Dual fuel combustion is a promising solution to lowering engine emissions and enabling higher fuel flexibility. High cyclic variability is one of the main challenges of dual fuel combustion which can compromise emissions, operational stability, and drivability. Recurrence quantification analysis (RQA) is a potent tool for the quantification of cyclic variability, however the application of RQA to engine cylinder pressure data represents methodological challenges. In this work, a systematic RQA approach for cyclic pressure variations is proposed with two main contributions: (1) a methodology to determine RQA parameters to capture inter-cycle recurrences, and (2) a sliding window technique to enable real-time crank-angle-resolved analysis. This methodology was demonstrated using four firing conditions with increased cyclic variability from the most stable neat diesel case to highly unstable dual fuel cases with increasing percentage of natural gas substitution. Phase-space reconstruction enabled qualitative investigation using recurrence plots. The results showed that RQA can capture nuances in the cyclic dynamics not readily captured using averaged integrated quantities such as the indicated mean effective pressure, cumulative heat release, or combustion phasing. The median recurrence rate (RR) was shown to consistently decrease with increased cyclic variability. Determinism (DET) decreases then increases with higher cyclic variability. This finding reveals the paradox that lower repeatability (or RR) does not necessarily lead to lower predictability (or DET) as is sometimes the case with partially misfiring cycles being followed by more robust recovery cycles. These results can have significant implications for real-time cycle-by-cycle engine control.

The transition toward sustainable energy systems necessitates innovative solutions that reduce greenhouse gas emissions while improving fuel efficiency in existing combustion technologies. Hydrogen has emerged as a promising clean energy carrier; however, its widespread deployment is limited by challenges associated with large-scale transportation and storage. This study investigates a practical alternative in which hydrogen-rich syngas produced via steam methane reforming (SMR) is directly integrated into the diesel engine intake, thereby eliminating the need for fuel transport, storage, and separation while supporting a more sustainable fuel pathway. A validated computational fluid dynamics (CFD) model was developed to examine the effects of varying SMR gas mixture ratios (0–20%) on engine combustion, performance, and emissions. The findings reveal that increasing the SMR fraction enhances in-cylinder pressure by up to 15.7%, heat release rate by 100%, and engine power output by 102.5% compared to conventional diesel operation. Additionally, under SMR20 conditions, CO2 emissions are reduced by approximately 12%, demonstrating the potential contribution of this approach to decarbonization and climate mitigation efforts. However, the rise in in-cylinder temperatures was found to increase NOx formation, indicating the necessity for complementary emission control strategies. Overall, the results suggest that direct SMR syngas integration offers a promising pathway to improve the environmental and performance characteristics of conventional diesel engines while supporting cleaner energy transitions.

In the pursuit of sustainable and clean energy, biofuels and nanofuels are increasingly investigated as practical solutions to improve diesel engine efficiency and emission characteristics. This study evaluates the effects of Nickel (III) oxide nanoparticles added to biodiesel (B20) and biodiesel-n-butanol (B20But10) blends on combustion, engine performance, emissions, and optimization using a single-cylinder, four-stroke, water-cooled, direct injection diesel engine. Experiments were conducted at multiple engine loads and nanoparticle concentrations ranging from 0 to 100ppm. At full load and 100ppm Ni₂O₃, peak in-cylinder pressure increased to 55.86bar for B20 and 55.45bar for B20But10, while maximum heat release rates reached 29.45 and 30.02J/°CA, indicating enhanced premixed combustion behavior. Brake thermal efficiency increased to 24.89% for B20 and 24.94% for B20But10, accompanied by reductions in brake specific fuel consumption to 0.309 and 0.333kg/kWh, respectively. Emission results showed reductions of 13-28% in HC, 8-43% in smoke opacity, and 12-21% in NOx. Response surface methodology was employed to develop both performance- and emission-oriented predictive models, yielding high reliability (R2 = 90.9-99.9%) and identifying optimal nanoparticle levels between 50 and 75ppm. Overall, Ni₂O₃-enhanced blends provide measurable performance and emission benefits without requiring engine modifications.

