Dionissios N. Assanis

A fully coupled computational fluid dynamics and multi-zone model with detailed chemical kinetics for the simulation of premixed charge compression ignition engines

Abstract Modelling the premixed charge compression ignition (PCCI) engine requires a balanced approach that captures both fluid motion as well as low- and high-temperature fuel oxidation. A fully integrated computational fluid dynamics (CFD) and chemistry scheme (i.e. detailed chemical kinetics solved in every cell of the CFD grid) would be the ideal PCCI modelling approach, but is computationally very expensive. As a result, modelling assumptions are required in order to develop tools that are computationally efficient, yet maintain an acceptable degree of accuracy. Multi-zone models have been previously shown accurately to capture geometry-dependent processes in homogeneous charge compression ignition (HCCI) engines. In the presented work, KIVA-3V is fully coupled with a multi-zone model with detailed chemical kinetics. Computational efficiency is achieved by utilizing a low-resolution discretization to solve detailed chemical kinetics in the multi-zone model compared with a relatively high-resolution CFD solution. The multi-zone model communicates with KIVA-3V at each computational timestep, as in the ideal fully integrated case. The composition of the cells, however, is mapped back and forth between KTVA-3V and the multi-zone model, introducing significant computational time savings. The methodology uses a novel re-mapping technique that can account for both temperature and composition non-uniformities in the cylinder. Validation cases were developed by solving the detailed chemistry in every cell of a KIVA-3V grid. The new methodology shows very good agreement with the detailed solutions in terms of ignition timing, burn duration, and emissions.

Characterizing the thermal sensitivity of a gasoline homogeneous charge compression ignition engine with measurements of instantaneous wall temperature and heat flux

Abstract An experimental study was performed to provide qualitative and quantitative insight into the thermal effects on a gasoline-fuelled homogeneous charge compression ignition (HCCI) engine combustion. The single-cylinder engine utilized exhaust gas rebreathing to obtain large amounts of hot residual gas needed to promote ignition. In-cylinder pressure, heat release analysis, and exhaust emission measurement were employed for combustion diagnostics. Fast response thermocouples were embedded in the piston top and cylinder head surface to measure instantaneous wall temperature and heat flux, thus providing critical information about the thermal boundary conditions and a thorough understanding of the heat transfer process. Two parameters determining thermal conditions in the cylinder, i.e. intake charge temperature and wall temperature, were considered and their effect on ignition and burning rate in an HCCI engine was investigated through systematic experimentation. The approach allowed quantitative analysis, and separating qualitatively different effects on the core gas temperature from the effects of near-wall temperature stratification. The results show great sensitivity to changes in wall temperature and such like, but a somewhat weaker effect of intake charge temperature on HCCI combustion. Variations of combustion phasing and peak burn rates due to wall temperature changes can be compensated if the intake charge temperature is varied in the opposite direction and with a factor of 1.11. The combustion stability limit of the HCCI engine depends more on wall temperature than on intake charge temperature. Analysis of a large number of individual cycles indicates that decreasing intake temperature retards timing, and the burn rates change primarily as a function of ignition timing. In contrast, lowering the wall temperature led to greater reduction in the bulk burn rate and greater increase in cyclic variability than expected simply as a result of retarded ignition, thus indicating significance of the thermal stratification in the near-wall boundary layer.

The effect of the stroke-to-bore ratio on combustion, heat transfer and efficiency of a homogeneous charge spark ignition engine of given displacement:

Abstract This study investigates how the selection of the stroke-to-bore (S/B) ratio affects combustion, heat transfer and overall efficiency in a homogeneous charge spark ignition (SI) engine of a given displacement. Initially, flame front area maps and wall areas in contact with burned gases are examined from a purely geometric point of view, for S/B ratios of 0.7, 1.0 and 1.3. Subsequently, a quasi-dimensional turbulent flame entrainment model is used to quantify the extent to which turbulence versus geometric factors are responsible for the observed combustion, heat transfer and cycle efficiency behaviour, as the S/B ratio varies. Calculations are performed for a range of engine speeds and loads, as well as for operation with 15 per cent exhaust gas recirculation (EGR). Results show that the S/B ratio has a significant effect on both turbulence levels and the geometric interaction of the flame front with the combustion chamber walls. In general, a longer stroke leads to higher thermal efficiency through faster burning and lower overall chamber heat loss. These effects are non-linear, being more dramatic when the S/B ratio is increased from below unity than from above unity. The potential of the long-stroke engine for brake fuel economy improvement can be exploited to the fullest at low speeds, while friction losses gradually diminish it at higher speeds.

