Knock Intensity Research Articles

Research on the Effect of Water Injection Pressure on Knocking Combustion of a Direct Injection Gasoline Engine

In this study, the research method of numerical simulation is used to explore the inhibition of different water injection pressures on knock combustion of turbocharged direct injection gasoline (GDI) engines by coupling computational fluid dynamics with a chemical-kinetics model. First, the ignition advance angle and compression ratio are increased to induce the GDI engine to knock, and then the influence of the water injection pressure on the in-cylinder, evaporation of water, and the knock of the gasoline engine are analyzed. The simulation results show that, compared with no water injection, the direct injection of water in the cylinder can significantly reduce the knock intensity. When the water injection pressure is greater than 40 bar, the knock intensity is less than 2 and the knocking is completely suppressed. In this work, the effects of different water injection pressures on knocking are explored by analyzing the effects of water injection pressure on water atomization, in-cylinder combustion, and the knocking mechanism. On the one hand, the evaporation rate of water increases with increasing water injection pressure and the quality of the liquid film generally improves. On the other hand, direct water injection can significantly reduce the distribution of CH2O in the end mixture, thereby reducing the generation of H2O2 and further suppressing the spontaneous combustion of the end mixture. At the moment of knock, when the water injection pressure is greater than 40 bar, the detonation mechanism of the no. 7 monitoring point does not produce a sudden change in HCO radicals. The water spray can effectively reduce the NOx emission, and the NOx emission under the water spray pressure of 120 bar is the lowest. However, after spraying water, it will increase CO emissions.

Experimental understanding of the relationship between combustion/flow/flame velocity and knock in a hydrogen-fueled Wankel rotary engine

Driven by issues such as global warming, there is currently great interest and significance in developing zero carbon-emission internal combustion engines. Limited by structural and sealing requirements, there are no experimental studies related to combustion velocity measurement of the hydrogen-fueled Wankel rotary engine (HWRE) with excellent power performance, which is important to investigate its knock characteristic. In this work, a combustion velocity measuring method of dual spark plug HWRE is proposed, besides the obtained combustion, flow and flame velocities are correlated with the knock to explore their influence on the HWRE knock characteristics. The study found that adopting dual spark plugs in HWRE is necessary to reduce the negative impact of the flame not being able to propagate against the flow field. Besides, excess air ratio affects the combustion velocity of HWRE by changing the flame velocity, while the engine speed affects the combustion velocity of HWRE by changing the flow velocity, which basically does not affect the flame velocity. The knock level of HWRE is influenced by its flame velocity and flow velocity, both of which are positively correlated to the knock intensity and the latter has a more significant effect.

Effect of fuel composition and EGR on spark-ignited engine combustion with LPG fueling: Experimental and numerical investigation

This paper presents an experimental and numerical investigation of a spark-ignited (SI) cooperative fuel research (CFR) engine fueled with different liquefied petroleum gas (LPG) fuels and exhaust gas recirculation (EGR). The effects of LPG fuel composition on engine combustion characteristics are initially evaluated at two different compression ratios (CR). Results show normal combustion at CR 7 and heavy knocking combustion at CR 10 for all the tested fuels, with a more substantial impact for the LPG fuel with high proportions of n-butane species. The Livengood-Wu (LW) integral method is then used to analyze the knock occurrence risk of individual fuel based on the reactivity of the tested fuels. The introduction of EGR then demonstrates the potential of knock intensity reduction below the borderline knock limit. A zonal-based kinetic interactions study is also performed to understand the knock mitigation effectiveness of EGR over the pressure–temperature domain relevant to SI engine operation. Finally, a multidimensional, computational fluid dynamics (CFD) simulation model is shown to predict the LPG combustion characteristics and presents the evolution of in-cylinder temperature and chemical species to demonstrate the development of end-gas autoignition events without and with EGR.

