Abstract

1

AJAY KUMAR GARG ENGINEERING COLLEGE

27th Km. Stone, Delhi-Hapur Bypass Road,

Ghaziabad-201009

A

REPORT

ON

POROUS MEDIUM ENGINE

SUBMITTED BY:

DEEPAK AGRAWAL

IIIth YEAR

MECHANICAL ENGINEERING

ROLL NO.:1202740059

2

ABSTRACT

This report describes application of a highly porous open cell structures to

internal combustion engines for supporting mixture formation and combustion processes. Porous structures, materials and their properties for engine

application are discussed in this paper. Especially application to a high temperature combustion processes are considered. Novel concepts for internal

combustion engines based on the application of porous medium technology are presented and discussed. The main attention is focused on the engine concepts

having.

Potential for homogeneous (nearly emissions free) combustion process under

variable engine operational conditions. It is shown that porous media can be used for a great variety of improvements in the combustion process. The key

role for NOx reduction and soot emission elimination is a homogeneous combustion in engine. This can be realized by homo generous mixture

formation, and a 3D ignition preventing from formation of a flame front having a temperature gradient a in the entire combustion volume. All these processes:

gas flow, fuel injection and its spatial distribution, vaporization, mixture homogenization, ignition and combustion can be controlled or positively influenced with the help of porous media/ceramic reactors.

Its use in ICEs is not without problems due to heat transfer during

compression, limited maximum temperature and possibiliy of delayed heat supply to expansion. A simple tool for cycle parameter assessment using

computerized T-s diagram was developed and presented earlier. The preliminary results of it are promising.

As the further step, a CFD model has been developed for an evaluation of a

PM combustion potential and its future optimization. A simulation method based on a moveable-grid, Runge-Kutta finite volume (FV) 2.5-D model

has required the introduction of new source terms concerning spatially distributed heat transfer and momentum sink due to drag in a PM domain. The

geometry of a PM insert is characterized using a “filament” model of its matrix characterized in 3 main directions. The temperature of a PM insert

surface is solved using the Fourier equation with estimated effective thermal conductivity of a solid phase. A combustion model uses momentum and specie sources near to a fuel entry to a PM insert accompanied by very fast

heat release for a part of fuel covered by oxygen available and complex chemical kinetics.

3

The detailed FVM model has provided preliminary results as a base for sensitivity analysis, cycle optimization and engine lay-out changes. Steep

temperature gradients may occur in a PM domain due to a gradual transport of

cylinder charge into a PM and a compression or combustion of pre-heated

gas. NOx formation might be limited only if high temperature occurs in the

zone of a rich mixture. Concerning efficiency, a premature heat supply to gas from a PM during compression is disadvantageous as well as an intensive heat

transfer during combustion. The model will be used in a future optimization of the PM combustion process

4

TABLE OF CONTENTS

1-INTRODUCTION .......................................................................5-6

2- POROUS MEDIUM TCHNOLOGY.............................................6-9

2.1- INTRODUCTION OF POROUS MEDIUM

TECHNOLOGY......................................................................6-7

2.2- HOMOGENEOUS COMBUSTION........................................8-9

3- CONCEPT OF PM ENGINE: IENTERNAL COMBUSTION

ENGINE .........................................................................................10-14

3.1- WITH A CLOSED PM CHAMBER………………………..11-12

3.2- WITH A OPEN PM CHAMBER…………………………...13-14

4- COGNERATION IN PM ENGINE………………………………15-16

5- MATERIAL USED FOR POROUS MEDIUM………………….17-18

6- SELECTION OF AVAILABLE PM MATERIALS ……………..19-22

7- APPLICATION OF PM ENGINE……………………………………23

8- PROBLEMS OF PM COMBUSTION……………………………….24

9- CONCLUSION AND FURTHER RECOMDATIONS……………...25

10- REFERANCES……………………………………………………….26

5

1- INTRODUCTION The old reciprocating internal combustion engine (ICE) has survived more

than hundred years with great success. Has it potential for further development? Will it be replaced

by fuel cells? There are needs to find all reserves unused up to now to improve ICE at least until a time of a fuel cell will come. The big disadvantage of an unsteady ICE combustion is the large inhomogeneity of temperature and concentration that leads to a

high rate of NOx formation in early burnt parts of

cylinder content.

