Nicholas P. Cernansky

Potential of Thermal Stratification and Combustion Retard for Reducing Pressure-Rise Rates in HCCI Engines, Based on Multi-Zone Modeling and Experiments

This work investigates the potential of in-cylinder thermal stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between thermal stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (Φ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing. For combustion phasing in the range of 7 - 10°CA after TDC, a linear thermal distribution is optimal since it produces a near-linear pressure rise. For other combustion phasings, non-linear distributions are required to achieve a linear pressure rise. Also, the total thermal width must be greater at higher fueling rates to avoid excessive pressure-rise rates. The study also shows that increasing the natural thermal width of the charge by 50% would allow the equivalence ratio to be increased from 0.44 to 0.60, with an associated increase of the IMEPg from 524 to 695 kPa for naturally aspirated operation. It was also found that the naturally occurring thermal stratification plays a major role in producing the experimentally observed benefit of combustion-timing retard for slowing the combustion rate. Reduced chemical-kinetic rates with combustion retard are found to play a lesser role.

A flow reactor study of neopentane oxidation at 8 atmospheres : Experiments and modeling

An existing detailed chemical kinetic reaction mechanism for neopentane oxidation [1] is applied to new experimental measurements taken in a flow reactor [2] operating at a pressure of 8 atm. The reactor temperature ranged from 620 K to 810 K and flow rates of the reactant gases neopentane, oxygen, and nitrogen were 0.285, 7.6, and 137.1 standard liter per minute (SLM), respectively, producing an equivalence ratio of 0.3. Initial simulations identified some deficiencies in the existing model and the paper presents modifications which included upgrading the thermodynamic parameters of alkyl radical and alkylperoxy radical species, adding an alternative isomerization reaction of hydroperoxy-neopentyl-peroxy, and a multistep reaction sequence for 2-methylpropan-2-yl radical with molecular oxygen. These changes improved the calculation for the overall reactivity and the concentration profiles of the following primary products: formaldehyde, acetone, isobutene; 3,3-dimethyloxetane, methacrolein, carbon monoxide, carbon dioxide, and water. Experiments indicate that neopentane shows negative temperature coefficient behavior similar to other alkanes, though it is not as pronounced as that shown by n-pentane for example. Modeling results indicate that this behavior is caused by the β-scission of the neopentyl radical and the chain propagation reactions of the hydroperoxyl-neopentyl radical.

1-Pentene oxidation and its interaction with nitric oxide in the low and negative temperature coefficient regions

Abstract The oxidation of 1-pentene and the effect of nitric oxide (NO) on this oxidation has been examined in a pressurized flow reactor facility. The experiments were conducted at 6 atmospheres over the temperature range of 600–800 K and an equivalence ratio of 0.4. 1-Pentene showed alkane type behavior exhibiting low temperature reactivity and strong negative temperature coefficient behavior. An examination of stable species composition revealed that hydrogen abstraction reactions leading to allyl radicals are more important than radical addition to the double bond. This is in contrast to previous studies on terminal olefins where the addition route was proposed as the dominant fuel consumption pathway. We believe that as carbon number of the terminal olefin increases alkane type reaction behavior occurs at the other end of the molecule and this is the reactivity observed under our experimental conditions. The presence of small concentrations of nitric oxide (0–500 ppm mole fraction) significantly altered 1-pentene oxidation at these temperatures. The effect of NO is a result of the competion between its promoting effect through reaction with HO 2 · radicals and its retarding effect through reaction with OH · radicals. Thus the effect depends strongly on the underlying fuel oxidation chemistry which generates the HO 2 · and OH · radicals and the concentration of the nitric oxide added. An explanation for this behavior is presented in the paper.

Chemistry of Fuel Oxidation Preceding End-Gas Autoignition

Abstract A comparison is made between measured species concentrations in a spark ignition engine and predictions from a numerical model using detailed chemical kinetics. Gas samples are extracted from the end gas at times just prior to autoignition of n-butane/air oriso-butane/air mixtures. Concentration histories of stable species are ohtained through gas chromatographic analysis. A detailed chemical kinetics model is used to predict species concentrations in an idealized end gas. The chemical reactions leading to formation of the relevant species are identified. The relative distribution of intermediate products predicted by the model is in good agrrement with the experimental measurements. Chemical kinetic differences between autoignition of n-butane, a straight chain hydrocarbon, and iso-butane. a branched chain hydrocarhon. are discussed.

