Simulation Of The Effects On Gasoline CAI Combustion And Gas Exchange Process Based On Dual Exhaust Valve Opening Strategy

Energy resource and environment protection are two problems that sustainable development will face. The engines of automobile not only become the main energy consumption source but also the largest emission pollution resource.Homogeneous Charge Compression Ignition (HCCI)/Controlled Auto Ignition (CAI) engines have generated considerable interest as an alternative combustion system to Spark Ignition (SI) and Compression Ignition (CI) engines. It is a hybrid concept, where a premixed charge is ignited by compression heat to inherit the benefits of SI and CI engine operations. HCCI engines offer advantages of extremely low oxides of nitrogen (NOx) and particulate matter emissions, while maintaining high thermal efficiencies at low load. However, the challenges from the auto-ignition control difficulty and narrow operation region limit the practical application of this technology.Up to now,many HCCI experimental and modeling researches have been done, and many modeling approaches have guided the experiments. According to the previous studies, it becomes apparent that one (or a combination) of four methods can be used to facilitate the CAI combustion process, the use of recycling of burnt gases is considered probably the most appropriate method to achieve CAI combustion in a 4-stroke production gasoline engine. Variable valvetrains which develop well currently are considered the most promising technological solution to realize this improvement.The simulation of CAI engines is a new area and a complete engine simulation code with detailed chemical kinetics of CAI combustion is not yet available. In this article,the simulation of the CAI engine was carried out by combining the Aurora detail chemical kinetics code from the CHEMKIN combustion package with the one dimensional engine simulation software GT-Power, and also the mathematical software MatLab. The aim of this paper is to analyse the influence of the dual exhaust valve opening (DEVO) strategy on the gas exchange process and the combustion process.The study indicates the following conclusion:1. In this article the strategy which allows the products to escape the combustion chamber in the usual fashion and then to re-induct them from the exhaust port during the intake stroke is used. We refer to this as DEVO strategy. The model was applied to analyse the influence of valve timing range on the gas exchange process and combustion process at the speed of 1000r/min, the first three were single-parameter valve timing variations: Dual Exhaust Valve Opening (DEVO) timing, Dual Exhaust Valve Closing (DEVC) timing and Intake Valve Closing (IVC) timing. The results obtained indicate that when the DEVO is varied from 375°CA to 400°CA, the EGR quality in the cylinder was able to heat the fresh charge to achieve CAI. When it comes to 390°CA, the internal EGR rate will reach the maximum value. Then used the optimum DEVO timing, and changed the DEVC timing respectively, the internal EGR rate shows the same trend with DEVO. When the DEVC varied, the internal EGR rate can reach only nearly 50% at most. Because of no backflow when the DEVC timing is 568°CA, the internal EGR rate in the cylinder will reach the maximum, and the timing was fixed in the whole simulation. In order to re-induct more EGR quality, the IVC timing was changed respectively. The IVC has the strongest influence on the internal EGR rate, and the maximum EGR rate can reach 78.9%. The gas exchange process determines the quantity of EGR which in turn affects the engine parameters and the cylinder charge properties, hence the control of the CAI combustion process.The changes in EGR quality with the variations in DEVO and IVC timing are also investigated. For CAI combustion the internal EGR rate can change from 39.2% to 78.9%. The model is also applied to analyse the influence of speed on internal EGR rate and forecast the range of the speed and the load of the CAI engine.The model is also applied to analyse the effects of combustion boundaries changed, which is used to know more about the DEVO strategy. Firstly, a simulation was carried out to study the possibility of widening the operating range by intake air heating. Reducing the requirement of the internal EGR rate at very light load is also the aim. The intake air temperature was heated from 300K to 350 K. As expected, the results showed that the increase in intake air temperature widens the valve timing region. It is shown that for the same engine load, the EGR rate required with intake air of 350 K is lower. Secondly,the fuel used in the model is changed from gasoline to ethanol,and the modeling speed was 1000r/min. The results obtained indicate that the trend of EGR rate changed with valve timing is similar to that of gasoline's. The results show that when internal EGR rate is lower than 47%, the temperature at the IVC is lower than 498K,this temperature is too low to achieve CAI combustion. When the internal EGR rate is higher than 78%, the concentration of charge is too low, misfire appears. Therefore the internal EGR rate range to realize CAI is 47%-78%. The range of the speed and the load of the CAI engine which is smaller than that of gasoline's was also forecasted.