Investigation On Knocking Characteristics And Suppression Of Spark Ignition Two-Stroke Engine Fuelled With Kerosene

The spark ignition(SI)two-stroke engine is widely used in power system of small flight devices and UAVs due to the high power to weight ratio and low maintenance cost.With the implementation of the“single fuel” policy,SI engines under the military background urgently need to develop alternative fuels for gasoline.Compared with gasoline,aviation kerosene such as RP-3 has higher physical stability and obvious advantages in storage and transportation.In addition,the widespread application of aviation kerosene in the military makes the research of aviation kerosene as an alternative fuel for SI engines has been highly valued in UAVs and military field.However,there are certain differences in the physical and chemical properties of kerosene and gasoline.The combustion characteristics of the two fuels in SI engines are different as well.The low octane number of kerosene leads to knocking combustion phenomenon to be particularly serious.Knock is a key factor hindering the improvement of thermal efficiency of SI engines.Severe knocking combustion will not only lead to the deterioration of engine power and economy,but even cause serious irreversible engine damage.The research on the knocking combustion of SI kerosene engines is one of the core topics in this technical field.In order to reveal the knock combustion mechanism of SI kerosene engine,and then propose knock suppression strategies,a series of studies were carried out in this thesis.Based on a port fuel injection(PFI)two-stroke SI engine,one-dimensional model and three-dimensional model of the engine were established.An engine test bench was built,and some related research were carried out.The numerical simulation models of the engine working process and combustion process were established,and the various modules in the model were calibrated by test data.The results prove that the established 1-D model can accurately simulate the engine performance under various working conditions and predict knocking combustion.The 3-D model can accurately reflect in-cylinder working process through thermodynamic parameters and simulate the process of knocking combustion.The knock phenomenon of SI two-stroke kerosene engine was studied by numerical simulation.The difference of combustion process between kerosene and gasoline was compared.The mechanism of different ignition advance angle,mixture concentration and flame kernel size on knock was quantitatively analyzed.The research shows that the heat release process of gasoline combustion is more intense under the same working condition,and the pressure and temperature of the unburned mixture are higher than that of kerosene.However,the kerosene has low auto-ignition temperature and slow flame propagation speed,which leads to more obvious knock tendency.The position of the initiation of knock and high knock intensity are located in the cylinder wall area which is farthest from the spark plug or normal flame front.The position of the maximum knock intensity in the cylinder will keep changing with the change of crankshaft angle,resulting in the violent oscillation of the cylinder pressure.There are two main factors that affect the knock of the mixture concentration,namely the flame propagation speed and the heat release in the cylinder.The relationship between the two factors is mutually offset.Increasing the flame kernel diameter will lead to an increase of the knock intensity,but due to the increase in the flame propagation speed,the mass fraction of the mixture at the end of the knock is reduced,thereby reducing the distribution of knock combustion in the cylinder.In order to propose effective knock suppression strategies,the effects of different parameters on the engine performance and knock were studied.The results show that due to the physical and chemical properties of kerosene,the method of retarding the ignition advance angle alone for the SI kerosene engine cannot eliminate the knock.A reasonable ignition-injection coordinated control strategy should be designed.Increasing intake pressure has little effect on engine power improvement,and it will cause serious scavenging loss of PFI two-stroke engine and increase the tendency of knock.Therefore,the intake boost is not suitable for the PFI two-stroke engine.Under the strategy of lower scavenging efficiency and larger ignition advance angle,the engine knock can be well suppressed without excessive power loss.A proper reduction of the bore-stroke ratio can not only improve the engine power performance,but also reduce the exhaust temperature and knock tendency by accelerating the flame propagation process.The arrangement and spacing of dual spark plugs have great influence on the incylinder combustion.Optimizing the position of spark plugs is one of the effective methods to suppress the engine knock.Combined with a self-developed electronic control system,the engine test bench was built,and the kerosene engine cold start was studied.The engine performance and knocking combustion under different key control parameters were analyzed.The test results show that retarding the ignition advance angle can suppress knock.However,the exhaust temperature will exceed the standard due to the slow combustion speed and the retarded combustion phase,so the method is limited in practical application.Blending ethanol can enhance the antiknock performance of spark ignition two-stroke kerosene engine.Meanwhile,blending ethanol in kerosene will accelerate the flame propagation in cylinder and improve the combustion stability.