This study experimentally investigates the influence of Al?O? nanoparticle addition on the combustion, performance, and emissions of emulsified Jatropha biodiesel in a compression-ignition engine. An emulsified fuel blend comprising 88% Jatropha methyl ester (JME), 10% water (v/v), and 2% surfactant (B20W10) was prepared using ultrasonication, into which Al?O? nanoparticles were dispersed at concentrations of 25 ppm and 50 ppm. Tests were conducted at varying loads under constant speed to evaluate performance, combustion, and emission characteristics. Among the tested fuels, B20W10Al50 yielded the best outcomes, achieving a 2.03% increase in brake thermal efficiency (BTE) and a 3.84% reduction in brake specific fuel consumption (BSFC) compared to diesel, with statistical analysis confirming the significance of these improvements. Combustion analysis showed a modest increase in peak in-cylinder pressure for B20W10Al50. Emission reductions were substantial relative to diesel: unburned hydrocarbons decreased by 40%, CO by 66.7%, NOx by 22.7%, and smoke opacity by 41.7%. These findings demonstrate that nanoparticle-assisted emulsification can address the common biodiesel trade-offs between efficiency and NOx formation. The study highlights B20W10Al50 as a promising formulation for sustainable transport applications, while also noting the need for further research on long-term nanoparticle stability, injector compatibility, and durability under real-world operating conditions.

In the pursuit of cleaner and more efficient internal combustion engines, dual-fuel strategies combining diesel and hydrogen are gaining increasing attention. This study employs detailed computational fluid dynamics (CFD) simulations to investigate the behaviour of pilot diesel injections in dual-fuel diesel–hydrogen engines. The study aims to characterize spray formation, ignition delay and early combustion phenomena under various energy input levels. Two combustion models were evaluated to determine their performance under these specific conditions: Tabulated Well Mixed (TWM) and Representative Interactive Flamelet (RIF). After an initial numerical validation using dual-fuel constant-volume vessel experiments, the models are further validated using in-cylinder pressure measurements and high-speed natural combustion luminosity imaging acquired from a large-bore optical engine. Particular attention was given to ignition location due to its influence on subsequent hydrogen ignition. Results show that both combustion models reproduce the experimental behavior reasonably well at high energy input levels (EILs). At low EILs, the RIF model better captures the ignition delay; however, due to its single-flamelet formulation, it predicts an abrupt ignition of all available premixed charge in the computational domain once ignition conditions are reached in the mixture fraction space.

Controlling in-cylinder airflow is critical for enhancing fuel-air mixing, improving combustion efficiency, and reducing pollutant emissions in diesel engines. This study established a steady-flow intake test bench and used a four-valve diesel engine equipped with a helical/tangential dual intake system as the research subject. By integrating steady-flow measurements with computational fluid dynamics (CFD) simulations, the effects of helical/tangential intake valve lift asymmetry on in-cylinder flow, combustion, and emission characteristics were systematically investigated. The results establish clear design criteria for helical/tangential valve lift asymmetry: one intake valve should operate at the maximum lift (12 mm), while the other must be no less than two-thirds of the maximum (≥8 mm), with a lift difference exceeding 2 mm. Three representative asymmetry combinations—H9/T12, H12/T8, and H10/T12—were examined, all of which significantly enhanced intake performance. Compared with the baseline (H12/T12), these combinations increased the flow coefficient by up to 14.3% and the swirl ratio by up to 11.0%. Furthermore, they improved turbulent kinetic energy (TKE), reduced pumping losses, and enhanced overall intake efficiency, thereby promoting more uniform fuel–air mixing. In terms of combustion and emissions, the H9/T12 combination demonstrated the most favorable trade-off. Relative to the baseline, it increased peak cylinder pressure by 15.6% and cumulative heat release by 8.1%. Although nitrogen oxides (NO x ) emissions rose by 6.3%, soot and carbon monoxide (CO) emissions were simultaneously reduced by 21.4% and 12.0%, respectively. The helical/tangential intake valve lift asymmetry strategy provides an effective pathway for simultaneously improving combustion efficiency and optimizing emission characteristics. This study offers valuable technical guidance for the design of high-efficiency, low-emission diesel engines, contributing to future advancements in clean and sustainable engine technologies.

