Investigation into the Relationship between Super-Knock and Misﬁres in an SI GDI Engine

: The super-knock poses new challenges for further increasing the power density of spark ignition (SI) engines. The critical factors and mechanism connecting regarding the occurrence of super-knock are still unclear. Misﬁre is a common phenomenon in SI engines that the mixture in cylinder is not ignited normally, which is often caused by spark plug failure. However, the effect of misﬁre on engine combustion has not been paid enough attention to, particularly regarding connection to super-knock. The paper presents the results of experimental investigation into the relationship between super-knock and misﬁres at low speed and full load conditions. In this work, a boosted gasoline direct injection (GDI) engine with an exhaust manifold integrated in the cylinder head was employed. Four piezoelectric pressure transducers were used to acquire the data of a pressure trace in cylinder. The spark plugs of four cylinders were controlled manually, of which the ignition system could be cut off as demanded. In particular, a piezoelectric pressure transducer was installed at the exhaust pipe before the turbocharger to capture the pressure traces in the exhaust pipe. The results illustrated that misﬁres in one cylinder would cause super-knock in the other cylinders as well as the cylinder of itself. After one cylinder misﬁred, the unburned mixture would burn in the exhaust pipe to produce oscillating waves. The abnormal pressure ﬂuctuation in the exhaust pipe was strongly correlated with the occurrence of super-knock. The sharper the pressure ﬂuctuation, the greater the intensity of knock in the power cylinder. The cylinder whose exhaust valve overlapped with the exhaust valve of the misﬁred cylinder was prone to super-knock.


Introduction
At present, no power unit has a higher power density than an internal combustion (IC) engine. The IC engine has a wide power coverage, high efficiency and high reliability. For a long time in the future, the IC engine will be the most important power device and widely used in the world [1]. With increasing pressure of energy and emission regulations, there is an urgent need to improve thermal efficiency and reduce emissions of engines [2]. The combination of high boost and direct injection is considered to be one of the most promising approaches to improve the efficiency of spark ignition (SI) engine. The characteristics of these new technologies are higher in-cylinder pressure and faster flame propagation speed, and constantly expand the operating boundary of the IC engine. With the continuous improvement of power density, there is an occasional and destructive knock named superknock in the low speed and heavy load conditions [3,4]. Super-knock with a high level of destructiveness is an important obstacle to the development of high power density and thermal efficiency engines [5,6].
The experimental setup is shown in Figure 1. Experiments were conducted with a 1.5-L four-stroke, GDI SI engine with a four-cylinder inline design and a turbocharger. The fuel was RON 92# gasoline. The exhaust manifold (4-1 type) of the engine was integrated into the cylinder head. The ignition timing was 1-3-4-2. The engine was coupled to an alternating current dynamometer (AVL DynoRoad 202/12 Sx) to adjust the operating parameters, such as engine speed and torque. Four piezoelectric pressure transducers (AVL ZI33 Y3M) were flush mounted on the cylinder head to acquire the in-cylinder pressure data. The natural frequency of the pressure transducer is approximately 150 kHz, with a resolution of 0.1 kPa. The high frequency pressure oscillation in the exhaust pipe was measured by the Kistler type 6041B water-cooled pressure transducer, and the transducer was cooled by Kistler 2621 conditioning system. Pressure signal was triggered using a Energies 2021, 14, 2099 3 of 18 digital encoder coupled to the crankshaft with a resolution of 0.1 • CA. The signals were sent to an AVL 4P4G charge amplifier and acquired by an AVL combustion analyzer.
Energies 2021, 14, x FOR PEER REVIEW 3 of 19 was measured by the Kistler type 6041B water-cooled pressure transducer, and the transducer was cooled by Kistler 2621 conditioning system. Pressure signal was triggered using a digital encoder coupled to the crankshaft with a resolution of 0.1 °CA. The signals were sent to an AVL 4P4G charge amplifier and acquired by an AVL combustion analyzer.

Experimental Methodology
The experiment is divided into two parts. The first part collected the in-cylinder pressure of the four cylinders synchronously. In the second part, the three cylinders' pressure and exhaust pipe pressure curves were collected synchronously. The one key point of the experiments was to control the ignition timing by cutting off the ignition coil of the electronic ignition system. A manual control method was adopted in this experiment. The schematic diagram of the control circuit is shown in Figure 2. The computer controlled the ignition coil through an electric relay. When the relay was disconnected, the spark plug could not receive the spark signal from the electronic control unit (ECU). With a total of 4 channels, the spark plugs of four cylinder all could be controlled. Since super-knock occurs randomly, engine cycles have to be recorded continuously to capture super-knock cycles. The process of the test, in terms of the maximum amplitude of pressure oscillations (MAPO) time history of the four different cylinders, is shown in Figure 3. Pressure traces of 500 successive engine cycles misfired were recorded. Super-knock and pre-ignition could be distinguished by cylinder pressure. In Figure 3, the ignition coil of cylinder 3 was manually turned on and off. When the ignition coil was turned off, the MAPO was rapidly reduced. After the ignition coil was on, the MAPO returned to its normal value. In order to avoid the contingency of the experimental results, the experiments also were conducted on another engine of the same model to repeat the same ones.

