Fuel-air Equivalence Ratio Research Articles

Operational feasibility of a spark ignition engine which is subjected to VTEC management strategy

ABSTRACT A validated spark ignition engine combustion model was used to investigate the operational feasibility of subjecting a 5.734 l, V8 engine to the Honda variable (valve) timing and electronic lift control (VTEC) management strategy. The numerical results were found to be in good agreement with measured data during the compression and expansion strokes. However, the results of the numerical model overestimated the measured data by approximately 10%–15% during the combustion phase. The reason for this discrepancy could be due to the fact that the turbulence levels within the combustion chamber during the experiment was not specified. Therefore, we accounted for turbulence in the simulation with a flame speed multiplying factor, . The investigations cover operations at low, mid-range and full power periods; for fuel-air equivalence ratios, of . The results of the study show that operational viability of a 5.734 l, V8 engine when subjected to the VTEC engine management scheme was achievable, provided high levels of turbulence were maintained, especially for extra lean mixtures, that is, for fuel-air equivalence ratio, . This work shows that the values of the flame speed factor, have to be carefully selected in order to achieve complete combustion.

Experimental study of lean spark ignition combustion using gasoline, ethanol, natural gas, and syngas

In the development of internal combustion engines, engineers and researchers are facing the challenge of improving engine efficiency while reducing harmful exhaust emissions. Previous research has shown that lean combustion is one of the viable techniques that can improve engine efficiency while effectively reducing exhaust emissions. Lean burn engines operate at low burned gas temperatures and can achieve high thermal efficiency based on favorable mixture thermodynamic properties. However, under high dilution levels, a lean misfire limit is reached where the combustion process becomes unstable and incomplete combustion starts to occur. Instability significantly affects engine efficiency, driveability, and exhaust emissions, which limit the full potential of lean burn engines. The lean misfire limit is not only dependent on engine design but also on fuel properties. Therefore, fuels that are conducive to lean combustion can provide the opportunity for enhanced efficiency and reduced emissions. Spark ignited (SI) combustion with conventional gasoline has shown to have relatively narrow range of fuel-air equivalence ratio; therefore, it is desired to explore the lean limit of SI combustion by using alternative fuels, which can also contribute to the reduction of greenhouse gas emissions from transportation and power generation.Experiments were conducted on a Cooperative Fuel Research (CFR) engine with varying fuel-air equivalence ratio (φ) to assess the engine performance and emissions with three alternative fuels, natural gas, ethanol, and syngas, at compression ratio of 8:1 and engine speed of 1200 rev/min. Equivalence ratio was varied by decreasing the mass of fuel while keeping the mass of air the same. The lean misfire limit was defined as the equivalence ratio where the CoV of IMEP across multiple consecutive engine cycles was greater than 5%. It was found that syngas can maintain stable combustion at extremely lean conditions and has the lowest lean misfire limit. Natural gas combustion achieved a lower lean misfire limit than gasoline and ethanol. Gasoline and ethanol had similar lean misfire limits, but it was found that gasoline helped the engine to achieve higher load and fuel conversion efficiency compared to the three alternative fuels.

An investigation on the potential of dedicated exhaust gas recirculation for improving thermal efficiency of stoichiometric and lean spark ignition engine operation