To avoid this, the homogeneous combustion after compression ignition (HCCI) offers a solution if

enormous air excess is used and suitable self-ignition properties of fuel are available. It limits

temperatures occurring in the whole combustion space simultaneously and the time of their action. Turbocharging is necessary to compensate for

decreased specific power due to high air excess. Unfortunately, there are problems with a control of timing and burning rate of HCCI. Burning should

not take place till fast expansion starts to limit temperature increase in already combusted layers.

It means that low real compression ratio is used despite the fact that the geometrical compression ratio is high (the temperature at the end of

compression must be high enough to ignite fuel-air mixture with a reasonable ignition delay).

The aim of the current contribution is to find tools to analyze the possibilities of porous media combustion in ICEs with limited

temperature (to prevent nitrogen oxides formation) but simultaneously aiming to burn efficiently all rests of hydrocarbons, soot and carbon

monoxide in a hot PM insert. The potential to achieve a real homogeneous combustion in a porous medium (PM) is analyzed.

The problems of a combustion simulation are well-known. The links between chemistry (specie conservation), rate-of-heat release ROHR (energy

conservation) and the expansion of burned mixture causing additional

6

acceleration of flame front (momentum conservation) bring about an unstability of numerical algorithm, suppressed currently by both the artificial

or natural damping features of FV numerical schemes. Numerical diffusion is achieved as both artificial, i.e., intentional one or natural, i.e. false one caused

by too rough FV mesh. Unfortunately, the big gradients of concentration and temperature demand fine meshes. The integration requires extraordinary small

time step, moreover, called for by a stiffness of chemical kinetic equations. In this situation, a PM combustion with temperature stabilization due to

overheated solid phase and its damping drag creates a problem that is not interesting only as itself but it may elucidate other problems of a general

flame simulation.

7

2. POROUS MEDIUM TECHNOLOGY

2.1. Introduction to porous medium technology

In this technology a highly porous structures having open cells are considered,

with porosity higher than 80%, and typically higher than 90%. This makes the porous media transparent for gas flow, spray and flame.

Porous medium (PM) technology is here defined as an utilization of specific and unique features of highly porous medium for supporting of individual processes (mixture formation, ignition and combustion) realized in engine.

Most of these processes perform in PM-volume drastically different manner from this observed in a free volume.

Selected features of the porous medium permit its attractive application to the following

En err circulation in engine cycle in the form of hot burned gases recirculation

or combustion energy this may significantly influence thermodynamic properties of the charge in the cylinder and may control the ignitability

(activity) of the charge. This energy recirculation may be performed under different pressure and temperature conditions during the engine cycle.

Additionally, this heat recuperation may be used for controlling the combustion temperature level.

Fuel injection in PM-volume: especially unique features of liquid jet

distribution and homogenization throughout the PM-volume (effect of multi-jet splitting) [3] is very attractive for fast mixture formation in the PM-volume.

Fuel vaporization in PM-volume: combination of large heat capacity of the

PM-material, large specific surface area with excellent heat transfer in PM-volume make the liquid fuel vaporization very fast and complete. Here two

different conditions of the process have to be considered: vaporization with presence and with lack of oxygen.

Mixing and homogenization in PM-volume: unique features of the flow properties inside3D-structures allow very effective mixing and in PM-volume.

3D-thermal-PM-ignition (if PM temperature is at least equal to ignition

temperature under certain thermodynamic properties and mixture composition): there is a new kind of ignition, especially effective if the PM-volume creates

automatically the combustion chamber volume.

Heat release in PM-volume under controlled combustion temperature (properties of homogeneous combustion): there is only one known kind of

8

combustion that permits homogeneous combustion conditions almost

independently of the engine load with possibility of controlling of the

combustion temperature level.

