Multi-mode Combustion Research Articles

Knock characteristics and combustion regime diagrams of multiple combustion modes based on experimental investigations

Spark assisted compression ignition (SACI) is a kind of combustion mode that bridges the gap between homogeneous charge compression ignition (HCCI) under low load and spark ignition (SI) under high load. In a multi-mode combustion engine, different combustion modes are used under different engine loads and the transition between combustion modes is necessary. To realize transitions among the three combustion modes, this work studies combustion and knock characteristics of HCCI, SACI and SI, and proposes region diagrams that divide the operation regions of different combustion modes. The results show that knocking combustion of HCCI and SI is totally different, and SACI knock combines the features of both HCCI knock and SI knock. Further analysis indicates that compression ignition (CI) of SACI relies on both initial thermodynamic state and flame propagation. And the more auto-ignition is dependent on flame propagation, the more uncontrollable CI process becomes. Region diagrams show that HCCI works with high i-EGR ratios, and an HCCI knock region exists with moderate i-EGR ratios. Transition from normal SACI to HCCI knock can be completed even in two cycles. Therefore, air-fuel mixture and i-EGR should be carefully organized to avoid this region. In the region diagram of SACI-SI, there is an instable transition region between SACI and SI, which is caused by instability of flame propagation due to such amount of EGR and the reliability of CI process on flame propagation. Fuel stratification is helpful to suppress knock, advance combustion phase by earlier spark timing, and improve combustion stability. But it cannot eliminate the unstable region. Twin spark adds another flame kernel into combustion chamber and greatly improves combustion stability. With this strategy, stable transition between SACI and SI modes can be achieved.

Thermodynamic efficiency assessment of gasoline spark ignition and compression ignition operating strategies using a new multi-mode combustion model for engine system simulations

Advanced combustion strategies for gasoline engines employing highly dilute and low-temperature combustion modes, such as homogeneous charge compression ignition and spark-assisted compression ignition, promise significant improvements in efficiency and emissions. This article presents a novel, reduced-order, physics-based model to capture advanced multi-mode combustion involving spark ignition, homogeneous charge compression ignition and spark-assisted compression ignition operating strategies. The purpose of such a model, which until now was unavailable, was to enhance existing capabilities of engine system simulations and facilitate large-scale parametric studies related to these advanced combustion modes. The model assumes two distinct thermodynamic zones divided by an infinitely thin flame interface, where turbulent flame propagation is captured using a new zero-dimensional formulation of the coherent flame model, and end-gas auto-ignition is simulated using a hybrid approach employing chemical kinetics and a semi-empirical burn rate model. The integrated model was calibrated using three distinct experimental data sets for spark ignition, homogeneous charge compression ignition and spark-assisted compression ignition combustion. The results demonstrated overall good trend-wise agreement with the experimental data, including the ability to replicate heat release characteristics related to flame propagation and auto-ignition during spark-assisted compression ignition combustion. The calibrated model was assessed using a large parametric study, where the predicted homogeneous charge compression ignition and spark-assisted compression ignition operating regions at naturally aspirated conditions were representative of those determined during engine testing. Practical advanced combustion strategies were assessed relative to idealized engine simulations, which showed that efficiency improvements up to 30% compared with conventional spark-ignition operation are possible. The study revealed that poor combustion efficiency and pumping work are the primary mechanisms for efficiency losses for the advanced combustion strategies evaluated.

Multi-mode combustion process monitoring on a pulverised fuel combustion test facility based on flame imaging and random weight network techniques

