Relationship between Super-Knock and Pre-Ignition Zhi Wang, Hui Liu, Tao Song, Yunliang Qi, Xin He, Shijin Shuai and JianXinWang ME 769 Presentation Sibin Kurunthottikkal Philip 02/26/2015

Super-Knock in Gasoline engines More than 60% vehicles equipped with gasoline engines. High boost and direct injection have advantages in power density and fuel consumption and have become a main tendency of gasoline engine for energy saving in recent years. However, a new engine knock mode, called super-knock, mega-knock, or low-speed pre-ignition (LSPI), occurs at low-speed high-load operating regime, as shown in Figure 1. Super-knock can suddenly damage the engine due to the extremely high peak pressure and pressure oscillation. The common knock suppression methods, including retard sparking timing, enrich mixture, and enhance wall heat transfer, are not effective for super-knock. Currently, there are more than 1 billion vehicles in the world

Experimental Methodology Pre-ignition represents the combustion of the fuel–air mixture triggered by ‘‘hot-spot’’ other than spark prior to the spark timing. Conventional engine knock is due to auto-ignition of the end-gas before flame propagation consumes end-gas in the cylinder. Super-knock is the severe engine knock triggered by pre-ignition. Turbocharged GDI engine was used to study super-knock and pre-ignition that were compared with conventional knock and normal combustion in detail. Numerical simulation was carried out to further analyze the combustion behavior and flame propagation mode. Super-knock often occurs under the low-speed and high-load engine operating conditions, so the minimum-speed operating point of maximum engine torque is selected as the super-knock test point, whose operating parameters are listed in Table 2.

Experimental recorded data a) Spark electrode breakup b) Exhaust valve melt c) Piston ring land broken In this study, cycles with the maximum cylinder pressure higher than 10.5 MPa and pressure oscillation higher than 2.0 MPa are considered as super-knock cycles based on the criteria in Table 3. Since super-knock occurs randomly, enough engine cycles have to be recorded continuously to capture super-knock cycles. In this study, pressure traces of 5000 engine cycles were recorded, which were post proceeded to capture super-knock cycles. Figure 3 shows the pressure traces of 20 engine cycles, among which three super-knock cycles were observed. Compared with normal combustion, the peak pressure of super-knock can exceed three times higher. This extremely high pressure may lead to fatal engine damage. The spark electrode was broken by the pressure wave oscillating in the combustion chamber, whose amplitude could exceed 2.0 Mpa The exhaust valve was melt due to the high temperature and high speed of combustion gas. When the super-knock occurs, the local high pressure is formed. The pressure waves propagate in the combustion chamber and impinge on the cylinder wall, and the wave is reflected and its amplitude doubles at walls

Comparison of Super-Knock, Conventional Knock and normal combustion The amplitude of the maximum pressure rise of super-knock (∆p = 12.0 MPa) is more than an order of magnitude higher than conventional knock (∆p = 0.5 MPa). From the heat release rate, the combustion process of super-knock: 1. Pre-ignition stage: occurs before TDC 2. Slow heat release stage: the initial stage of heat release rate from 0 CA ATDC to 17 CA ATDC is low due to flame propagation 3. Fast heat release stage: large amount of unburned mixture releases heat almost simultaneously, leading to a detonation like combustion It is obvious that the most important difference between super-knock and conventional knock is the amplitude of the pressure rise at the start of pressure oscillation. The amplitude of the maximum pressure rise of super-knock (Dp = 12.0 MPa) is more than an order of magnitude higher than conventional knock (Dp = 0.5 MPa). From the heat release rate, the combustion process of super-knock mainly consists of the following three phases. 1. Pre-ignition stage: pre-ignition event occurs before TDC 2. Slow heat release stage: the initial stage of heat release rate from 0 CA ATDC to 17 CA ATDC is low due to flame propagation 3. Fast heat release stage: large amount of unburned mixture releases heat almost simultaneously, leading to a detonation like combustion. The pressure rise could exceed 10 MPa within 0.5 CA and follows a drastic in-cylinder pressure oscillation.

Characteristics of Super-Knock and Pre-Ignition When cylinder pressure higher than 10.5 MPa and pressure oscillation higher than 2.0 MPa were identified, 50 cycles of cylinder pressure data prior to and 50 cycles after this cycle were saved. In this study, a total of 15 data sets containing superknock cycles were obtained. Each data set consists of 100 engine cycles. For normal combustion cycles, the SOCs are located in a deviation band, whose mean value is 25 CA ATDC with deviation range of +/-7 CA. The cycles with SOC earlier than 18 CA ATDC are considered as pre-ignition cycles, which are highlighted inside the rectangular region in Figure 6. The cycles with Dp higher than 2.0 MPa are considered as super-knock cycles, which are highlighted inside the rectangular region in Figure7. In order to quantitatively analyze the knocking intensity (KI) in this article, engine knock is classified into the following three categories based on Dp: Cycle 954,1388 super-knock Dp >2 MPa. Cycle 1135 Heavy-knock is defined as 0.2 MPa > Dp \>2 MPa. 1390 Slight-knock is defined as Dp < 0.2 Mpa. cycle 1073 is non-knock case with Dp of 0 MPa because of the smooth cylinder pressure trace. Pre-ignition occurs more frequently than super-knock. Pre-ignition may not cause super-knock, but pre-ignition always happens for a super-knock cycle. It was also seen that there was no strong correlation between knock intensity and unburned mixture energy & unburned mixture temperature & pressure Pre-ignition occurs more frequently than super-knock. Pre-ignition may not cause super-knock, but pre-ignition always happens for a super-knock cycle.

