Sebastian Mosbach

Influence of Injection Timing and Piston Bowl Geometry on PCCI Combustion and Emissions

Premixed Charge Compression Ignition (PCCI), a Low Temperature Combustion (LTC) strategy for diesel engines is of increasing interest due to its potential to simultaneously reduce soot and NOx emissions. However, the influence of mixture preparation on combustion phasing and heat release rate in LTC is not fully understood. In the present study, the influence of injection timing on mixture preparation, combustion and emissions in PCCI mode is investigated by experimental and computational methods. A sequential coupling approach of 3D CFD with a Stochastic Reactor Model (SRM) is used to simulate the PCCI engine. The SRM accounts for detailed chemical kinetics, convective heat transfer and turbulent micro-mixing. In this integrated approach, the temperature-equivalence ratio statistics obtained using KIVA 3V are mapped onto the stochastic particle ensemble used in the SRM. The coupling method proved to be advantageous in terms of computational expense and emission prediction capability, as compared with direct coupling of CFD and chemical kinetics. The results show that the fuel rich pockets in the late injection timing are desirable for triggering auto-ignition and advancing the combustion phasing. Furthermore, the model is utilised to study the impact of combustion chamber design (open bowl, vertical side wall bowl and re-entry bowl) on PCCI combustion and emissions. The piston bowl geometry was observed to influence the in-cylinder mixing and the pollutant formation for the conditions studied. INTRODUCTION Low Temperature Combustion (LTC) modes such as Homogeneous or Premixed Charge Compression Ignition (HCCI/PCCI) are receiving increased attention due to their potential for simultaneously reducing soot and NOx emissions from Direct Injection (DI) diesel engines. PCCI mode involves premixed combustion of a highly diluted or lean mixture and the combustion process is primarily controlled by the chemical kinetics. Thus, the control of ignition timing and burning rate in PCCI combustion is fundamentally more challenging than in a conventional compression ignition DI diesel engine governed mainly by physical processes such as fuel-air mixing. Furthermore, for the cases where the airfuel charge is often not purely homogeneous, the influence of fuel-air mixing on combustion also needs to be taken into account. In addition to experimental studies, a variety of computational modelling approaches based on multidimensional computational fluid dynamics (CFD) have also been applied to investigate early direct injection PCCI combustion. The detailed chemical kinetics and the flow description in PCCI mode are relatively decoupled, when compared to conventional diesel combustion. This fact has been exploited by sequential solvers based on CFD and multi-zone combustion models [1-3]. In a multi-zone approach, the computational cells having similar temperature and composition histories are grouped into a relatively small number of zones (~10). The chemical kinetics solver is applied to each zone, assumed as a well stirred reactor. Flowers et al. [2] modified the multi-zone model to This is Computational Modelling Groups latest version of the paper. For the published version please refer to http://www.sae.org/technical/papers/2009-01-1102

REAL-TIME EVALUATION OF A DETAILED CHEMISTRY HCCI ENGINE MODEL USING A TABULATION TECHNIQUE

A storage/retrieval scheme has been implemented for a Stochastic Reactor Model (SRM) for Homogeneous Charge Compression Ignition (HCCI) engines which enables fast evaluation in transient multi-cycle simulations. The SRM models combustion, turbulent mixing, and convective heat transfer during the closed-volume part of the engine cycle employing detailed chemical kinetics. In contrast to previously developed storage/retrieval techniques which tabulate chemistry only, our method stores, retrieves, and interpolates output quantities of the entire internal combustion engine model, i.e. the SRM. These quantities include ignition timing, cumulative heat release, maximum pressure rise rate, and emissions of CO, CO2, unburnt hydrocarbons, and NOx, as functions of equivalence ratio, octane number, and inlet temperature for instance. The new tool is intended to be used for performing a variety of otherwise exceedingly expensive computational tasks such as multi-cycle multi-cylinder simulations, transient operation and control, optimization of engine operating parameters, design of experiments, and identification of parameters for achieving stable HCCI operation over a wide range of conditions. Using transient control as an example, we show that, when coupled to a commercial 1D CFD engine modelling package, the tabulation scheme makes such simulations feasible and convenient.

