Combustion Knock Research Articles

Effect of Spark Ignition Timing and Water Injection Temperature on the Knock Combustion of a GDI Engine

A turbocharged downsizing spark ignition (SI) engine cooperating with an in-cylinder direct injection technology is one of the most effective ways to improve the power and economy of gasoline engines. However, engine knock has limited the application and development of the downsizing of gasoline engines. Water injection technology can effectively suppress the knock. In this study, a method of numerical simulation was used to explore the effect of the water injection temperature on the combustion and suppression of the knock. First of all, the knock of the gasoline engine was induced by advancing the spark timing. Then, when the other conditions were the same, different water injection temperatures were set. The results show that lowering the water injection temperature reduced the knock intensity in the cylinder, but increasing the water injection temperature made the water distribution more uniform, and the peak values of each monitoring point were more consistent. The circulating work power increased with the increase of the water injection temperature. For emissions, as the temperature of the water injection increased, the emissions of soot and unburned hydrocarbons (UHCs) decreased, and NOx slightly increased.

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.

Effect of relative humidity on water injection technique in downsized spark ignition engines

Combustion knock is a major barrier to achieving high thermal efficiency in spark ignition engines. Water injection was recently identified as a potential way of overcoming this barrier. To evaluate its general applicability, experiments were performed on a downsized three-cylinder spark ignition engine, varying the humidity of the intake air, the water injection timing, and the engine speed. The minimum quantity of injected water required to maintain a given load (and thus level of engine performance) was determined under each set of tested conditions. The knock-suppressing effects of water injection were found to be related to changes in the fuel–air mixture’s specific heat ratio (kappa) rather than evaporative cooling, and to therefore depend on the total quantity of water in the cylinder rather than the relative humidity per se. The total quantity of water in the cylinder was also shown to be a key determinant of advancement in combustion phasing and particulate emissions under various conditions.

The effect of cooled EGR on combustion and load extension in a kerosene spark-ignition engine

It’s currently a trend to use aviation kerosene as the fuel for aviation spark-ignition reciprocating engines because of its good safety compared with gasoline during the transportation, storage and use. However, when converting the gasoline engines to kerosene, power density is constrained by knock combustion for its lower octane number, especially for four-stroke engines. In this study, cooled exhaust gas recirculation (EGR) was adopted to extend the output of a kerosene engine at knock limits based on a four-cylinder, four-stroke, horizontally opposed engine. First, the effects of cooled EGR on combustion characteristics are studied, including the in-cylinder pressure, heat release rate, knock intensity etc. Then, load extension via cooled EGR was experimental analyzed. The results show that when the EGR rate increased from 0% to 10%, the indicated mean effective pressure (IMEP) at knock limits improved by 7%. At the same time, because of higher specific heat ratio and reduced heat transfer loss, specific fuel consumption decreased by 8.1%, and the indicated thermal efficiency increased from 23.5% to 25.6%.

Effects of reactivity inhomogeneities on knock combustion in a downsized spark-ignition engine

The reactivity inhomogeneities are inevitably generated in the cylinder with the implementation of knock suppression strategies in downsized engines with direct-injection spark ignition. However, the mechanism of its influence on combustion has not yet been fully understood. A fundamental study was conducted by large eddy simulation to comprehensively study the effects of reactivity inhomogeneity formed by an independent variable of fuel stratification or temperature stratification on the in-depth mechanism of end-gas autoignition in a downsized spark ignition engine. The turbulent flame propagation is determined by an improved G-equation turbulent combustion model, and the detailed chemistry mechanism of a primary reference fuel is employed to observe the detailed reaction process in the end-gas autoignition process. Knock suppression results are observed in the temperature stratification case. In a large temperature gradient case, the premature autoignition timing caused by higher local temperature provides sufficient fresh gas for spontaneous combustion, leading to a better power performance compared to slight temperature gradient cases. Results of fuel stratification show that the knock is suppressed and gradually disappears with increasing ΔΦ. However, the subsequent reduction of peak pressure and the lengthening of combustion duration results in a terrible drop in power performance. The impact of flame propagation velocity dominates the pressure evolution compared to the effect of a highly local rich mixture reducing the ignition delay time under present conditions. This work will give an underlying understanding of the knock mechanism through different strategies.

