Knock Model Research Articles

The interactions between IC engine thermodynamics and knock

The development of high efficiency spark-ignition internal combustion engines is often constrained by the occurrence of knock. Knock may result in engine damage, lower performance, and lower efficiency. The options for preventing knock often involve lower compression ratios, lower boost, retarded spark timing, and other design choices that are detrimental to engine performance and efficiency. Since knock is largely a function of the thermodynamic state of the unburned zone, the occurrence of knock is expected to be a strong function of the engine thermodynamics. The purpose of the current work is to couple a simple knock model with a comprehensive engine cycle simulation to determine the interactions between the engine thermodynamics and knock. This work has explored the effects of engine parameters such as compression ratio (4–12), engine speed (500–2500rpm), inlet pressure (50–100kPa), exhaust gas recirculation (0–25%), combustion duration and heat transfer on knock. In each case, the occurrence of knock is connected to the cylinder pressures and the gas temperatures of the unburned zone. For example for a compression ratio of 12, to avoid knock the brake thermal efficiency decreased from 36.5% to 34% due to retarding the combustion.

Development of a RANS-Based Knock Model to Infer the Knock Probability in a Research Spark-Ignition Engine

Development of a RANS-Based Knock Model to Infer the Knock Probability in a Research Spark-Ignition Engine

A RANS knock model to predict the statistical occurrence of engine knock

In the recent past engine knock emerged as one of the main limiting aspects for the achievement of higher efficiency targets in modern spark-ignition (SI) engines. To attain these requirements, engine operating points must be moved as close as possible to the onset of abnormal combustions, although the turbulent nature of flow field and SI combustion leads to possibly ample fluctuations between consecutive engine cycles. This forces engine designers to distance the target condition from its theoretical optimum in order to prevent abnormal combustion, which can potentially damage engine components because of few individual heavy-knocking cycles.A statistically based RANS knock model is presented in this study, whose aim is the prediction not only of the ensemble average knock occurrence, poorly meaningful in such a stochastic event, but also of a knock probability. The model is based on look-up tables of autoignition times from detailed chemistry, coupled with transport equations for the variance of mixture fraction and enthalpy. The transported perturbations around the ensemble average value are based on variable gradients and on a local turbulent time scale. A multi-variate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported; this results in the prediction of an earliest knock probability preceding the ensemble average knock onset, as confirmed by the experimental evidence. The proposed model estimates not only the regions where the average knock is promoted, but also where and when the first knock is more likely to be encountered.The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine with optical access is presented and the analysis of the knock statistical occurrence obtained by the proposed model adds an innovative contribution to overcome the limitation of consolidated “average knock” analyses typical of a RANS approach.

Large-Eddy Simulation of Cycle-resolved Knock in a Turbocharged SI Engine

The paper presents a numerical study of cycle-to-cycle variability in a turbocharged GDI engine. The Large-Eddy Simulation technique is adopted in this study in conjunction with the recent ISSIM-LES model for spark-ignition, allowing a dedicated treatment of both the flame kernel formation and flame development phases.Numerical results are compared with an extended dataset of experimental test-bed acquisitions, where the engine is operated at knock-limited spark advance.The agreement of both ensemble averaged combustion pressure history and of its standard deviation confirm the validity of the adopted numerical framework able to correctly quantify the degree of CCV measured by the experiments.Knock tendency is evaluated by means of an in-house developed knock model, based on a tabulation technique for AI delays of the same RON98 gasoline as the one used in experiments. The results confirm the knock-free condition of the experimental KLSA, for which the cycle-resolved knock signature is extremely weak just for the cycles in the highest band of the CCV-affected combustion. The visualization of the pressure wave allows to identify the exhaust side as the most knock-prone region.Finally, spark-advance is increased by 3 CA with respect to the experimental edge-of knock limit, in order to simulate an experimentally prevented operating condition. Local pressure measurements mimicking flush-mounted transducers confirm the severe knock damage related to this condition. The predictive capability of the combustion CCV and of the adopted knock model confirm the heavy and recurrent cycle-resolved knock damage.

