Super-knock Induced By Misfires And Damage Of Piston By Detonation Waves

The internal combustion(IC)engine has wide power coverage,high thermal efficiency and high reliability.For a long time in the future,the IC engine will be the most important power and widely used in the world.With the pressure of energy and emission regulations,there is an urgent need to improve thermal efficiency and reduce emissions of engines.The combination of high boost and direct injection is considered to be one of the most promising approaches to improve efficiency.The characteristics of these new technologies are higher in-cylinder pressure and faster flame propagation speed,and constantly expand the operating boundary of IC engines.However,the continuous expansion of the operating boundary makes the state of the mixture in the cylinder become sensitive and unstable.The probability of abnormal combustion phenomenon increases,which limits the expansion of the operating boundary and the improvement of the downsizing degree.The safe operation boundary seriously restricts the improvement of thermal efficiency of IC engines.With the continuous improvement of power density,there is an occasional and destructive knock named super-knock in the low speed and heavy load condition.Super-knock is an important obstacle to the development of the high power density and thermal efficiency engine.The critical factor and mechanisms connecting with the occurrence of super-knock are still unclear.At the same time,it cannot clearly reveal the cause of rapid failures of combustion chamber components after super-knock.This investigation mainly focused on the mechanism of super-knock induced by misfires and piston failures.By means of experiment and numerical simulation,the following three parts of work were carried out:(a)The engine bench test method is used to study the relationship between a misfire and super-knock.By controlling the on-off of the ignition coil manually,one cylinder of the engine misfired.Then we could observe the abnormal combustion phenomenon of other cylinders caused by misfires,capture the working cycle of super knock,and analyze which cylinders will have superknock.Furthermore,a pressure sensor was installed on the exhaust pipe to monitor the abnormal pressure fluctuation in the exhaust pipe,and reveal the mechanism of engine super knock induced by misfire.(b)Based on the detonation bomb device(DBD),the heat load of the piston under detonation conditions was collected,and the serious ablation of the piston was reproduced on the DBD.Then,the process of the piston heat load increasing sharply was revealed by using the finite element numerical simulation method.Finally,the heat transfer process of the piston wall under the detonation wave was studied by using the coupled fluid-solid numerical simulation method.(c)The damage mechanism of piston material by detonation wave was revealed based on the DBD.The aluminum alloy specimen is installed in the detonation chamber as a piston to apply detonation load.The failure characteristics of the piston were reproduced on the DBD.The failure mechanism of the piston was revealed when the detonation occurs.After the above research,the conclusions are as follows.1.A misfire in one cylinder could induce super-knock on a turbocharged GDI engine with an integrated exhaust pipe.There was a destructive detonation combustion phenomenon in cylinders.The maximum cylinder pressure can exceed 200 bar and the sudden increase of pressure can exceed 100 bar,accompanied by severe pressure oscillation.The abnormal combustion usually occurred in the next and last ignition cylinder corresponding to the misfired cylinder.The misfired cylinder itself would occur super knock.There was a correlation between the knock and the abnormal fluctuation of exhaust pipe pressure.The more severe the abnormal pressure fluctuation in the exhaust pipe was,the greater the knock intensity was.The existence of exhaust overlapping period between the misfired cylinder and other cylinders can lead to super knock.There are two main reasons for spontaneous combustion and even super knock of misfire cylinder: one is the wet wall of fuel in the cylinder.The second is the increase of high-temperature exhaust gas in the cylinder.Therefore,misfire is an important factor affecting the occurrence of super knock under low speed and heavy load.The results show that there was a strong correlation between a misfire and super knock,which could explain that the accidental and random characteristics of super knock were the accidental and random characteristics of misfire caused by early failure of the spark plug.The results are significant for avoiding and controlling super knock.2.Continuous detonations lead to the accumulation of temperature of the specimen and the weakening of its strength.The average heat flux density of the piston wall could be measured by the heat capacity method.The one detonation heat absorbed by the specimen accounts for 20.9% of the total heat released by fuel.The efficiency of heat transfers from gas to wall in combustion chamber is very high.The average heat flux density of the combustion chamber wall could reach 79.5 MW / m.The phenomenon of piston material being seriously ablated was reproduced on the DBD.After several consecutive detonations,the surface of the original smooth and flat specimen has become rough and sandblasted.It can be seen that the specimen surface has obvious material melting appearance,and at the same time,the micro appearance was sandblasted and had many pore structures,which means that the material was also affected by pressure waves while melting.The results of fluid-solid coupling numerical simulation showed that the position of detonation wave passing through was the position of maximum heat flux.When there were severe pressure waves,the heat flux on the wall is obviously higher than that without pressure wave,and the heat flux at the converging position of detonation wave was maximum.When super detonation occurs,the main reasons for serious ablation of piston surface in a short time are as follows: a)the detonation wave in combustion chamber enhances wall heat transfer;b)detonation occurs in continuous cycles;c)the thermal conductivity of piston material is limited.3.The specimens were always damaged at their central and edge region.The results were consistent with the typical characters of piston failure in real engines.The pressure collected at the cylinder head does not represent the pressure on the piston.The detonation wave converges at the center and forms an overpressure of more than100 MPa,which is much higher than the pressure collected at the cylinder head.Therefore,the pressure collected from the cylinder head position cannot explain the mechanism of serious piston failures.This result could explain why the measured cylinder pressure during knock can destroy the piston material even though it is lower than the strength value of piston material being destroyed.It is found that when the clearance was 12 mm,the center deflection was the largest.The center convergence of detonation waves is the strongest when the clearance was 12 mm.For detonation load,ZL108 has the weakest resistance,while hard aluminum alloy 2A12 and forged aluminum alloy 6082 have much stronger anti-deformation ability for detonation load.When detonation combustion occurs,the piston resistance of the alloy is relatively weak.The stress concentration position on the piston specimen is the most prone to serious failure.