Effect of Fuel Properties on Knocking Combustion in an Optical Rapid Compression Machine

Knocking combustion is the main bottleneck when downsizing and supercharging spark-ignition engines for pursuing higher thermal efficiency. The effects of thermodynamic conditions and thermal and/o...

Knock combustion investigation on a two-stroke spark ignition UAV engine burning RP-3 kerosene fuel

Purpose The purpose of this paper is to investigate the knock combustion characteristics, including the combustion pressure, heat release rate (HRR) and knock intensity of aviation kerosene fuel, that is, Rocket Propellant 3 (RP-3), on a port-injected two-stoke spark ignition (SI) engine. Design/methodology/approach Experimental investigation using a bench test and the statistical analysis of data to reflect the knock combustion characteristics of the two-stroke SI unmanned aerial vehicle (UAV) engine on RP-3 kerosene fuel. Findings Under the full load condition of 4,000 rpm, at the ignition timing of 25 degree of crank angle (°CA) before top dead centre (BTDC), the knock combustion is sensitive to the thinner mixture; therefore, the knock begins to occur when the excess air ratio is larger than 1.0. When the excess air ratio is set as 1.2, the knock obviously appears with the highest knock intensity. At the excess air ratio of 1.2, better engine performance is obtained at the ignition timing range of 20-30 °CA BTDC. However, the ignition timing at 30° CA BTDC significantly increases the peak combustion pressure and knock intensity with the advancing heat release process. Practical implications Gasoline has a low flash point, a high-saturated vapour pressure and relatively high volatility, and it is a potential hazard near a naked flame at room temperature, which can create significant security risks for its storage, transport and use. The authors adopt a low-volatility single RP-3 kerosene fuel for all vehicles and equipment to minimise the number of different devices using various fuels and improve the military application safety. Originality/value Most two-stroke SI UAV engines for military applications burn gasoline. A kerosene-based fuel for stable engine operation can be achieved because the knock combustion can be effectively suppressed through the combined adjustment of the fuel amount and spark timing.

Numerical Simulation of Knock Combustion in a Downsizing Turbocharged Gasoline Direct Injection Engine

Engine knock has become the prime barrier to significantly improve power density and efficiency of the engines. To further look into the essence of the abnormal combustion, this work studies the working processes of normal combustion and knock combustion under practical engine operating conditions using a three-dimensional computation fluid dynamics (CFD) fluid software CONVERGE (Version 2.3.0, Convergent Science, Inc., Madison, USA). The results show that the tumble in the cylinder is gradually formed with the increase of the valve lift, enhances in the compression stroke and finally is broken due to the extrusion of the piston. The fuel droplets gradually evaporate and move to the intake side under the turbulent and high temperature in the cylinder. During the normal combustion process, the flame propagates faster on the intake side and it facilitates mixture in cylinder combustion. During the knock combustion simulation, the hotspots near the exhaust valve are observed, and the propagating detonation wave caused by multiple hotspots auto-ignition indicates significant effects on knock intensity of in-cylinder pressure.

Quantitative evaluation of n-butane concentration on knock severity of a natural gas heavy-duty SI engine

Knocking combustion of a natural gas spark ignition engine with the addition of n-butane was investigated. Specially, the effects of n-butane concentration on knock severity were quantitatively evaluated with engine full-load at various engine speeds. The results show that with an increase in n-butane concentration, the peaks of in-cylinder pressure and heat release rate increase, combustion duration is shortened, and CA50 is advanced. At high speeds, n-butane concentration has a more pronounced influence on combustion and knock. With increased n-butane concentration, cyclic pressure traces oscillate more and more violently, the maximum pressure rise rate (MPRR) and the maximum amplitude of pressure oscillation (MAPO) in some cycles increase significantly. Here the MPRR threshold of light knock cycle is defined at 1.0 MPa/CAD, and the one of heavy knock cycle is 4.0 MPa/CAD. Meanwhile the MAPO threshold of light knock cycle is set at 0.1 MPa, and the one of heavy knock cycle is 0.3 MPa. Based on these thresholds and the probability of knock severity, light and heavy knock conditions are identified. Moreover, the combustion close to light knock may increase thermal efficiency, and heavy knock has caused the erosion of aluminum piston crown and top land in the experiment.