Simulating a Homogeneous Charge Compression Ignition engine fuelled with a DEE/EtOH blend

We numerically simulate a Homogeneous Charge Compression Ignition (HCCI) engine fuelled with a blend of ethanol and diethyl ether by means of a stochastic reactor model (SRM). A 1D CFD code is employed to calculate gas flow through the engine, whilst the SRM accounts for combustion and convective heat transfer. The results of our simulations are compared to experimental measurements obtained using a Caterpillar CAT3401 single-cylinder Diesel engine modified for HCCI operation. We consider emissions of CO, CO2 and unburnt hydrocarbons as functions of the crank angle at 50% heat release. In addition, we establish the dependence of ignition timing, combustion duration, and emissions on the mixture ratio of the two fuel components. Good qualitative agreement is found between our computations and the available experimental data. The performed numerical simulations predict that the addition of diethyl ether to ethanol neither spreads out the combustion nor lowers light-off temperatures significantly, both in accordance with experimental observations.

Dual injection homogeneous charge compression ignition engine simulation using a stochastic reactor model

Abstract Multiple direct injection (MDI) is a promising strategy to enable fast-response ignition control as well as expansion of the homogeneous charge compression ignition (HCCI) engine operating window, thus realizing substantial reductions of soot and NOx emissions. The present paper extends a zero-dimensional-probability-density-function-based stochastic reactor model (SRM) for HCCI engines in order to incorporate MDI and an improved turbulent mixing model. For this, a simplistic spray model featuring injection, penetration, and evaporation sub-models is formulated, and mixing is described by the Euclidean minimal spanning tree (EMST) sub-model accounting for localness in composition space. The model is applied to simulate a gasoline HCCI engine, and the in-cylinder pressure predictions for single and dual injection cases show a satisfactory agreement with measurements. From the parametric studies carried out it is demonstrated that, as compared with single injection, the additional second injection contributes to prolonged heat release and consequently helps to prevent knock, thereby extending the operating range on the high load side. Tracking the phase space trajectories of individual stochastic particles provides significant insight into the influence of local charge stratification owing to direct injection on HCCI combustion.

A Detailed Chemistry Multi-cycle Simulation of a Gasoline Fueled HCCI Engine Operated with NVO

A previously developed Stochastic Reactor Model (SRM) is used to simulate combustion in a four cylinder in-line four-stroke naturally aspirated direct injection Spark Ignition (SI) engine modified to run in Homogeneous Charge Compression Ignition (HCCI) mode with a Negative Valve Overlap (NVO). A portion of the fuel is injected during NVO to increase the cylinder temperature and enable HCCI combustion at a compression ratio of 12:1. The model is coupled with GT-Power, a one-dimensional engine simulation tool used for the open valve portion of the engine cycle. The SRM is used to model in-cylinder mixing, heat transfer and chemistry during the NVO and main combustion. Direct injection is simulated during NVO in order to predict heat release and internal Exhaust Gas Recycle (EGR) composition and mass. The NOx emissions and simulated pressure profiles match experimental data well, including the cyclic fluctuations. The model predicts combustion characteristics at different fuel split ratios and injection timings. The effect of fuel reforming on ignition timing is investigated along with the causes of cycle to cycle variations and unstable operation. A detailed flux analysis during NVO unearths interesting results regarding the effect of NOx on ignition timing compared with its effect during the main combustion.

Two-stage Fuel Direct Injection in a Diesel Fuelled HCCI Engine

Two-stage fuel direct injection (DI) has the potential to expand the operating region and control the autoignition timing in a Diesel fuelled homogeneous charge compression ignition (HCCI) engine. In this work, to investigate the dual-injection HCCI combustion, a stochastic reactor model, based on a probability density function (PDF) approach, is utilized. A new wall-impingement sub-model is incorporated into the stochastic spray model for direct injection. The model is then validated against measurements for combustion parameters and emissions carried out on a four stroke HCCI engine. The initial results of our numerical simulation reveal that the two-stage injection is capable of triggering the charge ignition on account of locally rich fuel parcels under certain operating conditions, and consequently extending the HCCI operating range. Furthermore, both simulated and experimental results on the effect of second injection timing on combustion indicate that there exists an optimal second injection timing to gain maximum engine output work for a given fuel split ratio.