Effects of charge motion on knocking combustion under boosted high load condition of a medium-duty gasoline engine

Due to higher power density and strong swirl flow field, knock tendency is more severe in medium duty (MD) gasoline engines derived from diesel engine platforms. How to design the charge motion reasonably, so as to increase the thermal efficiency and suppress knock simultaneously needs to be further discussed in combination with the actual combustion boundary conditions. The investigations in this study focus on the charge motion roles on knock and thermal efficiency under a high load/low speed operating condition using numerical simulations. The results show that the mechanisms of inducing knocking combustion and the effects of tumble ratio (TR) on knock intensity (KI) are different under swirl, inclined swirl and tumble motion conditions. With strong swirl configurations, weaker turbulence intensity at the bottom of the combustion chamber reduces the convective heat transfer between the fresh charge and hot exhaust gas, resulting in an obvious high temperature region, and inducing the first hot spot. Increasing TR can effectively suppress knock by shortening the residence time of end gas. With inclined swirl and strong tumble configurations, the position of hot spots is determined by the asymmetrical shape of the flame front, which is induced by large-scale tumble motion around the spark plug close to TDC. The end gas reactivity is higher and the temperature distribution in the end gas region is more uniform with the increase of TR, resulting in a sharp increase in auto-ignition velocity and thus KI. Due to the suppression of knock, reduction of heat losses as well as promotion of flame speed, an optimized inclined swirl motion based on the flat cylinder head is more appropriate for MD gasoline engines.

Bayesian Estimation and Control of Engine Knocking Level for Transient Operation

The contribution of this article is to propose a knock controller based on the estimation of the distribution quantile of the knock intensity measurements. Despite the quantile estimation randomness, the corrections undertaken by the controller are moderated by the level of confidence in the quantile estimation. This strategy offers a fast control ensuring a better compromise between the engine efficiency and the prevention of knock phenomenon. The design of the controller reduces the engine torque fluctuations and is suited for engine transient operations.

A knock study of hydrogen-fueled Wankel rotary engine

Hydrogen-fueled Wankel rotary engine (HWER) is an excellent power device with superior power and emission performances. Besides, unlike hydrogen-fueled reciprocating piston engines, HWRE is less prone to backfire due to its structural advantages. However, limited by its long combustion chamber, knock, one of abnormal combustion in engines, still hinders the development of HWER. Hence, based on this consideration, the goal of this work is to investigate the knock of HWRE from different aspects, including ignition timing, spark plug number, spark plug location, excess air ratio and engine speed, to provide relevant information for the design of hydrogen-specific WRE. The results show that the knock intensity gradually increases as the ignition timing is advanced, the excess air ratio is decreased and the engine speed is increased, but the knock duration shows different variations. Adopting dual spark plugs tends to lead to stronger knock caused by unstable combustion of hydrogen, while only adopting leading spark plug tends to lead to the knock caused by auto-ignition. When dual spark plugs are used, the leading spark plug is more responsible for the knock caused by unstable combustion and the trailing spark plug is more responsible for the knock caused by auto-ignition.

Review on Methodologies Used for Knocking Detection and Intensity Evaluation in Internal Combustion Engines

Abstract: Extracting the knocks characteristics is the key of ignition control in auto-ignition compression engines, especially those severely suffered from abnormal consumption in the engine chamber. Most of the advanced detecting methods of knocking are generally related to the two main variables, including engine cylinder pressure signals and the engine cylinder block vibration signals, commonly measured using piezoelectric accelerometer sensors. Being familiar with different approaches capable of determining the engine knocking occurrence probability as well as providing suitable methods for knock intensity evaluation is significantly helpful to propose a comprehensive predicting model for measuring the level of oscillation pressure waves frequencies originated by the knocks. Following by predicting the knock intensity and extracting the knock feature, optimizing the engine operation in terms of fuel consumption, thermal efficiencies, and lifetime duration would be possible. For this aim, the current paper is addressed with overviewing upon the researches performed in the regard of developing high accurate models and optimized knock detection approaches. Keywords: auto-ignition, Internal combustion engine, Knocking detection approach, Knock intensity evaluation.

Analyzing characteristics of knock in a hydrogen-fueled Wankel rotary engine

Hydrogen-fueled Wankel rotary engine (HWRE) as a promising power device overcomes some of the drawbacks of the hydrogen-fueled reciprocating engine. However, its elongated combustion chamber makes it prone to knock, and little research has been done in this area. Hence, to understand the knock characteristics of HWRE, the present work focus on parameters of HWRE knock at 2000r/min, an excess air ratio of 1.2 and an ignition timing of −7°CA ATDC. The main results are as follows: As with knock intensity, knock duration and maximum pressure rising rate also can characterize the knock of HWRE because of the logarithmic or linear relationship among three parameters. For the knock intensity, two parameters are needed to precisely characterize the HWRE knock, while only one parameter is needed for the knock duration and the maximum pressure rising rate. Considering the computing cost, the maximum pressure rising rate is better than others. The knock intensity has a significant influence on the timed sequence of peak knock pressure and peak in-cylinder pressure. Besides, the mechanism of backfire caused by the knock of HWRE is different from that of the hydrogen-fueled reciprocating piston engine, which results from the inter-cylinder flame leakage.