<div>This study investigated the combustion processes in hydrogen dual-fuel operation using hydrotreated vegetable oil (HVO) and diesel fuel as pilot fuels. The visualizations of hydrogen dual-fuel combustion processes were conducted using hydroxyl radical (OH*) chemiluminescence imaging in an optically accessible rapid compression and expansion machine (RCEM), which can simulate a compression and expansion stroke of a diesel engine. Pilot injection pressures of 40 and 80 MPa and injection quantities of 3, 6 mm<sup>3</sup> for diesel fuel and to match the injected energy, 3.14, 6.27 mm<sup>3</sup> of HVO were tested. The total excess air ratio was kept constant at 3.0. The RCEM was operated at a constant speed of 900 rpm, with in-cylinder pressure at top dead center (TDC) set to approximately 5.0 MPa. Results demonstrated that using HVO as pilot fuel, compared to diesel fuel, led to shorter ignition delay and combustion duration. OH* chemiluminescence imaging revealed that longer ignition delays observed with diesel fuel resulted in pilot mixture ignition downstream near the piston bowl wall, followed by flame propagation into the hydrogen–air mixture. In contrast, the shorter ignition delays characteristic of HVO caused the pilot mixture to ignite between the injector and the piston bowl wall, with subsequent flame propagation into the hydrogen premixture.</div>

Research on cylinder pressure identification method based on physical information

This study differs fundamentally from prior investigations on WPO-diesel and WPO-DEE blends by combining combustion-resolved experimentation with predictive modeling, thereby advancing WPO utilization from empirical testing toward optimization-oriented engine integration. It examines the performance, combustion, and emission characteristics of a single-cylinder variable compression ratio (VCR) diesel engine fueled with ternary blends of diesel, waste plastic oil (WPO), and diethyl ether (DEE). WPO was produced via catalytic pyrolysis of LDPE waste and blended with diesel at 15%, 20%, 25%, and 30% by volume, while DEE was maintained at a constant 10% to improve ignition quality, volatility, and atomization. Engine tests were performed at a constant speed of 1500 rpm under variable loads ranging from 2 to 12 kg to evaluate the influence of blend composition and operating conditions on brake thermal efficiency (BTE), brake specific fuel consumption (BSFC), combustion development, and regulated emissions (CO, HC, NOx, CO₂). The D65B25DE10 blend (65% diesel, 25% WPO, 10% DEE) demonstrated the best overall performance among the tested fuels, achieving a 22.22% reduction in CO and an 11.88% reduction in HC emissions compared with diesel, although BTE decreased by 6.93% and BSFC increased by 6.03% at full load. Combustion analysis revealed extended ignition delay and higher peak cylinder pressure for higher-WPO blends, while DEE improved vaporization and supported more complete oxidation. To complement the experimental work, a feed-forward artificial neural network (ANN) model with a 6-12-6 architecture was developed using blend ratio, load, compression ratio, and speed as inputs to predict BTE, BSFC, and emissions. The ANN achieved strong correlation with experimental data (R2 > 0.97), confirming its suitability for performance prediction and blend optimization. The combined experimental and computational approach offers a comprehensive framework for evaluating WPO-based fuels, extending beyond previous binary blend studies by revealing the synergistic effects of DEE in ternary blends and establishing a robust ANN model for predictive optimization. This methodology demonstrates the potential of WPO-based fuels to reduce fossil diesel dependence while promoting sustainable waste-to-energy utilization.

