High Reactivity Fuels Research Articles

Experimental study on high fatty acid WCO biodiesel effects on RCCI engine performance, and emissions, considering fuel oxygen level

One of the significant challenges of RCCI engines is increasing the amount of UHC and combustion phasing control. On the other hand, a substantial challenge for biodiesel fuel is increasing NOx production. The low-temperature combustion (LTC) strategy is an effective method of reducing NOx. In this experimental study, an RCCI engine with diesel-gasoline dual fuel was experimentally evaluated under medium load conditions (BMEP = 20 bar) with a waste cooking oil (WCO) biodiesel mixture. The effect of changing WCO biodiesel fuel to diesel ratio (with different volume ratios of 0, 5, 10, and 20%) was investigated. The results showed that with a 5% increase in WCO fuel volume share, due to the higher cetane number of WCO fuel, the start of combustion (SOC) occurred about 4 crank angle degrees earlier. By increasing the volume ratio of biodiesel from 5 to 10% (B10), the amount of CO emissions suddenly increased by 33%; due to the increase in the viscosity of the high reactivity fuel (HRF), the fuel is not well atomized, causing poor cold combustion. In the case of using B20 fuel, the amount of unburned hydrocarbons (UHC) is greatly reduced due to the high pressure inside the cylinder.

Comparative analysis of combustion and emission characteristics in RCCI engines using alcohol fuels with different carbon numbers.

Alcohol fuels with different carbon numbers such as propanol (C3), butanol (C4) and pentanol (C5) have lately become popular in both conventionally and RCCI-operated diesel engines thanks totheir high cetane number (CN) and oxygen content along with lower latent heat of evaporation, which are useful for reducing the high CO/HC emissions, whereas RCCI mode still suffers from these emissions. Therefore, in this study, these three alcohol fuels with carbon numbers ranging from C3 to C5 were employed as low-reactivity fuel (LRF) in a single-cylinder RCCI engine under a constant engine speed of 2400rpm and varying loadings (from 20 to 60% of full load at 20% intervals) and premixed ratios (from 0 to 60% with 15% intervals) when using B7 as high-reactivity fuel (HRF). In the experimental study, the effect of oxygen content, latent heat of evaporation, and cetane number which changes linearly with the carbon number of alcohols used, on exhaust emissions, were analyzed. When compared to conventional diesel mode (CDM), RCCI mode using alcohol fuels increased HC and CO emissions but decreased smoke opacity at a great level. The greatest reduction in both CO emissions and smoke opacity was recorded with the use of propanol which has the highest oxygen content, while pentanol with the lowest latent heat of evaporation had success in reducing HC emissions. In addition, NOx emissions were reduced by up to 60% when butanol was used as low-reactivity fuel. Ignition delay increased more during RCCI operation with propanol having an octane number of 118, whereas butanol and pentanol have 92 and 78, respectively. It was concluded that the properties of alcohol fuels such as the oxygen content and latent heat of evaporation had a significant effect on HC/CO emissions in RCCI engines.

Applications of machine learning techniques to broaden operating envelope of biodiesel-fueled HCCI engines

A significant shortcoming of Homogeneous charge compression ignition (HCCI) engines is their narrow operating envelope, particularly with high-reactivity fuels like biodiesel. Additionally, the compositional variability of biodiesels, reflected in cetane number variations based on different source feedstocks, poses another challenge. While various strategies have successfully extended the maximum load limit in diesel-HCCI engines, they have not been adequately explored in biodiesel-fueled HCCI engines. This study is the first to comprehensively investigate the interplay of biodiesel composition, cetane number, engine compression ratio, and charge dilution strategy for extending the operating envelope of a light-duty HCCI engine. Given the impracticality of controlling multiple variables experimentally, this work focuses on the potential of machine learning (ML) algorithms to predict the operational limits of an HCCI engine using neat biodiesels derived from diverse sources. Among the ML models explored, artificial neural networks were the most accurate in predicting the minimum stable load, with prediction errors of 3.5% (calibration) and 3.9% (validation). Support vector machine models predicted the maximum load (with and without dilution) with errors below 5%. Notably, biodiesel produced from a blend of linseed, karanja, coconut, and mustard oils in specific proportions (42%, 3%, 25%, and 30% mass, respectively) yielded a wide HCCI operating range from 0.4 to 3.25 bar (without dilution) and up to 3.67 bar BMEP with charge dilution. This study highlights the novelty and practicality of using ML to predict and extend the operating envelope for biodiesel-fueled HCCI engines, demonstrating their suitability for future applications.