Numerical study on auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions

Knocking combustion for spark-ignition engine is related to auto-ignition of a portion of the unburned fuel-air mixture. In this investigation, the detailed chemical kinetic mechanism was engaged to numerically study the auto-ignition characteristics of hydrogen-enriched methane under engine-relevant conditions. Compared with the experimental data, the USC Mech 2.0 mechanism obtained the closest agreement, and it was adopted in the sensitivity analysis, the rate of production (ROP) analysis and the reaction pathway analysis. Results showed that at high temperature and high pressure, the ignition delay times (IDs) of CH4/H2 fuel blends reduce significantly (by two orders of magnitude at most) with rising hydrogen fraction, but the decline rate is not so obvious (within 37.3%) at low temperature. The sensitivity analysis indicated that at high temperature the reaction (R1) and reactions (R2, R3) promote each other while at low temperature only the reaction (R3) unilaterally facilitates the reaction (R12). The ROP analysis implied that the decrease of IDs of methane by hydrogen addition is realized through increasing the H, O, and OH radical production. Interestingly, the IDs for CH4/H2 fuel blends show different trends at different temperature, which decrease (by 47.5% at most) at low temperature but increase (by 132.7% at most) at high temperature as the equivalence ratio rises. The sensitivity analysis showed that the ignition kinetics for CH4/H2 fuel blends more depend on the CH4 concentration at low temperature but oxygen concentration at high temperature. This investigation not only supplements the fundamental combustion studies of CH4/H2 fuel blends in elevated pressures, but also reveals the influence mechanism of hydrogen addition and equivalence ratio on the IDs of CH4/H2 fuel blends at high pressure. More importantly, it may offer fundamental insights for the control of knocking combustion of spark-ignition (SI) engine.

The knock study of high compression ratio SI engine fueled with methanol in combination with different EGR rates

Methanol, with a high octane index, is considered to be one of the most ideal fuels for high compression ratio engines. In this paper, the SAGE models combined with methanol chemical reaction kinetics (consisting of 46-species and 247-elementary reactions) are used to study knock combustion. Knock combustion is simulated in a high compression ratio SI methanol engine with different EGR rates. The results show that the introduction of EGR into the combustion chamber can reduce the knock intensity and delay the knock onset time. Without EGR, the intake side of the combustion chamber is prone to knocking mostly. As the EGR rate increases, the exhaust side of the combustion chamber is the most likely to knocking. OH radicals can be considered an indicator of the temperature, and H2O2 can be regarded as an indicator of the propagating flame front. OH marks the knock phase duration and HCO marks the onset of knocking. The reaction intensities of CH2O, H2O2, CO and OH species are higher than other species during knocking combustion. The greater the reaction rate is, the greater the knock intensity will be.

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.

Numerical Investigation on Knock Combustion in a Diesel–Dimethyl Ether Dual-Fuel Engine

Numerical investigation on knock was conducted in a direct-injection diesel engine with port premixed dimethyl ether (DME) fuel. Knock combustion at various engine operating conditions with a fixed speed was simulated, and the effects of premixed DME ratio, injection timing, and exhaust gas recirculation (EGR) rate on knock suppression were studied based on the multidimensional computational fluid dynamics (CFD) analysis. A reduced diesel–DME kinetic mechanism coupled with the CFD model was used to obtain local pressure and radical concentration at various monitoring locations within the computation domain. A method with a band-pass filter was then employed to deal with pressure data, and maximum amplitude of pressure oscillation was adopted as an evaluation index of the knock intensity. The analysis of species concentration was also used to quantify knock here. The research results indicated the profiles of pressure and major intermediate species could provide useful information to quantify and predict knock for DME–diesel dual-fuel premixed compression-ignition combustion. Moreover, using EGR was an effective way to reduce the knock intensity and delay the knock onset. A lower DME premixed ratio and retarded injection timing could also mitigate the knock.

