High Cyclic Variability Research Articles

Combustion features under different center of heat release of a diesel engine using dimethyl carbonate/diesel blend

ABSTRACTExperiments were performed in a single cylinder common-rail diesel engine that adopts a low temperature premixed charge compression ignition (PCCI) mode. Combustion features of dimethyl carbonate (DMC)-diesel blends under various centers of heat release (COHRs) were revealed in details. With retarding of COHR, all the peaks of pressure and pressure rise rate and bulk gas temperature are postponed and declined in sequence. Normally, the crank angle of peak pressure is quite close to the COHR, while the peak of bulk gas temperature appears about 7°CA after COHR as a rule. The prolongation can be demonstrated at every stage of combustion such as q10 and q90 with the COHR being put backward. In addition, the heat release of diesel is completely slower than that of D10 fuel at various stages. Unfortunately, retarding of COHR implies a declining thermal efficiency of engines as well as a higher cyclic variation in general. Nevertheless, D10 blend has higher thermal efficiency than diesel thanks to high oxygen content of DMC and low boiling point that prompts better fuel atomization and complete combustion. Meanwhile, the cyclical variation of D10 is greater than diesel fuel owing to the low heat value, high latent heat of vaporization, and poor flammability of DMC. As a total, a comprehensive understanding of PCCI combustion features under different COHRs can be conducive to conducting effective management of combustion process and manipulating the subsequent emission performance to a favorable level.

Fast Computation of Combustion Phasing and Its Influence on Classifying Random or Deterministic Patterns

The classification between a sequence of highly variable combustion events that have an underlying deterministic pattern and a sequence of combustion events with similar level of variability but random characteristics is important for control of combustion phasing. In the case of high cyclic variation (CV) with underlying deterministic patterns, it is possible to apply closed-loop combustion control on a cyclic-basis with a fixed mean value, such as injection timing in homogeneous charge compression ignition (HCCI) or spark timing in spark ignition (SI) applications, to contract the CV. In the case of a random distribution, the high CV can be avoided by shifting operating conditions away from the unstable region via advancing or retarding the injection timing or the spark timing in the mean-sense. Therefore, the focus of this paper is on the various methods of computing CA50 for analyzing and classifying cycle-to-cycle variability. The assumptions made to establish fast and possibly online methods can alter the distribution of the calculated parameters from cycle-to-cycle, possibly leading to incorrect pattern interpretation and improper control action. Finally, we apply a statistical technique named “permutation entropy” for the first time on classifying combustion patterns in HCCI and SI engine for varying operating conditions. Then, the various fast methods for computing CA50 feed the two statistical methods, permutation and the Shannon entropy, and their differences and similarities are highlighted.

Performance of a hydrogen-enriched ethanol engine at unthrottled and lean conditions

Concerning the throttling loss under part load conditions, it is feasible to further improve the engine thermal efficiency through operating the engine under the unthrottled condition and controlling its load by changing the excess air ratio. However, the narrow flammability of ethanol may lead the ethanol engine to encounter high cyclic variations under unthrottled and lean conditions. The addition of hydrogen is potentially helpful for solving this problem. In this test, the engine was run under an speed of 1400rpm and unthrottled conditions. The hydrogen volume fractions in the intake were respectively kept at 0% and 3%. For a given hydrogen blending level, the ethanol flow rate was reduced to enable the engine to run under lean conditions. The results showed that the engine efficiency was improved with the blending of hydrogen. The highest thermal efficiency was improved by 6.07% after blending 3% hydrogen to the intake air. The addition of hydrogen could increase the engine torque output at lean conditions. Both cooling and exhaust losses were decreased after the hydrogen enrichment while adopting the lean combustion strategy. The hydrogen addition contributed to the extended lean burn limit and decreased cyclic variation under lean conditions. HC and CO emissions were decreased whereas NOx emissions were increased after the blending of hydrogen.

