Knock Intensity Research Articles

Analysis of onset and severity of knock in SI engine based on in-cylinder pressure oscillations

This paper, based on pressure wave equation coupled with KIVA-3V combustion codes with reduced kinetics mechanism, analyzed and discussed the onset and severity of knock identified by different knock metrics in SI engine running at different speeds. The calculated results demonstrated that the knock onset defined by pressure oscillations characteristics here was approximately consistent with that defined by heat release. Meanwhile, the knock onset shifted backwards in high-speed region where relatively high flame propagation speed prevented the occurrence of auto-ignition, and thus knock intensity, characterized by knock metrics, MAPO and KI20, decreased observably. Visualized 3D calculation results were presented to show the flame propagation and pressure oscillation characteristics. Finally, the correlation between engine speed and the onset and severity of knock indicated that knock onset was highly correlated to engine speed, the knock intensity judged by MAPO and KI20 was accurate basically, and KI20 knock metric was better due to more pressure oscillations characteristics information contained. The comparison between experimental and calculated results indicated that the onset and intensity of knock can be characterized and quantified accurately with high-frequency pressure oscillations characteristics calculated with pressure wave equation.

Study of knock in a high compression ratio spark-ignition methanol engine by multi-dimensional simulation

Methanol as an alternative fuel is considered to be one of the most favorable fuels for internal combustion engines. In this paper, knock in a high compression ratio spark-ignition (SI) methanol engine was studied by using multi-dimensional simulation. Knocking combustion under various engine operating conditions were simulated, and the effects of spark timing, EGR (Exhaust Gas Recirculation) techniques, mixture concentration and combustion chamber shape for suppressing knock in a high compression ratio spark-ignition methanol engine were studied based on the multi-dimensional simulation analysis. The results showed that it could not fully suppress engine knock only by retarding spark timing methods because of the high compression ratio, with the increase of EGR rate, the knock intensity was found to be greatly suppressed and the peak pressure was reduced and postponed. Meanwhile, knock intensity had a maximum point, rich mixture and lean mixture can help reduce knock intensity and the knock occurrence timing was delayed. Finally, in order to reduce engine knock intensity, two feasible new combustion chambers were proposed.

Dynamic knock detection and quantification in a spark ignition engine by means of a pressure based method

In spark ignition engines, knock onset limits the maximum spark advance. An inaccurate identification of this limit penalises the fuel conversion efficiency. Thus it is very important to define a knock detection method able to assess the knock intensity of an engine operating point.Usually, in engine development, knock event is evaluated by analysing the in-cylinder pressure trace. Data are filtered and processed in order to obtain some indices correlated to the knock intensity, then the calculated value is compared to a predetermined threshold. The calibration of this threshold is complex and difficult; statistical approach should be used, but often empirical values are considered.In this paper a method that dynamically calculates the knock threshold necessary to determine the knock event is proposed. The purpose is to resolve cycle by cycle the knock intensity related to an individual engine cycle without setting a predetermined threshold.The method has been applied to an extensive set of experimental data relative to a gasoline spark-ignition engine.Results are correlated to those obtained considering a traditional method, where a statistical approach has been used to detect knock.

Characteristics Study of Knocking, Fuel Consumption and Emissions with Cooled External EGR on SIDI Turbo Charged Gasoline Engine

Based on 2.0L spark ignition direct-injection(SIDI) turbo charged gasoline engine with variable valve timing(VVT), the spark timing effects on combustion process, fuel consumption, knock limitation and emissions were studied with external exhaust gas recirculation(EGR) on different loads and speeds. For the purpose of easily comparing data results, the location of 50% mass fraction burned(CA50) was widely used in this paper. The study results show that CA50 is changing linearly with spark timing. To reach the same combustion phase, spark timing need to be further advanced with external EGR. Combustion variability of IMEP(COV) increase sharply with retarding of CA50 with external EGR. The external EGR can greatly improve knock limitation and knock intensity(KI). To reach the best fuel consumption, CA50 need to be advanced to 6 °CA to 8 °CA for both with external EGR and without external EGR. NOx emissions increase with advancing of spark timing. HC show different trends when retarding spark timing with and without external EGR.

Hydrogen resistance to knock combustion in spark ignition internal combustion engines

The results of investigations focusing on knock combustion analysis of a hydrogen-fueled engine have been presented in the paper. Knock intensity was determined as the intensity of the in-cylinder combustion pressure pulsations (recorded with a sampling frequency of 100 kHz) and filtered through high-pass filtering with cut-off frequency of 3.5 kHz. The research was conducted on the CFR engine with a variable compression ratio ranging from 6 to 14. The research has shown a rapid increase in pressure pulsations amplitude was observed while the compression ratio was changed from 11 to 12. This was interpreted as a result of in-cylinder hydrogen-air mixture self-ignition at the end of the spark ignition controlled combustion. Supporting this observation the theorem of dual nature of hydrogen knock combustion was postulated. Intensity of the pressure pulsations that accompany normal combustion without hydrogen self-ignition was in an exponential correlation with the compression ratio, which directly translates into a similar correlation of the pulsations and temperature of hydrogen-air mixture at the moment of ignition.

