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Experimental Investigation of Cyclic Variability on Combustion and Emissions of a High-Speed SI Engine
Abstract
Cyclic combustion variability (CCV) is an undesirable characteristic of spark ignition (SI) engines, and originates from variations in gas motion and turbulence, as well as from differences in mixture composition and homogeneity in each cycle. In this work, the cycle to cycle variability on combustion and emissions is experimentally investigated on a high-speed, port fuel injected, spark ignition engine. Fast response analyzers were placed at the exhaust manifold, directly downstream of the exhaust valve of one cylinder, for the determination of the cycle-resolved carbon monoxide (CO) and nitric oxide (NO) emissions. A piezoelectric transducer, integrated in the spark-plug, was also used for cylinder pressure measurement. The impact of engine operating parameters, namely engine speed, load, equivalence ratio and ignition timing on combustion and emissions variability, was evaluated. The variations in mixture stoichiometry were found to have a strong effect on engine combustion variability. Rich cyclic mixture compositions exhibit lower coefficient of variation (COV) for the indicated mean effective pressure (IMEP) and NO emissions (COVNO) compared with lean mixtures. The mean value of CO emission was found to be mainly affected by stoichiometry while COVCO is affected by lambda fluctuations. At higher engine loads, maximum cylinder pressure and IMEP are increased, while COVIMEP decreased. Furthermore, ignition timing was found to strongly affect combustion and NO emissions, as it is related with early flame kernel development and thereby with flame propagation. Maximum braking torque (MBT) operation exhibits maximum IMEP and minimum COVIMEP. Compared to MBT operating conditions, advanced ignition timing leads to higher maximum cylinder pressure, higher NO and COVNO, while retarded ignition timings lead to lower maximum cylinder pressure, lower NO concentration and higher NO variability (COVNO).
... Some of the main factors on which it depends include mixture composition, oxygen availability, burning rate, the peak combustion temperature, and residence period in a critical temperature window. All these factors are known to vary in every cycle leading to cyclic variations in engine-out emissions [31,123,124]. Milkins et al. [103] indicated a definite relation of HC and CO emissions with the combustion rate, which varies due to cycle-to-cycle variations in a SI engine. Thus, a reduction in CCV could result in a decrease in CO and HC emissions. ...
... Analyzers with a fast response of an order of few milliseconds are required for cycle resolved measurement of emissions level. The chemiluminescence detector (CLD) was used for measurements of NO emissions, and the Non-Dispersive Infra-Red (NDIR) detectors were used for measurement of CO and carbon dioxide (CO 2 ) emissions [31,123]. The nature of cyclic variations in NO, CO, and CO 2 emissions is investigated for a highspeed PFI SI engine [123]. ...
... The chemiluminescence detector (CLD) was used for measurements of NO emissions, and the Non-Dispersive Infra-Red (NDIR) detectors were used for measurement of CO and carbon dioxide (CO 2 ) emissions [31,123]. The nature of cyclic variations in NO, CO, and CO 2 emissions is investigated for a highspeed PFI SI engine [123]. An attempt is made to relate cyclic emission variations with cyclic variations in IMEP and heat release [31]. ...
... Pollutants formation during engine combustion is correlated both with mixture properties such as air-fuel equivalence ratio, residual gas fraction and fuel properties as well as with the combustion type. At normal combustion conditions (deflagration), NOx formation is primarily controlled by oxygen availability under lean or rich conditions while within the stoichiometric window formed NOx concentration is correlated with the combustion burn rate [24]. Under no knocking combustion, previous studies showed that as in-cylinder peak pressure is increased, NOx emissions are proportionally increased [24e26]. ...
... A similar procedure was followed to detect HC concentration at each combustion cycle, taking into account the variations in signal delay, due to the different positioning of the sampling probes for NO and HC species. More details on the exact process per pollutant can be found in open literature [24,26,51], [52]. ...
... Fig. 9 represents the relationship between maximum cylinder pressure and NO emissions for 100 consecutive cycles under normal combustion conditions (ignition timing 5 BTDC). The trend of these points is found close to linear, validating similar observations from previous studies [24,46,53]. In fact, deviations from this linear correlation can be related with the cyclic dispersion of mixture parameters such as residual gas fraction, fuel trapped mass or airfuel equivalence ratio at each combustion cycle. ...
The aim of this experimental study is to investigate the pollutants formation and cyclic emission variability under knocking combustion conditions. A great number of studies extensively describe the phenomenon of knock and its combustion characteristics as well as the effect of knock on engine performance; however the impact of knocking combustion on pollutants formation and how it affects cyclic emission variability has not been previously explored. In this study, an optical single cylinder SI research engine and fast response analyzers were employed to experimentally correlate knocking combustion characteristics with cyclic resolved emissions from cycle to cycle. High-speed natural light photography imaging and simultaneous in-cylinder pressure measurements were obtained from the optical research engine to interpret emissions formation under knocking combustion. The test protocol included the investigation of the effect of various engine parameters such as ignition timing and mixture air/fuel ratio on knocking combustion and pollutant formation. Results showed that at stoichiometric conditions by advancing spark timing from MBT to knock intensity equal to 6 bar, instantaneous NO and HC emissions are increased by up to 60% compared to the MBT operating conditions. A further increase of knock intensity at the limits of pre-ignition region was found to significantly drop NO emissions. Conversely, it was found that when knocking combustion occurs at lean conditions, NO emissions are enhanced as knock intensity is increased.
... It is known from mean cycle studies that CO emissions are mainly controlled by the overall mixture equivalence ratio [20]. However the impact of air/fuel ratio on the actual scatter of CO levels has been investigated in the literature only from the stochastic point of view [21]. On the other hand, NO formation is linked to oxygen availability, burn rate, and maximum cylinder pressure [5,22e24]. ...
... Cyclic variations of combustion and pressure development [25], cyclic fluctuations of mixture air/fuel ratio [26], and heterogeneities of cylinder charge [27] are the main sources of NO variation. One study [25] showed that NO formation exhibits a non-linear correlation with indicated mean effective pressure (IMEP), while a more recent study investigated the impact of various engine parameters such as equivalence, load, and ignition timing on NO CEV [21]. However, a detailed experimental study that explains the origins of NO variability into deterministic, which can be controlled, and stochastic, which cannot be controlled, has not yet been conducted. ...
... The code initially transforms the recorded signals to physical magnitudes and then matches incylinder pressure with pollutants traces (NO, NO x , CO, CO 2 ) for each individual engine cycle. Details of the algorithm developed have been presented in an earlier study [21]. The uncertainty of the calculated parameters presented in the results section, namely the cumulative heat release and the IMEP, has been determined applying the RMS (root mean square) method, taking into consideration the individual error of the measured parameters, and it is estimated within 1% of the calculated value. ...
