Transparent Combustion Chamber Research Articles

Influence of nickel powders on burning behaviors of single-based propellant in variable-pressure and constant-pressure combustion conditions

Single-based propellants containing 0.4 wt%, 0.8 wt% and 1.6 wt% nickel powders(NPs) were prepared and investigated in closed bomb tester and transparent combustion chamber, which can provide variable-pressure and constant-pressure conditions for propellant burning, respectively. In closed bomb tester, propellants burned to a pressure above 5–220 MPa. The results showed that burning rate of propellants was increased by 19.0 %, 12.5 % and 11.9 % in 5–8 MPa, 8–16 MPa and 20–200 MPa, when 1.6 wt% NPs used in propellant. Meanwhile, propellants burning in combustion chamber were with a constant pressure of 1.0 MPa, 1.5 MPa, 2.0 MPa, 3.0 MPa and 5.0 MPa. The burning rate of propellants can be increased by NPs in 1.0–5.0 MPa. While the catalytic efficiency(Z) had high dependence on NPs contents and burning pressure. At 5.0 MPa, 0.4 wt%, 0.8 wt% and 1.6 wt% of NPs enhanced burning rate of propellants by 42 %, 44 % and 68 % than that of original propellants. In pressure below 3.0 MPa, burning rate of propellants cannot be increased with 0.4 wt% and 0.8 wt% NPs, but that can be enhanced with 1.6 wt% NPs, suggested that higher NPs content and higher pressure were conducive to catalytic effect, which was significant with the pressure above 5 MPa. Furthermore, after NPs incorporating into propellant, the distance from flame zone to propellant burning surface was significantly reduced. And the potential catalytic mechanism was proposed.

Investigating the Origins of Cyclic Variability in Internal Combustion Engines Using Wall-Resolved Large Eddy Simulations

Abstract Modern internal combustion engines (ICE) operate at the ragged edge of stable operation characterized by high cycle-to-cycle variations (CCV). A key scientific challenge for ICE is the understanding, modeling, and control of CCV in engine performance, which can contribute to partial burns, misfire, and knock. The objective of this study is to use high-fidelity numerical simulations to improve the understanding of the causes of CCV. Nek5000, a leading high-order spectral element, open source code, is used to simulate the turbulent flow in the engine combustion chamber. Multicycle, wall-resolved large-eddy simulations (LESs) are performed for the General Motors (GM), Transparent Combustion Chamber (TCC-III) optical engine under motored operating conditions. The mean and root-mean-square (rms) of the in-cylinder flow fields at various piston positions are validated using particle image velocimetry (PIV) measurements during the intake and compression strokes. The large-scale flow structures, including the swirl and tumble flow patterns, are analyzed in detail and the causes for cyclic variabilities in these flow features are explained. The energy distribution across the different scales of the flow are quantified using one-dimensional (1D) energy spectra, and the effect of the tumble breakdown process on the energy distribution is examined. The insights from this study can help us develop improved engine designs with reduced cyclic variabilities in the in-cylinder flow leading to enhanced engine performance.

Experimental and Numerical Investigation of Ultra-Wet Methane Combustion Technique for Power Generation

Abstract Using steam as a heat carrier and working media has merits to increase electric efficiency up to 60% and decrease NOx emission to single-digit compared to dry gas turbine cycles. These attribute primarily to the physical properties of steam as having high heat capacity to reduce local flame temperature, and hence reduce emissions by inhibiting the thermal NOx forward reaction rate. In this work, ultrahigh steam content with a steam-to-air mass ratio of up to 40% is premixed with methane–air mixture before entering into a swirl-stabilized high pressure (HP)-burner for combustion. A significant change of flame from V-shape (attached) to M shape (detached) is observed through a transparent combustion chamber whilst changing steam content. The measurement of chemiluminescence OH* is conducted with an intensified CCD-camera bandpass filtered at 320 nm. Following these measurements, large eddy simulation (LES) is used to capture reacting flow features. Reasonably well agreements between experimental data and numerical results are obtained for both attached and detached flames in terms of the OH* distribution. Slight inconsistency of OH* intensity is mainly due to uncollected wall temperature, which leads to either over- or underprediction of chemical reaction rate depending on the experimental flame positions. Distributed flame front is clearly identified with LES for wet methane combustion associated with 35% steam-to-air ratio corresponding to a high Karlovitz number flame. Slightly unstable combustion is observed when the steam-to-air ratio exceeds 40% featuring an onset of flame blow-off. In addition, interaction between precessing vortex core (PVC) and the flame is presented for different level of steam dilution, and conclusions are drawn regarding the flame stabilization. The in-depth understanding of the ultrawet combustion is an important step toward the use of sustainable, steam-diluted biosyngas for electricity production.

