06 research methodology

7/28/2019 06 Research Methodology

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RESEARCH METHODOLOGY

It impossible to appreciate the unique elements that compose HCCI engines without understanding the

physical, chemical, and thermodynamic theories that permit the designs overall function. Fuel

chemistry, reactive processes, and kinetic factors are critical aspects of the HCCI concept. These

complexities lead to the necessity of powerful control systems and advanced computers within theHCCI design. From a mechanical perspective, engine block design, crank angle, and valve timing are

of particular importance. The challenge of the HCCI design is arguably as much an issue of chemistry

as it is of mechanics. The HCCI engine, when compared to its diesel counterpart, is quite robust

because of the versatile array of possible fuels [B.Heywood et, al., 2006].

As HCCI engines boast behavior bordering on a perfectly isochoric system; this permits high

compression ratio fuels to be utilized. On this note, one of the biggest challenges is finding the most

efficient and appropriate fuel for combustion.

A multitude of fuels can be used in an HCCI engine, an extreme sensitivity to fuel quality and

composition is evident [R.H.Munoz et,2005]. Further, the slim margin of ideal operating conditions

often leads to misfires and incomplete combustion [Luckert et,al.,2006]. Under a high load (low

air/fuel ratios), HCCI engines are notoriously loud and produce acoustic oscillations characteristic ofknocking in the typical gasoline engine [Luckert et,al.,2006]. Though still operable, such an engine

would be unattractive for most consumer applications. On these grounds, the occurrence of knocking

provides a lower limit and misfiring provides an upper limit for air to fuel ratios. HCCI also faces

hypersensitivity to temperature. Factors such as low temperature and variations in oxygen content can

cause ignition delays or unusual combustion characteristics. Due to the unique nature of the HCCI

combustion stroke, the engine is intrinsically difficult to control [Flynn et,al., 2000]. Having discussed

HCCIs sensitivity to the conditions of the system and the surroundings, it is clear that ignition control

is one of the central challenges of the technology. This paper will focus on three elemental factors of

ignition control: variable spark/auto-ignition processes, variable valve timing, and passive methods of

temperature control.

Several modeling approaches have been developed to explore the influences of in homogeneities in

temperature and composition and the role of turbulence in HCCI engines. These include multiple-zone

models [Flynn et,al.,2000], stochastic models [Yang et,al.,2009], and multidimensional models

(computational fluid dynamics CFD) [Hyvonen et,al2003]. Complex-chemistry CFD studies that

have explored hydrodynamic effects include Refs. [Hyvonen et,al2003] and [H.Munoz et,al.,2005];

there a characteristic-timescale turbulent combustion model was used to account for

turbulence/chemistry interactions. The aim of this research is to elucidate further the role of

turbulence/chemistry interactions (TCI) on HCCI auto ignition and emissions. Toward that end, a

state-of-the-art TCI model (a transported probability density function PDF method [Luckert

et,al.,2006]) will be implemented in an unstructured-mesh finite volume CFD solver, including

detailed chemistry. The focus is on establishing trends and sensitivities rather than on quantitativecomparison with experimental measurements. In this synopsis , the feasibility of gasoline fueled HCCI

combustion will be analyzed by combining VVT and , n-heptane direct injection. The effects of

various intake valve timings, fuel injection timings, n-heptane and diesel quantities on the NOx and

HC emissions will being the find out by a CFD simulation technique.

CFD Simulation

Using numerical simulations, it is possible to calculate the temporal behavior of every variable

of interest at any place inside the computational domain. This allows the obtainment of a detailed

knowledge of the relevant processes and is a prerequisite for their improvement. There are threeclasses of models that can be used in numerical simulations of in-cylinder processes. If very shortcalculation times are necessary, so-called thermodynamic models are used. These zero-dimensional

models, which do not include any spatial resolution, only describe the most relevant processes without


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Providing insight into local sub-processes. Very simple sub-models are used, and a prediction of

pollutant formation is not possible. The second class of models are the phenomenological models,

which consider some kind of quasi-spatial resolution of the combustion chamber and which use more

detailed sub-models for the description of the relevant processes like mixture formation, ignition and

combustion. These phenomenological models may be used to predict integral quantities like heat

release rate and formation of nitric oxides (NOx). The third class of models is the computational fluiddynamics (CFD) models. In CFD codes, the most detailed sub-models are used, and every sub-process

of interest is considered. This class of models is the most expensive regarding the consumption of

computational power and time. The turbulent three-dimensional flow field is solved using the

conservation equations for mass, momentum and energy in combination with an appropriate

turbulence model. The CFD codes are especially suited for the investigation of three-dimensional

effects on the in-cylinder processes, like the effect of tumble and swirl, the influence of combustion

chamber geometry, position of injection nozzle, spray angle, number of injection holes, etc.

