Knock Characteristics Research Articles

Experimental Investigation on Knock Characteristics from Pre-Chamber Gas Engine Fueled by Hydrogen

Hydrogen-fueled engines require large values of the excess air ratio in order to achieve high thermal efficiency. A low value of this coefficient promotes knocking combustion. This paper analyzes the conditions for the occurrence of knocking combustion in an engine with a turbulent jet ignition (TJI) system with a passive pre-chamber. A single-cylinder engine equipped with a TJI system was running with an air-to-fuel equivalence ratio λ in the range of 1.25–2.00, and the center of combustion (CoC) was regulated in the range of 2–14 deg aTDC (top dead center). Such process conditions made it possible to fully analyze the ascension of knock combustion until its disappearance with the increase in lambda and CoC. Measures of knock in the form of maximum amplitude pressure oscillation (MAPO) and integral modulus of pressure oscillation (IMPO) were used. The absolute values of these indices were pointed out, which can provide the basis for the definition of knock combustion. Based on our own work, the MAPO index > 1 bar was defined, determining the occurrence of knocking (without indicating its quality). In addition, taking into account MAPO, it was concluded that IMPO > 0.13 bar·deg is the quantity responsible for knocking combustion.

Experimental study of spatial distribution of knock events in a turbocharged spark-ignition engine

An experimental investigation on the spatial distribution of knock events in a turbocharged spark-ignition engine for hybrid vehicle applications was conducted by using a multichannel fiber optic method. The knock positions were detected under different conditions to investigate the influence of crucial engine design and operating parameters on the knock characteristics including the spatial distribution in the combustion chamber and its relationship to knock intensity. The measured data reveal that the spatial distribution in the engine with a port fuel injection (PFI) system is mainly located on the exhaust side with insignificant influence of engine speed and load, which is attributed to the elevated thermal load around the exhaust valves. However, the knock events under gasoline direct-injection (DI) conditions were found to occur in more scattered locations with more occurring on the engine front-end and rear-end sides. These results indicate that the in-cylinder fuel-air mixing process may have a significant impact on the knock occurrence spots under DI conditions. The knock positions of the engine with different excessive air ratios, injection timings, and intake-valve timings were also detected, indicating that engine operating parameters have complex influences on the knock-region distribution in a DI engine. In addition, experiments were also carried out in two different cylinders to verify the cylinder-to-cylinder variations in knock regions which may be caused by the engine cooling design. Furthermore, no apparent correlations were observed between the knock position and the knock intensity by analysis of the experimental data.

Investigation of boost pressure and spark timing on combustion and NO emissions under lean mixture operation in hydrogen engines

Hydrogen may become a replacement for liquid fossil fuels, contributing to greenhouse gas emissions reductions by improving the thermal efficiency of boosted lean burn spark ignition engines. Single-zone engine combustion models are simple, but can yield useful results as a step in the design process for developing alternative fuel systems. The single-zone thermodynamic model is advanced by implementing a laminar flame speed sub-model to investigate combustion, an extended Zeldovich mechanism for nitric oxide emissions, and incorporating the Livengood-Wu integral model for knock characteristics. The results were validated using published experiments giving satisfactory predictions between simulation and experiment for spark timing variation, manifold air pressure, and equivalence ratios. A detailed analysis of boosted lean burn strategies showed that nitric oxide emissions increased with boosted pressure, hence emissions can be controlled through optimizing the excess air ratio and the start of combustion. Further techniques to achieve high thermal efficiency and to prevent knock for boosted lean burn hydrogen SI engine are discussed.

Controlled ignition of low-carbon gas-engine fuels based on natural gas and hydrogen: process kinetics