Combustion systems need to be operated under a range of different conditions to meet fluctuating energy demands. Reliable monitoring of the combustion process is crucial for combustion control and optimisation under such variable conditions. In this paper, a monitoring method for variable combustion conditions is proposed by combining digital imaging, PCA-RWN (Principal Component Analysis and Random Weight Network) techniques. Based on flame images acquired using a digital imaging system, the mean intensity values of RGB (Red, Green, and Blue) image components and texture descriptors computed based on the grey-level co-occurrence matrix are used as the colour and texture features of flame images. These features are treated as the input variables of the proposed PCA-RWN model for multi-mode process monitoring. In the proposed model, the PCA is used to extract the principal component features of input vectors. By establishing the RWN model for an appropriate principal component subspace, the computing load of recognising combustion operation conditions is significantly reduced. In addition, Hotelling’s T2 and SPE (Squared Prediction Error) statistics of the corresponding operation conditions are calculated to identify the abnormalities of the combustion. The proposed approach is evaluated using flame image datasets obtained on the PACT 250kW Air/Oxy-fuel Combustion Test Facility (PACT 250kW Air/Oxy-fuel CTF). Variable operation conditions were achieved by changing the primary air and SA/TA (Secondary Air to Territory Air) splits. The results demonstrate that, for the operation conditions examined, the condition recognition success rate of the proposed PCA-RWN model is over 91%, which outperforms other machine learning classifiers with a reduced training time. The results also show that the abnormal conditions exhibit different oscillation frequencies from the normal conditions, and the T2 and SPE statistics are capable of detecting such abnormalities.

Experimental study of glow plug assisted compression ignition

The purpose of this study is to investigate the effects of glow plug assist on a four-cylinder compression ignition (CI) engine fueled with dieseline. Pressure sensor glow plugs (PSG) are utilized to assist low temperature combustion (LTC) at low to medium load range, and the glow plugs are required for fast response and accurate output control. Therefore, a glow plug control unit (GPCU) is developed, and a closed-loop power feedback control algorithm is used. Equipped with PSGs and GPCU, the engine is tested at three speeds under varying loads. As a part of multi-mode combustion for CI engines, glow plug assisted combustion (GA-CI) appears earlier combustion phases and higher peak in-cylinder pressure. GA-CI can effectively reduce cycle-to-cycle variations and misfire is avoided, especially at low load conditions. The nature of the glow plug assisting process is to enhance the in-cylinder temperature and fuel reactivity. Plus, glow plugs also play a role in triggering the auto-ignition of the pre-mixture. With glow plug assist, NOx emissions rise slightly but are still below 0.2g/kWh, whereas particulate matter (PM) emissions drop sharply and decrease to under 0.02g/kWh. Since CO emissions and HC emissions decrease as well, the combustion efficiency is significantly enhanced, with the maximum being over 98%.

Effects of High-harmonic Components on the Rayleigh Indices in Multi-mode Thermo-acoustic Combustion Instability

This paper presents the characteristics of non-fundamental multi-mode combustion instability and the effects of high-harmonic components on the Rayleigh criterion. Phenomenological observations of multi-harmonic-mode dynamic pressure waves regarding the intensity of harmonic components and the source of wave distortion have been explained by introducing examples of second- and third-order harmonics at various amplitudes. The amplitude and order of the harmonic components distorted the wave shapes, including the peak and the amplitude, of the dynamic pressure and heat release, and consequently the temporal Rayleigh index and its integrals. A cause-and-effect analysis was used to identify the root causes of the phase delay and the amplification of the Rayleigh index. From this analysis, the skewness of the dynamic pressure turned out to be a major source in determining whether multi-mode instability is driving or damping, as well as in optimizing the combustor design, such as the mixing length and the combustor length, to avoid unstable regions. The results can be used to minimize errors in predicting combustion instability in cases of high multi-mode combustion instability. In the future, the amount of research and the number of applications will increase because new fuels, such as fast-burning syngases, are prone to generating multi-mode instabilities.

Multimode combustion in a mild hybrid electric vehicle. Part 2: Three-way catalyst considerations

This is the second of a two-part study that discusses multimode combustion in a mild hybrid electric vehicle. Homogeneous charge compression ignition (HCCI) combustion oxidizes the oxygen storage capacity (OSC) of the three-way catalyst (TWC), thereby removing the TWC's ability to convert NOx under lean conditions. Despite prolonged operation in HCCI mode, enabled by the electric motor, the depletion of the OSC causes significant penalties in fuel economy and the amounts of tailpipe NOx are substantial. Counter-intuitively, it is seen that decreasing the sizes of both HCCI regime and OSC results in reduced tailpipe NOx while maintaining fuel economy benefits.

