This numerical simulation investigated the impact of pre-chamber fuel injection timing, injection angle, and exhaust gas recirculation (EGR) on combustion and emissions in a heavy-duty diesel engine using a pre-chamber combustion system. The results illustrate that the pre-chamber combustion system exhibits the potential to enhance the indicated thermal efficiency (ITE) of a diesel engine, while specific pre-chamber configurations can concurrently lower both NOx and soot emissions. The optimized cases show that the sustained turbulence generated by the pre-chamber enhances the redistribution of the unburned mixtures in the cylinder. The sustained turbulence leads to a broader range of gas mixtures near the equivalence ratio(Phi) of 1, resulting in complete combustion and lower NOx and soot emissions. In the optimized pre-chamber combustion system (with pre-chamber injection timing (PIT) at 0 degrees CA ATDC and pre-chamber injection angle (PIA) at 54.5 degrees), introducing 3% EGR results in a reduction of 11.61% in NOx emissions and 9.97% in soot emissions compared with the baseline engine.
Accurate and comprehensive reconstruction of in-cylinder combustion process is essential for timely monitoring of engine combustion state. This article developed a method based on the zero-dimensional (0-D) physical model integrated with big data. The traditional 0-D prediction model based on cumulative fuel mass is improved, the factor of in-cylinder temperature is introduced to adjust the heat release rate, which solves the problem of difficulty in calibrating the heat release rate. Then, convolutional neural network-gated recurrent unit (CNN-GRU), as a deep neural network, including a special convolutional layer and a gated recurrent unit (GRU) neural network is designed for the parameters to be calibrated in the model. The 0-D predictive combustion model is constructed by combining the physical model with CNN-GRU, the combustion process is simplified and reconstructed. The fitting results show that the 0-D physical model based on improved cumulative fuel mass approach is an effective method to reflect the heat release law. Under non-calibration conditions, the root mean square error (RMSE) value of peak firing pressure (PFP) based on CNN-GRU prediction model is 0.5862. The prediction model is a promising method to realize online fitting and optimization of combustion process.
Combustion performance experiments were carried out on a single-cylinder diesel engine to study the influ-ences of a lateral swirl combustion system(LSCS)and a multi-swirl combustion system(MSCS)on engine performance.Combined with simulation analysis,the fuel-air mixing and combustion characteristics were revealed for the LSCS and the MSCS respectively.Experimental results show that under low loads and high excess air coefficients,the MSCS ob-tains better combustion performance than the LSCS with the maximum reduction in fuel consumption of 3.6 g/(kW·h),in soot emission of 0.13 g/(kW·h)and in combustion duration of 2.6° CA.However,under heavy loads and low excess air coefficients,the LSCS shows better combustion performance than the MSCS with the maximum reduction in fuel con-sumption of 2.6 g/(kW·h),in soot emission of 0.56 g/(kW·h)and in combustion duration of 2.8° CA.Simulation results indicate that,when the engine load decreases or the excess air coefficient increases,the fuel spray penetra-tion ability becomes weakened,and the circular ridge of the MSCS chamber improves fuel-air mixing quality more effectively.However,when the engine load increases or the excess air coefficient decreases,the fuel spray pene-tration ability is enhanced.The split-flow creation of the LSCS chamber improves the fuel-air mixing quality more evidently,while the circular ridge of the MSCS chamber hinders the diffusion of fuel spray.
The contribution of dimethyl ether(DME) to the ignition delay times(IDTs) of ammonia(NH3) was investigated behind reflected shock waves. The experiments were performed at a pressure of 0.14/1.0 MPa, temperature range of 1150-1950 K, equivalence ratio of 0.5/1.0/2.0, and NH3/DME mixing ratios of 100/0, 95/5, 90/10, and 70/30. It was observed that the addition of DME decreased the IDTs and promoted the reactivity of NH3. With the increase of DME, the effect of the equivalence ratio on the IDTs of NH3 decreased. Under higher temperature and pressure conditions, the promoting effect of DME on the ignition of NH3 was weakened. An updated mechanism is proposed to reveal the promoting effect of DME on the ignition of NH3. Mechanisms from the literature were compared against the measurements, and the updated kinetic mechanism was validated with experimental data. Good agreement between measurements and simulations were shown. Chemical kinetic analyses were performed to interpret the interactions between DME and NH3 during fuel ignition. The numerical analysis indicated that the promotion effect of DME is primarily due to an increase of the rate of production and concentration of the radical pool, especially the OH radical pool. The large number of OH radicals generated by the reaction HO2 + CH3 = OH + CH3O during the early oxidation of DME is key to the NH3 consumption and early initiation of the chain reaction.
In order to make high pressure common rail system of the marine diesel engine select injection rate in a flexible way, the double-lift electronically controlled injector, whose needle valve has two upper limit positions, was proposed. An AMESim model of the injector was established, and based on the model, injection quantity characteristics and needle displacement, injection rate, pressure of the delivery chamber with different control timing of the injector under different injection modes and rail pressure were investigated. The results show that, the lift of needle valve is capable of being switched under different rail pressure, enabling the injector to realize three basic injection modes of low lift, high lift and boot. Compared with high lift injection mode, the injector under low lift injection mode has higher control accuracy on injection quantity, and the injection quantity changes almost linearly with the IPW. When the injector is in low lift injection mode, the balance point of delivery chamber pressure fluctuation due to the needle opening will be slightly higher than that of high lift injection mode. The variation of average injection rate with rail pressure under low lift injection mode is smaller than that under high lift injection mode, thus the injection rate fluctuation of low lift is less affected by delivery chamber pressure fluctuation. When the injector is in boot injection mode, lift switching delay is used to describe the time lag of lift switching signal to injection signal. As the value of lift switching delay increases, the change of injection quantity with rail pressure and injection pulse widths transits from high lift injection mode to low lift injection mode. The start time of lift switching has significant effects on the fluctuation of delivery chamber pressure and injection rate in the subsequent fuel injection process.