Experimental Methodology
The experiment is divided into two parts. The first part collected the in-cylinder pressure of the four cylinders synchronously. In the second part, the three cylinders' pressure and exhaust pipe pressure curves were collected synchronously. The one key point of the experiments was to control the ignition timing by cutting off the ignition coil of the electronic ignition system. A manual control method was adopted in this experiment. The schematic diagram of the control circuit is shown in Figure 2. The computer controlled the ignition coil through an electric relay. When the relay was disconnected, the spark plug could not receive the spark signal from the electronic control unit (ECU). With a total of 4 channels, the spark plugs of four cylinder all could be controlled. Since super-knock occurs randomly, engine cycles have to be recorded continuously to capture super-knock cycles. The process of the test, in terms of the maximum amplitude of pressure oscillations (MAPO) time history of the four different cylinders, is shown in Figure 3. Pressure traces of 500 successive engine cycles misfired were recorded. Super-knock and pre-ignition could be distinguished by cylinder pressure. In Figure 3, the ignition coil of cylinder 3 was manually turned on and off. When the ignition coil was turned off, the MAPO was rapidly reduced. After the ignition coil was on, the MAPO returned to its normal value. In order to avoid the contingency of the experimental results, the experiments also were conducted on another engine of the same model to repeat the same ones.
The other key point is to capture abnormal exhaust pressure fluctuations. The installation of exhaust pipe pressure transducer is shown in Figure 4. The water channel was further designed on the mounting base of the transducer, and two sets of cooling water circulation ensured the normal operation of the pressure transducer. The combustion analyzer used in this bench only has four channels. In order to collect cylinder pressure and exhaust pipe pressure synchronously, the exhaust pipe pressure signal will replace the cylinder pressure signal of a certain cylinder during the experiment.  The other key point is to capture abnormal exhaust pressure fluctuations. The installation of exhaust pipe pressure transducer is shown in Figure 4. The water channel was further designed on the mounting base of the transducer, and two sets of cooling water circulation ensured the normal operation of the pressure transducer. The combustion analyzer used in this bench only has four channels. In order to collect cylinder pressure and exhaust pipe pressure synchronously, the exhaust pipe pressure signal will replace the cylinder pressure signal of a certain cylinder during the experiment.   The other key point is to capture abnormal exhaust pressure fluctuations. The installation of exhaust pipe pressure transducer is shown in Figure 4. The water channel was further designed on the mounting base of the transducer, and two sets of cooling water circulation ensured the normal operation of the pressure transducer. The combustion analyzer used in this bench only has four channels. In order to collect cylinder pressure and exhaust pipe pressure synchronously, the exhaust pipe pressure signal will replace the cylinder pressure signal of a certain cylinder during the experiment. Standard test procedure was strictly used to enable consistent and statistically reproducible results of all variants of the tests. The test was performed by conducting experiments at low engine speeds (1500-2000 r/min) and wide open throttle (WOT) conditions. In this study, the engine tests were controlled by the original ECU of the engine. The parameters of fuel injection and ignition have been calibrated to the optimal values before these tests. Engine tests were performed after warming the coolant and oil temperatures to 90 ± 3 and 93 ± 3 • C, respectively. A 100% engine load with WOT was kept constant during the tests. The intake air temperature after the Intercooler was maintained at 40 ± 3 • C. To capture the phenomenon of pre-ignition and super-knock under low speed and high load, the experiment selected 6 speeds between 1500 and 2000 r/min with WOT. Under each speed and load, misfire was performed on each of the four cylinders sequentially.  Standard test procedure was strictly used to enable consistent and statistically rep ducible results of all variants of the tests. The test was performed by conducting exp ments at low engine speeds (1500-2000 r/min) and wide open throttle (WOT) conditio In this study, the engine tests were controlled by the original ECU of the engine. The rameters of fuel injection and ignition have been calibrated to the optimal values bef these tests. Engine tests were performed after warming the coolant and oil temperatu to 90 ± 3 and 93 ± 3 °C, respectively. A 100% engine load with WOT was kept const during the tests. The intake air temperature after the Intercooler was maintained at 40 °C. To capture the phenomenon of pre-ignition and super-knock under low speed a high load, the experiment selected 6 speeds between 1500 and 2000 r/min with WOT. der each speed and load, misfire was performed on each of the four cylinders sequentia

Results
This part mainly focuses on abnormal combustion and knock induced by misfi The observation of the abnormal combustion was mainly made on in-cylinder press traces. The analysis of the conclusion is based on the correspondence among the ignit timing and strokes of the four cylinders. The ignition timing was 1-3-4-2. The correspo ence among the strokes of the four cylinders is shown as Figure 5.

Results
This part mainly focuses on abnormal combustion and knock induced by misfires. The observation of the abnormal combustion was mainly made on in-cylinder pressure traces. The analysis of the conclusion is based on the correspondence among the ignition timing and strokes of the four cylinders. The ignition timing was 1-3-4-2. The correspondence among the strokes of the four cylinders is shown as Figure 5. Standard test procedure was strictly used to enable consistent and statistically reproducible results of all variants of the tests. The test was performed by conducting experiments at low engine speeds (1500-2000 r/min) and wide open throttle (WOT) conditions. In this study, the engine tests were controlled by the original ECU of the engine. The parameters of fuel injection and ignition have been calibrated to the optimal values before these tests. Engine tests were performed after warming the coolant and oil temperatures to 90 ± 3 and 93 ± 3 °C, respectively. A 100% engine load with WOT was kept constant during the tests. The intake air temperature after the Intercooler was maintained at 40 ± 3 °C. To capture the phenomenon of pre-ignition and super-knock under low speed and high load, the experiment selected 6 speeds between 1500 and 2000 r/min with WOT. Under each speed and load, misfire was performed on each of the four cylinders sequentially.

Results
This part mainly focuses on abnormal combustion and knock induced by misfires. The observation of the abnormal combustion was mainly made on in-cylinder pressure traces. The analysis of the conclusion is based on the correspondence among the ignition timing and strokes of the four cylinders. The ignition timing was 1-3-4-2. The correspondence among the strokes of the four cylinders is shown as Figure 5.