The effect of ethanol blending on mixture formation, combustion and soot emission studied in an optical DISI engine

Abstract In various research studies, ethanol blended fuels have shown reduced particulate matter (PM) emissions in comparison to gasoline and its surrogate fuels in direct-injection spark-ignition (DISI) engines. However, there are also studies reporting increased particulate concentration for fuels with low ethanol content. In this work the mixture formation and sooting combustion behavior of isooctane and the mixture E20 (20 vol% of ethanol in isooctane) is analyzed for catalyst heating operation. These operating conditions are critical as they strongly contribute to overall soot emissions in driving cycles. Simultaneous high speed imaging of OH ∗ –chemiluminescence and natural soot luminosity measurements are performed in combination with primary particle concentration measurements using a laser induced incandescence (LII) sensor in the engine exhaust duct. At these operating conditions E20 exhibits a higher sooting tendency as compared to isooctane. In order to identify the reason for increased soot formation, the mixture formation process is analyzed by planar laser induced fluorescence (LIF) measurements. The results show that soot was formed in fuel rich regions with incomplete evaporated fuel droplets remaining from the injection event. A different evaporation process of E20 fuel spray and mixing behavior is indicated showing a more compact rich mixture cloud with surrounding lean areas near the spark plug region. This mixture stratification is characterized by higher cyclic variations and constitutes a significant source of soot formation.

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.

Real-time internal residual mass estimation for combustion with high cyclic variability

In this work, a physics-based method of estimating the residual mass in a recompression homogeneous charge compression ignition engine is developed and analyzed for real-time implementation. The estimation routine is achieved through in-cylinder pressure and exhaust temperature measurements coupled with energy and mass conservation laws applied during the exhaust period. Experimental results on a multicylinder gasoline homogeneous charge compression ignition engine and dynamic analysis demonstrate the estimation routine’s ability to perform across a wide range of operating conditions as well as on a cycle-by-cycle basis for highly variable combustion phasing data.

The Influence of Operation Pressure on the Long-Term Stability of Salt-Cavern Gas Storage

The operation pressure of underground salt-cavern gas storage directly affects its stability. Because of seasonal demand and other emergency reasons, the gas storage working pressures always change from high to low or from low to high cyclic variation. In order to analyze the effect of gas storage pressure changing on its long-term stability, considering the salt rock creep, a 3D finite element model was built using the software Abaqus. Moreover, the deformation and analyzed results of the storage under 0 MPa, 4 MPa, 6 MPa, 8 MPa, 10 MPa, and 12 MPa and also circulating changes pressure operation were given in the 10-year creep. It concluded that how working pressures have effect on long-term stability of salt-cavern gas storage. The research results indicated that the long-term creep performance of underground salt cavern gas storage is affected by internal pressure, the smaller the internal pressure creep is, the more obvious the creep and the greater deformation of gas storage are. The greater the internal pressure is, the smaller the deformation of the gas storage is. The low pressure and excessive high pressure must be avoided during the operation of gas storage. These results have an important significance on determining the reasonable pressure of gas storage operation and ensure the long-term stability of gas storage.

Quantifying Cyclic Variability in a Multicylinder HCCI Engine With High Residuals

Cyclic variability (CV) in lean homogeneous charge compression ignition (HCCI) combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multicylinder engine. Experiments are performed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combustion analysis method that estimates the unburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as air flow and wall temperature, between cylinders strongly influence the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.

Large Eddy Simulation of Turbulent Combustion in a Spark-Assisted Homogenous Charge Compression Ignition Engine

A large eddy simuation (LES) model is presented for simulation of spark-assisted homogeneous charge compression ignition combustion. The model is based on tabulated chemical kinetic rate for ignition and flame surface density for flame propagation, taking into account interaction between flame propagation introduced by the spark and auto-ignition due to charge compression. The model is used to simulate the combustion process in an experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. The model is shown to be able to predict the combustion behavior observed in previous engine experiments. With low initial temperature, the SI flame mode prevails; with high initial temperature, the HCCI mode prevails. With moderate initial temperature, the SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of the observed high cyclic variation in the experiments.

