Combustion Knock Research Articles

Experimental Assessment of the Performance and Fine Particulate Matter Emissions of a LPG-Diesel Dual-Fuel Compression Ignition Engine

The present work is focused on the assessment of the performance and fine particulate matter emissions (PM2.5) of a turbocharged four-cylinder direct injection diesel engine operating under dual-fuel mode with Liquefied Petroleum Gas (LPG). For load levels of 30%, 60% and 100%, measurements were taken, keeping the engine speed constant at 2200, 2500 and 3200 rpm, while the engine knock detonation was detected through a non-invasive internal system. According to experimental measurements, the abnormal knock combustion occurred at full load operation with a maximum LPG energy fraction of ~60%. The brake fuel conversion efficiency increased by 2.6% with an LPG energy fraction of 10%, where a fuel saving of 11.9% was achieved with respect to the diesel-only operation. The reduction of diesel consumption was around 50% with respect to 100% diesel operation at full load operations, where the highest brake fuel conversion efficiency was achieved. The brake fuel conversion efficiency decreased as LPG addition increased for all the engine loads. Regarding emissions, PM2.5 decreased with the addition of LPG. However, HC and CO emissions increased as LPG injection was higher. NOx emissions and exhaust gas temperatures were reduced for operation with higher LPG fractions, except for full load levels at 2200 and 2500 rpm.

Pyrolysis oil blended n-butanol as a fuel for power generation by an internal combustion engine

The article discusses results from the investigation of the n-butanol-pyrolysis oil blend, which can be considered a potential fuel for the internal combustion spark-ignition engine operating in a power generation set. The n-butanol-pyrolysis oil blends were prepared at two ratios of 3:1 (25%) and 1:1 (50%) by volume, respectively. As reference fuels for combustion tests in the engine, regular gasoline (gasoline EU 95) and n-butanol were proposed. The combustion tests were conducted on the single-cylinder research engine at a compression ratio of 11:1 and an equivalence ratio of 1. The experimental analysis was focused on engine performance, combustion phases, knock occurrence, and exhaust emissions. As observed, combustion got slower for butanol-pyrolysis oil blends. CO emission was similar to tests with reference fuels. Unburnt hydrocarbons increased with pyrolysis oil increased to 50% in a blend. NO emissions were reduced. It was found that n-butanol blended pyrolysis oil at a ratio of 3:1 can be successfully applied as the fuel to the spark-ignition engine. Additionally, it was found that this blend is more resistant to combustion knock compared to regular gasoline 95. Hence, the engine can work at a higher compression ratio without any malfunctions caused by combustion knock.

Knock combustion characteristics of an opposed‐piston two‐stroke gasoline engine

AbstractThe spark plug of an opposed‐piston two‐stroke (OP2S) gasoline engine is arranged on the side wall of the cylinder liner, far from the center of the combustion chamber; the ignition core of the mixture is offset, the flame propagation distance is increased, the combustion duration is prolonged, and the knock tendency is severe. In this paper, a quasi‐dimensional two‐zone combustion model is used in GT‐Power software to establish a thermodynamic process simulation model and a knock prediction model is included to analyze the effect and matching of the compression ratio, ignition timing, and other thermodynamic process parameters on the knock intensity and engine performance. The extended coherent flame model combustion model is coupled with the Huh–Gosman spray model in AVL‐Fire software, and the An B knock model is used to establish an in‐cylinder combustion model and analyze the flame propagation and the knock response rate of the flat and pit piston during the combustion process. With an increase of the compression ratio, the temperature and pressure of the mixture in the combustion chamber increase at the time of ignition, which leads to the knocking combustion in the cylinder. With an increase of the ignition advance angle, the in‐cylinder pressure and temperature increase rapidly, which increases the likelihood of knocking combustion. In comparison with the pit piston combustion chamber, the flame propagation speed of the flat piston combustion chamber is relatively slow, which increases the knock tendency. The results show that lowering the compression ratio and delaying the ignition can reduce the in‐cylinder knock tendency by setting a compression ratio of 10.5 and an ignition advance angle of 20°CA. When the pit piston is used to organize the squish and inverse squish before and after the inner dead center, the flame propagation process can be promoted. The knock response rate of pit piston is 24.7% lower than that of flat top piston.

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.

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

Effect of injection timing on knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures

The knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures are still unclear, especially under different injection timings. Therefore, this study illustrates the above issues through CONVERGE simulation. The results show that with the start of injection (SOI) sweeps from −7°CA to −32°CA, a large amount of liquid-phase fuel adheres to the wall, and the wet-wall ratio of fuel at SOI = −32°CA is as high as nearly 30%. The fuel film evaporates slowly, coupled with the effect of low temperature on chemical reactions, the high-temperature ignition (HTI) is delayed seriously until the end of injection. The amount of premixed mixture formed during long ignition delay is significantly increased, but its uniformity is better and the concentration is more suitable for ignition. Once HTI is triggered, high-frequency and strong pressure oscillation occurs in the cylinder, and the maximum oscillation amplitude is as high as nearly 10 MPa, far exceeding the threshold of destructive knock combustion. Delayed fuel injection can effectively alleviate the above problems, such as the best when the SOI in this study is −17°CA. In addition, HC emissions are positively correlated with the amount of fuel film, but the trend of CO quantity with injection timing shows the opposite result. NOx emission increases as the injection timing advances, while soot is the opposite, because the mixture concentration is leaner at the earlier SOI and the expanded high-temperature region leads to an accelerated oxidation rate of soot.

