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The effect of Di-tertiary Butyl Peroxide (DTBP) on Primary Reference Fuels (PRFs) in Homogeneous Charge Compression Ignition (HCCI) engines was investigated numerically and was compared with trends from previous experimental observations. A detailed kinetic mechanism for PRF combustion containing more than a thousand species and four thousand reactions was combined with a twenty one species, sixty-nine reaction mechanism for DTBP decomposition. This mechanism predicted the observed experimental trends reasonably well and was used to examine how DTBP addition acts to advance combustion timing and to induce hot ignition for lean and high octane number mixtures. The study suggests that DTBP's predominant mode of action for low Octane Number (ON) fuels is thermal, while for high ON fuels it is chemical. The extended kinetic model compiled for this study and the results obtained can be used to aid in the understanding and development of tailored additives for HCCI engines.

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.

This paper examines the similarity in the pre-ignition chemistry and reactivity behavior of two and four-stoke homogenous charge engines, analyzes the sources of UHC and CO, and describes an approach with the potential to substantially reduce UHC and CO in these systems. The approach is based on experiments on a two-stroke engine at no load conditions and on a four-stroke engine both with high levels of internal exhaust gas recirculation (EGR). By increasing internal EGR in an unloaded spark ignited two-stroke engine, UHC was reduced from 7800 ppm to 3000 ppm, CO decreased from 3% to 0.2%, and cyclic variability was diminished. These results demonstrate that stable engine operation can be obtained with additional internal EGR. Similar improvements were obtained at stoichiometric and lean conditions. Further experiments and modeling indicate that the main source of UHC and CO emissions is the fuel trapped in crevices.

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.

The role of post-combustion oxidation in influencing exhaust hydrocarbon emissions from spark ignition engines has been identified as one of the major uncertainties in hydrocarbon emissions research [l]*. While we know that post-combustion oxidation plays a significant role, the factors that control the oxidation are not well known. In order to address some of these issues a research program has been initiated at Drexel University. In preliminary studies, seven gaseous fuels: methane, ethane,ethene,propane,propene, n-butane, 1-butene and their blends were used to examine the effect of fuel structure on exhaust emissions. The results of the studies presented in an earlier paper [2] showed that the effect of fuel structure is manifested through its effect on the post-combustion environment and the associated oxidation process. A combination of factors like temperatures, fuel diffusion and reaction rates were used to examine and explain the exhaust hydrocarbon emission levels.

Time resolved exhaust port sampling results show that the gas mixture in the port at exhaust valve closing contains high concentrations of hydrocarbons. These hydrocarbons are mixed with hot in-cylinder gases during blowdown and can react either via gas phase kinetics in the exhaust port/runner system or subsequently on the exhaust catalyst before they are emitted. Studies were conducted on a single cylinder, four stroke engine in our laboratory to determine the interaction between the hot blowdown gases and the hydrocarbons which remain in the exhaust port. A preselected concentration and volume of hydrocarbon tracers (propane, propene, n-butane, and 1-butene) in either oxygen/nitrogen mixtures or pure nitrogen were injected into the exhaust port just behind the exhaust valve to control the initial conditions for any potential oxidation in the port.

Inevitably a fraction of the hydrocarbon fuel in spark ignition engines escapes in-cylinder combustion and flows out with the burned products. Post combustion oxidation in the cylinder and exhaust port may consume a part of this fuel and plays an important role in determining exhaust emission levels. This paper presents results from experiments designed to identify the factors that control post-combustion oxidation. Regulated exhaust components and detailed hydrocarbon species were measured using seven neat hydrocarbons and four blends as fuel. The fuels were selected to compare the relative rates of mixing and chemical kinetics. The results indicate that exhaust temperature, diffusion rates and fuel kinetics each play a complicated role in determining emission levels.

A reduced chemical kinetic model by Li et al. [1]* for predicting primary reference fuels' reactivity and autoignition behavior was modified to apply to the butanes, and it was correlated to experimental results from the non-fired engine cycles under skip fired conditions. The fuels examined in this work were neat n-butane and n-butane/iso-butane blends (10, 20, and 48 percent by volume iso-butane). In our initial work using measured pressure data from the first skip cycle, we modified Li et al.'s model by only adjusting the fuel specific rate parameters of the alkylperoxy radical (RO2·) isomerization reaction, the reaction of aldehydes with OH·, and the reaction forming cyclic ethers. In this work, analysis was extended to the second skip cycle and additional oxidation rate parameters with high fuel sensitivity were adjusted. Several reactions, which are not significant in butane oxidation, were temporarily made to be inactive in the model.

