Combustion Mode Research Articles

Effect of liquid ammonia HPDI strategies on combustion characteristics and emission formation of ammonia-diesel dual-fuel heavy-duty engines

Reducing carbon emissions from internal combustion engines has become a crucial topic due to the pressure brought about by global warming. Ammonia, a carbon-free fuel, has significant potential for heavy-duty diesel engine applications. However, the laminar flame speed of ammonia fuel is relatively slow, resulting in suboptimal combustion performance. Therefore, the present study focuses on ammonia-diesel dual-fuel heavy-duty engines. A novel liquid ammonia high-pressure direct injection (HPDI) strategy has been developed to address the challenges associated with poor combustion performance and increased unburned ammonia emissions in ammonia-diesel dual-fuel internal combustion engines. Firstly, a three-dimensional numerical simulation method was utilized to establish and verify a model of a heavy-duty internal combustion engine that incorporates HPDI of liquid ammonia. Afterward, numerical studies comparing the engine's combustion and emission characteristics are conducted with varied ammonia energy fractions, liquid ammonia injection timings, and liquid ammonia injection directions. The results indicate that, compared to the pure diesel mode, the novel liquid ammonia HPDI strategy can alter the combustion mode, significantly improving fuel–air mixture efficiency. This leads to more thorough combustion. After replacing 80% of the fuel energy input with ammonia, there is still an increase of 8.9% in indicated mean effective pressure and a 10.6% improvement in indicated thermal efficiency. Moreover, modifying the HPDI strategy reduces greenhouse gas emissions effectively, and due to the thermal de-NOx reaction, there is a notable decrease in NOx emissions. Furthermore, by altering the liquid ammonia injection timing and direction, finer control of fuel combustion and pollutant generation can be achieved, thereby reducing unburned ammonia emissions in conditions of high ammonia energy fractions.

Numerical investigation of premixed combustion of producer gas with hydrogen injection in a cyclonical chamber

ABSTRACT This paper employs computational fluid dynamics (CFD) simulations with an eddy dissipation concept model (EDC) and a skeletal reaction mechanism (SK17) to analyze the impact of hydrogen addition on the premixed combustion mode of producer gas (PG) fuel in a cyclonic flow combustion chamber. The results, validated against measured wall temperature data, demonstrate a minor discrepancy between simulation and experiment. The studied result showed that injection of hydrogen into PG fuel combustion elevates the temperature of the combustion process, which can be indicated by the average temperature across the combustion chamber, outlet temperature of exhaust gas, and reactive radical species. However, increasing H2 injection over 20% results in strongly increasing CO formations. Compared to a fixed thermal input case, hydrogen injection with a variable input power resulted in a higher combustion temperature and increased emissions of both CO and NO. The maximum CO emission reached approximately 1246 ppm, while NO peaked at around 75 ppm, both observed at a 30% hydrogen injection rate. The key finding of this study revealed that direct injection of hydrogen to the combustion under the condition of a given equivalence ratio is unfavorable due to insufficient oxygen for the reaction, leading to an increase in CO emissions. The optimal condition would be a hydrogen injection below 20%, which produces low CO and NO emissions.

Stratified premixed combustion optimization of a natural gas/biodiesel dual direct injection engine

The natural gas (NG) stratified premixed combustion mode (NSPC) demonstrates significant potential for improving thermal efficiency and lowering emissions when compared to the NG mixing-limited combustion mode (NMLC). In this study, the NSPC for a NG/biodiesel dual direct injection engine was optimized through numerical simulation. The initial evaluation of biodiesel as the pilot fuel reveals its capacity to reduce soot and CO emissions while simultaneously increasing thermal efficiency in comparison with pure diesel. The subsequent examination delved into the effects of the injection timing of biodiesel and natural gas on mixture distribution, combustion, and emissions characteristics. The findings indicates that a well-balanced combination of moderate injection timing of natural gas and slightly delayed injection timing of biodiesel results in significantly lowered CO and soot emissions while maintaining high thermal efficiency, but with a trade-off of increased NOx emissions. The engine performance was further refined through the integrated application of exhaust gas recirculation (EGR) and pilot fuel pre-injection. Achieving a high thermal efficiency of 42.5%, minimal CO emissions at 1.5 g/kWh, soot emissions below 1 × 10−4 g/kWh, and NOx emissions less than 2 g/kWh, is achieved by configuring the start of injection for natural gas and biodiesel at −56°CA ATDC and −10°CA ATDC, respectively, with a biodiesel pre-injection ratio of 30%, and an EGR rate of 36%.