Depending on the application of a porous medium, a combination of

thermal and mechanical properties of the materials as well as their

inner structure and pores size have to separately be chosen and

optimized for supporting of particular engine process. If the porous

medium is used directly for controlling the combustion process under

high pressure conditions, above requirements become especially

critical. The probe presented in figure 1 is characterized by the

following parameters: porosity=91.88%,connection density =0.0311

per mm3 (represents a number of connections or junctions per cubic

millimetre)

For applications considered in this paper typical pore size is higher

than 1mm, and usually is of order of 3mm large. This pore size is

often expressed by the pore density “ppi” – pore per linear inch.

Typical pore density useful for applications reported in this paper is

from 8 to30ppi. The pore shape and pore density depends on the

basic foam used for manufacturing of final foams (e.g. PU-foam for

ceramic foams

The volume of a highly porous structure may be divided in to pore

volume (free volume for gas),

Material volume, hollow tube junctions, and micro porosity

9

2.2 HOMOGENEOUS COMBUSTION

Homogeneous combustion in an IC engine is defined as a process characterized

by a 3Dignition

of the homogeneous charge with simultaneous-volumetric-combustion, hence, ensuring a homogeneous temperature field. According to the definition given

above, three steps of the mixture formation & combustion may be selected that define the

ability of a given combustion system to operate as a homogeneous combustion system:

Homogenization of charge.

Ignition conditions.

Combustion process & temperature field.

Four different ignition techniques may be selected:

Local ignition (e.g., spark plug).

Thermal self-ignition (e.g., compression ignition).

Controlled auto-ignition (e.g., low temperature chemical ignition).

3D-thermal PM self-ignition (3D-grid-structure of a high temperature).

The last considered ignition system has been recently proposed by Durst and Welcas (3) and uses a 3D-structured porous medium (PM) for the volumetric

ignition of homogeneous charge. The PM has homogeneous surface temperature over the most of the PM-volume, higher than the ignition temperature. In this case the PM-volume defines the combustion chamber

volume. Thermodynamically speaking, the porous medium is here characterized by a high heat capacity and by a large specific surface area. As a

model, we could consider the 3D-structure of the porous medium as a large number of “hot spots” homogeneously distributed throughout the combustion

chamber volume. Because of this feature a thermally controlled 3D-ignition can be achieved. Additionally, the porous medium controls the temperature

level of the combustion chamber permitting the NOx level control almost independently of the engine load or of the (A/F) ratio. Let us consider the four

possible combustion modes of a homogeneous charge:

Homogeneous charge with local ignition.

Homogeneous charge with compression ignition.

Homogeneous charge with controlled auto-ignition.

Homogeneous charge with 3D-thermal self-ignition in PM-volume.

10

In the case of local ignition we could not fulfill the requirements of the ignition defined for homogeneous combustion. In this case a flame kernel will be

followed a flame propagation in the combustion chamber. In the case of compression ignition a multi-point ignition can be achieved, except the near-

wall areas. On one hand side, this process (if volumetric) would be related to very high-pressure gradients in the cylinder. On the other hand, any non-

homogeneity of the charge, any hot spots in the combustion chamber, and colder area near the cylinder and piston walls will make the ignition process

not controllable

11

3-CONCEPT OF THE PM-ENGINE: INTERNAL

COMBUSTION ENGINE WITH MIXTURE FORMATION

AND HOMOGENEOUS COMBUSTION IN POROUS

REACTOR

PM-engine is here defined as an internal combustion engine with a homogeneous combustion process realized in a porous medium volume. The

following individual processes of PM engine are realized in porous medium volume: internal heat recuperation, fuel injection, fuel vaporization, mixing

with air, homogenization of charge, 3D-thermal self-ignition, and a homogeneous combustion. The TDC (Top Dead Centre) compression volume is equal to the PM volume which creates the engine combustion

chamber. Outside the PM-volume there is no combustion present in the cylinder. PM-engine may be classified with respect to the timing of heat

recuperation in engine as: � Engine with periodic contact between PM and cylinder (so-called closed

PM-chamber).

� Engine with permanent contact between PM and cylinder (so-called open

PM-chamber).

Another classification criterion concerns the positioning of the PM-reactor in

engine. Here, three possible localizations may be selected: engine head, cylinder, and piston

(Figure). Interesting feature of PM-engine is its ability to operate with different liquid- and gaseous fuels. Independently of the fuel used, this engine is a

3DPM- thermal self-ignition engine. Finally, the PM engine concept may be applied to both two- and four-stroke cycle engines.