To suppress knock for improving thermal efficiency (ηth) of spark ignition (SI) engines, external exhaust gas recirculation (EGR) has been used. However, the use of EGR reduces flame speed, which leads to observed increase of cycle-to-cycle variations of SI combustion. To overcome this problem by reforming the fuel to add reactive compounds such as H2 and CO to the intake charge, dedicated EGR (D-EGR) concept is proposed, which uses a portion of the cylinders of a multi-cylinder engine to produce the entirety of the EGR consisted mainly of H2 and CO. To maximize the potential of D-EGR and provide new insights for improving ηth of SI engines, this study computationally investigates the use of D-EGR over a wide range of fuel-air equivalence ratios in four-cylinder engine (one D-EGR cylinder and three normal cylinders) for stoichiometric and lean operation. For the computations of laminar burning velocity (SL) and burned gas temperature (Tb), PREMIX in CHEMKIN-PRO were conducted with GRI-Mech 3.0 which is a detailed chemical-kinetic mechanism for methane (CH4).The results show that for D-EGR cylinder, both SL and Tb decrease with an increase of fuel/air equivalence ratios at D-EGR cylinder (ϕD-EGR) because the lower O2 as the more fuel is supplied to D-EGR cylinder for higher ϕD-EGR. ηth at D-EGR cylinder (ηth_D-EGR) decreases from 34.8% to 15.0% with an increase of ϕD-EGR from 1.0 and 3.0, regardless of changes in the amount of gross indicated work (Wg_D-EGR) and heat transferred to the combustion chamber (Qc_D-EGR) at D-EGR cylinder. For stoichiometric combustion at normal cylinder (ϕNormal = 1.0), both SL and Tb increase almost linearly with an increase of ϕD-EGR in the ϕD-EGR = 1.0–2.4 range. Furthermore, ηth at normal cylinder (ηth_Normal) increases from 34.8% to 51.9% between ϕD-EGR = 1.0 and 3.0. As a result, ηth of four-cylinder engine (ηth_Engine) with one D-EGR cylinder and three normal cylinders increases with an increase of ϕD-EGR, and the highest ηth_Engine (39.4%) is achieved for ϕD-EGR = 1.9. For lean combustion at normal cylinder (ϕNormal = 0.9, 0.8, 0.7, 0.6, 0.5), the higher SL and Tb are observed for the higher ϕD-EGR, similar to stoichiometric combustion (ϕNormal = 1.0). Eventually, by a combination of D-EGR addition and lean combustion, the highest ηth_Engine of 42.9% is obtained for ϕNormal = 0.7 with ϕD-EGR of 2.0. This corresponds to a 16.0% increase of ηth_Engine relative to the baseline which is a stoichiometric charge consisted of CH4 and air without any EGR (ηth_Engine = 26.9%).

A comparative study on partially premixed combustion (PPC) and reactivity controlled compression ignition (RCCI) in an optical engine

Partially premixed combustion (PPC) and reactivity controlled compression ignition (RCCI) are two new combustion modes in compression-ignition (CI) engines. However, the detailed in-cylinder ignition and flame development process in these two CI modes were not clearly understood. In the present study, firstly, the fuel stratification, ignition and flame development in PPC and RCCI were comparatively studied on a light-duty optical engine using multiple optical diagnostic techniques. The overall fuel reactivity (PRF number) and concentration (fuel-air equivalence ratio) were kept at 70 and 0.77 for both modes, respectively. Iso-octane and n-heptane were separately used in the port-injection (PI) and direct-injection (DI) for RCCI, while PRF70 fuel was introduced through direct-injection (DI) for PPC. The DI timing for both modes was fixed at –25°CA ATDC. Secondly, the combustion characteristics of PPC and RCCI with more premixed charge were explored by increasing the PI mass fraction for RCCI and using the split DI strategy for PPC. In the first part, results show that RCCI has shorter ignition delay than PPC due to the fuel reactivity stratification. The natural flame luminosity, formaldehyde and OH PLIF images prove that the flame front propagation in the early stage of PPC can be seen, while there is no distinct flame front propagation in RCCI. In the second part, the higher premixed ratio results in more auto-ignition sites and faster combustion rate for PPC. However, the higher premixed ratio reduces the combustion rate in RCCI mode and the flame front propagation can be clearly seen, the flame speed of which is similar to that in spark ignition engines but lower than that in PPC. It can be concluded that the ratio of flame front propagation and auto-ignition in RCCI and PPC can be modulated by the control over the fuel stratification degree through different fuel-injection strategies.