This study contributes to the understanding of cycle-to-cycle emissions variability (CEV) in premixed spark-ignition combustion engines. A number of experimental investigations of cycle-to-cycle combustion variability (CCV) exist in published literature; however only a handful of studies deal with CEV. This study experimentally investigates the impact of CCV on CEV of NO and CO, utilizing experimental results from a high-speed spark-ignition engine. Both CEV and CCV are shown to comprise a deterministic and a stochastic component. Results show that at maximum break torque (MBT) operation, the indicated mean effective pressure (IMEP) maximizes and its coefficient of variation (COVIMEP) minimizes, leading to minimum variation of NO. NO variability and hence mean NO levels can be reduced by more than 50% and 30%, respectively, at advanced ignition timing, by controlling the deterministic CCV using cycle resolved combustion control. The deterministic component of CEV increases at lean combustion (lambda = 1.12) and this overall increases NO variability. CEV was also found to decrease with engine load. At steady speed, increasing throttle position from 20% to 80%, decreased COVIMEP, COVNO and COVCO by 59%, 46%, and 6% respectively. Highly resolved engine control, by means of cycle-to-cycle combustion control, appears as key to limit the deterministic feature of cyclic variability and by that to overall reduce emission levels.
... Variation in NO formation is primarily dependent on excess oxygen availability, fuel burn rate, peak combustion chamber temperature, and peak cylinder pressure [22][23][24]. It was reported in some studies that the fluctuations in NO formation are primarily due to cyclic fluctuations, heterogeneity of air/fuel ratio, and variations in ignition timings [25][26][27]. ...
... However, in LI mode, SoC advanced to 2.12°CA bTDC at 0.66 bar and 3.42°CA aTDC at 3.96 bar BMEP. It was reported that advancing SoC could possibly mitigate the problem of cyclic variations in the engine [26]. In LI mode, 30HCNG showed the earliest SoC amongst all HCNG mixtures at MBT ST and λ = 1.2. ...
In this experimental study, a prototype laser ignited engine was developed and fuelled with hydrogen enriched compressed natural gas (HCNG; mixture of H2 and CNG). Engine performance, emissions and combustion characteristics were compared for laser ignition (LI) and spark ignition (SI) modes. The maximum brake torque (MBT) timing was employed to reduce cycle-to-cycle variations (CCV) and to improve the engine performance, combustion and emission characteristics. Composition of HCNG mixture was changed by dynamically blending H2 with CNG on a volumetric basis. MBT timing was determined for naturally aspirated engine in conventional SI mode, by varying the spark timing (ST) between 22° CA bTDC to 46° CA bTDC and by varying relative air–fuel ratio (λ) from rich-to-lean. Optimum torque for HCNG mixtures was observed at 31° CA bTDC. The 10% mass fraction burn (MFB10) duration reduced continuously with advancing ignition timing up to 31°CA bTDC, which then increased with further advanced ST. For any particular ST, MFB10 and MFB90 decreased with increasing H2 enrichment of CNG. At high λ (lean mixtures), CCV would be higher due to lower engine efficiency and increased emissions. LI mode exhibited lower coefficient of variation in indicated mean effective pressure (COVIMEP) compared to SI mode. The COVIMEP for all HCNG mixtures were within 2% up to λ = 1.2 and it increased further with increasing λ but the variation was within ± 6%. Lower CCV (< 2% of COVIMEP), reduced emissions and improved brake thermal efficiency (BTE) were observed at 31°CA bTDC MBT timing compared to other spark timings. This experimental study indicated that laser ignition is a suitable technology for deployment in HCNG engines.
... To conclude the discussion of this section, it is important to give some details on the influence of water injection on the cycle variability that has an effect on torque, thermo-dynamic efficiency as well on fuel consumption. As well known, a reduction of the cyclic variability may reduce intrinsically the knock tendency because of a lower dispersion between lower and fast cycles [25,26,27]. In addition, experimental studies have shown as the reduction in the cyclic variability contributes also to decrease engine noise. ...
... On the contrary, a decreasing trend along the spark advance was observed. Results indicate that for the considered commercial light duty SI engine, the different operating conditions give a CoV IMEP below 5% that guarantees an acceptable combustion stability and should cause a negligible impact on fuel consumption all over the engine operating plane [26]. Moreover, for each investigated engine speed, at W/G=0.30 and the most advanced spark timings, the IMEP CoV is comparable to the gasoline reference cases. ...
The potential benefits of water injection on performance and emissions were investigated on a downsized PFI twin-cylinder turbocharged spark ignition engine. Experiments were carried out at high load condition (~15.5 bar IMEP) within the engine speed range from 3500 to 4500rpm with a step of 500 rpm. For each test case the effect of the injected water quantity on combustion and exhaust emissions was investigated by sweeping from 10%w to 30%w the water to gasoline ratio. The water was injected at the same timing as the gasoline by a low pressure injection system external controlled. Tests were performed at WOT conditions exploring, for each operating condition, a spark sweep from knock-free up to knock-limited operation. Compared to the full gasoline reference case, the water injection allowed to advance extensively the spark timing without knock occurrence. The 20% water to gasoline mass fraction gave the best improvements in terms of IMEP. The reduction of combustion temperature, due to the water injection, coupled to the spark timing advance without knock, led to a stable reduction of the temperature at the turbine inlet.
... With emission legislation becoming stricter around the world [1], the development of the spark-ignition (SI) engine primarily focuses on the improvement of combustion to achieve lower emissions and better fuel efficiency. Cyclic fluctuations in the air-to-fuel equivalence ratio, residual gas fraction and turbulence, even at nominally steady state operating conditions, result to a variance of the burn rate and of engine-out emissions [2][3][4]. The variance in engine-out emissions because of cyclic variability compromises the optimization of the engine, in particular considering the NO trade-off with fuel efficiency [5][6][7][8][9][10]. ...
... Experimental data from previous research [2], conducted at the Laboratory of Applied Thermodynamics (LAT), were utilized for the validation of the proposed emission model. An in-line fourcylinder high-speed PFI SI engine from a motorcycle (HONDA CDR600RR) was employed (Table 1). ...
