Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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THERMALLY STRATIFIED REGENERATIVE COMBUSTION CHAMBER AND
METHOD FOR MODIFYING A COMBUSTION CHAMBER IN AN INTERNAL
COMBUSTION ENGINE AND RESULTING ENGINE
[01] TECHNICAL FIELD
[02] The present disclosure relates generally to the field of reciprocating
piston
engines, and more particularly relates to combustion chamber modifications and
charge ignition modifications that result in improved combustion in existing
reciprocating piston, internal combustion engines, particularly such engines
operating at lean-burn conditions.
[03] BACKGROUND
[04] Large, stationary, so-called "legacy" natural gas fuel burning,
reciprocating
piston, combination or integrated internal combustion engines and compressors
driven by such engines have been used to pump natural gas through distribution
pipelines for more than 100 years following conversion of such compressor
engines to burn natural gas fuels instead of liquid fuels or steam.
[05] Some layouts of such combination engine-compressors can be observed in
patents to Mueller US 2,514,287; Scheiterlein US 2,917,226; and Heater et al.
US 4,091,772. More recent examples of the combustion chamber arrangement of
such engines can be observed in pending U.S. patent application publication US
2010/0319655 of McClendon. Additional description of the legacy engines can
be found in the report sponsored by Engines and Energy Conversion Lab entitled
"ERLE Cost Study of the Retrofit Legacy Pipeline Engines to Satisfy 1/2 g/BHP-
HR NOx", Rev. 1, May 21, 2009, involving a study performed by Engines and
Energy Conversion Lab, National Gas Machinery Laboratory (An Institute of
Kansas State University), Advance Technology Corporation and Hoerbiger.
[06] A further review of legacy Cooper-Bessemer Type GMV Integral-Angle Gas
Engine-Compressors may be found at "An ASME Historic Mechanical Engineering
Landmark" published by the ASME History and Heritage Committee in August
2006 for Knox County Historical Museum, Mount Vernon, Ohio. Another
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publication describing such engines may be observed in Bourn, Gingrich, and
Smith's "Advanced Compressor Engine Controls to Enhance Operation, Reliability
and Integrity", Southwest Research Institute, San Antonio, TX, USA, per Doe
Award No. DE-FC26-03NT41859, SwRi Project No. 03.10198, March 2004.
[07] Legacy engines of the type discussed above have served well and continue
in service up to the present time. On the other hand, they still suffer from
certain
disadvantages that have required further study and research to overcome. To
name a few such disadvantages, the engines are prone to be difficult to start
when cold; run roughly when cold, with mechanical stresses imposed on moving
parts such as pistons and bearings and with preignition events that damage
spark
igniters; run with relatively high variation of peak firing pressure and
variation
of timing of peak firing pressures of combustion cycles; emit excess NOx,
unburned hydrocarbons and excess CO; and run at efficiencies that are less
than
theoretically possible due to compromises imposed on the operating conditions
of the engines.
[08] Thus, a review of reciprocating piston, internal combustion engine art
reveals
various attempts to improve lean-burn combustion in the combustion chamber of
such engines by utilizing prechambers to initiate a torch-like output to cause
ignition
in lean-burn fuel-air mixtures.
[09] There is thus a need for improved combustion chamber design for such
engines to overcome or avoid the described disadvantages and to bring the
emissions of the engines into line with modern emission standards. Obtaining
such improvements, however, must not come at a cost of excess downtime for
the engines, which operate around the clock, or major modifications of the
engine components which would require costly and lengthy trials and research
to
prove feasibility and demonstrate successful results. Such engines are no
longer
manufactured, and repair and overhauling of the engine often require
manufacturing parts to replace worn out elements and components that are no
longer readily available.
[10] SUMMARY
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[11] A method for improving combustion in a main combustion chamber of an
internal combustion engine and a resulting engine are provided. The engine
includes a
main combustion chamber arranged between a head and a reciprocating piston. A
heat
retaining element is provided between the head and the main combustion
chamber.
The heat retaining element is configured to reduce heat transfer from the main
combustion chamber into the engine head. A precombustion chamber is provided,
which includes a reaction chamber. The reaction chamber is configured to be
provided with a secondary charge of air/fuel and a first spark igniter. The
reaction
chamber communicates with the main combustion chamber via a plurality of
discharge channels configured to discharge fuel radical species from the
reaction
chamber into the main combustion chamber. The fuel radical species is
generated
from the secondary charge. The heat retaining element is a self-supporting
structure
coupled to the head. The heat retaining element includes a head-facing portion
substantially corresponding in shape to a portion of the head-facing the main
combustion chamber.
[12] Additionally, a method is provided for improving combustion in a
combustion
chamber of a reciprocating piston internal combustion engine. The engine
includes a
main combustion chamber arranged between a head and a reciprocating piston.
The
method includes providing a heat retaining element between the head and the
main
combustion chamber, the heat retaining element being configured to reduce heat
transfer from the main combustion chamber into the engine head. The heat
retaining
element is a self-supporting structure coupled to the head. The heat retaining
element
includes a head-facing portion substantially corresponding in shape to a
portion of the
head facing the main combustion chamber. The heat retaining element is
provided
such that a gap is formed between the head-facing portion of the heat
retaining
element and the portion of the head facing the main combustion chamber.
[13] A heat retaining element is also provided, the heat retaining element
being
configured to be installed between a head and a main combustion chamber of an
internal combustion engine, the combustion chamber of the engine being between
a
head and a reciprocating piston. The heat retaining element is configured to
reduce
heat transfer from the main combustion chamber into the engine head. The heat
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retaining element is a self-supporting structure coupled to the head and
includes a
head-facing portion substantially corresponding in shape to a portion of the
head
facing the main combustion chamber. The heat retaining element is provided
such that
a gap is formed between the head-facing portion of the heat retaining element
and the
portion of the head facing the main combustion chamber.
[14] The objective of the disclosed concept is to provide a scheme for
redesigning the
main combustion chamber of legacy engines such as described above involving
only
modifying the head of the engine and part of the combustion chamber in the
head area
by using simple solutions involving changing and adding components to the
engine
head area that lead to avoidance of the disadvantages discussed above and
improvement of operating smoothness and efficiency of the engines.