The potential wide use of environmentally friendly low-carbon gas fuel based on natural gas, hydrogen, their mixtures, and syngas in power generation and transport requires detailed information about the kinetics of ignition of these gases at temperatures below 1000 K, at which fuel ignition occurs in internal combustion engines (ICEs) and gas turbines. The same temperature range is also important for monitoring the storage and transportation conditions of these fuels. Although there are quite a few studies addressing the ignition of classical gas fuels such as methane or hydrogen, there is an obvious lack of works dealing with real natural gases and gas mixtures. Furthermore, even for methane and hydrogen, data on the ignition at high temperatures (T > 1000 K), which have been mainly gained by the shock-wave method for highly diluted mixtures, are at variance with the kinetic estimations for real conditions of operating with them or their use in ICEs. Considering the ignition characteristics at T < 1000 K is also important for syngas, the largest-scale base product of gas chemistry and the main industrial source of hydrogen. The pronounced discrepancies between the extrapolation of the results obtained for high-temperature ignition to lower temperatures and the results of kinetic modelling of these processes make it necessary to analyze their causes. This review addresses new experimental results on the ignition of methane–alkane and methane–hydrogen mixtures (real gas fuels) and kinetic modelling of these processes, which reveal significant changes in the ignition behaviour at T < 1000 K. These changes in the ignition process upon the variation of the temperature, pressure, and composition of the mixture are related to significant changes in the methane and hydrogen oxidation mechanisms in this temperature range. They are mainly caused by changes in the kinetics and, hence, the role of peroxide compounds and radicals in methane and hydrogen oxidation following temperature and pressure variation. The established features bring about the question of the adequacy of the existing criteria for assessing the knock resistance of gas engine fuels, primarily those containing hydrogen, when they are used in ICEs, and for assessing their explosiveness and measures taken for their safe handling. The review considers the possible methods for improving the knock characteristics of natural and associated gases to meet the requirements of power equipment manufacturers.<br> The bibliography includes 128 references.

Numerical study on knock characteristics and mechanism of a heavy duty natural gas/diesel RCCI engine

In this paper, the knock phenomenon of reactivity controlled compression ignition (RCCI) engine fueled with natural gas/diesel was numerically studied. The knock mechanism of the RCCI engine is explained and the strategy of suppressing knock is put forward. The knock characteristics were studied by setting monitoring points in different spaces positions of the cylinder. The results show that the pressure oscillation amplitude at the center and edge of the cylinder is large under the high load condition of RCCI engine. In addition, the knock mechanism was studied by using pressure difference method, maximum amplitude of pressure oscillation, important components, temperature isosurface, pressure fluctuation and heat release rate. The results show that the knock of RCCI engine is mainly caused by the end-gas auto-ignition. The pressure difference results show that the characteristic frequency is consistent with the natural resonance mode (0,1) of the cylindrical combustion chamber. On this basis, the effects of pilot oil injection timing and compression ratio on engine knock are further studied. It is confirmed that diesel knock and end-gas knock may exist simultaneously in the same cycle when RCCI engine knock occurs. And diesel knock occurs before top dead center, and end-gas knock occurs after top dead center. Proper adjustment of pilot oil injection timing and reduction of compression ratio can effectively suppress engine knock.

Effects of Water Injection on Combustion Emission and Knock Characteristics of Turbocharged Direct Injection Gasoline Engine

Effects of Water Injection on Combustion Emission and Knock Characteristics of Turbocharged Direct Injection Gasoline Engine

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.

Cylinder-to-cylinder variation of knock and effects of mixture formation on knock tendency for a heavy-duty spark ignition methanol engine

The knock characteristics of a high-pressure port fuel injection heavy-duty SI engine fueled with pure methanol were investigated experimentally and numerically. This paper aims to provide the optimization direction of the injection strategy from the perspective of knock suppression and thermal efficiency improvement. Both experiment and simulation prove that injection during the intake stroke with high injection pressure can mitigate knock because the strong interaction between spray and intake flow helps to form the more homogeneous and lower temperature mixture which prolongs the ignition delay of end-gas. As the injection timing (αinj) varies from 320°CA to 480°CA, the combustion speed and thermal efficiency increase first and then decrease, while the knock tendency is the opposite. Take the combustion at αinj of 320°CA as a baseline, the faster combustion speed at αinj of 380°CA caused by the homogeneous mixture contributes to the higher thermal efficiency, while that of 480°CA caused by the higher temperature further enhances the knock tendency. There is a significant cylinder-to-cylinder variation of the knock tendency. The knock tendency of the cylinder with the less fresh air charge is lower because of the lower in-cylinder temperature and pressure.