Fluid and Combustion Dynamics in Axisymmetric and Perisymmetric Supersonic Combustion Flowpaths

An experimental comparison of the fluid and combustion dynamics between axisymmetric and perisymmetric ethylene-fueled supersonic flowpaths was made in a Mach 4.5 high-enthalpy flow. The perisymmetric shape can be characterized as roughly elliptical, with sharp corners at its major axis vertices. High-speed imaging of the flow luminescence and wall pressure measurements were performed simultaneously. Four different combustion modes (scram mode, scram-ram transition, ram mode, and unstart) were observed by altering the global equivalence ratio. Time-resolved measurements revealed that the mode transitions for the perisymmetric model occurred earlier than the axisymmetric model. The instantaneous data showed transient phenomena such as upstream flame propagation in the combustor, reverse flame propagation into the isolators, and the propagation of the pseudo-shock. In addition, an inlet buzz was observed for the perisymmetric model. The time-resolved high-speed images and the pressure data revealed that inlet unstart and restart repeatedly occurred during the inlet buzz. The results of a Fourier transform analysis indicated that acoustic resonance within the flowpath was the primary mechanism for the inlet buzz of the perisymmetric model.

Experimental study on the influence of the wall cavity on stability of kerosene two-phase rotating detonation combustion

The combustion characteristics in annular rotating detonation combustors are experimentally investigated using 40% oxygen-enriched air and liquid aviation kerosene (RP-3) as reactants. Two types of wall cavity structures, embedded in the outer and the inner wall of the annular combustors, respectively, are employed to study their effects on detonation stability. The characteristics of typical combustion modes observed in the annular combustors are firstly analyzed. The regime of typical combustion modes is then examined in the phase diagram of mass flow rate and equivalence ratio. Results show that the single-wave mode regime can be affected by wall cavities. In comparison to the combustor without a wall cavity, the regime is expanded for the cavity in the inner wall but significantly reduced for the cavity in the outer wall. When the single-wave mode forms, the cavities embedded in the inner wall result in a significant increase of the apparent detonation wave velocity but a decrease in the velocity fluctuation rate, and the effects are changed with the axial location of the cavities. The axial location of the cavity can also influence the stable propagation location of the detonation wave. Additionally, the analysis of the high-frequency pressure signals indicates that the cavities do not affect the initiation process of the formation of the combustion modes. However, the variation of oxidizer mass flow rate and equivalence ratio can evidently influence the evolution time of different combustion modes after ignition. The study indicates that the wall cavity structure has a significant influence on the propagation behavior of rotating detonation waves in the combustors, and it can be utilized to enhance the stability of kerosene two-phase rotating detonation combustion.

Developments in the RCCI engines powered by several alternative fuel types: An overview

The perspective on internal combustion engines (ICEs) has changed because of the step-by-step decrease in oil reserves, environmental concerns, and many other reasons. However, the use of these engines will continue in the coming years. The development of ICEs has accelerated in recent years. As a result, studies on systems that can improve performance and reduce pollution in engines are carried out by many researchers. The basis of the studies carried out in this context is the change in the combustion mode. In other words, the emergence of different combustion modes is given importance by researchers. Among these, many studies have been made on the implementation of the reactivity-controlled compression ignition (RCCI) combustion mode. RCCI engine uses two different fuels. These fuels are low-reactivity (alcohol, gasoline, etc.) and high-reactivity (diesel, biodiesel, etc.) fuels. In addition, the fact that different alternative fuel types can be employed in these engines has made them popular all over the world. This review presents the results of the effects of parameters such as fuel ratio, different injection strategies, exhaust gas recirculation (EGR) rate, compression ratio, and piston geometry on the performance, emissions, and combustion behavior of RCCI engines using both conventional and alternative fuels. This study has been compiled to provide an overview of the current status of the RCCI concept and to offer benefits to researchers.