[15] Proposed is the substitution of a Goossak-type pre-combustion radical
producing
reaction chamber and ignition system with a main combustion chamber
regenerative
heat retainer element or system for the gas jet igniter arrangement currently
used in the
head of the internal combustion engine of legacy engines and similar engines
using the
same fuel and ignition technology.
[16] The numerous other advantages, features and functions of embodiments of a
method for improving start-up and operating combustion in a main combustion
chamber of a reciprocating piston internal combustion engine and embodiments
of a
resulting improved engine will become readily apparent and better understood
in view
of the following description and accompanying drawings. The following
description is
not intended to limit the scope of the method for improving start-up and
operating
combustion in a main combustion chamber of a reciprocating piston internal
combustion engine and embodiments of a resulting improved engine, but instead
merely provides exemplary embodiments for ease of understanding.
[17] A feature of the embodiments and examples described herein includes a
method
of improving start-up and operating lean-burn combustion in a main combustion
chamber of a reciprocating piston, internal combustion engine having, for
example, a
main block and a fluid-cooled head, such combustion chamber being as defined
by a
variable volume above each engine piston, wherein the following steps are
provided:
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[18] (a) a heat retaining element distinct from the engine main block and head
located
within the head of the engine is provided that retains heat of combustion of
each
combustion cycle for transfer to charge of a subsequent combustion cycle;
[19] (b) the heat retaining element is installed in the head as a self-
supporting structure
having a head-facing portion having a shape substantially corresponding to the
shape
of that portion of the main combustion chamber defined by the head with a
clearance
gap between at least the head-facing portion and the head before engine
operation, with
the size of the clearance gap being arranged to be varied in dependence on
temperature
of the heat retainer after engine start-up, so that the heat transfer rate
between the heat
transfer element and the head is varied as a function of the size of the
clearance gap
during engine operation to optimize the rise in temperature of a layer of fuel-
air in
contact with the heat retaining element during the latter stage of the
compression
stroke.
[20] The size of the clearance gap of step (b) is varied by using natural
thermal
expansion and contraction of the heater retainer within the head during engine
operation. The gap may be reduced down to zero during engine operation to
effectively
cause increased heat transfer between the heat retainer and the head under
engine
operation conditions that cause high heating of the heat retainer while
maintaining the
ability to transfer heat to a layer of fuel-air in contact with the element.
[21] The engine head and heat retainer have respectively a head and heat
retainer
thermal diffusivity, a head and heat retainer thermal capacity, and a head and
heat
retainer heat transfer coefficient. The heat retainer is constructed from a
material that
has at least one of: a lower heat retainer thermal diffusivity than the head
thermal
diffusivity, a heat retainer heat capacity greater than the head heat
capacity, and a heat
retainer heat transfer coefficient lower than the head heat transfer
coefficient. The
engine that is suitable for use of the present disclosure may be a fluid-
cooled, two-
stroke, direct injected, natural gas fuel lean burning, engine that in one
configuration
preferably includes at the head of the engine a precombustion chamber having a
volume, with the precombustion chamber being provided with a spark igniter
within
the precombustion chamber volume, and receiving a charge of secondary air/
fuel each
combustion cycle of the engine.
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[22] The current precombustion chamber and flame torch igniter in the engines
uses a
spark ignited precombustion chamber supplied with a rich volume of igniter gas
that is
fired in timed relationship with the main combustion chamber combustion cycle,
to
cause a jet stream or "torch" of hot burning fuel to be injected into the main
combustion chamber to which a main charge has been admitted to ignite the main
charge for each combustion event. The torch igniter arrangement is needed
primarily
due to the lean burn conditions in the main combustion chamber and the other
conditions within the main combustion chamber.
[23] The current precombustion chamber and gas torch igniter is modified by
removing the nozzle end of the igniter and substituting for same a Goossak
type
reaction chamber with an end cap providing multiple discharge orifices with
sharp
entry and exit edges, defined lengths, and defined sizes for the orifices, in
combination
with using a defined ratio of reaction chamber volume to main combustion
chamber
volume, a defined reaction chamber air/fuel mixture, a defined main combustion
air/fuel mixture, and a defined pressure differential between the reaction
chamber and
the main combustion chamber.
[24] Optionally, a spark igniter in the main combustion chamber and an
auxiliary
electrical heater may be used in the modification of the basic cylinder head.
[25] The regenerative heat retaining element or system in the main combustion
chamber may be a coating, solid element, or other device located in the
modified head
end of the combustion chamber only, with adequate sealing between the
combustion
chamber and fuel injector, reaction chamber orifice cap, spark igniter and
other objects
in communication with the main combustion chamber. Assuming the material of
the
head 11 has a defined thermal diffusivity, thermal capacity and heat transfer
coefficient, the material of the regenerative heat retainer 20 will be
selected to have
one or more of a lower thermal diffusivity a (sq. ft. /hr), higher thermal
capacity Cp x p
(Btu/ft3 F) and lower heat transfer coefficient k (Btu/hr ft. F) than the head
11. An
electrical heater arrangement may be imbedded into the heat retainer or the
head of the
engine for adding heat to strategic areas of the main combustion chamber.
[26] Obtained is a more stable and reliable ignition timing and charge firing,
a more
stable yet shorter combustion event, lower combustion temperatures with
reduced
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NOx, improved coefficient of variation (COV) expressed as standard deviation
of peak
firing pressure (PFP) from combustion event to combustion event each
combustion
cycle, as well as location of PFP, lower carbon monoxide (CO) emission and
improved
specific fuel consumption (SFC), all without sacrificing power, and likely
improving
power.