Multimode combustion in a mild hybrid electric vehicle. Part 1: Supervisory control

This is the first of a two-part simulation study that discusses the application of a multimode combustion engine in a mild hybrid electric vehicle (HEV). The torque assist, offered by the electric motor, can be used to extend the residence time in the homogeneous charge compression ignition (HCCI) regime, before returning to spark-ignition (SI) combustion. To enable multimode operation in the HEV, the supervisory control strategy has to maintain the battery's state-of-charge while accounting for the SI/HCCI combustion mode switch. In this study four supervisors are discussed which extend the baseline equivalent consumption minimization strategy by the mode switching decision.

Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part I—Frequency and mode analysis

This paper presents the results of combustion instability characteristics of H2/CO/CH4 synthetic gases and synthetic natural gases by testing a scale-downed industrial GE7EA gas turbine that features a partially-premixed swirl-stabilized combustion. H2/CO/CH4 syngases ranging from 0% to 100% and synthetic natural gases containing H2 from 0% to 30% were tested and analyzed in view of H2, which has distinctive combustion characteristics of fast burning and high flame temperature. Interestingly, as features of the partially-premixed flames that were adapted to inhibit flashback from fast-burning H2-containing syngas, high multi-mode combustion instabilities such as 2nd, 3rd, 4th, and over were observed without fundamental longitudinal mode. To investigate these high multi-mode combustion instabilities, combustion properties such as the laminar burning velocity, adiabatic flame temperature and ignition delay time were calculated for the all tested fuels and their dependence within them and dependence on instability frequencies were closely analyzed. Test results demonstrated that H2 contents are the root cause of the high-order instability mode and shifting within modes, since fast burning H2 affects the chemical time and flame temperature of syngas more than that of CO and CH4. The relationships between the combustion properties have also illustrated that the sensitive change of combustion properties are responsible for generating flame oscillations of high multi-mode and sensible frequency and mode shifting. Therefore, the laminar burning velocity, adiabatic flame temperature and ignition delay time should be considered as important parameters when designing a gas turbine combustor; moreover, deciding operating conditions for new fuels and careful management of fuel quality, particularly in view of H2, is required when operating integrated gasification combined cycle plants and synthetic natural gas plants.

Mild HEV with Multimode Combustion: Benefits of a Small Oxygen Storage

This simulation study discusses the application of a multimode combustion engine in a mild hybrid electric vehicle (HEV) with three-way catalytic converter (TWC). Operation in the lean combustion mode homogeneous charge compression ignition (HCCI) results in oxidation of the oxygen storage capacity (OSC) of the TWC. Thereby, the TWC’s ability to convert NOx under lean conditions is removed. Succeeding depletion of the OSC under rich spark-ignition (SI) conditions is required, which results in significant fuel efficiency penalties. In case of a mild HEV the torque assist from the electric motor is able to extend the residence time in HCCI, thereby reducing the number OSC depletion events. The applied supervisory controller, which decides when to switch between SI and HCCI, is based on the equivalent consumption minimization strategy (ECMS) and incorporates the fuel penalties associated with mode switching and OSC depletion. It is shown that, while the impact of the OSC depletion on drive cycle fuel economy of the mild HEV is still significant, it is much smaller than in case of the vehicle without electric motor. The prolonged operation in lean HCCI mode leads to substantial amounts of tailpipe NOx for all drive cycles tested. In a case study two modifications to the system hardware are introduced with counterintuitive results. First, the HCCI regime is further constrained to conditions where engine-out NOx levels are extremely low. Second, the size of the OSC is significantly reduced, allowing a much faster and less inefficient depletion. Associated drive cycle results show a substantial reduction in tailpipe NOx while fuel economy benefits can be maintained.

