| The governments have tightened fuel consumption and emissions regulations while facing energy crises and environmental problems.Driven by the strict rules and market requirements,automotive powertrains are transitioning away from ICE vehicles（ICEVs）to the diversification tendency.Despite these challenges,the ICEs have accumulated a hundred years of production experience,and their electrification also provides a rare opportunity to maximize their strength.As for the technical solutions,increasing compression ratios is a primary method for thermal efficiency enhancement.However,the increase in compression ratios causes abnormal combustion phenomena regarding preignition and detonation.Water injection（WI）technology is well-known as a combustion control method with high precision and rapid response.This research designed the heated water injection（HWI）system with waste heat recovery based on a turbocharged GDI engine.A temperature control valve and an exhaust flow valve were applied to control water injection temperature in a closed loop.The engine control system was developed based on a field-programmable gate array.A high-speed data acquisition card was utilized for the experimental data collection.A series of experimental and simulation studies on the HWI technology with waste heat recovery（WHR）was conducted.The main work and conclusions in this dissertation are depicted as follows:a)Research on the heat transfer and recovery characteristics of a helically coiled heat exchangerA waste heat recovery system featuring a helically coiled heat exchanger was first designed,considering the exhaust energy availability and the HWI prototype characteristics.The influence of the flow state and physical parameters on heat transfer and waste heat recovery characteristics was investigated based on a controlled burner test bench.The results show that the shell-side heat convection of the helically coiled heat exchanger is in the laminar sate（0）?[300,1800])due to the less constraint in space.Increasing exhaust flow rate dramatically improves the shell heat transfer coefficient,and the overall heat transfer coefficient increases by 75.4%when the shell Reynolds number rises from 559 to 1648（T=400°C）.Meanwhile,the tube-side heat convection of the helically coiled heat exchanger is in the turbulent sate（0）?[3500,7000])because of its small diameter and rapid flow.The increase in the mass flow rate in coiled tubes enhances the tube heat transfer coefficient,while the overall heat transfer coefficient increases by only 3.0%.From the perspective of thermal resistance,the increases in exhaust flow rate and temperature of shells reduce the thermal resistances of shells by 43.0%and 11.4%,respectively,thereby improving the heat transfer characteristic of the heat exchanger.Although the tube thermal resistance reduces by 64.5%,it only accounts for 2%of total thermal resistance because of the enormous difference in thermal conductivity between water and exhaust.In addition,the increases in the exhaust flow rate and temperature in shells substantially improve the waste heat recovery.To be specific,the waste heat recovery capacity rises from 3.74 k W to 6.79 k W when the exhaust temperature in shells increases from 400°C to 600°C（volume flow rate 100 m3/h）;the waste heat recovery increases to 7.28 k W when the exhaust flow rate in shells increased from 100m3/h to 300 m3/h（T=400°C）.Therefore,the thermal resistance of shells is the principal restriction in the heat transfer capacity within the experimental range.It is vital to improving the shell-side Reynolds number by optimizing the heat exchanger structure,which enhances the waste heat recovery of the HWI system.b)Experimental and theoretical research on the influence of HWI technology with WHR on combustion and emissions characteristics in a gasoline engineSecondly,the influence of HWI technology with WHR on the engine performance was investigated based on the engine test bench,including combustion,energy distribution,cycle vibration,waste heat utilization,and emissions.A theoretical thermodynamic model,named internal combustion Rankine cycle,was established based on the experimental data.The results show that the HWI can speed up the water evaporation rate and improve combustion efficiency and stability,reducing unburned and exhaust losses.The elevated injection temperature lowers the duration of flame kernel formation,which shortens the ignition delay prolonged by conventional water injection.The CA10 is advanced 4.5°CA to-3°CA ATDC under the same spark advance compared to conventional water injection.The mechanism of HWI for combustion control is to lower flame propagation speed and improve the specific heat ratio of the air-fuel mixture.The degree of constant volume combustion increases from78.48%to 90.08%,increasing WFR from 0 to 1.14.Moreover,the HWI can reduce the production of NOx and CO emissions and effectively lower HC emissions by 26.2%as the water injection temperature increases to 205℃.However,an excessive water injection mass and an advanced or retarded water injection timing may worsen the ignition and flame propagation,increasing the unburned loss and the misfire probability.According to the theoretical results based on the internal combustion Rankine cycle,the HWI technology with WHR increases the theoretical efficiency because of the specific heat ratio,which increases by approximately 0.06 since the specific heat capacity of water is 1.8 times higher than that of CO2.Considering the coupled relationship between the HWI and the exhaust gas,the theoretical thermal efficiency gain is approximately 7.66%（with a compression ratio of 10）due to the increase in the specific heat ratio.The optimum theoretical thermal efficiency increases up to 59.71%with a compression ratio of 16.c)Research on the influence of water injection strategy on water-fuel consumptionFinally,an evaluation index for the energy conservation potential of water injection is proposed to optimize water consumption and minimize the negative influence on energy conservation.Based on the proposed evaluation index,an optimization approach to assessing the trade-off relationship between water and fuel consumption.The approach based on Brent’s method is adopted to search for the optimum combination parameters of the objection function.A comprehensive vehicle dynamics model is established and validated to evaluate the energy conservation potential of proposed water injection strategies.The results present that water injection can exploit the superiority in fuel economy under high torque demand,e.g.,rapid acceleration and high speed（>80 km/h）.Taking the simulation results of the WLTC as a representative,which comprehensively considers urban,extra-urban,and other dynamic conditions,water injection improves fuel economy by 5.2%under the fuel cost strategy.However,this strategy also consumes a large amount of water,up to 1.08L/100 km.Water injection strategy can be selected according to the customer demands and vehicle design purposes.Two novel water injection strategies are compared and analyzed based on the proposed evaluation index for the fuel/water consumption co-optimization potential.The usage cost strategy considers the tolerance for water consumption from the view of customers,and another space cost strategy considers the influence of water tank volume on the compact arrangement in passenger vehicles.The WLTC test results illustrate that the proposed strategies can improve the effectiveness of water injection and decrease inefficient water consumption.However,they slightly deteriorate its fuel/water consumption co-optimization potential.Compared with the fuel cost strategies,the usage cost strategy decreases water consumption to half（0.58L/100 km）,and its energy conservation ability decreases slightly from 7.7%to 4.8%（WLTC）.Furthermore,the water consumption of the space cost strategy reduces dramatically to a quarter（0.29 L/100 km）,and its energy conservation ability decreases by 13.5%to 4.5%（WLTC）. |