In situ sensor for cycle-resolved measurement of temperature and mole fractions in IC engine exhaust gases

We present a multi-species mole fraction and temperature sensor for in situ exhaust gas diagnostic of internal combustion (IC) engines. The sensor is based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) and incorporates four optical channels - two miniature White cells and two double-traversal cells - with base lengths of 6 cm. It has been demonstrated at a hot air test stand and in the exhaust manifold of a single-cylinder research engine, with measured temperatures of up to 1000 K. Stable operation was achieved with absorption lengths of up to 192 cm (test stand) and 97 cm (engine). Employing time-division multiplexed detection, six species were measured simultaneously in the engine exhaust, at wavelengths ranging from 1.4 µm to 5.2 µm: water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), nitrogen dioxide (NO2) and nitric oxide (NO). The effective measurement rate was as high as 1 kHz, and cycle-to-cycle variations were clearly detected. We show the correlation of the air-fuel equivalence ratio with the spectroscopically measured mole fraction of each species. At a cycle-resolved rate, detection limits for the legally regulated species NO and NO2 were 1 ppm and 4 ppm, respectively. The sensor is intended to help improve the understanding of IC engine emission behavior during fast transients.

Optical diagnostics on the reactivity controlled compression ignition (RCCI) with micro direct-injection strategy

Micro direct-injection (DI) strategy is often used to extend the operation range of the reactivity controlled compression ignition (RCCI) to high engine load, but its combustion process has not been well understood. In this study, the ignition and flame development of the micro-DI RCCI strategy were investigated on a light-duty optical engine using formaldehyde planar laser-induced fluorescence (PLIF) and high-speed natural flame luminosity imaging techniques. The premixed fuel was iso-octane and an oxygenated fuel of polyoxymethylene dimethyl ethers (PODE) was employed for DI. The fuel-air equivalence ratio of DI was kept at 0.09 and the premixed equivalence ratio was varied from 0 to 1. RCCI strategies with early and late DI timing at –25° and –5° crank angle after top dead center were studied, respectively. Results indicate that the early micro-DI RCCI features a single-stage high-temperature heat release (HTHR). The combustion in the low-reactivity region shows a combination of flame front propagation and auto-ignition. The late micro-DI RCCI presents a two-stage HTHR. The second-stage HTHR is owing to the combustion in the low-reactivity region that is dominated by flame front propagation when the premixed equivalence ratio approaches 1. For both early and late micro-DI RCCI, the intermediate-temperature heat release (ITHR) of iso-octane, indicated by formaldehyde, takes place in the low-reactivity region before the arrival of the flame front. This is quite different from the flame front propagation in spark-ignition (SI) engine that shows no ITHR in the unburned region. The DI fuel mass is a key factor that affects the combustion in the low-reactivity region. If the DI fuel mass is quite low, there is more possibility of flame front propagation; otherwise, sequential auto-ignition dominates. The emergence of the flame front propagation in micro-DI RCCI strategy reduces its combustion rate and peak pressure rise rate.

Research on the hot surface ignition of hydrogen-air mixture under different influencing factors

To further reveal the pre-ignition characteristics of hydrogen internal combustion engine, the effect of hot surface characteristic parameters on the ignition characteristics of hydrogen-air mixture was investigated in this research. Based on the prototype of the constant volume combustion bomb with an overhead glow plug, the duration from the heating of the hot surface to the combustion of hydrogen-air mixture, the so-called heating duration, was firstly researched under different fuel-air equivalence ratio, initial temperature, initial pressure, hot surface temperature, and hot surface area, and the influence of each factor on the heating duration was analyzed. The results show that the order of the effect of each factor on the hot surface ignition is as follows: hot surface temperature > initial pressure of hydrogen-air mixture > equivalent ratio > initial temperature of hydrogen-air mixture > hot surface area. The influence of the hot surface characteristic parameters on the heating duration was further analyzed in detail. On this basis, the relationship among the critical ignition temperature, the heating duration and the hot surface area was researched and established. The results show that the heating duration is the only major factor affecting the critical ignition temperature. Finally, the research results were applied to analysis the pre-ignition in hydrogen internal combustion engine.