Understanding the Dynamic Evolution of Cyclic Variability at the Operating Limits of HCCI Engines with Negative Valve Overlap

An experimental study is performed for homogeneous charge compression ignition (HCCI) combustion focusing on late phasing conditions with high cyclic variability (CV) approaching misfire. High CV limits the feasible operating range and the objective is to understand and quantify the dominating effects of the CV in order to enable controls for widening the operating range of HCCI. A combustion analysis method is developed for explaining the dynamic coupling in sequences of combustion cycles where important variables are residual gas temperature, combustion efficiency, heat release during re-compression, and unburned fuel mass. The results show that the unburned fuel mass carries over to the re-compression and to the next cycle creating a coupling between cycles, in addition to the well known temperature coupling, that is essential for understanding and predicting the HCCI behavior at lean conditions with high CV.

Controlling cyclic combustion timing variations using a symbol-statistics predictive approach in an HCCI engine

Cyclic variation of a Homogeneous Charge Compression Ignition (HCCI) engine near misfire is analyzed using chaotic theory methods and feedback control is used to stabilize high cyclic variations. Variation of consecutive cycles of θ Pmax (the crank angle of maximum cylinder pressure over an engine cycle) for a Primary Reference Fuel engine is analyzed near misfire operation for five test points with similar conditions but different octane numbers. The return map of the time series of θ Pmax at each combustion cycle reveals the deterministic and random portions of the dynamics near misfire for this HCCI engine. A symbol-statistic approach is used to predict θ Pmax one cycle-ahead. Predicted θ Pmax has similar dynamical behavior to the experimental measurements. Based on this cycle ahead prediction, and using fuel octane as the input, feedback control is used to stabilize the instability of θ Pmax variations at this engine condition near misfire.

Reducing the idle speed of a spark-ignited gasoline engine with hydrogen addition

Reducing idle speed is an effective way for decreasing engine idle fuel consumption. Unfortunately, due to the increased residual dilution and dropped combustion temperature, spark-ignited (SI) gasoline engines are prone to suffer high cyclic variation and even stall at low idle speeds. This paper investigated the effect of hydrogen addition on the performance of an SI gasoline engine at reduced idle speeds of 600, 700 and 800 rpm. The test results shows that cyclic variation was raised with the decrease of idle speed but reduced obviously with the increase of hydrogen energy fraction ( β H 2 ) . Decreasing idle speed and adding hydrogen were effective for reducing engine idle fuel consumption. The total fuel energy flow rate was effectively dropped from 30.8 MJ/h at 800 rpm and β H 2 = 0% to 17.6 MJ/h at 600 rpm and β H 2 = 19.9%. Because of the dropped fuel energy flow rate causing the reduced combustion temperature, both cooling and exhaust losses were markedly reduced after decreasing idle speed and adding hydrogen. HC and CO emissions were dropped with the increase of β H 2 , but increased after reducing idle speed. However, NOx emissions were decreased after reducing idle speed and adding hydrogen, due to the dropped peak cylinder temperature.

Natural gas spark ignition engine efficiency and NOx emission improvement using extreme exhaust gas recirculation enabled by partial reforming

Natural-gas (NG), spark ignition (SI) engines have widespread application in the power generation and upstream oil and gas industries. The manufacturers of these engines are being challenged to meet increasingly stringent nitrogen oxide (NO x) emission regulations without sacrificing fuel conversion efficiency. SI engines may be operated with air—fuel mixtures lean of stoichiometric to achieve higher thermal efficiency and to reduce NO x emissions. Compared with new combustion strategies such as homogeneous charge compression ignition, however, lean SI combustion suffers from a somewhat limited tolerability to mixture dilution and high cyclic variations. Alternatively, NO x emissions may be reduced by using exhaust gas recirculation (EGR) to dilute a stoichiometric air—fuel mixture. This paper investigates the application of reformer gas (RG) to enable a higher mixture dilution of an NG SI engine using EGR. It was found that RG enrichment allows an increase in the EGR dilution of a stoichiometric NG—air mixture from 12 per cent to more than 35 per cent. The optimal level of RG enrichment directly compensates for the combustion phasing retardation effect of EGR. Increasing the RG fraction in the mixture beyond the optimal value adversely affected the combustion process and fuel conversion efficiency. The experimental data suggest that NO x emissions comparable with the forthcoming 2010 US Environmental Protection Agency heavy-duty diesel engine regulations may be achieved using EGR, RG enrichment, and a three-way catalytic converter (TWC). An energy balance showed that there is the potential to increase the overall system fuel conversion efficiency slightly owing to a more optimized combustion process after taking into account the energy losses associated with fuel reforming. The approach of EGR, fuel reforming, and a TWC is suitable for retrofits because it can be accomplished without modifying the engine geometry.

