Variation In Ignition Delay Research Articles

Analysis of spray/wall impingement using an ECN single-hole injector and a controlled-temperature wall under realistic engine conditions

Spray–wall interactions (SWI) directly affect fuel–air mixture and emissions formation. Therefore, they are considered among the most critical physical processes in engine research nowadays. However, the physics of the wall film formation, propagation, and breakup is not fully understood yet. This work aims to use a thermoregulated steel wall to study the spray–wall interaction phenomenon and its influence on the macroscopic spray behavior. A single-hole injector known as “Spray D” in the Engine Combustion Network was used. n-Dodecane was employed as fuel and the wall has been positioned in four different configurations, varying angle and distance respect to the injector tip. For this work, not only diesel combustion is reproduced in the test rig due to ambient gas engine-like thermodynamic conditions, but wall temperature has been controlled to emulate characteristic values that could be found in the piston of an internal combustion engine. This implies that the spray–wall heat transfer is simulated and its effects on ignition and spray development can be analyzed. Heat flux was measured by employing high-speed thermocouples fitted in the wall and by the use of an one-dimensional transient wall heat model. Three high speed cameras were simultaneously used to observe the SWI, one for the Schlieren optical technique which allows to study the vapor phase of the spray and to determine the ignition delay, another one to observe the natural luminosity of the flame, and finally, an intensified camera was used to determine the lift-off length by observing the chemiluminescence of the OH*. An interesting finding obtained in this work was a boundary layer formation due to the thermal diffusion that contributes to cool down the spray and to delay the high-temperature chemical reactions. Results show a substantial increment of the heat flux and the wall temperature variation with both ambient temperature and density by increasing the flame temperature and gas entrainment. The exposure to the cold wall affects the ignition delay variation with the injection pressure and the wall distance. It was found that the wall temperature (in the range of tested conditions) did not affect the lift-off length location. • The exposure to the cold wall delayed the ignition. • Flame thickness around the wall undergoes a narrowing at lower wall temperatures. • The injection pressure increments the convective coefficient. • A boundary layer due to thermal diffusion was formed on the wall.

Improvement in Ignition Delay Characteristics of Diesel Engine Combustion by Varying Combustion Chamber Air Pressure

Complete and clean combustion is always desirable for better performance of engine and less emissions. An experimental work is carried in constant volume combustion chamber for getting conditions like diesel engine combustion to study the ignition delay characteristics of diesel engine combustion by varying combustion chamber air pressure. In this experimental work, air pressure of combustion chamber varied from 10 to 25 bar, hot surface temperature inside the combustion chamber varied from 350°C to 550°C and fuel injection pressures varied from 100 to 200 bar for hollow cone spray and solid cone spray . For this work a set-up is made in which the flame detection is done by digital storage oscilloscope using an optical method. The findings of the work suggests that combustion chamber air pressure and injection pressure are significantly varies the values of ignition delay at a particular hot surface temperature. It is also find that on increasing the values of combustion chamber air pressure and injection pressure, ignition delay values are decreases although the variation in ignition delay is less at higher injection pressure and combustion chamber air pressure.

Variation in Ignition Delay with Changing the Environment inside the Combustion Chamber for Different Sprays

Ignition delay is very important parameter which influences the entire combustion process and emissions generated inside the engine. An experimental study was conducted to observe the variation in ignition delay with changing environment inside the combustion chamber for different sprays. Two types of nozzles namely pintle nozzle and single hole nozzle were used for creating hollow cone spray and solid cone spray respectively. This study was carried at different injection pressures (100 bar, 150 bar and 200 bar), different HST (350°C, 450°C and 550°C) and different air pressures (10 bar, 15 bar, 20 bar and 25 bar) for obtaining the value of ID. A digital Oscilloscope is used to record ID and optical method was used for detecting the flame. Results of the experiments show that air pressure, injection pressure and HST all are responsible for variation in ID but HST is more strongly affect the ID compared to other parameters. However at high value of HST and injection pressures variation in ID is less. Variations in the values of ID are more for Solid cone spray than Hollow Cone spray although ID is decreased for both sprays at almost all injection pressures. The dependency of types of spray is more prominent in lower temperature ranges compared to higher temperature ranges.