Study on the ion current formating process under engine knocking conditions

In the last decades, ion current sensing has been widely recognized as a promising approach to the engine knock/super knock detection. However, ion current formating mechanism under engine knocking conditions is yet to be revealed, which substantially hinder the further development of ion current based knock detecting technology. In this paper, the ion current signals detected under engine knocking conditions are investigated both experimentally and numerically. In a spark ignition engine, the ion current signal and combustion pressure are analyzed in both time and frequency domain. Besides, a multi-physics CFD model, which incorporates compressible fluid dynamics, neutral/charged species reaction kinetics and flame plasma hydrodynamics, is constructed in this work. Then the combustion process, the flame - pressure wave interaction and the motion of the charged species are analyzed, and the key features of the ion current signal under knocking conditions are studied. The multi-physics model indicates that the in-cylinder distribution of positive and negative charged species is almost the same due to the ambipolar diffusion of the electrons, and then the ion current can be analyzed via observing the positive ion flux solely. By combining the experimental and numerical results, it is revealed that the ion concentration is correlative to the temperature in a non-linear way, and the ion current mainly responds to the low frequency pressure oscillation mode. Correspondingly, the relation between the ion current oscillating amplitude and the knock intensity is non-linear. For knocking at high load conditions, both the amplitude of ion current signal and the ion concentration can respond to the oscillation of local pressure and knocking intensity. In particular, the signal oscillation becomes insignificant when knocking intensity KI < 0.85.

The effects of compression ratio and combustion initiation location on knock emergence by using multiple pressure sensing devices

Knock research is an essential study because knock limits spark-ignition (SI) engine power output, durability, noise, fuel consumption, and emission performance. The thermal efficiency of an engine can be improved by increasing the compression ratio, but at the same time it leads to the possibility of end gas autoignition inside the chamber. The objective of this work was to show the effect of three different compression ratios (CRs) 8.5, 9.5, and 10.5 on the mechanism of knock. In order to achieve this four equispaced spark plugs were fitted around the circumference of a special metal liner to initiate flame fronts from those plugs by firing with different spark approaches (e.g. spark timing, number, and location). In addition, six pressure sensors were installed at various locations to precisely record the autoignition event by monitoring pressure oscillations, combustion characteristics, and knock intensity. In-cylinder pressure and heat release rate (HRR) analyses were applied to distinguish the knocking combustion for the three different CRs. The results showed that a greater number of spark plugs produced more stable combustion even for a delayed spark timing (ST) than a single spark plug, but also increased pressure oscillations, HRR and knock propensity with higher CRs due to an increase in pressure and temperature inside the cylinder. The maximum amplitude of pressure oscillation (MAPO) method was used to determine the knock intensity, and it was observed that at the lowest CR of 8.5 three spark plugs gave higher MAPO values compared to four spark plugs for the same operating conditions. However, four spark plugs instigated higher MAPO values than three spark plugs for the higher CRs of 9.5 and 10.5 because, with the increase of CR and spark plug numbers, the pressure and temperature were increased during the combustion event, which was coupled with the compression effect on the unburned end gas mixture. Additionally, Fast Fourier transform (FFT) and wavelet methods were implemented to observe the frequencies induced during knock. It was found that sparking four plugs provoked various vibration modes in different amplitude ranges with the different CRs.