Abnormal Combustion without Pressure Oscillation n
To explain the effect of misfires on engine combustion, the results at 1500 r/min and WOT were presented. Figure 6 shows the engine test results of four cylinders' pressure trances under misfires occurring in one cylinder. Figure 6a-d show the four cylinders' pressure curves under misfires at cylinder 1, cylinder 3, cylinder 4, and cylinder 2, respectively. This sequence is the same as the ignition timing (1-3-4-2). The spark timing at 1500 r/min and WOT was 7.5 • CA before top dead center (BTDC), which was controlled by the original ECU of the engine, and not adjusted to induce knock. Figure 6 indicates one regular pattern: misfire in one cylinder would induce abnormal combustion in the next ignition cylinder corresponding to the misfired cylinder. The Pmax (maximum cylinder pressure) in the next ignition cylinder corresponding to the misfired cylinder increased significantly compared to that in the other two cylinders. In Figure 6b, cylinder 3 misfired, the Pmax of cylinder 4 was larger than the Pmax of cylinder 1 and cylinder 2 by 47.3 bar. In addition, the combustion phase of the next ignition cylinder corresponding to the misfired cylinder advanced significantly compared to that in the other two cylinders. The in-cylinder pressure profile of cylinder 4 significantly deviates from the normal combustion cycle of cylinder 1 and cylinder 2 at −9 • CA BTDC ( Figure 6b). However, the in-cylinder pressure profile of cylinders 1 and 2 deviates from the normal motor cycle of cylinder 3 at −18 • CA BTDC.
WOT were presented. Figure 6 shows the engine test results of four cylinders' pressure trances under misfires occurring in one cylinder. Figure 6a-d show the four cylinders' pressure curves under misfires at cylinder 1, cylinder 3, cylinder 4, and cylinder 2, respectively. This sequence is the same as the ignition timing (1-3-4-2). The spark timing at 1500 r/min and WOT was 7.5 °CA before top dead center (BTDC), which was controlled by the original ECU of the engine, and not adjusted to induce knock. Figure 6 indicates one regular pattern: misfire in one cylinder would induce abnormal combustion in the next ignition cylinder corresponding to the misfired cylinder. The Pmax (maximum cylinder pressure) in the next ignition cylinder corresponding to the misfired cylinder increased significantly compared to that in the other two cylinders. In Figure 6b, cylinder 3 misfired, the Pmax of cylinder 4 was larger than the Pmax of cylinder 1 and cylinder 2 by 47.3 bar. In addition, the combustion phase of the next ignition cylinder corresponding to the misfired cylinder advanced significantly compared to that in the other two cylinders. The in-cylinder pressure profile of cylinder 4 significantly deviates from the normal combustion cycle of cylinder 1 and cylinder 2 at −9 °CA BTDC ( Figure 6b). However, the in-cylinder pressure profile of cylinders 1 and 2 deviates from the normal motor cycle of cylinder 3 at −18 °CA BTDC.  Figure 7 shows the pressure rise rate and heat release rate of the four cylinders when cylinder 3 ( Figure 6b) was misfired. Figure 7 shows that the maximum pressure rise rate of cylinder 4 could reach 10 bar/°CA, which was much larger than the pressure rise rate of the other two cylinders. The heat release of cylinder 4 was obviously advanced, and the combustion duration was shortened compared to those of the other cylinders. These phenomena occurred because the thermodynamic state of the unburned gas in cylinder 4 was enhanced when cylinder 3 was misfired.  Figure 7 shows the pressure rise rate and heat release rate of the four cylinders when cylinder 3 ( Figure 6b) was misfired. Figure 7 shows that the maximum pressure rise rate of cylinder 4 could reach 10 bar/ • CA, which was much larger than the pressure rise rate of the other two cylinders. The heat release of cylinder 4 was obviously advanced, and the combustion duration was shortened compared to those of the other cylinders. These phenomena occurred because the thermodynamic state of the unburned gas in cylinder 4 was enhanced when cylinder 3 was misfired. Figure 8 shows four cylinders' pressure curves under misfire occurring in one cylinder (1500 r/min, WOT). Figure 8 indicates the other regular pattern: misfire in one cylinder would induce abnormal combustion in the last ignition cylinder corresponding to the misfired cylinder. The Pmax of the last ignition cylinder corresponding to the misfired cylinder increased significantly compared to that of the other two cylinders. In Figure 8c, cylinder 4 was misfired, and the Pmax of cylinder 3 was larger than the Pmax of cylinder 1 and cylinder 2 by 17.2 bar. In addition, the combustion phase of the next ignition cylinder corresponding to the misfired cylinder advanced compared to that of the other two cylinders. The characteristics of pressure rise rate and heat release rate of abnormal combustion were similar to those in Figure 7.   Figure 8 indicates the other regular pattern: misfire in one cylinder would induce abnormal combustion in the last ignition cylinder corresponding to the misfired cylinder. The Pmax of the last ignition cylinder corresponding to the misfired cylinder increased significantly compared to that of the other two cylinders. In Figure 8c, cylinder 4 was misfired, and the Pmax of cylinder 3 was larger than the Pmax of cylinder 1 and cylinder 2 by 17.2 bar. In addition, the combustion phase of the next ignition cylinder corresponding to the misfired cylinder advanced compared to that of the other two cylinders. The characteristics of pressure rise rate and heat release rate of abnormal combustion were similar to those in Figure 7.     Figure 8 indicates the other regular pattern: misfire in one cylinder would induce abnormal combustion in the last ignition cylinder corresponding to the misfired cylinder. The Pmax of the last ignition cylinder corresponding to the misfired cylinder increased significantly compared to that of the other two cylinders. In Figure 8c, cylinder 4 was misfired, and the Pmax of cylinder 3 was larger than the Pmax of cylinder 1 and cylinder 2 by 17.2 bar. In addition, the combustion phase of the next ignition cylinder corresponding to the misfired cylinder advanced compared to that of the other two cylinders. The characteristics of pressure rise rate and heat release rate of abnormal combustion were similar to those in Figure 7.