Effect of DEE Oxygenate on Diesel in Algal Biodiesel-Diesel Blends on the Combustion Phenomenon of DI Compression Ignition Engines

Biodiesel derived from Stoechospermum marginatum is analysed in a CI engine to understand its feasibility to be used as fuel. In this study, Soxhlet extraction method was applied to extract the bio oil from the brown sea weed (S. Marginatum) with n-hexane as solvent. Considering the low FFA content through titration, base single stage transesterification process with methanol and NaOH at molar ratio of 8:1 is used and it yielded 84.5% of bio diesel. The test fuel is prepared with processed algae bio diesel and Di-Ethyl Ether in different blend ratios namely D80AB20, D70AB20DEE10 and D60AB20DEE20. The variation in combustion aspects are evaluated with constant concentration of algal bio diesel at 20% with variation in diesel and oxygenate concentrations. Consequential variation is observed in the combustion behaviour of CI engine on fuelling with test fuels. Comparable in-cylinder pressure is observed in bio-diesel blends and oxygenated blends. D70AB20DEEE10 exhibits superior heat release rate and pressure rise at all loads. The peak in-cylinder pressure and variation in ignition delay are also analysed in this study.

An investigation on ignition-delay modelling for control

The ignition delay is an important quantity in low temperature combustion concepts where a prolonged ignition delay gives an enhanced fuel-air mixing process, favourable for decreased particulate and NOx emission levels. This article experimentally evaluates three different low-order physics-based correlation models and their ability to predict the ignition delay for the purpose of model-based controller design. The models relate the state of the gas mixture after the point of fuel injection in order to predict the ignition delay. The experiments were all performed on a six-cylinder direct-injection Scania diesel engine with a fuel mixture of 80 vol% gasoline and 20 vol% n-heptane. The results showed that ignition-delay variation was not easily predicted when the injection timing was varied close to top-dead-centre but that the model performance satisfactorily during intake-condition variations. The results also showed that a more complex model did not necessarily give an increased prediction performance.

Block Diagram Model for the Simulation of an Electromagnetic Rail Accelerator System

Rail accelerators are superior to classical gas-driven accelerators with regard to attainable terminal velocity, ignition delay variation, and controllability. The behavior is generally described with a system of nonlinear differential equations, which can be solved in many ways. This is done in order to describe an existing launcher, to predict its performance when parameters change, or to estimate the properties of such a system in the design phase. This paper presents a simplified electromechanical system of differential equations in a novel form. This form enables scientists to solve the equations using one of the following software: Scilab, MATLAB, or a similar one. The model is robust and parameter changes have a low impact on the solving time. This makes it a reliable tool for parametric studies. The model comprises pulsed power capacitors, the pulse-forming network, cables, and the launcher itself. At first, the electrical part is described with its circuit equivalent and coupled with the mechanical process. Second, the system is analytically transposed to a standard form. This form is then transferred into the simulation software, which solves the equations. It can simply be adapted to other launchers and modified to comprise nonlinear side effects. The simulation results are compared with experimental results from the augmented rail accelerator at the French-German Research Institute of Saint-Louis, France.

Global Reduced Model for Conventional and Alternative Jet and Diesel Fuel Autoignition

A global reduced model for the autoignition of conventional petroleum-derived and alternative, hydroprocessed and Fischer–Tropsch, jet and diesel fuels is presented. The model is based on a seven-step model from the literature (Zheng, J.; Miller, D. L.; Cernansky, N. P. A Global Reaction Model for the HCCI Combustion Process, SAE Paper 2004-01-2950, 2004) and includes steps to predict oxidation and autoignition in the three temperature regimes of interest to engine applications, the high-, low-, and negative-temperature-coefficient (NTC) regimes. Here we have included dependence of reaction rate parameters on the derived cetane number (DCN) to predict the influence of fuel variability on autoignition and optimized the reduced model to best fit target shock tube ignition delay time data for jet and diesel fuels. The standard deviation between model predictions for ignition delay and the target shock tube data set is ±21% and the model captures all experimental trends for ignition delay variation with DCN, temperature, pressure, and fuel–air equivalence ratio in all temperature regimes. Comparisons to experimental ignition delay time data not contained in the target data set show model–experiment deviation similar to the comparisons with the target data, with global standard deviation of ±20–30% and differences of at most a factor of 2. The model comparisons with shock tube data suggest that the model is suitable for autoignition prediction for fuels containing large quantities of aliphatic compounds with DCN = 30–80 for 650–1300 K, 8–80 atm, and fuel–air equivalence ratios of 0.25–1.5.

Control of ignition delay in microfabricated hot-wire igniters: Simulation, fabrication, and experimental results

The presented work studied thermal interactions between laminated, metal-foil, hot-wire igniters and exothermic solid-state, composite chemical systems in order to demonstrate precise timing control of a thermal ignition process. The study includes FEA modeling, device fabrication, and characterization to demonstrate control microthruster ignition delays to within 2 ms. The modeling included studies of total ignition delay as well as the ignition delay variation versus process variations. Microthrusters were then fabricated via printed circuit board lamination, and the ignition performance was characterized. The characterization showed agreement with modeling to within 2-sigma for most cases. And the characterization demonstrated that the ignition delay could be controlled to within 0.36 and 0.84 ms for the best case. Furthermore, this performance was demonstrated with a small battery supply (200–600 mAh) and minimal electronics in the ignition system. This work extends the use of current microthrusters to short-lifetime applications that need high forces delivered in millisecond time intervals.