B212 均一予混合気へのアルコール濃度勾配付与によるHCCI燃焼制御(OS-10:内燃機関(1))

To study the effect of the ethanol concentration gradient on knocking in n-heptane/air homogeneous mixture, we carried out the compression and auto-ignition experiment by using rapid compression machine (RCM). The luminescence of flame was captured by High speed camera. Experimental results were that the luminescence progressed from top (lean) to bottom (rich) in cylinder with the ethanol concentration gradient and when the ethanol concentration gradient was over 0.1, the knock intensity was smaller than that of homogeneously mixed ethanol.

A cumulative-summation-based stochastic knock controller

In this paper, an analysis of knock signals suggests that the knock intensity is a cyclically uncorrelated random process, and that it is therefore not possible to control individual cycles to a specified knock intensity in a deterministic manner. A new knock control algorithm is therefore developed on the basis of a stochastic interpretation of the knock signal, and on the basis of a control objective specified as a certain percentage of knocking cycles. Unlike traditional controllers, the new algorithm does not respond to knock events provided that these are occurring within a specified tolerance of the target knock rate. The new controller uses the cumulative summation of knock events to make this determination, thereby avoiding the slow transient response times sometimes associated with ‘stochastic’ knock controllers. When a spark adjustment is deemed necessary, the magnitude of the control action is scaled according to the likelihood ratio of the observed events since the last spark adjustment was made. A theoretical analysis of the new controller is presented and a simulation tool which is closely based on experimental data is used to assess its performance. The results show that the new controller is able to achieve the same target knock rate as a traditional controller while operating at a more advanced mean spark angle. There is also less cyclic variance about this mean and the regulatory response is significantly improved. The transient response to overly advanced or retarded conditions is similar to the traditional controller. These results suggest that the new controller will deliver increased torque and engine efficiency under knock-limited conditions without increasing the risk of engine damage.

Description of knock onset in SI turbocharged gas engine

Knock is an abnormal combustion process occurring in SI engines. To describe engine knock reliably is still challenge. Based on measured in cylinder pressure, the techniques of knock onset determination are introduced. The findings about knock intensity are discussed. Simulation of knock phenomena is mentioned. The results from model and experiment are compared.

Compression ignition of directly injected natural gas with entrained diesel

A new fuel injector prototype for heavy-duty engines has been developed to use direct-injection natural gas with small amounts of entrained diesel as an ignition promoter. This ‘co-injection’ is quite different from other dual-fuel engine systems where diesel and gas are introduced separately. In an engine with co-injection, diesel and gas are injected simultaneously through one set of nozzle holes as the piston approaches top dead centre. Most of the combustion is non-premixed, as in a conventional diesel engine, but the natural gas supplies over 90 per cent of the energy for typical operating conditions. Reliable compression-ignition can be attained, but two injections per engine cycle are often needed to minimize engine knock. The present paper focuses on 800 r/min light-load operation (equivalence ratio between 0.05 and 0.22) with a single injection per cycle, in order to better understand how the ignition process is affected by in-cylinder conditions and the gas/diesel ratio. Two techniques were used to explore the data: response surface methodology and power-law fits. These methods both showed above a certain diesel/gas ratio that ignition delay approached that of pure diesel injections, but significant knock would occur if the total fuel energy of the injection was high. With high injection pressure and low cylinder pressure, the region of allowable diesel flow (i.e. the region with low knock intensity and high combustion efficiency) was increased for cases with low to moderate gas flow compared with low injection pressure. Injection pressure had little effect at high cylinder pressure, and had no significant effect on ignition delay.

Application of a new parametric model-based filter to knock intensity measurement

In this paper, a new parametric model-based filter is proposed for impact-induced fault intensity measurement. The filter designing steps are based on advanced parametric modeling and decomposition of the non-stationary random vibration signals, extracted from a mechanical system. One of the important innovations of this work throughout filter designing is proposing a method for identifying the most proper latent component of the signal in the undamaged state using instant modules of the signal. By considering this fact that knock is one of the important defects in the spark ignition engines, the proposed filter is applied for knock intensity measurement as a case study. For evaluation purpose, the proposed method is compared with other previous methods in literatures.