A detailed comparison of ethanol–diesel direct fuel blending to conventional ethanol–diesel dual-fuel combustion

Dual-fuel combustion is a well-known measure to enable the combustion of low-reactivity fuels (LRF) in compression-ignited engines with high thermal efficiency through a pilot injection of a high-reactivity fuel (HRF). In most cases, the LRF is introduced into the intake manifold and therefore premixed with the air before entering the combustion chamber during the intake stroke (premixed charge operation, PCO). In this work, this approach is investigated for bioethanol-diesel dual-fuel combustion using external and internal exhaust gas recirculation (EGR) to improve emissions and engine efficiency. In addition, PCO is compared to an alternative concept in which bioethanol and diesel are blended shortly upstream of the high-pressure pump (premixed fuel operation, PFO) at variable mixing ratios. The results show that higher ethanol shares of up to 70% can be achieved at low engine load when using PCO, while at medium and high load, the maximum energy share of ethanol is higher with PFO. While PCO is limited by engine knock, PFO rather suffers from the reduction in cetane number. In PCO, external and internal EGR allow for a reduction of unburned hydrocarbons (up to − 82%) and carbon monoxide (up to -60%), while nitrous oxide emissions are simultaneously lowered by up to − 65%. Both with and without EGR, PFO shows low emissions of unburned hydrocarbons and carbon monoxide (similar to conventional diesel combustion) and a significant reduction in nitrous oxide and soot formation. Brake thermal efficiency (BTE) drops in both modes compared to conventional diesel combustion, in PCO operation due to unburned and partially unburned fuel and in PFO due to increased friction in the high-pressure fuel pump caused by an increased fuel flow.

Optical study on the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode

Employing high-reactivity fuels such as polyoxymethylene dimethyl ethers (PODE) are effective ways to improve ammonia-based engine performance. However, abnormal combustion (i.e., knocking combustion) may be encountered at reactivity-controlled compression ignition (RCCI) mode. In this study, the auto-ignition and knocking behaviors of ammonia/PODE RCCI mode were studied in a highly-strengthened optical engine at different injection timing conditions, with synchronization measurement of in-cylinder pressure and high-speed photography. The port-injection of ammonia and direct injection of PODE were employed at an energy ratio of 80/20. Results shows that knock intensity is significantly increased with the advancement of injection timings under elevated thermodynamic conditions, accompanied by steep pressure peaks and pressure oscillations. Combustion visualization elucidates the relationship between auto-ignition (AI) progress and knock intensity. Pressure oscillations under knocking combustion are closely correlated to the interplay between different AI flame fronts. Further analysis identifies two distinct combustion characteristics, manifested in the most AI locations and the initial AI flame speed. Numerical results indicate that different AI behaviors are primarily determined by the degree of inhibition in PODE pre-flame reactions by ammonia dehydrogenation, which affects AI kernel development. The slow initial auto-ignition flame results in the formation of a sufficiently active premixed mixture in the unburned region before the initial AI flame front arrives. Once the second AI flame occurs, the active premixed mixture undergoes rapid combustion, leading to strong pressure waves.

Stage-wise kinetic analysis of ammonia addition effects on two-stage ignition in dimethyl ether