Large-Eddy simulation of lean and ultra-lean combustion using advanced ignition modelling in a transparent combustion chamber engine

The need for Internal Combustion Engines (ICEs) to face near-future challenges of higher efficiency and reduced emissions is leading to a renewed interest towards lean-combustion. Several operational issues are associated to lean combustion, such as an abrupt increase of combustion cycle-by-cycle variability (CCV), leading to unbearable levels of Indicated Mean Effective Pressure (IMEP) variation and to misfiring cycles. Significant potential in the wide-scale establishment of lean combustion might come from Large Eddy Simulation (LES), which is able to elucidate the relationships between local physical processes (e.g. velocity magnitude, Air to Fuel Ratio (AFR), etc.) and early combustion progress (e.g. 1%) in unprecedented manners. To this aim, an improved ignition model for LES is proposed in the paper. Two premixed propane-air lean strategies are selected from the wide TCC-III database. A lean-stable (λ=1.10, also named lean) and lean-unstable (λ=1.43, also named ultra-lean) conditions are simulated, highlighting the model capability to well reproduce the sudden rise in CCV for increased mixture dilution. Explanations are given for the observed behaviour and a hierarchical quest for CCV dominant factors is presented. Finally, the different role of local flow field is highlighted for the two cases, and the comparison of optical acquisitions of OH* emission against simulated flame iso-surface up to 1% burn duration reinforce the simulation fidelity. The study shows the investigation possibilities of innovative combustion strategies given by advanced LES simulations. The understanding of turbulent combustion dynamics and the knowledge of the related lean-burn instabilities are key enabler for the exploration of new efficient lean-burn operations.

Application of a zonal hybrid URANS/LES turbulence model to high and low-resolution grids for engine simulation

A zonal hybrid unsteady Reynolds-averaged Navier–Stokes/large eddy simulation (URANS-LES) Zonal detached-eddy simulation (ZDES) model is applied to internal combustion engine (ICE) simulation and comparisons of predicted flow morphology and variability are carried out against on the transparent combustion chamber (TCC-III) particle image velocimetry (PIV) data set for motored conditions. To this aim, a previously developed model derived from a standard seamless-detached eddy simulation (DES) formulation is adopted for two different grid resolutions. In particular, two zonalization choices are evaluated based on previous single-grid results, in order to assess the model outcomes based on the joint turbulence treatment/grid density: the seamless-DES mode is applied (1) only to the cylinder (TCC-Z1) and (2) to the cylinder and intake port (TCC-Z2). Multi-cycle simulations (50 samples) are carried out and the results are compared to experimental data in terms of PIV images using multiple quality indices on multiple planes ( Y = 0 and X = 0). Finally, comparison of predicted mean flow fields is extended to standard URANS mode. Results show that the use of a cylinder-only seamless-DES treatment on a relatively coarse grid results in a quantitative agreement between simulated and measured (PIV) flow fields, both in terms of average morphology and flow variability, whereas the extension of the DES mode to the intake port does not introduce relevant variations. Quality indicators seem to be moderately sensitive to the grid resolution, thus confirming the adaptive potential of a ZDES–like model and promoting the use of DES–type turbulence modelling even on relatively low-resolution grids. The analysis of average fields compared to URANS simulations highlights the benefit for both grids of a scale-resolving ZDES modelling when the same underlying turbulence model ( k-ε RNG) is used. This study reinforces the recommendation in the use of hybrid URANS-LES models to simulate ICE flows. The adopted ZDES formulation based on the two-equation k-ε RNG model shows that high-quality results can be obtained even on engineering-grade grids, both in terms of average and cycle-to-cycle variation. The numerical results obtained using the two grids with variable resolution are consistent, and this further promotes a wider adoption of this class of models to simulate engine flows in industrial applications.