Measurements of the auto-ignition of n-heptane/toluene mixtures using a rapid compression machine

Ignition delay times of undiluted stoichiometric n-heptane/toluene mixtures have been measured in a rapid compression machine (RCM) for pure n-heptane and for an increasing concentration of toluene by liquid volume (25–100%) in the mixture, mainly at two different molar densities (150 and 180 mol/m 3). Initial pressures and temperatures were varied respectively in the 0226–0298 bar range and in the 299–352 K range, in order to allow compressed pressures and temperatures respectively in the 7.95–10.38 bar range and in the 710–814 K range. No ignition was observed for pure toluene due most probably to the variation of the final temperature and pressure in the combustion chamber during the very long ignition times of pure toluene at the experimental conditions considered in this work. The ignition delay variation at fixed temperature and the relative variation of the two-stage ignition times are reported. It is shown how the mixture behavior seems to be governed by the n-heptane chemistry for low toluene amount, while an increase of toluene concentration causes longer ignition delay times. Numerical modeling of the experiments with a recent Lawrence Livermore chemical mechanism for surrogate fuels shows that the results agree fairly well in the case of low and mid proportion of toluene by liquid volume. The single-zone model has allowed to extend the temperature range of investigation, showing that the low temperature range is the most influenced by the presence of toluene.

Understanding Ignition Delay Effects With Pure Component Fuels in a Single-Cylinder Diesel Engine

In order to better understand how future candidate diesel fuels may affect combustion characteristics in diesel engines, 21 pure component hydrocarbon fuels were tested in a single-cylinder diesel engine. These pure component fuels included normal alkanes (C6–C16), normal primary alkenes (C6–C18), isoalkanes, cycloalkanes/-enes, and aromatic species. In addition, seven fuel blends were tested, including commercial diesel fuel, U.S. Navy JP-5 aviation fuel, and five Fischer–Tropsch synthetic fuels. Ignition delay was used as a primary combustion metric for each fuel, and the ignition delay period was analyzed from the perspective of the physical delay period followed by the chemical delay period. While fuel properties could not strictly be varied independently of each other, several ignition delay correlations with respect to physical properties were suggested. In general, longer ignition delays were observed for component fuels with lower liquid fuel density, kinematic viscosity, and liquid-air surface tension. Longer ignition delay was also observed for component fuels with higher fuel volatility, as measured by boiling point and vapor pressure. Experimental data show two regimes of operation: For a carbon chain length of 12 or greater, there is little variation in ignition delay for the tested fuels. For shorter chain lengths, a fuel molecular structure is very important. Carbon chain length was used as a scaling variable with an empirical factor to collapse the ignition delay onto a single trend line. Companion detailed kinetic modeling was pursued on the lightest fuel species set (C6) since this fuel set possessed the greatest ignition delay differences. The kinetic model gives a chemical ignition delay time, which, together with the measured experimental ignition delay, suggests that the physical and chemical delay period have comparable importance. However, the calculated chemical delay periods capture the general variation in the overall ignition delay and could be used to predict the ignition delay of possible future synthetic diesel fuels.

Homogeneous charge compression ignition of binary fuel blends

The present experimental investigation aims to understand the homogeneous combustion chemistry associated with binary blends of three surrogate components for practical fuels, including toluene, isooctane, and diisobutylene-1 (DIB-1). Specifically, high-pressure autoignition characteristics of the three neat fuel components as well as the fuel blends of toluene + isooctane and toluene + DIB-1 are studied herein. Experiments are conducted in a rapid compression machine at compressed pressures varying from 15 to 45 bar and under low to intermediate temperatures. To obtain insights into interactions among fuels, the relative proportion of the two neat fuels in the reactive mixtures is systematically varied, while the total fuel mole fraction and equivalence ratio are kept constant. Experimental results demonstrate that ignition delays for neat toluene are more than an order of magnitude longer than those for neat isooctane. Whereas DIB-1 has ignition delays shorter than those for isooctane at higher temperatures, at temperatures lower than 820 K DIB-1 shows a longer ignition delay. Although the ignition delays of binary blends lie in between the two extremes of neat components, the variation of ignition delay with the relative fuel proportion is seen to be highly nonlinear. Especially, a small addition of isooctane or DIB-1 to toluene can result in greatly enhanced reactivity. In addition, the effect of DIB-1 addition to toluene is more significant than the effect of isooctane addition. Furthermore, in the compressed temperature range from 820 to 880 K, ignition delay of the toluene + isooctane blend shows greater sensitivity to temperature than that of isooctane.

