Combustion Of Dual-fuel Engine Research Articles

Modelling the evaporative cooling effect from methanol injection in the intake of internal combustion engines

Renewable methanol is one of the most promising alternative fuels for internal combustion engines. However, its much higher latent heat of vaporization compared to traditional fossil-based hydrocarbon fuels poses new challenges. On both spark-ignition (SI) engines and dual-fuel (DF) engines, methanol can be introduced through injectors installed in the intake path, with its evaporation then causing a cooling effect to the intake air flow. While this is beneficial in mitigating knock with both SI and DF operations, it could potentially lead to cold-starting issues in SI engines and incomplete combustion in DF engines. To properly model the in-cylinder behaviour, the mixture temperature after methanol injection needs to be accurately predicted. A simple yet effective methanol evaporation model based on the liquid droplet and film evaporation mechanisms is thus proposed here to quantify the evaporative cooling effect from methanol injection. The model first treats the methanol spray as a group of droplets with identical size, and after reaching the wall the model assumes that the remaining methanol forms a thin liquid film. The evaporation rates and the consequent temperature drops of these two modes are calculated separately. The calculation results indicate that only a negligible amount of methanol evaporates as droplets, with the predicted temperature drops agreeing well with the validation datasets when taking only the film evaporation into account. The key factor for this good agreement is that the balance between the heat and mass transfer needs to considered when evaluating the surface temperature of a liquid methanol film. The proposed model also suggests that the droplet evaporation can be greatly improved with an injection angle nearly parallel to the air flow, or a finer initial droplet size. Both measures are more effective than increasing the air temperature before injection.

Optical engine investigation of ammonia combustion enhancement with polyoxymethylene dimethyl ethers (PODE₃) as pilot fuel

Ammonia, gaining attention as a carbon-free fuel amid declining fossil fuel reserves, faces challenges like combustion instability and misfiring in its application in internal combustion engines. In this study, polyoxymethylene dimethyl ethers (PODE3), a clean, renewable, and electrically synthesized fuel with high cetane number and oxygen content, is evaluated as a pilot fuel for ammonia in dual-fuel engines. The study, performed in a dual-fuel optical engine, investigates the use of PODE₃ compared to diesel as direct injection fuels for ammonia combustion, focusing on the impact of PODE3 injection timing and dwell (the interval between dual-stage injections) on the ignition characteristics of ammonia. The results demonstrate that, compared to diesel, using PODE3 enhances ammonia combustion in dual-fuel engines. This is evident from increased in-cylinder pressure, higher apparent heat release rate (AHRR), advanced combustion phases, and improved indicated mean effective pressure (IMEP), enhancing overall stability. PODE3 also effectively minimizes soot generation, evidenced by a 71.5 % reduction in spatially integrated natural luminosity (SINL), accompanied by a 16.9 % increase in peak flame area. Adjusting PODE3 injection timing is crucial; initial retardation improves combustion performance, marked by higher in-cylinder pressure and AHRR, as well as expanded flame area. However, over-delaying injection timing reduces these gains. The optimal performance is achieved under the SOI-25 condition, balancing combustion and flame development, though some areas remain unburnt. In a dual-stage injection strategy using PODE3, where the second injection timing remains fixed at −25 °CA ATDC, the critical adjustment of advancing the first injection timing to −35 °CA ATDC markedly enhances combustion performance compared to a single injection condition. This setting reduces ignition delays, increases peak in-cylinder pressures, and improves stability, which collectively contribute to a significant 20.4 % increase in the peak flame area, particularly between adjacent fuel jets. Expanding upon this, advancing the first injection timing further to −45 °CA ATDC leads to a decrease in peak in-cylinder pressure and AHRR, accompanied by a notable 40.3 % reduction in the peak flame area. This research affirms the advantages of PODE3 over diesel in ammonia-powered dual-fuel engines, markedly enhancing combustion performance and environmental sustainability, where the injection strategy of PODE3 emerges as a crucial element.

