Abstract
At present, the emissions of
internal combustion engines can only be improved by catalytic treatments of the
exhaust gases. Such treatments, however, result in high costs and relatively
low conversion efficiency. This suggests that a new combustion technique should
be developed to yield improved primary combustion processes inside the engine
with drastically reduced exhaust gas emissions. In this paper,. We report on such
a technique that is applicable to direct injection, internal combustion
engines, either diesel or gasoline fuelled. This technique is based on the
porous-medium (PM) combustion technology previously developed in the laboratory
for steady state household and industrial combustion processes. It is shown
that the PM combustion technique can be applied to internal combustion engines,
i.e. it is demonstrated that improvements obtained in steady state combustion
are also realizable in unsteady combustion processes. Theoretical
considerations are presented for internal combustion engines, indicating that
an overall improvement in thermal efficiency can be achieved for the PM engine.
This is explained and the general performance of the new PM engine is
demonstrated for a single-cylinder, air-cooled, direct injection diesel engine.
Verification experiments are described that were carried out as part of the
present study. Initial results are presented and an outlook is given on how the
present developments might continue in the future.
Introduction
Gradually came the days
when man started imagining huge & started striving for bringing his
imagination to the day of light Today after having reached such heights of
advancement we the human beings are still in thirst of technology indeed very
desperately. One such field for current discussion and interest of brilliant
brains is that how we developed such a engine which would give non-zero
emission and as well as less fuel consumption to with stand under wild range of
speed and load? the answer to this
question is homogeneous combustion in I.C. engine using porous medium
Technology.
The process of mixture
formation, ignition & combustion in conventional engine is not effective
due to the lack of mechanisms for homogenous combustion process.
Two parameters will be
required for future internal combustion engine i.e. non-zero emission level
& low fuel consumption. These parameter is strongly dependant the process
of mixture formation & combustion which are difficult to controlled in a
conventional engine combustion system.
The question is remain
unsolved is the method for realization of homogenous combustion in IC engine,
specially if the variable engine operational conditions are considered.
So here porous medium
concept is introduced in order to overcome above difficulty. PM utilize the
special features of highly porous media to support and controlled the mixture
formation and combustion process in IC engine.
Main requirements for future engine
Basic requirements for
future clean internal combustion (I.C.) engine concern very low that is exhaust
emissions level for both gaseous and particulate matter components under as low
as possible fuel consumption. Internal combustion engine has to operate in a
wide range of speeds and loads and should satisfy selected requirements under
all operational conditions. For vehicle application, the following conditions
are required for future engine:
Operation with a
homogeneous stoichiometric charge for high power density Operation with a homogeneous-lean charge for
low specific fuel consumption
Realization of
homogeneous combustion, for all mixture compositions for the lowest combustion
emissions.
For significant
reduction of specific fuel consumption and for a near-zero combustion emissions
especially attractive would be realization of engine operating with a
lean-homogeneous charge at part loads, assuming that the combustion process is
homogeneous.
HOMOGENEOUS COMBUSTION
Homogeneous combustion in an
IC engine is defined as a process characterized by a 3D-ignition 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 and combustion may be selected that define the
ability of a given combustion system to operate as a homogeneous combustion
system
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
POROUS MEDIUM (PM) TECHNOLOGY
The porous medium
technology for IC engines means here the utilization of specific features of a
highly porous media for supporting and controlling the mixture formation and
combustion processes in I.C. engines. The employed specific features of PM are
directly related to a very effective heat transfer and very fast flame
propagation within the PM. close view of a magnified 3D-structure of SiC
ceramic foam is given in Figure
New concept of mixture preparation
for homogeneous combustion in engines using porous medium technology
Different R&D activities of the author using porous materials (highly porous 3D-structures) (see LSTM at University of Erlangen-Nürnberg and Promos GmbH in Erlangen) indicated unique features of this technology for mixture formation and combustion processes, also as applied to IC engines.