Prediction Modeling and Analysis of Knocking Combustion using an Improved 0D RGF Model and Supervised Deep Learning

The knock phenomenon is one of the major hindrances for enhancing the thermal efficiency in spark-ignited engines. Due to the stochastic behavior of knocking combustion, analytical cycle studies are required. However, there are many problems to be addressed with regard to the individual cycle analysis of in-cylinder pressure data. This study thus proposes novel, comprehensive and efficient methodologies for evaluating the knocking combustion in the internal combustion engine. The proposed methodologies include a filtering method for the in-cylinder pressure, the determination of the knock onset, and the calculation of the residual gas fraction. Consequently, a smart knock onset model with high accuracy could be developed using a supervised deep learning that was not available in the past. Moreover, an improved zero-dimensional (0D) estimation model for the residual gas fraction was developed to obtain better accuracy for closed system analysis. Finally, based on a cyclic analysis, a knock prediction model is suggested; the model uses 0D ignition delay correlation under various experimental conditions including aggressive cam phase shifting by a dual variable valve timing (VVT) system. Using the proposed analysis method, insight into stochastic knocking combustion can be obtained, and a faster combustion speed can lead to a higher knock intensity in a steady-state operation.

Experimental research on operational process of petrol engine on lean air-and-fuel mixtures on the edge of combustion knock

The results of experimental research are provided regarding the operational process of VAZ-21124 petrol engine with the compression ratio of 12.0 on lean air-and-fuel mixtures with various values of heating the air charge at intake, and of the ignition advance angle. In the course of performing the experimental research, the pressure of gases in the engine cylinder was registered, as well as the temperature of the air charge in the intake manifold, the ignition advance angle, and the concentration of toxic components in the exhaust gases. The obtained pressure indicator diagrams were processed using a special program for determining indices of the operating cycle, cycle consumption of fuel and air, air excess factor, and heat release rate. The obtained cycle consumption of fuel and air, as well as the air excess factor were compared to the ones measured experimentally. As a result, possible boundaries of leaning of air-and-fuel mixture were revealed, as well as the values of heating the air charge, and of the ignition advance angle, for intensification of the process of combustion till the knock moment. When the air charge temperature at the intake was raised from 29°C to 112°C;, the ignition advance angle retarded from 35.5 to 15 degrees, and the air excess factor decreased from 1.55 to 1.41 respectively.

The influence of mixing ratio of low carbon mixed alcohols on knock combustion of spark ignition engines

Alcohol fuels, owing to the characteristics of oxygen-containing and high octane number, are considered as one potential alternative fuel additive in spark ignition (SI) engines. In this paper, the ethanol and n-butanol are blended with gasoline to form the new low-carbon mixed alcohols-gasoline blended fuels. The effect of different mixing proportions of ethanol and n-butanol in gasoline on knock combustion and fuel economy of SI engine was studied by experiment and simulation. The results showed that with the increase of n-butanol ratio, brake specific fuel consumption (BSFC) increase, but knock index (KI) and knock intensity decrease. Moreover, crank angle delays under the same knocking condition when the engine fueled with E5-Bx and B5-Ey fuel, and the crank angle is delayed more significantly with the increase of engine speed. In addition, when the ethanol and n-butanol are blended as the same proportion in gasoline (i.e. x = y), the KI and knock intensity of E5-Bx fuel is lower than that of B5-Ey, which indicates that the B5-Ey fuel has better antiknock property. But the BSFC of B5-Ey is slightly greater than that of E5-Bx under the same operating condition, that is, the B5-Ey fuel has higher fuel consumption.