Physical Models and Numerical Methods

Three-dimensional, time-dependent Reynolds-averaged simulations will be performed using anunstructured, deforming-mesh, compressible finite-volume CFD solver. A Lagrangian particle Monte

Carlo method is being implemented to solve the modeled composition PDF transport equation using a

consistent hybrid particle/finite-volume method [M.Zaccardi et, al.,2009], thereby explicitly

accounting for turbulent fluctuations in composition and enthalpy (temperature) about the local cell-

mean values. A standard gradient transport approximation will be used together with a two-equation k

turbulence model to account for transport by turbulent velocity fluctuations, and a simple pair-

exchange model will be used for molecular mixing [Barroso.G et, al.,2005]. Thermo chemistry will be

implemented using the Matlab/Fortran libraries [Hillion et, al.,2011]. Chemical mechanisms being

used is n-heptane. The material property will be generated in simulation library.A zero-dimensional simulations will be done to verify and analyze the NOx emission and chemistry

influence on ignition delays, temperature, pressure etc. Direct in-cylinder fuel injection will bemodeled using stochastic Lagrangian models that are based on the spray formulation. As spray parcels

evaporate in accordance with the spray vaporization model, new PDF notional particles will be

introduced at the exact location of each vaporizing parcel with the correct mass, composition (pure

fuel), and temperature. This captures in a natural and direct manner the high levels of local gas-phase

concentration and temperature fluctuations that arise from spray vaporization. To isolate the roles of in

homogeities and un mixedness, results from three levels of modeling will be compared: In general, 0D

adiabatic models predict advanced ignition, a high rate of pressure rise, and low unburned

hydrocarbon and CO emissions compared to higher-order models or experiments.CFD without

turbulence/chemistry interactions (CFD wo/PDF) captures mean flow effects, wall heat transfer, and

in homogeneities in the mean fields; CFD with turbulence/chemistry interactions (CFD w/PDF)

captures the effects of turbulent fluctuations about the local mean, in addition.

Through this synopsis, an attempt is made to improve HCCI control using CFD simulation for HCCI

engine modeling. Depending on the purpose, HCCI engine models will be classified into three types.

The first is the multidimensional computational fluid dynamics (CFD) model, which will be used for

the optimization of fuel injection timing, mixture generation and combustion chamber design. The

second is the zero-dimensional multi- zone model coupled with detailed chemical kinetics, which will

be normally used for understanding the HCCI combustion process. The final one is the single-zone

simplified dynamic control oriented model, which is normally used for control strategy development.

We will be developing CFD-based methods that accommodate detailed thermo chemistry and

turbulence/ chemistry interactions, while remaining computationally practicable. The essential

elements are chemical mechanisms. A consistent hybrid particle/finite-volume compositionprobability density function (PDF) method, and storage/retrieval-based chemistry acceleration.

A zero dimensional model will also selected to study engine Performance and auto ignition

characteristics of , n-heptane oxidation; because of its simplicity and its ability to predict performance


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characteristics of HCCI engine [Flynn et,al.,2000]. This model will be used for the closed cycle with

full chemical kinetic considerations both in compression and expansion processes.

Fuel Injection Technique

The thermal and fuel distribution are the key variables controlling the initiation of HCCI combustion.Fuel injection has the potential to control the combustion process by altering the local fuel

concentration. Various injection strategies have been explored for HCCI engines, including port fuel

injection, early direct injection, late direct injection in combination with a split injection strategy.

The technique here we are using is Direct injection.

The work described in this presentation will be conducted on a six cylinder, direct fuel

injected, direct ignited research engine equipped with hydraulically actuated variable valve timing.