Implementing detailed chemistry and in-cylinder stratification into 0/1-D IC engine cycle simulation tools

Employing detailed chemistry into modern engine simulation technologies has potential to enhance the robustness and predictive power of such tools. Specifically this means significant advancements in the ability to compute the onset of ignition, low and high temperature heat release, local extinction, knocking, exhaust gas emissions formation etc. resulting in a set of tools which can be employed to carry out virtual engineering studies and add additional insight into common IC engine development activities such as computing IMEP, identifying safe/feasible operating ranges, minimizing exhaust gas emissions and optimizing operating strategy. However the adoption of detailed chemistry comes at a greater computational cost, this paper investigates the means to retain computational robustness and ease of use whist reducing computational timescales. This paper focuses upon a PDF (Probability Density Function) based model based on the Stochastic Reactor Model (SRM), which has gained increasing attention from academics and industry for its capabilities to account for in-cylinder processes such as chemical kinetics, fuel injection, turbulent mixing, heat transfer etc. whilst retaining in-cylinder stratification of mixture composition (i.e. fuel equivalence ratio) and temperature. Among the techniques considered here are: a standard KIVA 3V simulation, down-sampling from 3D CFD composition-space to stochastic particles using sequential coupling of KIVA 3V and SRM, the use of detailed chemical kinetics within SRM, chemical mechanism reduction, down-sampling of a chemical mixture space within the SRM, and parallelization of chemistry solution within SRM. The experimental engine setup studied is that used by Cao et. al. [1], employing Premixed Charge Compression Ignition (PCCI), which is a Low Temperature Combustion (LTC) strategy for diesel engines. This paper demonstrates how equivalent results can be achieved with a reduction in computational time from 28 days to 10 minutes. In order to enable engineers to more easily exploit SRM’s capabilities in the IC engine development process, it has been coupled with an industrystandard 1D engine cycle simulation tool (Ricardo WAVE) and a working example is presented.

HCCI Combustion Control Using Dual-Fuel Approach: Experimental and Modeling Investigations

A dual-fuel approach to control combustion in HCCI engine is investigated in this work. This approach involves controlling the combustion heat release rate by adjusting fuel reactivity according to the conditions inside the cylinder. Experiments were performed on a single-cylinder research engine fueled with different ratios of primary reference fuels and operated at different speed and load conditions, and results from these experiments showed a clear potential for the approach to expand the HCCI engine operation window. Such potential is further demonstrated dynamically using an optimized stochastic reactor model integrated within a MATLAB code that simulates HCCI multi-cycle operation and closed-loop control of fuel ratio. The model, which utilizes a reduced PRF mechanism, was optimized using a multi-objective genetic algorithm and then compared to a wide range of engine data. The optimization objectives, selected based on relevance to this control study, were the cylinder pressure history, pressure rise rate, and gross indicated mean effective pressure (IMEPg). The closed-loop control of fuel ratio employed in this study is based on a search algorithm, where the objective is to maximize the gross work rather than directly controlling the combustion phasing to match preset values. This control strategy proved effective in controlling pressure rise rate and combustion phasing while not needing any prior knowledge or preset information about them. It also ensured that the engine was always delivering maximum work at each operation condition. This is in a sense analogous to the use of maximum brake torque timing in spark-ignition engines. The dynamic model allowed for convenient examination of the dual-fuel approach beyond the limits tested in the experiments, and thus helped in performing an overall assessment of the approach’s potential and limitations.

The future of computational modelling in reaction engineering

In this paper, we outline the future of modelling in reaction engineering. Specifically, we use the example of particulate emission formation in internal combustion engines to demonstrate what modelling can achieve at present, and to illustrate the ultimately inevitable steps that need to be taken in order to create a new generation of engineering models.

Studying the Influence of Direct Injection on PCCI Combustion and Emissions at Engine Idle Condition Using Two dimensional CFD and Stochastic Reactor Model

A detailed chemical model was implemented in the KIVA3V two dimensional CFD code to investigate the effects of the spray cone angle and injection timing on the PCCI combustion process and emissions in an optical research diesel engine. A detailed chemical model for Primary Reference Fuel (PRF) consisting of 157 species and 1552 reactions was used to simulate diesel fuel chemistry. The model validation shows good agreement between the predicted and measured pressure and emissions data in the selected cases with various spray angles and injection timings. If the injection is retarded to -50° ATDC, the spray impingement at the edge of the piston corner with 100° injection angle was shown to enhance the mixing of air and fuel. The minimum fuel loss and more widely distributed fuel vapor contribute to improving combustion efficiency and lowering uHC and CO emissions in the engine idle condition. Finally, the coupling of CFD and multi-zone Stochastic Reactor Model (SRM) was demonstrated to show improvement in CO and uHC emissions prediction.