Methanol has emerged as one of the most promising carbon-neutral renewable fuels for internal combustion engines, offering a viable pathway to reduce greenhouse gas emissions and meet increasingly stringent emission standards. Developing advanced combustion strategies for compression-ignition engines has become imperative in alignment with global carbon neutrality initiatives. Methanol-diesel dual-fuel engines operating under reactivity-controlled compression ignition (RCCI) mode represent a groundbreaking solution, enabling ultra-low emissions without compromising thermal efficiency. To resolve the critical challenge of achieving emission-economy equilibrium in dual-fuel engines, a three-dimensional (3D) computational fluid dynamics (CFD) model of a methanol/diesel reactivity-controlled compression ignition (RCCI) engine was developed. The coupled effects of intake air temperature (IAT), intake pressure (IP), and methanol substitution ratio (MSR) on the combustion process and emission formation mechanisms were systematically investigated. A second-order regression model was developed using the Box-Behnken response surface methodology, with equivalent brake-specific fuel consumption (ESFC) and NOx, CO, and HC emissions as multi-objective functions for multi-parameter co-optimization. The results reveal that elevating IAT from 320 K to 360 K and increasing IP from 210 kPa to 230 kPa leads to increased peak cylinder pressure and indicated mean effective pressure (IMEP). Appropriate elevation of IAT and IP significantly reduces ESFC and HC emissions by enhancing combustion efficiency and mixture homogeneity. However, the increase in IAT leads to a simultaneous increase in NOx emissions and Soot concentration. Notably, the IAT exhibits a more pronounced regulatory effect on combustion phasing and heat release rate than the IP. Under constant intake pressure conditions, increasing MSR and IAT synergistically advance start of combustion and center of combustion, reducing ESFC while elevating peak cylinder pressure and IMEP. The simultaneous increase in IAT and MSR promotes a significant rise in NOx formation rate and emissions, while HC emissions gradually decline. Moreover, the synergistic control of MSR and IP effectively hinders Soot formation, revealing the coupled interaction mechanisms between operational parameters in emission control strategies. Dominant control over pollutant emissions is attributed to the coupled effects of IAT and MSR, whereas IP demonstrates a secondary influence. Nonlinear coupling interactions between MSR and intake parameters govern HC and NOx emission trends. Optimal NOx reduction is achieved via moderate IP combined with lower IAT. However, intermediate IAT coupled with elevated IP enhances Soot oxidation rates, decreasing Soot emissions. The second-order response surface models constructed via response surface methodology exhibit strong goodness-of-fit and predictive capability, with both R² and R²_adj values exceeding 0.98, while maintaining differences between R² and R²_pred below 0.2 across all models. In addition, under the optimal parameter set obtained through multi-objective optimization, the discrepancy between predicted values and simulation results remains below 4%. Response surface analysis reveals IAT and MSR as the dominant factors governing engine emissions, whereas IP exhibits relatively minor effects. Furthermore, the interactive effects between these parameters exhibit distinct nonlinear contributions to pollutant emissions. Under the optimal parameter combination of an intake temperature of 325.5 K, intake pressure of 230 kPa, and MSR of 34.5%, the ESFC reaches 204.04 g/(kW·h), with NOx, CO, and HC emissions reduced to 9.74, 8.58, and 10.98 g/(kW·h), respectively. Compared to the fuel-economy-optimal condition B9, this strategy achieves a 48.65% reduction in NOx emissions and a 10.95% decrease in CO emissions, while maintaining a marginal 3.09% increase in ESFC within constrained boundaries. These findings establish a theoretical foundation for multi-parameter synergistic optimization of control parameters in methanol/diesel RCCI engines to balance fuel economy and emission performance.

High-efficiency after-treatment has been applied to diesel engines for near-zero emissions against carbon peaking and carbon neutrality in modern agriculture. The diesel particulate filter (DPF) system is one of the most effective and mainstream technologies for particulate matter (PM). Among them, the pressure drop can increase with the increase of soot load in the DPF, leading to the decline of engine performance. Therefore, it is very necessary to remove the deposited particle for the regeneration of the DPF. However, the active and passive regeneration of the DPF system is closely related to a critical temperature range. The exhaust temperature can be expected at the light-off temperature of the diesel oxidation catalyst (DOC) for the high hydrocarbons (HC) conversion efficiency, in order to meet the DPF regeneration temperature during the engine operation. The exhaust thermal management is critical to the DOC inlet temperature for the downstream DPF regeneration. Unfortunately, the DOC inlet temperature is lower than the light-off temperature at low speed and load. Thermal management is required for diesel engines to rapidly improve DOC inlet temperature. Most previous studies have examined the influence of fuel injection timing, intake throttling and different post-injection strategies on exhaust gas temperature. In this study, a systematic investigation was made on the synergistic effects of main injection timing (MIT), fuel injection pressure (FIP), intake throttling, post-injection timing (PIT), and post-injection quantity (PIQ) on DOC inlet temperature, exhaust gas temperature (EGT), brake specific fuel consumption (BSFC), engine performance and emissions at different operation conditions. The collaborative mechanism was proposed to optimize the fuel injection, intake, and operating parameters, in order to improve DOC inlet temperature, fuel economy and emission performances. Parametric experiments were performed in the conditions of the MIT, FIP, throttle valve opening, PIT, and PIQ at low speed and low-to-medium