This study proposes a novel emissions model for the prediction of spark ignition (SI) engine emissions at homogeneous combustion conditions, using post combustion analysis and a detailed chemistry mechanism. The novel emissions model considers an unburned and a burned zone, where the latter is considered as a homogeneous reactor and is modeled using a detailed chemical kinetics mechanism. This allows detailed emission predictions at high speed practically based only on combustion pressure and temperature profiles, without the need for calibration of the model parameters. The predictability of the emissions model is compared against the extended Zeldovich mechanism for NO and a simplified two-step reaction kinetic model for CO, which both constitute the most widespread existing approaches in the literature. Under various engine load and speed conditions examined, the mean error in NO prediction was 28% for the existing models and less than 1.3% for the new model proposed. The novel emissions model was also used to predict emissions variation due to cyclic combustion variability and demonstrated mean prediction error of 6% and 3.6% for NO and CO respectively, compared to 36% (NO) and 67% (CO) for the simplified model. The results show that the emissions model proposed offers substantial improvements in the prediction of the results without significant increase in calculation time.
... One of the fundamental challenges of spark-ignition combustion is the inherent cyclic variability in the combustion process [30]. This variability in burn-rate leads to variability in end-gas conditions, causing cycles with combustion phasing advanced of the mean cycle to experience higher in-cylinder pressures and temperatures, thereby increasing their likelihood of end-gas autoignition and knock. ...
Propane has demonstrated significant potential for reductions in greenhouse gas and pollutant emissions in medium-and heavy-duty engine applications, but further improvements require accurate, compact, and scalable chemical kinetic mechanisms to design the next generation of propane fueled engines, particularly at the boosted operating conditions necessary to meet the power density demand of medium-and heavy-duty applications. In this work, six key chemical reactions were identified in a reduced mechanism with 70 species and 352 reactions through a sensitivity analysis performed at conditions typical of thermodynamic trajectories observed in a high compression ratio, long stroke engine operated on propane from throttled to boosted operating conditions. While the original mechanism was validated against rapid compression machine (RCM) data, it was found to over-predict experimental autoignition tendencies in 2-zone, 0-D SI engine simulations performed in Chemkin Pro. Subsequently, a genetic algorithm approach was used to optimize the six reaction rate parameters within established uncertainty bounds by performing RCM simulations and comparing to two independent sets of literature ignition delay times for propane, thus generating two new kinetic mechanisms. The first optimization achieved a mean absolute percent error (MPE) reduction in 2nd stage ignition delay of 61.4% in seven generations , while the second optimization utilized a newer experimental RCM dataset, and achieved MPE reduction of 56.7% in seven generations, and further marginal improvement to 57.8% reduction in 34 generations. The two mechanisms were then evaluated again in the 2-zone 0-D SI engine model in Chemkin Pro comparing typical mean and knocking cycle trajectories, and it was found that the second optimized mechanism provided better prediction of knock onset at the representative conditions evaluated in this work, particularly for higher load operating conditions.
... 31,32 The different physical and chemical properties of biofuels widely affect the combustion characteristics. 14,33 Many researches were conducted to measure the diesel engine performance running with different types of bio oils. Puhan et al. 34 and Godiganur et al. 35 studied the diesel engine performance fueled by Mahua biodiesel. ...
Castor biodiesel (CBD) was manufactured by slow pyrolysis of oil from highly yielded seeds with anhydrous sodium hydroxide catalyst. An experimental study of engine's performance, emissions and combustion characteristics using bio-diesel blended with gas oil in volumetric ratios of 0, 10, 25, 50, 75, and 100% at different loads was performed. Increase of CBD percentage in the blend led to a reduction in engine's thermal efficiency, cylinder pressure, net heat release rate, and smoke emission. The exhaust gas temperature, specific fuel consumption, unburned hydrocarbon, CO, and nitrogen oxide emissions were increased with the increase of CBD ratio. Biodiesel showed the maximum increase in specific fuel consumption by 10% and the thermal efficiency was decreased by 10.5% about pure diesel. Smoke emissions were decreased for CBD100 by 12% about gas oil. The maximum increases in NO x , CO, HC emissions, and exhaust gas temperature for CBD 100 were 22, 34, 48, and 11%, respectively related to diesel oil. The maximum reductions in cylinder pressure and net heat release rate were 5 and 13% for CBD100 about gas oil, respectively. Biodiesel percentage of 10% showed near values of performance parameters and emissions to gas oil, so, it is recommended as the optimum percentage.
... 21 The combustion characteristics have been affected by the physical and chemical properties of biofuels. 11,22 Thus, biofuels with higher hydrogen contents produced lower cycle to cycle variations and burning rates because of the higher flame speed obtained, as mentioned by Attai et al. 4 Thus, cyclic variation has two distinctive natures as deterministic, stochastic and this would simply contribute to address and classify the combustion variations. 23,24 The cyclic variability was revealed by observing the cylinder pressure development for several cycles. ...
Pyrolysis of castor oil with anhydrous sodium hydroxide as a catalyst was performed to produce Catalytic Castor pyrolytic oil (CCPO). The physical and chemical properties of the pyrolytic and gas oils were recorded according to ASTM standards. Gas oil was blended with castor pyrolytic oil at different volumetric ratios of 0%, 25%, 75%, and 100% as CCPO00, CCPO25, CCPO75, and CCPO100, respectively. Coefficient of variation (COV) of combustion parameters proved to be a profound method of assessing combustion characteristics and engine performance. COV of combustion parameters (IMEP, P max , and dP/dY max) for gas oil blends with pyrolysis oil were measured. Recorded pressure crank angle traces of 150 consecutive cycles were used for COV's determination. A single cylinder diesel engine equipped with calibrated measuring techniques was used at different engine loads. Higher volumetric blending ratios of pyrolytic oil with diesel oil increased the COV's within an acceptable range of engine operating conditions. Minor modifications might be valuable for engines fueled by pyrolysis oil blends to obtain smoother, lower noise operation, and combustion stability.
... Cyclic variability in SI engines has been extensively examined by several research groups, trying to understand its main causes and ways of resolving these [7]. To this aim, a large diversity of experiments have been conducted focusing on a variety of parameters [8], such as the measurement of the velocity field, flame propagation and specifications [9][10][11][12], and the heat losses to the electrodes of the spark plug [13]. Recently, large focus was also given on the variability of emissions, which requires the use of advanced measuring equipment with a high sampling rate to capture the highly transient effects [14]. ...
A methodology for determining the cyclic variability in spark-ignition (SI) engines has been developed recently, with the use of an in-house computational fluid dynamics (CFD) code. The simulation of a large number of engine cycles is required for the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) to converge, usually more than 50 cycles. This is valid for any CFD methodology applied for this kind of simulation activity. In order to reduce the total computational time, but without reducing the accuracy of the calculations, the methodology is expanded here by simulating just five representative cycles and calculating their main parameters of concern, such as the IMEP, peak pressure, and NO and CO emissions. A regression analysis then follows for producing fitted correlations for each parameter as a function of the key variable that affects cyclic variability as has been identified by the authors so far, namely, the relative location of the local turbulent eddy with the spark plug. The application of these fitted correlations for a large number of engine cycles then leads to a fast estimation of the key parameters. This methodology is applied here for a methane-fueled SI engine, while future activities will examine cyclic variations in SI engines when fueled with different fuels and their mixtures, such as methane/hydrogen blends, and their associated pollutant emissions.