Two-Stage Turbocharging for the Downsizing of SI V-Engines

One of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking.This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range.It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intake-exhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered.The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool.The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine.

Fuel Efficiency Optimization for a Divided Exhaust Period Regulated Two-Stage Downsized Spark Ignition Engine

In our previous paper, a new gas exchange concept termed divided exhaust period regulated two-stage (DEP R2S) system has been proposed. In this system, two exhaust valves in each cylinder are separately functioned with one valve feeding the exhaust mass flow into the high-pressure (HP) manifold, while the other valve evacuating the remaining mass flow directly into the low-pressure (LP) manifold. By adjusting the timing of the exhaust valves, the target boost can be controllable while improving the engine's pumping work and scavenging is attainable which results in better fuel efficiency from the gas exchange perspective. This paper will continue this study by adding an appropriate knock model to examine the benefits this concept could bring to the combustion phasing. The results at full load showed that under knock limited spark advance (KLSA) and fully optimized exhaust valve timing condition, the DEP R2S system benefited from lower pumping loss and better scavenging due to the reduced backpressure and improved pulsation interference despite suffering from reduced expansion ratio and expansion work. The combustion phasing was advanced across the engine speed which is mainly attributed to the reduced residual and the reduced requirement of gross indicated mean effective pressure (IMEP). The net brake-specific fuel consumption (BSFC) was observed to improve by up to 3% depending on the engine operating points. At part load, the DEP R2S system could be used as a mechanism to extend the “duration” of the exhaust valve. This will reduce the recompression effect of the exhaust residuals during the beginning and the end of the exhaust stroke compared to the original R2S model with late exhaust valve opening and early exhaust valve opening. In addition, increased internal exhaust gas recirculation (EGR) due to the increased overlap between the LP and the intake valve is also beneficial for the improved pumping mean effective pressure (PMEP) as the throttle can be further opened to reduce the corresponding throttling loss. The average net BSFC improvement is expected to be approximately 6–7%.

Knock and Cycle by Cycle Analysis of a High Performance V12 Spark Ignition Engine. Part 2: 1D Combustion and Knock Modeling

Knock and Cycle by Cycle Analysis of a High Performance V12 Spark Ignition Engine. Part 2: 1D Combustion and Knock Modeling

Quasi-dimensional analysis of combustion, emissions and knocking in a homogeneous GDI engine

A quasi-dimensional model is developed with the surrogate mechanism of isooctane and n-heptane to predict knock and emissions of a homogeneous GDI engine. It is composed of unburned and burned zone with the latter divided into multiple zones of equal mass to resolve temperature stratification. Combustion is based on turbulent entrainment and burning in a spherically propagating flame with the entrainment rate interpolated between laminar and turbulent flame speed. Validation is performed against measured pressure traces, NOx and CO emissions at different load and rpm conditions. Comparison is made between predictions by the empirical knock model and the chemistry model in this work. There is good agreement for pressure, NOx, CO and knock for the test engine. Promising results are obtained through parametric study with respect to octane number and engine load by the chemistry knock model.

A quasi-dimensional model for SI engines fueled with gasoline–alcohol blends: Knock modeling

As knock is one of the main factors limiting the efficiency of spark-ignition engines, the introduction of alcohol blends could help to mitigate knock concerns due to the elevated knock resistance of these blends. A model that can accurately predict their autoignition behavior would be of great value to engine designers. The current work aims to develop such a model for alcohol–gasoline blends. First, a mixing rule for the autoignition delay time of alcohol–gasoline blends is proposed. Subsequently, this mixing rule is used together with an autoignition delay time correlation of gasoline and an autoignition delay time correlation of methanol in a knock integral model that is implemented in a two-zone engine code. The predictive performance of the resulting model is validated through comparison against experimental measurements on a CFR engine for a range of gasoline–methanol blends.The knock limited spark advance, the knock intensity, the knock onset crank angle and the value of the knock integral at the experimental knock onset have been simulated and compared to the experimental values derived from in-cylinder pressure measurements.

