Fabian Mauss

Supercharged Homogeneous Charge Compression Ignition

The Homogeneous Charge Compression Ignition (HCCI) is the third alternative for combustion in the reciprocating engine. Here a homogeneous charge is used as in a spark-ignited engine, but the charge is compressed to autoignition as in a diesel. The main difference compared with the Spark Ignition (SI) engine is the lack of flame propagation and hence the independence from turbulence. Compared with the diesel engine, HCCI has a homogeneous charge and hence no problems associated with soot and NOdx formation. Earlier research on HCCI showed high efficiency and very low amounts of NOdx, but HC and CO were higher than in SI mode. It was not possible to achieve high IMEP values with HCCI, the limit being 5 bar. Supercharging is one way to dramatically increase IMEP. The influence of supercharging on HCCI was therefore experimentally investigated. Three different fuels were used during the experiments: iso-octane, ethanol and natural gas. Two different compression ratios were used, 17:1 and 19:1. The inlet pressure conditions were set to give 0, 1, or 2 bar of boost pressure. The highest attainable IMEP was 14 bar using natural gas as fuel at the lower compression ratio. The limit in achieving even higher IMEP was set by the high rate of combustion and a high peak pressure. Numerical calculations of the HCCI process have been performed for natural gas as fuel. The calculated ignition timings agreed well with the experimental findings. The numerical solution is, however, very sensitive to the composition of the natural gas. (Less)

Investigation of combustion emissions in a homogeneous charge compression injection engine: Measurements and a new computational model

The CO and hydrocarbon emissions of a homogeneous charge compression injection engine have been explained by inhomogeneities in temperature induced by the boundary layer and crevices according to a stochastic reactor model. The boundary layer is assumed to consist of a thin film (laminar sublayer) and a turbulent buffer layer. The heat loss through the cylinder wall leads to a significant temperature gradient in the boundary layer. The partially stirred plug flow reactor (PaSPFR) model, a stochastic reactor model (SRM), has been used to model turbulent mixing between the boundary layer, crevices, and the turbulent core and to account for the chemical reactions within the combustion chamber. The combustion of natural gas in the engine is described by a detailed chemical mechanism that is incorporated in the SRM. Molecular diffusion induced by turbulent mixing is described by the simple interaction by exchange with the mean (IEM) mixing model. The turbulent mixing intensity that describes the decay of the species and temperature fluctuations is estimated from measurements of the velocity fluctuations and the integral length scale of the turbulent flow in the engine. Pressure, CO emissions, and unburned hydrocarbons are also measured. Comparison between the mean quantities obtained from the SRM and these measurements show very good agreement. It is demonstrated that the SRM clearly outperforms a previous PFR-based one-zone model. The PaSPFR-IEM model captures the pressure rise that could not be described exactly using a simple one-zone model. The emissions of CO and hydrocarbons are also predicted well. Scatter plots of the marginal probability density function of CO 2 and temperature reveal that the emissions of hydrocarbons and CO can be explained by stochastic particles that undergo incomplete combustion because they are trapped in the colder boundary layer or in the crevices.

Automatic reduction procedure for chemical mechanisms applied to premixed methane/air flames

An existing skeletal mechanism for laminar premixed methane/air flames has been used as a starting point for further automatic reduction by quasi-steady-state approximation (QSSA) for species with short chemical lifetimes and/or minor influence on the chemical system. Individual species are ranked with respect to static and dynamic characteristics according to a level of importance (LOI) measure obtained from their chemical lifetimes, diffusion velocities and flame-zone residence times in combination with a species sensitivity measure. The maximum element mass fraction and the maximum enthalpy occupied by a certain molecular species are constrained in order to limit the mass and energy deficiency caused by QSSA. Maximum values of lifetime and LOI are accumulated over the entire flame length for a range of fuel/air equivalence ratios . Species with low LOI are selected for QSSA, and their concentrations are calculated iteratively by solving the coupled algebraic system. Kinetic models with a varying degree of reduction are then automatically generated and implemented as FORTRAN source code by setting different lower LOI and element mass fraction limits. It is found that the lifetime and LOI measure differ due to the inclusion of sensitivity counteracting the rise in lifetime at low temperatures. The species ranking by the LOI disfavors reasonably stable species, which are removed from the system. The laminar burning velocities as predicted by the most strongly reduced mechanism with five global reaction steps show very good agreement with detailed calculations. The profiles of steady-state species also agree well if, the corresponding species lifetime is short.