Comparative study on low ambient temperature regulated/unregulated emissions characteristics of idling light-duty diesel vehicles at cold start and hot restart

Since it is common for vehicles with idle engines to be stored indoors during the winter season to remain warm, better constraints are needed for idling emissions at low ambient temperature. CH4 (methane), C2H6 (ethane), C2H4 (ethylene), HNCO (isocyanic acid), N2O (nitrous oxide), NH3 (ammonia), C3H6 (propylene), and HCHO (formaldehyde) from many unregulated exhaust emissions were selected and measured by a Fourier Transform Infrared Spectroscopy (FTIR) analyzer in this study. This paper investigates the idling regulated and unregulated emissions of Euro 5/6 diesel vehicles at cold start and hot restart near 0 °C conditions. The impact of cold start and hot restart on NOx idling emissions was lower than CO and HC idling emissions in diesel vehicles. With regard to unregulated emissions, high fuel–air equivalence ratios and longer ignition delay in the idle cold start condition may increase incomplete combustion and affect the thermal cranking processes, leading to the increase of CH4, C2H4 and C3H6. Furthermore, as the formation of N2O is considerably affected by the equivalence ratio and cylinder temperature, a high fuel-air equivalence ratio at low ambient temperature in the cold start condition resulted in the increase of N2O emissions.

Spark-induced breakdown spectroscopy of methane/air and hydrogen-enriched methane/air mixtures at engine relevant conditions

An experimental study of temporally and spatially resolved spark-induced breakdown spectroscopy (SIBS) of methane and hydrogen-enriched methane mixtures is reported for premixed combustion in internal combustion engines. Experiments were conducted at quiescent conditions in a small constant volume chamber, varying pressure, stoichiometry and hydrogen admixture rates. Spectral emissions of hydroxyl (OH) at 306 nm, NH at 336 nm, the cyanogen (CN) band at 388 nm and the nitrogen second positive system (N2) were spatially resolved on a time-integrated setup. OH emissions were found to be strongest between the electrodes, whereas CN emissions were more pronounced at the center and ground electrode. Metal emission lines were observed, partially interfering with the radicals of interest. Energy dispersive X-ray spectroscopy showed nickel, copper and iron shares in the noble metal electrodes, confirming the origin of these metal emissions. Simultaneously to time-integrated spectra, temporally-resolved spectra at a frame rate of 100 kHz were recorded during the glow phase of the electrical discharge. Comparison of both spectra revealed a good agreement in terms of spectral characteristics and resolved species. Temporally-resolved spectra highlighted the strong dependence on the electric discharge characteristics of the inductive coil ignition system in the early phase of ignition. Ratios of CN/OH and CN/NH were found to correlate with the fuel-air equivalence ratio, but shot to shot repeatability was up to a factor 3 larger than the dependence on the mixture composition. Moreover, these ratios changed with increasing pressure at ignition timing, and with the higher pressure metal emission lines became more pronounced. Additionally, at pressures higher than 2 bar, the equilibrium position shifted to the disadvantage of the second positive system of nitrogen, suppressing this emission. Hydrogen admixture to methane of up to 50 vol% were found to not significantly alter the spectral signature between 300 and 400 nm, only marginally affecting the signal ratios of CN/OH and CN/NH.

Role of global fuel-air equivalence ratio and preheating on the behaviour of a biogas driven dual fuel diesel engine

The practice of the green renewable fuels in diesel engines is increasing concerning the progressive deterioration of the green environment and the scarcity of the fossil fuels. In these aspects, several experiments in dual fuel mode (DFM) at different global fuel–air equivalence ratio (Φglobal), intake charge preheating and loads have been carried out considering biogas as the inducted renewable fuel. The Φglobal was varied from 0.30 to 0.89 from part to higher loads with biogas flow rate (BFR) of 0.67–3.99 kg/h. The maximum diesel replacements (DR) and biogas energy share (BES) are found to be 92.49% and 97.55%, respectively. However, at higher Φglobal, the brake thermal efficiency (BTE) reduces drastically. Although with preheating, there is an increment of BTE by 5.72% and 2.60% at loads of 4.36 N·m and 21.78 N·m, respectively. At lower Φglobal in DFM and with preheating, the diesel like trends of BTE is achieved. Overall combustion behaviour deteriorates at higher Φglobal. However, it significantly improved with the preheating and at controlled Φglobal. Higher cycle-to-cycle variation of cylinder peak pressure (CPP) is noticed at higher Φglobal. With preheating, at part and higher loads the reduction of CO of 29.41% and 65.49% are estimated. At higher load (21.78 N·m) and with preheating a reduction of 53.33% of HC is noticed. The drastic reduction of NOx is observed with the increment of Φglobal. The superior performance is achieved at each of the tested loads at the optimum Φglobal and with preheating.