load, or medium speed and low load. The results showed that the intake throttle valve and post-injection strategies shared better effects on the increase of EGT, whereas, the MIT was retarded with the decrease in FIP. Therefore, multi-objective optimization was conducted for the intake throttling coupled with post-injection strategies at low speed and low load using the response surface method combined with the Box-Behnken design. Then, the optimal input parameters were determined for the maximum DOC inlet temperature and the minimum BSFC, NOx, and smoke emissions. The input parameters of the engine were chosen to be the intake mass flow, PIT and PIQ, while the target variables were the DOC inlet temperature, BSFC, NOx and smoke emissions. The RSM-based prognostic model showed a better correlation with the mean absolute percentage error of less than 5%, and all coefficients of determination were above 0.97. There was a different influence of these input parameters on individual responses, depending on their contributions. The two topmost contributing factors to the DOC inlet temperature were intake air mass and PIQ. Intake air mass was the highest contribution to the DOC inlet temperature and smoke emission, whereas, the PIQ was the highest contribution to the BSFC and NOx emission. At the medium level (0) of post-injection timing (30 °CA), the lower intake airflow mass and higher post-injection quantity were achieved in the highest value of DOC inlet temperature, while the comparatively larger amount of intake airflow mass coupled with a medium level of post-injection quantity were used to achieve the lower NOx and smoke emission. At the medium level (0) of intake air flow (100 kg/h), there was a sharp increase of BSFC, where the post-injection timing was retarded and the post-injection quantity increased simultaneously. Multi-objective optimization showed that the maximum DOC inlet temperature was predicted as 253.3 ℃, as well as the lowest value of BSFC, NOx and smoke emissions were predicted as 272.6 g/(kW·h), 7.53 g/(kW·h), and 1.68 mg/m³, respectively. The optimized value of intake airflow mass was 87 kg/h, post-injection timing was 29 °CA, and post-injection quantity was 5.4 mg. This finding can provide a strong reference to optimize the exhaust thermal management for the exhaust gas temperature and emission performances of diesel engines.

Diesel engines are required for the combination of low or zero-carbon renewable alternative fuels and high-efficiency combustion strategies, particularly with the increasingly stringent emission regulations and carbon dioxide (CO₂) emission limits. Methanol-diesel dual-fuel reactivity controlled compression ignition (RCCI) combustion can be expected to realize higher thermal efficiency and ultra-low nitrogen oxides (NO_x) and soot emissions. However, it is still lacking on the performance of the methanol-diesel dual-duel RCCI at high altitudes. This study focused on the combined effects of methanol substitution rate (MSR), main injection timing (MIT) and altitude environment on the combustion process, performance, and emission in the methanol-diesel RCCI engines. The parametric experiments were performed to change the MSR, MIT and altitude at 1 800 and 3 200 r/min. Firstly, the impacts of MSR at different altitudes (2 000, 1 000, and 0 m) on combustion, engine fuel economy and emissions were investigated under various engine speed and load conditions. A comparison was made with the conventional diesel combustion (CDC) running. Secondly, the effects of MIT at different altitudes on the combustion process, emissions and performance characteristics were experimented with, while the MSR was maintained constant. The results showed that the maximum in-cylinder pressure and peak heat release rate gradually increased with the increase in MSR, while the start of combustion (SOC) and CA50 were advanced. The equivalent brake specific fuel consumption (ESFC), the NOx and soot emissions were reduced significantly, whereas, the brake thermal efficiency (BTE), the THC and CO emissions increased at different altitudes. With the MSR increased from 0 to 20% at 1 800 r/min engine speed at 100% load, the maximum in-cylinder pressure increased by 1.72 MPa on average, the PHRR increased by 25.08 J/(°) on average, the ESFC decreased by an average of 4.67%, the BTE increased by an average of 4.90%, the NOx and soot emissions decreased by an average of 16.63% and 50% respectively, and the THC and CO emissions increased 142.03 mg/m³ and 388.18 mg/m³ on average at 0, 1 000 and 2 000 m altitude. With the MSR increased from 0 to 7% at 3200 r/min engine speed, the ESFC decreased by 1.76% on average, the BTE increased by an average of 1.79%, the NOx and soot emissions decreased by an average of 8.17% and 20.70%, respectively, at different altitudes. As the altitude increased from 0 m to 2 000 m, the PHRR was reduced by 4.80 and 8.08 J/°, the CA50 retarded 1.44° and 1.43°, the BTE dropped by 0.82% and 0.68%, the ESFC increased by 2.10% and 1.99%, the NO_x emissions decreased by 10.61% and 7.35%, the opacity smoke increased by 26.54% and 32.12%, the THC emissions increased by 29.88% and 15.45%, and the CO emissions increased by 22.42% and 18.15%, respectively, at 1 800 r/min with 20% MSR and at 3 200 r/min with 7% MSR. As the MIT was advanced while the MSR was kept constant at different altitudes, the maximum cylinder pressure and PHRR gradually increased, the CA50 was close to the TDC position due to advanced combustion phasing, BTE gradually increased, ESFC and soot emissions were reduced, and the NOx, THC, and CO emissions increased. As the MIT was advanced from -1.5° to -7.5° at 1 800 r/min with 15% MSR, the ESFC was reduced by approximately 8.27% on average, the BTE increased by 9.08% on average, while the opacity smoke decreased by an average of 90.94% at 0, 1 000, 2 000 m altitudes. The main injection timing of diesel fuel can appropriately increase to improve the thermal efficiency and fuel economy of methanol-diesel RCCI engines at high altitudes. This finding can provide a basis to optimize the control parameters for the better combustion and emission performance of methanol-diesel RCCI engines under altitude environments.