Comparison of Near-Wall Flow and Heat Transfer of an Internal Combustion Engine Using Particle Image Velocimetry and Computational Fluid Dynamics

In this study, computational fluid dynamics (CFD) modeling capability of near-wall flow and heat transfer was evaluated against experimental data. Industry-standard wall models for RANS and large-eddy simulation (LES) (law of the wall) were examined against the near-wall flow and heat flux measurements from the transparent combustion chamber (TCC-III) engine. The study shows that the measured, normalized velocity profile does not follow the law of the wall. This wall model, which provides boundary conditions for the simulations, failed to predict the measured velocity profiles away from the wall. LES showed a reasonable prediction in peak heat flux and peak in-cylinder pressure to the experiment, while RANS-heat flux was closer to experimental heat flux but lower in peak pressure. The measurement resolution is higher than that of the simulations, indicating that higher spatial resolution for CFD is needed near the wall to accurately represent the flow and heat transfer. Near-wall mesh refinement was then performed in LES. The wall-normal velocity from the refined mesh case matches better with measurements compared with the wall-parallel velocity. Mesh refinement leads to a normalized velocity profile that matches with measurement in trend only. In addition, the heat flux and its peak value matches well with the experimental heat flux compared with the base mesh.

Characterization of radiative heat transfer in a spark-ignition engine through high-speed experiments and simulations

A combined experimental and Large-Eddy Simulation (LES) study of molecular radiation is presented for combustion in a homogeneous pre-mixed spark-ignition engine. Molecular radiation can account for ~10% of the engine heat loss and could have a noticeable impact on the local conditions within the combustion chamber. The Transparent Combustion Chamber (TCC) engine, a single-cylinder two-valve research engine with a transparent liner and piston for optical access, was used for this study. High-speed infrared emission spectroscopy and radiative post-processing of LES calculations have been performed to gain insight into the timescales and magnitude of radiative emissions of molecular gases during the combustion process. Both the measurements and simulations show significant Cycle-to-Cycle Variations (CCV) of radiative emission. There is agreement in the instantaneous radiative spectrum of experiment and simulation, but the crank-angle development of the radiative spectrum shows disagreement. The strengths and limitations of the optical experiments and radiative simulations are seen in the results and suggest pathways for future efforts in characterizing the influence of molecular radiation. In particular, focusing on the relative changes of the spectral features will be important as they contain information about the thermochemical properties of the gas mixture.

Correlation of CCV Between In-Cylinder Swirl Ratio and Polar Velocity Profile in Valve Seat Region Using LES Under Motored Engine Condition

An analysis of Transparent Combustion Chamber (TCC3) engine Large-Eddy Simulation (LES) result was carried out to investigate Cycle-to-Cycle Variation (CCV) correlation between in-cylinder swirl ratio and flow in the valve seat region of the intake port to address a challenging question on “What causes CCV of in-cylinder flow”. Polar Velocity (PV) profile, mean velocities normal to a ring-shaped cutting surface in the valve seat region, is calculated to depict intake port flow. A Net Polar Velocity (NPV) can be defined by performing the vector sum of the polar velocity around the intake valve. A standard deviation of PV is also calculated from azimuthal distribution of PV magnitudes relative to its mean value. The analysis of 18 LES cycles of TCC3 engine with a two-valve, pancake-shaped combustion chamber shows that similar CCV of in-cylinder swirl ratio patterns are observed at different crank angles from Intake Valve Opening (IVO) to Exhaust Valve Opening (EVO). Further analysis shows clear correlations of CCV between in-cylinder swirl ratio and NPV magnitude and the standard deviation of PV at selected crank angles from IVO to EVO. The correlations get significantly better with the ring-shaped cutting surface moves from up-stream to downstream of the valve-seat region. This study reveals that the CCV of in-cylinder swirl ratio is built up gradually from upstream to downstream in the intake port and valve-seat region. Further evaluation of the analysis method is planned for a four-valve engine as an evaluation metric for better engine intake port design and combustion chamber optimization.

