• Energy Research

Abstract A single-cylinder heavy-duty optical engine is used to characterize dual-fuel (DF) combustion. In experiments, methane is applied as the main fuel while directly injected pilot diesel ignites the premixed methane-air mixture close to the top-dead center (TDC). In the present study, diesel-methane DF combustion is analyzed as a function of (1) the methane equivalence ratio, (2) initial charge temperature, and (3) the quantity of pilot diesel. Experiments are conducted at 1400 rpm and a load of 9–10 bar IMEP, and DF combustion is visualized in the engine through Bowditch-designed optical access. Meanwhile, a high-speed camera records temporally resolved natural luminosity (NL) color images of the combustion event. The results of the study suggest that DF combustion based on the apparent heat release rate (HRR) data consists of three overlapping combustion stages, where the level of overlap depends on mixture fractions of both pilot-diesel and methane in the in-cylinder charge. The stages are identified by analyzing the second derivative of HRR data. The study revealed that during the first stage, most of the pilot diesel burns in the premixed mode, and that the ignition delay time (IDT) directly influences the burnt charge mixture fraction of pilot diesel and entrained premixed methane-air mixture. In addition, the first-stage combustion is visualized as initial flame kernels originating from pilot-diesel sprays. IDT is found to be especially sensitive to the methane equivalence ratio and initial charge temperature. Furthermore, the concentration of methane and the quantity of pilot diesel in the charge distinctively influence combustion duration trends.

The demand for renewable energy sources is gradually escalating due to the spontaneously growing population and global economic development. The access to fossil fuels is gradually declining due to the limited available reserves. Hence, renewable energy resources, technology choice, and energy policy are always being revised due to the modernization of society. Meanwhile, the liquid energy sources such as methyl ester from locally produced vegetable oils are readily accepted by many countries globally, although it is currently being blended (up to 20%) with diesel. Oxides of nitrogen are the most substantial emissions from diesel engines produced due to high combustion temperature. The addition of alcohol in the fuel reduces the NOxformation since alcohols have high latent heat of evaporation. The present study’s primary purpose is to investigate the effect of different alcohol types on engine performance and emission characteristics. For this purpose, seven test fuels and neat diesel were used. The test fuels P20 (20% palm biodiesel with 70% neat diesel and 10% alcohol on a volume basis), D70P20E10, D70P20Pr10, D70P20B10, D70P20Pe10, D70P20H10 were prepared and tested on a single-cylinder, 4-stroke, DI-diesel engine at different speeds at 100 % load. The P20E10 ternary fuel blend illustrated the most practical combination of all the bioethanol-based blends, which considerably improves the BTE, BSFC and reduces NOxformation at high speed compared to other types of alcoholic fuel blends. Also, the P20E10 fuel blend improved the cloud point of neat diesel.

A novel combustion mode, namely tri-fuel (TF) combustion using a diesel pilot to ignite the premixed methane–hydrogen–air (CH4–H2–air) mixtures, was experimentally investigated under various H2 fractions (0%, 10%, 20%, 40%, 60%) and ultra-lean conditions (equivalence ratio of φ= 0.5). The overarching objective is to evaluate the effect of H2 fraction on flame characteristics and engine performance. To visualize the effect of H2 fraction on the combustion process and flame characteristics, a high-speed color camera (Photron SA-Z) was employed for natural flame luminosity (NFL) imaging to visualize the instantaneous TF combustion process. The engine performance, flame characteristics, and flame stability are characterized based on cylinder pressure and color natural flame images. Both pressure-based and optical imaging-based analyses indicate that adding H2 into the CH4–air mixture can dramatically improve engine performance, such as combustion efficiency, flame speed, and flame stability. The visualization results of NFL show that the addition of H2 promotes the high-temperature reaction, which exhibits a brighter bluish flame during the start of combustion and main combustion, however, a brighter orangish flame during the end of combustion. Since the combustion is ultra-lean, increasing the H2 concentration in the CH4–air mixture dramatically improves the flame propagation, which might reduce the CH4 slip. However, higher H2 concentration in the CH4–air mixture might lead to a high-temperature reaction that sequentially promotes soot emissions, which emit a bright yellowish flame.

Abstract Cyclic variations constitute an inherent consequence of the flow, thermal and concentration field variations between cycles. They are understood to lead to lower efficiency and higher emissions. The current investigation aims to evaluate the cycle-to-cycle variations (CCVs) based on 2D visualization and cylinder pressure in an optically accessible heavy-duty engine fueled with methane (main fuel) and diesel (pilot fuel). A high-speed color camera is employed to measure the combustion behavior based on natural luminosity (NL). Proper orthogonal decomposition (POD) is applied to reconstruct and analyze the images. The POD-based coefficient of variation (COV) is implemented to evaluate the cyclic variability, along with the pressure-based and global intensity-based COV. This coefficient is then adopted to discriminate the coherent and incoherent parts from the fluctuations in the luminosity field. The POD-based and global intensity-based COV presents the variations in the luminosity field, which can provide information on chemical kinetics, while pressure-based COV provides a general description of the cyclic fluctuation of thermodynamics. To extract more information from the NL images, the color-intensity COV analysis based on the intensity separated from RGB channels is adopted to estimate the CCVs from the aspect of spectral emissions (excited and ionized radicals in the flame). Finally, the effects of methane lambda, pilot fuel rate and charge air temperature on the CCVs were analyzed systematically. The results revealed that richer methane conditions has an inhibitive effect on the CCVs. The appearance of the CCVs were determined by the ignition characteristics of the pilot fuel. A critical point was found in charge air temperature, when the charge air temperature lower than the critical point, the increase of the charge air temperature has a promotive effect on the CCVs; after that, it has an inhibitive effect on the CCVs.