Figure: Possible locations of PM-reactor in PM-engine concept

12

3.1-CONCEPT OF THE PM-ENGINE WITH A CLOSED PM

CHAMBER

Let us start this analysis with a case of closed PM chamber, i.e.

engine with a periodic contact between working gas and PM-heat

recuperator (Figure 19). At the end of the expansion stroke (Figure

19e) the valve controlling timing of the PM chamber closes and fuel

may be injected in the PM volume. This chamber is a low pressure

chamber and a long time is available for fuel supply and its

Figure 19: Principle of the PM-engine operation with a closed chamber; 1-intake valve, 2-exhaust valve, 3-PM-chamber valve, 4-fuel injector, 5-piston

vaporization in the PM-volume. Simultaneously, other processes may perform in the cylinder volume. These processes may be continued through exhaust,

intake and compression strokes (Figure 19a). Near the TDC of compression (Figure 19b) the valve in PM-chamber opens

and the compressed air flows from the cylinder to the hot PM containing fuel vapours. Very fast

mixing of both gases occurs before mixture igniting in the whole PM-volume (Figure 19c). The resulting heat release process performs simultaneously in the

whole PM volume. Three necessary conditions for a homogeneous combustion are here fulfilled: homogenization of charge in PM-volume, 3D-thermal self-

ignition in PM-volume and a volumetric combustion with a homogeneous

13

temperature field in PM-volume. Additionally, the PM deals as a heat capacitor and controls the combustion temperature level

3.2- CONCEPT OF THE PM-ENGINE WITH AN OPEN PM

CHAMBER

Another possible realization of the PM-engine is a combustion system characterized by a permanent

contact between working gas and PM-reactor. In this case it is assumed that the PM-combustion chamber is mounted in the engine head, as shown in Figure

20. During the intake stroke it is weak influence of the PM-heat capacitor on the in cylinder air thermodynamic conditions.

Also during early compression stroke, only small amount of air contact the hot

PM. This heat exchange process (non-adiabatic compression) increases with continuing compression timing, and at the TDC the whole combustion air is

closed in the PM volume. Near the TDC of compression the fuel is injected into PM volume and very fast fuel vaporization and mixing with air occur in

3D-structure of PM. E2 E3

PM-Reactor

E1 E4

E5

Figure: Energy balance of PM-reactor in PM-engine with open

chamber: E1=energy supplied from compression; E2=energy supplied with fuel;

E3=energy losses; E4=energy supplied from PM to the air; E5=energy transported with burned gases

14

Figure: Principle of the PM-engine operation with an open chamber; 1-intake valve, 2-exhaust

valve, 3-fuel injector, 4-piston

A 3D-thermal self-ignition of resulting mixture follows in PM-volume together

with a volumetric combustion characterized by a homogeneous temperature

distribution in PM-combustion zone. Here, the energy balance in the PM

reactor defines thermodynamic conditions of the engine cycle (Fig. 21). During

following expansion stroke the heat is transferring into mechanical work

(Figure 21e). Again, all necessary conditions for homogeneous combustion are

fulfilled in the PM-combustion volume. An example of the PM-engine head

with open chamber and PM reactor mounted in the engine head is shown in

Figure 22. These first experimental investigations on the real PM-engine

indicated its potential for (near) zero emission operation in a very wide range

of charge compositions and engine rates.

15

Especially noticeable is that no soot emissions were observed together with

close-to zero NOx emissions (~100mg/kWh), even for nearly stoichiometric

charges. Noticeable was also extremely low combustion noise of the engine

with combustion in porous reactor. However, there were two technical

problems limiting obtained results:

Material problem: for this first engine realization a SiC ceramic reactors have

been chosen.

Engine head without reactor

CR injector

SiC reactor

Complete engine head with reactor

`

Intake valve Exhaust valve

SiC reactor

Figure: View of the PM-engine head with open chamber built on the basis of a single-cylinder DI-

Diesel engine

One problem was to mount this ceramic reactor in engine, and on the other

hand side the available reactor (material) quality was not very high (at least for

this application).