Super-Knock in Fired Cylinder
The in-cylinder pressure curves shown in Figures 6 and 8 have no obvious oscillations. However, super-knock was also detected. Figure 9 shows the four cylinders' pressure curves under misfire occurring in cylinder 2. The corresponding operating point was 2000 r/min and WOT. The spark timing is 4.5 • CA BTDC. In Figure 9, cylinder 2 was misfired, leading to super-knock in cylinder 1, which was the next ignition cylinder corresponding to the misfired cylinder. The Pmax of cylinder 1 increased sharply, accompanied by intense pressure oscillations. Compared with normal combustion, the peak pressure of super-knock could exceed three times higher. A strong detonation wave, which rapidly swept through the pressure transducer, leading to the pressure "discontinuity" of a correspondingly rapid pressure rise of 107.86 bar. This result indicated that misfire in one cylinder will induce super-knock in the next ignition cylinder corresponding to the misfired cylinder. It should be noted that the exhaust stroke of the misfired cylinder corresponds to the power stroke of the knock cylinder. This result indicated that misfire in one cylinder would induce super-knock in the cylinder before starting the power stroke.

Super-Knock in Fired Cylinder
The in-cylinder pressure curves shown in Figures 6 and 8 have no obvious oscillations. However, super-knock was also detected. Figure 9 shows the four cylinders' pressure curves under misfire occurring in cylinder 2. The corresponding operating point was 2000 r/min and WOT. The spark timing is 4.5 °CA BTDC. In Figure 9, cylinder 2 was misfired, leading to super-knock in cylinder 1, which was the next ignition cylinder corresponding to the misfired cylinder. The Pmax of cylinder 1 increased sharply, accompanied by intense pressure oscillations. Compared with normal combustion, the peak pressure of super-knock could exceed three times higher. A strong detonation wave, which rapidly swept through the pressure transducer, leading to the pressure "discontinuity" of a correspondingly rapid pressure rise of 107.86 bar. This result indicated that misfire in one cylinder will induce super-knock in the next ignition cylinder corresponding to the misfired cylinder. It should be noted that the exhaust stroke of the misfired cylinder corresponds to the power stroke of the knock cylinder. This result indicated that misfire in one cylinder would induce super-knock in the cylinder before starting the power stroke.  Figure 10 shows the four cylinders' pressure curves under misfire occurring in cylinder 2. The corresponding operating point was 2000 r/min and WOT. In Figure 10, cylinder 1 was misfired, leading to super-knock in cylinder 2. The pressure amplitude of cylinder 2 could reach 162.85 bar, accompanied by intense pressure oscillations. This result indicates that misfire in one cylinder would induce super-knock in the last ignition cylinder corresponding to the misfired cylinder. The exhaust stroke of the misfired cylinder corresponds to the intake stroke of the knock cylinder. Misfire in one cylinder would induce super-knock in the cylinder before starting intake stroke. It was worth noting that the compression curves of the four cylinders in Figure 10 were different before ignition, and there is a great non-uniformity in each cylinder. In particular, the compression curve of misfire cylinder is the lowest, which is more obvious in Figure 9. In Figure 9, the TDC pressure of misfire cylinder (Cylinder 2) was 25 bar lower than that of knock cylinder (Cylinder 1).  Figure 10 shows the four cylinders' pressure curves under misfire occurring in cylinder 2. The corresponding operating point was 2000 r/min and WOT. In Figure 10, cylinder 1 was misfired, leading to super-knock in cylinder 2. The pressure amplitude of cylinder 2 could reach 162.85 bar, accompanied by intense pressure oscillations. This result indicates that misfire in one cylinder would induce super-knock in the last ignition cylinder corresponding to the misfired cylinder. The exhaust stroke of the misfired cylinder corresponds to the intake stroke of the knock cylinder. Misfire in one cylinder would induce super-knock in the cylinder before starting intake stroke. It was worth noting that the compression curves of the four cylinders in Figure 10 were different before ignition, and there is a great non-uniformity in each cylinder. In particular, the compression curve of misfire cylinder is the lowest, which is more obvious in Figure 9. In Figure 9, the TDC pressure of misfire cylinder (Cylinder 2) was 25 bar lower than that of knock cylinder (Cylinder 1). Figure 11 shows the four cylinders' pressure curves under misfire occurring in cylinder 1. The corresponding operating point was 2000 r/min and WOT. In Figure 11, cylinder 1 was misfired, leading to super-knock in cylinder 2 and cylinder 3, respectively. The pressure amplitude of cylinder 2 and cylinder 3 could reach 162.30 bar, accompanied by intense pressure oscillations. This result indicated that misfire in one cylinder will simultaneously induce super-knock in the next and last ignition cylinder corresponding to the misfired cylinder. The exhaust stroke of the misfired cylinder corresponds to the power and intake stroke of the knock cylinder. The exhaust stroke of the misfired cylinder corresponds to the power and intake stroke of the knock cylinder. This result indicates that misfire in one cylinder would simultaneously cause knocking occurrence of the cylinders before starting the power and intake stroke, respectively. Similar to Figures 9 and 10, the compression curves of four cylinders in Figure 11 are obviously different, and the compression curve of misfire cylinder is the lowest. The instability of the cylinder pressure curve is mainly due to the occurrence of misfire and super-knock, the combustion state of each cylinder becomes unstable, and there is a great cycle variation. With the occurrence of misfire, the temperature in misfired cylinder decreased, which leaded to the decrease in compression pressure. However, the increase in the temperature in the knock cylinder leads to the increase in the compression pressure, which leads to super-knock. In addition, the mixture of the misfired cylinder entered the exhaust pipe and was ignited, which would produce abnormal pressure fluctuations in the exhaust pipe and enhance exhaust interference.  Figure 11 shows the four cylinders' pressure curves under misfire occurring in cylinder 1. The corresponding operating point was 2000 r/min and WOT. In Figure 11, cylinder 1 was misfired, leading to super-knock in cylinder 2 and cylinder 3, respectively. The pressure amplitude of cylinder 2 and cylinder 3 could reach 162.30 bar, accompanied by intense pressure oscillations. This result indicated that misfire in one cylinder will simultaneously induce super-knock in the next and last ignition cylinder corresponding to the misfired cylinder. The exhaust stroke of the misfired cylinder corresponds to the power and intake stroke of the knock cylinder. The exhaust stroke of the misfired cylinder corresponds to the power and intake stroke of the knock cylinder. This result indicates that misfire in one cylinder would simultaneously cause knocking occurrence of the cylinders before starting the power and intake stroke, respectively. Similar to Figures 9 and 10, the compression curves of four cylinders in Figure 11 are obviously different, and the compression curve of misfire cylinder is the lowest. The instability of the cylinder pressure curve is mainly due to the occurrence of misfire and super-knock, the combustion state of each cylinder becomes unstable, and there is a great cycle variation. With the occurrence of misfire, the temperature in misfired cylinder decreased, which leaded to the decrease in compression pressure. However, the increase in the temperature in the knock cylinder leads to the increase in the compression pressure, which leads to super-knock. In addition, the mixture of the misfired cylinder entered the exhaust pipe and was ignited, which would produce abnormal pressure fluctuations in the exhaust pipe and enhance exhaust interference.