Ignition and combustion characteristics of micro/nano-Al and Al@Ni alloy powders at elevated pressures

Nanosizing and alloying of aluminum (Al) promisingly improve ignition and combustion performance of Al-based propellants. To aim this, lased-induced ignition and combustion characteristics of micro/nano-Al and aluminum@nickel (Al@Ni) alloy powders at elevated pressures (0.1–3.0 MPa) were investigated and the pressure effect on these characteristics was correspondingly evaluated. Comparative results demonstrate that nanosizing and alloying significantly shorten ignition delay and total burn time at elevated pressures, with ignition delay reduction of 98.04% and 52.28% at 1.0 MPa for nano-Al and Al@Ni alloy compared to micro-Al counterpart. Nanosizing and alloying can decrease crucial pressure values (‘mid-pressure extinction domains’) influencing ignition delay, and promote the transfer from the surface to gaseous combustion mode. Maximum combustion temperatures of nano-Al are higher than micro-Al and Al@Ni alloy. During combustion, the combustion of Al@Ni alloy is further intensified due to the microexplosion, while possibly inhibited at higher pressures.

Study on the formation and ignition law of ignition core in ammonia atmosphere diesel cylinder

Ammonia fuel does not contain carbon and is a kind of very good zero-carbon alternative fuel for diesel engines. However, ammonia is difficult to be ignited in diesel engine. The dual-fuel combustion mode of ammonia and diesel is an effective technical means to solve the problem. In this paper, the 186FA diesel engine test bench was built, and the ammonia/diesel dual fuel combustion mode was formed by injecting 0 %, 10 %, 20 %, 30 %, 40 %, and 50 % ammonia into the inlet and direct injection of diesel into the cylinder. The in-cylinder pressure and heat release laws were measured and analyzed at 2700r/min, 25 % load and 75 % load. Based on the analysis of the oxidation in ignition process of ammonia gas and diesel, a computational fluid dynamics model of the ignition process was established. By comparing the temperature contour surface in the cylinder and the concentration contour surface of OH radical, the paper put forward the ignition core evaluation index suitable for diesel/ammonia dual fuel. The formation law of the ignition core of diesel injected in homogeneous low reactive ammonia atmosphere in the cylinder was analyzed, and the influence of ammonia on the ignition process of diesel was revealed. The results show that the ignition delay period is prolonged after adding ammonia, and there is an exponential function relationship between the ignition delay period and the atmospheric concentration of ammonia. At 1200 K, there is an obvious parabolic relationship between ammonia concentration and OH radical concentration. The ignition core area was increased with the increase of ammonia concentration. Compared with pure diesel mode, the single ignition core at 75 % load of 2700 r/min is 3.6 times larger at 50 % ammonia atmosphere concentration. The ignition core will move up the diesel bundle with the increase of ammonia concentration. The study of chemical reaction kinetics showed that the reaction NH3 + OH = NH2 + H2O displayed positive sensitivity when ammonia concentration was less than 10 % and negative sensitivity when ammonia concentration was more than 10 %. The difference in sensitivity is the reason why the phase of the maximum burst pressure in the 10 % ammonia atmosphere is slightly earlier.

Experimental study on the effects of water addition in methanol on the performance of diesel-methanol diffusion combustion on a high-speed engine

Methanol is regarded as a low-carbon renewable fuel which is friendly to the environment. For purpose of investigating the impact of water introduction in methanol on diesel-methanol diffusion combustion, experimental tests were performed on a refitted dual-fuel high-speed diesel engine test bench. This research compared the differences between diesel-methanol diffusion combustion (DMDC) mode and diesel-hydrous methanol diffusion combustion (DHMDC) mode with a water mass content of 20%. The tests were performed by maintaining the engine speed of 3000 rpm and varying engine load as well as methanol injection timing (MIT). The results indicate that the heat release process in DHMDC mode begins later but is more rapid and concentrated compared to DMDC mode. The addition of 20% water in methanol does not significantly decreases the peak average in-cylinder temperature. However, it decreases the NOx emissions by up to approximately 40%. Water introduction leads to higher CO and THC emissions in DHMDC mode, but the retardation of MIT helps mitigate the increase in CO and THC emissions. The exhaust losses in DHMDC mode decrease by about 5%–8%, while the heat transfer losses increase by over 20%, resulting in a lower indicated thermal efficiency (ITE) in comparison with DMDC mode. The effects of the 20% water addition in methanol are sensitive to changes in engine load and MIT. Higher engine load and appropriately retarded MIT facilitate more efficient and cleaner combustion in DHMDC mode.