Combustion and emission characteristics of a lean burn natural gas engine

Lean burn is an effective way to improve spark ignition engine fuel economy. In this paper, the combustion and emission characteristics of a lean burn natural gas fuelled spark ignition engine were investigated at various throttle positions, fuel injection timings, spark timings and air fuel ratios. The results show that ignition timings, the combustion duration, the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) and engine-out emissions are dependent on the overall air fuel ratio, spark timings, throttle positions and fuel injection timings. With the increase of the air fuel ratio, the ignition delays and combustion duration increases. Fuel injection timings affect ignition timings, combustion duration, IMEP, and the COV of the IMEP. Late fuel injection timings can decrease the COV of the IMEP. Moreover, the change in the fuel injection timings reduces the engine-out CO, total hydrocarbon (THC) emissions. Lean burn can significantly reduce NOx emissions, but it results in high cyclic variations.

Oral contraceptives and premenstrual symptoms: Comparison of a 21/7 and extended regimen

The purpose of this study was to assess the incidence and severity of premenstrual-type symptoms in patients converted from a 21/7 oral contraceptive (OC) regimen to an extended regimen. This was a single center prospective analysis of the single item Scott and White (S&W) Mood Scale and the Penn State Daily Symptom Report (DSR17) during a 21/7-day followed by a 168-day extended regimen of an OC containing 3 mg of drosperinone and 30 microg of ethinyl estradiol (DRSP/EE). Of the 114 patients who began the study, 111 completed the preextension 21/7 phase of the study. There were significant differences in severity in the DSR17 and the S&W mood scale among days of the cycle. (P < .0001) The highest values in both scales occurred during the 7-day hormone free interval (HFI) of the 21/7 cycles (P < .001). Of the 111 patients who completed the 21/7 phase of the study, 102 (92%) completed the 168-day extended regimen. During the extended phase of the study, subjects were divided into 2 groups: those with a 100% increase in symptoms from the first half to the second half of the last 21/7 cycle were labeled as high cyclic variability, whereas those with lesser or no cyclic change were labeled as low cyclic variability. There were 55 (54%) with increased cyclic variability in mood scores peaking during the 7-day HFI. Premenstrual-type symptoms measured by both the S&W mood scale and the DSR17 instrument decreased during the extended DRSP/EE OC regimen (P < .0001) compared with the preceding 21/7 cycle, with the greatest improvement detected in the sixth month of continuous OCs (P < .003). The patient group with greatest cyclic variability during the 21/7 regimen demonstrated the most improvement during the 168-day regimen (P < .0001). The single item S&W mood scale was significantly (P < .05) correlated to each of 17 elements of the DSR17 with Spearman R correlation coefficients of 0.25 to 0.57. The greatest correlation coefficient (Spearman's R = 0.66) is with the sum of all 17 items. A 168-day extended regimen of DRSP/EE led to a decrease in premenstrual-type symptoms compared with the 21/7-day regimen.

Combustion Limits and Efficiency in a Homogeneous Charge Compression Ignition Engine