... On the other hand, stoichiometric and lean mixtures exhibit higher NOx emission variability. As it is revealed in Fig. 11, stoichiometric and lean mixtures lead to higher COV of IMEP, translated into more intense combustion variability, which leads to higher NOx emission variability [49]. Fig. 18 shows the influence of the engine operating equivalence ratios on the indicated specific HC emissions (g/kW h) for three different values of the spark plug gaps. ...
... At increasing the air-fuel ratio (AFR), the cyclic variability becomes stronger. The influence of engine parameters, namely engine load, speed, AFR and spark advance (SA), to the cyclic combustion variability has been investigated by Ref. [6] where it has been shown that SA affects cyclic combustion variability mostly when engine geometry is determined. ...
In this paper, a spark advance self-optimization strategy is presented for lean-burn operation mode of spark-ignition (SI) engine which aims on-board combustion phase tuning to achieve high efficiency under a probability constraint of knocking events. Firstly, the effects of spark advance (SA) on combustion phase under lean-condition are analyzed in a statistical perspective based on experiments. Then, based on conclusion of the analysis, a SA control scheme, which combines extremum seeking loop with likelihood-based knock limit control loop, is proposed to optimize SA for maximal fuel economy with knock probability threshold. Finally, experimental validation results are demonstrated that are conducted on a test bench with a V6 commercial SI engine.
... Cycle-by-cycle variations (CCVs) are in fact commonly observed during the combustion process of a SI engine [19,20]. They mainly cause fluctuations in the rate of heat release, turning in oscillations in the Indicated Mean Effective Pressure (IMEP), power output, fuel consumption and exhaust emissions [21]. ...
In this paper, the results of an extensive experimental analysis regarding a twin-cylinder spark-ignition turbocharged engine are employed to build up an advanced 1D model, which includes the effects of cycle-by-cycle variations (CCVs) on the combustion process. Objective of the activity is to numerically estimate the CCV impact primarily on fuel consumption and knock behavior. To this aim, the engine is experimentally characterized in terms of average performance parameters and CCVs at high and low load operation. In particular, both a spark advance and an air-to-fuel ratio (?) sweep are actuated. Acquired pressure signals are processed to estimate the rate of heat release and the main combustion events. Moreover, the Coefficient of Variation of IMEP (CoVIMEP) and of in-cylinder peak pressure (CoVpmax) are evaluated to quantify the cyclic dispersion and identify its dependency on peak pressure position. In parallel, the experimentally tested engine is fully schematized in a 1D framework. The 1D model, developed in the GT-Power(TM) environment, makes use of user defined sub-models for the description of combustion, turbulence and knock phenomena. 1D analyses are carried out for various engine speeds, load levels, ? ratios, and spark timings, without changing any tuning constant. In a first stage, the model is validated in terms of overall engine performance parameters, and ensemble-averaged pressure traces inside the cylinder, and within the intake and exhaust ducts, as well. A more detailed comparison is also performed with reference to the average rate of heat release in different operating conditions. In a subsequent step, the effects of CCVs are introduced in the model in terms of Gaussian distributed modifications of the burning rate, predicted with reference to the ensemble-average operation. Consistently with the experimental data, applied burning rate are modified to simulate a train of pressure cycles statistically equivalent to the measured ones. The influence of the CCVs on the instantaneous peak position, air flow rate, and Indicated Specific Fuel Consumption (ISFC) is consequently investigated on a cycle-bycycle basis, and compared to the average operation. Numerical analyses show that CCVs cause a reduced ISFC penalty, which can be considered significant only in case of delayed combustions and increased CoVs. Knock limited spark advance is also identified with and without CCV, highlighting some additional fuel economy penalties
Due to the wide application of SI engines, there is an urgent need to improve their thermal efficiency. Lean combustion technology is an advanced technology that effectively improves the efficiency of ignition engines. However, the change of the combustion cycle deteriorates with the increase of the dilution of the mixture, so reducing the influence of the combustion variations during lean combustion is the key issue to increase the excess air ratio. In the lean limit combustion, the combustion stability and heat release rate can be improved by choosing the appropriate spark advance and spark discharge energy. In this paper, a self-adaptive multi-stage ignition system is designed, which can realize the control of adaptive ignition energy and SA according to the real-time operating condition. Through experiments, the effects of ignition energy and SA on the cycle-to-cycle variations are analyzed, and it is verified that the system effectively improves the lean combustion thermal efficiency and can ensure the combustion stability under lean combustion.
In order to improve the combustion stability of ethanol engine under lean-burn condition, the experiment was carried out on a spark ignition engine with ethanol intake port injection and hydrogen direct injection. To study the effects of ignition timing, excess air ratio and hydrogen addition ratio on cycle-by-cycle variations of ethanol engine, five different ignition timings, five different excess air ratios and five different hydrogen addition ratios were applied at low speed of 1500 r/min and small throttle opening of 9%. The experimental results showed that the cyclic variation of the peak in-cylinder pressure decreased with the advance of ignition timing, and the cyclic variation of the indicated mean effective pressure decreased first and then increased. When the ignition timing is fixed at 10°CA BTDC, the cyclic variation of the peak in-cylinder pressure, the peak pressure change rate and the indicated mean effective pressure decreased with the increase of hydrogen addition ratio. 15% hydrogen addition can reduce the coefficient of variation of peak in-cylinder pressure by 75%. When the hydrogen addition ratio is 10%, the ethanol engine can run steadily under the lean-burn condition with excess air ratio of 1.5.
In this work cyclic variability is examined in a methane-fueled spark-ignition engine, focusing on its influence on performance and emissions. A recent numerical approach within the ignition model of an in-house computational fluid dynamics (CFD) code has been expanded, with the aim to increase the accuracy when estimating the effect of this highly-complex phenomenon in engines. Apart from focusing solely on the flame propagation process and the related small-scale turbulence, the effect of each cycle on the next one has also been included. This is accomplished by developing a single-zone thermodynamic model applied only during the engine open cycle, in order to accelerate the calculations, while the engine closed cycle is simulated by the CFD code. A methodology for estimating the coefficient of variation (COV) of indicated mean effective pressure (IMEP) is used based on the simulation of a large number of consecutive engine cycles, until the COV of IMEP converges. The variability of the engine performance and nitric oxide (NO) and carbon monoxide (CO) emissions is evaluated, showing that this methodology of incorporating the open cycle brings a significant effect only on the emissions, mostly to NO varying up to 70% compared to the average cycle one, unlike the engine performance where a negligible difference of COV of IMEP is disclosed compared to the use of only the closed cycle. Overall the calculated emissions show a large variability, especially for NO, revealing that the additional computational effort for this kind of multi-cycle simulations is justified to providing accurate details.