Abstract Ammonia (NH3) has gained considerable attention as a promising carbon-free hydrogen carrier fuel for internal combustion engines, but its direct use in compression-ignition engines presents challenges, often requiring high-reactivity fuels to ignite the premixed NH3/air mixture and initiate combustion. This study focuses on the ignition process of binary NH3 and dimethyl ether (DME) mixtures, as DME is a carbon-neutral, high-reactivity fuel. A key novelty of this paper is the comparison of the ignition processes of DME and NH3/DME mixtures from a temporal, process-oriented perspective, analyzing chemical kinetics across distinct ignition phases rather than focusing solely on instantaneous reactions at discrete time points. The stage-wise analysis reveals that NH3 has minimal impact on the control mechanism governing the two-stage ignition process of DME. Specifically, DME still largely depends on OH radical proliferation during low-temperature oxidation (LTO), which releases heat as the reaction progresses. As the temperature increases, LTO branching pathways gradually shift to chain-propagation pathways, reducing overall reaction activity. The reactivity and temperature rise rate of the system are then governed by the H2O2 loop mechanism before thermal ignition. However, ammonia significantly extends the ignition delay of DME by competing with OH radicals, which are critical for DME oxidation, thus inhibiting ignition. As the ignition reaction proceeds, ammonia kinetics become more involved. For example, nitrogen-containing species from NH3 oxidation, such as NO, NO2, and NH2, react with CH3OCH2 to form CH3OCHO, reducing the flux through the LTO pathway of DME. While ammonia reaction pathways also produce OH radicals, this occurs at the expense of HO2 and H radicals, leading to H2O2 formation. Overall, these findings demonstrate the substantial impact of ammonia addition on DME ignition, highlighting the need for further research to better understand NH3/DME binary fuel ignition and to optimize the design and operation of NH3/DME dual-fuel engines for improved efficiency and reliability.

Numerical Investigations on Reducing Unburned Hydrocarbon and Carbon Monoxide Emissions in Reactivity-Controlled Compression Ignition Using Partial Reactivity Stratification with Alternative Fuels and Additive

<div>A numerical investigation has been performed in the current work on reactivity-controlled compression ignition (RCCI), a low-temperature combustion (LTC) strategy that is beneficial for achieving lower oxides of nitrogen (NO<sub>x</sub>) and soot emission. A light-duty diesel engine was modified to run in RCCI mode. Experimental data were acquired using diesel as HRF (high-reactivity fuel) and gasoline as LRF (low reactivity fuel) to check the accuracy and fidelity of predicted results. Blends of ethanol and gasoline with DTBP (di-tert-butyl peroxide) addition in a small fraction on an energy basis were used in numerical simulations to promote ignitability and reactivity enhancement of PFI charge. Achieving stable, smooth, and gradual combustion in RCCI is challenging at low loads, especially in light-duty engines, due to misfiring and poor combustion stability. DTBP is known for enhancing cetane number and accelerating combustion, and it is mixed in a PFI blend to avoid combustion deterioration. The factors governing reactivity stratification to achieve optimal combustion phasing were investigated in the present study. DTBP decomposition and its low-temperature oxidation chemistry were found to be responsible for affecting combustion phasing, heat release patterns, and emission trends. DTBP additive and different in-cylinder strategies were applied and studied to reduce unburned emissions. Adopting a multiple injection approach utilizing dual-pulse assisted in reducing HC and CO levels. It enhances combustion quality by providing adequate control over combustion phasing. Altering operating parameters like intake temperatures reduced HC, CO, and soot emissions by 97.6%, 57.6%, and 52.8%, respectively, compared to baseline gasoline/diesel RCCI data. Optimizing the injection timings of the first and second pulse helps achieve optimal combustion phasing and a 72.95% reduction in NO<sub>x</sub> emissions. The higher injection pressure of DI helped lower the CO and soot emissions by 53.33% and 51.84%, respectively.</div>

Enhancing the usability range of acetylene in CI engines through split injection of high-reactive fuel under RCCI operation: Optimized operability using an MCDM approach

Reactivity Controlled Compression Ignition (RCCI) has emerged as a promising solution to address the challenges of advanced combustion techniques in diesel engines, balancing combustion control and emission reduction. This study investigates the impact of split-injection high-reactivity fuel (HRF) dosing in RCCI to enhance charge homogeneity at lower injection timing advancements. It systematically explores progressive advancements in HRF injection timing, distinguishing between main and pilot injections. The main injection varied from 20° bTDC to 70° bTDC, while the pilot injection, relative to the main injection, spans 25°-40° CA ahead with a 5° progression. As an LRF, Port-injected Acetylene supplements the process with variable premixed ratios. Further, using a multi-criteria decision-making method (AHP), the study optimizes operating conditions, proposing a main injection at 20° bTDC, a pilot injection 40° CA ahead, and 70 % LRF participation. This optimized condition yields significant benefits such as 32.99 % higher brake thermal efficiency (BTE), 160 % longer premixed duration, 18.65 % lower maximum temperature, 89.41 % lower NO emissions, 91.83 % lower hydrocarbon (HC) emissions, 91.24 % lower CO emissions and 97.20 % lower smoke opacity compared to conventional diesel combustion (CDC) operation. The results highlight the advantages of multiple fuel injections in RCCI, improving blending characteristics at lower injection angles.