Experimental investigation and gray relational optimization of engine parameters to improve the output characteristics of an ammonia biodiesel powered dual fuel combustion engine

An innovative approach to this research is to evaluate the effects of different concentrations of biodiesel with premixed ammonia energy in a dual-fuel combustion engine and determine the best operating conditions for optimal engine responses. Experimental results of a 40% ammonia energy premixing fraction with direct injection of a biodiesel blend of 30% (40%AEPF + B30) at 80% load improved brake thermal efficiency (BTE) by 8.42% while reducing brake specific energy consumption (BSEC) by 13.28%. It also reduces the hydrocarbon (HC) emission by 16.8%, the carbon monoxide (CO) emission by 15.06%, and the smoke opacity by 26.8%. As opposed to this, carbon dioxide (CO2) and nitrogen oxide (NOx) emissions are elevated by 21.5% and 16.54%, respectively, relative to the 40% ammonia energy premixing fraction with direct injection of diesel (40%AEPF + Diesel). Thereafter, the ANOVA technique is also utilized to analyze the impact of operational parameters, and the findings show that the responses are statistically significant. Additionally, the results are utilized in a grey relational analysis (GRA) based optimization process to determine the appropriate operational variables. According to optimization results, 40%AEPF + B30 at 80% load provides the most efficiency and fuel economy. It is capable of being used as an alternate fuel instead of conventional diesel.

Development of an ammonia-biodiesel dual fuel combustion engine's injection strategy map using response surface optimization and artificial neural network prediction

The study intends to calibrate the compression ignition (CI) engine split injection parameters as efficiently. The goal of the study is to find the best split injection parameters for a dual-fuel engine that runs on 40% ammonia and 60% biodiesel at 80% load and a constant speed of 1500 rpm with the CRDi system. To optimize and forecast split injection settings, the RSM and an ANN model are created. Based on the experimental findings, the RSM optimization research recommends a per-injection timing of 54 °CA bTDC, a main injection angle of 19 °CA bTDC, and a pilot mass of 42%. As a result, in comparison to the unoptimized map, the split injection optimized calibration map increases BTE by 12.33% and decreases BSEC by 6.60%, and the optimized map reduces HC, CO, smoke, and EGT emissions by 15.68%, 21.40%, 18.82, and 17.24%, while increasing NOx emissions by 15.62%. RSM optimization with the most desirable level was selected for map development, and three trials were carried out to predict the calibrated map using ANN. According to the findings, the ANN predicted all responses with R > 0.99, demonstrating the real-time reproducibility of engine variables in contrast to the RSM responses. The experimental validation of the predicted data has an error range of 1.03–2.86%, which is acceptable.

LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.

Construction and verification of dual-fuel engine combustion model

In order to achieve real-time mapping and online optimization of the combustion process of a dual-fuel engine that is in prolonged operation, this paper is the first to combine the Wiebe function with a deep learning neural network to propose a zero-dimensional (0-D) combustion prediction model for a biodiesel-diesel dual-fuel engine. First, the parameters of the double Wiebe functions are calculated by the Pelican Optimization Algorithm (POA), and the operating parameters and Wiebe parameters are used as input and output parameters of neural networks, respectively, to construct parameter identification models. Then, the combustion process is simplified and reconstructed by combining the Wiebe function with the deep learning neural network, and a 0-D prediction model based on the hybrid model-driven and data-driven method is established, which can further obtain combustion results such as cylinder pressure curve and indicated mean effective pressure (IMEP). The results show that the coefficient of determination (R2) value of the dual-fuel engine 0-D prediction model based on the double Wiebe function combined with POA–CNN–Bi-LSTM is 0.9827, and the model has good prediction accuracy and generalization. The development of the combustion model provides reliable numerical model support for the online evaluation and optimization of dual-fuel engine performance.