Spark knock suppression in spark ignition engines with hydrogen addition under low and high engine speeds

The influence of hydrogen addition on spark knock characteristics is investigated under two engine speeds (2000 rpm and 4800 rpm) with experiments and chemical analyses to describe the effect of hydrogen addition on the high-speed knock as well as on the low-speed knock. The experimental results showed that the hydrogen addition of 15 HV% advances the knock limit by 3°CA at 2000 rpm, while by 1°CA at 4800 rpm, indicating that the knocking suppression effect weakens at higher engine speeds. The chemical kinetic analyses showed that hydrogen addition reduces the heat release rate under 900 K where the low temperature oxidation (LTO) appears, while it promotes the heat release rate above 900 K. These results suggest that the smaller effect of hydrogen addition at the higher engine speed is due to the smaller dependence of the LTO on the ignition process, with the shorter residence time below 900 K.

Investigation on the knock characteristics in a gasoline direct-injection engine port-injected with water-methanol blends

Combustion knock is one of the key factors limiting the performance of spark-ignition engines, and thus great emphasis is placed on developing methods for effective and efficient knock suppression. The majority of works in this area are focused on the influence of these methods on the engine parameters, while maintaining the same “knock level” represented by a certain statistical parameter of knock combustion. The knock level defined as the mean or other statistical parameter of a stochastic process does not represent its characteristics. Thus, it is not evaluated on a sufficient level to show how different solutions for the knock reduction affect the knock probability distribution; nor is it known whether a universal treatment to characterise this influence can be developed. In this work we aim to fill this gap, and by the analysis of the knock combustion in a single-cylinder direct-injection gasoline engine with co-injection of water, methanol, and their blends, conclude on the effects of different additives on the probability distribution of in-cylinder pressure-related parameters. We also applied a method to normalise the knock phenomenon by a model representation of its probability distribution. The analysis was performed for knock-limited spark advance conditions for different amounts of co-injected water, methanol, and their blends. The method to normalise the knock distribution was based on the log-normal probability distribution, and it was successfully applied to obtain the knock characteristics at a quantified characteristic limit of knock, which was impossible to precisely meet directly in the tests. The results revealed that depending on the co-injected additive and its amount, the knock distribution undergoes slight but systematic changes, in terms of distribution skewness and kurtosis. At the selected knock limit, the injection of methanol resulted in knock characteristics more skewed to the knock limit comparing to the baseline case. Water addition resulted in opposite trends. Blending water and methanol gave a similar knock peak-to-peak distribution as in the gasoline-only case.

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.

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.

Numerical investigation on knock characteristics and mechanism of large-bore natural gas dual-fuel marine engine

This work revealed the unique knock characteristics and mechanism of large-bore natural gas dual-fuel Marine engine with flame jet and strong swirl by a novel three-dimensional pressure difference method to further reduce the knock tendency. The knock characteristics were investigated by pressure monitoring points distributed in different spaces. The results indicate that, owing to the large cylinder bore, almost no pressure oscillations were found in the center position, whereas, severe pressure oscillations appeared at the edge. The pressure difference results illustrate that the movement of the high-pressure area and the low-pressure area tends to the low-frequency resonance mode (1,0), which manifests as an axial rotation following the swirl motion direction. Furthermore, the pressure difference method combined with the maximum amplitude of pressure oscillation (MAPO) method and energy distribution slices was adopted to explore the knock mechanism. The results show that the large-bore natural gas engine has a special knock mechanism which can neither be attributed to the end-gas self-ignition theory nor completely flame acceleration theory. The knock mechanism is that the flame jets from the prechambers cause the uneven initial pressure distribution, and the uneven pressure distribution is strengthened by the uneven heat release on the flame surface. This knock mechanism was confirmed by changing the pilot fuel mass and equivalence ratio subsequently. By the way, a knock suppression method by reducing the pilot fuel was proposed based on the knock mechanism.