ABSTRACT This paper delves into the design, analysis, and manufacturing of a modified aluminium connecting rod for the Peugeot XU7JP engine (an inline 4-cylinder, 1.8 L gasoline engine with a compression ratio of 9.3:1 and a maximum cylinder pressure of about 5.5 MPa). The present study focuses on achieving substantial weight reduction and improving engine performance through optimised geometry and material substitution. Utilising a heat-treated 7075 aluminium scandium alloy, the connecting rod weight was reduced by 41.6% – a significant decrease of 279 g compared to the traditional steel rod – resulting in marked improvements in dynamic behaviour, including reduced inertia, vibrations, and noise. Finite element analysis demonstrated that the modified aluminium rod exhibits lower displacement and stress concentrations while maintaining the necessary structural integrity under operational loads. Additionally, buckling analysis revealed a higher buckling coefficient for the aluminium rod, indicating superior resistance to instability under compressive forces. These theoretical and computational findings were validated through practical engine tests on a dynamometer, which showed a 17–25% improvement in engine power and torque when using the aluminium connecting rod, with the modified design delivering 114 hp and 166 N⋅m of torque compared to the steel rod 89 hp and 137 N⋅m of torque.

This study examines the combustion and emission behavior of a four-stroke Wankel rotary engine operated in Homogeneous Charge Compression Ignition (HCCI) mode using n-heptane fuel over excess-air ratios of λ = 2.2–2.8. A validated three-dimensional CONVERGE CFD model of the Mazda Renesis RX-8 13B MSP engine—verified against experimental spark-ignition pressure data—was used to assess HCCI performance. The simulations employed semi detailed chemistry (SAGE), Renormalization Group (RNG) k–ε turbulence modelling, and adaptive mesh refinement to capture the multi-stage auto-ignition process. The analysis encompassed key combustion and emission indicators, including in-cylinder and peak pressure, heat-release rate, cumulative heat-release behavior, maximum pressure-rise rate, combustion-phasing metrics crank angle at 10% of total heat released (CA10), crank angle at 50% of total heat released (CA50), crank angle at 90% of total heat released (CA90), combustion duration, and major exhaust species carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC) and nitrogen oxides (NOₓ). Stable HCCI operation was achieved only within a narrow range: λ = 2.2–2.8. Richer mixtures exceeded the 10 bar/°CA knock-related pressure-rise-rate limit, while leaner mixtures resulted in misfire. Increasing λ delayed auto-ignition, weakened high-temperature oxidation, and extended combustion duration. CO and HC emissions rose with λ due to reduced combustion temperature and strong near-wall quenching driven by the Wankel chamber’s high surface-to-volume ratio, whereas NOx remained extremely low and nearly eliminated for λ ≥ 2.4. Overall, the findings confirm that HCCI can be successfully realized in a Wankel rotary engine with ultra-low NOx emissions, provided operation remains within its narrow λ window. These results underscore the potential of Wankel HCCI concepts for lightweight aviation and unmanned aerial vehicle (UAV) propulsion, while highlighting the need for improved mixture preparation and combustion-phasing control for practical implementation.

It has long been known that inlet port geometry plays a crucial role in regulating in-cylinder flow processes, significantly affecting combustion efficiency and engine emissions. This paper elucidates the effects of an intake port geometry modification, specifically the implementation of a novel moving deflector to intensify tangential intake flow, on fluid flow patterns, combustion stage, and exhaust emissions in a spark-ignited internal combustion engine. The analysis was performed using multi-dimensional numerical modeling of reactive flow, where the numerical domain was extended to the complete intake system to explicitly encompass the modification. The numerical model was validated against experimental data, showing excellent agreement, with differences in peak in-cylinder pressure and peak rate of heat release (RHR) kept below 3% and the moment of peak pressure being nearly identical to the experimental results. During the induction stroke, the effects of implemented modification through intensification of intake jet were clearly legible, pursued by deflection of smaller side vortices in the vicinity of the bottom dead-center. During compression, the attenuation of the effects of the earlier established macro flow was encountered and some negative effects of the increased intake jet were elucidated. During combustion the existence of “flame dominated fluid flow” controlled primarily by turbulence diffusion was encountered. Negative effects on exhaust emissions were elucidated as well. As the combustion process in spark ignition internal combustion engines is primarily controlled by turbulent diffusion, proper identification of influential types of organized flows is a challenging but very important task. The advantages offered by the application of numerical modeling in these situations are clear.