Model-prediction and optimization of the performance of a biodiesel – Producer gas powered dual-fuel engine

Diesel engines have been blamed for harming the environment owing to toxic emissions that raise glasshouse gas (GHG) levels. This study intends to model-forecast and improve the emission and combustion parameters of a dual-fuel combustion engine fueled by biodiesel/diesel pilot and producer gas (PG), which burn cleaner than fossil diesel. To assure a high part of locally producible green fuel, biodiesel was made from waste cooking oil, and PG was made from Babool waste wood. Experiment data were obtained at varied engine loads, fuel injection timings, and pilot fuel mix ratios. Using the experimental data, a multi-layer perceptron architecture was used to create an Artificial Neural Network (ANN) based prediction framework to predict outcomes such as brake thermal efficiency, brake-specific energy consumption, oxides of nitrogen, carbon monoxide, unburnt hydrocarbons, and peak in-cylinder pressure. Statistical measures for the predictive model viz., R (0.964 – 0.998) and R2 (0.9292 – 0.996), and root mean square error (0.008 – 2.185) prove the model’s robustness. Using the desirability technique, the trade-off analysis between efficiency and emission showed that 74.37% engine load, injection timing of 27 degrees crank angle before top dead center (°CA bTDC), and a 20% biodiesel/diesel blend were the ideal operating conditions. An experimental investigation confirmed the prediction errors were fewer than 5%. Using this reliable hybrid approach of predicting and optimizing the performance of a dual-fuel engine, a considerable reduction in exhaust emissions with an acceptable level of engine performance was discovered, which would reduce the negative impact on the environment.

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.

A Reduced Reaction Mechanism for Diesel/2-Methyltetrahydrofuran Dual-Fuel Engine Application

2-methyltetrahydrofuran (MTHF2) has been recently regarded as a promising alternative engine fuel. However, the chemical reaction mechanism for MTHF2 combustion in the engine has not been reported to date. In this study, a reduced diesel/MTHF2 reaction mechanism with only 78 species among 233 reactions was constructed for diesel/MTHF2 dual-fuel engine simulations. Firstly, a diesel surrogate mechanism involving the sub-mechanisms of n-decane, iso-octane, methylcyclohexane (MCH), toluene, a reduced mechanism of C2-C3 species and a detailed mechanism of H2/CO/C1 was selected. Secondly, a skeletal MTHF2 mechanism containing 54 species and 294 reactions was formulated under engine-relevant conditions using combined mechanism reduction methods. Thirdly, a reduced sub-mechanism of MTHF2 oxidation with 11 species and 13 reactions was extracted and combined with the four-component diesel surrogate fuel mechanism. Subsequently, the reduced diesel/MTHF2 mechanism was obtained by improving the combined five-component mechanism based on sensitivity analysis. Finally, the proposed mechanism was validated with selected experimental data of ignition delay times, flame species concentrations and laminar flame speeds. In addition, the new measurements from diesel/MTHF2 dual-fuel engine combustion were obtained and then utilized to further assess the developed mechanism. Overall, the developed diesel/MTHF2 mechanism can be used for diesel and MTHF2 dual-fuel engine combustion simulation.

Development of a reduced n-heptane/toluene/tetrahydrofuran mechanism for dual-fuel engine combustion prediction

• A reduced n-heptane/toluene/tetrahydrofuran mechanism was constructed. • The reduce mechanism contains 65 species and 214 reactions. • Good performance was highlighted by this mechanism against experiments. • This mechanism can be used for dual-fuel diesel engine simulations. Tetrahydrofuran (THF) is being proposed as a promising alternative fuel for internal combustion engines. However, the reaction chemistry of THF for multi-dimensional engine simulations has not been well established. In this study, a reliable n-heptane/toluene/THF mechanism was developed to simulate combustion of diesel and THF dual-fuel engine. Firstly, a reduced THF reaction mechanism was obtained by employing various mechanism reduction methods. Secondly, a combined mechanism was constructed based on decoupling methodology, which contained the reduced oxidation sub-mechanisms of n-heptane, toluene and THF, a reduced C 2 -C 3 mechanism and a detailed H 2 /CO/C 1 mechanism. Thirdly, the reaction rate constants of selected reactions involved in the combined mechanism were optimized and the newly developed reduced tri-component mechanism consisting of 65 species among 214 reactions. Subsequently, validations of the developed mechanism were performed using the previously published experimental results of ignition delay times in shock tubes, species mole fractions in laminar premixed flames and jet-stirred reactors, and laminar flame speeds. Meanwhile, dual-fuel engine experiments were conducted and new experimental data of in-cylinder pressure and heat release rate were also used to verify the developed mechanism. The validation indicates that the n-heptane/toluene/THF mechanism of this work can be applied for predicting the combustion process of diesel and THF dual-fuel engine.