Study on pollutants formation under knocking combustion conditions using an optical single cylinder SI research engine

The aim of this experimental study is to investigate the pollutants formation and cyclic emission variability under knocking combustion conditions. 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 equivalence 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.

Characterizing nonlinear interaction between a premixed swirling flame and acoustics: Heat-driven acoustic mode switching and triggering

The aim of this present study is to examine the critical role of air-fuel equivalence ratio Ф and air flow rate on triggering self-excited thermoacoustic oscillations in a swirling combustor, which is widely applied in industry to achieve low combustion emissions. For this, experimental study of the effect of air-fuel equivalence ratio in a propane-burnt swirling combustor is conducted to gain insights on the nonlinear dynamics behaviors of the thermoacoustic oscillations. A series experiments are conducted by varying 1) the air flow rate and 2) the equivalence ratio. It is found that the air flow rate and the equivalence ratio play important roles on producing limit cycle thermoacoustic oscillations. The frequencies and amplitudes of these oscillations strongly depend on the equivalence ratio. In addition, the dominant thermoacoustic mode is found to switch from a higher frequency at ω3 to a lower one at ω1 for a given Φ, as the air flow rate Qa is varied. However, as Qa is set to a given value, increasing the equivalence ratio from 0.8 to 1.2 leads to the dominant frequency being shifted by approximately 20%. In general, the present study sheds lights on the nonlinear characteristics and behaviors of heat-driven acoustic oscillations in a swirling thermoacoustic system.

The Effect of Inlet Valve Timing and Engine Speed on Dual Fuel NG-Diesel Combustion in a Large Bore Engine

High load (18 bar IMEP) dual fuel combustion of a premixed natural gas/air charge ignited by directly injected diesel fuel was studied in a large bore gas engine. A nozzle design with low flow rate was installed to inject a small diesel volume (10.4 mm3) equal an energetic amount of about two percent. The effect of compression end temperature on ignition and combustion was investigated using valve timings with early IVC (Miller) and maximum charging efficiency (MaxCC). Furthermore, the engine speed was reduced (1500 rpm to 1000 rpm) for the Miller valve timing to analyze the impact of the chemical time scale on the combustion process. During all experiments, the cylinder charge density was kept constant adjusting the intake pressure and the resulting air mass flow. Unlike a typical reactivity-controlled compression ignition (RCCI) combustion process, the combustion phasing of the investigated pilot-ignited natural gas combustion was also sensitive to the injection timing besides the air-fuel equivalence ratio (AFER). As a consequence of the lower compression end temperature with Miller valve timing, the operating range of the AFER was shifted to significantly lower values (λ = 1.53 to 1.63) compared to MaxCC (λ = 1.88 to 2.08) and knocking combustion was no longer observed. Furthermore, the bijective relation between the mass fraction burnt 50% (MFB50%) as a function of the start of energizing (SoE) at a constant air-fuel equivalence ratio was no longer valid. The experiments with MaxCC valve timing revealed that this behavior is not necessarily a characteristic of the pilot ignition. In fact, conditions inhibiting the ignition and engine knock result in loss of the one-to-one pairing between SoE and MFB50%. The reduction of the engine speed extended the lean misfire limit (λ = 1.53 to 1.75) and improved the engine start behavior.

Predicting ignition delay times of C1-C3 alkanes/hydrogen blends at gas engine conditions

A comprehensive ignition delay time database of 3.1 million points using detailed chemistry is generated. The database includes ignition delay times for pure methane, ethane, propane and hydrogen, as well as for gas blends comprising of the mentioned gases. The database covers a broad range of gas engine applications; pressure 1–20 MPa, temperature 900–2500 K, air–fuel equivalence ratio (λ) 0.9–2.5 and exhaust gas recirculation (EGR) 0–30 m%. For gas blends up to 60 vol% to methane, it is observed that different gas blends with the same HC-ratios/methane-numbers, show similar ignition delay times. Ignition delay time correlations for pure CH4, C2H6, C3H8, H2 and the gas blends are suggested with mean deviation being as low as about 7%. A new blending methodology is developed to describe the C1-C3 alkanes/H2 blends at elevated pressure and temperature conditions. Correlations describing the effect of EGR on the ignition delay time are further developed.

