) and a 0.2 pp penalty at 15 bar IMEPn. The Gen 1 coating also showed an uHC and smoke penalty. An 80-micron thick Gen 2 coated piston showed an efficiency benefit of 0.9 pp at 6 bar IMEPn and 0.4 pp at 10 bar IMEPn, but an efficiency penalty of 0.5 pp at 15 bar IMEPn. The Gen 2 coating, which has a lower thermal inertia than the Gen 1 coating, displayed durability problems while the Gen 1 coating did not. A third piston was coated with a 120-micron thick Gen 2 coating everywhere except the high failure outer bowl region, which was coated with a 120-micron Gen 1 coating. This hybrid coating displayed good durability, an uHC emission penalty, a smoke benefit, an efficiency benefit of 0.1 pp at 6 bar IMEPn, an efficiency benefit of 0.4 pp at 10 bar IMEPn, and no change in efficiency at 15 bar IMEPn. These results indicate that a durable Gen 1/Gen 2 coating can provide an efficiency benefit to GCI combustion, and that further improvements on piston-spray interaction optimization and surface sealing of the TBC, as well as increasing the coated combustion chamber surface area (e.g. head and valves) can add further improve efficiency.

Effect of dual port-direct injection of syngas in a rotary engine

Using syngas as a primary fuel for internal combustion engines with spark ignition is a bridge technology for transitioning from carbon-based to hydrogen-based energy sources. There are two different ways of syngas delivery into combustion chamber: port injection or direct injection. In this study, the combination of these methods is considered for integration benefits of each injection technology and promotion of syngas utilization in rotary engines. In the novel concept of syngas dual injection, part of the fuel is delivered by a port injector during the induction stroke, while another part is directly injected during the compression stroke. The ratio of fuel mass delivered by port and direct injection is changed within the range of 0.0 (direct injection only) to 1.0 (port injection only) and chosen as a key parameter of concentration stratification. This study aims to investigate the optimal fuel injection strategy with numerical simulation of mixture formation, ignition, and combustion processes. It was shown that dual injection strongly influences turbulence intensity and mixture heterogeneity. The turbulence lengthscales generally increase with dual injection ratio growth. The direct injection case demonstrates maximum performance characteristics. However, the case with a small portion of directly injected fuel (10%–25%) increases fuel conversion efficiency by 5%–7% and decrease fuel consumption by 4%–6% with lower level of CO emission. The results show that the dual injection strategy provides flexibility in controlling the combustion process and emission reduction.

Enhancement of combustion performances and reduction of combustible species emission using an additional of combustion-reaction of engine

A new concept is presented to enhance the combustion performance and to reduce the combustible species gas emission of a combustion engine. Exhaust gases are still rich in typical combustible species whose chemical energy can be harvested to increase efficiency. The idea is to provide extra time to the combustion reactions by the addition of two-post-expansion-strokes to the Otto cycle. The chemical energy of the working-gas residual combustible species is converted into an additional heat-input. The new thermodynamic cycle, called the MUB-2 cycle, is similar to an Otto cycle but has two isochoric heat-inputs instead of one. The cycle has been experimentally tested and analytically evaluated. It is found that even though the engine’s thermal-conversion efficiency is not changed, however, the average values of combustion efficiency, fuel-conversion efficiency, heat-input per cycle, and indicated work per cycle are increased by 6.42 (%), while average levels of HC, CO, and H2 are reduced by 43.86 (%), 38.58 (%), and 38.58 (%), respectively. Other advantages of the MUB-2 cycle are higher heat input per cycle and more robustly work to the changes of air-fuel mixture homogeneity over the Otto cycle, the Atkinson-Miller cycle, and other cycles that only incorporate a single combustion process.

Energy, exergy and emission [3E] analysis of Mesua Ferrea seed oil biodiesel fueled diesel engine at variable injection timings

Biofuels can be used in diesel engine for power generation to address the twin problems: fossil fuel depletion and the rise of pollution. In this regard, the present work explores the possibility of enhancement of performance of Mesua Ferrea seed oil biodiesel run diesel Engine through operation at different injection timings and load settings. The investigation is carried out through energy, exergy, and emission (3E) analysis. For experimentation, a research test engine of 3.5 kW is considered. Four injection timings (20° bTDC, 23° bTDC, 25° bTDC, 28° bTDC) and five load settings (20%, 40%, 60%, 80%, 100%) were taken for the test. The results indicate that the retarded injection timing of 20° bTDC results in an improvement of fuel conversion efficiency, as well as a lowering of both exergy destruction and emissions for Mesua Ferrea seed oil biodiesel, run a diesel engine. At injection timing of 20° bTDC for 100% load, the maximum brake thermal efficiencies and maximum exergetic efficiency were found to be 26.11% and 30.32%, respectively under biodiesel mode as compared to 27.76% and 30.74%, respectively for diesel mode. For the same injection timing, the average drop in CO, CO2, and HC emissions were found to be 46.22%, 1.62%, and 39.67%, respectively whereas the average increase of NOX emission was found to be 8.08% in comparison to diesel mode.