16

4.0 COGENERATION IN PM-ENGINE

During ordinary operation of any engine approximately 70% of the heat value

is lost due to practical and physical limitation. The concept of cogeneration

now a day is quite established and implemented almost in every industry,

where power requirement is accompanied by the industrial process heating

(IPH). The term heat to power ratio is somehow a very interesting topics of

research, as in every industry the load matching is very important for proper

utilization of heat energy. Generally it is seen that the dynamic range to which

Q/W ratio can operate is limited due to certain parameters in working of any

power generation cycle, typical values for certain established systems are given

in the following table

Cogeneration

system

Q/W Power output

Back pressure

Steam Turbine

4.0-14.3 14-22 %

Steam turbine 2.0-10.0 22.0-40.0 %

Gas turbine 1.3-2.0 24-35 %

Combined cycle 1.0-1.7 34-40 %

Ordinary

reciprocating engine

1.1-2.5 33-53 %

PM-engine Yet to be explored Yet to be explored

Table. Efficiencies and dynamic power modulation of some established cogeneration technology

Range of Q/W can be raised by incorporating certain mechanisms in above

given systems like steam bleed or supplementary firing of fuel to match the

IPH requirements. The concept synonymous to supplementary fuel firing can

be used in PM-Engines here the advantage would be that the mechanical power

output is also more for a fixed quantity of fuel. A typical schematic is shown in

(FIG.4.A)

17

Pm with heat exchanger

Figure . A. Principle of the PM-engine cycle with cogeneration.

This concept uses the energy of PM combustion to develop shaft power and

also to make excess heat when its needed in the IPH a typical schematic above

shows the basic working principle of this new concept. Logically if we are able

to transfer heat in the finite time then the temperature of combustion chamber

can be maintained and hence the operation of PM engine is unaffected from

very lean F/A mixture to very high F/A mixture the power range and

operational characteristic remains workable

18

5-MATERIALS USED FOR POROUS MEDIUM

Since this technology depends on high temperature resistant porous

materials hence, identification and survey of such material is also

necessary. The most important material and for porous burner are

SiC (silicon carbide) foams as well as mixer-like structure made of

Al2O3 fibers, ZrO2 foams and C/ SiC structure. In some special

application chromiumiron alloys and nickel base alloys are also used.

Al2O3 and ZrO2 are having different manufacturing properties these

material can be used in temperature range of 1650 o C above, where

as metals and SiC material do not fall in this category, hence they are

used in comparatively low temperature applications. However they

possess outstanding characteristic with regard to thermal shock,

mechanical strength and heat transport capacity etc. The overall

performance of a porous body is strongly dependent on combination

of base material and porous structure itself. Aluminum oxide can be

used to a process temperatures of 1950oC, although the technical

temperature limit is 1700oC. Al2O3 – based materials show an

intermediate heat conductivity ranging from 5W/(m K) at 1000oC to

about 30W/(m K) at 20oC. Also Al2O3 shows an intermediate

thermal expansion and an intermediate resistance to thermal shock

and emissivity of 0.28 at 2000 o K. High quality SiC can be used to a

maximum process temperature 1600oC,a heat conductivity in rang of

20 W/(m K) at 1000 o C and 150 W/(m K) at 20oC,a very good

resistance to thermal shock and a very low thermal expansion and the

overall emissivity at 2000oK is

19

Figure : Examples of different porous structures

about 0.8 to 0.9. Temperature resistant metal alloys may be used for

temperature below 1250oC. Their properties features a high heat conductivity

ranging from 10 W/(m K) at 20oC to about 28 W/(m K) at 1000 o C, extremely

high thermal expansion and extremely good resistance to thermal shock. The

emissivity of metals varies strongly with the surface finish and varies form

0.045 at 200 K to polished nickel of 0.5 in stainless steal. Solid Zirconia present

a highest temperature resistance which ranges up to 2300 o C. Heat

conductivity of solid Zirconia is hardly temperature dependant and in the range

of 2 W/(mK) to 5 W/(m K). Good conduction and heat transport capacity, low

radiations, and intermediate dispersion properties makes it quit suitable from

high temperature application.