Investigation of combustion and particle number (PN) emissions in a spark induced compression ignition (SICI) engine for ethanol-gasoline blends

Spark induced compression ignition (SICI) is a promising method to achieve high thermal efficiency as well as a robust control of combustion. With the increase of compression ratio, the popularization of bioethanol and the development of NOx emission control technologies for lean combustion, it is easier to realize high-efficiency and clean SICI combustion under medium-to-high load. Combining split injection with various excess air ratio (λ), the effects of ethanol in fuels with the same RON value (93# and 80#E15) on combustion characteristics, indicated thermal efficiency and PN emissions were investigated in a high compression ratio (15.5), single-cylinder, four-stroke engine. For comparison, the performance of pure anhydrous ethanol (E100) was also studied in this engine. The results show that ethanol-contained fuels can accelerate the flame propagation but suppress the heat release rate (HRR) of autoignition, and autoignition does not even exist in E100 combustion. Consequently, CA50 is advanced but combustion duration is prolonged for ethanol-contained fuels. It is highlighted that the predominant contribution of ethanol in improving the indicated thermal efficiency is the optimization of CA50 rather than the combustion iso-volume. In addition, the knock intensity is effectively suppressed, and the combustion process can be optimized by split injection with appropriate EOI2 (the end of the second injection) and split ratio for gasoline and ethanol-blended fuel, which further improve the indicated thermal efficiency. PN emissions exhibit a generally positive correlation with knock intensity in SICI mode and SICI mode can reduce the particles generated by the spray-wall impingement and ultra-rich mixture near the wall. Lean combustion can simultaneously suppress flame propagation and autoignition and is more effective in PN emissions reduction for fuels of 93# and 80#E15 within the cycle-to-cycle variation (COV) limitation rather than E100. However, E100 demonstrates two orders of PN emissions smaller than 93# and 80#E15 due to its high oxygen fraction content.

The effect of ignition intensity and in-cylinder pressure on the knock intensity and detonation formation in internal combustion engines

Engine downsizing with a higher compression ratio and a higher boosting pressure improves the thermal efficiency of engines. However, with further downsizing of engines, knock intensity is aggravated until super knock occurs which is accompanied by a detonation wave that potentially can destroy engines rapidly. Therefore, it is essential to reveal the parameters that influence the knock intensity and the formation of a super knock. Self-designed detonation bomb experiments and corresponding numerical simulations were conducted to explore the end-gas combustion mode as well as the knock intensity problem. The experiment was conducted at three typical in-cylinder pressures for representing different mixture energy densities, which classify the knock phenomenon into subcritical (lower pressure), critical (medium pressure) and supercritical (higher pressure) conditions. Spark ignitions with low and high intensity realized by adjusting the spark ignition energy were given in each condition, respectively. The in-cylinder combustion process and pressure oscillation process were monitored by the synchronous acquisition of three pressure sensors that were installed in different positions of the chamber. It is found that irrespective of the low or high ignition intensity, the super knock as well as the detonation would not occur at the lower pressure, and only mild knock occurs. At the medium pressure, the high ignition intensity would result in a detonation wave, while the low ignition intensity would not. At the higher pressure, despite the intensity of ignition, a detonation wave as well as the super knock would always occur. Therefore, the ignition intensity and the in-cylinder pressure are two essential factors that affect the knock intensity of engines.

Visualization of end-gas auto-ignition during PREMIER combustion in a dual-fuel gas engine

The purpose of this study is to visualize PREMIER combustion phenomenon in a micro-pilot diesel fuel ignited dual-fuel engine with differentiating the causes of knocking and PREMIER combustion. The auto-ignition characteristics in the end-gas region were investigated with combustion visualization and in-cylinder pressure analysis using a compression-expansion machine (CEM) fueled with a natural gas. The micro-pilot diesel fuel injection timing used to generate PREMIER combustion occasionally induced knocking and pressure oscillations, while normal and PREMIER combustion did not cause the pressure to oscillate. The auto-ignition in the end-gas region of both knocking and PREMIER combustion was found to cause a second rise in pressure before the propagation premixed flame reached the cylinder wall. As the auto-ignition in PREMIER combustion was later than in knocking, the area and mass of the end-gas region were smaller. The KI factors, which represented the intensity of the pressure oscillations, were higher in the case of knocking than in the case of PREMIER combustion. As the knock intensity increases in CEM, the dominant frequencies of 10 kHz and 14.5 kHz are generated in addition to the dominant frequency of 6.4 kHz. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. Knocking caused a sudden pressure difference and imbalance between the flame propagation and end-gas regions, followed by pressure oscillations.