Experimental study on combustion and emissions of an ammonia/diesel dual-fuel engine using split-injection strategy

Blending high-reactivity fuels is recognized as an effective way to improve poor combustion performance of ammonia (NH3). In this work, pressure trajectories, flame evolutions, and emission characteristics of ammonia/n-dodecane dual fuels were studied in an optical engine, addressing split-injection strategy. Synchronization measurement of in-cylinder pressure and high-speed photography was performed for combustion evolutions, and FT-IR spectroscopy was adopted for nitrogen-based emissions. Results demonstrate that pre-injection timing and its energy ratio play a significant role in improving engine thermal efficiency and pollution emissions. Specifically, compared to the single-injection strategy, split-injection strategy leads to a reduced peak in-cylinder pressure, optimized combustion phasing, and shortened combustion duration, achieving an increase in the indicated mean effective pressure by 15.8 %. An early pre-injection timing with an energy ratio of 40 %∼60 % yields superior thermal efficiency compared to single injection at the same injection timing. For the delayed pre-injection timing, increasing pre-injection energy ratio is required to achieve optimal engine performance. Combustion visualizations show that the improved flame development under split-injection conditions is attributed to high-reactivity fuel stratification and enhanced premixing processes. Regarding nitrogen-based emissions, unburned NH3 and N2O emissions are insignificantly reduced, and NOx emissions correlate with the sensitivity of flame development to in-cylinder temperature. On the premise of ensuring stable combustion, all nitrogen-based emissions are reduced at a pre-injection energy ratio of 40 %, and an energy ratio of 60 % reduces N2O and unburned NH3 emissions while increasing NO and NO2 emissions. The current work can provide useful insights into the optimization and control of ammonia/diesel dual-fuel engines in terms of combustion and emissions.

Optimization on manifold injection in DI diesel engine fuelled with acetylene

Researchers demonstrated that implementing new combustion technology and optimising fuel quantity results in a significant reduction in traditional fossil fuel usage and emission levels. The Reactivity Controlled Compression Ignition (RCCI) combustion strategy is one of the low temperature combustion technologies, and it is used to reduce the overall combustion temperature while also providing better combustion control. This study looks into RCCI combustion technology, which uses conventional diesel fuel as the high reactivity fuel (HRF) injected through the injector and acetylene gas as the low reactivity fuel (LRF) injected into the cylinder via a modified inlet manifold alongside air. The modified engine setup was tested for performance, emissions, and combustion under various load conditions, as well as different mass flow rates of acetylene gas, a low reactivity fuel that is injected with air. The flow field of the low reactivity fuel at the inlet manifold is analysed using the Computational Fluid Dynamics principle, which is used to determine the best flow rate for improving combustion quality. According to the simulation results, the optimal acetylene flow rate is 3 Litres Per Minute (LPM), and experimentation shows that at 3 LPM acetylene injection, the brake thermal efficiency (BTE) improves by about 3.2%, and emissions such as carbon monoxide (CO), hydrocarbon (HC), smoke intensity, and oxides of nitrogen (NOx) are reduced by about 35%, 17%, 10%, and 21%, respectively.