Homogeneous Charge Compression Ignition Engine: A Simulation Study on the Effects of Inhomogeneities

A stochastic model for the HCCI engine is presented The model is based on the PaSPFR-IEM model and accounts for inhomogeneities in the combustion chamber while including a detailed chemical model for natural gas combustion, consisting of 53 chemical species and 590 elementary chemical reactions. The model is able to take any type of inhomogeneities in the initial gas composition into account, such as inhomogeneities in the temperature field, in the air-fuel ratio or in the concentration of the recirculated exhaust gas. With this model the effect of temperature differences caused by the thermal boundary layer and crevices in the cylinder for a particular engine speed and fuel to air ratio is studied. The boundary layer is divided into a viscous sublayer and a turbulent buffer zone. There are also colder zones due to crevices. All zones are modeled by a characteristic temperature distribution. The simulation results are compared with experiments and a previous numerical study employing a PFR model. In all cases the PaSPFR-IEM model leads to a better agreement between simulations and experiment for temperature and pressure. In addition a sensitivity study on the effect of different intensities of turbulent mixing on the combustion is performed. This study reveals that the ignition delay is a Junction of turbulent mixing of the hot bulk and the colder boundary layer. (Less)

ANALYSIS OF A NATURAL GAS FUELLED HOMOGENEOUS CHARGE COMPRESSION IGNITION ENGINE WITH EXHAUST GAS RECIRCULATION USING A STOCHASTIC REACTOR MODEL

Abstract Combustion and emissions formation in a Volvo TD 100 series diesel engine running in a homogeneous charge compression ignition (HCCI) mode and fuelled with natural gas is simulated and compared with measurements for both with and without external exhaust gas recirculation (EGR). A new stochastic approach is introduced to model the convective heat transfer, which accounts for fluctuations and fluid-wall interaction effects. This model is included in a partially stirred plug flow reactor (PaSPFR) approach, a stochastic reactor model (SRM), and is applied to study the effect of EGR on pressure, autoignition timing and emissions of CO and unburned hydrocarbons (HCs). The model accounts for temperature inhomogeneities and includes a detailed chemical mechanism to simulate the chemical reactions within the combustion chamber. Turbulent mixing is described by the interaction by exchange with the mean (IEM) model. A Monte Carlo method with a second-order time-splitting technique is employed to obtain the numerical solution. The model is validated by comparing the simulated in-cylinder pressure history and emissions with measurements taken from Christensen and Johansson (SAE Paper 982454). Excellent agreement is obtained between the peak pressure, ignition timing and CO and HC emissions predicted by the model and those obtained from the measurements for the non-EGR, 38 per cent EGR and 47 per cent EGR cases. A comparison between the pressure profiles for the cases studied reveals that the ignition timing and the peak pressure are dependent on the EGR. With EGR, the peak pressure reduces and the autoignition is delayed. The trend observed in the measured emissions with varying EGR is also predicted correctly by the model.

Evaluating the EGR-AFR Operating Range of a HCCI Engine

We present a computational tool to develop an exhaust gas recirculation (EGR) - air-fuel ratio (AFR) operating range for homogeneous charge compression ignition (HCCI) engines. A single- cylinder Ricardo E-6 engine running in HCCI mode, with external EGR is simulated using an improved probability density function (PDF)-based engine cycle model. For a base case, the in-cylinder temperature and unburned hydrocarbon emissions predicted by the model show a satisfactory agreement with measurements. Furthermore, the model is applied to develop the operating range for various combustion parameters, emissions and engine parameters with respect to the air-fuel ratio and the amount of EGR used. The model predictions agree reasonably well with the experimental results for various parameters over the entire EGR-AFR operating range thus proving the robustness of the PDF based model. The boundaries of the operating range namely, knocking, partial burn, and misfire are reliably predicted by the model. In particular, the model provides a useful insight into the misfire phenomenon by depicting the cyclic variation in the ignition timing and the in-cylinder temperature profiles. Finally, we investigate two control options, namely heating intake charge and trapping residual burned fraction by negative valve overlap. The effect of these two methods on HCCI combustion and CO, HC and NOdx emissions is studied. (Less)

