Search Results

Now more than ever, the increasing strictness of environmental regulation and the stronger need of higher efficiency standards are pushing for the development of cleaner and energy-efficient powertrains. HCCI engines are suitable candidates to achieve these objectives. Understanding the autoignition process and how it is affected by operating conditions is central to the development of these engines. In addition to experiments, detailed kinetic modeling represents a very effective tool for gaining deeper insight into the fundamentals of HCCI autoignition and combustion. Indeed, modeling activities are today widely used in engine design, allowing a significant reduction in prototype development costs and providing a valuable support to the improvement of control strategies.

The ignition delay times of toluene-oxygen-argon mixtures with fuel equivalence ratios from 0.5 to 1.5 and concentrations of toluene from 0.1 to 2.0% were measured behind reflected shock waves for temperatures 1270 to 1755 K and at a pressure of 2.4 ± 0.7 atm. A detailed chemical kinetic model has been developed on the basis of a kinetic mechanism proposed by Pitz et al. [1] to reproduce our experimental results as well as some literature data obtained in other shock tubes at pressures from 1.1 to 50 atm. It is found that the present chemical kinetic model could give better agreement on the pressure dependence of the ignition delay times than the previously proposed kinetic models.

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.

This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the two-stage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors. A framework is also proposed for understanding the heat release and emission formation processes.

The dynamic stage of combustion - the intrinsic process for pushing the compression polytrope away from the expansion polytrope to generate the indicator work output of a piston engine - was studied to reveal the influence of the air/fuel ratio on the effectiveness with which the fuel was utilized. The results of tests carried out for this purpose, using a 12 liter diesel engine, were reported last year [SAE 1999-01-0517]. Presented here is an analytic interpretation of the data obtained for part-load operation at 1200 and 1800 rpm. A solution is thus provided for an inverse problem: deduction of information on the dynamic features of the exothermic process of combustion from measured pressure record. Provided thereby, in particular, is information on the effectiveness with which fuel was utilized in the course of this process - a parameter reflecting the effect of energy lost by heat transfer to the walls.

Homogeneous Charge Compression Ignition (HCCI) engines are currently of great interest as a future alternative to Diesel and Spark Ignition engines because of HCCI's potential to achieve high efficiency with very low NOx emissions. However, significant technical barriers remain to practical implementation of HCCI engines: difficult-to-control combustion, low power density, rapid pressure rise, and high hydrocarbon and carbon monoxide emissions. To overcome some of these barriers, operational strategies that involve relaxing the constraint of truly “homogeneous” HCCI combustion have been studied. The phrase “Premixed Charge Compression Ignition” or “PCCI” combustion can be used to describe this class of combustion processes, in which combustion occurs similarly to HCCI engines as a non-mixing controlled, chemical kinetics dominated, auto-ignition process, but the fuel, air, and residual gas mixture need not be homogeneous.

This research investigates how the handling of mixing and heat transfer in a multi-zone kinetic solver affects the prediction of carbon monoxide and hydrocarbon emissions for simulations of HCCI engine combustion. A detailed kinetics multi-zone model is now more closely coordinated with the KIVA3V computational fluid dynamics code for simulation of the compression and expansion processes. The fluid mechanics is solved with high spatial and temporal resolution (40,000 cells). The chemistry is simulated with high temporal resolution, but low spatial resolution (20 computational zones). This paper presents comparison of simulation results using this enhanced multi-zone model to experimental data from an isooctane HCCI engine.

Previous studies have shown that fuels with higher laminar flame speed also have increased tolerance to EGR dilution. In this work, the effects of fuel laminar flame speed on both lean and EGR dilute spark ignition combustion stability were examined. Fuels blends of pure components (iso-octane, n-heptane, toluene, ethanol, and methanol) were derived at two levels of laminar flame speed. Each fuel blend was tested in a single-cylinder spark-ignition engine under both lean-out and EGR dilution sweeps until the coefficient of variance of indicated mean effective pressure increased above thresholds of 3% and 5%. The relative importance of fuel laminar flame speed to changes to engine design parameters (spark ignition energy, tumble ratio, and port vs. direct injection) was also assessed.

The ignition delay times of primary reference fuel (PRF) and toluene mixtures have been measured behind the reflected shock waves. The range of experiments covered combustion of fuel in diluted argon for stoichiometric mixtures, pressures of 2.5 atm, temperatures from 1200-1600 K, 0.4% of fuel concentration. The ignition delay times of n-heptane increased with the addition of toluene. However the ignition delay times of iso-octane decreased with the addition of toluene from 0 to 50% and increased from 50 to 100%. A detailed kinetic model with cross reactions considered in this study can not reproduce the trend of ignition delay times for iso-octane/toluene mixtures. From the reaction path analysis, it was suggested that cross reactions between alkenes and aromatics are required to account for these experimental results.

This research investigates a control system for HCCI engines, where equivalence ratio, fraction of EGR and intake pressure are adjusted as needed to obtain satisfactory combustion. HCCI engine operation is analyzed with a detailed chemical kinetics code, HCT (Hydrodynamics, Chemistry and Transport), that has been extensively modified for application to engines. HCT is linked to an optimizer that determines the operating conditions that result in maximum brake thermal efficiency, while meeting the peak cylinder pressure restriction. The results show the values of the operating conditions that yield optimum efficiency as a function of torque and rpm. The engine has high NOx emissions for high power operation, so the possibility of switching to stoichiometric operation for high torque conditions is considered. Stoichiometric operation would allow the use of a three-way catalyst to reduce NOx emissions to acceptable levels.

Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO.