Load expansion of a dieseline compression ignition engine with multi-mode combustion

This paper presents a study on the upper load expansion and combustion characteristics of a multi-mode combustion engine with dieseline. The partially premixed compression ignition (PPCI) mode with early direct injection and late-injection low temperature combustion (L-LTC) mode with near-top dead center (TDC) direct injection are studied and compared at various loads with sweeps of injection parameters and exhaust gas recirculation (EGR) rates. A single injection strategy and a double injection strategy with small pilot ratios are tested. For both combustion modes, the criteria are set to identify the load limit of clean combustion. The NOx limit is 200ppm, and the particulate matter number limit is 100×106/cm3. The maximum pressure rise rate is lower than 0.8MPa/°, and the maximum cylinder pressure is below 16MPa. In addition, the coefficient of variation for the indicated mean effective pressure (IMEP) should be less than 3% combustion stability. The results indicate that L-LTC has a higher load limit than PPCI for all the test fuels; therefore, the upper load limit of PPCI can be increased using L-LTC. However, the thermal efficiency of L-LTC is lower than that of PPCI. Therefore, a multi-mode combustion strategy is proposed to be the best option for the whole load range. For the small to medium load range, PPCI can achieve a low emissions level and with a higher thermal efficiency than L-LTC; therefore, PPCI is an ideal choice for this range. For the medium to high load range, the L-LTC mode is the only suitable mode due to its low emissions. In the high load range, a simultaneous reduction in NOx and PM is not possible for both PPCI and L-LTC; therefore, the conventional CI mode has to be used to ensure high fuel economy performance.

Combustion instability characteristics of H2/CO/CH4 syngases and synthetic natural gases in a partially-premixed gas turbine combustor: Part II—Time lag analysis

In continuation of part I, which reported the characteristics of high-frequency multi-mode combustion instability for various compositions of syngases, this study analyzes the instability mechanism using the characteristic length and time scales relating the particular combustion instability of high multi-modes. Here, time lag analysis based on the Rayleigh criterion has been used and the times of convection, acoustic, ignition delay, chemical reaction, and newly developed skewness are considered. The skewness time, which reflects the wave distortion resulting from the addition of a higher harmonic component of combustion instability, is obtained from the analytic derivation of the interval between single-mode and multi-mode waves. The proportion of skewness time in the total lagging time was varied from zero for complete single mode to 11% for high harmonic multi-mode according to the fuel variation. The effectiveness of skewness time on the time lag analysis was verified from the experimental results of 98 cases and it showed remarkable improvement in the prediction accuracy of multi-mode instability from 88.8% to 99.0%. From the analysis results, it can be concluded that skewness time should be considered as an important parameter when calculating characteristic time scales for high multi-mode, and this method can be usefully applied for the development of an optimal design for and safe operation of a syngas-fired gas turbine combustor.

Lean HCCI/Rich SACI Gasoline Combustion Cycling and Three-Way Catalyst for Fuel Efficiency and NOx Reduction

Multimode combustion (MMC) concepts using homogeneous charge compression ignition (HCCI) gasoline combustion at low loads and spark assisted compression ignition (SACI) gasoline combustion at medium loads have the potential for improved fuel efficiency relative to spark ignition (SI) gasoline combustion. Two MMC concepts are compared in this paper with respect to fuel efficiency and tailpipe NOx emissions. The first concept uses stoichiometric HCCI and SACI to allow standard three-way catalyst (TWC) operation. The second concept also uses HCCI and SACI, but cycles between lean and rich combustion and uses a TWC with increased oxygen storage capacity (OSC) for potentially even greater fuel efficiency improvement. This paper performs a preliminary comparison of the two MMC concepts by analyzing two scenarios: (1) cycling between stoichiometric HCCI at 2 bar BMEP (brake mean effective pressure) and stoichiometric SACI at 3 bar BMEP, and (2) cycling between lean HCCI at 2 bar BMEP and rich SACI at 3 bar BMEP. The effects of excess oxygen ratio during HCCI operation and the frequency of oxygen depletion events on TWC performance and fuel efficiency are investigated. Results show that MMC lean/rich cycling can achieve better fuel efficiency than stoichiometric HCCI/SACI cycling. NOx emissions are moderately higher, but may still be low enough to meet current and future emission regulations.