Funding Information: The authors would like to acknowledge the receipt of the following financial support for this research work and authorship. This work was financially supported by Academy of Finland (grant no. 13297248) Fortum-Neste Foundation (grant no. 2020050 and 20210032 ), and Merenkulun säätiö (grant no. 20210073). Publisher Copyright: © 2022 The Author(s) Methanol (MeOH) is a promising low-carbon liquid fuel to provide global energy security with a potential to achieve net-zero greenhouse gas emissions in transport sector. However, its utilization in diesel engines at high MeOH substitution ratios (MSR) suffers from misfire or high pressure rise rates owing to its distinct physio-chemical properties. This issue is addressed in the present study by adopting negative-valve overlap (NVO) and hot residual gases from the previous cycle. Experiments are performed in a single-cylinder heavy-duty CI engine for a constant MSR (90% energy based) and an engine speed of 1500 rpm. The aim of the study is to investigate the effects of 1) NVO period, 2) charge-air temperature (Tair), 3) MeOH lambda (λMeOH) on the MeOH-diesel dual-fuel (DF) combustion in NVO mode, and 4) to demonstrate the implications of NVO in yielding high net-indicated efficiency (ηind) together with low pollutant emissions at a wide range of engine operating loads (40–90%). The results show that the hot residual gases from the previous cycle enhance the reactivity of the fresh MeOH-air mixture by inducing slow oxidation processes before TDCf. The slow pre-flame oxidation processes are disruptive or oscillatory in nature, wherein NVO period, Tair and λMeOH can be used to control these processes and their induced reactivity enhancing capability. It is noticed that the pre-flame oxidation processes and the main combustion have a direct correlation between them. Based on the control strategy, the MeOH-diesel combustion in the NVO mode produced on average ηind of approx. 53% accompanied with very low NOx emission of 1.1 g/kWh at a wide range of engine operating loads (40–90%). Additionally, on average the combustion phasing (CA50) is maintained at ∼ 2 oCA aTDC, while the combustion stability remains high (COVIMEP ∼ 3.5%). Peer reviewed

Publisher Copyright: © 2024 The Authors Ammonia is increasingly recognized as a viable alternative fuel that could significantly reduce greenhouse gas emissions without requiring major modifications to existing engine technologies. However, its high auto-ignition temperature, slow flame speed, and narrow flammability range present significant barriers, particularly under high-speed combustion conditions. This review explores the potential of ammonia as a sustainable fuel for internal combustion engines, focusing on its advantages and challenge. The review draws on a wide range of studies, from NH3 production, application, to the combustion mechanisms, that explore various strategies for enhancing NH₃ combustion in both spark ignition and compression ignition engines. Fundamentals and key approaches discussed include using hydrogen and hydrocarbon fuels as combustion promoters, which have been shown to improve ignition and flame propagation. Literature on fuel injection strategies, such as port fuel injection, direct injection, and dual-fuel injection, are examined to highlight their influence on NH₃-air mixing and combustion efficiency. Furthermore, the review delves into advanced ignition technologies, such as low-temperature plasma ignition, turbulent jet ignition, and laser ignition, which are explored for the potential to overcome the ignition difficulties associated with NH₃. After a comprehensive analysis based on the literature, the intelligent liquid-gas twin-fluid co-injection system (iTFI) emerges as a promising approach, offering improved combustion stability and efficiency through better fuel-air mixture preparation. By synthesizing the existing research, this review outlines the progress made in NH₃ combustion and identifies areas where further study is needed to fully realize its potential as a sustainable fuel. Peer reviewed

Abstract In a dual-fuel (DF) combustion process, the ignition of the main fuel plays a crucial role on engine performance and emissions. In the present work, a pilot fuel is used to ignite a gaseous methane-air mixture. Three diesel-like pilot fuels are used comparing especially the cetane number (CN) and the aromatic content (AC). The experiments are conducted in a single-cylinder heavy-duty research engine keeping the total-fuel energy constant. Lean conditions are applied for the port-fuel injected methane-air mixture ( ∅ C H 4 = 0.52). The methane provides 97% of the total energy while 3% of the energy comes from a pilot fuel. The experiments are performed for two pilot-fuel injection pressures and two engine speeds. The results of the present work suggest that DF combustion consists of three overlapping combustion stages: (I) ignition of the pilot fuel, (II) burning of the main fuel in the vicinity of the pilot fuel, and (III) combustion of the remaining main fuel. It was observed that cetane number directly affects the peak heat-release rate (HRRpeak) during Stage I, whereas aromatic content influences HRRpeak during Stages II and III. A fuel with high cetane number and aromatic content provides high DF combustion efficiency, low methane slip, THC and CO emissions at the expense of high NOx emissions. An increase in the aromatic content is responsible for the increased NOx emissions. Based on the average performance trends of the pilot fuels, they can be rated as [high CN, high AC] > [Low CN, high AC] > [High CN, AC free].