Fundamental Research On Intelligent Cooling System For Vehicle Engines

In order to face the energy and environmental challenges and to meet the demanding customer needs, engine cooling system has been expanding and extending its functional envelop along with the rapid development of vehicle and engine technology. With the rigorous performance requirements, the technical roadmap of cooling system has been evolving, from an auxiliary component only for cooling as before, to a comprehensive thermal management system to help vehicle meet the demands of emission regulations, fuel economy, traction performance, ride comfort, reliability and durability during all operating conditions (e.g. start-up, driving, and stop). Intelligent cooling management is the key technology to take all crucial demands into consideration, and it is the very future technology with the evolution and development of cooling system. Intelligent cooling management is meant to coordinate different cooling components according to the variation of the road and engine conditions. Therefore, it is important to design and develop a systematic and coordinated control system, aiming to satisfy multiple objectives under all operating conditions of engine.This dissertation studied the fundamental problems of intelligent cooling system through experimental investigation and simulation. The control strategy of cooling system was designed to cover all operating conditions of engine, and then the validation experiments were completed. The target of control strategy is to adjust the temperature of coolant to realize accurate cooling, and to shorten the warm-up time during start-up and to avoid heat soak after engine shutdown. Therefore, the fuel economy and reliability of engine can be improved, and the pollutant emission can be reduced.The main contents of the thesis are as follows:1. The experimental platform of intelligent cooling system on passenger vehicles was established, which included the engine test system, vehicle mounted test system and wind-tunnel test system. Then the data fusion method among the three sub-systems was proposed using parameters such as ambient temperature, heat rejection of coolant, velocity of cooling air, engine speed and load. In addition, a new test method of small temperature difference was designed to improve the test accuracy of heat rejection of coolant based on compound thermal couple, it was validated that the measurement error is within the range of±0.04℃.2. The study of intelligent cooling system MAP was completed. Based on the three systems mentioned above, research have been completed on (1) engine heat balance,(2) vehicle windward cooling and (3) the characteristics of external cooling system (including heat rejection performance of cooling module, cooling pump and fan properties). Then the system MAP was designed and improved. The results show:with ambient temperature, vehicle speed, engine speed and load as the inputs, and cooling fan and pump speed as the outputs, the internal relation between inputs and outputs can be described using8equations. Based on these results, the MAPs of coolant heat rejection, pump and fan speed were made in order to minimize the total power consumption of cooling pump and fan. Then the MAPs were combined to the MAP of the entire cooling system.3. The cooling control technology of engine warm-up and post-cooling was studied. Using the electric heater as the heating device, different auxiliary heating schemes, including synchro heating, preheating and combined heating, were compared in the respect of the coolant temperature warm-up period and fuel consumption. An electric thermostat based on fuzzy control was designed to realize the accurate adjustment of coolant temperature. Finally, the post-cooling cooling control technology was studied based on combined regulation of electric fan and pump. The results show that the auxiliary heating schemes could apparently shorten the warm-up time and lower the fuel consumption. However, the effects of different schemes were different, with maximum time reduction of66.1%and fuel consumption reduction of32.7%, respectively. The coolant temperature fluctuation can be reduced to2.2℃with the application of electric thermostat, and it can rapidly approach to the steady target state, with the fluctuation less than1℃. The best post-cooling scheme is the running only pump at minimum speed, and it can avoid overheat after engine shutdown, as well as save power consumption.4. The numerical simulation was carried out to study engine internal cooling under steady conditions. The flow and heat transfer of coolant inside the water jacket was analyzed according to theories of Computational Fluid Dynamics and Numerical Heat Transfer. The results show that the mass flow rate of coolant has obvious and significant effect on the cooling status inside engine. There is a safety mass flow rate qcs during the working process of engine, considering the overheat area minimization as the criterion. A method of engine operating conditions division was proposed with heat rejection of coolant Qc as reference, and then the qcs-Qc curve was acquired and used to modify the cooling system MAP.5. The integrated control technology of intelligent cooling system was studied. The results and conclusions found above were composed to form the complete control strategy covering all operating conditions of engine. The control system was established and validated with tests under typical conditions. The results show that compared to the original cooling system, the control strategy combining MAP-based forward, PID-based backward and fuzzy control can reduce the warm-up time, adjust coolant temperature more accurately, and effectively avoid heat soak after engine shutdown, as well as lower the power consumption.