Oxidizer Flow Effects on the Flammability of Solid Combustibles

A study is presented on the effect of oxidizer flow characteristics on the piloted ignition and opposed flame spread of a slab of PMMA heated by an external radiant flux. The objective is to establish the basis for a potential new lest method that may be used to determine the flammability performance of solid materials in terms of their ignition delay, critical heat flux for ignition, and flame-spread rate, for varied oxidizer flow conditions. The proposed Forced-flow Ignition and flame-Spread Test (FIST) follows the concepts of the LIFT (ASTM E 1321-93), and thus consists of a combination of ignition delay and flame spread experiments as a function of an externally applied radiant flux, but incorporates controlled forced convection as the predominant mechanism for the gas-phase transport of heat and mass. PMMA slabs were tested to assess the applicability of the methodology, and results are presented for radiant fluxes ranging from 0 to 35 kW/m2, forced oxidizer flow velocities of 1.0, 1.75, and 2.5 m/s. and natural convection. Results of the variation of ignition delay as a function of free stream oxygen concentration (18 to 45%) are also presented. Following the LIFT methodology, the ignition delay and flame spread data are used to assemble flammability diagrams for PMMA at different oxidizer flow velocities. Additional natural convection tests were performed in the LIFT apparatus with the purpose of establishing a base line for comparison of results. It is shown that the resulting FIST and LIFT flammability diagrams are similar. However, the diagrams as well as the parameters extracted from them depend on the oxidizer flow velocity, resulting in families of flow-dependent flammability diagrams. As a result of the use of forced convection, it is considered that the FIST is suitable for characterizing solid material flammability in mixed regimes ranging from natural to predominantly forced convection, and at different oxygen concentrations. This could provide a means to more accurately rank the relative flammability of solid combustible materials that would be used in environments where the oxidizer flow differs from air in natural convection, such as areas with significant air currents, vitiated environments, or microgravity applications (space based facilities).

Cetane number vs. structure in paraffin hydrocarbons

By using a ‘reduced kinetic model’ for the spontaneous ignition of hydrocarbon–air mixtures, it is found that for n-paraffins in the diesel-fuel boiling range, there is a simple correlation between the ignition delay and the ratio of primary to secondary hydrogen atoms in the molecule. The reduced kinetic model also accounts successfully for the variation in ignition delay with chain-branching in isoparaffins, and for the acceleration of ignition in the presence of simple OH-radical sources.

Ignition of hydrogen-air mixing layer in turbulent flows

Autoignition of a hydrogen-air scalar mixing layer in homogeneous turbulence is studied using direct numerical simulation (DNS). An initial counterflow of unmixed nitrogen-diluted hydrogen and heated air is perturbed by two-dimensional homogeneous turbulence. The temperature of the heated airstream is chosen to be 1100 K, which is substantially higher than the crossover temperature at which the rates of the chain-branching and termination reactions are equal. Three different turbulence intensities are tested in order to assess the effect of the characteristic flow time on the ignition delay. For each condition, a simulation without heat release is also performed. The ignition delay determined with and without heat release is shown to be almost identical up to the point of ignition for all of the turbulence intensities tested, and the predicted ignition delays agree well within a consistent error band. It is also observed that the ignition kernel always occurs where hydrogen is focused, and the peak concentration of HO2 is aligned well with the scalar dissipation rate. The dependence of the ignition delay on turbulence intensity is found to be nonmonotonic. For weak to moderate turbulence, the ignition is facilitated by turbulence via enhanced mixing, while for stronger turbulence, whose timescale is substantially smaller than the ignition delay, the ignition is retarded due to excessive scalar dissipation, and hence diffusive loss, at the ignition location. However, for the wide range of initial turbulence fields studied, the variation in ignition delay due to the corresponding variation in turbulence intensity appears to be quite small.

The Spontaneous Ignition of Hydrocarbon Fuels in a Flowing System

An apparatus is described in which the ignition delays of fuel sprays have been measured over a range of pressures (30–60 atm.), temperatures (770–980°K), and velocities up to 70 ft/s (21 m/s), in a continuously flowing air stream. Experiments have been made over this range of conditions with 21 hydrocarbon fuels and the appropriate values of ignition delay for any given engine or fuel can be interpolated. For the engine designer, equations are recommended relating ignition delay, air pressure, and temperature for a range of commercial fuels from diesel fuel to a premium grade gasoline. Curves are presented indicating the variation of ignition delay with cetane number and octane number, and a relationship between these two parameters has been derived.