20

6-SELECTION OF AVAILABLE PM MATERIALS AND THEIR FEATURES IN APPLICATION TO ENGINE

PROCESSES

As already indicated in this section, there are number of important

parameters that have to be considered in selection of PM materials for

application to combustion processes realized in porous media. On one hand

side, features of PM that are directly related to the heat transfer and

combustion process are very important, e.g. specific surface area, heat

transport properties, heat capacity and transparency for fluid flow and flame

propagation. On the other hand, the thermal resistance and the mechanical

properties of PM structure under high pressures are important for

particular applications. Another parameter which must also be considered

is the pores structure. Generally, the most important parameters of PM for

application to combustion technology in can be selected as follows:

1. specific surface area

2. heat transport properties

3. heat capacity

4. transparency for fluid flow and flame

5. pores size, pores density and pores structure

6. thermal resistance

7. Electrical properties

8. PM material surface properties

6.1 VERY EFFECTIVE HEAT TRANSPORT

PROPERTIES

Heat transport properties of PM are characterized by efficient heat

conductivity and very effective heat radiation inside PM.

These excellent heat transport properties permit for combustion in porous

medium much higher combustion rates than for a free flame (approximately

10 to 20 times higher).

Additionally, there is strong cooling of the reaction zone and in

consequence the thermal NOx formation is significantly reduced (low-

temperature combustion).

21

Figure: Example of flat PM-burner indicating strong heat radiation

of the solid phase (SiC foam, T~1500K)

6.2 LARGE POROSITY AND LOW PRESSURE LOSSES

As already indicated, highly porous materials mean structures of porosity

over approx. 80%. Owing to this large porosity, the PM

materials are transparent for gas and liquid flows as well as for

flames. This transparency permits low pres velocit sure losses in fluid

(gas) flow through the PM volume. Pressure drop over the wire packing

versus bulk y for three different PM lengths (50,100 and 150mm for

constant packing density is shown in Figure 12.

L=50mm

L=100mm

L=150mm

Mean bulk velocity [m/s]

Figure 13: Pressure drop over the ceramic foams of different pore

densities (ppi)

22

6.3 THERMAL PROPERTIES OF PM MATERIALS

One of the most important features of PM materials used in combustion

technology is their high thermal resistance, and especially important

parameters are: maximum temperature,thermo shock resistance and heat

capacity. Example of glowing foam structures being under thermal test is

shown in figure.

Figure: Thermal test of PM reactors for application to engine

Electrically heated foams

A porous structure may also directly be electrically heated, resulting in a

homogeneous temperature field throughout the PM-volume as shown in

figure .

Figure : direct electrical heating of SiC-Reactor (TPM~1200K) (U=12V)

23

7-APPLICATION OF POROUS MEDIUM TECHNOLOGY TO MIXTURE FORMATION AND COMBUSTION IN

ENGINES

Four different concepts concerning applications of PM-technology to mixture

formation and combustion in IC engines are considered in this chapter: New combustion system with mixture formation and homogeneous

combustion in PM-volume, so-called “PM-engine concept”.

New mixture formation system, with heat recuperation, vaporization

and chemical recombination in PM-volume, so-called “MDI- concept”.

“Intelligent engine concept” based on the MDI- system permitting

homogeneous combustion conditions (in a free cylinder volume) in a wide

range of engine operational conditions.

Phased combustion system for conventional DI Diesel, with temporal and

spatial control of mixture composition by utilization of interaction

between Diesel jet and PM-structure, so-called “Two-stage combustion”.

Before describing new engine concepts with porous medium technology (as

applied to combustion process), it is necessary to mention that there is a

number of concepts already reported in the literature which describe

application of PM technology (see also Table 4). Another group of

systems that use a PM in engines concerns internal heat recuperation, but

not combustion process itself. The main goal of such PM application to

internal combustion engines is to influence the thermal efficiency of engine

by internal heat recuperation.

There are also concepts combining the heat regeneration and catalytic

reduction of toxic components, e.g. gaseous and particulates [9,10]. Heat

flux and energy recirculation in such an engine has in detail been described

in. In this case the heat recuperator is attached to a rod and moves inside the

cylinder, synchronized to the piston movement (Figure 17). For most of the

cycle the porous regenerator is located close either to the cylinder head or

to the piston surfaces. During the regenerative heating stroke, the porous

insert moves down, and during the regenerative cooling stroke, the porous

regenerator moves up toward the engine head.