A Skeletal Chemical Kinetic Model for the HCCI Combustion Process

ABSTRACT In Homogeneous Charge Compression Ignition (HCCI) engines, fuel oxidation chemistry determines the auto-ignition timing, the heat release, the reaction intermediates, and the ultimate products of combustion. Therefore a model that correctly simulates fuel oxidation at these conditions would be a useful design tool. Detailed models of hydrocarbon fuel oxidation, consisting of hundreds of chemical species and thousands of reactions, when coupled with engine transport process models, require tremendous computational resources. A way to lessen the burden is to use a “skeletal” reaction model, containing only tens of species and reactions. This paper reports an initial effort to extend our skeletal chemical kinetic model of pre-ignition through the entire HCCI combustion process. The model was developed from our existing preignition model, which has 29 reactions and 20 active species, to yield a new model with 69 reactions and 45 active species. The model combines the chemistry of the low, intermediate, and high temperature regions. All of the chemical reaction rate parameters come from published data. Simulations were compared with measured and calculated data from our engine operating at the following conditions: speed – 750 RPM, inlet temperature - 393 K to 453 K, fuel - 20 PRF, and equivalence ratio - 0.4 and 0.5. The simulations are generally in good agreement with the experimental data including temperature, pressure, ignition delay, and heat release. This demonstrates that the model has potential for predicting the behavior of HCCI engines, and may provide a way to include non-trivial chemistry in multi-zone CFD simulations.

Experimental studies of propane oxidation through the negative temperature coefficient region at 10 and 15 atmospheres

A turbulent, high-pressure flow reactor has been used in conjunction with a novel controlled cool-down (CCD) technique, in an experimental study of the detailed product distribution from propane oxidation at 10 and 15 atm, 600 K < T < 900 K, and equivalence ratio of 0.4. The species concentration profiles show the low-temperature hydrocarbon oxidation regime extending from approximately 680 to 770 K. They indicate peak species yields, corresponding to the maximum rate of reaction, occurring at approximately 720–723 K. At higher temperatures, approximately 730–780 K, reactivity slows and practically stops. Above 780 K, increasing species concentrations indicate the onset of intermediate temperature chemistry. Fundamental transitions in the reaction path and the dominant branching agent are shown by changes in the species yield profiles. Specifically, below 690 K, CO2 is observed to be the major product. Above 690 K, CO is the major product until approximately 740 K where propylene becomes the major product. The temperatures at which these transitions occur change with pressure. The observed transitions in the major products indicate shifts in the relative importance of the four chain branching mechanisms (i.e., branching via alkylhydroperoxide decomposition, acetaldehyde decomposition, acylhydroperoxide decomposition, and alkylhydroperoxy radical oxidation) brought about by both changes in temperature and pressure. The results of these experiments present a unique challenge to modeling the chemical kinetics because of the mechanistic transitions due to range of temperatures investigated in a single experiment.

Droplet size and equivalence ratio effects on spark ignition of monodisperse N-heptane and methanol sprays