[27] In accordance with an exemplary version of the invention, a method is
proposed
for improving start-up and operating combustion in a main combustion chamber
of a
water-cooled, two-stroke, direct fuel injected, natural gas fuel burning,
reciprocating
piston internal combustion engine that normally includes within a head of the
engine a
precombustion chamber having a volume, the precombustion chamber being
provided
with a charge of secondary air/fuel and a first spark igniter within the
precombustion
chamber volume, the precombustion chamber volume communicating with the main
combustion chamber via a jet orifice through which a burning flame torch or
jet of
ignited secondary charge is discharged into the main charge that has been or
is being
compressed each combustion cycle of the engine to ignite each main charge in
the
main combustion chamber, and optionally having a second spark igniter in the
main
combustion chamber at least to facilitate start-up of the engine, by:
[28] modifying the precombustion chamber by substituting a reaction chamber
and a
plurality of flame quenching reaction chamber discharge channels for the jet
orifice,
the reaction chamber channels being provided in an end cap closing the
reaction
chamber and having diameters, sharp entry and exit edges, lengths that are 0.9
to 1.6
times the channel diameters and that quench flame fronts in both entry and
exit
directions, a total combined cross-section area that is 0.02 to 0.03 times the
volume of
the main combustion chamber at minimum volume Vmin (with the piston at top
dead
center (TDC));
[29] configuring the volume of the reaction chamber so that it is 2-3% of the
volume
of the main combustion chamber at TDC,
[30] operating the engine using high energy radicals of spark-ignited,
partially
combusted, secondary charge generated in the reaction chamber and discharged
flamelessly through the discharge channels, each combustion cycle to cause
timed auto
ignition of each main charge after engine start-up;
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[31] providing a regenerative heat retainer element apart from the engine
structure and
second spark igniter within the head of the engine that retains heat of
combustion of
each combustion cycle for transfer to the main incoming charge in the next
combustion
cycle by slowing heat transfer from the main combustion chamber into the
engine
structure;
[32] optionally providing an auxiliary heat source apart from the second spark
igniter
in the head area of the engine and operating the auxiliary heat source to heat
each main
charge, each combustion cycle of the engine at least during engine start-up
and cold
running conditions; and
[33] operating the engine with excess air/fuel ratio (Lambda) in the main
combustion
chamber equal to 1.0 to 2Ø
[34] Structurally, the flame quenching channels are provided in an end cap
that, apart
from the channels, closes communication between the reaction chamber and the
main
combustion chamber. The end cap is also configured to have an interior volume
corresponding substantially to the desired volume of the reaction chamber
designed in
accordance with production or radical fuel species. Using the basic structure
of the
original precombustion igniter assembly of the existing engines and cutting
off the end
of the existing igniter and substituting a cap with discharge orifices
therefore enables
modification of the combustion chamber at minimum cost and complexity. The
outside
configuration of the existing igniter, apart from the cap, corresponds with
the
configuration of the existing igniter, so that the new reaction chamber and
cap can
simply be substituted for the existing igniter without modifying the head of
the engine
or the block of same.
[35] The second spark igniter may be connected directly to the heat retainer
element to
cause heat transfer from the second spark igniter to the heat retainer
element. The heat
retainer element may be fitted in the engine head with a clearance air gap
between the
heat retainer element and the head before engine start-up, with the size of
the gap
arranged to be varied in dependence on main combustion chamber temperature
after
engine start-up due to expansion and contraction of the heat retainer element,
so that
the heat transfer rate between the heat retainer element and the head is
varied as a
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function of the size of the air gap, which may be reduced to zero under some
engine
operating conditions.
[36] At least one auxiliary micro-chamber closely adjacent to and heated by
the main
combustion chamber may be provided, such micro chamber communicating with the
main combustion chamber via at least one micro chamber passage and receiving
heated
products of combustion during each combustion cycle via the micro chamber
passage,
and through which is discharge hot radicals derived from the products of
combustion
into the main combustion chamber during each subsequent intake event of each
combustion cycle of the engine to thereby seed each fresh main charge with the
radicals after a first combustion cycle of the engine.
[37] The numerous other advantages, features and functions of embodiments of a
method for improving start-up and operating combustion in a main combustion
chamber of a reciprocating piston internal combustion engine and embodiments
of a
resulting improved engine will become readily apparent and better understood
in view
of the following description and accompanying drawings. The following
description is
not intended to limit the scope of the method for improving start-up and
operating
combustion in a main combustion chamber of a reciprocating piston internal
combustion engine and embodiments of a resulting improved engine, but instead
merely provides exemplary embodiments for ease of understanding.
[38] Another aspect of the disclosure is an internal combustion engine adapted
to use
the above-described process, the engine including a block, one or more
reciprocating
pistons in the block, a fluid-cooled head, a main combustion chamber defined
by the
block and the head above each piston, each main combustion chamber portion
defined
by the head having a selected head chamber shape, and the described heat
retainer
preferably comprising in one embodiment, specifically a self-supporting
structure
secured in the head between the head and each respective piston, the heat
retainer
having at least a front surface facing towards a respective piston and a rear
surface that
faces the head and a least in part conforms substantially with the head
chamber shape,
at least a portion of the rear surface spaced from the head to define a gap
before engine
operation.
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[39] The heat retainer preferably is formed of a material and is configured so
that the
heat retainer expands as a function of combustion heat during engine operation
to
reduce the gap and thereby increase at least a rate of heat transfer between
the heat
retainer and the water-cooled head as a function of combustion heat during
engine
operation. The gap reduction in such engine may extend to zero. The head and
heat
retainer preferably also have respectively a head and heat retainer thermal
diffusivity, a
head and heat retainer thermal capacity, and a head and heat retainer heat
transfer
coefficient, with the heat retainer being constructed of a material that has
at least one
of: a lower heat retainer thermal diffusivity than the head thermal
diffusivity, a heat
retainer heat capacity greater than the head heat capacity, and a heat
retainer heat
transfer coefficient lower than the head heat transfer coefficient.
[40] The heat retainer in the afore described configurations creates what may
be
termed a "thermally stratified regenerative combustion chamber" in the sense
that the
heat retainer transmits or conducts heat of combustion from each combustion
cycle
into the engine block and head in different manners and rates, with lower
temperatures
occurring near the intersection of the head and block of the engine, or near
the lower
part of the combustion chamber, with higher temperatures occurring at the mid
and top
part of the heat retainer that may be spaced from the cooled head of the
engine, at least
until the heat retainer has expanded into contact with the head, at which
point the
separation gap would be zero. The mid and top part of the combustion chamber
thus
would function at a higher temperature than the lower part of the combustion
chamber.
The engine designer is thereby provided with a design tool to adjust the
operating
temperature of the combustion chamber to influence the characteristics of the
lean-
burn by designing the heat retaining element, including the material
constituting the
heat retaining element, and the gap in a manner that can produce a customized
thermally stratified regenerative combustion chamber which will be useful to
control
lean-burn combustion events within the combustion cycle of the engine.