Effect of Fuel Chemical Compositions on Combustion and Autoignition Characteristics in Gasoline Surrogate Fuel

In this study, three types of gasoline surrogate fuels were used to investigate the effect of fuel chemical composition on combustion and auto-ignition characteristics. For the study, an optically accessible engine capable of velocity analysis was used to observe the combustion and auto-ignition behavior. The results showed that the knock intensity of TRF with high toluene content was lower than that of S5R due to the OH consumption effect during combustion, but the knock suppression effect of toluene weakened at higher temperature and pressure conditions. Next, as the reason why auto-ignition did not occur only for PRF 82.6, the combustion rate analysis and chemical reaction numerical analysis indicate that the cold flame reaction and H2O2 reaction loop are more active in PRF 82.6 than in the other fuels, resulting in a shorter ignition delay and faster flame spread speed, and thus no auto-ignition occurred. Conversely, in TRF and S5R, the OH consumption effect by toluene and the suppression of the cold flame reaction and H2O2 reaction loop resulted in a slower flame spread velocity than in PRF 82.6, which created a time delay before auto-ignition and thus auto-ignition occurred. In addition, it can be inferred that the main reason for the frequent occurrence of wall auto-ignition in S5R is that, unlike PRF 82.6, this fuel does not have an NTC region.

Stochastic Bayesian optimization for predicting borderline knock

Engine knock is an undesirable combustion that could damage the engine mechanically. On the other hand, it is often desired to operate the engine close to its borderline knock limit to optimize combustion efficiency. Traditionally, borderline knock limit is detected by sweeping tests of related control parameters for the worst knock, which is expensive and time consuming, and also, the detected borderline knock limit is often used as a feedforward control without considering its stochastic characteristics without compensating current engine operational condition and type of fuel used. In this paper, stochastic Bayesian optimization method is used to obtain a tradeoff between stochastic knock intensity and fuel economy. The log-nominal distribution of knock intensity signal is converted to Gaussian one using a proposed map to satisfy the assumption for Kriging model development. Both deterministic and stochastic Kriging surrogate models are developed based on test data using the Bayesian iterative optimization process. This study focuses on optimizing two competing objectives, knock intensity and indicated specific fuel consumption using two control parameters: spark and intake valve timings. Test results at two different operation conditions show that the proposed learning algorithm not only reduces required time and cost for predicting knock borderline but also provides control parameters, based on trained surrogate models and the corresponding Pareto front, with the best fuel economy possible.

The impact on combustion knock in CI engine fueled with methanol-diesel-n-pentanol ternary blends

The knock intensity (KI) evaluation is critical during fuel and engine development. In the current study, the effect on combustion knock was investigated in CI engine fueled with methanol-diesel-n-pentanol blends. MnP5, MnP10, MnP15, MnP20, and MnP25 fuel blends were used in the present investigation. The cylinder pressure data was used to evaluate in the knock intensity (KI). Further, combustion parameters like heat release rate, ignition delay and combustion duration were also presented in this study. As per the results, the KI values for MnP5 were found to be higher than diesel at all loads, whereas these values for MnP15 were observed to be lower than diesel at 25% and 50 % load.

Effects of multiple spark ignition on engine knock under different compression ratio and fuel octane number conditions

Engine knock has long been one of the major obstacles for improving thermal efficiency of spark-ignition (SI) engines. An in-depth understanding of the engine knock mechanism and characteristics is vital for controlling knocking combustion. Experimental investigation of knock events is challenging given their stochastic nature. We employ a single-cylinder research engine equipped with a specialized metal liner with four circumferentially mounted spark plugs to generate multiple ignition sites, and achieve more controlled knock events. Six pressure transducers are mounted to collect the pressure signals from different locations of the cylinder. A series of spark strategies (e.g., spark number, location and timing) are applied to investigate the knock characteristics of different spark ignition strategies. The effects of compression ratio and fuel octane number are also explored. The experimental results show that the knock intensity first increases as the number of active sparks goes from one to three and decreases significantly with four spark ignition, and even below the double spark ignition in some cases. This is due to the trade-off between the mass fraction and temperature of end-gas: nearly 90% of the fuel energy is consumed at knock onset in the four spark ignition cases, and only a small proportion of energy is consumed by auto-ignition, thus limiting the knock intensity. Compared with the single spark case, multiple spark ignition generates higher power output and lower cycle-to-cycle variations. The knock suppressing effect of the four spark ignition strategy is enhanced by higher fuel octane number and lower compression ratio. This study provides a possible way to generate controllable knock and gives insights into the different knock mechanisms under multiple spark ignition conditions.