Experimental investigation and optimization of dual fuel combustion using diesel/gasoline and Bio-oil extracted from Co-thermal liquefaction of paint/biomass wastes: An approach towards waste to energy

A higher-grade transport fuel from both non-biodegradable paint waste and biomass plant Prosopis Juliflora is obtained in the most economical and eco-friendly way by hydrothermal liquefaction process. The yielded bio-oil is experimented on a diesel engine in dual fuel combustion (DFC) strategy and compared with the conventional diesel combustion (CDC). In DFC, the secondary fuels (gasoline, bio-oil, and premixed gasoline-bio-oil blends) are port injected at different premixed ratios (10, 20 and 30%), while the primary diesel fuel is directly injected into the cylinder. DFC of diesel - bio-oil (DB) exhibited higher brake thermal efficiency than premixed gasoline at all power outputs and performed closer to neat diesel. Lower smoke emission is observed in DB than neat diesel and the DFC of diesel-gasoline (DG) at all conditions. At 30% premixed ratio, the smoke opacity of DFC of diesel and premixed bio-oil - gasoline (DGB) is 65.7% and 68% respectively compared to that of CDC (76.4%). At all loads, average NOx emission of DGB (20% gasoline) is abated by 23.2, 7.8 and 12.9% than CDC and 20% of DB and DG respectively but almost equal to neat diesel at full load. The experimental data are used to train an artificial network (ANN) and the trained ANN is used to generate sufficient data for optimizing the fuel concentration using TOPSIS algorithm to realize an overall improved performance and emissions. The optimized combustion modes arrived from this study are: CDC for 25% load, DFC of DGB (D90G2B8) for 50% load and DFC of DB for 75% (D89B11) and 100% (D86B14) loads.

Numerical study on the flow characteristics and stability in the isolator of rotating detonation ramjet engine by different combustion modes and structure parameters

In this paper, numerical simulations are conducted to investigate the effects of the combustion modes and structure parameters of the combustion chamber of a rotating detonation ramjet engine(RDRE) generated within the isolator. The stability of the isolator in the combustion mode as a single or dual wave mode is evaluated. The influence of parameters such as the combustion chamber expansion angle and the outside diameter of combustion chamber is also taken into account. The results show that the propagation of the detonation wave in the combustion chamber in a dual-wave (DW) mode reduces the instability generated by the combustion chamber on the isolator. In the stability model, the fluctuation range of the single-wave (SW) mode accounts for 14.43 % of the total length of the isolator, while it is 4.67 % in the DW mode, which is reduced by 10.97 %. The DW mode is more capable of withstanding changes in the combustion state of the detonation chamber. In the SW mode, the back propagation pressure wave (BPPW) propagates to the inlet of the isolator, while the fluctuation range of the isolator in the DW mode only increases by 29.68 % and still maintains a stable working condition. Changing the length of the isolator and outer diameter of the combustion chamber, the DW mode can also accommodate changes in the combustion chamber structure without affecting the inlet flow status of the isolator.

Effects of Varying Equivalence Ratios on the Combustion Efficiency Characteristic of a Dual-Fuel Compression Ignition Engine by Changing Intake Pressures and Exhaust Gas Recirculation Rates