Numerical Study of Knocking Combustion in a Heavy-Duty Engine under Plateau Conditions

Diesel engine combustion becomes very rough and can lead even lead to deflagration under high altitude conditions, which is harmful to component durability. In this study, the effects of altitude on the main combustion characteristics—in-cylinder fluid flow, spray behavior, and pressure and temperature distribution—were analyzed with CFD. A numerical model was built on the CONVERGE platform and validated with the optical spray behavior and pressure trace measured by the test bench. The simulation results indicated that the decreases in compression pressure and temperature at 4.5 km led to an over 4 °CA longer ignition delay than those of 1 and 3 km. The combustion efficiency decreased from 90% to 47% when the combustion changed from normal combustion to knocking combustion due to severe spray impingement. The processes of end-gas ignition, sequential combustion, and pressure oscillation in knocking combustion were revealed by the numerical modeling results. These results indicate that super-knocking combustion exists in both spark-ignition (SI) engines and compression-ignition (CI) engines.

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.

Effects of Charge Motion on Knocking Combustion Under Boosted High Load Condition of a Medium-Duty Gasoline Engine

Effects of Charge Motion on Knocking Combustion Under Boosted High Load Condition of a Medium-Duty Gasoline Engine

Effect of Injection Timing on Knock Combustion and Pollutant Emission of Heavy-Duty Diesel Engines at Low Temperatures

Effect of Injection Timing on Knock Combustion and Pollutant Emission of Heavy-Duty Diesel Engines at Low Temperatures

Research on Destructive Knock Combustion Mechanism of Heavy-Duty Diesel Engine at Low Temperatures

Research on Destructive Knock Combustion Mechanism of Heavy-Duty Diesel Engine at Low Temperatures

Numerical and experimental study on knocking combustion in turbocharged direct-injection engines for a wide range of operating conditions

A combined experimental and numerical study is conducted on knocking combustion in turbocharged direct-injection spark-ignition engines. The experimental study is based on parameter variations in the intake-manifold temperature and pressure, as well as the air-fuel equivalence ratio. The transition between knocking and non-knocking operating conditions is studied by conducting a spark timing sweep for each operating parameter. By correlating combustion and global knock quantities, the global knock trends of the mean cycles are identified. Further insight is gained by a detailed analysis based on single cycles. The extensive experimental data is then used as an input to support numerical investigations. Based on 0D knock modeling, the global knock trends are investigated for all operation points. Taking into consideration the influence of nitric oxide on auto-ignition significantly improves the knock model prediction. Additionally, the origin of the observed cyclic variability of knock is investigated. The crank angle at knock onset in 1000 consecutive single cycles is determined using a multi-cycle 0D knock simulation based on detailed single-cycle experimental data. The overall trend is captured well by the simulation, while fluctuations are underpredicted. As one potential reason for the remaining differences of the 0D model predictions local phenomena are investigated. Therefore, 3D CFD simulations of selected operating points are performed to explore local inhomogeneities in the mixture fraction and temperature. The previously developed generalized Knock Integral Method (gKIM), which considers the detailed kinetics and turbulence-chemistry interaction of an ignition progress variable, is improved and applied. The determined influence of spark timing on the mean crank angle at knock onset agrees well with experimental data. In addition, spatially resolved information on the expected position of auto-ignition is analyzed to investigate causes of knocking combustion.

Determination of a most representative cycle from cylinder pressure ensembles via statistical method using distribution skewness

In internal combustion engine research, cylinder pressure measurements provide valuable information about the underlying thermodynamic and combustion processes, and are typically collected in ensembles of several 100 traces. Although in some particular fields of combustion research all traces are analyzed, in most cases only one trace is studied because analyzing all the traces is impractical due to the large number of collected samples. Instead, an ensemble-averaged pressure trace is commonly calculated and used for analysis. However, this pressure trace is highly smoothed and dynamic information is lost during the averaging process. With the average trace, pressure rise rates are lower and pressure oscillations such as the ones resulting from combustion knock are lost. In this work, a statistical method was developed to determine the “most representative cycle,” which is the cycle from the ensemble that has the pressure trace most representative of the engine operating condition. Eleven characteristic parameters are computed from each pressure trace and probabilistic distributions are obtained for each of the parameters using all the traces in the ensemble. Finally, the most representative cycle is selected by means of a cost function minimization. The benefits of this method are illustrated using experimental data from four very different engine platforms, under four different combustion modes and over a range of operating conditions.

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.