Accounting for combustion mode switch dynamics and fuel penalties in drive cycle fuel economy

A methodology is introduced to analyze the drive cycle fuel economy of a vehicle equipped with a multimode combustion engine, utilizing commercial cam phasers and two-step cam profile switching. The analysis is based on a longitudinal vehicle model with manual transmission. The engine model employs a finite state machine describing mode switches between two distinct combustion modes, namely, spark-ignited and homogeneous charge compression ignition. Preliminary combustion mode switch experiments were used to parameterize the model. The influence of mode switch fuel penalties on drive cycle fuel economy was quantified through application of the model to the federal test procedure (FTP-75), the highway fuel economy test (HWFET) and the US06 supplemental federal test procedure. The mode switches were analyzed individually in terms of their benefit on fuel economy and a distinction was made between harmful and beneficial mode switches. Mode switches were defined as harmful whether their fuel penalty is greater than the benefits originating from spending time in the subsequent homogeneous charge compression ignition mode. A parametric study was conducted to investigate the impact of harmful mode switches on fuel economy as a function of the fuel penalties during the switch. In the case of high fuel penalties, supervisory control becomes an important tool for minimizing the number of harmful mode switches. One possible supervisory strategy discussed is a smoothing strategy, in which a mode switch is delayed by introducing a dwell time.

Controlled Load and Speed Transitions in a Multicylinder Recompression HCCI Engine

A model-based control strategy to track combustion phasing during load and speed transitions in the homogeneous charge compression ignition (HCCI) operating region of a multimode combustion engine is presented in this paper. HCCI transitions can traverse regions of high cyclic variability (CV), even if the steady state transition end points are stable with low CV. A control-oriented HCCI model for both the stable, low CV region and the oscillatory, high CV, late phasing region is used to design a controller that uses valve and fuel injection timings to track combustion phasing. Novel aspects of the controller include nonlinear model-inversion-based feedforward and gain scheduled feedback based on unburned fuel that reduces transient CV. Transitions tested on a multicylinder HCCI engine include load transitions at fixed engine speeds, simultaneous load, and speed transitions, and select FTP75 drive-cycle transitions with high load slew rates. Good combustion phasing tracking performance is demonstrated, and misfires are prevented.

On Beneficial Mode Switch Decisions based on Short-term Engine Load Prediction

Multimode combustion in automotive engines have the potential to increase fuel efficiency by choosing the best combustion mode at each operating condition. When switches among the combustion modes are neither instantaneous nor lossless, a mode switch can only be beneficial if the associated residence time in the target mode is long enough to compensate for the fuel penalty of the switch. An accurate engine load prediction over a short-term horizon is thus necessary for deciding on a beneficial mode switch. This paper addresses this. In the application of spark ignited (SI) and homogeneous charge compression ignition (HCCI) combustion modes, prior work quantified fuel efficiency benefits and mode switch penalties. In this paper, it is estimated that, to result in a fuel economy benefit, the residence time in HCCI mode is required to be longer than 1.2 s. If it is possible to accurately predict engine load over such a time horizon, an optimal mode switch decision could be made. To this end, five receding horizon prediction methods, based on artificial neural networks and polynomial extrapolation, are tested with vehicle measurement data and compared in terms of their accuracy in predicting engine load as well as HCCI entry and exit events. It is shown that although the prediction accuracy is very low for visitations of the HCCI regime around 1.2 s, all the methods are able to determine if an immediate entry of exit will occur, and if a HCCI visitation is going to be very short. This finding is applied in a basic mode switch decision structure.

Fuel Economy of a Multimode Combustion Engine With Three-Way Catalytic Converter

Highly diluted, low temperature homogeneous charge compression ignition (HCCI) combustion leads to ultralow levels of engine-out NOx emissions. A standard drive cycle, however, would require switches between HCCI and spark-ignited (SI) combustion modes. In this paper we quantify the efficiency benefits of such a multimode combustion engine, when emission constraints are to be met with a three-way catalytic converter (TWC). The TWC needs unoccupied oxygen storage sites in order to achieve acceptable performance. The lean exhaust gas during HCCI operation, however, fills the oxygen storage and leads to a drop in NOx conversion efficiency. If levels of tailpipe NOx become unacceptable, a mode switch to a fuel rich combustion mode is necessary in order to deplete the oxygen storage and restore TWC efficiency. The resulting lean-rich cycling leads to a penalty in fuel economy. Another form of penalty originates from the lower combustion efficiency during a combustion mode switch itself. In order to evaluate the impact on fuel economy of those penalties, a finite state model for combustion mode switches is combined with a longitudinal vehicle model and a phenomenological TWC model, focused on oxygen storage. The aftertreatment model is calibrated using combustion mode switch experiments from lean HCCI to rich spark-assisted HCCI (SA-HCCI) and back. Fuel and emission maps acquired in steady-state experiments are used. Different depletion strategies are compared in terms of their influence on drive cycle fuel economy and NOx emissions. It is shown that even an aggressive lean-rich cycling strategy will marginally satisfy the cumulated tailpipe NOx emission standards under warmed-up conditions. More notably, the cycling leads to substantial fuel penalties that negate most of HCCI's efficiency benefits.