Super-Knock in Misfired Cylinder
Misfire in one cylinder would induce abnormal combustion not only in the other cylinders, but also in the misfired cylinder itself. Figure 12 shows that the mixture of misfired cylinder was ignited by compression near the TDC and produced a super-knock. The cor-

Super-Knock in Misfired Cylinder
Misfire in one cylinder would induce abnormal combustion not only in the other cylinders, but also in the misfired cylinder itself. Figure 12 shows that the mixture of misfired cylinder was ignited by compression near the TDC and produced a super-knock. The corresponding operating point was 1500 r/min and WOT. As shown in Figure 12, the in-cylinder pressure profile of cylinder 2 deviated from the normal combustion at the TDC. The maximum pressure amplitude could reach 125.74 bar, accompanied by intense pressure oscillations. Detonation should occur in the combustion chamber, which could cause serious damage to the piston. Compressed by the piston, the mixture rapidly reached a state of high temperature and high pressure [21]. Spontaneous ignition occurred at the TDC and, subsequently, the pressure profile deviated from the normal combustion, and jumped to the Pmax value. Different from Figures 9-11, the pressure during compression in cylinder 2 with misfire did not differ much from the others. This is because there is no unburned mixture in the exhaust pipe after knock in the misfired cylinder, so there is no exhaust interference. The cycle variation of the engine was improved. Figure 11. Super-knock in the next and last ignition cylinder corresponding to the misfired cylin der.

Super-Knock in Misfired Cylinder
Misfire in one cylinder would induce abnormal combustion not only in the other c inders, but also in the misfired cylinder itself. Figure 12 shows that the mixture of misfi cylinder was ignited by compression near the TDC and produced a super-knock. The c responding operating point was 1500 r/min and WOT. As shown in Figure 12, the in-c inder pressure profile of cylinder 2 deviated from the normal combustion at the TDC. T maximum pressure amplitude could reach 125.74 bar, accompanied by intense press oscillations. Detonation should occur in the combustion chamber, which could cause rious damage to the piston. Compressed by the piston, the mixture rapidly reached a st of high temperature and high pressure [21]. Spontaneous ignition occurred at the T and, subsequently, the pressure profile deviated from the normal combustion, a jumped to the Pmax value. Different from Figures 9-11, the pressure during compress in cylinder 2 with misfire did not differ much from the others. This is because there is unburned mixture in the exhaust pipe after knock in the misfired cylinder, so there is exhaust interference. The cycle variation of the engine was improved.   Figure 13 shows the phenomenon in which the misfired cylinder was ignited by compression before the spark timing. The corresponding operating point was 1500 r/min and WOT. The in-cylinder pressure profile of cylinder 2 significantly deviated from the normal combustion cycle at 30 • CA BTDC, which indicated a pre-ignition before the normal spark timing of 7.5 CA BTDC. The Pmax was 104.98 bar higher than the compression pressure at TDC, with pressure oscillations. Due to the pre-ignition flame propagation and piston motion, the end-gas was compressed to a state of high temperature and high pressure in a short time; subsequently, spontaneous ignition occurred. The chemical reaction and rapid heat release occur in a short time and within a small space. The heat released promoted pressure wave propagation in the combustion chamber, resulting in a strong pressure oscillation at approximately 20 • CA BTDC. There was no super-knock in Figure 13, so the engine works stably. The cycle variation is small, and the compression pressure curves basically coincide. timing of 7.5 CA BTDC. The Pmax was 104.98 bar higher than the compression pressure at TDC, with pressure oscillations. Due to the pre-ignition flame propagation and piston motion, the end-gas was compressed to a state of high temperature and high pressure in a short time; subsequently, spontaneous ignition occurred. The chemical reaction and rapid heat release occur in a short time and within a small space. The heat released promoted pressure wave propagation in the combustion chamber, resulting in a strong pressure oscillation at approximately 20 °CA BTDC. There was no super-knock in Figure 13, so the engine works stably. The cycle variation is small, and the compression pressure curves basically coincide.