Experimental study of knock combustion and direct injection on knock suppression in a high compression ratio methanol engine

Methanol is a competitive low-carbon fuel for spark ignition engines. Though it has higher knock resistance than gasoline fuel, knocking combustion is still an obstacle for the application of methanol in spark ignition engines with relatively higher compression ratios. In this work, experimental investigations into knocking combustion and suppression were conducted on a single-cylinder, naturally aspirated, SI engine with a compression ratio of 15. Firstly, the knocking characteristics of the methanol engine with stoichiometric mixture under port fuel injection mode were evaluated. Results showed that knocking combustion occurred under medium and high loads. Especially, under the condition of indicated mean effective pressure (IMEP) = 0.7 MPa, some random heavy knock cycles with ultra-high knock intensity were observed. The frequency of heavy knock was 2.5% and the highest maximum amplitude of pressure oscillations reached 2.39 MPa. This heavy knock was similar to the super-knock phenomenon in boosted direct injection gasoline engines, and it can not be eliminated by retarding spark timing or adopting fuel-lean combustion. Then, the mechanism of the heavy knock was studied by thermodynamic condition analysis. Results showed that the heavy knock was triggered by pre-ignition which was induced by some hot spots in the combustion chamber. Finally, to make the best use of the charge cooling effect, the methanol direct injection strategy was applied to suppress the knocking combustion. When the engine worked with full opening throttle and adopt a single injection, the maximum knock intensity decreased to 0.43 MPa×°CA at −350 °CA ATDC start of injection which was 5.1% of the maximum value during port fuel injection mode. With the co-optimization of the start of injection and spark timing, the engine with a stoichiometric mixture could operate within the knock intensity limit at full load. Under this operating condition, the crank angle of 50% mass fraction burned located at 12 °CA ATDC and IMEP = 1.05 MPa was achieved. What’s more, adopting the optimal split injection instead of a single injection, the knock intensity was further reduced by 14.3% with a slight reduction of 0.02 MPa in IMEP.

Knock and knock intensity models for boosted SI conditions with gasoline-ethanol blends

In this work, a previously developed, two-stage table-lookup kinetic method for surrogate Toluene Reference Fuels (TRF) was extended to gasoline-ethanol blends and exercised in a GT-POWER model of knock in several engines. Experimentally, the knock limit is normally determined by the magnitude of pressure oscillations, known as the knock intensity (KI), rather than the crank angle of initial pressure oscillations or the crank angle of auto-ignition, which is difficult to identify. A number of empirical expressions of knock intensity have been developed for modeling purposes which relate the knock intensity to the crank angle and conditions at auto-ignition, and are available in the literature. These models were implemented into a GT-POWER model and tested against experimental knock-limited data for gasoline and gasoline-ethanol blends. The KI models are phenomenological in nature and are based on varying conceptual views of the knock event. With calibration all methods gave satisfactory results at mid-speed conditions when compared with the experimental data. Some deviations were seen at low and high speeds. Low speeds are potentially important because both RON and MON testing occurs under such conditions. Implications of the results and their relationship to recent knock experimental work are discussed and suggestions offered for improved knock models.

Effects of direct water injection timings on knock suppression, combustion, and emission performance of high compression ratio hydrogen-enriched natural gas engine

In this study, an approach of direct water injection was applied to suppress knock in the high compression ratio (CR) hydrogen-enriched natural gas (NG) engine. A CFD model for the high CR hydrogen-enriched NG engine was established and its effectiveness was verified by experimental data. On this basis, effects of water injection (WI) timings on knock mitigation, combustion, and emission features of the hydrogen-enriched NG engine were studied in detail. Results show that due to the slower flame propagation speed, monitoring points No.3 and 4 (the area near the intake valve) is more likely to knock accompanied by the higher flow velocity and turbulent kinetic energy. Besides, with the advance of WI timing, KI (knock intensity) keeps nearly unchanged firstly and then has a little increase after the second phase of reduction, whose minimum occurs at −150 °CA. Meanwhile, IMEP, CA10-90, and indicated thermal efficiency (ITE) have a similar change tendency with the maximum at −150 °CA, and ITE reaches its peak of about 40.9% with an increment of 3%. From −30 °CA to −180 °CA, NOx decreases a lot, while HC and CO emissions increase to some extent. Therefore, proper WI timing can not only effectively reduce KI and NOx emission, but also improve thermal efficiency and realize high CR in spark-ignition NG engines further.