Combustion enhancement and emission reduction in RCCI engine using green synthesized CuO nanoparticles with Cymbopogon martinii methyl ester and phytol blends

Cymbopogon martini, also known as Palmarosa, is a grass species in the Cymbopogon genus, native to India and widely cultivated in tropical regions for oil production. Phytol, a constituent of chlorophyll, has garnered attention in the biofuels industry due to its potential for conversion into biodiesel through transesterification. Its abundance and renewable nature underscore its relevance for sustainable development. This study investigates the energy potential of waste Cymbopogon martinii Methyl Ester (CMME) in engine combustion systems using reactivity-controlled compression ignition (RCCI) in a single-cylinder, four-stroke diesel engine for low-temperature combustion. The process involves blending low-reactivity fuel (LRF), such as phytol (Acyclic diterpene alcohol and a constituent of chlorophyll), with high-reactivity fuel (HRF), namely CMME, in carefully monitored proportions. CMME25 is used as the primary fuel, supplemented with varying amounts of phytol (10 %, 20 %, and 30 %). Heat release rate (HRR) analysis indicates that phytol improves combustion efficiency up to a 20 % blend in RCCI mode. However, exceeding a 30 % phytol blend reduces brake thermal efficiency (BTE). RCCI mode also shows decreased nitrogen oxide and smoke emissions but increased hydrocarbon (HC) and carbon monoxide (CO) emissions. Improved homogeneity and ignition delay result in higher peak pressure and HRR compared to traditional methods. Further investigation focuses on the effects of CuO nanoparticles on a CMME25 and PHY20 % blend in RCCI mode. CuO nanoparticles (50–100 ppm) enhance brake thermal efficiency, improve combustion parameters (ignition delay, HRR, and cylinder pressure), and reduce emissions. The study concludes that a blend of CMME25+PHY20 % with 100 ppm CuO in RCCI mode is a promising alternative to traditional diesel fuel, offering a sustainable and efficient energy solution.

Technical evaluation of low-carbon fuels as a decarbonization pathway of the light-duty transport sector

The need to rapidly reduce greenhouse gas emissions from the transportation sector has renewed interest in low-carbon fuels, both from the production and utilization standpoint. The maturing of alternative processes to synthesize fuels from atmospheric CO2 and renewable non-edible feedstocks constitutes an interesting proposition, which needs to be matched by favorable drop-in properties and environmental performance once applied to internal combustion engines. To this end, the authors investigate the use of diesel-like high-reactivity and low-reactivity LCFs in a light-duty compression-ignition engine by experimentally assessing their implementation with existing hardware (drop-in) and discussing possible modifications to achieve improved results through calibration. The results revealed that high-reactivity fuels could operate under similar conditions to the standard diesel fuel, exhibiting similar efficiency and soot emissions combined with important reductions in NOx emissions. On the other hand, low-reactivity LCFs show advantages in promoting alternative combustion concepts because of the longer ignition delays, which make it possible to use lower injection pressures and have more stratified air–fuel mixtures, but at the same time, the results revealed that the implementation of these fuels might suffer from hardware limitations which worsens the combustion stability and the operation at low-load. Although the fuels tested exhibited variations in Tank-to-Wheel due to their carbon content and fuel economy, it was found that Well-to-Wheel CO2 intensity is eventually dominated by the renewable content of the fuel and the CO2 is greatly impacted by its production pathway, rather than the fuel consumption optimization. Finally, the study confirmed that, for the successful adoption of oxygenated low-carbon fuels (including alcohols), appropriate material selection in the engine fuel system and fuel additives are necessary to prevent components and seal performance degradation.

Direct numerical simulation of ammonia/n-heptane dual-fuel combustion under high pressure conditions

Ammonia (NH3) combustion has received increasing attention for its carbon-free nature. However, the utilization of ammonia in engines is constrained by its relatively low reactivity. To overcome this shortcoming, ammonia blending with high-reactivity fuels has been used. In the present work, direct numerical simulation (DNS) of ammonia/n-heptane dual-fuel combustion was performed to understand the effects of ammonia addition on the combustion behavior of n-heptane sprays. Three DNS cases with NH3 energy ratios of 0%, 20%, and 40% were considered. Two-stage combustion was observed in all cases. It was found that both the first-stage and second-stage ignition delay times increase with increasing NH3 energy ratio, and the peak of mean heat release rate (HRR) reduces with increasing NH3 energy ratio. For low-temperature combustion, ignition kernels were observed in the case with pure n-heptane while ignition occurs almost volumetrically in the cases with NH3 addition. The reaction zone of the cases with NH3 addition is thicker compared to that of the case with pure n-heptane. The reactions of NH3 involve radicals, such as OH, which affects the low-temperature combustion of n-heptane. In the stage of high-temperature combustion, individual ignition kernels with significant heat release rate were observed in the cases with NH3 addition, where the reaction zones are thin. In contrast, for the case with pure n-heptane, large heat release rate occurs simultaneously over the entire domain with thick reaction zones. Ignition occurs in regions characterized by low scalar dissipation rate for both low-temperature and high-temperature combustion in all cases. It was found that, though the contributions of NH3-related reactions to the total HRR are noticeable, the contributions of important elementary reactions to the total HRR in pure n-heptane combustion still prevail in the cases with NH3 addition, which indicates that n-heptane chemistry dominates that of NH3 in this work.