In general, a leaner mixture condition improves combustion efficiency in compression ignition (CI) combustion using diesel. However, in the case of leaner air–fuel mixture conditions, it disturbs flame propagation in spark ignition combustion using gasoline, i.e., low reactivity fuel, causing a decrease in combustion efficiency. Since dual-fuel combustion in a CI engine typically involves the use of high- and low-reactivity fuels together, the differing reactivity conditions in the cylinder become as important as the local equivalence ratio in the cylinder. Thus, there is a need to verify the effect of a leaner mixture condition on combustion efficiency in dual-fuel CI combustion. For this reason, this study experimentally evaluates the effects of varying equivalence ratios on the combustion efficiency of gasoline/diesel dual-fueled CI combustion in a 0.4-L single-cylinder engine under low-speed (1500 rpm) and low-load (total LHV 570 J/str) conditions. To vary the equivalence ratios, intake pressures and exhaust gas recirculation (EGR) rates were, respectively, changed under the part-load condition. The results emphasize that as the equivalence ratio becomes leaner by increasing the intake pressure, combustion efficiency worsens due to the low reactivity properties and certain flame propagation modes of gasoline combustion. On the contrary, increasing the EGR rate did not significantly influence combustion efficiency, but it effectively helped reduce nitrogen oxide (NOx) emissions. Based on these results, it is concluded that optimizing dual-fuel CI combustion to suppress NOx emissions is better achieved using EGR, rather than creating a leaner mixture condition.

Sustainability of flameless combustion mode with hydrogen addition to liquid kerosene fuel at high-intensity conditions

The application of hydrogen for fuel enrichment and blending has recently gained importance. The sustainability and performance enhancement through hydrogen addition in flameless combustion mode for liquid fuels needs further investigation to enhance our understanding. Therefore, the present study reports the effect of hydrogen addition on the flameless combustion of a kerosene-fired 30 kW swirl combustor with hydrogen addition. A divergence-shaped conical combustor with tangential air injection and asymmetric fuel injection scheme is adopted. The liquid fuel was injected using a solid-cone swirl nozzle with 34 μm Sauter Mean Diameter. The hydrogen addition was varied from 0 to 20% on a thermal input basis, and the thermal input of liquid fuel was maintained at a 30 kW level. Computational studies helped understand the flow physics behavior within the combustor volume. The hydrogen addition was observed to improve the reactant dilution ratio (Rdil). The maximum values of the Rdil increased from 4.47 to 5.44 with hydrogen addition. Hydrogen addition further helped extend the flameless combustion regime to φ = 0.3 without any air preheating. A uniformly distributed reaction with a reduced-temperature gradient is observed with extended combustion stability limits. A marginal increase in NOx emissions was observed with hydrogen addition, albeit within permissible limits, accompanied by ∼8–20 dB reduction in acoustic emissions.

RCCI combustion of ammonia in dual fuel engine with early injection of diesel fuel

Ammonia has gained considerable attention as a valuable, carbon-free alternative fuel for internal combustion engine (ICE) applications. The growing interest in ammonia's potential as an energy source stems primarily from its advantageous storage and transport properties. The reactivity-controlled compression ignition (RCCI) combustion mode presents a promising avenue for overcoming the challenges intrinsic to conventional diesel engines, such as elevated nitrogen oxides (NOx) and particulate matter (PM) emissions. Additionally, it addresses the limitations of both homogeneous charge compression ignition (HCCI) combustion (limited operational range) and premixed charge compression ignition (PCCI) combustion (diminished power output). The application of an ammonia-based RCCI combustion strategy emerges as a viable pathway for harnessing ammonia as a fuel. In this study, a numerical simulation on a single-cylinder heavy-duty engine operated in RCCI mode is conducted utilizing ammonia and diesel as fuels. The engine is operated at medium load and 910 RPM. The simulation results are validated against experimental data sourced from existing literature. The study investigates the impact of varying ammonia energy fractions, injection timing, and intake valve close temperature (TIVC) on key operational characteristics of the engine including in-cylinder pressure, heat release rate (HRR), combustion efficiency (CE), indicated mean effective pressure (IMEP), and emission levels. The results indicated that increasing the ammonia energy fraction within the RCCI combustion mode, transitioning from 30 % to 70 %, leads to a notable enhancement in IMEP, while maintaining low levels of carbon monoxide (CO), hydrocarbons (HC), unburned ammonia, and nitrous oxide (N2O) as greenhouse gases (GHG). Furthermore, increasing ammonia's energy fraction yielded a discernible reduction in NOx emissions and carbon dioxide (CO2). Conversely, the RCCI combustion mode attained superiority over the dual fuel combustion mode by simultaneously elevating CE and diminishing emissions of GHG, (CO2 and N2O) as well as unburned NH3.