Discussion
The last part revealed the relationship between misfire and super-knock. Each cylinder can interact with each other directly through the exhaust pipe This part further explains the relationship between misfire and super-knock based on the exhaust pipe pressure. Figure 14 shows the exhaust pressure and in-cylinder pressure curves without knocking after misfires in cylinder 1. When cylinder 1 misfired, the exhaust pressure peak generated by cylinder 1 disappeared, and the corresponding crankshaft angle range was from −180 to 0 °CA. However, engine did not always work normally after misfires, and super-knock was captured during the experiments. Take the case of the cylinder 1 as an example, the super-knock caused by misfire could be divided into three situations: (1) the misfire of cylinder 1 would lead to the super-knock in cylinder 3. The ignition timing was 1-3-4-2, so that the exhaust stroke of cylinder 1 corresponds to the power stroke of cylinder 2 (

Discussion
The last part revealed the relationship between misfire and super-knock. Each cylinder can interact with each other directly through the exhaust pipe This part further explains the relationship between misfire and super-knock based on the exhaust pipe pressure. Figure 14 shows the exhaust pressure and in-cylinder pressure curves without knocking after misfires in cylinder 1. When cylinder 1 misfired, the exhaust pressure peak generated by cylinder 1 disappeared, and the corresponding crankshaft angle range was from −180 to 0 • CA. However, engine did not always work normally after misfires, and super-knock was captured during the experiments. Take the case of the cylinder 1 as an example, the superknock caused by misfire could be divided into three situations: (1) the misfire of cylinder 1 would lead to the super-knock in cylinder 3. The ignition timing was 1-3-4-2, so that the exhaust stroke of cylinder 1 corresponds to the power stroke of cylinder 2 ( Figure 5); (2) misfire in cylinder 1 would cause super-knock in cylinder 2. The exhaust stroke of cylinder 1 corresponded to the intake stroke of cylinder 2 ( Figure 5); (3) super-knock would occur in the misfired cylinder. The above three situations would be discussed based on exhaust pipe pressure and cylinder pressure.  Figure 15 shows three different types of exhaust pressure fluctuations at 1700 r/min and WOT. The exhaust pressure was collected by the fourth channel of the AVL combustion analyzer (the system defaults to the pressure of cylinder 4), and the crankshaft angle range from −180 °CA to 0 °CA was the pressure fluctuation generated by the exhaust pulse of cylinder 1. In general, if cylinder 1 misfired, the mixture in the cylinder was not burned, the exhaust pressure was low, and there was no obvious pressure fluctuation, as shown  Figure 15 shows three different types of exhaust pressure fluctuations at 1700 r/min and WOT. The exhaust pressure was collected by the fourth channel of the AVL combustion analyzer (the system defaults to the pressure of cylinder 4), and the crankshaft angle range from −180 • CA to 0 • CA was the pressure fluctuation generated by the exhaust pulse of cylinder 1. In general, if cylinder 1 misfired, the mixture in the cylinder was not burned, the exhaust pressure was low, and there was no obvious pressure fluctuation, as shown in Figure 15a. At low speed and full load, the exhaust pipe presented a red hot state with the temperature high to 800 • C before the turbocharger, as shown in Figure 16. Therefore, when cylinder 1 misfired, the unburnt mixture discharged from cylinder might be ignited, or even deflagrated in the exhaust pipe. The pressure fluctuations caused by the combustion of the mixed gas in the exhaust pipe were captured in Figure 15b,c. Especially in Figure 15c, the amplitude and frequency of the pressure oscillations were higher.  The in-cylinder pressure curves of cylinder 3 corresponding to the abovementioned different exhaust pressure oscillations are shown in Figure 17. It can be seen from Figure 17 that when the exhaust pressure did not fluctuate significantly (Figure 15a), the mixture in cylinder 3 burned normally (Figure 17a). When the exhaust pressure fluctuated slightly (Figure 15b), cylinder 3 had a moderate knock (Figure 17b). When the exhaust pressure fluctuated sharply (Figure 15c), super-knock occurred in cylinder 3 (Figure 17c). From the above results, it could be inferred that the knock of cylinder 3 was related to the combustion state of the unburned mixture of cylinder 1 in the exhaust pipe. The sharper the pressure fluctuation, the greater the intensity of knock in cylinder 3. According to the pressure oscillation characteristics of Figure 15c, it could be inferred that a strong pressure wave was generated in the exhaust pipe. According to existing studies [18], its oscillation characteristics were similar to detonation waves, but because the acquisition frequency was low, only 10,200 Hz (1700 r/min, once every 0.1 • CA), no higher pressure peaks were acquired, and the acquisition frequency of detonation waves ordinarily required up to 1 MHz. However, the above results can at least conclude that the abnormal pressure fluctuation in the exhaust pipe was strongly correlated with the occurrence of super-knock in the power cylinder corresponding to the misfired cylinder.  The in-cylinder pressure curves of cylinder 3 corresponding to the abovementioned different exhaust pressure oscillations are shown in Figure 17. It can be seen from Figure  17 that when the exhaust pressure did not fluctuate significantly (Figure 15a), the mixture in cylinder 3 burned normally (Figure 17a). When the exhaust pressure fluctuated slightly (Figure 15b), cylinder 3 had a moderate knock (Figure 17b). When the exhaust pressure fluctuated sharply (Figure 15c), super-knock occurred in cylinder 3 (Figure 17c). From the above results, it could be inferred that the knock of cylinder 3 was related to the combustion state of the unburned mixture of cylinder 1 in the exhaust pipe. The sharper the pressure fluctuation, the greater the intensity of knock in cylinder 3. According to the pressure oscillation characteristics of Figure 15c, it could be inferred that a strong pressure wave was generated in the exhaust pipe. According to existing studies [18], its oscillation characteristics were similar to detonation waves, but because the acquisition frequency was low, only 10,200 Hz (1700 r/min, once every 0.1 °CA), no higher pressure peaks were acquired, and the acquisition frequency of detonation waves ordinarily required up to 1 MHz. However, the above results can at least conclude that the abnormal pressure fluctuation in the exhaust pipe was strongly correlated with the occurrence of super-knock in the power cylinder corresponding to the misfired cylinder.