The study examines the performance, combustion, and emission characteristics of a diesel engine fueled with sesame biodiesel methyl ester (SBME)–diesel blends containing 0–100% SBME by volume (i.e., 0%, 10%, 20%, 30%, and 40% SBME blended with conventional diesel. The findings highlight significant improvements in combustion efficiency and emissions reduction with SBME compared to diesel, with 20 SBME emerging as the optimal blend. For 20 SBME, Brake Thermal Efficiency improves by 9.57%, and specific fuel consumption decreases by 7.94%, while Mechanical Efficiency increases by 0.618%. Indicated Power rises for 10, 30, and 40 SBME but slightly declines for 20 SBME by 0.66%. Emissions analysis shows reductions in CO₂, NO, and HC emissions for 10 and 20 SBME, with decreases of up to 3.5%, 19.18%, and 36.55%, respectively, while higher blends result in increased emissions. Combustion analysis reveals enhanced Cylinder Pressure Maximum (CPM) and Net Heat Release (NHR) for 20 and 30 SBME, alongside a reduction in Cumulative Heat Release (CHR) across all blends. Additionally, exhaust gas temperature and heat loss through radiation increase at higher loads. These results demonstrate that 20 SBME offers a balanced improvement in engine performance, combustion characteristics, and environmental impact, making it a viable alternative to conventional diesel.

Biomass-derived nanoparticles offer a safer alternative to metallic oxides, yet their use in diesel–biodiesel blends remains limited. This study explores the catalytic effect of corn stover–derived nanobiochar (NBC) on the performance, combustion, and emissions of Croton macrostachyus (CMS) biodiesel, compared with Al₂O₃ and TiO₂ nanoparticles, at 2700 RPM and 0–80% load. The novelty lies in revealing NBC’s influence on CP, apparent HRR, and ID period in CMS biodiesel blends. Experiments were conducted using six fuel blends: B20 (20% CMS biodiesel + 80% diesel), B20NBC30, B20NBC50, B20NBC75 (with 30, 50, and 75 ppm NBC, respectively), B20A50 (with 50 ppm Al₂O₃), and B20T50 (with 50 ppm TiO₂). The result shows that the NBC additive reduced the NOx, CO, and HC emissions by up to 9.8%, 8.4%, and 27.3% respectively, and BSFC by 2.2% and increased BTE by 2.6% compared to the B20. Conversely, TiO₂ and Al₂O₃ were more effective in reducing HC emissions, achieving reductions of 45.9% and 29.5% respectively, while reducing BSFC and increasing BTE compared to NBC additive. In addition, B20NBC50 exhibited an average reduction in cylinder pressure (CP) of 0.5%, heat release rate (HRR) of 517.51 J/oCA with a 2.7% increment and the shortest ignition delay (ID) and advanced start of combustion (SoC) compared to the B20, B20NBC30, and B20NBC75, that makes NBC at 50 ppm as the optimum concentration. NBC reduced NOx due to its higher specific heat capacity, while Al₂O₃ and TiO₂ increased it, demonstrating NBC’s advantage and potential as a sustainable, biomass-derived alternative to metallic nanoparticles.