Dilution of fresh charge for reducing combustion knock in the internal combustion engine fueled with hydrogen rich gases

Hydrogen as potential engine fuel can appear either as a single gas or as a component in processing gases e.g. syngas, hythane and coke gas. The research in this paper investigates impact of combustible mixture dilution on abnormal combustion called knock in the reciprocating internal combustion engine. Dilution can be realized by either exhaust gas recirculation (EGR) or making the combustible mixture lean. Novelty of this work is a new metrics defined as dilution ratio, which makes it possible to compare knock reduction caused by either EGR or leaning the air-gas mixture to the engine. Two gaseous fuels were investigated: hydrogen and coke gas with 65% hydrogen. Conclusion based on the proposed dilution ratio states that, for hydrogen as the fuel, applying EGR is more effective in knock reduction than making the mixture lean. It was found that EGR strategy in the hydrogen fueled engine can reduce knock intensity from initial 40 kPa–20 kPa, whereas by leaning the mixture to the same dilution ratio, the knock is reduced to approximately 28 kPa. With respect to coke gas, it is proved that both EGR and lean mixtures influence on knock reduction at the same strength.

Analysis of auto-ignition characteristics of low-alcohol/iso-octane blends using combined chemical kinetics mechanisms

Knocking combustion is mainly caused by auto-ignition of the end gas in spark-ignition (SI) engines. It not only leads to severe engine damage, but also can reduce thermal efficiency and constrain engine performance. As high-octane gasoline additives, the auto-ignition characteristics of low-alcohols, such as methanol, ethanol and n-butanol, and their blends with iso-octane fuels are analyzed under the conditions of the end gas. Moreover, the effects of what mechanism that promots and inhibits low-alcohols effects on auto-ignition characteristics of iso-octane is studied based on detailed chemical kinetics mechanisms. First, the low-alcohols mechanisms are validated and selected by comparing the predicted ignition delay times of the end gas with the corresponding experimental data under different target conditions of lower temperature and high pressure. From there, the auto-ignition characteristics of low-alcohol are analyzed to predict the knock tendency of fuels. In addition, the reactivity of low-alcohol/iso-octane blends is analyzed by comparing the CA50 contours of different fuels under various intake temperature and pressure conditions. It is found that the n-butanol and iso-octane have stronger auto-ignition characteristics than methanol and ethanol, revealing that methanol and ethanol have higher anti-knock properties. For further explaining the effect of low-alcohols on the anti-knock performance of low-alcohol/iso-octane fuels, the key reaction pathways for the low-temperature reaction of low-alcohol/iso-octane fuels is studied. Studies show that metastable products, such as aldehydes and ketones, produced by the chain propagation can delay the auto-ignition process and reduce the low temperature reactivity of blended fuels, when methanol and ethanol fuels are added to iso-octane. But the reaction rate of the system is accelerated, and the ignition time of the blended fuel is significantly earlier than that of iso-octane after the addition of n-butanol in iso-octane.

Simulation of knock probability in an internal combustion engine.

In spark-ignition internal combustion engines, fluctuations of the in-cylinder pressure trace and the tendency of combustion knock are usually different from one cycle to another. These cycle-to-cycle variations are affected by the initial state at ignition time and the subsequent burning. The occurrence of the phenomena is unpredictable, and their stochastic nature offers challenges in the optimization of engine control strategies. In this paper, a simulator providing a series of cycle-to-cycle varied in-cylinder pressures is introduced. The Wiebe function and Livengood-Wu integration are used to describe the determinacy of combustion. Various means, including the Markov chain, are introduced to express the stochastic quantities during combustion. In addition, the combustion of a given knock probability is simulated.

The Effects of Equivalence Ratio on Pressure Wave Development during Knocking Combustion

A knocking combustion in a one-dimensional constant volume reactor has been simulated with a detailed chemical kinetic mechanism of n-heptane premixed gases by using the compressible Navier-Stokes equations. This study focuses on the impact of the various equivalence ratios (0.6–1.4) in the pressure wave development during knocking combustion in an account of different initial temperature (600–900K). The result demonstrates the autoignition process in the end-gas region and explains the knocking phenomenon on different equivalence ratio. In an adiabatic wall condition, the largest knocking intensity occurs in equivalence ratio 1.4 whereas the lowest knocking intensity found in equivalence ratio 0.6. Regarding to initial temperatures, a strong peak of knocking is generated around 650K in all the equivalence ratios (0.8–1.4). However, yet a small perceptivity of knocking is found in around 750K for each equivalence ratio. In case of smaller equivalence ratios, a weak knocking occurs, which can be identified by the behavior of the pressure wave generation in the end-gas region.