Knock Caused by Blow-by during the Period of Exhaust Valves Overlapped
Misfire in cylinder 1 would also cause super-knock in cylinder 2. Figure 18 shows the pressure curves of the three cylinders and exhaust pipe when cylinder 1 misfired. The corresponding operating conditions were 2000 r/min and WOT. In Figure 8a, cylinder 1 misfired, and cylinder 2 had a super-knock. The Pmax of cylinder 2 could reach 204.3 bar, accompanied by severe pressure oscillations. Compared with normal combustion, the

Knock Caused by Blow-By during the Period of Exhaust Valves Overlapped
Misfire in cylinder 1 would also cause super-knock in cylinder 2. Figure 18 shows the pressure curves of the three cylinders and exhaust pipe when cylinder 1 misfired. The corresponding operating conditions were 2000 r/min and WOT. In Figure 8a, cylinder 1 misfired, and cylinder 2 had a super-knock. The Pmax of cylinder 2 could reach 204.3 bar, accompanied by severe pressure oscillations. Compared with normal combustion, the peak pressure of super-knock was more than four times higher. The strong detonation wave quickly swept over the pressure transducer, causing the corresponding cylinder pressure to rise rapidly from 41.4 to 204.3 bar, and the pressure amplitude reached 162.9 bar, causing serious pressure "discontinuity". This result indicated that misfires in cylinder 1 caused super-knock in cylinder 2. The exhaust stroke of cylinder 1 corresponded to the intake stroke of cylinder 2.  It can be seen in Figure 18b that when cylinder 1 misfired and cylinder 2 had a superknock, the exhaust gas discharged from cylinder 1 did not cause the obvious pressure fluctuation in the exhaust pipe. Therefore, the knocking mechanism of cylinder 2 was different from that of cylinder 3. It could be seen from Figure 5 that when cylinder 1 started to exhaust, the exhaust valve opens. At this time, cylinder 2 was just the starting intake stroke, and its exhaust valve had not been completely closed. Therefore, cylinder 1 and cylinder 2 had an overlap period of the exhaust valve. At this time, the high temperature and high pressure gas in the exhaust pipe rushes into cylinder 2, causing a change in the thermal atmosphere of the mixture in cylinder 2, and then led to super-knocking occurrence in cylinder 2.

Knock Caused by Residual Gas
A misfire in one cylinder would also cause a super-knock in the misfired cylinder itself. The pressure curve of the misfired cylinder knocking is shown in Figure 19, and the working condition was 1700 r/min and WOT. The misfired cylinder was cylinder 1. In Figure 19a, super-knock occurred in the misfired cylinder. The Pmax could reach 208.34 bar, accompanied by severe pressure oscillations. Since the ignition signal of the first cylinder was cut off, the occurrence of super-knock could only be caused auto-ignition. It could also be seen in Figure 19a that the in-cylinder pressure profile of cylinder 1 significantly deviated from the normal combustion cycle. It could also be seen in Figure 19b that the exhaust gas discharged from cylinder 1 produce an obvious pressure pulse, which also showed that the combustion phenomenon occurred in cylinder 1. It can be seen in Figure 18b that when cylinder 1 misfired and cylinder 2 had a superknock, the exhaust gas discharged from cylinder 1 did not cause the obvious pressure fluctuation in the exhaust pipe. Therefore, the knocking mechanism of cylinder 2 was different from that of cylinder 3. It could be seen from Figure 5 that when cylinder 1 started to exhaust, the exhaust valve opens. At this time, cylinder 2 was just the starting intake stroke, and its exhaust valve had not been completely closed. Therefore, cylinder 1 and cylinder 2 had an overlap period of the exhaust valve. At this time, the high temperature and high pressure gas in the exhaust pipe rushes into cylinder 2, causing a change in the thermal atmosphere of the mixture in cylinder 2, and then led to super-knocking occurrence in cylinder 2.