The effects of the droplet size and the equivalence ratio on the ignition of monodisperse n-heptane and methanol sprays at atmospheric pressure were investigated using a capacitive discharge spark ignition system. The minimum ignition energy Emin of the sprays was measured over a range of droplet diameters, D = 30–57 μm, and equivalence ratios, φ = 0.44 – 1.8. As expected, results showed that Emin decreased with decreasing droplet size, with increasing equivalence ratio, and with increasing fuel volatility. While the same trends have been observed previously for polydisperse sprays, this is the first study to quantify these effects for monodisperse sprays in this size range. The minimum ignition energy was also measured for fully prevaporized n-heptane and methanol. An optimum gas phase equivalence ratio for prevaporized n-heptane ignition was found between 1.5 < φ < 2.0. No corresponding optimum was obtained for prevaporized methanol due to difficulties in generating fuel-rich methanol mixtures. comparison of the spray and prevaporized ignition results indicated the existence of an optimum droplet size, D < 30 μm, for the ignition of fuel-lean sprays for both fuels. Extension of the lean prevaporized ignition limit, φ = 0.55, was also observed for all sprays. Experimental ignition results were compared to the predictions of two existing ignition models for quiescent sprays: the characteristic time model for ignition of Peters and Mellor and the general ignition model of Ballal and Lefebvre. Both models, using a characteristic time approach, predicted the experimentally determined ignition energies accurately for most conditions. Model performance deteriorated, however, for leaner ration, φ < 0.7, and for smaller droplet size, D < 35 μm, with both models increasingly underpredicting Emin.

The oxidation of JP-8, Jet-A, and their surrogates in the low and intermediate Temperature regime at elevated pressures

Abstract The preignition reactivity behavior of five JP-8 samples, three Jet-A samples, one JP-7 sample, and four JP-8 surrogate mixtures was studied in a pressurized flow reactor in the low and intermediate temperature regime (600–800 K). A strong Negative Temperature Coefficient (NTC) behavior was observed to start near 692 K for all the fuels. No significant differences in the low-temperature oxidation behavior were observed between Jet-A and JP-8, suggesting that the two fuels can be used interchangeably for surrogate development. Two surrogates stood out as the best to mimic the low temperature reactivity of JP-8, the Hex-12 surrogate by Sarofim and coworkers (Eddings et al., 2005) and the S1 surrogate by Agosta (2002). The results showed that extreme care must be taken to ensure that the sample used for surrogate development is of average composition and properties, otherwise severe mistuning of the surrogate could occur.

Chemical kinetic modeling of high-pressure propane oxidation and comparison to experimental results

A pressure-dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high-pressure flow reactor. Experimental conditions range from 10–15 atm and 650–800 K and have a residence time of 198 ms for propane-air mixtures at an equivalence ratio of 0.4. The experimental results clearly indicate negative temperature coefficient (NTC) behavior. The chemistry describing this phenomenon is critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor. Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals, and important isomers (radicals) with O 2 were estimated using thermodynamic properties, with multifrequency quantum Kassel theory for k(E) coupled with modified, strong collision analysis for falloff. Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. Sensitivity analysis identified the key reaction steps that control the rate of oxidation in the NTC region. The model reasonably simulates the profiles for many of the major and minor species observed in the experiments. Numerical simulations show that some of the key reactions involving propylperoxy radicals are in partial equilibrium in this residence-time, temperature, and pressure regime. This indicates that their relative concentrations are controlled by a combination of thermochemistry and other rate-controlling reaction steps. Major reactions in partial equilibrium include C 3 H 7 +O 2 =C 3 H 7 O 2 , C 3 H 6 OOH=C 3 H 6 +HO 2 , and C 3 H 6 OOH+O 2 =O 2 C 3 H 6 OOH. This behavior means that thermodynamic parameters of the oxygenated species, which govern partial equilibrium concentrations, are especially important. QRRK/falloff results also show that the reaction of propyl radical and hydroperoxy-propyl radicals with O 2 proceeds, primarily, through pressure-stabilized adducts rather than chemically activated channels; thus, dissociation and isomerization rates of these adducts are important.

An Experimental Study of Propene Oxidation at Low and intermediate Temperatures

Abstract The oxidation of propene at low and intermediate temperatures (580–715 K) has been studied experimentally using a static reactor. Gas chromatographic analysis was used for stable species determination. The experimental results were used to postulate the main reaction paths of the mechanism. A region of negative temperature coefficient (NTC) was determined between 625–655 K and cool flames were indicated at temperatures below this range. Changes in the product distribution indicated that a transition in the mechanism was occurring across the region of NTC, from low to intermediate temperatures. The change in the mechanism with increasing temperature was attributed in part to the change in the position of equilibrium of CH3 + O2 = CH3O2, and also to the competition between radical addition to the propene and H abstraction from the propene.