Overset grids for fluid dynamics analysis of internal combustion engines

Overset grids is emerging as an increasingly important and powerful tool in fluid dynamics simulations. A model built with the overset mesh technique consists of multiple spatially superimposed grids, one for each moving element (valves and piston in case of internal combustion engines) plus one background grid (representing the fixed portions, i.e. cylinder, ports, manifolds, etc.). Boundary information is exchanged between these grids via interpolation of the flow variables.Several reasons make this technique essential for different purposes. In the field of CFD simulations of internal combustion engines, the ease of replacing moving components makes overset mesh an interesting tool in order to optimize the design of single components without generating new grids for the whole engine assembly.Valves and piston motions are generally characterized by close gaps and self-intersecting trajectories. Overset grids allow to easily control valve and piston motion preserving a high mesh quality in the cylinder, at the valve seats and thereabouts: these areas are proven to be critical for the CFD analysis because of the high spatial gradients in the velocity flow field.The major drawbacks of the overset grid technique applied to internal combustion engines are the mass conservation error and the additional computational load introduced by the interpolation process.The paper shows the application of overset grids for the analysis of in-cylinder flow and turbulence in a research optical engine called TCC-III (Transparent Combustion Chamber). The TCC engine work has been funded by General Motors through the General Motors University of Michigan Automotive Cooperative Research Laboratory, Engine Systems Division.The applicability of overset meshing technique is assessed in terms of computational efficiency (stability, scalability, robustness) and accuracy. In particular, the effects of grid topology, grid density and turbulence modelling are evaluated by means of the comparison between CFD results and high-speed particle image velocimetry (PIV) experimental data. The comparison between RANS (Reynold-averaged Navier-Stokes) CFD simulations and the experimental data show a very high level of accuracy in the representation of the most relevant turbulent structures inside the cylinder.

An Experimental and Simulation Study of Early Flame Development in a Homogeneous-charge Spark-Ignition Engine

An integrated experimental and Large-Eddy Simulation (LES) study is presented for homogeneous premixed combustion in a spark-ignition engine. The engine is a single-cylinder two-valve optical research engine with transparent liner and piston: the Transparent Combustion Chamber (TCC) engine. This is a relatively simple, open engine configuration that can be used for LES model development and validation by other research groups. Pressure-based combustion analysis, optical diagnostics and LES have been combined to generate new physical insight into the early stages of combustion. The emphasis has been on developing strategies for making quantitative comparisons between high-speed/high-resolution optical diagnostics and LES using common metrics for both the experiments and the simulations, and focusing on the important early flame development period. Results from two different LES turbulent combustion models are presented, using the same numerical methods and computational mesh. Both models yield Cycle-to-Cycle Variations (CCV) in combustion that are higher than what is observed in the experiments. The results reveal strengths and limitations of the experimental diagnostics and the LES models, and suggest directions for future diagnostic and simulation efforts. In particular, it has been observed that flame development between the times corresponding to the laminar-to-turbulent transition and 1% mass-burned fraction are especially important in establishing the subsequent combustion event for each cycle. This suggests a range of temporal and spatial scales over which future experimental and simulation efforts should focus.

Empirical investigation of spark-ignited flame-initiation cycle-to-cycle variability in a homogeneous charge reciprocating engine

This experimental study investigates the flame-initiation period variability in the spark-ignited homogeneous charge third-generation transparent combustion chamber optical engine. The engine was operated with lean, rich, and stoichiometric, propane and methane, with and without nitrogen dilution. These operating conditions were chosen to systematically change the unstretched laminar flame velocity and the Markstein number. Traditional pressure measures, apparent heat release analysis, particle image velocimetry, and OH* flame imaging were used to generate over 400 metrics for 750 cycles at each of the 34 tests at 11 operating conditions. A multivariate statistical analysis was used to identify the parameters important to the variability of the crank angle at 10% fuel mass fraction burned but could not reveal physical mechanisms or cause and effect. The analysis here revealed that the combustion-phasing cycle-to-cycle variability is established by the time of the notional laminar-to-turbulent flame transition that occurs by 1% mass burn fraction, measured here from the flame image growth. Both the Markstein number and stretched laminar flame speed were found to be important. The velocity magnitude and direction were found to correlate with fast and slow 10% fuel mass fraction burned as found in early literature. It was also revealed that the shear strength, a property of the strain rate tensor at the scales resolved here (1 mm), deserves further investigation as a possible effect on 10% fuel mass fraction burned.

Parallel methodology to capture cyclic variability in motored engines

Numerical prediction of cycle-to-cycle variability in spark ignition engines is extremely challenging for two key reasons: (1) high-fidelity methods such as large eddy simulation are required to accurately capture the in-cylinder turbulent flow field and (2) cycle-to-cycle variability is experienced over long time scales, and hence, the simulations need to be performed for hundreds of consecutive cycles. In this study, a methodology is proposed to dissociate this long time-scale problem into several shorter time-scale problems, which can considerably reduce the computational time without sacrificing the fidelity of the simulations. The strategy is to perform multiple parallel simulations, each of which encompasses two to three cycles, by effectively perturbing the simulation parameters such as the initial and boundary conditions. The proposed methodology is validated for the prediction of cycle-to-cycle variability due to gas exchange in a motored transparent combustion chamber engine by comparing with particle image velocimetry measurements. It is shown that by perturbing the initial velocity field effectively based on the intensity of the in-cylinder turbulence, the mean and variance of the in-cylinder flow field are captured reasonably well. Adding perturbations in the initial pressure field and the boundary pressure improves the predictions. It is shown that this new approach is able to give accurate predictions of the flow field statistics in considerably less time than that required for the conventional approach of simulating consecutive engine cycles.