Predicting the effects of direct-injected fuels co-powered by high-CO2 biogas on RCCI engine emissions using kinetic mechanisms and multi-objective optimization

High inert gas content in biogas resulted in poor burning and emissions attributes, though scarcely investigated in reactivity controlled compression ignition (RCCI). Established kinetic mechanisms were combined with multi-objective optimization to investigate, predict, and analyze emissions occurrence and trade-offs for reduced environmental impacts. The work examined the impact of direct-injected high reactivity fuels (HRF) and port-injected Biogas at various inert gas (carbon dioxide, CO2) rates (25 – 45% vol), biogas fractions (40 – 70%), speeds (1600 – 2000 rpm), and loads (4.5 – 6.5 bar IMEP) on emissions of RCCI engine, experimentally. The findings revealed that while engine speeds greatly decreased CO (carbon monoxide) and NOx (nitrate oxide) emissions with rising unburned hydrocarbon (UHC) regardless of HRF employed, higher engine load significantly reduced UHC emissions. Diesel-biogas reduces NOx emissions and performs better in reducing CO and UHC emissions at lower speeds than B5-biogas, except in low-level loads. Although increasing CO2 impact led to a reduction in UHC and CO emissions, the biogas proportion was the most significant variable. The main factor influencing increased NOx emissions was engine load, which is inversely correlated with reduced NOx and increased particulate emissions owing to high CO2 content and biogas proportion. The premixed mode's optimisation outcome confirms the trade-off reduction at 5.5 bar IMEP, 35.586% CO2, and 50% fraction. As a result, running the RCCI engine with direct-injected diesel co-powered in equal proportion with high-CO2 biogas cuts the emissions trade-off dramatically, limiting the environmental repercussions of the emissions.

Probing the effect of fuel components on the auto-ignition behavior of ammonia/natural gas blends: A case study of ethane addition

Ammonia (NH3) is carbon-free and is known as a promising renewable fuel to achieve carbon reduction and carbon neutrality. However, ammonia presents much lower fuel reactivity compared to hydrocarbon fuels. Thus, ammonia is often blended with high reactivity fuels in practical combustion facilities, and natural gas is a good candidate due to its low carbon emissions and massive reserves. Methane is often used to represent natural gas in previous experimental and simulation studies, while the larger alkanes in natural gas can affect the fuel reactivity of ammonia/natural gas blends. This study aims to explore the effects of fuel components on the auto-ignition behaviors of ammonia/natural gas blends. As methane and ethane are the most important species in natural gas, a blend of 90 % methane and 10 % ethane by volume is used to represent natural gas in this study. A low-pressure shock tube was used to measure the ignition delay times (IDTs) of ammonia/methane, ammonia/ethane, and ammonia/natural gas blends in ‘air’ mixtures, with different ammonia blending ratios, at stoichiometric condition, at 1 atm and over the temperature range 1200 – 1800 K. The experimental results show that the IDTs of fuel blends increase with ammonia blending ratios. Meanwhile, 10 % ethane in natural gas can influence the fuel reactivity of ammonia/natural gas blends. A detailed kinetic model proposed in our previous study is used to simulate these new IDTs in addition to experimental data available in the literature. Overall, the kinetic model can predict well the auto-ignition behavior of ammonia/methane, ammonia/ethane, and ammonia/natural gas blends over a wide range of experimental conditions. The simulation and sensitivity analyses results show that the interaction reaction between ammonia and ethane, C2H6 + ṄH2 〈=〉 Ċ2H5 + NH3, is important for predicting the fuel reactivity of ammonia/ethane blends below 1600 K at the studied conditions.