Engine behavior analysis on a conventional diesel engine combustion mode powered by low viscous cedarwood oil/waste cooking oil biodiesel/diesel fuel mixture – An experimental study

Binary biofuel is the best alternative source that completely replaces petroleum-based fuel. In this study, we have experimented with the waste cooking oil and cedarwood oil as biofuel in a DI CI engine for various proportions and related its combustion, emission, and performance characteristics to those of base diesel. This study aims to eliminate the utilization of fossil fuel in a diesel engine by introducing green binary fuel (low viscous fuel resulting from the blending of cedarwood oil with WCO biodiesel) successfully. The objective of the study is to convert cedarwood – WCO into green binary fuel and investigate its performance, emission, and combustion properties. The transesterification process is utilized for the enhancement of WCO as biodiesel. It occasioned a reduction in brake thermal efficiency as the addition of waste cooking oil in the blend increased. At the same time, the maximum value of BTE of 27.8% was attained for B10C90 (10% transesterified waste cooking oil and 90% cedarwood oil in volume), whereas it was 28.1% for diesel at maximum load conditions. The BSEC was 15.4 MJ/kW-hr for B10C90 and 12.8 MJ/kWhr for diesel. The emission characteristics, CO, HC, NOx, CO2, and smoke for B10C90 were 17.93 g/kWhr, 0.55 g/kWhr., 20.09 g/kWhr, 2210.9 g/kWhr, and 25.55%. Combustion features such as NHRR, burn duration, MPRR, combustion efficiency, Ignition delay, and coefficient of variance for B10C90 were 53.74 bar, 29.38 CAD, 4.71 bar/CAD, 99.7%, 7.01 CAD, and 4.73% respectively. It showed that B10C90 had comparable performance (BTE) and combustion values to mineral diesel with better emission characteristics.

Analysis of the combustion modes in a rocket-based combined cycle combustor using unsupervised machine learning methodology

Combustion mode analysis is essential to a rocket-based combined cycle (RBCC) combustor because it may experience multiple combustion modes during the operation. In this study, a method based on an autoencoder and a K-means algorithm was proposed for combustion mode analysis. Flame chemiluminescence images and schlieren images of three combustion modes observed in an RBCC combustor were used to evaluate this method. Two autoencoders that followed the same encoder–decoder architecture were developed separately to generate the latent space representations of flame chemiluminescence images and schlieren images. In the latent space, the centroids and boundaries of different combustion modes were determined using the K-means algorithm. Each autoencoder was trained using 750 images and tested using another 3000 images. The method achieved an accuracy up to 99% on both flame chemiluminescence images and schlieren images. The images generated by the decoder suggested that the autoencoder captured the important features (e.g., primary reaction zone and shock wave) of the reacting flow field. The autoencoder developed for flame chemiluminescence images also successfully detected the combustion mode transition during an ignition process, which suggested that it had the potential to monitor the combustion mode in a real time manner. However, the autoencoder failed on monitoring combustion mode transition when it came to the schlieren images because the optical access of the training data was not exactly the same. Therefore, it is essential to ensure that the optical accesses of different combustion modes are exactly the same when schlieren images are employed for combustion mode analysis.

Effects of swirling inflow on the stability and combustion mode of rotating detonations

In this study, a novel approach for enhancing the stability of rotating detonation waves (RDWs) with the use of a swirling inflow strategy is presented. A series of numerical simulations are carried out by solving the two-dimensional reactive Navier–Stokes equations. The effects of the swirling angle on the stability of the RDWs and the combustion mode are analyzed. The results show that the formation of the burnt gas bumps is suppressed by the implementation of a swirling inflow. The swirling inflow also contributes to an increased homogeneity of the reactant within the fuel refill zone. As a result, a remarkable enhancement of the stability of the RDWs in terms of their oscillations in heights and inclined angles is achieved without an apparent compromise of the heights of the RDWs. The propagation speeds of the RDWs are controllable within a wide range approximately from 81% to 114% of the Chapman–Jouguet detonation speed by adjusting the swirling angle. Moreover, the oscillations in the instantaneous fuel consumption rates of both detonative and deflagrative combustion are dominated by the oscillation in the height of the RDW; hence, the swirling inflow reduces the oscillations in these two fuel consumption rates and subsequently the detonation fraction. Consequently, the smoothness of the performance output in terms of specific impulses can be significantly improved with a reduced standard deviation of oscillation up to 84% by the implementation of swirling inflows, and the averaged specific impulse only encounters a small deficit of no more than 7.4%.