Modeling-optimization of performance and emission characteristics of dual-fuel engine powered with pilot diesel and agricultural-food waste-derived biogas

Gaseous-liquid fuel combinations of biogas as primary fuel and diesel as pilot fuel were employed to power a compression ignition (CI) engine in this study. The effects of variable input factors such as engine load, compression ratio, and pilot fuel injection advance on the dual-fuel engine's combustion, thermal, and emission performance were investigated. The data acquired during this lab-based testing was used to develop a prognostic model using a Bayesian optimization strategy for hyperparameter optimization and a contemporary ensemble Gaussian process regression (GPR) technique. During model testing, the error analysis revealed that the developed model could predict the experimental data rather correctly, with R2 values ranging from 0.9995 to 0.9999 and MSE values ranging from 0.00018 to 0.1334. The mean absolute error was discovered to be between 0.0091 and 0.3065. Consequently, the developed GPR model can effectively simulate the on-board performance, emission, and combustion characteristics of a diesel-biogas dual-fuel setting. As a consequence, the current work established the GPR model in the existing CI engine meta-modeling framework as a consistent, trustworthy, and robust system analytical approach for diesel-biogas dual fuel mode of operation.

Application of machine learning and Box-Behnken design in optimizing engine characteristics operated with a dual-fuel mode of algal biodiesel and waste-derived biogas

Waste-derived biogas and third-generation algal biodiesel are attractive alternative fuels to substitute fossil diesel in a diesel engine. However, using biodiesel as a pilot liquid fuel and biogas as the main fuel in a diesel engine is a complicated and highly non-linear process. The current study seeks to predict and optimize the combustion and exhaust emission characteristics of a variable compression dual-fuel combustion engine. Data from experiments were obtained at a variety of engine loads, compression ratios, pilot fuel injection pressures, and timings. A multi-layer perceptron network was employed to develop an Artificial Neural Network (ANN) based prognostic model using the experimental data. The developed prognostic model was used to estimate brake thermal efficiency, biogas flow rates, peak in-cylinder pressure, carbon dioxide, unburned hydrocarbons, oxides of nitrogen, and carbon monoxide. The predictive model's robustness is demonstrated by statistical metrics such as R (0.9723–0.988) and R2 (0.9453–0.9761), Nash-Sutcliffe model efficiency (94–97%), and mean absolute percentage error (0.013–0.128%), Kling-Gupta efficiency (0.9548–0.9836), and Theil's U2 model uncertainty (0.162–0.368). To optimize the parameters of dual-fuel combustion, the Multi-Output Response Surface Methodology (RSM) was employed. The trade-off assessment between emission and efficiency using the desirability approach revealed that 84% engine load, 244 bar of fuel injection pressure, 28 °BTDC of injection timing, and 17.5 compression ratio are the best-operating conditions for the test engine. An experimental investigation was used to corroborate the RSM research findings, and errors were less than 9%. It was revealed that ANN-linked RSM is a good hybrid technique for modeling, prediction, and optimization of the performance of a dual-fuel engine.

Experimental investigation on combustion and particulate emissions of the high compressed natural gas reactivity controlled compression ignition over wide ranges of intake conditions in a multi-cylinder engine using a two-stage intake boost system

Compressed natural gas (CNG)/diesel reactivity-controlled compression ignition was investigated to understand the combustion and emission characteristics over wide ranges of intake pressure and exhaust gas recirculation (EGR) rate at high load condition. Reactivity-controlled compression ignition is a dual fuel engine combustion strategy that employs in-cylinder fuel blending with two different reactivity fuels, and multiple injections for the in-cylinder fuel reactivity control to optimize the combustion phasing, duration, and magnitude as well as NOx and soot reduction. High CNG substitution was of interest since it could lead to greater CO2 and particulate matter reductions. The present 80% CNG substitution required substantial intake boosting due to the charge air displaced by CNG. The experimental engine is commissioned for the reactivity-controlled compression ignition regime using natural gas and diesel fuel with a two-stage turbocharging system. The intake boosting system achieved the intake pressure and EGR rate of 210 kPa and 25%, respectively, when the EGR valve was fully open. The intake pressure and EGR levels varied in the ranges of 150 to 210 kPa and 0 to 25%, respectively. The corresponding equivalence ratio ranged from 0.85 to 0.5. The intake system achieved the highest turbocharger efficiency of 56% at the maximum boosting operation. The highest CO2 reduction was 23.4% at the maximum boosting condition. The improvements under the enhanced intake condition were significant from the standpoints of both efficiency and emissions, including in terms of the particle concentrations.