Optical engine experiments on combustion and emission performance of n-dodecane/ammonia dual fuels

Ammonia is regarded as a promising carbon-free fuel, but it suffers from poor ignition and combustion as well as heavy emissions. High-reactivity fuels are often adopted as ignition boosters to solve these issues. In this work, the combustion and emission characteristics of n-dodecane and ammonia dual fuels were studied in an optical engine, with emphasis on the role of n-dodecane addition and injection timing. Simultaneous pressure acquisition, high-speed photography, and FT-IR spectroscopy were adopted for measurements. The results demonstrate the significance of ammonia energy ratio and n-dodecane injection timing in improving engine performance. Specifically, engine combustion is suppressed as ammonia energy ratio is raised, manifesting decreased peak pressure, postponed combustion phasing, and enlarged cyclic variations. Visualizations show that such changes are closely related to the evolutions of flame initiation and development. Meanwhile, there are always unburned regions which become pronounced at higher ammonia energy ratios, answering for the massive emissions of NH3 and NOx. Furthermore, with the advance of n-dodecane injection timing, engine performance is first improved and then suppressed, featuring nonmonotonic variations in in-cylinder pressure and combustion evolutions. Particularly, ignition delay and late-stage combustion duration are largely prolonged, which is associated with the weakness of fuel stratification and combustion mode transition. The advance of n-dodecane injection timing facilitates unburned NH3 reduction while increasing NOx and N2O emissions. High thermal efficiency while reduced pollution emissions can be achieved under suitable injection timing conditions. This work can provide useful insights into the combustion control of diesel and ammonia dual-fuel engines.

Experimental Characterization of Hydrocarbons and Nitrogen Oxides Production in a Heavy-Duty Diesel–Natural Gas Reactivity-Controlled Compression Ignition Engine

Reactivity-Controlled Compression Ignition (RCCI) combustion is considered one of the most promising Low-Temperature Combustion (LTC) concepts aimed at reducing greenhouse gases for the transportation and power generation sectors. Due to the spontaneous combustion of a lean, nearly homogeneous mixture of air and low-reactivity fuel (LRF), ignited through the direct injection of a small quantity of high-reactivity fuel (HRF), RCCI (dual-fuel) shows higher efficiency and lower pollutants compared to conventional diesel combustion (CDC) if run at very advanced injection timing. Even though a HRF is used, the use of advanced injection timing leads to high ignition delays, compared to CDC, and generates high cycle-to-cycle variability, limited operating range, and high pressure rise rates at high loads. This work presents an experimental analysis performed on a heavy-duty single-cylinder compression ignited engine in dual-fuel diesel–natural gas mode. The objective of the present work is to investigate and highlight the correlations between combustion behavior and pollutant emissions, especially unburned hydrocarbons (HC) and oxides of nitrogen (NOx). Based on the analysis of crank-resolved pollutants measurements performed through fast FID and fast NOx systems under different engine operating conditions, two correlations were found demonstrating a good accordance between pollutant production and combustion behavior: Net Cyclic Hydrocarbon emission—cyclic IMEP variations (R2 = 0.86), and Cyclic NOx—maximum value of the Rate of Heat Released (R2 = 0.82).

Investigations on reactivity controlled compression ignition combustion with different injection strategies using alternative fuels produced from waste resources