An experimental investigation of the sensitivity of the ignition and combustion properties of a single-cylinder research engine to spark-assisted HCCI

Spark-assisted homogeneous charge compression ignition (HCCI) combustion may be a method to improve the operation of HCCI engines. In the current study, the impact of spark assist on the fundamental properties of ignition and combustion was investigated in a single cylinder, optically-accessible research engine. Early port fuel injection and air preheating were used with indolene fuel in the study. The effects of a range of air preheat (T in = 256–281 °C), fuel/air equivalence ratio (ϕ = 0.38–0.62) and spark assist timing (10°−90° before top dead centre) conditions on maximum in-cylinder pressure and timing, cycle variability, indicated mean effective pressure (IMEP) and heat release rate were investigated. Additionally, high-speed imaging was used to capture the piston-view ignition and combustion events during spark-assisted and unassisted HCCI operation. Methods were developed and applied to the imaging sequences to quantify the physical characteristics (e.g. location of autoignition sites) and the rate of propagation of the reaction fronts formed during spark-assisted and unassisted HCCI operation. The imaging data show that autoignition sites appear with increasing frequency as air preheat temperature is increased. The addition of spark assist led to the formation of reaction fronts at all conditions that propagated outward from the spark electrode at average speeds between 1.9 and 4.3 m/s. The imaging data indicate the effects of spark assist are due to compression heating of the unburned gases by the propagating reaction fronts which also leads to more consistent location of autoignition. Comparison of the imaging and engine data show the initial formation of the reaction fronts are not significant sources of heat release. While the engine data show that spark assist can affect phasing, heat release rate, IMEP and engine stability at the marginal HCCI operating conditions studied, the results also indicate spark assist has a narrow temperature range where the changes will be significant compared to the effects of the inherent thermal stratification of the HCCI fuel/air charge.

A one-dimensional lean NOx trap model with a global kinetic mechanism that includes NH3 and N2O

Abstract Eaton has developed an aftertreatment system for medium- and heavy-duty diesel engines in response to the US 2010 regulations. This system consists of a fuel reformer, a lean NO x trap (LNT), and an ammonia selective catalytic reduction (SCR) catalyst in series. A transient, one-dimensional model of the system was developed to improve system performance, reduce experimental testing, and optimize system design. In this paper, the LNT portion of this model is presented. The model simulates flow, heat transfer, and chemical reactions in the LNT catalyst. A global LNT chemical kinetic mechanism was developed to simulate the key catalytic processes with the minimum number of reactions. The model can be used to predict LNT catalyst performance over a range of operating conditions and driving cycles. Simulated species concentrations and gas temperatures at the LNT outlet were compared with experimental data at three steady state engine conditions over a 13-mode test. The conditions were chosen to develop and test the model over a range of gas temperatures, space velocities, and species concentrations. The LNT model predicts species trends and magnitudes with reasonable accuracy in comparison with experimental data. The simulated LNT NO x conversion efficiency over the 13-mode test was 67 per cent, compared with 63 per cent for the experiment.

MODELING OF CHEMICAL AND MIXING EFFECTS ON METHANE AUTOIGNITION UNDER DIRECT-INJECTION, STRATIFIED CHARGED CONDITIONS

Fundamental experiments and direct numerical simulations indicate that reaction rates during the ignition delay of direct-injection, stratified charge mixtures are determined by both chemical reaction and mixing rates. Consequently, the commonly used, purely chemistry-dependent approach for predicting ignition is no longer valid when the mixing rate limits the reaction rates. This work proposes an improved approach for predicting the reaction rates during ignition delay which accounts for both chemistry and mixing. Initially, reaction rates are determined using only the chemical reaction rate. Subsequently, a transition is made to the use of a modified eddy dissipation concept in which reaction rates are determined based on interaction between chemical reaction rates and mixing rates. To illustrate and validate the model, compression ignition under stratified charged conditions is considered for methane/air mixtures with temperatures ranging from 1200 to 1500 K. The predictions agree well with the experimental results qualitatively as well as quantitatively. Furthermore, by comparing our approach against a chemistry-only model, conditions where mixing plays an important role in predicting reaction rates during the ignition period are identified.