Performance Characteristic of Permanent Magnet Linear Generator for Energy Conversion with Renewable Fuels on Free-Piston Engines

Renewable fuels attract more attention owing to reduction of both air pollution and greenhouse gas emissions compared to fossil fuels. The free-piston engine generator is a new energy conversion with compact structure, multifuel combustion mode possibility, and less friction loss, and it can be applied to a variety of renewable fuels. This study investigated the effect of operating parameters on power generation and motion characteristics for free-piston engine generator. A test bench and a detailed and validated numerical model of the free-piston engine generator were presented. Results show that with the increase of the average velocity, the output power root mean square (RMS) and efficiency of the generator gradually increase. The output power RMS grows rapidly, whereas the generator efficiency gradually stabilizes. Under the same average velocity but different strokes and frequencies, the generator power and efficiency are the same. This result means that the multiple-point operating condition can achieve the same output power and generator efficiency goals. Moreover, the optimal operating conditions can be selected comprehensively, considering factors such as fuel properties, combustion efficiency, and performance. The influence of the generator on motion characteristics of the system under pure resistive load and high-frequency operation should be noted.

Toward the ultimate efficiency of methane to syngas conversion by partial oxidation: A moving bed reactor with parallel preheating of reactants

The conversion of methane to syngas by partial oxidation combined with steam reforming in a counterflow moving bed reactor is theoretically considered. The conversion proceeds in filtration combustion mode with the non-premixed flows of steam–methane and oxidant gas. The flow of solid granular heat carrier is divided in two subflows used to preheat, respectively, steam–methane flow and oxidant gas flow prior to the reaction. The calculations are performed under assumption of thermodynamic equilibrium established in the reaction products. For the main control parameters – the flow rates of oxidant gas, steam, and solid heat carrier, related to the flow rate of methane – the parametric domain has been determined that provides a highly efficient conversion in the absence of soot in the products. Efficient heat recuperation with the solid heat carrier makes attainable conversion with the oxygen-to-methane molar ratio as low as 0.43. The flow arrangement with separate preheating of the reactants prior to reaction promises a highly efficient method of converting hydrocarbons to syngas or hydrogen.

Optimizing combustion and emissions in natural gas/diesel dual-fuel engine with pilot injection strategy

For the clean and efficient operation of agricultural power, a non-road common-rail engine was modified to achieve natural gas/diesel dual-fuel (NDDF) combustion mode. Under low load conditions, NDDF combustion deteriorates and fuel economy decreases due to lower pressure and temperature inside the cylinder. Therefore, effects of injection pressure, pilot injection timing (SOIpilot) and pilot injection quantity (Qpilot) on in-cylinder combustion, emission characteristics and fuel economy of NDDF engine with pilot injection strategy were investigated at low load. Results show that with the increment of injection pressure, the diesel fuel beam penetration distance increased, prompting the diesel more quickly and fully mixed with fresh charge. The maximum in-cylinder pressure increased, the combustion center (CA50) moved forward, the ignition delay period was shortened, and soot, HC, C2H4 and C3H6 emissions were reduced, while BTE was improved. When SOIpilot moved forward, the pilot injected diesel and natural gas were more fully mixed, which promoted the generation of activation radicals and combustion intensity of main injected diesel. Pmax first increased and then decreased, the peak value of first heat release rate (HRRP1) gradually decreased, the peak value of second heat release rate (HRRP2) continuously increased, the combustion duration decreased, and the soot, CO and HC emissions decreased. At SOIpilot = −30 °CA ATDC, unregulated emissions reached the lowest level, while BTE was 35.55 %. With the increment of Qpilot, both Pmax and HRRP1 increased, CA05 and CA50 moved forward, combustion duration was extended, COV decreased significantly, and all emissions except NOx decreased, especially aldehyde emissions.