Knock Caused by Residual Gas
A misfire in one cylinder would also cause a super-knock in the misfired cylinder itself. The pressure curve of the misfired cylinder knocking is shown in Figure 19, and the working condition was 1700 r/min and WOT. The misfired cylinder was cylinder 1. In Figure 19a, super-knock occurred in the misfired cylinder. The Pmax could reach 208.34 bar, accompanied by severe pressure oscillations. Since the ignition signal of the first cylinder was cut off, the occurrence of super-knock could only be caused auto-ignition. It could also be seen in Figure 19a that the in-cylinder pressure profile of cylinder 1 significantly deviated from the normal combustion cycle. It could also be seen in Figure 19b that the exhaust gas discharged from cylinder 1 produce an obvious pressure pulse, which also showed that the combustion phenomenon occurred in cylinder 1. There were two main reasons why a misfired cylinder could be ignited by compression. The one was the fuel impingement on the piston pit and the linear portion of the cylinder. As the plug did not spark, the fuel on the wall could not be burned or taken away by strong airflow, resulting in a continuous increase in the wall wetting of the combustion chamber. The other one was the increasing in the amount of high temperature residual gas. The discharged unburned mixture deflagrated in the exhaust pipe and blows high-temperature exhaust gas back to the combustion chamber. According to the existing research, whether the super-knock was induced by the pre-ignition or by spark ignition was due to the spontaneous heat release of the unburned mixture auto-ignition under high temperature and pressure conditions. Consequently, the auto-ignition led to intense pressure oscillation. Then, the interaction between the pressure wave and the flame in the flame preheating zone would also promote the auto-ignition of the mixture, which finally changed the combustion mode from normal combustion to detonation combustion. However, the cumulative effects of fuel and conditions had been ignored. At low speed and high load conditions, misfire was an important factor for super-knock and pre-ignition.
The randomness of super-knock was mainly reflected in the following two aspects: first is the randomness of misfire of spark plug. As the spark plug does not fail suddenly, its loss of function shows a gradual growth. That is to say that misfire will occur when the spark plug starts to lose regular function at the early failure stage. Therefore, misfire occurrence will not take place regularly when it comes out. Misfire itself is random and accidental. The other is the randomness of the combustion of the unburned mixture in exhaust pipe. It can be found from the experimental results that the abnormal pressure fluctuation in the exhaust pipe does not always exist. It also is random and accidental. Finally, a super-knock suppression method is proposed: On-Board Diagnostics (OBD) of engine misfire is added to the engine control strategy. The ion current method has been successfully used to monitor engine misfire [22]. As long as the engine misfire occurs, a strict torque limit is imposed on the engine until the fault is eliminated, such as replacement of the spark plug. At the same time, the use of integrated exhaust pipes should be avoided. It is recommended to use the 4-2 and -1-3 in one common exhaust pipe design or to increase the length of the exhaust pipe and weaken the exhaust pulse interference of each There were two main reasons why a misfired cylinder could be ignited by compression. The one was the fuel impingement on the piston pit and the linear portion of the cylinder. As the plug did not spark, the fuel on the wall could not be burned or taken away by strong airflow, resulting in a continuous increase in the wall wetting of the combustion chamber. The other one was the increasing in the amount of high temperature residual gas. The discharged unburned mixture deflagrated in the exhaust pipe and blows high-temperature exhaust gas back to the combustion chamber. According to the existing research, whether the super-knock was induced by the pre-ignition or by spark ignition was due to the spontaneous heat release of the unburned mixture auto-ignition under high temperature and pressure conditions. Consequently, the auto-ignition led to intense pressure oscillation. Then, the interaction between the pressure wave and the flame in the flame preheating zone would also promote the auto-ignition of the mixture, which finally changed the combustion mode from normal combustion to detonation combustion. However, the cumulative effects of fuel and conditions had been ignored. At low speed and high load conditions, misfire was an important factor for super-knock and pre-ignition.
The randomness of super-knock was mainly reflected in the following two aspects: first is the randomness of misfire of spark plug. As the spark plug does not fail suddenly, its loss of function shows a gradual growth. That is to say that misfire will occur when the spark plug starts to lose regular function at the early failure stage. Therefore, misfire occurrence will not take place regularly when it comes out. Misfire itself is random and accidental. The other is the randomness of the combustion of the unburned mixture in exhaust pipe. It can be found from the experimental results that the abnormal pressure fluctuation in the exhaust pipe does not always exist. It also is random and accidental. Finally, a super-knock suppression method is proposed: On-Board Diagnostics (OBD) of engine misfire is added to the engine control strategy. The ion current method has been successfully used to monitor engine misfire [22]. As long as the engine misfire occurs, a strict torque limit is imposed on the engine until the fault is eliminated, such as replacement of the spark plug. At the same time, the use of integrated exhaust pipes should be avoided. It is recommended to use the 4-2 and -1-3 in one common exhaust pipe design or to increase the length of the exhaust pipe and weaken the exhaust pulse interference of each cylinder.

Conclusions
A turbocharged GDI engine with an integrated exhaust pipe was used to verify the relationship between misfire and super-knock. The spark plugs of four cylinders were controlled manually. Four piezoelectric pressure transducers were used to monitor the in-cylinder pressure data. In order to further explain the relationship between misfire and super-knock, a piezoelectric pressure transducer was installed at the exhaust pipe to capture abnormal exhaust pressure fluctuations. Based on the interaction between engine cylinders, this study further deepens the effect of the engine misfire on the knock. The results are applicable to turbocharged GDI engines with an 4-1 type integrated exhaust pipe. The main conclusions could be summarized as follows.

1.
A misfire in one cylinder would induce super-knock in the other cylinders. The maximum pressure amplitude could reach 200 bar, accompanied by intense pressure oscillations. The abnormal combustion usually occurred in the next and last ignition cylinder corresponding to the misfired cylinder.

2.
The unburned mixture of the misfired cylinder could be ignited by compression near the TDC, and even induce super-knock. At low speed and full load conditions, misfire was an important factor to induce super-knock.

3.
A misfire in one cylinder could induce pre-ignition. The in-cylinder pressure profile significantly deviated from the normal combustion cycle at 30 • CA BTDC, which indicated a pre-ignition before the normal spark timing of 7.5 CA BTDC. After the occurrence of combustion, the exhaust gas discharged from the misfired cylinder produced an obvious pressure pulse.

4.
After one cylinder misfired, the unburned mixture would burn in the exhaust pipe to produce pressure waves. The abnormal pressure fluctuation in the exhaust pipe was strongly correlated with the occurrence of super-knock. The sharper the pressure fluctuation, the greater the intensity of knock in the power cylinder.

5.
Cylinders whose exhaust valve overlapped with the exhaust valve of the misfired cylinder was prone to super-knock. The high temperature and high-pressure gas in the exhaust pipe would rush into the cylinder, and the thermal state in the cylinder would change abruptly, which could easily cause pre-ignition of the homogeneous mixture and even super-knock in the cylinder under intake stroke.