Exaust thermal management control of diesel engines via intake throttling coupled with post injection strategy

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.