24

8-PROBLEMS OF A PM COMBUSTION Although a PM combustion brings advantages for a steady flame, it is not simple to transfer them to a specific unsteady conditions of an ICE. The

obvious thermodynamic problems associated with the unsteady fast combustion in a limited space of PM insert are the following: The limited maximum gas temperature may decrease thermal efficiency

if the temperature stabilization caused by a PM is not compensated by changes in the thermodynamic cycle pattern.

A heat supply occurs during compression without a controlled access of

gas into a PM. It is generally disadvantageous. The control of compressed gas access into the PM insert (if used according to Fig. 1)

suffers from the vulnerability of the valve exposed to combustion products. Big forces loading its gear create further problems. The irreversibilities due to pressure differences at the valve may decrease

cycle efficiency. From these reasons the control valve is not assumed in this contribution.

The heat accumulated in a porous medium extends a heat supply period

to late expansion, which is generally not welcome in piston engines, if

no use of the enthalpy of exhaust gas is provided.

The combustion duration might be increased with the same result. The

currently achieved rates-of-heat-release in steady flow burners reach the

power density of 35 kW.dm-3 (corrected to a gas density after

compression), whereas diesel combustion chamber rates are one order

more (400-500 kW.m-3) if combustion lasts for usual 45-60 deg of a

crank angle (CA).

High initial gas temperature inside a PM insert may cause extraordinary

high temperature at the end of compression as well.

Heat transfer and thermal stress problems may occur in engine parts

neighboring with a combustion chamber.

The mixture formation concept is limited obviously to an internal one

(injection of fuel or a rich mixture into a PM just before combustion). A sufficient homogeneity of a mixture should be ensured.

After thermodynamics of PM process is optimized, some design problems follow, e.g., thermal stress treatment, cooling and start pre-heating devices.

25

8.0 CONCLUSION AND FURTHER RECOMMENDATIONS

The current research of an unsteady PM combustion has been aimed at checking the relevance and potential of it in the nfurther development of an

ICE considering emissions and efficiency. Simple idealized tools as well as an advanced CFD ones were developed for theoretical sensitivity analysis

They have been used for preliminary simulations aiming to their calibration and future optimization applications.

The experience obtained up to now has shown the following facts:

The cycles with limited maximum temperature and a significant isothermal

heat supply (afterburning) associated with heat storage to a solid body require for a good thermal efficiency.

The main conclusion is that the careful optimization of an uncontrolled

PM burner using well-known results from direct-injection engines should be provided based on the results from developed simulation tools. Lean

mixture and turbo charging seem to be prerequisites for it, a use of high maximum pressures is unavoidable.

For the near future, the current research will continue with

0-D comprehensive simulation using details from the CFD 3-D model

aiming to find an optimum volume and air excess for a PM ICE of a

standard lay-out concerning engine power and efficiency;

The development of a model for unsteady heat transfer to PM filaments (at

high frequency temperature changes) in the 3-D model.

26

9-REFERANCES

1. Adil SMH , Hura V, Mehrotra A ,Khaliq A “Recent Advancements of

Porous Medium Combustion Technology in IC Engines and a New Concept of Cogeneration in PM-Engine”, COGNIZANCE ’04 – Annual Technical

Festival, IIT Roorkee, 19th to 21th March 2004,Roorkee (UA).

2. Balvinder Budania, Virender Bishnoi “A New Concept of

I.C.Engine with Homogeneous Combustion in a Porous Medium”

3. Macek Jan, Polášek Miloš, Josef Božek Research Center, Czech

Technical University in Prague, Czech Republic “Porous medium combustion

in engines may contribute to lower nox emissions” Paper code: F02V147

4. Prof.Dr.-Ing. Miroslaw Weclas Institut für Fahrzeugtechnik Georg-

Simon-Ohm-Fachhochschule Nürnberg Keßlerplatz 12 D-90489 Nürnberg “Potential of porous medium combustion technology as applied to internal

combustion engines”