[41] In the engine, a spark igniter may be provided in each main combustion
chamber
as afore described preferably connected directly to the heat retainer. The
engine
contemplated, moreover, will be a reciprocating piston, water-cooled, two-
stroke,
direct injected, natural gas fuel lean burning engine that normally includes
at the head
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of the engine adjacent each main combustion chamber a precombustion chamber
having a volume, the precombustion chamber being arranged to receive in the
volume
a charge of secondary air/fuel during each combustion cycle of the engine, a
spark
igniter in the precombustion chamber arranged to be cyclically ignited in
timed
relationship with the combustion cycle of the engine. The precombustion
chamber will
communicate with a respective main combustion chamber via one or more jet
orifices
or ports through which a burning flame jet of secondary charge ignited by the
spark
igniter or high energy radicals resulting from partial combustion of the
secondary
charge in the precombustion chamber is periodically discharged into the main
charge
that has been or is being compressed each combustion cycle of the engine to
ignite
each main lean charge in the main combustion chamber.
[42] The numerous other advantages, features, and functions of embodiments of
a
method for improving start-up and operating combustion in a main combustion
chamber of a reciprocating piston internal combustion engine and embodiments
of a
resulting improved engine will become readily apparent and better understood
in view
of the following description and accompanying drawings. The following
description is
not intended to limit the scope of the method for modifying or resulting
modified
engine and the components thereof, but instead merely provides exemplary
embodiments for ease of understanding.
[43] BRIEF DESCRIPTION OF THE DRAWINGS
[44] These and other features, aspects, and advantages of the present
disclosure will
become better understood with regard to the following description, appended
claims,
and accompanying drawings where:
[45] FIG. 1. is a schematic cross-section view in elevation of a prior art
combustion
chamber of a reciprocating piston I.C. engine with an exemplary surrounding
head area
of the engine.
[46] FIG. 2 shows the combustion chamber of FIG. 1 with various elements
implementing the present invention.
[47] FIGS. 3 and 4 show a perspective and cross-section view taken along line
4-4,
respectively, a reaction chamber cap with discharge channels or orifices.
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[48] FIG. 5 shows a variation of the combustion chamber of FIG. 2.
[49] FIG. 6 is a schematic vertical cross-section view of a combustion chamber
of another
exemplary two-stroke, direct injected, water-cooled, natural gas lean-burning,
reciprocating piston engine having installed therein an embodiment of a heat
retainer.
[50] FIG. 7 is a detail view of a spark igniter directly connected to the heat
retainer.
[51] FIGS. 8a and 8b show an embodiment of a shape of a head-facing portion of
a heat
retainer corresponding to a shape of a portion of the head-facing the main
combustion
chamber.
[52] FIG. 9 shows an example of a thermally stratified regenerative combustion
chamber.
[53] FIG. 10 shows an example of reduced spark ignition energy.
[54] It should be noted that the drawing figures are not necessarily drawn to
scale, but
instead are drawn to provide a better understanding of the components thereof,
and are not
intended to be limiting in scope, but rather to provide exemplary
illustrations. It should
further be noted that the figures illustrate exemplary embodiments of method
for
modifying or resulting modified engine and the components thereof, and in no
way limit
the structures or configurations of the method for modifying or resulting
modified engine
and the components thereof according to the present disclosure.
[55] DETAILED DESCRIPTION
[56] With reference to FIG. 1, there is illustrated schematically an existing
combustion
chamber 10 in a water-cooled head structure 11 of a known engine, specifically
an
integrated engine-compressor engine of the legacy variety originally made by
the Cooper-
Bessemer Company used to pump natural gas through distribution pipelines, such
engines
being described in the Background section above. The upper block portion of
such an
engine is illustrated and described in published international patent
application number
PCT/US2009/035771 published as WO 2009/114327 Al, and U.S. patent application
publication number US 2012/0118262 Al, for a description of the upper block
structure
and basic head arrangement of such engine.
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[57] The legacy Cooper-Bessemer legacy engine is a large displacement, two-
stroke,
natural gas burning, turbocharged engine in which the air of each charge is
supplied by
air inlet ports (not shown) that are opened when the piston approaches its
bottom
position, and the gas fuel is directly injected into the combustion chamber by
a fuel
injector 12 located at the top area of the combustion chamber 10. Exhaust of
combustion products is through exhaust outlet ports (not shown) also located
in the
cylinder wall adjacent the bottom position of the piston. Ignition of each
air/fuel charge
is accomplished by using a precombustion chamber 14 to ignite a smaller rich
charge
mixture that is discharged via jet orifice 16 into the main combustion chamber
10 in
the manner of a jet flame or torch to ignite a charge already admitted into
the main
combustion chamber 10.
[58] The precombustion chamber 14 and torch ignition scheme are needed because
in
such engines the combustion takes place under lean burn conditions where
another
ignition means such as a spark igniter would not ignite each charge
dependably,
resulting in inefficient operation and undesired exhaust emissions. Also,
because of the
burning characteristics of natural gas fuel, the temperature of the combustion
chamber
of such engines tends to be lower than optimum for stable, uniform ignition
and
combustion, resulting in cold starting and running conditions that produce
mechanical
stresses and undesirable exhaust emissions and variations in peak firing
pressure
timing. Such legacy engines also operate at high horsepower output ratings at
relatively low RPM in the 300-500 range, at compression ratios of 4-8 to one,
which
further creates challenges to optimize combustion in terms of stable, uniform
peak
firing pressure and timing, exhaust emissions, cold start running and overall
smooth
operation. Due to the operating conditions, power output usually must be
compromised
to limit emissions or otherwise cause the engine to run at best available
power.
[59] The present invention has for an objective to modify such existing
combustion
chambers in a simple, effective manner not requiring complete tear-down of the
engine, using the existing head structure and fuel delivery system to improve
combustion characteristics of the engine and reduce undesirable emissions,
such a CO
and NOx.
