Exhaust Gas Recirculation Research Articles

Thermocatalytic hydrogen production by integrated multi-stage steam methane fuel processing with an exhaust gas recirculation loop in a high-temperature fuel cell power plant

Abstract Exhaust gas recirculation (EGR) has been identified as a key adaptation for gas turbines to mitigate the efficiency penalty of amine-based postcombustion carbon capture. By reducing the exhaust gas flow rate and increasing CO2 concentration, EGR enhances capture efficiency while also significantly lowering NOx emissions and residual O2, both of which influence amine stability. However, the impact and limitations of EGR in dynamic operation remain largely unexplored, representing a critical gap in operability assessments. Understanding these aspects can facilitate the development of control strategies and optimization algorithms where EGR serves as an additional control variable to mitigate pollutant emission peaks. To address this, a 3 kWe microgas turbine (mGT) setup with enhanced EGR was tested under sudden EGR variations, while monitoring emissions. Additionally, emissions were measured during hot starts with different pre-applied recirculation levels. Results indicate that NOx peaks occur during EGR transitions, and high EGR rates prevent successful turbine startup. Identifying these transient behaviors, in terms of emissions and combustion stability, provides initial insights into defining operability boundaries for EGR application. The development of control algorithms to manage EGR levels is left for future work.

<div>Ducted fuel injection (DFI) was tested for the first time on a production multi-cylinder engine. Design-of-experiments (DoE) testing was carried out for DFI with a baseline ultra-low sulfur diesel (ULSD) fuel as well as three fuels with lower lifecycle carbon dioxide (CO<sub>2</sub>) emissions: renewable diesel, neat biodiesel (from soy), and a 50/50 blend by volume of biodiesel with renewable diesel denoted B50R50. For all fuels tested, DFI enabled simultaneous reductions of engine-out emissions of soot and nitrogen oxides (NOx) with late injection timings. DoE data were used to develop individual calibrations for steady-state testing with each fuel using the ISO 8178 eight-mode off-road test cycle. Over the ISO 8178 test, DFI with a five-duct configuration and B50R50 fuel reduced soot and NOx by 87% and 42%, respectively, relative to the production engine calibration. Soot reductions generally decreased with increasing engine load. Hydrocarbon and carbon monoxide emissions tended to increase with DFI but were not excessive over the ISO 8178 test. Brake-specific energy consumption generally increased with DFI due to the use of retarded injection timings and exhaust-gas recirculation to achieve the desired NOx reductions but was less than or equal to that for conventional diesel combustion with ULSD at a similar NOx level. Significant deposits were encountered on one cylinder when running at idle with the ULSD fuel only, but this was mitigated by replacing the corresponding fuel injector (which showed deformation at the exits of two of its orifices) and using a fuel detergent additive in subsequent testing. In all, the engine was successfully operated for over 300 hours in the DFI configuration. Research areas for improved DFI implementation are identified.</div>

This review establishes ammonia/dimethyl ether (NH3/DME) dual‐fuel combustion as a transformative decarbonization pathway, moving beyond simple fuel blending to address critical combustion bottlenecks. Its distinctive contribution lies in mechanistically elucidating how DME's kinetic interactions stabilize high‐ratio ammonia combustion. Key innovations include: 1) achieving a 99% ammonia energy ratio via high‐pressure dual‐fuel injection, yielding a 46.5% reduction in CO2 emissions while maintaining over 49% indicated thermal efficiency; 2) revealing DME's critical role in generating OH radicals via H2O2 decomposition pathways, thereby overcoming ammonia's slow reaction kinetics and reducing ignition delay by 92%; and 3) resolving the NOx–NH3 emission tradeoff through optimized injection phasing (–6 to –2° CA) and exhaust gas recirculation strategies compliant with Tier III standards. These findings provide a practical roadmap for maritime applications leveraging existing infrastructure, directly advancing IMO 2050 decarbonization targets.

