| In response to the increasingly strict emission standards and fuel consumption regulations, currently, supercharged downsizing is becoming one of the main directions for the development of SI engines. On the other hand, it has been proved to be an effective way to reduce CO2 emissions from SI engines by utilizing new biomass alternative fuels. However, with the improvement in the downsizing degree of SI engines, the possibility of knock rises significantly. Meanwhile, regarding alternative fuels, engine performance optimization and knock suppression are important aspects need taking into consideration. EGR(Exhaust Gas Recirculation) technology has been widely accepted as an efficient strategy to suppress knock and optimize alternative fuel performance. In order to further explore the distinctions among different alternative fuels for engine performance and knocking combustion characteristics, thus coming up with available methods to prevent supercharged downsizing SI engine from knock phenomenon. In this work, based on a single cylinder SI engine, we conducted detail investigations on engine performance and knock while using gasoline and new biomass alternative fuels(including n-butanol and 2-methylfuran). The effects of compression ratio, intake pressure and intake air temperature and EGR ratio are included as well. The influence mechanism of above-mentioned factors is analyzed by carrying out numerical simulation with AVL-FIRE.Firstly, based on the maximum brake torque(MBT) ignition timing, we conducted experimental studies to discuss the influences of different compression ratio, inlet pressure and inlet temperature on SI engine fueled with various fuels. The results indicated that: inlet pressure distinguishes among other parameters for the best performance of promoting BMEP, followed by compression ratio, especially for the case of 2-methylfuran(MF). The increased inlet temperature usually leads to the decrease of BMEP, but in the case of n-butanol, engine performance could be improved when inlet temperature is increased properly.Secondly, their impacts on knocking combustion characteristic are discussed for these three fuels, and experimental results showed that knock intensity counts on inlet pressure greatly, followed by compression ratio, and it is not so sensitive to inlet temperature. n-butanol shows greater knock intensity than gasoline while under different compression ratios. With the increase of inlet pressure, the maximum pressure oscillation shows an up-ward trend for both gasoline and n-butanol, especially for the former. For the case of 2-methylfuran, surface ignition is more likely to occur while under the high compression ratio and large inlet pressure. It is concluded that after end-gas auto-ignition, n-butanol and 2-methyl furan could arrive at the maximum pressure oscillation relatively faster and then drops dramatically, and their average detonation energy is smaller than gasoline.To study the effects of EGR on detonation, for three fuels, according to the same engine knock intensity, we analyzed the impacts of EGR rates on engine performance and detonation while under various compression ratios, inlet pressure and inlet temperature. It is found that EGR works well while the average energy is small with the same maximum pressure oscillation. EGR gives the best performance in terms of knock suppression while under the high compression ratio and large inlet pressure, however, engine performance and fuel economy would get worse in the meanwhile. On a basis of the identical ignition timing, n-butanol and 2-methylfuran has shorter end-gas auto-ignition delay time than gasoline, and n-butanol shows the highest knock intensity, followed by gasoline and 2-methylfuran. Hence, applying larger ignition advance crank angle to improve engine performance is suitable for 2-methylfuran.Finally, we conducted simulation investigation on knocking combustion process for SI engines fueled with gasoline and n-butanol. Under Fire environment, quantitative analysis is performance to discuss the influence mechanism of different compression ratio, inlet pressure and inlet temperature for knocking combustion. It is found that these parameters exert influences on end-gas auto-ignition timing, location, the corresponding pressure and chemical reaction rate, among them the inlet pressure affects chemical reaction rate greatly. n-butanol shows lower unburned gas mixture mass fraction than the corresponding gasoline at the happening of auto-ignition combustion, while its chemical reaction rate is higher than gasoline. Since n-butanol releases less energy during auto-ignition combustion, its maximum pressure oscillation is lower than gasoline as well. Based on the identical ignition timing, for both gasoline and n-butanol, their end-gas auto-ignition timing, unburned gas mixture mass fraction and chemical reaction rate share the same decrease trend while applying EGR, especially for the case of gasoline. As a consequence, EGR gives better performance for gasoline from the viewpoint of knock suppression.In conclusion, improving inlet pressure and compression ratio are two main strategies to promote the power performance of supercharged downsizing SI engines, and increased inlet pressure can promote BMEP significantly. While under the large compression ratio and inlet pressure, engine can work under the relatively high knock intensity, thus giving good power performance. Regarding n-butanol and 2-methylfuran, due to the fact that they can suppress knock significantly, engines could work under higher compression ratio and inlet pressure, thus leading to the improvement in power output. EGR shows good performance to inhibit knock of n-butanol. For SI engines fueled with n-butanol can obtain higher inlet pressure and compression ratio by applying EGR. On the other hand, for the case of 2-methylfuran, EGR exerts relatively small influence on engine performance and knock. Knock suppression can be achieved by adopting large EGR rate. |
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