Experimental study on the effect of water port injection on the combustion and emission characteristics of diesel/methane dual-fuel engines

In this study, the effect of water port injection on the combustion and emission characteristics of diesel/methane dual-fuel combustion was studied experimentally. Conventional dual-fuel engine combustion was extended and explored as tri-fuel combustion by introducing a water port injection. The experiments were conducted on a four-cylinder engine at 1600 r/min and low load with a brake mean effective pressure of 0.42 MPa. Six methane substitution ratios (MSRs) (0%, 10%, 20%, 30%, 40%, and 50%) were programmed. At each MSR point, five water-to-diesel in pure diesel mode mass ratios (WDMRs) (0%, 25%, 45%, 65%, and 85%) were arranged. The experimental results indicated that methane addition decreased the combustion intensity and retarded the combustion process of the diesel engine. In addition, methane addition and water port injection prolonged the ignition delay of the diesel engine. The larger the MSR, the longer was the ignition delay for the corresponding WDMR. Similarly, the larger the WDMR, the longer was the ignition delay for the corresponding MSR. For emissions, the number concentration of particles decreased owing to methane addition, whereas it increased owing to water injection. Notably, for the maximum MSR (50%) and WDMR (0.85) utilization, the maximum total number concentration was 94474 #/cm3, which is much lower than the minimum total number concentration in a pure diesel engine (118549 #/cm3). Furthermore, both methane addition and water injection significantly decreased the nitrogen oxide (NOx) emissions; however, they increased the CO and HC emissions. In summary, adding methane coupled with water injection is an effective method to decrease NOx and particle emissions simultaneously.

Applying detailed chemistry for the dual-fuel engine combustion process simulation using CFD approach

Modern research in the area of internal combustion engines is focused on searching and investigating the technologies that will improve fuel efficiency and decrease emissions. The application of dual-fuel engines is considered a potential solution to these problems. In the dual-fuel engine, the natural gas-air mixture is ignited by a small amount of diesel fuel directly injected into a combustion chamber. This paper aims to develop a detailed chemistry mechanism for 3D simulation of the combustion process of a dual-fuel engine, providing sufficient convergence with the experimental data. It should be noted that sufficient convergence must also be provided in terms of such parameters as pilot fuel ignition delay and premixed air-fuel mixture flame propagation speed. In the course of the research, the analysis of the most commonly used detailed chemistry mechanisms for calculation of the combustion process and mechanisms’ disadvantages was performed. The results obtained with the use of the detailed mechanisms were compared with the results obtained without using detailed chemistry and with the experimental data as well.

Development of a reduced primary reference fuel-PODE3-methanol-ethanol-n-butanol mechanism for dual-fuel engine simulations

Polyoxymethylene dimethyl ethers (PODEn) have attracted worldwide attention, which could be adopted as an alternative or additive of diesel in compression ignition engines. This work focuses on the implementation of PODE for dual-fuel engine combustion. The effects of four dual-fuel combinations on the engine combustion performance and emissions were studied, including PODE-isooctane, PODE-methanol, PODE-ethanol, and PODE-n-butanol. First, a new reduced mechanism of primary reference fuel-PODE3-methanol-ethanol-n-butanol was developed and validated. This mechanism was then employed for the three-dimensional numerical investigation. The results demonstrated that at an exhaust gas recirculation (EGR) rate of 45%, PODE-n-butanol and PODE-isooctane generated higher thermal efficiencies than PODE-methanol and PODE-ethanol, primarily due to the higher reactivity of n-butanol and isooctane compared to methanol and ethanol. The highest thermal efficiency of 47.0% was obtained using PODE-NC4H9OH with a start of injection timing of −7.5°. With an EGR rate of 52%, NOx emissions of all cases met the regulation limit, which, however, were achieved at a sacrifice of engine efficiency; additionally, the highest thermal efficiency of 43.3% was achieved using PODE-CH3OH with an injection timing of −17.5°.