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[60] Specifically, an exemplary modification or conversion in accordance with
an
embodiment of the invention is shown in FIG. 2, where the modified main
combustion
chamber 18 is provided with a solid, fitted regenerative heat retainer 20 in
the form of
self-supporting shaped insert that is provided in close-fitting relationship
within the
existing head combustion chamber structure 11 above the piston of the engine
in each
cylinder of the engine to define the modified combustion chamber 18 between
the heat
retainer and the piston below. The heat retainer 20 will be formed of a
material that
will have the desired thermal properties needed to heat each new incoming
charge of
air/fuel by using the heat of combustion of the previous charge after the
first
combustion event. Specifically, assuming the material of the head 11 has a
defined
thermal diffusivity, thermal capacity and heat transfer coefficient, the
material of the
regenerative heat retainer 20 will be selected to have one or more of a lower
thermal
diffusivity, higher thermal capacity and lower heat transfer coefficient than
the head
11.
[61] For example, assuming the head 11 to be made of cast iron, the heat
retainer could
be made of aluminum. With the thermal properties of aluminum, each new charge
after
the first will be heated to a higher degree before ignition than the charges
admitted to
the existing prior art combustion chamber 10, given the high heat retention
properties
of aluminum. Heat retainers made of other appropriate materials could be used
provided the heat retention and transfer properties of the heat retainer
maintain the heat
of combustion of the main combustion chamber to the desired degree to achieve
the
advantages of the invention.
[62] As shown in FIG. 2, an auxiliary electrical heater element 22 may be
imbedded in
the heat retainer 20 to add heat to the combustion chamber 18 during cold
start
conditions to facilitate easier ignition and smooth running of the engine
during warm-
up and steady state operation. An appropriated control and power supply (not
shown)
would be provided for the heating element 22. Optionally, the heater may be
provided
within the head adjacent the combustion chamber.
[63] In addition, micro chambers 24 could be provided around the inner
periphery of
the heat retainer 20 with micro chamber passages 26 providing communication
between the combustion chamber 18 and the micro chambers 24 in accordance with
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the micro chamber designs described in U.S. patent Nos. 5,322,042; 5,862,788
and
6,178,942. The micro chambers provide fuel radicals for enhancing auto-
ignition of the
charges admitted into the combustion chamber 18 and otherwise provide the
benefits
described in the above-mentioned U.S. patents. Notably, the location of the
micro chamber
passages 26 in the heat retainer 20 advantageously avoids the need to modify
the head or
pistons of the existing engine being modified to provide the micro chambers.
[64] For the further possible improvement of the existing engine, the
precombustion
chamber 14 shown in FIG. 1 is modified by using instead a reaction chamber for
generating fuel radical species and replacing the end thereof having the jet
orifice 16 with
a new reaction chamber cap element 28 that is shown in more detail in FIGS. 4
and 5. The
cap element 28 includes discharge channels 30 in an otherwise closed end wall
32. The
radical producing reaction chamber 34 is substituted for the prior art
precombustion
chamber 14 and the precombustion chamber spark igniter 36 is provided to
initiate partial
combustion reaction of separate fuel supplied to the reaction chamber 34 in
timed
relationship with the combustion cycle of the main combustion chamber 18. As
described
above, preferably the existing igniter is replaced by the inventive reaction
chamber that is
configure to fit precisely where the prior igniter was located, using the same
connection
arrangement, e.g., a threaded connection, as the igniter. The newly added cap
28 on the
reaction chamber provides the desired discharge orifices for the fuel radical
species
generated in the reaction chamber 34 with minimum alteration of the basic head
structure
of the existing engine.
[65] The principle of operation of the reaction chamber 34 is in accordance
with principles
described in the patent of Mallory U.S. 2,148,357 and patents of Goossak GB
911,125
(1962), US 3,092,088; US 3,230,939, and US 3,283,751 as modified in accordance
with
the description above of the inventive reaction chamber and discharge
orifices. Also,
reference is made to the U.S. patent to Failla et al. US 4,898,135 for a
further description
of the principles of operation of the reaction chamber 34.
[66] For an exemplary engine provided with the modified main combustion
chamber 18
having a defined main combustion chamber volume Vmin at piston top dead center
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(TDC) position, an exemplary reaction chamber may have a volume of 2-3% of the
main combustion chamber volume Vmin. The channels 30 would have sharp entry
and
exit edges 38, 40, respectively, and be configured to quench any flame
attempting to
propagate through the cap discharge channels 30 by having lengths that are 0.9
to 1.6
times the diameters of the channels 30. The combined total cross-sectional
area of the
cap discharge channels would be 0.02 to 0.03 times the volume Vmin of the
modified
combustion chamber 18.
[67] Ignition of the charge in the modified main combustion chamber 18 would
be
produced by first igniting a relatively rich air/fuel charge (e.g., Lambda of
0.4 to 0.7)
in the reaction chamber using the reaction chamber spark igniter 36 in timed
relationship with the combustion cycle in the main combustion chamber 18,
thereby
causing discharge of high energy fuel radicals out of the cap discharge
channels 30 into
the main combustion chamber 18 which react with a relatively lean air/fuel
mixture in
the main combustion chamber 18 (Lambda 1.0 to 2.0) to cause autoignition of
the
charge in accordance with the known principles of radical induced ignition as
described in the aforementioned patents.
[68] The use of radicals instead of the flame torch principle of the prior art
described
above enables improved quality and timing of ignition of relatively lean
charge
mixtures in the main combustion chamber 18, smoother burning of the charge,
ignition
of the charge at lower temperature due to leaner conditions, reduction of
coefficient of
variation (COV) of peak firing pressure (PFP); reduction of location (timing-
wise) of
the PFP; and reduction of specific fuel consumption (SFC) vs. NOx trade-off as
compared with spark or flame ignition (flame front ignition).
[69] The heat retainer 20 may be installed in the modified combustion chamber
18
with a defined air gap 42 (see FIG. 5) between the top side of the heat
retainer 20 and
the adjacent upper side of the existing head structure 11. The gap further
impedes heat
transfer between the heat retainer 20 but the heat transfer will be varied as
a function of
the heat of the combustion chamber 18 and the temperature of the heat retainer
20 due
to expansion of the heat retainer 20 during engine operation that effectively
reduces the
size of the gap 42 theoretically down to zero. As the heat retainer 20 expands
against
the upper head structure 11, of course the heat retainer will be cooled at
that region,
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causing the heat retainer to contract to re-establish the gap 42, thereby
providing a self
regulating effect on the temperature of the heat retainer 20 in dependence on
main
combustion chamber operating temperature.