Reactivity-controlled compression ignition (RCCI) is a promising low-temperature combustion (LTC) strategy that results in low oxides of nitrogen (NOx) and soot emissions while maintaining high thermal efficiency. At the same time, RCCI leads to increased unburned hydrocarbon (HC) and carbon monoxide (CO) emissions in the exhaust, particularly under low loads. The current work experimented novel port-injected RCCI (PI-RCCI) strategy to overcome the high unburned emission limitations at low load conditions in RCCI. PI-RCCI is a port injection strategy in which low-reactivity fuel (LRF) is injected using a low-pressure injector, and the high-reactivity fuel (HRF) is injected through a high-pressure common rail direct injection (CRDI) injector. The low volatile HRF is injected into a heated fuel vaporizer maintained at 180°C in the intake manifold during the suction stroke. Modifying a single-cylinder, light-duty diesel engine with the necessary intake and fuel injection systems allows engine operation in both RCCI and PI-RCCI modes. Alternative fuels from waste resources such as waste cooking oil biodiesel (WCO) and plastic waste oil (WPO) are used as the HRF and LRF fuel in RCCI and PI-RCCI. To achieve maximum thermal efficiency in RCCI, the premixed energy ratio and the start of injection of the direct-injected fuel are optimized at all load conditions. The engine performance and exhaust emissions characteristics in PI-RCCI are compared with RCCI as a baseline reference. The results show a 70% and 48% reduction in CO and HC emissions, respectively, in PI-RCCI than in RCCI. Further, the brake thermal efficiency (BTE) was enhanced by around 20%, and the brake-specific fuel consumption (BSFC) was reduced by 13% in PI-RCCI. The NOx emissions decreased without any considerable changes in soot emission in PI-RCCI. The current study shows that fuels derived from waste resources can be used in RCCI and PI-RCCI modes with better engine performance and lower emissions.

Comparison of direct and port injection of methanol in a RCCI engine using diesel and biodiesel as high reactivity fuels

In this work, methanol dual-direct injection (DI2 mode) was compared with the port injection of methanol (RCCI mode) when biodiesel and diesel fuel were employed as HRFs (High Reactivity Fuels). This study presents an innovative dual-direct injection strategy as an alternative method to the commonly utilized port injection strategy in RCCI engines, and evaluates its impact on both engine performance and pollutant emissions. The engine was operated at a constant speed of 2400 rpm by a single-cylinder diesel research engine, and BMEP of 1.57, 3.15, and 4.72 bar (20 %, 40 %, and 60 % of max torque) with methanol was injected using both the DI2 and RCCI modes while diesel fuel and biodiesel were injected directly. In reactivity-controlled compression ignition (RCCI) mode, NOx emissions were reduced by 60 % and 62 % for diesel fuel and biodiesel, respectively, while the reduction accounted as 92 % and 80 % in smoke opacity. However, CO and unburned HC emissions were measured as higher for advanced modes than conventional mode using diesel fuel and biodiesel. According to experimental results obtained for DI2, unburned HC and CO emissions were reduced (up to 63 % and 22 %) when compared with RCCI mode. In addition, for DI2, NOx emissions were slightly higher than in RCCI mode, mainly due to the high injection pressure (50 bar) of DI2 mode. In comparison to conventional diesel injection (CDI), cylinder pressure and heat release rate (HRR) decreased as the premixed ratio (Rp) increased at 20 % load, while cylinder pressure and HRR increased with load and Rp increased. In both diesel and biodiesel experiments, ignition delay increased as Rp increased. The ignition delay was found to be longer in RCCI mode than in DI2 mode. It was concluded that DI2 was a beneficial way to control inefficient combustion during RCCI operation fuelled by diesel fuel and biodiesel.

Observation of two different cool flame regimes of diethyl ether in a counterflow burner

This short communication reports, for the first time, the existence of two different self-sustaining cool flame regimes of diethyl ether (DEE) in a diffusion counterflow burner: a weaker autoignition-assisted cool flame near the fuel burner and a normal diffusion cool flame near the stagnation plane. The results show that the normal diffusion cool flame extinction limit increases monotonically with the fuel mole fraction, while the autoignition-assisted cool flame approaches a plateau and can exist at a fuel mole fraction below the normal diffusion cool flame. It is shown that both flame regimes are governed by the same low-temperature chain-branching reaction pathway of DEE. By using in situ laser diagnostics, entrainment of unburned fuel stream to oxygen stream at the outer edge of the fuel burner is identified as the governing physical mechanism causing a partially premixed self-sustained hollow cool flame structure. The results reveal that when a fuel with high low-temperature reactivity, two different cool flame regimes can be observed in a counterflow flame experiment. Future studies with high-reactivity fuels in a counterflow burner must ensure to distinguish between the two self-sustaining cool flame regimes. Moreover, the existence of these different cool flame regimes needs to be examined so that they would not trigger an uncontrolled combustion phasing in advanced engines fed with high low-temperature reactivity fuels.