Overview of the Engine, Setup and Model

Diesel Engine

3.2 Overview of the Engine, Setup and Model

20 InGT-Power, the modeling and simulation is broadly done in the following steps:

I. Importing required templates into a project 2. Defining the objects to create parts

3. Placing the parts in required sequence 4. Linking the parts

5. Case setup to run several cases in one simulation 6. Simulation of the model

7. Post-processing using GT-Post.

21

-_GeUfll!

____ TCtrle

NG Supply

Fig. 3.1. Schematic diagram of the experimental setup of Rahman (2003).

2. A model of gas flow in the inlet and exhaust port of the engine. This must include comprehensive mapping of the discharge coefficients of the flow in both directions at all discontinuities in the ducting.

3. A model of the thermodynamic and gas dynamic behavior with in each cylinder of the engine while the valves or ports are open and closed.

4. A model of the engine friction characteristics, so that the predicted values of the indicated performance characteristics may be converted to measure engine output data, such as brake mean effective pressure, power, or brake specific fuel consumption.

GT-Power features an object-based code design that provides a powerful model building facility and reduces user effort. Models are built by a graphical user interface, GT-ISE (In- tegrated Simulation Environment), that simplifies the task of managing object libraries and building, editing, executing and post-processing models. GT-ISE minimizes the amount of in- put data entry, as only unique geometrical elements must be defined; this reduces required data input Models are built by point-and-click GUI from a library of GTI-suppliedor user-defined reusable components, known as 'objects'.

I .

/'

. "

(

Exhaust VaJvuCon,

'--1

--'11

I

ⁱ

I

InjProfileConn (Diesel)

EngCyrlnder 3

EngCyIlnder 4

V"""""""

Pipe

InjRateConn (Methane) InjRateConn (Carbon-dioxide)

Intake Air

(3001(, I _. ~RH)

Ii

EnglnecrankTraln

Fig. 3.2. Schematic of the model of the diesel engine.

~

\

^.

•.•...•..