[70] A spark igniter 44 may be provided in the modified main combustion
chamber 18,
which preferably will be directly threaded to the heat retainer 20 to maximize
the
initial heating of the latter during cold start of the engine when the spark
igniter would
be used for igniting each charge of the main combustion chamber 18.
[71] By utilizing all or some of the inventive improvements to modify existing
prior art
legacy engine combustion chambers, disadvantages or the existing engines may
be
overcome and operating efficiencies improved while undesired emissions are
decreased and performance is improved. Research suggests that using the
improvements of the inventive concept, the engine power may be improved on the
order of 20% and running speed for the power obtained may be reduced on the
order of
100RPM, while improving emissions of CO, NOx and unburned HC. Control over
timing of ignition of each combustion cycle also is better achieved using the
inventive
concepts. By rendering the timing of PFP and the value of PFP from combustion
cycle
to combustion cycle, the engine will operate more uniformly with less cyclic
irregularity, thereby offering the possibility of using the engine for
electrical power
generation which requires cycle regularity for optimum electrical power
generation.
Most significantly, all the improvements may be obtained in a simplified
manner
involving only modification of the head area of the existing engines, thereby
avoid
costly and lengthy tear down and rebuilding of the main engine block and
contained
components.
[72] With reference to the embodiment shown in FIG. 6, a representative or
exemplary
engine block 50 is shown in vertical cross-section to expose a main combustion
chamber 18 lying between a reciprocating piston 54 and a head 16. The piston
54
reciprocates in a cylinder 58 in the block 50, and in a typical engine, a
plurality of such
pistons and cylinders will be provided within the block. The piston is
connected by a
connecting rod 59 to an output crankshaft (not shown) and both the block 50
and head
16 of the engine are typically liquid cooled, the coolant circulating through
coolant
passages 60 in the head 16 and block 50.
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[73] The engine represented in the drawing is a two-cycle engine, with air
supplied via
an air inlet 62 communicating with inlet ports 64 and exhaust discharged via
exhaust
ports 66 communicating with exhaust outlet 68 in a conventional manner, such
engines
being typical and known to internal combustion engine designers.
[74] Fuel for each combustion charge, in this example, a gaseous fuel such as
natural
gas, is supplied by direct injection via fuel injector 12 in timed
relationship with each
compression event in the main combustion chamber 18, so that at ignition of
the
charge the appropriate air/fuel ration is established for proper ignition and
combustion
in a conventional manner.
[75] For ignition, both a spark igniter 44 and a precombustion chamber igniter
31 may
be used, with the precombustion chamber including a precombustion chamber
proper
34 to which a rich mixture of air/fuel precombustion charge is supplied (not
shown),
and in which the precombustion charge is ignited by a precombustion spark
igniter 36.
Upon ignition of the precombustion charge in timed relationship with the
intended
combustion event in the main combustion chamber 18, a high energy jet of
ignited
precombustion charge is discharged in a jet stream through one or more
precombustion
chamber outlet orifices 30 that provide communication between the
precombustion
chamber proper 34 and the main combustion chamber 18. The high energy jet of
flame
or partially combusted radicals of fuel is used to ignite the main charge in
the main
combustion chamber in a conventional manner. The spark igniter 44 (see detail
in FIG.
7) in such an engine may be used to ignite each charge during start-up of a
cold engine,
or may be used under operating conditions requiring such ignition or
combustion
enhancement. In addition as previously described, the precombustion chamber
may be
omitted.
[76] In summary, the motion of the piston after start-up rotation of the
engine
crankshaft (not shown) forces air into the main combustion chamber 18 via the
inlet
ports 64, which air may be pressurized (turbocharged or supercharged), or
naturally
aspirated or circulated, and gaseous fuel is injected directly into the main
combustion
chamber 18 via the fuel injector 12. The precombustion chamber 34 receives
precombustion charge of air and fuel and ignites same by a precombustion spark
igniter to produce a hot, highly energetic jet of gas aimed at the main
combustion
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chamber which in turn ignites the charge now in the main combustion chamber 18
in
timed relationship with the intended combustion cycle of the engine. Coolant
circulates
through the block 50 and head 16 to control the temperature of the structures
and
inherently the temperature of the main combustion chamber to varying degrees,
depending on the location being considered within the combustion chamber.
[77] A regenerative heat retainer 20 is disposed in the head 16 of the engine
between
the head proper and the piston below, so that the main combustion chamber 18
now is
defined by the volume between the heat retainer 20, the piston 54 and the
cylinder 58.
The heat retainer is configured to essentially conform in shape to the
original head area
of the main combustion chamber, but with a selected gap 42 between the heat
retainer
20 and the liquid cooled head 16. The heat retainer is also configured to
preserve the
original compression ratio of the engine, although the engine designer could
alter the
compression ratio using the heat retainer if desired, simply by increasing or
reducing
the volume of the main combustion chamber by altering the size of the heat
retainer 20.
[78] The heat retainer is installed in the head as a self-supporting structure
having a
head-facing portion 43 having a shape substantially corresponding to the shape
of that
portion 45 of the main combustion chamber defined by the head with a clearance
gap
between at least the head-facing portion and the head before engine operation,
as
shown in the overturned head 16 showing the portion 45 of the head-facing the
main
combustion chamber in FIG. 8a and the heating retainer 20 having head-facing
portion
43 in FIG. 8b.
[79] The material and thermal properties of the head will be taken into
account in
designing the heat retainer 20, and the following considerations will be
evaluated or
implemented when designing and installing the heat retainer 20.
[80] It will be assumed that the head 16 possesses a known head thermal
diffusivity,
head thermal capacity and head heat transfer coefficient, all thermal
properties that
may be calculated or derived from known information and data, depending on the
material of the head. On the basis of such head thermal properties, the
regenerative
heat retainer will be configured to have a lower heat retainer thermal
diffusivity than
the head thermal diffusivity, a heat retainer heat capacity greater than the
head heat
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capacity, and a heat retainer heat transfer coefficient lower than the head
heat transfer
coefficient.