Abstract This study examines the performance, combustion, emission, vibration, and acoustic behavior of a Common Rail Direct Injection (CRDI) engine fueled with diesel and hydrogen–ammonia dual‐fuel blends (FS1–FS4) at 4–8 L/min flow rates. The FS4 blend (8 L/min NH 3 + 8 L/min H 2 ) achieved the highest brake thermal efficiency of 31.55%, which is 34.4% higher than diesel, and reduced brake‐specific energy consumption by 12.4%. Cylinder pressure increased to 72.68 bar and the maximum heat release rate improved by 15.7%. In emission analysis, NO x levels increased slightly by 3.42%, while CO and HC emissions decreased by 14.6% and 11.3%, respectively, compared to diesel. Vibration and exhaust noise levels fell by 11.29% and 4.5%, demonstrating smoother operation. Gene expression programming (GEP) accurately predicted performance and NO x trends with R 2 = 0.996–0.998, confirming model reliability. The results highlight the promise of hydrogen–ammonia blends, particularly FS4, for improving energy efficiency, lowering fuel consumption, and reducing mechanical stress while maintaining acceptable emission trade‐offs in CI engines.

This study presents a comprehensive evaluation of waste cooking oil biodiesel blends (10%, 20%, and 25% by volume) in a single-cylinder, water-cooled diesel engine operating at constant 1500 rpm under varying load conditions. FTIR spectroscopy revealed characteristic biodiesel signatures, including strong ester carbonyl absorption at 1740 cm⁻¹ and weaker hydrocarbon peaks (2800-3000 cm⁻¹) as compared to diesel. The biodiesel, synthesized through optimized transesterification (0.85% KOH catalyst, 6:1 methanol-to-oil ratio, 60°C reaction temperature, 600 rpm agitation speed, and 1-hour duration), exhibited key fuel properties including a calorific value of 38.45 MJ/kg and viscosity of 4.5 cSt. So, pure biodiesel is not recommended for this engine without modifications. Performance analysis revealed that while the 25% blend achieved 12.57% higher indicated power at 4 kW brake power compared to diesel, it suffered a 9.60% reduction in mechanical efficiency and a 19.61% decrease in brake thermal efficiency. The specific fuel consumption increased progressively with blend ratio, reaching 36.94% higher values for the 25% blend relative to diesel. Combustion characterization demonstrated significant differences, with peak cylinder pressure increasing by 5.77% (72.4 bar vs. 68.5 bar for diesel) and ignition delay shortening by 2.1 crank angle degrees for the 25% blend at full load conditions. The cumulative heat release rose by 5.93%, while net heat release decreased by 12.40% due to elevated exhaust gas temperatures that were 42.29% higher than diesel. Notably, the 20% blend emerged as the optimal compromise, delivering a 3.62% increase in peak cylinder pressure (63.2 bar) with only a 7.01% mechanical efficiency penalty. The smoke emissions from every fuel blend tested fell below the maximum level permitted by the ISO 11614 standard, confirming their environmental safety. These findings provide critical insights into the trade-offs between enhanced combustion characteristics and reduced thermal efficiency when utilizing WCO biodiesel blends in conventional diesel engines

HCCI combustion is one of the combustion strategy that can help to suppress the emission of NOx and soot in diesel engine. Among the challenges faced are the homogenous mixture preparation, combustion phase and auto ignition control and limited working range. The low volatility of the diesel is one of the hurdle in preparing a homogenous mixture for auto ignition of HCCI engine. The experimental work focus on the impact of intake air temperature on the performance, combustion behavior and emission on HCCI mode. Air assisted injector of 5 bar with port fuel injection method was used on a single cylinder 4 stroke diesel engine. The intake air was heated at different temperatures of 450C,500C,550C and 600C. Highest BTE was recorded by intake temperature 600C with maximum efficiency of 22.3% at 20% load. Highest fuel efficiency also was showed by intake temperature 600C.The working range for intake temperature 600C is limited to 20% as beyond that load knocking occurred during combustion. Higher intake temperature increase ignition timing and in-cylinder pressure which contributed the knocking issue. Lowest emission of HC was also observed via intake temperature 600C. Intake temperature 450C contributed the lowest NOx with value ranging from 8 to 14 ppm but recorded the highest CO with values from 0.18 to 0.41% .