Effect of Valve Opening/Closing Setup on Computational Fluid Dynamics Prediction of Engine Flows

In computational fluid dynamics (CFD) simulations of internal combustion engines, one of the critical modeling parameters is the valve setup. A standard workaround is to keep the valve opens at a certain clearance (minimum valve lift), while imposing a solid boundary to mimic valve closure. This method would yield a step change in valve lift during opening and closing event, and different valve event timing than hardware. Two parametric studies were performed to examine (a) the effect of the minimum valve lift and (b) the effect of grid resolution at the minimum valve lift on predicted in-cylinder flow fields in Reynolds-averaged Navier–Stokes (RANS) simulations. The simulation results were compared with the state-of-the-art particle image velocimetry (PIV) measurement from a two-valve transparent combustion chamber (TCC-3) engine. The comparisons revealed that the accuracy of flow simulation is sensitive to the choice of minimum valve lift and grid resolution in the valve seat region. In particular, the predicted in-cylinder flow field during the intake process was found to be very sensitive to the valve setup. A best practice CFD valve setup strategy is proposed as a result of these parametric studies. The proposed CFD valve setup was applied to large eddy simulation (LES) of TCC-3 engine and preliminary results showed noticeable improvement already. Further evaluation of the valve setup strategy for LES simulations is ongoing and will be reported in a separate report.

Identification of Large-Scale Structure Fluctuations in IC Engines using POD-Based Conditional Averaging

Cycle-to-Cycle Variations (CCV) in IC engines is a well-known phenomenon and the definition and quantification is well-established for global quantities such as the mean pressure. On the other hand, the definition of CCV for local quantities, e.g. the velocity or the mixture distribution, is less straightforward. This paper proposes a new method to identify and calculate cyclic variations of the flow field in IC engines emphasizing the different contributions from large-scale energetic (coherent) structures, identified by a combination of Proper Orthogonal Decomposition (POD) and conditional averaging, and small-scale fluctuations. Suitable subsets required for the conditional averaging are derived from combinations of the the POD coefficients of the second and third mode. Within each subset, the velocity is averaged and these averages are compared to the ensemble-averaged velocity field, which is based on all cycles. The resulting difference of the subset-average and the global-average is identified as a cyclic fluctuation of the coherent structures. Then, within each subset, remaining fluctuations are obtained from the difference between the instantaneous fields and the corresponding subset average. The proposed methodology is tested for two data sets obtained from scale resolving engine simulations. For the first test case, the numerical database consists of 208 independent samples of a simplified engine geometry. For the second case, 120 cycles for the well-established Transparent Combustion Chamber (TCC) benchmark engine are considered. For both applications, the suitability of the method to identify the two contributions to CCV is discussed and the results are directly linked to the observed flow field structures.

RANS and Large Eddy Simulation of Internal Combustion Engine Flows—A Comparative Study

A comparative cold flow analysis between Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) cycle-averaged velocity and turbulence predictions is carried out for a single cylinder engine with a transparent combustion chamber (TCC) under motored conditions using high-speed particle image velocimetry (PIV) measurements as the reference data. Simulations are done using a commercial computationally fluid dynamics (CFD) code CONVERGE with the implementation of standard k-ε and RNG k-ε turbulent models for RANS and a one-equation eddy viscosity model for LES. The following aspects are analyzed in this study: The effects of computational domain geometry (with or without intake and exhaust plenums) on mean flow and turbulence predictions for both LES and RANS simulations. And comparison of LES versus RANS simulations in terms of their capability to predict mean flow and turbulence. Both RANS and LES full and partial geometry simulations are able to capture the overall mean flow trends qualitatively; but the intake jet structure, velocity magnitudes, turbulence magnitudes, and its distribution are more accurately predicted by LES full geometry simulations. The guideline therefore for CFD engineers is that RANS partial geometry simulations (computationally least expensive) with a RNG k-ε turbulent model and one cycle or more are good enough for capturing overall qualitative flow trends for the engineering applications. However, if one is interested in getting reasonably accurate estimates of velocity magnitudes, flow structures, turbulence magnitudes, and its distribution, they must resort to LES simulations. Furthermore, to get the most accurate turbulence distributions, one must consider running LES full geometry simulations.