Energy recirculation 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 its ignitability (activity). This energy recirculation
may be performed under different pressures and temperatures 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
Fuel vaporization in PM-volume:- Cmbination
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.
Mixing and homogenization in PM-volume:- Unique
features of the flow properties inside 3D-structures allow very effective
mixing and homogenization 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 the combustion chamber volume.
Heat release in PM-volume:- Under controlled combustion
temperature that permits homogeneous combustion conditions almost independently
of the engine load with possibility of controlling the combustion temperature
level.
PRINCIPLE OF THE PM-ENGINE
The
PM-engine is here defined as an internal combustion engine with the following
processes realized in a porous medium: internal heat recuperation, fuel
injection, fuel vaporization, mixing with air, homogenization of charge,
3D-thermal self-ignition followed by a homogeneous combustion. PM-Engine may be
classified with respect to the heat recuperation.
One of the most
interesting features of PM-engine is its multifuel performance. Independently
of the fuel used, this engine is a self-ignition engine characterized by its
3D-thermal ignition in porous medium. Finally, the PM-engine concept may be
applied to both two- and four-stroke cycles. Owing to the differences in
thermodynamic conditions, the PM-engine cycle has to be separately analyzed for
closed and open chambers
PM-engine with closed chamber
Let us start an analysis of the PM-engine cycle with a case of closed PM chamber, i.e. engine with a periodic contact between working gas and PM-heat recuperator. At the end of the expansion stroke the valve controlling timing of the PM-chamber closes and fuel is injected in the PM-volume. This volume represents in thermodynamic sense a low pressure chamber and a long time is available for fuel injection and its vaporization in the PM. These processes may continue through exhaust, intake and compression strokes (see Fig.)
Near the TDC of compression the valve in PM-chamber opens and the compressed
air flows from the cylinder into the hot PM volume containing fuel vaporous.
Very fast mixing of the gaseous charge occurs and the resulting mixture is
ignited in the whole PM volume. The resulting heat release process performs
simultaneously in the whole PM volume. The three essential conditions for a
homogeneous combustion are here fulfilled: homogenization of charge in
PM-volume, 3D-thermal self-ignition in PM and volumetric combustion with a
homogeneous temperature field in PM-volume. Additionally, the PM-material deals
as a heat capacitor and, hence, controls the combustion temperature.
Advantages of PM Technology
1)Very low emissions level due
to homogeneous combustion and controlled temperature in the PM-combustion
zone (e.g. NOx between 100 and 300mg/kWh for the (A/F) ratio from 1 to 5;. CO
can be reduced by several times; (almost) eliminated soot formation).
2)
Theoretically higher cycle efficiency due to similarity to the Carnot
cycle.
3)
Very low combustion noise due to significantly reduced pressure peaks.
4) Nearly constant and
homogeneous combustion temperature field in the PM- volume.
5)
Very fast combustion.
6)
Multi-fuel system.
7) May operate with homogeneous
charge: from stoichiometric to very lean mixture compositions.
8)
Weak effect of in-cylinder flow structure, turbulence or spray
atomization
Conclusion
There is no doubt that
the future of internal combustion engine is related to the homogeneous
combustion process in a wide range of engine operational conditions.
This technique shows potential for a near-zero combustion
emissions (especially NOx and soot) as well as high cycle efficiency (low fuel
consumption). Moreover, this kind of combustion system is less fuel specific.
However, the realization of homogeneous combustion in IC engine under variable
loads and speeds will probably require new concepts for mixture formation and
controlled ignition conditions under different engine loads. The future engine
operating with a homogeneous combustion process in a wide range of load and
speed will require variable temperature history during the compression stroke,
variable TDC compression temperature, completely vaporized fuel prior the
ignition process, variable mixture composition (A/F ratio), variable reactivity
(ignitability) of the charge, homogeneity of the charge, volumetric ignition
conditions, variable heat capacity of the cylinder content. But the research
has been carried out to make this process more & more economical.