In this study, the characteristics of the cyclic variations of three different low-temperature combustion modes including reactivity-controlled compression ignition (RCCI), homogeneous charge compression ignition (HCCI), and partially premixed combustion (PPC) were experimentally investigated at low-to-medium load in a compression-ignition engine. First, the impact of fuel properties on the cyclic variations of RCCI was discussed by utilizing polyoxymethylene dimethyl ethers (PODEs) (P100), diesel/PODEs blend with 30% PODEs by volume (P30), and diesel (P0) as the direct-injection high-reactivity fuel, while gasoline (G) with the research octane number of 98 as the premixed low-reactivity fuel. The results indicated that PODEs could effectively reduce the cyclic variations of RCCI compared to diesel. By varying the premixed ratio and the start of injection timing in the cylinder, it was found that the sensitivity of the cyclic variations of G/P100 RCCI to premixed ratio was lower than that of G/P0 RCCI. In G/P0 RCCI, the cyclic variations of indicated mean effective pressure with premixed ratio of 70% were significantly higher than that with premixed ratio of 90%, and the cyclic variations of CA50 (crank angle of 50% accumulative heat release) considerably reduced with earlier start of injection timing from 40 to 60°CA BTDC. Then, by comparing the cyclic variations of G/P100 RCCI, gasoline HCCI, and gasoline PPC at the same CA50 and the indicated mean effective pressure around 4.5 bar, it was found that the cyclic variations of CA50 of HCCI were much lower than that of RCCI, but they were higher than that of PPC.
In this work cyclic variability is examined in a methane-fueled spark-ignition (SI) engine. A new numerical approach is developed and integrated into the ignition model of an in-house computational fluid dynamics (CFD) code, with the aim to estimate the effect of this highly-complex phenomenon in engines. This sub-model focuses on small-scale turbulence, with its main feature concerning the prediction of the flame propagation once the relative position of the initial flamelet and the local turbulent eddy is defined. This is accomplished by using a blending of the laminar and turbulent flame speeds at the spark cell, according to the flame surface that lies outside the local eddy, smoothing the transition from the laminar to turbulent combustion regimes. A methodology for estimating the coefficient of variation (COV) of indicated mean effective pressure (IMEP) is then proposed based on the simulation of a large number of closed engine cycles. A validated CFD code including this sub-model is applied for the cyclic variability estimation of a SI engine fueled with methane, for which a complete set of measured data is available. The numerical results are then compared with the measured data, focusing on the minimum and maximum peak pressure and IMEP. The final result is the calculation of the COV of IMEP, once the COV value of the multi-cycle simulations converges according to the criteria selected. Overall, the numerical results are close to the measured data with a relative difference of COV of IMEP about 25%, showing that the developed code can be used to estimate, at least qualitatively, the main parameters of cyclic variability in engines, with the intention to introduce in a next version more mechanisms that contribute to this phenomenon. The ultimate goal is then to direct attention to the effects of different fuels on the cyclic variations and the associated pollutant emissions.
In this paper, an in-cylinder air-fuel ratio control problem for lean burn mode operation of spark ignition engine is investigated with cycle-based model considering the cycle-to-cycle coupling effects of residual gas compositions. The statistical properties of parameter variation of the cyclic transient model are calibrated based on experiments of lean burn operating modes. With the calibrated model, a model-based predictive control strategy is proposed to improve the preciseness of in-cylinder air-fuel ratio at lean-burn operation including transient operation. To analyze the combustion parameters and its stability, the cycle-based indicators, such as heat transfer, indicated mean effective pressure (IMEP), combustion efficiency, residual gas fraction, peak pressure, crank angle at 50 heat release (CA50) and NOx emission are adopted. The consecutive cycle co-relation is also addressed to analyze the stochastic behavior at lean combustion. Finally, experimental validation are performed and demonstrated on a full-scaled gasoline engine test bench to show the effectiveness of proposed lean-mode control scheme and combustion stabilities.
Cyclic variability of spark ignition engines is recognized as a scatter in the combustion parameter recordings during actual operation in steady state conditions. Combustion variability may occur due to fluctuations in both early flame kernel development and in turbulent flame propagation with an impact on fuel consumption and emissions. In this study, a detailed chemistry model for the prediction of NO formation in homogeneous engine conditions is presented. The Wiebe parameterization is used for the prediction of heat release; then the calculated thermodynamic data are fed into the chemistry model to predict NO evolution at each degree of crank angle. Experimental data obtained from literature studies were used to validate the mean NO levels calculated. Then the model was applied to predict the impact of cyclic variability on mean NO and the amplitude of its variation. The cyclic variability was simulated by introducing random perturbations, which followed a normal distribution, to the Wiebe function parameters. The results of this approach show that the model proposed better predicts mean NO formation than earlier methods. Also, it shows that to the non linear formation rate of NO with temperature, cycle-to-cycle variation leads to higher mean NO emission levels than what one would predict without taking cyclic variation into account.
The control of transient emissions from turbocharged diesel engines remains an important objective to manufacturers, since newly produced engines must meet the stringent criteria concerning exhaust emissions levels as dictated by the legislated Transient Cycles. In the present work, experimental tests are conducted on a medium-duty, turbocharged and after-cooled diesel engine in order to investigate the behavior and formation mechanism of nitric oxide (NO), smoke and combustion noise emissions under various transient operating schedules including acceleration, load change and starting. To this aim, a fully instrumented test bed was set up in order to record and research key engine and turbocharger variables during the transient events. The main parameters measured are nitric oxide concentration and smoke opacity (both using ultra-fast response analyzers) as well as combustion noise. Various other variables were monitored, such as in-cylinder pressure, engine speed, fuel pump rack position, boost pressure and turbocharger speed. The main focus of the experimental investigation was devoted to engine acceleration tests representative of automotive and truck applications, commencing from various engine speeds and loads. The experimental test pattern also included load increases and (cold and hot) startings. Analytical diagrams are provided to explain the behavior of exhaust emissions development in conjunction with turbocharger and governor/fuel pump response. Turbocharger lag was found to be the main cause for the emissions peak values observed during all transient events. During starting, the lack of air and its mismatch with fueling caused excessive black smoke, identified by the extremely high values of exhaust gas opacity.