[81] This will result in retention of heat of combustion within the main
combustion
chamber in which the heat retainer is installed to a greater extent than
occurred in the
unmodified main combustion chamber. With the gap 42 provided between the heat
retainer 20 and the head 16, the modified main combustion chamber with the
heat
retainer also will be thermally stratified between the lower and upper parts
of the
combustion chamber, with the relatively cooler part of the combustion chamber
located
at the lower part thereof, and the hotter part near the top area thereof. This
feature
enables the engine designer to take into account the ignition and combustion
properties
of the air/fuel charge in the main combustion chamber, the direction of the
precombustion jet discharged from the precombustion chamber and other effects
that
may be desirable towards enhancing the efficiency of combustion of the engine
or
uniformity of the peak firing pressures over sequential combustion cycles. The
gap 42
is varied as a function of operating temperature within the main combustion
chamber
18 due to the expansion and contraction of the heat retainer 20, thereby
providing
another control function over the operating temperature of the main combustion
chamber 18. When the gap 42 is zero, of course, the liquid cooled head 16
contacts the
heat retainer at its upper end and cools the heat retainer in that area,
resulting
eventually in contraction of the heat retainer to reopen the gap 42, with the
cycle
repeating depending on operating conditions of the engine.
[82] Although the heat retainer is described herein as provided with a water-
cooled
head, it is not limited to use with a water-cooled head, but may be provided
in an
engine including engines cooled by other liquids or an air-cooled head, or a
head
cooled by other various mechanisms or even with uncooled engines.
[83] The heat retainer in the afore described configurations creates what may
be
termed a "thermally stratified regenerative combustion chamber" in the sense
that the
heat retainer transmits or conducts heat of combustion from each combustion
cycle
into the engine block and head in different manners and rates. As shown in
FIG. 9,
lower temperatures occur near the intersection of the head and block of the
engine, or
near the lower part of the combustion chamber, for example, at position A on
flange 46
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of heat retainer 20. Higher temperatures occurring at the mid and top part of
the heat
retainer 20 may be spaced from the cooled head of the engine, at least until
the heat
retainer has expanded into contact with the head, at which point the
separation gap
would be zero. The mid, for example, at position B and top part of the
combustion
chamber at position D thus function at a higher temperature than the lower
part of the
combustion chamber. The engine designer is thereby provided with a design tool
to
adjust the operating temperature of the combustion chamber to influence the
characteristics of the lean-burn by designing the heat retaining element,
including the
material constituting the heat retaining element, and the gap in a manner that
can
produce a customized thermally stratified regenerative combustion chamber
which will
be useful to control lean-burn combustion events within the combustion cycle
of the
engine.
[84] In the engine, a spark igniter 44 may be provided in each main combustion
chamber as afore described preferably connected directly to the heat retainer.
For
example, as shown in FIG. 9, spark igniter 44 may be positioned at position C.
[85] In one example during operation of a natural gas fuel burning internal
combustion
engine, thermocouples were placed at positions A, B, C, and D, as shown in
FIG. 9.
During operation, the thermocouples measured a temperature of 103 C at
position A,
294 C at position B, 229 C at position C, and 210 at position D.
[86] The engine contemplated, moreover, will be a reciprocating piston, water-
cooled,
two-stroke, direct injected, natural gas fuel lean burning engine that
normally includes
at the head of the engine adjacent each main combustion chamber a
precombustion
chamber 34 having a volume, the precombustion chamber being arranged to
receive in
the volume a charge of secondary air/fuel during each combustion cycle of the
engine,
a spark igniter in the precombustion chamber arranged to be cyclically ignited
in timed
relationship with the combustion cycle of the engine. The precombustion
chamber will
communicate with a respective main combustion chamber via one or more jet
orifices
or ports 30 through which a burning flame jet of secondary charge ignited by
the spark
igniter or high energy radicals resulting from partial combustion of the
secondary
charge in the precombustion chamber is periodically discharged into the main
charge
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that has been or is being compressed each combustion cycle of the engine to
ignite
each main lean charge in the main combustion chamber.
[87] The engine as modified or constructed in accordance with the embodiments
described herein will run with lower exhaust gas NOx, lower rate of misfire,
lower fuel
consumption, lower coefficient of variation (COV) of the location of Peak
Firing
Pressure over the operating range of the engine, lower COV Indicated Mean
Effective
Pressure (IMEP) over the operating range of the engine. The better control and
utilization over thermal transfer of heat of combustion by the thermally
stratified
regenerative combustion chamber results in the above characteristics of such
an
engine.
[88] In an exemplary engine, the head 16 could be made of cast iron and the
regenerative heat retainer 20 could be made of a self-supporting machined or
otherwise
shaped steel, with the spark igniter 44, for example, threaded directly into
the heat
retainer 20 as shown in the detail of FIG. 7. The fuel injector 12 likewise
could be
directly threaded to the heat retainer 20 as shown in FIG. 6. The heat
retainer 20 would
be sealed against leakage by direct metal-to-metal contact or by appropriate
gasket
material as needed. The thickness of the heat retainer 20 would be determined
by
appropriate calculation and iteratively based on the materials of the head 16
and heat
retainer 20, as well as the combustion chamber operating conditions, fuel used
in the
charges and other relevant parameters for any given engine so that the thermal
diffusivities, heat capacities, heat transfer coefficients of the head 16 and
heat retainer
20 would be matched to achieve the purposes set forth above.
[89] Although heat retainer 20 may be made of machined or otherwise shaped
steel, as
described above, heat retainer 20 may also be made of various steel or steels,
or other
metals, alloys, or materials, either machined, cast, shaped, or otherwise
formed. For
example, heat retainer 20 may be made of aluminum or an aluminum alloy,
titanium, a
magnesium alloy, or an alloy including at least one of chromium, nickel, iron,
molybdenum, cobalt, or tungsten.