Numerical Analysis of the process after Pre-Ignition 1D compressible multi-component reactive flow model was employed. The cylinder is simplified to a pipe with dimensionless diameter and two boundaries. The model focuses on the principle of flame and pressure evolution in high-pressure and temperature combustible gas. When the initial pressure and temperature are in the range of normal combustion, the pressure wave has little influence on flame propagation. In this case, the hot spot induces deflagration. When the initial temperature of unburned mixture increases to a high level similar to the end-gas condition, pressure wave couples with flame propagation causing detonation of the unburned mixture 1D compressible multi-component reactive flow model was employed. The cylinder is simplified to a pipe with dimensionless diameter and two boundaries. All the reaction and wave propagation are calculated in the pipe. The model focuses on the principle of flame and pressure evolution in high-pressure and -temperature combustible gas. The hot-spot is assumed spherical with an initial radius (r = 2 mm) and an elevated initial temperature (Tp = 1800 K) at its center, as shown in Figure 15. The gases outside the hot-spot area are homogenously mixed. When the initial pressure and temperature are in the range of normal combustion, the pressure wave has little influence on flame propagation. In this case, the hot spot induces deflagration. When the initial temperature of unburned mixture increases to a high level similar to the end-gas condition, pressure wave couples with flame propagation causing detonation of the unburned mixture

Deto-curve Two Types of engine super-knock: Super-knock induced by pre-ignition followed by deflagration of the end-gas is similar to conventional knock and usually causes moderate pressure oscillation Deto-knock described as hotspot-induced deflagration followed by hot-spot-induced detonation (HDD) in the end-gas Figure 18 shows the condition of the gas mixture that leads to either deflagration or detonation based on the 1D compressible multi-component reactive flow model. The flame speed was used to distinguish between deflagration and detonation. From the numerical analysis above, two different types of engine super-knock could exist. The first type is the super-knock induced by pre-ignition followed by deflagration of the end-gas. This type of super-knock is quite similar to conventional knock and usually causes moderate pressure oscillation. The second type of super-knock exhibits significantly higher magnitude of pressure oscillation than that of the first type due to the detonation of the end-gas and is designated as ‘‘deto-knock.’’

Discussion on detonation mode Figure 19 illustrates a possible process of detoknock. First, pre-ignition occurs before TDC due to a local hot-spot (deposit, oil, oil–gasoline etc.) in the combustion chamber. The local heat release triggers the flame front and the flame propagates from the hotspot to the rest of the mixture. Then, electric SI occurs. If the spark plug is in unburned zone, the second flame front propagates from the cylinder center to the cylinder wall. The rapid expansion of the burned gas combined with the upward movement of the piston compress the unburned mixture to higher temperature (about 1000 K) and pressure (about 10 MPa) rapidly. Finally, a second hot-spot in the end-gas induced the detonation of the high temperature and high pressure of the unburned mixture as indicated in Figure 19. Thus the existence of hot-spot in the end-gas is the key factor that ultimately triggers detonation.

Classification of knocking combustion SI combustion can be divided into normal combustion (non-knock) and engine knock according to pressure oscillation. Engine knock can be classified into two categories: conventional knock and super-knock. There are two types of super-knock, end-gas deflagration and end-gas detonation (deto-knock), according to auto-ignition mode. Pre-ignition is the origin of abnormal combustion. Preignition can lead to end-gas detonation (super-knock), end-gas deflagration (super-knock, heavy-knock, slight-knock), and turbulent flame propagation (nonknock).

Summary A one-dimensional model was set up to numerically simulate the possible combustion process of the end-gas after pre-ignition. Pre-ignition may lead to super-knock, heavy-knock, slight-knock, and non-knock. Two distinct end-gas combustion modes are identified depending on the pressure and temperature of the mixture: deflagration and detonation. Hot-spot in the mixture at typical near TDC pressure and temperature condition can only induce deflagration. Hot-spot in the unburned end-gas mixture at temperature and pressure conditions above ‘‘detocurve’’ may induce detonation. There are two types of super-knock: End-gas deflagration, which causes moderate pressure oscillation. Deto-knock, which exhibits significantly high magnitude of pressure oscillation, detonation occurs in the end-gas. The mechanism of deto-knock may be described as hot-spot induced deflagration to HDD.