We perform a sensitivity analysis to investigate the influence of material input parameters on the pressure, stresses, and displacement of an isotropic porous solid cylinder representing hard mechanical systems such as the bone. We model the system using the governing equations of Biot's poroelasticity in cylindrical polar coordinates, where the solutions are found by enforcing radial boundary conditions. The sensitivity analysis is carried out on the solutions for the pressure, stress components and displacement using ranges of the investigated parameters representative of the bone. Our study finds that the time [Formula: see text] has the highest influence on the pressure, the stress components and displacements. We find that the Poisson ratio ν plays a greater role than shear μ in the pressure response, and the shear μ counts more than the other parameters in the radial and circumferential stresses. There are key joint interactions between the Biot's coefficient α and the Poisson ratio ν, the non-dimensionalised radius [Formula: see text] of the bone, and the Biot's modulus M when investigating interstitial pressure, which is a key value in bone remodelling and fracture healing. This study paves the way to a deeper understanding of the interplay of all the parameters that are necessary to capture the true behaviour of hard mechanical systems such as the bone and its potential remodelling.

An experimental verification of the effectiveness of applying a compromise fuel injection advance angle (FIAA) was conducted on a test stand for a PD1M type diesel generator unit. The study included an analysis of injection pressure changes at various FIAA values, as well as tests of the installation with the ESUVT.01 electronic fuel injection control system under load. Additionally, modeling of the diesel engine’s working process was performed using the “Diesel-RK” software package, followed by processing of the obtained data. Comparison of test results in locomotive characteristic modes at fuel injection advance angles of 14° and 29° crankshaft rotation showed that using a compromise FIAA value ensures a reduction in the locomotive’s average operational fuel consumption by 7-10 %, depending on operating conditions. Furthermore, decreasing the advance angle positively affects the reduction of maximum cylinder pressure and exhaust gas temperature, indicating an increase in the overall effectiveness of this approach.

The diesel substitution ratio is a key parameter influencing the combustion characteristics and energy conversion efficiency of hydrogen diesel dual-fuel free-piston engines. This study develops a thermodynamic hydrodynamic coupled model for a dual-fuel free engine to investigate the effects of five substitution ratios (15%, 20%, 25%, 30%, and 35%) on in-cylinder mixture formation, combustion characteristics, and emission performance. The key novelty of this work lies in employing this fully coupled combustion-dynamics model to systematically optimize the hydrogen–diesel substitution ratio, which explicitly captures the critical feedback between combustion and the piston’s unique motion. The cumulative heat release served as the key quantitative metric. The analyzed parameters included the gas mixture fraction, turbulent kinetic energy, flow trajectories, in-cylinder pressure and temperature, combustion reaction rate, unburned equivalent ratio, cumulative heat release and its rate, heat release rate, and emission mass. The results demonstrate that the engine’s overall performance is optimal at a substitution ratio of 25%. At this ratio, a peak volumetric mixture fraction of 0.0088 was achieved with a broad distribution range, indicating significantly improved spatial fuel uniformity. The flow field exhibited organized swirl patterns that enhanced fuel dispersion. The peak in-cylinder pressure reached 7.2 MPa, which was 0.044 MPa higher than that of the 20% group. The combustion temperature remained stable, with a peak value of 1606 K, exceeding the 20% and 30% groups by 7 K and 16 K, respectively. The heat release phase was well-synchronized with the piston motion, ensuring a high proportion of premixed combustion for thorough fuel oxidation. Although nitrogen oxide (NOx) emissions were slightly higher, the reduction in soot was substantially greater than in the 20% group, leading to overall superior performance compared to the other substitution ratios. This study develops a thermodynamic hydrodynamic coupled model for a dual-fuel free-piston engine by leveraging the interaction between piston motion and combustion. This paper presents a novel strategy for optimizing the substitution ratio in a free piston engine via a fully coupled combustion-dynamics model.