Abstract The present study highlights the impact of exhaust gas recirculation (EGR) on the performance, emissions, and combustion characteristics of a diesel engine using blends of diesel, n‐butanol, and waste‐cooking vegetable oil (WCVO). The experiments are performed in a single‐cylinder, 4‐stroke water‐cooled diesel engine under variable engine loads (0, 4, 8, 12, and 16 N) with a constant speed of 1500 rpm (compression ratio of 18:1). Three WCVOs (Ground nut, Palm, and Sunflower) in volumes of 20% each along with n‐butanol at 10% and diesel at 70% are considered for the analysis. The diesel engine adjusts the EGR at 15% and 25%. For these cases, performance features such as brake thermal efficiency (BTE), brake‐specific fuel consumption, brake‐specific energy consumption, and exhaust gas temperature are evaluated. The BTE was observed to decrease with the WCVO blends due to their higher oxygen content and lower calorific value. The BTE value is also increased for the Palm WCVO at the 15% EGR with the increase in engine load. The emission parameters like nitrogen oxides (NOx), carbon monoxide, carbon dioxide, oxygen, and smoke opacity are also tracked, and it was found that the NOx and particulate matter inside the engine cylinder are reduced using the EGR. For all loads, the Sunflower (20%) with 15% EGR reduces the NOx by 5%–10% compared to diesel. The combustion parameters, like cylinder pressure and heat release rate, are also analyzed for all fuel blends.

Improving the efficiency of spark-ignited (SI) engines while simultaneously reducing emissions remains a critical challenge in meeting global energy demands and increasingly stringent environmental regulations. Lean burn combustion is a proven strategy for increasing efficiency in SI engines. However, the air dilution level is limited by the mixture’s ignition ability and poor combustion efficiency and stability. A promising method to extend the dilution limit and ensure stable combustion is the implementation of an active pre-chamber combustion system. The pre-chamber spark-ignited (PCSI) engine facilitates stable and rapid combustion of very lean mixtures in the main chamber by utilizing high ignition energy from multiple flame jets penetrating from the pre-chamber (PC) to the main chamber (MC). Together with the increase in efficiency by dilution of the mixture, nitrogen oxide (NOX) emissions are lowered. However, at peak efficiencies, the NOX emissions are still too high and require aftertreatment. The use of exhaust gas recirculation (EGR) as a dilutant might enable simple aftertreatment by using a three-way catalyst. This study experimentally investigates the use of EGR as a dilution method in a PCSI engine fueled by methane and analyzes the benefits and drawbacks compared to the use of air as a dilution method. The experimental results are categorized into three sets: measurements at wide open throttle (WOT) conditions, at a constant engine load of indicated mean effective pressure (IMEP) of 5 bar, and at IMEP = 7 bar, all at a fixed engine speed of 1600 rpm. The experimental results were further enhanced with numerical 1D/0D simulations to obtain parameters such as the residual combustion products and excess air ratio in the pre-chamber, which could not be directly measured during the experimental testing. The findings indicate that air dilution achieves higher indicated efficiency than EGR, at all operating conditions. However, EGR shows an increasing trend in indicated efficiency with the increase in EGR rates but is limited due to misfires. In both dilution approaches, at peak efficiencies, aftertreatment is required for exhaust gases because they are above the legal limit, but a significant decrease in NOX emissions can be observed.

This study investigates the reduction of nitrogen oxide (NOx) emissions in diesel engines using computational simulations performed with Diesel-RK. Three in-cylinder control strategies were evaluated: exhaust gas recirculation (EGR), direct water injection, and variations in combustion chamber depth. Simulations were conducted on a six-cylinder, four-stroke diesel engine at engine speeds ranging from 1000 to 3000 rpm. The results show that increasing EGR lowers NOx emissions by reducing peak combustion temperatures through oxygen dilution and enhanced mixture heat capacity. Water injection was found to be the most effective strategy, reducing NOx to nearly negligible levels across all speeds by combining evaporative cooling, heat absorption, and oxygen displacement. Changes in combustion chamber depth had a moderate influence, with shallower chambers improving swirl and mixing, thereby lowering local temperature peaks and suppressing NOx formation. Overall, the findings highlight water injection as the most effective standalone strategy, while combining it with optimized chamber geometry and moderate EGR offers a promising integrated approach for NOx control in diesel engines.