Large Eddy Simulation (LES) for IC Engine Flows

Numerical computations are carried out using an engineering-level Large Eddy Simulation (LES) model that is provided by a commercial CFD code CONVERGE. The analytical framework and experimental setup consist of a single cylinder engine with Transparent Combustion Chamber (TCC) under motored conditions. A rigorous working procedure for comparing and analyzing the results from simulation and high speed Particle Image Velocimetry (PIV) experiments is documented in this work. The following aspects of LES are analyzed using this procedure: number of cycles required for convergence with adequate accuracy; effect of mesh size, time step, sub-grid-scale (SGS) turbulence models and boundary condition treatments; application of the proper orthogonal decomposition (POD) technique.

Evaluation of a new computational fluid dynamics model for internal combustion engines using hydrogen under motoring conditions

The present work conducts a preliminary evaluation of a new CFD (computational fluid dynamics) model, which is under development at the authors' laboratory. Using this model, it is feasible to understand how the intake manifold and in-cylinder geometry affect the in-cylinder flow field and the mixing processes taking place in an Otto (spark-ignition) engine. The model is applied on a high-swirl, two-valve, four-stroke, transparent combustion chamber engine running under motoring conditions. To investigate the fuel–air mixing process, hydrogen is injected in the intake manifold. To evaluate the model three case studies are examined. First, the model is applied to simulate the external mixing in the intake manifold with a tee-mixer injection system. Secondly, the transient gas flow field in the intake manifold and engine cylinder is examined over the complete engine cycle. Finally, the transient mixing process in the intake manifold and the spatial and temporal distribution of species concentrations inside the cylinder are numerically computed using the developed model. To validate the model, the results obtained through the test cases examined are compared either with available experimental data or with simulated results, which are obtained using a commercially available CFD code applied under the same conditions.

Optical investigations of fuel deposition burning in ported fuel injection (PFI) spark-ignition (SI) engine

Abstract The present paper discusses the results obtained by experimental investigations of the diffusion-controlled flame near the valves due to fuel film deposition in a single cylinder, ported fuel injection (PFI), four-stroke spark-ignition (SI) engine with a four-valve production head. The engine was equipped with a transparent combustion chamber with a wide quartz window in the bottom. Optical techniques, based on 2D-digital imaging and natural emission spectroscopy, were used to follow the flame propagation and the chemical species involved in the fuel deposition burning near the valves. Two-colour pyrometry was applied to evaluate the flame temperature and soot concentration. Finally, exhaust particle size distributions were measured and correlated with the fuel deposition burning. The effects of different operating conditions in terms of speed, spark timings, and injection phasing were tested. Moreover, the effect of multi-component fuel was investigated by the comparison between commercial gasoline and pure iso-octane.

THE INTERACTION OF AN ANNULAR AIR JET WITH A METHANOL SPRAY FLAME IN A CYLINDRICAL COMBUSTION CHAMBER

An experimental investigation into the interaction of an annular air jet with a burning methanol fuel spray confined in a cylindrical, transparent combustion chamber has been undertaken. Gas-phase ...

Evaluation of Knocking Combustion by an Ion Current System and Optical Diagnostics of Radical Species(S.I. Engines, Combustion Diagnostics)

In spark ignition engines, knock can be defined as self-ignition of a certain portion of the unbumed gas beyond the flame front. This abnormal combustion releases a chemical energy that excites within the cylinder volume pressure oscillations that are responsible of engine noise and sometimes damage. Therefore, knock is undesirable process that has to be avoided. The present paper aims to evaluate the knocking combustion by an ion current system and optical diagnostics of radical species. The ion signal was also compared with the in-cylinder combustion pressure and the engine vibration. The optical measurements were based on 2D UV-Visible digital imaging of combustion process, UV chemiluminescence and spectrally resolved emissions from radical species. The optical techniques were applied for their potential to provide accurate measurements over a wide range. In particular, in this paper the spatial and temporal evolution of flame front and radical species such as OH and HCO were correlated to pressure and ion-current sensing during knocking combustion. The engine used during experiments was an optically accessible single cylinder, ported fuel injection, four-stroke spark-ignition engine with a four-valve production head. All the measurements were realized in the transparent combustion chamber equipped with a wide quartz window (diameter=57 mm) in the bottom of the chamber. The engine operated at 1000 rpm with stoichiometric mixture and wide-open throttle.