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In this study a 1-dimensional computational model of a Fe-Zeolite catalyst, implementing conservation of mass, species and energy for both gas and catalyst surface phases has been developed to simulate emissions conversion performance. It is applied to both a fresh catalyst and one that has been aged through exposure to the exhaust system of a Heavy Duty Diesel engine performing in the field for 376K miles. Details of the chemical kinetics associated with the various NOx reduction reactions in the two Fe-Zeolite configurations have been examined and correlated with data from a synthetic gas rig test bench. It was found that the Standard reaction, (4NH3 + 4NO + O2 → 2N2 + 6H2O), which is one of the main reactions for NOx reduction, degraded significantly at the lower temperatures for the aged system.

Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel.

The development of surrogate mixtures that represent gasoline combustion behavior is reviewed. Combustion chemistry behavioral targets that a surrogate should accurately reproduce, particularly for emulating homogeneous charge compression ignition (HCCI) operation, are carefully identified. Both short and long term research needs to support development of more robust surrogate fuel compositions are described. Candidate component species are identified and the status of present chemical kinetic models for these components and their interactions are discussed. Recommendations are made for the initial components to be included in gasoline surrogates for near term development. Components that can be added to refine predictions and to include additional behavioral targets are identified as well. Thermodynamic, thermochemical and transport properties that require further investigation are discussed.

One option for ignition control of Homogeneous Charge Compression Ignition (HCCI) engines is to use small amounts of ignition-enhancing additives to alter the ignition properties. Di-tertiary Butyl Peroxide (DTBP) is one such additive and it has been suggested as a cetane improver in diesel engines. In this study, the effects of DTBP on spark ignition (SI) primary reference fuels (PRFs, n-heptane and iso-octane) and their blends (PRF20, PRF50, PRF63, PRF87 and PRF92) were investigated during HCCI engine operation. Experiments were run in a single cylinder CFR research engine for three inlet temperatures (410, 450 and 500 K) and several equivalence ratios (0.28 - 0.57) at a constant speed of 800 rpm and a compression ratio of 16.0. Experimental results show that ignition delay time, cycle to cycle variation, and stable operating range were all improved with the addition of less than 2.5% DTBP by volume.

The generally accepted explanation of autoignition in engines is that the reactivity is driven by temperature, where autoignition occurs after the mixture has reached some critical temperature (approx. 1000 K) by a combination of self-heating due to preignition reactions and compression heating due to piston motion and flame propagation. During the course of our investigations into autoignition processes and homogeneous charge compression ignition we have observed some ignitions that begin at much lower temperature (< 550 K). In this paper we describe these observations, our attempts to investigate their origins, and an alternative explanation that proposes that traditional models may be missing the chemistry that explains this behavior. Finally, applications of lower temperature chemical reactions are discussed.

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.

This paper presents a new global reaction model to simulate the Homogeneous Charge Compression Ignition (HCCI) combustion process. The model utilizes seven equations and seven active species. The model includes five reactions that represent degenerate chain branching in the low temperature region, including chain propagation, termination and branching reactions and the reaction of HOOH at the second stage ignition. Two reactions govern the high temperature oxidation, to allow formation and prediction of CO, CO2, and H2O. Thermodynamic parameters were introduced through the enthalpy of formation of each species. We were able to select the rate parameters of the global model to correctly predict the autoignition delay time at constant density for n-heptane and iso-octane, including the effect of equivalence ratio.

NOx and soot emissions remain critical issues in diesel engines. One method to address these problems is to achieve homogeneous combustion at lower peak temperatures - the goal of research on controlled autoignition. In this paper n-heptane is used to represent a large hydrocarbon fuel and some of the effects of internal and external EGR, oxygen concentration, and engine speed on its combustion have been examined through simulation and experiment. Simulations were conducted using our existing skeletal chemical kinetic model, which combines the chemistry of the low, intermediate, and high temperature regimes. Experiments were carried out in a single cylinder, four-stroke, air cooled engine and a single cylinder, two stroke, water cooled engine. In the four-stroke engine experiments the effects of EGR were examined using heated N2 addition as a surrogate for external EGR and engine modifications to increase internal EGR.

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.

Homogeneous Charge Compression Ignition (HCCI) engines have the possibility of low NOx and particulate emissions and high fuel efficiencies. In HCCI the oxidation chemistry determines the auto-ignition timing, the heat release rate, the reaction intermediates, and the ultimate products of combustion. This paper reports an initial effort to apply our reduced chemical kinetic model to HCCI processes. The model was developed to study the pre-ignition characteristics (pre-ignition heat release and start of ignition) of primary reference fuels (PRF) and includes 29 reactions and 20 active species. The only modifications to the model were to make the proscribed adjustments to the fuel specific rate constants, and to enhance the H2O2 decomposition rate to agree with published data.

Recently, studies were conducted on a single cylinder, four stroke engine to investigate the effect of temperature and local mixedness on exhaust port hydrocarbon oxidation. To examine the effect of temperature, hydrocarbon tracers (propane, propene, 1-butene, n-butane, and n-pentane) were individually injected into the exhaust port just behind the exhaust valve for operating conditions that provided different exhaust port temperatures. For the local mixedness experiments, tracer mixtures (propane + n-butane, 1-butene + n-butane, propene + n-butane) were injected into the exhaust port just behind either a normal exhaust valve or a shrouded exhaust valve. The concentration of tracers and their reaction products were measured using gas chromatography of samples withdrawn from the exhaust stream. The tracer consumption behavior with changing port temperature confirmed that there is a minimum port temperature for hydrocarbon oxidation.