The most important thermodynamic property used in heat release calculations for engines is the specific heat ratio. The functions proposed in the literature for the specific heat ratio are temperature dependent and apply at or near stoichiometric air–fuel ratios. However, the specific heat ratio is also influenced by the gas composition in the engine cylinder and especially becomes important for lean combustion engines.In this study, temperature and air–fuel ratio dependent specific heat ratio functions were derived to minimize the error by using an equilibrium combustion model for burned and unburned mixtures separately. After the error analysis between the equilibrium combustion model and the derived functions is presented, the results of the global specific heat ratio function, as varying with mass fraction burned, were compared with the proposed functions in the literature. The results of the study showed that the derived functions are more feasible at lean operating conditions of a spark ignition engine.
We propose a simple, physically oriented model that explains important characteristics of cyclic combustion variations in spark-ignited engines. A key model feature is the interaction between stochastic, small-scale fluctuations in engine parameters and nonlinear deterministic coupling between successive engine cycles. Prior-cycle effects are produced by residual cylinder gas which alters volume-average in-cylinder equivalence ratio and subsequent combustion efficiency. The model's simplicity allows rapid simulation of thousands of engine cycles, permitting in-depth statistical studies of cyclic variation patterns. Additional mechanisms for stochastic and prior-cycle effects can be added to evaluate their impact on overall engine performance. We find good agreement with our experimental data.
Simultaneous measurements of early flame speed and local measurements of the major parameters controlling the process are presented. The early flame growth rate was captured with heat release analysis of the cylinder pressure. The local concentration of fuel or residual gas were measured with laser induced fluorescence (LIF) on isooctane/3-pentanone or water. Local velocity measurements were performed with laser doppler velocimetry (LDV).
The results show a significant cycle to cycle correlation between early flame growth rate and several parameters. The experiments were arranged to suppress all but one important factor at a time. When the engine was run without fuel or residual gas fluctuations, the cycle to cycle variations of turbulence were able to explain 50 % of the flame growth rate fluctuations. With a significantly increased fluctuation of F/A, obtained with port fuelling, 65% of the growth rate fluctuation could be explained with local F/A measurements. With a homogeneous fuel/air-mixture but with a high concentration of residual, a correlation could be obtained between local residual concentration and combustion.
An experimental study has been performed in order to evaluate the relative contribution of several relevant parameters on the cyclic variability in spark ignition engines. The cyclic variability has been examined via five major different pressure-related identifier, i.e. P{sub max}, {theta}{sub Pmax}, IMEP, (dp/d{theta}){sub max} and {theta}(dp/d{theta}){sub max}. Due to their high sensitivity to small fluctuation in the operating conditions, the two latter were found to be problematic in the characterization of the cyclic variability. The identifier (dp/d{theta}) max was found to be highly correlated to P{sub max}. MBT ignition resulted in minimal cyclic variability. Variations in the ignition timing were found, however, to be less important under early ignition condition than under retarded ignition condition. The standard deviation of the spark jitter was found to be around 1.7 degrees and therefore cannot be ignored. Noticeable cyclic variations were observed also under motoring conditions which indicated that the role of the valves and rings leakage cannot be neglected. The spark energy and spark duration were found to be less important than suggested in the literature. On the contrary, the spark plug kind and its orientation were found to be very significant in determining the cyclic variability. This supports the ideamore » that random fluctuations in the flow field due to the turbulence of the flow in the cylinder are important. These spatial fluctuations, that are also time-dependent, contribute to the imperfect mixing of the cylinder content, partial stratification, random convection of the spark kernel away from the electrodes, random heat transfer from the burning kernel to the spark electrodes, etc.« less
In this paper a combustion diagnostic method is presented where measured pressure data is used to calculate the heat release, local temperatures and concentrations of NO and other species. This is done by a multizone model where the lambda value, i.e. 1/equivalence ratio, in each zone can be chosen arbitrarily. In homogenous charge engines lambda is given by the global air/fuel ratio. The local lambdas during initial combustion in stratified charge and diesel engines have to be estimated either as an average value or with a chosen distribution. One new zone of each local lambda is generated and the temperature, volume and species in all old zones are updated at each time step of calculation. In this paper the model is demonstrated by using pressure data from premixed and direct injected stratified charge natural gas SI engines and from a DI diesel engine. The pre-mixed data is used to validate the model as such while the ambition in the stratified charge and diesel cases has been to find the average local lambda that gives the same NOx emission as measured. The emphasis in the latter cases has been to study the influence on average local lambda of the duration of the fuel injection. Early injected fuel seems to burn at slightly leaner mixtures than later injected. (Less)
Cyclic variability in spark ignition engine combustion, especially at high dilution through lean burn or high EGR rates, places limits on in-cylinder NOx reduction and thermal efficiency. Flame wrinkling, resulting from interactions with turbulence, is a potential source of cyclic variations in turbulence. Previous studies have shown that flame kernels are subject to significant distortions when they are smaller than the integral length scale of turbulence. With the assumption that flame development is not subject to noticeable variations, after it reaches the integral length scale, the authors have shown that turbulent-burning-caused combustion variability can be successfully modeled as a function of laminar flame speed and turbulence intensity. This paper explores the contributions of flame wrinkling to flame kernel growth variation. As the kernel growth problem is complex, this study only explores one of the many aspects of the problem. The complete description of the phenomenon requires consideration of additional effects, which are discussed here. However it is shown that geometric variations of the flame kernel can potentially constitute a significant portion of variability. These variations can be considered to be present at all operating conditions at almost constant magnitude. Although the problem is non-linear, studying individual phenomena can provide useful information in understanding variability.
For spark-ignition gasoline engines operating under the wide speed and load conditions required for light duty vehicles, ignition quality limits the ability to minimize fuel consumption and NOx emissions via dilution under light and part load conditions. In addition, during transients including tip-outs, high levels of dilution can occur for multiple combustion events before either the external exhaust gas can be adjusted and cleared from the intake or cam phasing can be adjusted for correct internal dilution. Further improvement and a thorough understanding of the impact of the ignition system on combustion near the dilution limit will enable reduced fuel consumption and robust transient operation. To determine and isolate the effects of multiple parameters, a variable output ignition system (VOIS) was developed and tested on a 3.5L turbocharged V6 homogeneous charge direct-injection gasoline engine with two spark plug gaps and three ignition settings. External exhaust gas recirculation (EGR) was used to explore combustion near dilution limits with various ignition system configurations. Tests were conducted under two engine operating conditions, a road load of 2.6 bar BMEP and engine speed of 1500 rev/min and a light load of 1.5 bar BMEP and engine speed of 1000 rev/min. At these two conditions, ignition tests were performed with various external EGR rates. The results show that the spark plug gap size is the dominant factor for the two engine conditions, and that the smaller gap size plug has a lower EGR tolerance than the larger gap size. Additionally the results show that increasing total duration via continuous or discontinuous discharges improves combustion stability and EGR tolerance. Finally ignition system requirements are summarized for dilute low load combustion in the context of the overall requirements for an ignition system in DI boosted engines.