Effects of octane number on autoignition and knocking phenomena in a stratified mixture

The effects of the fuel concentration gradient and the octane number on the autoignition and knocking phenomena in a stratified mixture were studied experimentally on a using a rapid compression machine using stratified mixtures of air and fuels n-heptane, iso-octane, n-hexane, and n-pentane with different octane numbers (0, 100, 25, and 62, respectively). In the chamber, the lower the vertical location, the richer the fuel concentration of the mixture. The mixture contains no gradient in the horizontal direction. The experimental results show that rapid spread of the flame is caused not by flame propagation but by sequential autoignition. Although ignition delays of a stratified mixture are not dependent on the fuel concentration gradient in the mixture, they are constant as long as mean equivalence ratio is the same, and they decrease with the decreasing mean equivalence ratio. In excess of certain gradient value, the knock intensity is smaller as the gradient becomes larger for all fuels tested regardless of their octane number.

On the Reduction of a 3D CFD Combustion Model to Build a Physical 0D Model for Simulating Heat Release, Knock and Pollutants in SI Engines

In the automotive industry, today's major objectives concern the reduction of pollutant emissions and fuel consumption while improving performance and driveability. For this purpose, during the last decade, the classical engine has evolved towards a very complex system combining many hi-tech components with advanced control strategies. Optimizing the whole engine system and controlling its behaviour has then become a real challenge for car manufacturers. In this context, powertrain simulation tools have been shown to be an undisputable support during all stages of the engine development from concept design to control strategies development and calibration. However these tools require sophisticated models to be efficient, especially in the combustion chamber where combustion and pollutant formation processes take place. This paper presents a 0D physical combustion model devoted to the prediction of heat release, knock and pollutants in SI engines. The originality of the model derives from the fact it is based on the reduction of the 3D CFD E-CFM (Extended Coherent Flame Model) model developed at IFP. The CFM formalism distinguishes two zones: the fresh and the burnt gases, which are separated by a flame front and are both described by their temperature, mass and composition. The proposed model is an important evolution of the CFM-1D model previously published. It computes the rate of consumption of the fresh gases and is based on the calculation of the flame front surface using the real engine geometry and a 0D derivation of the flame surface density approach. Pollutants (CO and NOx) are computed both through the flame front an within the burnt gases using a reduced kinetic scheme and a classical extended Zel'dovitch mechanism. The knock timing calculation is performed in the fresh gases zone describing the evolution of an auto-ignition precursor and a simple correlation is used to estimate the corresponding knock intensity. The whole model is validated against experimental data at several steady state operating points for a single-cylinder engine. Parametric variations around optimal engine settings are also performed. A good agreement with experiments is observed, showing the interest of reducing 3D CFD models to build predictive 0D models for engine system simulations.

Knock Characteristics in Liquefied Petroleum Gas (LPG)−Dimethyl Ether (DME) and Gasoline−DME Homogeneous Charge Compression Ignition Engines

The knock characteristics in an engine were investigated under homogeneous charge compression ignition (HCCI) operation. Liquefied petroleum gas (LPG) and gasoline were used as fuels and injected at the intake port using port fuel injection equipment. Dimethyl ether (DME) was used as an ignition promoter and was injected directly into the cylinder during the intake stroke. Different intake valve timings and fuel injection amounts were tested to verify the knock characteristics of the HCCI engine. The LPG is more suitable for high-load operation of HCCI than gasoline because of its high latent heat of vaporization and octane number. The knock intensity of LPG−DME combination was lower than that of gasoline−DME, because of the lower self-ignitability and the higher latent heat of vaporization of the total injected fuel. The ringing intensity (RI) increased as the intake valve open (IVO) timing was advanced. This is attributed to higher volumetric efficiency and residual gas, which promotes combustion. The k...

Weak knock characterization and detection for knock control

Knock is a fundamental phenomenon in combustion engines, and knock control is central in any engine management system. Better understanding of knock, and weak knock in particular, is considered from two main aspects: knock detection and knock characterization. The aim of knock detectors is both to detect knock and to estimate the crank angle at knock onset. Focusing on weaker knock than before, it is shown that knock detectors and algorithms have to take into account other characteristics of knock traces than the standard model. It is shown that the best-performing knock detector of those investigated is one that supervises changes in signal variance, except for low signal-to-noise ratios where it is advantageous to use also the oscillation frequency. Regarding characterization, an important result is that in a wide range of intensities there is an almost linear dependence between the logarithmic normalized knock energy and the rate of cycles with knock. This means for example that a knock controller can use feedback on the rate of cycles with knock instead of knock intensity and vice versa; both can in combination with better detection provide possibilities for smoother and more anticipatory control schemes.