Exploring the usability of biohydrogen and biodiesel in internal combustion engines with exhaust gas recirculation: RSM-ANN-Based prediction and optimization

Abstract Reducing the carbon impact of power-generation and industrial gas turbines can be achieved through blending of low-carbon fuels as well as carbon capture. In gas turbines, carbon capture is facilitated by the use of exhaust gas recirculation (EGR), where exhaust gases are mixed with inlet gases to increase carbon dioxide concentrations in the exhaust, which in turn increases the efficiency of carbon capture technologies. However, increasing the diluent fraction of the reactants can decrease flame static stability and lead to flame elongation, potentially affecting combustion efficiency and emissions. In this study, we measure emissions in a single-nozzle model combustor at a range of EGR levels and compositions for blends of natural gas (NG) and hydrogen (H2) to understand the impact of EGR on combustion efficiency. Different blends of diluents that mimic the effects of EGR are tested with oxygen mole fractions from 21% (no diluent injection) down to 15%. Blends of NG and H2 are tested with up to 40% H2 by volume. Flame imaging is used to better understand the connection between EGR, fuel composition, flame stabilization, and combustion efficiency. As EGR level increases, the flame becomes longer and more diffuse. H2 blending aids flame stabilization and combustion efficiency. The experimental results are complemented by detailed chemical kinetic modeling to identify the changes in reaction pathways that are driven by high levels of diluents and various fuel compositions. We conclude with a discussion of the impact of low combustion efficiency on both cycle efficiency as well as the performance of downstream carbon capture systems.

Low temperature reactivity controlled compression ignition strategy using compressed natural gas-isobutanol blends and exhaust gas recirculation for enhanced emission control

In this study, the effects of carbon nanotube (CNT)-blended fuels on engine noise and vibration levels were experimentally investigated in a single-cylinder, air-cooled, direct-injection diesel engine. CNTs were introduced as single-wall (SWCNT) and multi-wall (MWCNT) variants at concentrations of 25 ppm and 50 ppm. Tests were performed at engine loads of 0%, 25%, 50%, 75%, and 100%, and EGR (Exhaust Gas Recirculation) rates of 0%, 10%, and 20%. The results showed a clear trend of increasing noise and vibration levels with rising engine load. For instance, the noise level for D100 fuel rose from 94.84 dB (0% load) to 96.71 dB (100% load), while SW50 increased from 95.92 dB to 97.80 dB, and MW50 from 95.12 dB to 97.61 dB. Regarding vibration, D100 increased from 96.23 m/s² to 96.94 m/s², whereas SW25 showed a rise from 89.17 m/s² to 101.90 m/s², and MW50 maintained more stable values from 98.28 m/s² across the load range. Increasing the EGR rate generally reduced both acoustic parameters, especially under low-load conditions. Notably, MW50 fuel yielded the most consistent reduction in vibration, while SW50 tended to amplify noise at full load. These findings suggest that MWCNTs, particularly at higher concentrations, offer improved vibration mitigation, whereas SWCNTs may enhance noise under certain conditions. The combined use of CNT additives and EGR presents a promising strategy for tuning diesel engine acoustic behavior.

Abstract This research presents an innovative dual-fuel methodology that incorporates hydrogen fumigation at a sustained flow rate of 1.0 L/min in conjunction with biodiesel synthesized from waste Sapota (Achras zapota) seed oil, aiming to improve the operational efficiency and environmental efficacy of a compression-ignition engine. The synthesis of Sapota seed oil methyl ester (SOME) was accomplished through the transesterification process of Soxhlet-extracted oil, resulting in a yield of 91.5%. The biodiesel produced demonstrated a calorific value of 39.1 MJ/kg, a kinematic viscosity of 4.5 mm²/s, and a flash point of 148°C, in compliance with ASTM D6751 specifications. An experimental analysis was conducted on a 5.2 kW single-cylinder diesel engine utilizing diesel–SOME blends (B10, B20, and B30). Among the evaluated blends, the B20 configuration with hydrogen fumigation exhibited the most favorable outcomes, enhancing brake thermal efficiency from 28.4% (baseline diesel) to 33.2% and decreasing brake-specific fuel consumption by 6.4%. Significant reductions in emissions were also recorded, with carbon monoxide diminishing by 38%, unburned hydrocarbons by 34%, and smoke opacity by 25%. Conversely, nitrogen oxides emissions escalated by 9.6% due to increased in-cylinder temperatures attributed to the elevated flame speed of hydrogen, underscoring the necessity of integrating aftertreatment systems such as exhaust gas recirculation or selective catalytic reduction. This study represents the inaugural report on the amalgamation of SOME with hydrogen fumigation within a compression-ignition engine framework, demonstrating a technically feasible strategy for waste valorization and the generation of low-emission, high-efficiency energy without necessitating modifications to the engine. The findings provide significant contributions to the progress of sustainable and cleaner internal combustion technologies.