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 analysis of bio-butanol in TISI engine based on chemical reaction kinetics

The knocking combustion of n-butanol (bio-butanol) combined with exhaust gas recirculation (EGR) in turbocharge direct injection spark ignition (TISI) engine with high compression ratio is studied. Mainly analyze the influence of EGR rate and composition on knock combustion and the indication effect of unconventional emission of free radicals on knock. The research results show that adding appropriate EGR can reduce the engine knock intensity, but the introduction of a large amount of EGR can cause the knock intensity to increase. When the EGR rate is 8%, the MAPO is less than 0.1 MPa, indicating that knocking combustion does not occur in the cylinder. When the EGR rate is increased to 15%, the MAPO is greater than 0.15 MPa, indicating that knocking combustion is intensified. Knock combustion is mainly concentrated on the intake side, which is related to the flame propagation speed. The addition of EGR can reduce the temperature in the cylinder, mainly because the specific heat capacity of H2O and CO2 in the EGR component is relatively large, which can significantly reduce the temperature of the mixture. The variation of CH2O, CH3CHO and C2H4 can reflect the knock intensity of the mixture in the cylinder. In conclusion, the introduction of EGR into the TISI engine can effectively reduce the knock intensity. The variation of mixture composition can better reflect the knock region and knock intensity.

Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.

Online Spark Timing Optimization With Complex High-Fidelity Combustion Phasing, Knock, and Coefficient of Variation of IMEP Models

Abstract The combustion phasing of spark ignition (SI) engines is traditionally regulated with map-based spark timing (SPKT) control. The calibration of these maps is time-consuming for SI engines with a high number of control actuators. This paper proposes three online SPKT optimization algorithms that can utilize control-oriented semiphysics-based combustion models making the SPKT control algorithm more adaptive to different engine designs. These three SPKT optimizers do not require model inversion and derivative information. These methods also preserve the dependence between combustion phasing, knock, and coefficient of variation (COV) of indicated mean effective pressure (IMEP) models to avoid evaluating combustion models multiple times within one iteration. The two-phase and constraint relaxation methods are derived from direct search optimization theories. The recursive least square (RLS) polynomial fitting method can be considered as a virtual extreme seeking (ES) process that converts the original “black” box nonlinear constrained optimization into the solution of three low-order polynomial equations. Although these three online SPKT optimization approaches have unique properties making them preferable with certain types of combustion models, simulation and test results show that all of them can find the optimal SPKT with less than 10 evaluations of the combustion models. This fact makes it possible to implement the proposed model-based SPKT control strategy in future engine control units (ECUs).

Detection of Knocking Combustion Using the Continuous Wavelet Transformation and a Convolutional Neural Network

The phenomenon of knock is an abnormal combustion occurring in spark-ignition (SI) engines and forms a barrier that prevents an increase in thermal efficiency while simultaneously reducing CO2 emissions. Since knocking combustion is highly stochastic, a cyclic analysis of in-cylinder pressure is necessary. In this study we propose an approach for efficient and robust detection and identification of knocking combustion in three different internal combustion engines. The proposed methodology includes a signal processing technique, called continuous wavelet transformation (CWT), which provides a simultaneous analysis of the in-cylinder pressure traces in the time and frequency domains with coefficients. These coefficients serve as input for a convolutional neural network (CNN) which extracts distinctive features and performs an image recognition task in order to distinguish between non-knock and knock. The results revealed the following: (i) The CWT delivered a stable and effective feature space with the coefficients that represents the unique time-frequency pattern of each individual in-cylinder pressure cycle; (ii) the proposed approach was superior to the state-of-the-art threshold value exceeded (TVE) method with a maximum amplitude pressure oscillation (MAPO) criterion improving the overall accuracy by 6.15 percentage points (up to 92.62%); and (iii) The CWT + CNN method does not require calibrating threshold values for different engines or operating conditions as long as enough and diverse data is used to train the neural network.

Investigation on Effects of Ignition Configurations on Knocking Combustion Using an Optical Rapid Compression Machine under Lean to Stoichiometric Conditions

ABSTRACT Multiple ignition is beneficial to the improvement of engine thermal efficiency since it helps to shorten combustion duration and enhance lean-combustion stability. However, more researches are still needed to further understand knocking combustion under multiple ignition conditions, especially under lean-burn conditions. This study presents an investigation into the knocking characteristics under lean to stoichiometric conditions using multiple ignition on an optical rapid compression machine (RCM). Up to six ignition configurations were tested under three equivalence ratios (0.7/0.85/1). High-speed photography and high-frequency pressure acquisition were used to record the combustion processes. The results showed that the burning velocity increased nonlinearly with the increase of spark plug numbers while the knocking intensity (KI) first decreased and then increased, exhibiting a “U-shaped curve” trend. The burned mass fraction (BMF) at the instant of end-gas auto-ignition was also affected by ignition configuration. Generally, the smaller the BMF is at the instant of auto-ignition, the stronger the KI becomes. Furthermore, the key product during iso-octane oxidation, ·OOQOOH, was selected as an indicator to conduct chemical kinetics analysis on end-gas oxidation process. Results showed that the ignition configuration had strong impact on the heat release during flame propagation, which further influenced the reaction process of end-gas by influencing its low-temperature reactions. Under moderate spark plug number (around 3), the heat release during the flame propagation was most intensive, which further suppressed the low-temperature reactions of the end-gas and thus weakened auto-ignition intensity.