A computer simulation of the four-stroke spark-ignition engine cycle has been developed for studies of the effects of variations in engine design and operating parameters on engine performance, efficiency, and NO emissions. Predictions made with the simulation have been compared with data from a single-cylinder CFR engine over a range of equivalence ratios, spark-timings and compression ratios. Predicted fuel consumption and NO emissions showed acceptable quantitative agreement with the data. A parametric study of the effect of variations in load, speed, combustion duration, timing, equivalence ratio, exhaust gas recycle and compression ratio was then carried out for a 5. 7 l displacement engine. Pairs of parameters were selected for analysis, and variations in brake specific fuel consumption, brake specific NO emissions and mean exhaust temperature were determined.
Experimental data was obtained from a Rover K4 optical access engine and analyzed with a combustion analysis package. Cyclic NO values were calculated by mass averaging the measurements obtained by a fast NO analyzer. While the mass averaged results were used as the basis of comparison for the model, results indicate that mass averaging a fast NO signal is not nearly as critical as mass averaging a fast FID signal. A computer simulation (ISIS - Integrated Spark Ignition engine Simulation) was used to model the NO formation on a cyclic basis by means of the extended Zeldovich equations. The model achieves its cyclic variability through the input of experimentally derived burn rates and a completeness of combustion parameter, which is based on the Rassweiler and Withrow method of calculating mass fraction burned and is derived from the pressure-crank angle record of the engine. The cycle-by-cycle NO modeling results are found to compare well with the experiments in terms of the mean value for all cycles, the general slope of the NO vs. Pmax line, and the maximum and minimum values (or spread). The modeled results do not, however, exhibit the same degree of scatter as the experimental data. This is attributed to the model utilizing a constant temperature at inlet valve closure and constant residual mass fraction for all cycles.
Lean combustion in spark-ignition engines has long been recognised as a means of reducing both exhaust emissions and fuel consumption. However, problems associated with cycle-by-cycle variations in flame initiation and development limit the range of lean-burn operation. An experimental investigation was undertaken in order to quantify the effects of spark energy released and initial flame kernel growth on the cyclic variability of IMEP and crank angle at which 5% mass fraction was burned in a Honda VTEC-E, stratified-charge, pentroof-type, single-cylinder, optically accessed, spark-ignition engine. Simultaneous CCD images of the flame at the spark plug were acquired from two orthogonal views (one through the piston crown and one through the pentroof) on a cycle-by-cycle basis during the first 40 crank angle degrees after ignition timing, for isooctane port injection at an air to fuel ratio of 22, engine speed of 1500 RPM, 30% volumetric efficiency and 40° crank angle spark advance. The stage of the first 40° crank angle after ignition timing corresponded to the average time of 5% mass fraction burned. Image acquisition was synchronised with in-cylinder pressure, spark voltage and spark current sampling, for each of the following four spark-plug orientations: upstream, downstream and two crossflow positions of the ground electrode relative to the mean ensemble-averaged velocity vector at the spark-plug location at ignition timing. The measurements showed large cycle-by-cycle variations in flame size, shape and location. The flame size (projected enflamed areas through the two views and estimated enflamed volume) at 40° crank angle after ignition timing was found to correlate with coefficients as high as -0.96 with the crank angle of 5% mass fraction burned and 0.85 with IMEP. This degree of correlation between the flame size and the crank angle of 5% mass fraction burned or IMEP, reduced as the crank angle after ignition timing decreased, but correlations were still as large as -0.64 between the flame size at 10° crank angle after ignition timing and the crank angle of 5% mass fraction burned. The crossflow orientations produced the lowest levels of cyclic variability in IMEP, the upstream one the highest and the downstream orientation performed in between. The spark duration was found to vary from 12° to 22° crank angle on a cycle-by-cycle basis, being 19° crank angle on average, and this in conjunction with the behaviour of the flame-size correlations with the crank angle of 5% mass fraction burned suggests that the "quality" of a cycle is determined within the short time of the spark discharge. High spark energies were generally associated with short durations of the spark event and always faster than average initial flame kernel growth cycles. The correlation coefficient between spark energy and flame volume was as high as 0.76 at 10° crank angle after ignition timing, decreasing to 0.59 at 20° crank angle after ignition timing. The displacement of the flame luminous centre was not found to produce very strong correlation coefficients with the crank angle of 5% mass fraction burned (<0.55), but the correlations with the spark energy and, mainly, duration were quite strong (0.5-0.7), especially for the later stage of 20° crank angle after ignition timing rather than the earlier one of 10° crank angle after ignition timing. Although mostly associated with low spark energies, long spark durations could also produce fast cycles presumably because they provided an ignition window large enough to mask the effects of in-cylinder variations. For cycles with a given spark energy, the ones with the longest duration always produced higher than average initial flame kernel growth cycles.
A study has been conducted to measure the transient HC, NOx, CO, CO2 and particulate emissions from a modern 1.6-liter, Euro IV-stage turbocharged Gasoline Direct Injection (GDI) passenger car engine. The tests were conducted using ultra-fast-response analyzers with millisecond response times so that the real-time effects of the individual combustion events and the ECU's start strategy could be studied. The results show that through the use of an aggressive cold start calibration strategy, the catalyst is very efficient after light-off at about 30s. However, during this same period, there are signs of partial misfires and rich AFR excursions, both of which contribute to the overall tailpipe emissions. The data from the fast-response analyzers allowed clear discrimination between rich events and partial misfires and would allow appropriate calibration actions to be taken. NOx conversion was seen to be highly transient, with very big changes in tailpipe NOx emissions occurring corresponding to the fuelling control system AFR switching frequency. Under lean AFR bias conditions, particularly after a lean AFR disturbance, significant NOx breakthrough was seen. ECU optimization using fast-response analyzers could further improve this vehicle's emissions calibration.
A novel continuous inductive discharge ignition system has been developed that allows for variable duration ignition events in SI engines. The system uses a dual-coil design, where two coils are connected by a diode, combined with the multi-striking coil concept, to generate a continuous current flow through the spark plug. The current level and duration can be regulated by controlling the number of re-strikes that each coil performs or the energy density the primary coils are charged to. Compared to other extended duration systems, this system allows for fairly high current levels during the entire discharge event while avoiding the extremely high discharge levels associated with other, shorter duration, high energy ignition systems (e.g. the plasma jet [1 , 2], railplug [3] or laser ignition systems [4 , 5 , 6 , 7 , 8]. Tests on a single cylinder of a 4-cylinder engine indicated that the addition of the continuous discharge mode resulted in a larger improvement in burn rate and stability than was achieved by either one coil in multi-strike mode or both coils in multi-strike mode with simultaneous discharge. The improvement in burn rate and stability translated into improved fuel consumption, particularly at low specific power levels and at high engine speeds. Finally, the improvement in EGR tolerance at low load conditions is seen as an important enabler for high dilution engines, as the requirement for engine stability during throttle tip-out is an important limiting factor in the ultimate EGR levels targeted by engine designers.