Deactivation of a diesel oxidation catalyst due to exhaust species from rich premixed compression ignition combustion in a light-duty diesel engine

Abstract Low-temperature premixed-charge compression ignition (PCI) can significantly reduce both nitric oxide and nitrogen dioxide (NO x ) and particulate matter emissions in compression ignition engines through a range of engine operating conditions. Exhaust hydrocarbons and carbon monoxide can be removed with a diesel oxidation catalyst (DOC). Although PCI normally utilizes a globally fuel-lean mixture, it is independent of equivalence ratio provided that local combustion temperatures are sufficiently low. A more fuel-rich PCI mode of operation could be useful in exhaust after-treatment strategies such as providing carbon monoxide and hydrocarbons for regeneration of a lean NO x trap (LNT). In a previous study, it was found that a rich PCI strategy deactivates a platinum-based DOC within seconds and may allow excessive harmful emissions to be passed into the environment. This study attempts to quantify the effects of different species representative of those found in rich PCI exhaust on a platinum-based DOC in a background of exhaust from an engine operating in a lean PCI regime. Excess carbon monoxide, propane, propylene, and methane were injected in varying concentrations while catalyst outlet temperature, carbon monoxide, and hydrocarbon conversion were measured for a period of 200 s. Of the injected species, it is shown that propylene has the greatest deactivation effect on the catalyst followed by carbon monoxide, both in terms of time and concentration. Propane is found not to deactivate the catalyst even in very globally fuel-rich conditions whereas methane acts as an inert gas over the catalyst in the temperature range of interest. It is concluded from the study that high concentrations of carbon monoxide do not act alone in the poisoning process for the rich PCI condition. The presence of some partial oxidation products such as unsaturated hydrocarbons can also have an adverse effect on DOC performance.

Modelling of heat transfer in internal combustion engines with variable density effect

Heat transfer is one of the major factors affecting the performance, efficiency, and emissions of internal combustion engines. As convection heat transfer is dominant in engine heat transfer, accurate modelling of the boundary layer heat transfer is required. In engine computational fluid dynamics (CFD) simulations, the wall function approach has been widely used to model the near-wall flow and temperature field. The present paper suggests a modified wall function approach to model heat transfer in internal combustion engines. Special emphasis has been placed on introducing the effect of variable density and variable viscosity in the model formulation. A non-dimensional temperature corrector is suggested to incorporate the variable density effect on the wall function approach. The suggested model is applied in KIVA-3V and is validated against experimental data of a homogeneous charge compression-ignition engine, showing improved predictions for pressure and emissions compared with the standard wall function model.

Speciated hydrocarbon emissions and the associated local ozone production from an automotive gasoline engine

Abstract Concentrations of total and individual exhaust gas hydrocarbon species were measured from a contemporary automotive gasoline engine to gain insight into how an engines operating conditions affect the combustion and post-flame oxidation processes and to estimate the ozone-forming potential of these hydrocarbons. Both the customary method of estimating maximum ozone production using maximum incremental reactivity (MIR) factors and a new method of estimating actual local ozone production (LOP) were used to quantify the harmfulness of the exhaust hydrocarbons. Depending on local atmospheric conditions, LOP estimations are about 2–10 times less than the method of maximum ozone production using MIR factors. Per unit of engine output, retarded spark timing and higher engine load reduce the LOP of catalyst-in hydrocarbons, while the air-fuel ratio does not strongly affect the LOP of catalyst-in hydrocarbons. LOP is increased during a start-up and drastically decreased by the catalytic converter, once the catalyst is heated to its operating temperature and the engine is run at stoichiometry.

Modeling of SCR NH3 Storage in the Presence of H2O

Diesel engines offer excellent fuel economy, but this comes at the expense of higher emissions of nitrogen oxides (NOx ) and Particulate Matter (PM). To meet current emissions standards, diesel engines require aftertreatment devices. Concepts using combinations of catalysts are becoming more common in aftertreatment systems to reduce the cost and size of these aftertreatment systems. One combination is an LNT-SCR system where the LNT releases NH3 during a regeneration to be used by the SCR catalyst for further NOx reduction. This involves rich-lean cycling of the exhaust stream, which alters species concentrations in the exhaust. Most notably H2 O and CO2 levels can vary from 4%–14% during lean-rich cycling. An investigation was performed using multiple Temperature Programmed Desorption (TPD) experiments to determine how H2 O and CO2 affect NH3 storage capacity of an Fe-based zeolite SCR catalyst. It was determined that H2 O and CO2 inhibit NH3 storage capacity of the SCR catalyst. This inhibition has shown a linear dependence on H2 O and CO2 concentration at constant temperature. It was also determined that H2 O is a much stronger inhibitor of NH3 storage capacity then CO2 . Additional Temperature Programmed Desorption (TPD) experiments, were run where H2 O and CO2 concentration (0%, 6%, and 10%) and the initial storage temperature (200°C, 250°C, 300°C, 350°C) were varied. Results suggest the addition of a reaction that creates competition for active sites on the catalyst between H2 O and NH3 . The additional reaction allows H2 O and NH3 to be stored on open catalytic sites and has improved model accuracy by accounting for large changes in H2 O, CO2 , and temperature.Copyright