Formulation of a Knock Model for Ethanol and Iso-Octane under Specific Consideration of the Thermal Boundary Layer within the End-Gas

Formulation of a Knock Model for Ethanol and Iso-Octane under Specific Consideration of the Thermal Boundary Layer within the End-Gas

A predictive model for knock onset in spark-ignition engines with cooled EGR

A predictive knock model is crucial for one dimensional (1-D) engine cycle simulation that has been proven to be a powerful tool in both optimization of the conceptual design and reduction of calibration efforts in development of spark-ignition (SI) engines. With application of advanced technologies such as exhaust gas recirculation (EGR) in modern SI engines, update of knock model is needed to give an acceptable prediction of knock onset. In this study, bench tests of a turbocharged gasoline SI engine with cooled EGR system operated under knocking conditions were conducted, the cylinder pressure traces were analyzed by the band-pass filtering technique, and the crank angle of knock onset was determined by the signal energy ratio (SER) and image processing method. A knock model considering multi-variable effects including pressure, temperature, EGR ratio and excess air ratio (λ) is formulated and calibrated with the experimental data using the multi-island genetic algorithm (GA). The calculation method of the end gas temperature, the impacts of the ratio of specific heats as well as the temperature at the intake valve closure on the end gas temperature are discussed. The performance of the new model is compared with the widely-used phenomenological knock models such as Douaud–Eyzat model and Hoepke model. While the widely-used knock models fail to give acceptable predictions when increasing EGR with fuel enrichment operations, the new model predicts the knock onset with high accuracy under the fuel enrichment conditions both with and without EGR. Performance of the newly developed model is also examined in a different engine and a higher predictive accuracy of the new model than other models is obtained.

Probability density function approach coupled with detailed chemical kinetics for the prediction of knock in turbocharged direct injection spark ignition engines

In this work a new knock model is derived which accounts for the inherent feature of knocking combustion, namely that it is a stochastic phenomenon. It provides the probability of autoignition and distinct criteria to determine the mean knock onset as well as the relative number of knocking cycles. For modeling purposes an ignition progress variable is proposed to determine the reactive state of the unburnt fuel–air mixture and the occurrence of autoignition. Statistical information of this quantity is introduced by presuming a clipped Gaussian probability density function (PDF). Its shape is defined by the Favre mean and variance of the ignition progress variable for which transport equations are derived. The chemical source terms that appear in these equations are closed by employing a presumed PDF approach to account for turbulence chemistry interaction. A clipped Gaussian PDF distribution for temperature and a β-PDF for mixture fraction are employed. Hence, the impact of temperature and mixture fraction fluctuations on the ignition progress variable is accounted for. The chemical source terms are evaluated based on tabulated chemistry incorporating detailed chemical kinetics. For the assessment of the knock model a spark timing sweep was performed on the engine test bench for a full-load operating point at n=2000rpm. In-cylinder flow simulations including gas exchange, mixture formation, combustion, and knock were carried out and the results are compared with experimental data. It is shown that the knock model is able to predict the mean knock onset with reasonable accuracy and that the impact of a spark timing sweep on the number of knocking cycles is well captured.

균일혼합기 가솔린 직분사 엔진의 다중 영역 유사차원 해석을 통한 배기 및 노킹 예측

A quasidimensional program is developed for a four stroke cycle homogeneous GDI (Gasoline Direct Injection) engine. It includes models for spray, burning rate and chemistry to predict knock and emissions. With early injection a homogeneous GDI engine goes through spark ignited, turbulent premixed combustion as in PFI (Port Fuel Injection) engines. The cylinder charge is divided into unburned and burned zone with the latter divided into multiple zones of equal mass to resolve temperature stratification. Validation is performed against measured pressure traces, NOx and CO emissions at different load and RPM conditions. Comparison is made between an empirical knock model and predictions by the chemistry model in this work.