A computational study of supersonic combustion in strut injector and hypermixer flow fields

Achieving sufficiently high combustion efficiency and stability in supersonic combustion is extremely challenging and highly dependent on the fuel-injection and mixing strategies adopted. A viable approach to this is the strut injector, which by inducing flow recirculation, facilitates flame stabilization in the strut-wake. In this investigation we examine in detail the flow, mixing, self-ignition and flame stabilization mechanisms of conventional and alternating-wedge injection struts. In order to analyze these, we consider NAL’s supersonic combustor, equipped with two conventional two-stage injection struts, and an alternating-wedge injection strut, in conjunction with ONERA’s vitiation air heater. Experimental results, including spontaneous flame images, wall-pressure and Planar Laser Induced Fluorescence (PLIF) images of hydroxyl (OH) are here combined with computational results based on finite-rate chemistry Large Eddy Simulation (LES) with skeletal hydrogen-air reaction mechanisms. The spontaneous flame images and the predicted flame structures for both injector-strut types agree well qualitatively, demonstrating that combustion LES captures the overall features of the experiments. Detailed comparisons between experimental data and computational results for the wall pressure and for mean and rms OH-PLIF cross-sections show acceptable agreement, indicating that the LES results can be used to further study the intrinsic features of the flame structure and the stabilization mechanism. These results indicate significant differences in flow and flame structures between both two-stage injection struts and the alternating-wedge injection strut tested. More specifically, the longitudinal vorticity introduced by the alternating-wedge injection strut increases the combustion efficiency but results in an intermittent auto-ignition phenomenon. For the two-stage injection struts combustion consists of auto-ignition pockets surrounded by self-igniting fronts embedded in a background of non-premixed flames or stirred reactors. In contrast, the alternating-wedge injection strut vigorous combustion is observed proceeding through a multi-mode (auto-ignition, non-premixed, premixed) combustion event.

Enhanced heat release analysis for advanced multi-mode combustion engine experiments

Advanced combustion strategies, such as Homogeneous-Charge Compression-Ignition (HCCI) and Spark-Assisted HCCI or Spark-Assisted Compression-Ignition (SACI) hold considerable promise for improving engine efficiencies while maintaining low pollutant emissions, yet few models exist that accurately include the important chemical and physical mechanisms of these advanced combustion strategies. Further, experimental data from advanced combustion engine experiments are not well represented using conventional spark ignited analytical tools. This paper presents new methods for advanced combustion analysis that integrate previous analytical methods with new algorithms to capture the unique features of advanced combustion strategies like SACI.The new analytical capabilities were incorporated into a program which was designated the Advanced Combustion Engine Heat Release Analysis (ACE-HRA) tool. The models developed and applied in ACE-HRA were assessed by comparison with high fidelity engine simulations of HCCI and SACI. The high fidelity simulations provided data sets with detailed predictions of heat release rates, temperatures, auto-ignition timing, flame speeds and other key parameters not resolved or measured in engine experiments. The sensitivity of ACE-HRA estimates to model input data was quantified for important engine performance parameters. The sensitivity analysis showed that estimates for in-cylinder masses have the largest overall impact on the ACE-HRA results (e.g. ±10% variation led to changes on the order of ±5–10% in peak rate of heat release, burn duration and peak temperature). Noticeable differences in peak heat release rate and ringing intensity were also observed when comparing cycle-by-cycle analysis against ensemble-average analysis, which has implications on how the results are interpreted and applied in modeling work.After validating ACE-HRA with the high-fidelity simulations, ACE-HRA was applied to interpret the data from a recent experimental study of SACI combustion. The ACE-HRA methods were used to infer the effects of flame propagation on in-cylinder gradients and cycle-to-cycle variability, and to provide quantitative estimates for the associated changes in the end-gas burn rate. The trends observed, such as the decrease in burn rate for later auto-ignition and higher burn fraction by flame, provide more in-depth understanding of SACI combustion and demonstrate the insight that can be revealed by the new ACE-HRA tool.