Combustion phasing is characterized by CA50, the crank angle position where 50 per cent of the total heat release occurs. CA50 = ( a + b) [OI - OI0] where OI is the octane index defining the fuel auto-ignition quality given by OI = (1 - K) RON + K MON = RON - KS and S is the sensitivity of the fuel, (RON - MON). K, ( a + b), and OI0, which is the requirement of the engine, can be estimated from in-cylinder thermodynamic parameters and engine operating conditions. If OI is much smaller or greater than OI0, combustion will reach, respectively, the ‘knock limit’ with unacceptably high rate pressure rise or the ‘instability limit’ with unacceptably high cyclic variation. Nineteen different gasoline-like fuels with RON &gt; 60 have been tested at many different operating conditions in a single-cylinder homogeneous charge compression ignition (HCCI) engine. This generates a data set across which the correlations between CA50 and other operating parameters such as the equivalence ratio and the inlet pressure Pin are broken. Regression analysis shows that the maximum pressure rise rate (MPRR) increases with increasing φ and Pin and decreases with increasing CA50 and S if other parameters are kept constant. X = 1.55CA50 - 52 can be used to discriminate between conditions of high and low cyclic variation. For X &lt; 0, the coefficient of variation (COV) of indicated mean effective pressure (i.m.e.p.) is less than 10 per cent in about 90 per cent of the cases, but if X&gt;0, COV of i.m.e.p. is less than 10 per cent in only about 12 per cent of the cases. The i.m.e.p. increases with fuel energy content per cycle. The indicated fuel conversion efficiency ηi increases with increase in i.m.e.p. With other parameters kept constant, ηi decreases with increasing φ, intake air density, and intake temperature and increases with increasing CA50 and maximum pressure. An HCCI engine can be operated with an acceptable noise level and cyclic variation only within a narrow range of CA50 between about –5 and +10° from top dead centre.

A record of long- and short-term environmental and climatic change during OAE3: La Luna Formation, Late Cretaceous (Santonian–early Campanian), Venezuela

The La Luna Formation was deposited under anoxic/dysoxic conditions in a tropical epicontinental sea on the northwest South America margin. Sedimentological, micropaleontological and geochemical evidence provides insights into factors that influenced the sedimentation and controlled the accumulation of organic-rich deposits at decimeter and meter scales during the youngest of the Cretaceous oceanic anoxic events (OAE). The La Luna Formation consists of an alternation of black marlstones interbedded with black limestones and black marly limestones. The benthic foraminifera assemblages indicate sedimentation in the upper neritic to upper bathyal environment. These rocks contain large amounts of organic matter. It is interpreted that a combination of warm global and rainy climate and the presence of bathymetric barriers caused poor circulation and low rates of water column ventilation during a high sea level in the early Santonian leading to the preservation of carbon-rich deposits in this region. During the late Santonian, a cooling-trend in global climate increased wind strength and upwelling; this change probably reduced runoff causing a weakening of the pycnocline and destabilized the stratification in the water column providing a progressive increase in oxygen in the water column and on the sea floor and a decrease in total organic carbon preservation in a shallower basin. These changes and the establishment of full mid- and deep-water exchange in response to the deepening and widening of the Equatorial Atlantic Gateway could have been important mechanisms for ending the epeiric sea anoxia. Changes through time in the vanadium–nickel fraction, planktonic and benthic foraminifera assemblages, productivity proxy elements, and lithological characteristics support this model. Superimposed on the general trend, variations in calcium carbonate and total organic carbon percentages at the scale of tens of centimeters reveal high frequency cyclic variations, which apparently correspond to the main frequencies of orbital forcing. This cyclicity is interpreted as a primary depositional signal and is the result of orbital-controlled fluctuations in terrigenous dilution and variations in oxygen concentration at the base of the water column. Variations in the intensity of upwelling can be observed, but they did not control the cyclicity.

The Effect of Hysteretic Damping on Turbogenerator Shaft Torsional Oscillations

Electromechanical interaction between an electrical system and turbogenerator units may, in certain circumstances, lead to shaft oscillatory behaviour which has an onerous effect on the life expenditure of the shaft. The rate of decay of the oscillations produced as a result of a system disturbance is determined by both the electrical and mechanical components of damping. Due to the advances made in the modelling of electrical machines over the past fifty years the effect of the electrical component of damping is well understood and easily incorporated into a system model. The mechanical components of damping are known to be variable, but nevertheless are usually represented as constant viscous coefficients in the shaft system equations of motion. The present paper proposes a method for the evaluation of a variable damping coefficient which depends on the property of the shaft material when undergoing high stress cyclic variations. The effect of this term is to more rapidly reduce the high torque oscillations and reduce low torque oscillations less rapidly. This effect has been observed in practice and hitherto not predicted from simulation studies. The investigation relies on previous work when taking account of the effects of various steam conditions. Emphasis is placed on the derivation of a shaft model which includes equivalent variable viscous damping and representative results are collected to show the effect of this important parameter on the pattern of derived shaft torques based on a representative 600 MW turbo-generator.