Cycle Combustion Variation (CCV) is an undesirable characteristic of Spark Ignition (SI) engines as a result of random perturbation of the combustion parameters, the equivalence ratio and the turbulence of the flow. Previous studies investigated the impact of some engine parameters on nitric oxide (NO) variability as well as the impact of these parameters on engine performance. The impact of these parameters on the mean value of NO distribution due to CCV and the amplitude of the NO variation are very important for engine tuning in order to meet the emission limits. In this study a detailed chemical mechanism for the prediction of NO formation in a homogeneous engine combustion chamber is presented. The mechanism receives as an input the thermodynamic analysis from a commercial Engine Simulation Tool (EST) and calculates the NO chemical kinetics for each crank angle step. Data found in literature were used for the validation of model approach. The second step of this study is the investigation of the nitric oxide variation due to CCV. Assuming that the combustion parameters values form a normal distribution, random perturbations for these parameters were introduced in the thermodynamic model. The varied thermodynamic data are then imported in the detailed chemical mechanism and NO distributions are estimated. The main target of this study is the investigation of the CCV impact on NO emissions as well as the correlation of imep variation and nitric oxide variation due to CCV.
Experimental data were obtained from a Rover K4 optical access engine and analysed with a combustion analysis package. Cyclic NO values were calculated by mass averaging the measurements obtained by a fast NO analyser. The modelled NO results were compared with the experimental values on plots of NO versus peak cylinder pressure, Pmax. The model achieves its cyclic variability through the input of experimentally derived burn rates and a completeness of combustion parameter. This is based on the Rassweiler and Withrow method for calculating the mass fraction burned and is derived from the pressure—crank angle record of the engine. The cycle-by-cycle NO modelling results are found to compare well with the experiments in terms of the mean value for all cycles, the general slope of the NO versus Pmax line and the maximum and minimum values (or spread). The modelled results do not, however, exhibit the same degree of scatter as the experimental data. This is attributed to the model utilizing a fixed blow-by rate, a constant temperature at inlet valve closure and a constant residual mass fraction for all cycles at a given operating point.
This paper investigates a technique of calculating the completeness of combustion on a cycle- by-cycle basis. The technique introduces the normalized pressure rise due to the combustion parameter, ψ to describe the completeness of combustion. This parameter is based on the Rassweiler-Withrow method of calculating the mass fraction burned and is derived from the pressure-crank angle record of the engine.
Experimental data were obtained from a Rover K4 optical access engine and analysed with a combustion analysis package. A computer simulation was then used to model the data on a cycle-by-cycle basis, both with and without the completeness of combustion parameter. The paper discusses the conditions under which it is suitable to model mean engine cycles, compared with the need to model cycle-by-cycle variability, and comments on the situations in which each type of modelling would be most appropriate. The engine simulation model is also used to investigate cycle-by-cycle variability of NO emissions that have recently been obtained experimentally.
The successful aspects of this investigation are that the cycle-by-cycle variability in the completeness of combustion can be determined by use of the parameter ψ, that the inclusion of the completeness of combustion parameter improves the simulation's ability to model the experimental data both in a statistical sense (the coefficient of variation of the indicated mean effective pressure) and on a cycle-by-cycle basis and that cycle-by-cycle NO modelling results are found to compare well with experiment.
Cyclic variations in pressure and burning time in a single cylinder spark-ignition engine have been determined as a function of equivalence ratio and engine speed. For the same operating conditions, measurements of turbulence intensity are available and these have been used with an estimate of the integral scale to estimate the magnitude of the Taylor microscale. It has been shown that the standard deviation in the burning time associated with the early stages of burning is predictable from knowledge of the Taylor microscale and the laminar burning velocity. This result is an implication of the Tennekes model of small scale turbulence and the Chomiak explanation of high flame propagation rate in regions of concentrated vorticity.
The operating range of lean-burn SI engines is limited by the level of cycle-by-cycle variability in the early flame development, which typically corresponds to the 0–5% mass fraction burned. An experimental investigation was undertaken to study this flame variability in an optical, stratified-charge, SI engine close to the lean limit of stable operation (A/F=22). Double-exposed flame images acquired through either a pentroof window (“tumble plane” of view) or the piston crown (“swirl plane” of view) were processed to calculate the intra-cycle flame growth and convection rates under 1500 RPM low-load conditions. Projected flame-boundary analysis was also performed to investigate the effect of flame shape/wrinkling on the subsequent timing of 5% mass fraction burned on a cycle-by-cycle basis. The images showed that the flame always preserved its shape while growing in size (even when it had been initiated with a highly convoluted shape); image processing demonstrated the manner with which the flame-growth speed varied as the flame propagated and approached the pentroof and piston-crown walls for slow, “typical” or fast burning cycles. It was found that it was beneficial to have a high convection velocity in the swirl plane of flow during the first 10° CA after ignition timing (corresponding to less than 0.1% mass fraction burned), but after this stage it was beneficial to have a moderate convection velocity for the flame. However, on the tumble plane of flow, a high convection velocity was preferable up to 30° CA after ignition timing (corresponding, typically, to 1% mass fraction burned). Slow development of a flame was associated with higher stretch rates for the same flame radius than fast-developing flames during the period of growth from 3 to 6 mm in radius (about 0.1–1% mass fraction burned). Extended analysis of the projected flame front's shape and its wrinkling showed that the fastest lean-condition flames had contour characteristics similar to those of the flames recorded for stoichiometric conditions. This suggested that the fastest lean flames on a cycle-by-cycle basis might have been richer than the average in the vicinity of the spark plug at ignition.
A detailed chemical mechanism to predict NO cycle-to-cycle variation in homogeneous engine combustion E-COSM'12-IFAC Workshop on Engine and Powertrain Control
- A Karvountzis-Kontakiotis
- L Ntziachristos
Karvountzis-Kontakiotis, A. and Ntziachristos, L., " A detailed
chemical mechanism to predict NO cycle-to-cycle variation
in homogeneous engine combustion, " E-COSM'12-IFAC
Workshop on Engine and Powertrain Control, Simulation and
Modeling, vol. 3, pp. 408-415, 2012, doi:10.3182/20121023-3FR-4025.00046.