Investigation into the cause of high multi-mode combustion instability of H2/CO/CH4 syngas in a partially premixed gas turbine model combustor

In this paper, the fuel composition effects of H2/CO/CH4 syngas (0–100% for each component gas) on the self-excited high multi-mode (mainly the 3rd and 4th with their harmonics) combustion instability (CI) characteristics have been studied in a partially premixed swirl-stabilized gas turbine model combustor by investigating phase-resolved high-speed OH∗ planar laser induced fluorescence (OH-PLIF) and OH∗ chemiluminescence at a rate of 12.5kHz and analyzing the proper orthogonal decomposition (POD), spatio-temporal Rayleigh index (RI), and time-lag. Phase-synchronized OH-PLIF images suggested critical clues of CI driving mechanisms, including the periodic alternation of flame attachment/detachment and vortex coupling with everlasting flames at the outer recirculation zone due to high hydrogen fuels’ high reactivity. These PLIF results also demonstrated that the relatively short mixing length and highly fuel-dependent flame length are the root causes of a high instability-mode and sensible mode-shift. Thus, PLIF images were used to calculate the flame length by obtaining the intensity-weighted centroid for the more precise time-lag analysis. The spatio-temporal RI results indicated that the fuel composition affects the location and intensity of CI driving/damping, RI frequency, and instability mode and frequency. POD analysis from high-speed OH∗ images showed that the distinct coherent structures and large roll-up of flames are responsible for generating flame oscillations for each mode. High cross-correlation between the POD modes showed the convection of these coherent structures and axially-alternative swirl-like flame motions. At some particular compositions of H2/CH4/CO, high multi-mode CI was observed (e.g., the 3rd, 4th, and 6th modes appear simultaneously) and the original time-lag model could not be applicable, since the acoustic pressure wave is not simply sinusoidal but largely distorted. Thus, a new time-lag model using skewness time (τskew) was proposed to reflect the distortion from multi-mode CI and the accuracy of this model was verified using the experimental data.

A Comparison of Valving Strategies Appropriate For Multimode Combustion Within a Downsized Boosted Automotive Engine—Part I: High Load Operation Within the Spark Ignition Combustion Regime

As future downsized boosted engines may employ multiple combustion modes, the goal of the current work is the definition of valving strategies appropriate for moderate to high load spark ignition (SI) combustion and at low to moderate loads for spark assisted compression ignition (SACI) combustion for an engine with variable valve timing capability and fixed camshaft profiles. The dilution and unburned gas temperature requirements for SACI combustion can be markedly different from those of SI; therefore it is important to ensure that a given valving strategy is appropriate for operation within both regimes. This paper compares one-dimensional (1D) thermodynamic simulations of rated engine operation with positive valve overlap (PVO) and a baseline negative valve overlap (NVO) camshaft design in a boosted automotive engine with variable valve timing capability. Several peak lifts and valve open durations are investigated to guide the down-selection of camshaft profiles for further evaluation under SACI conditions in a companion paper. While the results of this study are engine specific, rated performance predictions show that the duration of both the intake and exhaust camshafts significantly impacts the ability to achieve high load operation. While it was noted that the flow through the exhaust valves chokes for the majority of the exhaust stroke for peak exhaust lifts less than 8 mm, the aggressive engine rating of 194 kW at 5250 rpm could be achieved with peak intake lifts as low as 4 mm and baseline duration. Therefore, camshafts with peak lifts of 8/4 mm exhaust/intake were down-selected to facilitate multimode combustion operation with high levels of PVO. Analysis of high load operation with the down-selected camshafts indicates that peak unburned gas temperatures remain low enough to mitigate end-gas knock, while other variables such as peak cylinder pressure, turbine inlet temperature, and turbocharger speed are all predicted to be within acceptable limits.