Laminar flame properties of C1-C3 alkanes/hydrogen blends at gas engine conditions

The use of fuel blending is encouraged in order to achieve more flexibility in gas engines. In order to design such engines effectively, relevant information about the laminar flame speeds and laminar flame thickness are necessary. Hydrodynamic and thermo-diffusive instabilities at gas engine conditions prevent the acquisition of reliable data experimentally. One-dimensional numerical simulations with detailed chemistry can be a solution. A huge database of laminar flame speeds is generated covering a broad range of gas engine applications, pressure (p) 0.1–20 MPa, fresh gas temperature (Tu) 300–1100 K, air-fuel equivalence ratio (λ) 0.9–2.5, methane 100–60 vol%, ethane 0–40 vol%, propane 0–40 vol%, hydrogen 0–30 vol% and exhaust gas recirculation (EGR) 0–30 m%. The detailed reaction mechanisms GRI 3.0 and AramcoMech 1.3 are used for the generation of flame speed data for the mentioned conditions. A laminar flame speed correlation for 100% hydrogen extending up to elevated pressure and temperature conditions is developed. A blending law based on Le Chatelier’s rule is investigated. It is observed that the HC-ratio has a very determining effect for the laminar flame properties of different C1-C3 alkane blends. Based on this observation, it was possible to derive a very efficient correlation for both laminar flame speed and laminar flame thickness for the group of natural gas blends with methane, ethane, propane and hydrogen, and as well as including relevant EGR, which corresponds within 7% accuracy to the calculated database of about 73,000 points. The developed laminar flame speed correlation is incorporated in an engine process simulation code. It is validated with the measured in-cylinder pressure traces from the single cylinder research engine experiments for different gas blends and EGR ratios.

Kinetic modeling and experimental study on the combustion, performance and emission characteristics of a PCCI engine fueled with ethanol-diesel blends

The combustion process in the Premixed Charge Compression Ignition (PCCI) engine is basically restricted by the in cylinder charged mixture components. Also, the homogeneity of the charged mixture is determining the quality and process of the chemical reaction during the first stage of combustion which establish the auto-ignition process. In the present work, the engine experimental setup is equipped with a new suggested modification on the original fuel system device in order to produce a perfect commixture of diesel/ethanol at different blends ratio with the charged air. The obtained laboratory results are used to validate the simulation's data of the PCCI engine ignition. The prediction is performed using a detailed kinetic reaction mechanism. The simulation study has been achieved to predict the auto-ignition timing and the combustion characteristics of the PCCI engine fueled with different blends of ethanol and diesel at different volume percentage. The obtained results show that the premixed ratio of the ethanol in the ethanol/diesel fuel blends can be used to control the auto-ignition timing and the combustion characteristics at different engine air/fuel ratios. Also, the main pathway of this work is to establish the influence of the engine operating parameters which including the premixed ratio, fuel–air equivalence ratio on the engine performance, combustion and emission characteristics of the PCCI engine. These effects are studied and traced through the simulation result data of the in-cylinder pressure, temperature, and gas phase heat release at different a premixed ratio of ethanol-diesel fuels blends of 0, 10, 20, 30,40 and 50% (by volume).

High-pressure pyrolysis and oxidation of ethanol

The pyrolysis and oxidation of ethanol has been investigated at temperatures of 600–900 K, a pressure of 50 bar and residence times of 4.3–6.8 s in a laminar flow reactor. The experiments, conducted with mixtures highly diluted in nitrogen, covered fuel-air equivalence ratios (Φ) of 0.1, 1.0, 43, and ∞. Ethanol pyrolysis was observed at temperatures above 850 K. The onset temperature of ethanol oxidation occurred at 700–725 K over a wide range of stoichiometries. A considerable yield of aldehydes was detected at intermediate temperatures. A detailed chemical kinetic model was developed and evaluated against the present data as well as ignition delay times and flame speed measurements from literature. The model predicted the onset of fuel conversion and the composition of products from the flow reactor experiments fairly well. It also predicted well ignition delays above 900 K whereas it overpredicted reported flame speeds slightly. The results of sensitivity analyses revealed the importance of the reaction between ethanol and the hydroperoxyl radical for ignition at high pressure and intermediate temperatures. An accurate determination of the rate coefficients for this reaction is important to improve the reliability of modeling predictions.

