Theoretical And Experimental Study On The Water Injection Of A TGDI Engine

For gasoline engines,the technique of downsizing combined with boosting is an effective low cost technical solution and a major development orientation.However,it is accompanied by the emergence of some adverse effects(e.g.,high heat load and severe knock tendency).In practice,reducing compression ratio and over-fueling are adopted to restrict knock.It is generally accepted that reducing compression ratio is against the improvement of thermal efficiency and over-fueling is not beneficial to fuel economy of gasoline engines.Water introduction has obvious advantages on reducing emissions,improving performance and thermal efficiency,especially on the restricting of knock,because of the higher latent heat and specific heat capacity of water.Charge cooling and the decrease of the specific heat ratio are the primary influence of water injection on engines,but there are not studies on the issue,analyzing the difference of the two influences can provide development of turbocharged engines with theoretical supports.Based on the thermodynamics of engine operations,this paper,combining theoretical analysis,experiments and simulation,studied the mechanism of water introduction impacting engine operations and reducing emissions and provided foundations for the application of water introduction on engines.The primary contents and conclusions are as follows:(1)Thermodynamic analysis of water injection on turbocharged gasoline direct injection engines.This study established a mathematical model of turbocharged gasoline engines with water introduction,and from the perspective of thermodynamic cycle,analyzed the effects of the technique on the thermodynamic process of turbocharged gasoline engines.1)Injecting water into the intake pipe is an isenthalpic process,the moisture content of intake air increases The added water absorbs heat at the time of evaporation,and the air temperature decreases.The decrease in sensible heat of air is equal to the increase in the latent heat of vapor.The total enthalpy of air unit remains constant during the humidification.2)The charge cooling effect of water introduction reduces the initial temperature and pressure of working medium.The specific heat capacity of mixture increases with water introduction thus the temperature and pressure of working medium decreased.3)Theorectical study indicated that water injection had no effect on the thermal efficienct of engines,but it has a great impact on the in-cylinder temperature.The compression ratio and ignition timing of gasoline engines could be improved with the help of water injection.(2)The effects of water introduction on the performance and economy of gasoline engines is experimentally studied in this paper.The results indicate that the improvement of engine performance increases with larger water-to-fuel mass ratio,and the power and torque improves with speed.The thermal efficiency gets a certain improvement at all speeds and the improvement increases with water-to-fuel ratio.(3)In view of the great significance of how water introduction affects the thermodynamic process of gasoline engines,this study,from the perspective of the first law,deeply analyzes the effects of WI on the thermal balance of a GDI engine.At operations with water introduction,the decrease of the temperature of in-cylinder working medium leads to a temperature drop in engine parts thereby plays a significant role in heat transfer loss and exhaust losses,and the increase of water-to-fuel mass ratio enhances the effect.An increase of the air flow rate can be noticed after water injection,therefore,the pumping work increases and the pumping loss ratio decreases.However,the pumping loss ratio is relatively small in comparison with other energy losses,particularly at large throttle angle and high load with water injection operations,leading to a trivial effect of water injection on pumping loss ratio.Because the increase of in-cylinder pressure,the friction loss increases.In conclusion,the sum of decrease of the latter three loss ratios is greater than the increase of exhaust loss ratio that directly improves thermal efficiency.(4)This study considered the second law of thermodynamics and analyzed the effects of water injection on the irreversibility of a turbocharged gasoline direct injection engine,improving the analysis of gap between the real and the ideal process.At operations with WI,the thermal availability of the intake air slightly decreased due to the decrease in the air temperature resulting from the evaporation of the injected water and the chemical availability increased because of the increase in the mass of the injected fuel per cycle;as a result,the total in-cylinder availability before ignition increased with the rise in the WFR.Althought the availability deduction caused by the irreversibility of combustion increased during operations with WI due to the change in the in-cylinder temperature,but the availability transferred by the heat transfer and the availability in the exhaust gas decreased,as well as the increase in the mass of the working medium enhanced the performance of the gasoline engine and improved the availability transferred by the work transfer,therefore the thermal efficiency of the second law increased.(5)Four technical solutions are designed by conducting experimental tests and numerical simulations to deeply assess the different impacts of high latent heat of evaporation as well as the change of working medium in gasoline engines.The decrease of in-cylinder pressure at the end of compression is mainly benefit from the change of working medium.Charge cooling has a smaller impact to increasing specific heat on the variation of in-cylinder temperature and pressure,though the effect of the change of working medium is more evident.The increase in water-to-fuel mass ratio enhances the effects.The tendency of peak in-cylinder pressure is opposite to the pressure at the end of compression.Therefore,the thermodynamics of engines is primarily influenced by the change of the working medium.The charge cooling effect merely reduced the intake air temperature and had a minor impact on in-cylinder thermodynamics.