Effects of mixture formation strategies on combustion in dual-fuel engines – a review

The article presents an overview of technical solutions for dual fuel systems used in internal combustion engines. It covers the historical and contemporary genesis of using two fuels simultaneously in the combustion process. The authors pay attention to the value of the excess air coefficient in the cylinder, as the ignitability of the fuel dose near the spark plug is a critical factor. The mixture formation of compression ignition based systems are also analyzed. The results of research on indirect and direct injection systems (and their combinations) have been presented. Research sections were separated based to the use of gasoline with other fuels or diesel oil with other fuels. It was found that the use of two fuels in different configurations of the fuel supply systems extends the conditions for the use of modern combustion systems (jet controlled compression ignition, reactivity controlled compression ignition, intelligent charge compression ignition, premixed charge compression ignition), which will enable further improvement of combustion efficiency.

Safe operation of dual-fuel engines using constrained stochastic control

Premixed combustion strategies have the potential to achieve high thermal efficiency and to lower the engine-out emissions such as NOx. However, the combustion is initiated at several kernels which create high pressure gradients inside the cylinder. Similarly to knock in spark ignition engines, these gradients might be responsible of important pressure oscillations with a harmful potential for the engine. This work aims to analyze the in-cylinder pressure oscillations in a dual-fuel combustion engine and to determine the feedback variables, control actuators, and control approach for a safe engine operation. Three combustion modes were examined: fully, highly, and partially premixed, and three indexes were analyzed to characterize the safe operation of the engine: the maximum pressure rise rate, the ringing intensity, and the maximum amplitude of pressure oscillations (MAPO). Results show that operation constraints exclusively based on indicators such as the pressure rise rate are not sufficient for a proper limitation of the in-cylinder pressure oscillations. This paper explores the use of a knock-like controller for maintaining the resonance index magnitude under a predefined limit where the gasoline fraction and the main injection timing were selected as control variables. The proposed strategy shows the ability to maintain the percentage of cycles exceeding the specified limit at a desired threshold at each combustion mode in all the cylinders.

Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, this study focuses on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH4) on pilot-fuel ignition. The simulations reveal that CH4has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH4entrained into the spray decreases, the initial oxidization of CH4consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH4oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

Experimental, numerical and exergy analyses of a dual fuel combustion engine fuelled with syngas and biodiesel/diesel blends

This work investigates the effects of addition of syngas and biodiesel on a reactivity controlled compression ignition (RCCI) engine fuelled with diesel. Scanning electron microscopy (SEM) of exhaust particulate matter has been done to obtain particulate matter (PM) morphology. Energy and exergy analyses have been performed to observe energy and availability shares, and to provide directions for the energy recovery systems. Closed cycle combustion simulations have been performed to complement the experimental results and for an improved understanding of in-cylinder dynamics. Chemical and physical properties of three biodiesel samples have been analysed using elemental analysis, Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry. Based on the initial study, used cooking oil based biodiesel blend (B20, 20% biodiesel) has been chosen in experiments. The optimal operating conditions for syngas/diesel and syngas/B20 in RCCI mode for different operating parameters have been investigated. Injection pressure, injection timing and pre-injection mass ratio have been modified to get improved combustion efficiency at mid-load. Syngas/diesel mode with an injection timing of 19° before top dead centre (bTDC) shows slightly higher brake thermal efficiency (BTE) with 22% and 77% lower oxides of nitrogen (NOx) and PM respectively as compared to conventional diesel combustion. In syngas/B20 mode, a maximum BTE of 24% has been observed for the case with a pre-injection at 50° bTDC with 30% mass fraction and 18° bTDC main injection timing. Syngas/diesel shows a reduction in primary soot particle count by about 67% and contains larger aggregates as compared to neat diesel.