Automobile emissions have significantly intensified environmental degradation, contributing to climate change. Extensive research is underway to identify alternative fuels that can be renewable and enhance engine efficiency while minimising emissions. This study evaluates the performance and emission characteristics of a diesel engine powered by novel ternary blends of diesel, biodiesel derived from waste cooking oil (WCO), waste plastic oil (WPO), and ethanol (DBE blend). The blends were tested at varying load conditions and cold Exhaust Gas Recirculation (EGR) rates of 7% and 14% on a single-cylinder, 4-stroke, and 3.2kW power CRDI (Common rail direct injection) diesel engine. Biodiesel was synthesised via a two-step esterification process, meeting ASTM standards. Key findings include improved BTE and reduced emissions with increased ethanol content. The D40CB10E10 blend demonstrates the highest BTE among the biodiesel blends. EGT decreases with rising ethanol content compared to diesel. The D60CB20E20 blend exhibited the lowest NOx emissions (9.98% lower than diesel) and the lowest smoke density. Ethanol's oxygenation and heat of vaporisation improved combustion, reducing EGT and CO emissions. However, HC emissions increased with ethanol. These results demonstrate that DBE blends can enhance engine efficiency and reduce emissions, offering a sustainable alternative to conventional diesel.

This paper aims to analyse the evaluation of alternative technologies that comply with Nitrogen Oxides (NOx) emissions limitations of ships. After Maritime Pollution Convention (MARPOL) Annex VI NOx regulations came into force, shipowners have increasingly turned to alternative technologies and fuels that would comply with Tier II and Tier III limits. Selecting the most suitable sustainable alternative technology is a critical and complex task for the shipowners/ship operators as many important criteria must be evaluated. Consequently, in this paper, Multi-Criteria Decision-Making Methods (MCDM) – namely Fuzzy Analytic Hierarchy Process (FAHP) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS)-are used to identify the best NOx emission reduction technology and assist decision-makers. Based on the weighted criteria and the characteristics of alternative technologies, the TOPSIS method identified the best alternative technologies for the new and existing ships as Exhaust Gas Recirculation (EGR) (Ci = 0.7) and Selective Catalytic Reduction Systems (SCR) (Ci = 0.684).

Machine learning-enhanced optimization of exhaust gas recirculation strategies for superior diesel engine performance and emissions control: A synergistic experimental and computational study

This study numerically investigates the effect of exhaust gas recirculation (EGR) and fuel-premixing ratio on the PAH emissions (naphthalene, benzo[a]pyrene, phenanthrene, acenaphthene, pyrene, benzo perylene, chrysene, and benzo [g,h,i]perylene), its precursors (such as C2H3, C2H2, C4H5, C4H3, C6H5, C6H6), and their toxicity potential from conventional diesel and hydrogen/diesel dual-fuel (HDDF) engine. The aim of this investigation is to understand the effect of EGR mass fraction and hydrogen energy share (HES) on the toxicity potential of PAHs and total PAH mass emission in conventional diesel and dual-fuel combustion under different loading conditions. This study is focused on the thermal, chemical, and dilution effects of EGR with and without HES on PAH formation. The simulations are performed on ANSYS Forte using a detailed chemical mechanism of diesel surrogate (66.8% n-decane/33.2% alpha-methylnaphthalene). The reaction mechanism used for simulation consists of 189 species and 1392 reactions. Results demonstrate that as the EGR increases from 10 to 30% in conventional diesel combustion, the toxicity equivalent potential of PAHs increases by 25% at lower engine load. However, at a fixed engine load and a constant EGR level of 30%, increasing the hydrogen energy share (HES) to 30% results in a 33% reduction in the toxicity equivalent potential of PAHs. This reduction highlights the potential benefits of hydrogen addition alongside EGR in future hydrogen-diesel dual-fuel engines. Additionally, the incorporation of hydrogen significantly reduces the mass emission of PAHs.