Effects of flame propagation speed on knocking and knock-limited combustion in a downsized spark ignition engine

Engine knock remains the key factor that ultimately restricts the peak thermal efficiency in modern downsized spark ignition (SI) engines. The flame propagation speed can affect knock tendency by the residence time and thermodynamic state of end-gas, which are competing with each other. The investigations in this study focus on flame propagation speed impacts on engine knock characteristics and thermal efficiency under a high load/low speed operating condition using three-dimensional numerical simulations. The results show that knock intensity (KI) increases first and thereafter decreases with the increase of SI flame speed under knocking condition. Behind the nonmonotonic relationship, the control mechanisms of end-gas auto-ignition are different. The low to moderate speed SI combustions are linked to local hot spots. Higher KI is caused by the interaction between the pressure waves induced by these hot spots. Then with the acceleration of SI flame, a homogeneous autoignition event occurs along the reaction front, leading to more intense pressure fluctuations. At the high-speed SI combustion, local hot spots disappear gradually, and the end-gas autoignition is dominated by flame-induced sequential auto-ignition. The corresponding KI decreases due to the reduction of available fresh gases. A posteriori analysis is also performed applying theories proposed by Zeldovich and Bradley. It’s shown that an increase in the SI flame speed results in lower first and thereafter higher temperature gradient in end-gas, leading to a drastic change in the auto-ignition behaviors. In addition, the relative effects of knock suppression and combustion duration reduction on thermal efficiency improvement are clarified.

Determination of Weak Knock Characteristics for Two-Stroke Spark Ignition UAV Engines Based on Mallat Decomposition Algorithm

Two-stroke spark ignition (SI) unmanned aerial vehicle (UAV) engines do not allow heavy knock and require a certain knock safety margin. However, weak knock can help the engine increase power output and reduce fuel consumption. To accurately extract the knock characteristics of engine vibration signals under the condition of weak knock, a signal feature extraction method based on the Mallat decomposition algorithm was proposed. Mallat decomposition algorithm can decompose the signal into two parts: a low-frequency signal and a high-frequency noise signal. The decomposed high-frequency noise is eliminated, and the low-frequency signal is retained as the characteristic domain signal. Simulation results show the effectiveness of the proposed algorithm. The engine vibration signal of a two-stroke SI UAV engine was decomposed into the low-frequency signal and the high-frequency signal by the Mallat decomposition algorithm. The low-frequency signal is taken as the knock characteristic domain signal component, and the wavelet packet energy method is used to verify the correctness of the obtained signal component. The relative energy parameter is calculated by using the knock characteristic domain signal component, which can be used as the determination index of knock intensity. This method provides a reference for the weak knock control of two-stroke SI UAV engines.

Production of motor benzene via plastic processing as perspective direction of "green" fuel power industry

The paper reviews the topicality and practical significance of the implementation of processing technology of plastic with the purpose of obtaining raw material for manufacturing motor benzene. It is shown that this case is the advantageous strategy for the damage reduction to the environment from the oil-gas complex and usage of commercial gasoline based on the fossil hydrocarbon crude and for the utilization of plastic wastes as well. The perspective technologies of plastic processing including pyrolysis, catalytic cracking, hydrothermal liquefaction based on the bibliographic review and the recommendations on the improvement of benzene component obtained as a basis for industrial production of motor benzene are presented. In the capacity of the last ones, the implementation of alcohol mixed octane-increasing additives based on the ethanol is justified. It is shown that using alcohol not only increases octane number of the fuel, but also saves expensive hydrocarbon components, allows reducing the toxic high-octane aromatic hydrocarbons in the gasoline, improves the distribution of octane numbers by benzene fractions based on the reformat, as well as expands the crude base for fuel production due to the non-oil raw. The studies showed that adding 10 % of alcohol is the optimum concentration, as on the one hand it provides the growth of knock characteristics and phase stability of the gasoline, on the other hand does not require additional regulation of motor engine for using such fuel.