Recently, we reported the development of a new reduced chemical kinetic model for predicting reactivity and autoignition behavior of primary reference fuels in a motored research engine. The predicted oxidation behavior (ignition delay, preignition heat release, and evolution of key chemical species) is in fairly good agreement with experiments. In addition, the model reproduced the experimentally observed dependence of overall reactivity on charge density and manifold inlet conditions. This paper reports our initial effort to apply this new reduced chemical kinetic model to other fuels. Specifically, the model was tested using neat n-butane and n-butane/iso-butane blends (10, 20, and 48 percent by volume iso-butane) under skip fired conditions. The only adjustments made in the model were to the fuel specific rate parameters of the RO2· isomerization reaction, the reaction of aldehydes with OH·, and the reaction forming cyclic ethers.

A reduced chemical kinetic model has been developed for the prediction of major oxidation behavior of primary reference fuels (PRF's) in a motored engine, including ignition delay, preignition heat release, fuel consumption, CO formation and production of other species classes. This model consists of 29 reactions with 20 active species and was tuned to be applicable for the neat PRF's, 87 PRF and 63 PRF, and at various engine conditions. At the motored engine condition where detailed species data were generated, the model reproduces the ignition delay and the preignition heat release quite well (to within 15%). Fuel consumption and CO formation predictions differed from experiments by at most 25% for all of the four fuels. Predictions for other species classes generally agreed with experiments. As inlet temperature was varied, the experimentally observed negative temperature coefficient (NTC) behavior of iso-octane and 87 PRF was reproduced by the model.

This experimental study was conducted in a motored research engine to investigate the effect of blending methanol and ethanol on hydrocarbon oxidation and autoignition. An 87 octane mixture of primary reference fuels, 87 PRF, was blended with small percentages of the alcohols to yield a constant gravimetric oxygen percentage in the fuel. The stoichiometric fuel mixtures and neat methanol and ethanol were tested in a modified single-cylinder engine at a compression ratio of 8.2. Supercharging and heating of the intake charge were used to control reactivity. The inlet gas temperature was increased from 325 K to the point of autoignition or the maximum achievable temperature of 500 K. Exhaust carbon monoxide levels and in-cylinder pressure histories were monitored in order to determine and quantify reactivity.

Autoignition chemistry of n-heptane, iso-octane and an 87 octane blend, 87 PRF, was studied in a single-cylinder modified Wisconsin model AENL engine under motored conditions. Use of a fast-acting sampling valve and gas chromatographic analysis allowed measurement of in-cylinder gas composition during the ignition process. Crank angle resolved species evolution profiles were generated for all three fuels at a fixed inlet temperature of 376 K. For n-heptane, the measurements were made during a cyclically repeatable two stage ignition process up to the point of hot ignition (the second stage ignition). These n-heptane experiments were run at ø = 0.3 to avoid excessive pressure rise at hot ignition which might damage our engine. iso-Octane and 87 PRF were run at stoichiometric equivalence ratio which did not have a second stage ignition, and species were measured only during the first stage of ignition.

This paper presents results of studies investigating the effect of octane enhancing ethers on the reactivity of an 87 octane mixture of primary reference fuels, 87 PRF, in a motored engine. 87 PRF was blended with small percentages of MTBE, ETBE, TAME and DIPE based on a constant gravimetric oxygen percentage in the fuel. The experiments were conducted in a modified single-cylinder Wisconsin AENL engine at compression ratios of 5.2 and 8.2. Supercharging and heating of the intake charge were used to control reactivity. The inlet gas temperature was increased from 320 K, where no reactivity occurred, until either autoignition occurred or the maximum temperature of the facility was reached. Exhaust carbon monoxide levels and in-cylinder pressure histories were monitored in order to determine and quantify reactivity.

The detailed chemical reactions leading to autoignition of n-pentane are investigated in this study. A single-cylinder engine operating in a nonfired mode was used. The engine is supercharged and the temperature of the inlet fuel/air mixture is varied. By increasing the inlet manifold temperature, at a given inlet manifold pressure, the fuel/air mixture can be made to undergo autoignition. In-cylinder pressure and temperature profiles were measured. Gas samples from the combustion chamber were extracted and analyzed using gas chromatography techniques. The detailed chemical reaction mechanisms explaining the products from the different stages of the fuel oxidation process are presented. It is speculated that the generation of OH radicals from the peroxide (QOOH) decomposition is responsible for the autoignition of the n-pentane fuel/air mixture.