Detailed Modeling of soot formation in a partially stirred plug flow reactor

The purpose of this work is to propose a detailed model for the formation of soot in turbulent reacting flow and to use this model to study a carbon black furnace. The model is based on a combination of a detailed reaction mechanism to calculate the gas phase chemistry, a detailed kinetic soot model based on the method of moments, and the joint composition probability density function (PDF) of these scalar quantities. Two problems, which arise when modeling the formation of soot in turbulent flows using a PDF approach, are studied. A consistency study of the combined scalar-soot moment approach reveals that the molecular diffusion term in the PDF-equation can be closed by the IEM and Curl-type mixing models. An investigation of different kernels for the collision frequency of soot particles shows that the influence of turbulence on particle coagulation is negligible for typical flame conditions and the particle size range considered. The model is used as a simple toot to simulate a furnace black process, which is the most important industrial process for the production of carbon blacks. Despite the simplifications in the modeling of the turbulent flow reasonable agreement between the calculated soot yield and data measured in an industrial furnace black reactor is achieved although no adjustments were made to the kinetic parameters of the soot model. The effect of the mixing intensity on soot yield and different soot formation rates is investigated. In addition the influence of different operating conditions such as temperature and equivalence ratio in the primary zone of the reactor is studied. (Less)

Detailed soot modeling in turbulent jet diffusion flames

An approach for modeling the interaction between the formation and oxidation of soot, the radiative heat loss, and the flowfield in turbulent jet diffusion flames is presented. These interactions are modeled by the flamelet library approach in the framework of prescribed probability density functions (PDFs). The formation and oxidation of soot is calculated from a detailed chemical soot model. The laminar flamelet concept is applied to model the rates of soot particle inception, soot volume dependent surface growth, and oxidation, as well as species and temperature fields, Radiative heat transfer from the soot particles and the gas-phase species, CO2 and H2O, decreases the peak flame temperature, which in turn influences the flamelet structures. Experiments from Young et al. on a turbulent ethylene diffusion flame are used to validate the modeling approach. The calculated fields of mean mixture fraction, temperature, and soot volume fraction are found to be in agreement with the experimental data. The spatially resolved rates of soot inception, surface growth, and oxidation are presented. The maximum rates of the surface independent production occur in the fuel-rich region at a radial position of about 10 mm. In contrast, the maximum rates of surface growth and oxidation are found on the centerline with the oxidation occurring at a higher location in the flame. The total rate has its maximum on the centerline, whereas soot formation and destruction balances on the slightly rich side near the stoichiometric contour. This shows the strong interaction of soot formation and oxidation in the flame. A sensitivity analysis of the calculated soot volume fraction on different model parameters is presented. The rate coefficient of the heterogeneous surface growth reaction is the most sensitive parameter. A 20% increase of this rate leads to a 65% increase of the calculated maximum soot volume fraction.

Comparison of automatic reduction procedures for ignition chemistry

In this paper, we present a comparison between the reduced mechanisms obtained through a computational singular perturbation method (CSP) and the reduced mechanisms obtained through a lifetime analysis based only on the diagonal elements of the Jacobian matrix and a species sensitivity The two methods are used for the analysis of autoignition, which is an interesting test situation because of the sensitivity of ignition to the radical pool and the smaller range of timescales expected. It is found that the steady-state species selected by the two methods are in good agreement. The mechanisms are reduced to a 10-step mechanism when CSP is applied and an 11-step mechanism in the case of the simpler lifetime analysis. Both mechanisms are compared with the detailed mechanism and experimental data and are found to reproduce the physical and chemical parameters very well. This shows that for a large part of the timescale range, the system is close to linear. The comparison shows the advantage of the CSP method as being somewhat more accurate. However, the simpler lifetime analysis is of sufficient accuracy and of more convenience when applied to a system requiring a considerable reduction in computational time, as is the case when applying online reduction. (Less)

Development of adaptive kinetics for application in combustion systems

In this paper, an automatic method for reducing chemical mechanisms during run time based on the quasi-steady-state assumption (ASSA) is presented. The method uses a lifetime analysis of the chemical species which can be set to steady state according to a ranking procedure. Steady-state species concentrations are computed by algebraic rather than differential equations, thus yielding a significant reduction in the computational effort. In contrast to previous reduction schemes in which chemical species were selected only when they were in steady state throughout the whole process, the present method allows for species to be selected at each operating point separately generating an adaptive chemical kinetics scheme. The mechanism can change during the simulation run. This ensures that the optimal reduced mechanism is used at each time step leading to a very efficient and accurate procedure. The method is used for calculations of a natural gas fueled engine operating under homogeneous charge compression ignition (hCCI) conditions. We discuss criteria for selecting steady-state species and the influence of these criteria on the results, such as concentration profiles and temperature. A full mechanism with 53 species can be reduced to a minimun of 14 non-steady-state species while still reproducing the physical behavior of the detailed mechanism with good agreement.