Knock detection in a quasi-dimensional SI combustion simulation using ignition delay and knock combustion modeling

In spark ignition (SI) engines, knock suppression is essential to improve fuel economy. Integrating new technologies into already complex downsized turbocharged engines increases the number of design and control variables substantially and requires extensive engine testing. Utilizing simulation-based study in an engine development process is crucial to reduce expensive and time-consuming engine tests. A quasi-dimensional (quasi-D) two-zone SI engine model is typically used in SI engine simulations. However, the usefulness of quasi-D-based simulation tools is often limited by the predictive capability of knock models. In this study, a novel knock model intended to be implemented in quasi-D SI engine simulation was developed. The model comprises a two-stage ignition delay model, a knock combustion model, and a knock index model. The knock model simulates the heat release spike during a knock combustion, which can be observed from the apparent heat release rate generated with the low-pass filtered cylinder pressure data. The model was calibrated and validated against engine experimental data. Its capability of knock detection and knock-limited spark timing estimation was demonstrated under various conditions of octane numbers, air–fuel equivalence ratios, and coolant temperatures with toluene primary reference fuel (TRPF). The knock model was also able to capture different knock characteristics between TPRF and primary reference fuel (PRF) in modern downsized turbocharged SI engines. The proposed methodology represents a useful tool to be employed during the development phase of an engine to reduce the experimental efforts while improving the engine performance at knock borderline.

Numerical study of the effect of injection timing on the knock combustion in a direct-injection hydrogen engine

Knock combustion is one of the main barriers to the use of hydrogen as an engine fuel. This paper focuses on the effect of injection timing on knock characteristics in a direct-injection hydrogen engine by numerical simulation. Firstly, the mixture distribution and process of flame propagation were described under different injection timings. We found that both the uniformity of mixture and the flame speed increase with the advance of injection timing. Secondly, for single injection, delaying injection timing makes the knock intensity decrease first and then increase. Besides, autoignition spot can appear either near the cylinder wall or at the bottom of piston pit when the combustion chamber is ω-shaped. Finally, the results showed that a coupling between pressure wave and flame front causes heavy pressure oscillations. Thermophysical properties and the Unburned Mass Fraction (UBMF) are both critical factors that influence knock significantly and knock intensity increases with UBMF.

A real-time pressure wave model for knock prediction and control

This article develops a control-oriented real-time zero-dimensional knock pressure wave model for spark-ignited engines based on the reaction-based two-zone combustion model developed earlier. This knock pressure wave model is capable of predicting the in-cylinder pressure oscillations under knocking combustion in real-time and can be used for the model-based knock prediction and control. A pressure wave equation including the knock deadening behavior is proposed, simplified, and used to calculate the pressure perturbations generated by knock combustion. The boundary and initial conditions at knock onset are analyzed and the analytic solution of the pressure wave equation is obtained. The model is calibrated and validated over two engine operating conditions at knock limit. The chemical kinetic-based Arrhenius integral and the maximum amplitude of pressure oscillations are used as the evaluation methods for knock onset and intensity prediction, and the knock frequency is studied using a fast Fourier transform of the filtered in-cylinder pressure oscillations. The simulation results show that the proposed knock pressure wave model is able to predict the knock onset timing, frequency, and intensity accurately under two engine operating conditions. Furthermore, the knock characteristics associated with gas mixture properties at intake valve closing is analyzed based on the experimental data, and their effect to knock cycle-to-cycle variation is also studied for the proposed model.

Simulations of engine knock flow field and wave-induced fatigue of a downsized gasoline engine

A mathematical correlation is developed, based on the thermodynamic model of a downsized gasoline engine, to establish the numerical relationship among the thermodynamic parameters of the combustion chamber. In the developed numerical model, the in-cylinder pressure curves of various operation conditions are simulated by varying the air–fuel ratio in the cylinder, and the associated knock characteristics are recorded. The accuracy of the numerical simulation results is verified against the knock excitation experiment. Then, based on the Rover K16 gasoline engine, a simulation model is developed to simulate the engine knock in the combustion chamber and observe the force acting on the top surface of the piston. The results show that the forces, which act on the piston top surface, are varying at various locations at the same time, and the largest forces occur at the edge of the piston and followed by the piston centre. Then, by comparing the thermo-mechanical coupling strength of the piston under different operating conditions, the results show that the occurrence of the knocking does not exceed the piston’s strength limit. However, the stress and deformation value of the piston is increased significantly, and the failure point of the piston position is changed. Finally, based on the calibrated strength results, the piston durability is predicted for various engine knock conditions. The results show that there is an initial damage of piston in the process of detonation at the surface of the piston pin hole and the joint of the piston cavity. The gasoline engine finally has a predicted mileage of 2,53,440 km continuously, which meet the prescribed mileage of 2,20,000 km.