Flame Dynamics Intermittency in the Bistable Region Near a Subcritical Hopf Bifurcation

We report experimental evidence of thermoacoustic bistability in a lab-scale turbulent combustor over a well-defined range of fuel–air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts,” i.e., sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions (PDFs) of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bistable region. The gain and phase between acoustic pressure and heat release rate fluctuations are evaluated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of the corresponding statistics is new and particularly informative. It shows that the system is characterized, in average, by a nearly constant gain and by a drift of the phase as function of the oscillation amplitude. This finding may suggest that the bistability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the nonconstant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

Experimental study of equivalence ratio and fuel flow rate effects on nonlinear thermoacoustic instability in a swirl combustor

Industrial combustion systems such as power generation gas turbines, rocket motors, furnaces and boilers often face the problem of large-amplitude self-excited pressure oscillations that occur due to the onset of thermoacoustic instability. To prevent the onset of such instability, understanding the effects of fuel–air equivalence ratio ϕ, and fuel flow rate Vf on self-excited nonlinear thermoacoustic oscillations is of fundamental and practical importance. Experimental investigation of the roles of these parameters on triggering thermoacoustic instability in a swirl combustor has received very little attention. In this work, we design a swirling thermocoustic combustor and conduct a series of experimental tests. Autocorrelation and recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace are performed. These experimental tests allow us to study the effect of the fuel–air equivalence ratio ϕ on the onset of thermoacoustic instability by varying the fuel volume flow rate Vf. We demonstrate that the fuel volume flow rate and the equivalence ratio play different but critical roles on generating thermoacoustic instability at different frequencies and amplitudes. Maximum sound pressure level can be as high as 135 dB. In addition, mode switching, (i.e. frequency swap) is found to occur between approximately ω3≈510 Hz and ω1≈170 Hz, depending on the equivalence ratio ϕ. Furthermore, the dominant frequency corresponding to the maximum amplitude is shown to be shifted by approximately 20%, as the fuel flow rate Vf is increased and the combustion condition is changed from lean to rich. These findings are quite useful for designing a feedback control strategy to stabilize an unstable combustor. The present work opens up an applicable means to design a stable swirling combustor.

Investigations on air-fuel mixing and flame characteristics of biodiesel fuels for diesel engine application

In this study, the spray and combustion phenomena of biodiesels were investigated in a constant volume combustion chamber (CVCC). Mineral diesel was used as a baseline fuel and biodiesels derived from waste cooking oil, Karanja oil, and Jatropha oil were utilized to investigate the effect of fuel properties on spray and combustion processes. Experiments were performed at high temperature and pressure conditions in order to simulate the atmospheric environment of a diesel engines. Test fuels were injected at an injection pressure of 80 MPa using a common-rail equipped solenoid injector. Macroscopic evaporation characteristics were analyzed by high-speed shadowgraphy technique under evaporating conditions. The representative droplet size distribution and Sauter mean diameter (SMD) were measured using the Phase Doppler Interferometry (PDI) technique, which was applied to study the spray atomization characteristics of the fuels. The air-fuel equivalence ratio in the spray was calculated using mathematical correlations. The quantitative estimations of soot generation in the spray flames were compared using Hue number analysis. From the shadowgraphy images, the biodiesels showed slower air-fuel mixing characteristics than the baseline diesel due to their inferior volatility. While diesel evaporated abruptly after the fuel injection, the biodiesels showed dense liquid regions in the center of the spray plume. Biodiesels also exhibited larger SMD than the baseline mineral diesel in the fuel spray because of their higher density, viscosity, and surface tension. Despite having poor spray atomization characteristics, the calculated equivalence ratio of biodiesels was lower than that of the baseline diesel. This trend was attributed to the oxygen content of biodiesel. The flame luminosity and visible spray flame duration of biodiesels were lower than those of diesel, while the biodiesel spray flames exhibited lower sooting tendency than the baseline diesel.