This study explores the impact of intake air cooling intensity, defined by heat exchanger effectiveness (HEE) and exhaust gas recirculation (EGR), on the energy and environmental performance of a turbocharged compression ignition (CI) engine. Experimental investigations were conducted on a 1.9-litre CI engine operating at 2000 rpm under three brake mean effective pressure (BMEP) conditions (0.2, 0.4, and 0.6 MPa), which correspond to part-load engine operation. HEE was varied at 0%, 50%, and 100%, in both EGR-on and EGR-off modes. Additional numerical simulations were carried out using AVL BOOST software to analyze combustion dynamics, including engine operating cycle modeling to validate the accuracy of the combustion analysis. The results demonstrate that increasing HEE significantly improves cylinder filling and excess air ratio, leading to enhanced combustion efficiency and lower in-cylinder temperatures. This, in turn, reduces specific NOx emissions by approximately 40% with EGR and approximately 60% without EGR; however, under EGR-on conditions, the reduced combustion intensity leads to increased smoke and unburned hydrocarbon emissions—particularly at high cooling intensities. This effect is primarily associated with the engine control unit’s (ECU) limitations on intake air mass flow to maintain the target EGR ratio. Integrated control of HEE and EGR systems improves engine performance and reduces emissions across varying conditions, while highlighting trade-offs that inform the refinement of air management strategies.

"Experimental Investigation on the Effect of Injection Timing and EGR on Performance and Emissions of a Compression Ignition Engine Fueled with Diesel and Blends of Waste Cooking Oil Biodiesel Start of Injection (SOI) or injection timing refers to the crank angle degree (CAD) at which the fuel injection into the combustion chamber commences, typically before Top Dead Center (bTDC) of the compression stroke. The timing of fuel injection plays a critical role in the combustion process, influencing the engine's power output, fuel economy, and emission levels. Diesel engines are widely adopted across defense, transport, power generation, and agricultural sectors due to their high efficiency, durability, and torque characteristics. However, the growing environmental concerns and the scarcity of fossil-based diesel fuels have steered research toward cleaner, renewable alternatives such as biodiesel. Biodiesel, derived from renewable feed stocks such as waste cooking oil (WCO), not only supports energy diversification but also significantly reduces carbon monoxide (CO), unburned hydrocarbons (HC), and particulate matter (PM) emissions when compared to conventional diesel (D100). The only major drawback observed is a tendency for increased nitrogen oxide (NOx) emissions due to higher oxygen content. To mitigate NOx emissions, exhaust gas recirculation (EGR) is employed effectively as an in-cylinder NOx control strategy. In this study, a single-cylinder, four-stroke direct injection diesel engine is tested using pure diesel (D100) and two biodiesel blends: WCOBD20 (20% biodiesel, 80% diesel) and WCOBD40 (40% biodiesel, 60% diesel). The experiments are carried out at a constant injection pressure of 220 bar and at three different injection timings: 21°, 23°, and 25° bTDC. EGR flow rates of 0%, 10%, and 20% are applied at part-load operation to observe their effect on NOx reduction. Engine performance metrics such as brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), and brake power (BP) are measured along with emission characteristics like CO, HC, NOx, smoke opacity, and exhaust gas temperature (EGT). In addition, combustion parameters including ignition delay, peak pressure, combustion duration, and heat release rate are analyzed. The outcomes of this investigation aim to offer deeper insights into optimizing injection timing and EGR strategy for achieving cleaner combustion and better efficiency while using sustainable biodiesel-diesel blends in compression ignition engines.

Isolating the influence of exhaust gas recirculation on the ignition and combustion reaction for combustion control of polyoxymethylene dimethyl ethers (PODEn)/gasoline under various fuel distribution conditions

Numerical studies on a novel air path design featured with the sequential turbocharging and the low-pressure exhaust gas recirculation for the marine two-stroke engine fulfilling the IMO Tier Ⅲ regulation