_.•.

~~~- .

L-., ,_

4

_~- •

23 EndEnvironment object is used to describe !be ambient environment, both for intake and exhaust, boundary conditions of pressure, temperature and composition. In this object, ei!ber of the static pressure or stagnation pressure can be imposed in bar, and temperature is imposed in K. Composition of the environment is defined in Fstateinit object which specifies the initial condition at the start of the simulation, and subsequently, FpropGasCom- bust and FprsMixtureCombust objects are used to estimate gas and mixture properties, respectively. In case of gaseous fuel thermodynamic and transport properties are also estimated

, using these objects, however, FpropLiqIncomp object is used to provide liquid fuel pr0p- erties for the simulation. Detailed of all the objects used in the present study are presented in Appendix B.

The intake system starts with an ambient condition EndEnvironment going into a smooth orifice. The orifice has been made effectively smooth by setting the forward and re- verse discharge coefficient to I. Multiple pipes attribute are used to model a bundle of pipes of different diameter. However, Pipe object used to accomplish all the calculations where a user has to provide the following data:

• Pipe size (diameters at inlet and outlet end and length)

• Surface roughness

• Estimated wall temperature

• Pressure loss coefficient etc.

The intake and exhaust manifolds are modeled as cast iron using heat conduction objects to solve for the wall temperatures. The intake and exhaust ports are modeled using !be recom- mendations for the wall temperature, heat transfer multiplier and friction multiplier listed in the GT-Power User's Manual (Gamma-Technologies 2(01). The heat transfer from internal fluids to pipe and walls is calculated using a heat transfer coefficient. The heat transfer coef- ficient is cal~ulated at every time step from the fluid velocity, the thermo-physical properties and the wall surface finish. The surface roughness attribute in pipe component can have a very strong influence on the heat transfer coefficient These are calculated by Reynolds Number.

I

24 The friction for smooth walls is given by :

16 ReV

0.08

Re°.25

V

in laminar region, ReV

<

2000

in turbulent region, ReV> 4000 (3.1)

with a transitional region in between. Surface roughness data for some common materials are presented in Table 3.1.

Table 3.1. Surface roughness factors for some engineering materials.

Materials Sand roughness (mm)

Drawing tubing metal 0.0015 - 0.0025

Smooth plastic fiberglass 0.0025

Flexible smooth rubber 0.025

Galvanized metals, smooth finish 0.025

Commercial steel 0.046

Wrought iron 0.046

Asphalted cast iron 0.12

Galvanized metals, normal finish 0.15

Steel pipe with light rust 0.25

Cast iron 0.26

Steel pipe with heavy rust 1.0

The heat transfer coefficient is calculated at every time step from the fluid velocity, the thermo-physical properties and the wall surface finish. The surface roughness attribute in pipe component can have a very strong influence on the heat transfer coefficient, especially for very rough surface such as cast iron or cast aluminum. The heat transfer coefficient is arrived at through the use of the Colburn analogy (Gamma-Technologies 2(01).

(3.2)

where

25

Cj =friction coefficient p

=

fluid density

Uej j =effective outside boundary layer Cp =specific heat

Pr =Prandtl number

The Colburn analogy is used for turbulent, laminar and transitional flow. The internal heat transfer coefficient, the predicted fluid temperature, and the internal wall temperatore are used to calculate the total heat transfer the user must enter a wall temperature for pipes and flows- plits, which is either used as a fixed value throughout the simulation or as the initial wall temperatore if the wall temperatore is to calculate.

Details of the engine is addressed by En gcy li nde r object. Hence, En gCy1Geomobject is invoked by Engcylinder object to specify the geometry of the engine cylinder and crank- train. Following inputs are required in the object:

• Bore

• Stroke

• Connecting rod length

• Wrist pin offset

• Compression ratio

• TDC clearance height.

Val veConn object is used to provide profile of the cams driving the intake and exhaust valves. In ^jPro ^fileConn object is used to model the injection rate profile of liquid fuel into the engine cylinder. Two In jRateConn objects are used to model the introduction of natural gas and car!l?n-dioxide at any required composition and rate to produce biogas.

EngCylHeat Tr object is invoked by Engcylinder object and this object describes parameters for the in-cylinder heat transfer models used to calculate heat transfer from the

26 engine cylinder and engine crank -ease parts. In the present study, Woschni model is used to estimate the engine cylinder heat transfer. EngCombDIWiebe object is used to implement DI Weibe combustion model which is invoked by Engcylinder object to estimate the com- bustion rate for the engine using three term Wiebe function. Details of the DI Wiebe function used in the present study is addressed in ~ 3.2.1. Engine's crank-train is specified in the object EngineCrankTrain, which models the crank-slider mechanism that translates the cylinder pressure into the crankshaft torque, summing each cylinder's torque according to its timing.

Following inputs are required for this object:

• Engine type - 2-stroke or 4-stroke

• Number of cylinders

• Configuration of the cylinders - in-line or V

• Speed or load specification

• Engine speed

• Crankshaft Inertia

• Firing order

• Engine friction object

EngFrictionCF object is used to specify the parameters of Chen-flynn engine friction and is briefly presented in ~ 3.2.2.

3.2.1 Combustion Modeling with DI Weihe Model

In the present study, Direct-Injection Weibe. Combustion Model is used to impose the com- bustion rate using a three term Wiebe function (the superposition of three normal with Weibe curves). The purpose of using three functions is to make it possible to model pre-ignition (the large initial sllike) and larger tail. The Weibe model used by GT-Power model is:

27 Inputs: Sal

=

Start of injection ID

=

Injection delay

Dp

=

Premix duration DM =Main duration

DT

=

Tail duration Fp

=

Premix fraction

FT

=

Tail fraction Ep

=

Premix exponent

EM

=

Main exponent ET

=

Tail exponent

Calculated

Constants:

_FM

=

Main Fraction

WCp = Wiebe premix Constants WCM

=

Weibe main Constants WCT

=

Wiebe Tail Constants

FM ⁼ (I-Fp -FT) _(3.3)

[ D rEP

WCp = p - 0.1051/ Ep _(3.4)

2.3021/Ep

[ DM rEM

WCM = 0.1051/EM _(3.5)

2.3021/EM

WCT [ DT _ 0.1051/ET]-Er _(3.6)

Z.30Z1/Er

Hence, combustion calculation is the cumulative combustion, nurmalized to1.0. The combus- tion starts at 0.0 (0.0% burned) and progresses to 1.0 (100% burned).

HHR(O) = Fp

[1-

^{exp{-WCp(O - Sal - ID)Ep}]}

⁺

FM [1-exp{-WCM(O-SOI-ID)EM}]

+

FT

[1 -

^exp{-WCT(O - Sal - ID)Er}] (3.7)

This model is semi predictive if the user specifies default option for the ignition delay or if attributes ..Attributes set to "def' is calculated from the injection profile, air fuel ratio, pressure and temperature. Therefore, to obtain meaningful results the injection geometry, cylinder pres_

sure, injection rate and injection timing must be carefully specified because they will effect the combustion rate.

28 3.2.2 Friction Modeling with Cben-F1yn Model

In GT-POWER, Chen-Flynn model is used to calculate engine friction using the following equation:

fmep =a+ PF. Pmax

+

b.vp+ c.v~ _(3.8) where a

=

Constant part of fmep

P F =Peak Cylinder Pressure Factor Pmax

=

Maximum Cylinder Pressure b =Mean Piston Speed Factor

c =Mean Piston Speed Squared Factor vp =Mean Piston Speed

The EngineFr ictionCF reference object is used to model friction in the engine. Aux- iliary loses (such as water pumps, radiator fans etc) can also be lumped into the EngFric- tionCF reference object This is an empirically derived model that states total engine friction is a function of peak cylinder pressure, mean piston speed and mean piston speed squared.