We have developed an artificial neural network (ANN) based combustion model and have integrated it into a fluid mechanics code (KIVA3V) to produce a new analysis tool (titled KIVA3V-ANN) that can yield accurate HCCI predictions at very low computational cost. The neural network predicts ignition delay as a function of operating parameters (temperature, pressure, equivalence ratio and residual gas fraction). KIVA3V-ANN keeps track of the time history of the ignition delay during the engine cycle to evaluate the ignition integral and predict ignition for each computational cell. After a cell ignites, chemistry becomes active, and a two-step chemical kinetic mechanism predicts composition and heat generation in the ignited cells. KIVA3V-ANN has been validated by comparison with isooctane HCCI experiments in two different engines.

This paper shows a numerical evaluation of fuels and additives for HCCI combustion. First, a long list of candidate HCCI fuels is selected. For all the fuels in the list, operating conditions (compression ratio, equivalence ratio and intake temperature) are determined that result in optimum performance under typical operation for a heavy-duty engine. Fuels are also characterized by presenting Log(p)-Log(T) maps for multiple fuels under HCCI conditions. Log(p)-Log(T) maps illustrate important processes during HCCI engine operation, including compression, low temperature heat release and ignition. Log(p)-Log(T) diagrams can be used for visualizing these processes. The paper also includes a ranking of many potential additives. Experiments and analyses have indicated that small amounts (a few parts per million) of secondary fuels (additives) may considerably affect HCCI combustion and may play a significant role in controlling HCCI combustion.

To facilitate the growing interest in hydrogen combustion for internal combustion engines, computer models are being developed to simulate gaseous fuel injection, air entrainment and the ensuing combustion. This paper introduces a new method for modeling the injection and air entrainment processes for gaseous fuels. Modeling combustion is not covered in this paper. The injection model uses a gaseous sphere injection methodology, similar to liquid droplet injection techniques used for liquid fuel injection. In this paper, the model concept is introduced and model results are compared with correctly- and under-expanded experimental data.

Homogeneous charge compression ignition (HCCI) is a new combustion technology that may develop as an alternative to diesel engines with high efficiency and low NOx and particulate matter emissions. This paper describes the HCCI research activities being currently pursued at Lawrence Livermore National Laboratory and at the University of California Berkeley. Current activities include analysis as well as experimental work. On analysis, we have developed two powerful tools: a single zone model and a multi-zone model. The single zone model has proven very successful in predicting start of combustion and providing reasonable estimates for peak cylinder pressure, indicated efficiency and NOx emissions. This model is being applied to develop detailed engine performance maps and control strategies, and to analyze the problem of engine startability. The multi-zone model is capable of very accurate predictions of the combustion process, including HC and CO emissions.

This work investigates a control system for HCCI engines, where thermal energy from exhaust gas recirculation (EGR) and compression work in the supercharger are either recycled or rejected as needed. HCCI engine operation is analyzed with a detailed chemical kinetics code, HCT (Hydrodynamics, Chemistry and Transport), that has been extensively modified for application to engines. HCT is linked to an optimizer that determines the operating conditions that result in maximum brake thermal efficiency, while meeting the restrictions of low NOx and peak cylinder pressure. The results show the values of the operating conditions that yield optimum efficiency as a function of torque and RPM. For zero torque (idle), the optimizer determines operating conditions that result in minimum fuel consumption. The optimizer is also used for determining the maximum torque that can be obtained within the operating restrictions of NOx and peak cylinder pressure.

Single cylinder engine experiments and chemical kinetic modeling have been performed to study the effect of variations in fuel, equivalence ratio, and intake charge temperature on the start of combustion and the heat release rate. Neat propane and a fuel blend of 15% dimethyl-ether in methane have been studied. The results demonstrate the role of these parameters on the start of combustion, efficiency, imep, and emissions. Single zone kinetic modeling results show the trends consistent with the experimental results.

Homogenous Charge Compression Ignition (HCCI) is a new engine technology with fundamental differences over conventional engines. HCCI engines are intrinsically fuel flexible and can run on low-grade fuels as long as the fuel can be heated to the point of ignition. In particular, HCCI engines can run on “wet ethanol:” ethanol-in-water mixtures with high concentration of water, such as the high water content ethanol-in-water mixture that results from fermentation of corn mash. Considering that much of the energy required for processing fermented ethanol is spent in distillation and dehydration, direct use of wet ethanol in HCCI engines considerably shifts the energy balance in favor of ethanol.

A study of the influence of dilution, attained by air excess, upon the dynamic stage of combustion - the nucleus of a work producing cycle - in a diesel engine, is reported as a sequel of SAE 2000-01-0203. While the latter has been restricted to variation in dilution obtained by bleeding air compressed by the supercharger, here the scope of engine tests was expanded by incorporating an additional stage of compression. Besides revealing the mechanism of the dynamic stage, the paper demonstrates that its effectiveness is a linear function of the air excess coefficient, irrespectively how it is attained.

Injection spray dynamics is known to be of great importance when modeling turbulent multi-phase flows in diesel engines. Two key aspects of spray dynamics are liquid breakup and penetration, both of which are affected by the initial sizes of the injected droplets. In the current study, injection of liquid n-heptane is characterized with initial droplet sizes with diameters on the order of 0.10 - 0.25 nozzle diameters. This is done for a Reynolds Averaged Navier-Stokes (RANS) RNG k-ε turbulence model with a minimum grid size of 125 μm and for a Large Eddy Simulations (LES) viscosity turbulence model with a minimum grid size of 62.5 μm. The results of both turbulence models are validated against non-reacting experimental data from the Engine Combustion Network (ECN). The results show that the injected droplet sizes have a significant impact on both liquid and vapor penetration lengths.