CT2-1: Application of a Reduced Elementary Reaction Scheme to Three-dimensional Numerical Simulation of Knocking Phenomenon in a Spark Ignition Engine Fueled by LPG-DME Mixture(CT: Combustion, Thermal and Fluid Science,General Session Papers)

In this study, the authors tried to understand the knocking phenomenon in a spark ignition engine fueled by LPG-DME mixture. The authors' original 3-D CFD code "GTT" linked to the CHEMKIN-II subroutines, called "GTT-CHEM", was used for not only understanding this knocking phenomenon but also estimating the knock intensity, which is usually measured by the completely different way on the test bed, from the CFD results. Then, a several different shapes of piston cavity were examined by both CFD and actual engine tests. As to the 3-D chemical kinetics calculation technique, for largely saving the computational time with little deterioration of accuracy, the authors constructed a reduced elementary reaction scheme for LPG-DME mixture by combining and reducing the GRI-Mech's scheme for LPG (mostly propane) and the Curran's scheme for DME. This reaction scheme was employed in the GTT-CHEM code, into which a modified version of the Kong's turbulent combustion model based on elementary reactions was incorporated. The GTT-CHEM code with the original reduced elementary reaction scheme was able to reproduce the knocking phenomenon reasonably well in the LPG-DME mixture fueled spark ignition engine. Finally, after actual engine experiments about different piston cavities, it has been found that a knocking intensity estimation index derived from HCHO behavior in CFD results is a good prophetic index to expand the knocking limit.

LPG-DME 성층혼합 압축착화 엔진

The combustion characteristics of a liquefied petroleum gas-di-methyl ether (LPG-DME) compression ignition engine was investigated under homogeneous charge and stratified charge conditions. LPG was used as the main fuel and injected into the combustion chamber directly. DME was used as an ignition promoter and injected into the intake port. Different LPG injection timings were tested to verify the combustion characteristics of the LPG-DME compression ignition engine. The combustion was divided into three region which are homogeneous charge, stratified charge, and diffusion flame region according to the injection timing of LPG. The hydrocarbon emission of stratified charge combustion was lower than that of homogeneous charge combustion. However, the carbon monoxide and nitrogen oxide emission of stratified charge combustion were slightly higher than those of the homogeneous charge region. The indicated mean effective pressure was reduced at stratified charge region, while it was almost same level as the homogeneous charge combustion region at diffusion combustion region. The start of combustion timing of the stratified charge combustion and diffusion combustion region were advanced compared to the homogeneous charge combustion. It attributed to the higher cetane number and mixture temperature distribution which locally stratified. However, the knock intensity was varied as the homogeneity of charge was increased.

110 各種燃料におけるノック強度特性に関する研究(OS-1 ガソリンHCCI機関,火花点火機関)

110 各種燃料におけるノック強度特性に関する研究(OS-1 ガソリンHCCI機関,火花点火機関)

Estimation of knock intensity in Spark-Ignition engines by using wavelet transform

Knock in spark ignition (SI) engines is an undesirable mode of combustion, causing high combustion pressure pulses associated with vibration of the engine block. This phenomenon limits performance, durability and fuel economy of SIengines. In this paper the author propose; a new knock detection method called WBKD (Wavelet Based Knock Detection), which is based on wavelet transform. Real data experiments confirmed the utility of the proposed approach for knock intensity estimating.

04/02986 Development of an empirical correlation for combustion durations in spark ignition engines: Bayraktar, H. and Durgun, O. Energy Conversion and Management, 2004, 45, (9–10), 1419–1431

An average energy method for quantifying knock intensity has been developed with a shifting window period; this is called the logarithmic knock intensity (LKI). To establish the engine knock conditions, a statistical analysis has been undertaken in real-time and a 1000-cycle LKI mean has been recommended as a proper criterion for defining the knock intensity. A method for eliminating the speed dependence of the LKI has also been developed. Derived from the above method, a multi-microprocessor based Knock Detector with LCD and LED displays and a D/A output has been developed.

Prediction of knock limited operating conditions of a natural gas engine

Computer models of engine processes are valuable tools for predicting and analyzing engine performance and allow exploration of many engine design alternatives in an inexpensive fashion. In the present work, a zero-dimensional, two zone thermodynamic model was used to determine the knock limited operating conditions of a natural gas engine. Experimentally based burning rate models were used for flame initiation and propagation calculations. A knock model was incorporated with the zero-dimensional model. Comparison of the measured and calculated cylinder pressure data indicated that the model is able to match the measured cylinder pressure data with less than 8% error in magnitudes if the computations are started at the experimental spark timing. The knock predictions agreed with the measurements also. With the established knock model, it is possible not only to investigate whether knock is observed with changing operating and design parameters, but also to evaluate their effects on the maximum possible knock intensity.