HCCI combustion will be initiated by early closing of the exhaust valve to retain exhaust in the

cylinder, adding both increased temperature and pressure to the subsequent compression and

combustion processes.

The auto-ignition timing of HCCI combustion depends on the charge temperature, pressure

and concentration history during the compression process, and its unique reaction kinetics. Fuelselection has a significant impact on both engine design and control strategies of HCCI combustion,Intheory, HCCI engines can be operated with any fuel, but the optimal fuel depends on the combustion

controlled strategies and the operating conditions. Fuel volatility and auto-ignition temperature are the

key parameters to be considered in order to gain high fuel efficiency.

Different gasoline range fuels could be investigated which varied primarily in their motor

cetane number. Trends were found that higher cetane number resulted is slower combustion, lower

peak pressures, improved fuel efficiency, and lower NOX emissions. These trends may also show

some fuel chemistry or property effects which could not be sorted out with the number of fuels tested.

Other work is relative to spark assist of HCCI combustion to improve operating range, stability, andcontrol.

To evaluate these strategies, computational modeling tools can provide significant insight in acost- and time-effective manner.

The main work in this research is a simulating the different engine parameters. Engine

simulation with CFD software is used to find ways to improve the efficiency and operating range.

Engine parts are designed in the Computer-Aided Design (CAD) program Pro-E/CATIA. To evaluate

the strength of engine components, Ansys will be used both for the dynamic simulation and Finite

Element Analysis (FEA). Post-processing of data will be done in Ansys CFX.

This work describes the modeling strategy for constructing performance maps for HCCI

engine. A numerical model will be developed and used to test different mechanisms like VVT,

injection port models and for different fuels designs to overcome the challenges. While less accurate

than experimental test, this approach can be used to quickly screen various designs and guide

experimental study which are more expensive and timeconsuming. Also the constructed performance

maps can be used to estimate fuel economy over regulatory drive cycle without doing the vehicle test.

When constructing the performance map, the equivalence ratio and timing for exhaust valve

closing(EVC) are optimized at each operating point of engine speed and load to minimize fuel

consumption. Other objective functions could be chosen such as pollutant emissions, but fuel

consumption is selected because it is practically important and can be reliably predicted with the

model. Equivalence ratio affected the engine output usual way, i.e burning more fuel produced more

work. The valve timing is used to retain various amounts of residual gases in the cylinder in order to

alter the charge temperature and moisture composition and thereby control the ignition timing to

achieve the stable engine operation over a range of speed/load conditions.For this investigation, the modeling Is based on an open system first law analysis, with steady state

compressible flow relations used to model the mass flow through the intake and exhaust valves.


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Volume Rate EquationThe in-cylinder volume and its derivative are given by the following well-known slider-crank

formulas:

with

where is the rotational speed of the crankshaft, a is half of the stroke length,L is the connecting rod

length,B is the bore diameter and Vc is the clearance volume at top dead center. Manifold, m2, and

from exhaust manifold to cylinder,m

3,as shown in Fig 2.

Fig 2: Valve Mass Flows: left - induction with intake and exhaust valves open, rightexhaust[ H.Stanglmaier et,al.,1999]

Valve Flow EquationsThe mass flow through the valves consists of flow from intake manifold to cylinder, m1, from

cylinder to exhaust manifold, m2, and from exhaust manifold to cylinder, m3, as shown in Figure 1.

Equations for these mass flow Equations for these mass flow rates are developed using a

compressible, steady state,one-dimensional, isentropic flow analysis for a restriction, where real gas

flow effects are included by means of a discharge coefficient, CD. The relations for the mass flows

are:

The CFD simulation strategy for this project is as follows:

The combination of variable valve timing (VVT) and gas fuel injection of high cetane number will be

chosen to control the HCCI combustion phase in this study.


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While analyzing with the CFD simulation tool a regular diesel will be injected at intake port as main

fuel, while small amount of , n-heptane will also be injected directly into the cylinder as an ignition

promoter for the control of ignition timing.

Different intake valve timings will be analyzed for combustion phase control. Regular gasoline will be

analyzed for HCCI operation and emission characteristics with various engine conditions.

This paper investigates the steady-state and transient state combustion characteristics of the HCCIengine with VVT and, n-heptane as a fuel, to find out its benefits in exhaust gas emissions.

06 research methodology

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