Heat Transfer, Knock Modeling and Cyclic Variability in a “Downsized” Spark-Ignition Turbocharged Engine

AbstractIn the present paper a combined procedure for the quasi-dimensional modelling of heat transfer, combustion and knock phenomena in a “downsized” Spark Ignition two-cylinder turbocharged engine is presented. The procedure is extended to also include the effects consequent the Cyclic Variability. Heat transfer is modelled by means of a Finite Elements model. Combustion simulation is based on a fractal description of the flame front area. Cyclic Variability (CV) is characterized through the introduction of a random variation on a number of parameters controlling the rate of heat release (air/fuel ratio, initial flame kernel duration and radius, laminar flame speed, turbulence intensity). The intensity of the random variation is specified in order to realize a Coefficient Of Variation (COV) of the Indicated Mean Effective Pressure (IMEP) similar to the one measured during an experimental campaign. Moreover, the relative importance of the various concurring effects is established on the overall COV. A kinetic scheme is then solved within the unburned gas zone, characterized by different thermodynamic conditions occurring cycle-by-cycle. In this way, an optimal choice of the “knock-limited” spark advance is effected and compared with experimental data. Finally, the CV effects on the occurrence of individual knocking cycles are assessed and discussed.

Cycle-by-Cycle Analysis, Knock Modeling and Spark-Advance Setting of a “Downsized” Spark-Ignition Turbocharged Engine

Cycle-by-Cycle Analysis, Knock Modeling and Spark-Advance Setting of a “Downsized” Spark-Ignition Turbocharged Engine

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.

Prediction of Autoignition Site in Spark Ignition Engines

A three-dimensional knock model is developed to predict the knock occurrence and autoignition site in spark ignition engines. Weller's 1-equation combustion model and a flame wall quenching model a re applied to simulate flame propagation. A Reduced kinetic model developed by Keck and Hu is used to simulate autoignition in the unburned gas region. Engine experiments are performed to calibrate the reduced kinetic model and to verify the knock model. A spark plug integrated pressure transducer and 11 ion probes are installed in the cylinder head to detect knock occurrences, flame arrival angles, and autoignition sites. Autoignition site can be determined from the knock onset angle determined by cylinder pressure and flame arrival time determined by ion probes. Before the reduced kinetic model is combined with the combustion model, it was calibrated with measured cylinder pressure in a spark ignition engine. The cylinder pressure and flame arrival angles of numerical results are compared with the experiments. The numerical results are in good agreement with the experiments. The knock model developed in this study can predict knock occurrence and autoignition sites with reasonable accuracy.

Hydrocarbon ignition: Automatic generation of reaction mechanisms and applications to modeling of engine knock

A computational technique is described which automatically develops detailed chemical kinetic reaction mechanisms for large aliphatic hydrocarbon fuel molecules. This formulation uses the LISP language to apply general rules which identify the chemical species produced, the reactions between these species, and the elementary reaction rates for each reaction step. Reaction mechanisms for cetane ( n -hexadecane) and most alkane fuels C 7 and smaller are developed using this automatic technique, and detailed sensitivity analyses for n -heptane and cetane are described. These reaction mechanisms are then applied to calculation of knock tendencies in internal combustion engines. The model is used to study the influence of fuel molecule size and structure on knock tendency, to examine knocking properties of fuel mixtures, and to determine the mechanisms by which pro-knock and anti-knock additives change knock properties.

Acetaldehyde oxidation in the negative temperature coefficient regime: Experimental and modeling results

Acetaldehyde oxidation has been studied in experiments at temperatures of 553 and 713 K carried out in a low pressure, static reactor and in numerical modeling calculations using a detailed chemical kinetic reaction mechanism. The results of the experimental study were used to construct and validate the reaction mechanism, which was then used to examine acetaldehydeoxidation in the negative temperature coefficient regime between 550 and 900 K. This mechanism was also tested against independent measurements of acetaldehyde oxidation carried out by Baldwin, Matchan, and Walker. The overall rate of reaction and the properties of the negative temperature coefficient regime were found to be sensitive to the competition between radical decomposition reactions and the addition of molecular oxygen to acetyl and methyl radicals, including particularly During these experiments, an upper limit to the rate of decomposition ofCH3O2H was measured at 553 K. Implications of the results for future kinetic modeling of engine knock are discussed.