Highly Active Interfacial Sites in SFT‐SnO2 Heterojunction Electrolyte for Enhanced Fuel Cell Performance via Engineered Energy Bands: Envisioned Theoretically and Experimentally

Extending the ionic conductivity is the pre‐requisite of electrolytes in fuel cell technology for high‐electrochemical performance. In this regard, the introduction of semiconductor‐oxide materials and the approach of heterostructure formation by modulating energy bands to enhance ionic conduction acting as an electrolyte in fuel cell‐device. Semiconductor (n‐type; SnO2) plays a key role by introducing into p‐type SrFe0.2Ti0.8O3‐δ (SFT) semiconductor perovskite materials to construct p‐n heterojunction for high ionic conductivity. Therefore, two different composites of SFT and SnO2 are constructed by gluing p‐ and n‐type SFT‐SnO2, where the optimal composition of SFT‐SnO2 (6:4) heterostructure electrolyte‐based fuel cell achieved excellent ionic conductivity 0.24 S cm−1 with power‐output of 1004 mW cm−2 and high OCV 1.12 V at a low operational temperature of 500 °C. The high power‐output and significant ionic conductivity with durable operation of 54 h are accredited to SFT‐SnO2 heterojunction formation including interfacial conduction assisted by a built‐in electric field in fuel cell device. Moreover, the fuel conversion efficiency and considerable Faradaic efficiency reveal the compatibility of SFT‐SnO2 heterostructure electrolyte and ruled‐out short‐circuiting issue. Further, the first principle calculation provides sufficient information on structure optimization and energy‐band structure modulation of SFT‐SnO2. This strategy will provide new insight into semiconductor‐based fuel cell technology to design novel electrolytes.

Development and evaluation of a skeletal mechanism for EHN additized gasoline mixtures in large Eddy simulations of HCCI combustion

Advanced Low Temperature Combustion modes, such as the Sandia proposed Additive-Mixing Fuel Injection (AMFI), can unlock significant potential to boost fuel conversion efficiency and ultimately improve the energy conversion of internal combustion engines. This is a novel improved combustion process that is enabled by supplying small (<5%) variable amounts of autoignition improver to the fuel to enhance the engine operation and control. Common, diesel-fuel ignition-quality enhancing additive, 2-ethylexyl nitrate (EHN), is doped into gasoline to enable Sandia LTGC + AMFI combustion. This manuscript focuses on the development of a reduced sub-mechanism for EHN chemical kinetics at engine relevant conditions that is implemented into a skeletal mechanism for chemical kinetic studies of gasoline surrogate fuels. The mechanism validation utilized zero-dimensional numerical simulations and comparison to shock tube ignition-delay data of pure and EHN-doped n-heptane. Additional validation is presented with Homogeneous Charge Compression-Ignition (HCCI) engine data of pure and EHN-doped research-grade E10 gasoline. Then, the mechanism was deployed in a 3-D computational fluid dynamics (CFD) using Large Eddy Simulations (LES) to model the HCCI engine experiments of 0.4% vol EHN additized E10 gasoline at several equivalence ratios. Simulations showed a very good performance of the mechanism, and the model accurately reproduced (a) the ignition point, (b) combustion phasing, (c) combustion duration, and (d) the peak of the heat release rates of the engine experiments. The results show that EHN promotes Low-Temperature Heat Release, ultimately driving the gasoline to autoignite at thermodynamic conditions where the fuel would not otherwise ignite. Overall, this work demonstrates a viable reduced chemical-kinetic mechanism for EHN and shows that it can be combined with a skeletal gasoline mechanism for CFD-LES analysis of well-mixed LTGC that matches well with experimental results. The CFD-LES analysis also shows the spatial distribution of EHN-fuel interactions that control the autoignition throughout the combustion chamber.

Ammonia energy storage for hybrid electric aircraft

This work considers the development of a hybrid electric vertical takeoff and landing aircraft with an NH3 engine. An NH3 cracker with an external heat supply from the engine and a high-temperature catalytic reactor is needed to crack part of the NH3 in H2 and N2. The engine has two turbochargers in cascade, an intercooler, and an SCR catalyst with additional NH3 injection before the catalyst to control emissions. The H2 is used to ignite an NH3 mixture which may also be enriched with hydrogen to address particularly slow-burning points. The proposed learn burn engine which features direct injection and jet ignition of H2 may have power densities above 100 kW per liter and fuel conversion efficiencies approaching 50%. This engine can be further specialized to work on a narrow range of speeds and loads, ideally, one single point, to produce electricity on board the aircraft. The energy storage per unit mass of the complete system is expected to reduce vs. diesel but not dramatically. Importantly, it is much better than the storage of energy in a battery.

Determination of a heat transfer correlation for small internal combustion engines

Small internal combustion engines (ICEs) with displacements < 100 cm3 have greater surface area-to-volume ratios that promote heat transfer through the combustion chamber walls, subsequently lowering the fuel conversion efficiency. As a result, employing common heat transfer correlations developed for larger engines to estimate wall heat transfer yields inaccurate results. Therefore, it is necessary to review literature correlations and determine their applicability for small ICEs. To this end, this paper first presents a literature review highlighting the characteristic length scales, characteristic velocities, and the properties of the working fluid for ICE convective heat transfer correlations. This information is used within a zero-dimensional ICE model to determine an appropriate heat transfer expression for a relatively small two-stroke ICE (53.2 cm3). Through parameter optimization, by matching the gross indicated power over 85 experimental data points using MATLAB’s fmincon routine, the developed expression (Nu=0.449Re3/4Pr1/3) shows good confidence with respect to the power (R2 = 0.924, RMS = 0.257) and an in-cylinder pressure data trace (R2 = 0.991, RMS = 1.52). A following parametric study varying the bore-to-stroke (BtS) ratio for small engines (0.15 cm3 to 100 cm3) finds a localized choice of the BtS and ignition timing at each displacement volume based on the minimum heat transfer due to the combination of a changing surface area and heat transfer coefficient. In the future, including the effects of volumetric efficiency and tuned exhaust pipes can enhance the scope of the proposed model.