[90] The embodiments described herein have particular advantages when applied
to a
two-stroke, reciprocating piston, natural gas lean-burning, integrated engine-
compressor as exemplified by legacy Cooper-Bessemer engines (e.g., Cooper-
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Bessemer Type GMV Integral-Angle Gas Engine-Compressor) that compress and
pump natural gas from gas fields or storage units through gas transmission
lines to
other storage stations or end users. Such engines make substantial horsepower
while
operating at relatively low RPM on the order of 300-500 RPM and compression
ratios
of 4-8 to one. These so called "legacy" engines are notorious for difficult
starting and
stable running when cold started, run with peak firing pressure variation that
is less
than desirable, suffer from bearing wear due to such operating
characteristics, poor
ignition resulting from uneven charge mixture variations and heating, and
undesirable
NOx and CO emissions. These engines use a precombustion chamber with igniter
and
hot burning jets discharged from the precombustion chambers to ignite each
charge,
without assistance from a spark igniter in the main combustion chamber after
start-up.
[91] Although various embodiments and examples disclosed herein describe heat
retainer 20 being used in a natural gas fuel burning engine, heat retainer 20
is not
limited to natural gas fuel burning engines, but may also be used in engines
using
other gaseous fuels, including, natural gases having various amounts of
methane,
high-methane natural gas, ethane, propane, or mixture of these or other
gaseous
fuels. Further, heat retainer 20 may be implemented in other engines fueled by
other forms of fuel, such as liquid fuels, including gasoline, kerosene,
diesel fuel,
JetA, JP4, JP5, JP8, JP10, methanol, ethanol, or mixture of these or other
liquid
fuels.
[92] Another advantage of the described embodiments is the promotion of
Enhanced Radical Ignition (ERI). ERI is a combination of two concepts: radical
ignition (RI) assisted by a regenerative heat retaining element (RHRE), the
heat
retaining element acting as an in-cylinder heat source required to enable auto-
ignition because of the low compression ratio/ temperature inherent in 2-
stroke
Legacy engines. Without the presence of the high temperature RHRE, it has been
shown by simulation that radical species created in a modified PCC (MPCC),
fail to
fully ignite fuel injected into the combustion chamber and misfire occurs.
[93] The ERI process with NOx producing flame front eliminated is applicable
to
2-stroke engines using modified radical producing MPCCs. To eliminate the
flame
front, ignition must start throughout the combustion chamber in what is
sometimes
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called a "volume mode" of combustion. Accomplishing this at the "cold"
starting
temperature of the low compression ratios of the Legacy's requires an in-
cylinder
heat source rather than increased compression ratio. Improved performance,
based
on RHRE has documented up to a larger bore, for example, a 8.5 inch AJAX
brand DP42 NG engine.
[94] Additionally, improved performance, based on RHRE has been shown with
Small Development Engine (SDE), for example, with 2.5 inch bore has also been
documented.
[95] Regenerative Heat Retaining Element (RHRE) has also proven useful in
state-
of-the-art engines, for example, those used in 2-stroke Unmanned Vehicle
Engines
(UAV) using heavy fuels. RHRE engines have been built and tested yielding
exceptionally stability, reduction in emissions and fuel consumption.
[96] In RHRE Legacy engines, after a brief start-up on spark-ignition (or
after later
refinements with heating elements imbedded within the RHRE), the RHRE retains
heat from the previous combustion cycle and serves as the ignition aid to
radical
species created in the MPCC for fully controlled auto-ignition of NG.
[97] Another factor involved in carrying out NG auto-ignition in 2-stroke
engines,
known from research and development over many years, is the appreciable
carryover of exhaust products from cycle to cycle in these engines. Run-on
after
ignition cut off is attributed to residual exhaust radical species and
residual exhaust
thermal energy. Simulation studies in conventional engines show elimination of
most of these potential RI species occurs during the exhaust of a 2-stroke
combustion cycle. With ERI, MPCCs aid in storage of a fraction of these
potential
RI species, and enable their reactivation from a state known as frozen
equilibrium
during compression and aid auto-ignition in the following ignition event.
[98] The remaining carryover species still in the combustion chamber in the
next
compression cycle also contribute to the ignition process by being reactivated
from
frozen equilibrium when heated by compression and the RHRE. Thus the essence
of the RHRE 2-stroke SI ignition consists of two interrelated processes. The
first is
retention of heat from the previous combustion cycle and the second is to use
of
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that heat to reactivate key residual chemical species naturally created late
in
combustion and quenched to frozen equilibrium during expansion of the previous
combustion event. Many of these residuals would have been exhausted from the
engine as contaminants without the presence of the RHRE. Instead they become
part of the ignition process on being reactivated to radical species by the
recycled
RHRE heat and enable radical assisted spark ignition (RASI). RASI has been
observed experimentally and the associated spark ignition energy (SIE)
measured
to be lower. RASI has been observed while measuring SIE and changing fuel/air
ratio, that SIE required falls to zero if the threshold of Radical Ignition
(RI) is
reached. RASI experiments have measured a 33% reduction in radical assisted
spark ignition voltage while a 1% reduction in the baseline caused engine
instability.
[99] An example of reduced spark ignition energy is shown in FIG. 10, showing
a
NG test of the AJAX brand DP42 engine comparing the rate of heat release and
spark ignition energy of the RHRE built with the baseline at 496 RPM. The RHRE
maximum rate of heat release is 43% greater while its spark ignition energy is
90%
lower. Both of these traits define the primary characteristics of the RHRE. As
shown in FIG. 10, a significant difference in magnitude of spark discharge,
baseline
is much greater using the same spark plug.
[100] While turbo chargers are used to increase the pressure of each charge of
air/fuel,
the increased loading on bearings and piston components and preignition in the
combustion chamber, particularly during cold start-up, decreases the operating
duty
cycle of the engine between maintenance cycles and overhauls, and increases
NOx
emissions and unburned hydrocarbons in the exhaust stream. The tendency is to
operate the engines to avoid these disadvantages by retarding timing of
ignition from
an optimum timing that could produce best power and economy.
[101] The regenerative heat retainer 20 produces rapid heating of the
combustion
chamber from a cold start condition of the engine without the need for
boosting the air
supply by turbocharging, for example, and creates a charge mixture capable of
ignition
at leaner air/fuel ratios. Extant ignition timing may then be retarded for
better power
while maintaining a more uniform, consistent peak firing pressure location
with
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reduced NOx emissions. Fuel consumption is further optimized by to the extent
that
the regenerative heat retainer will enable the engine to operate at lower
exhaust CO,
NOx for a given power output in view of the above considerations.
[102] The incorporation of a heat regeneration element in the combustion
chamber of
an AJAX brand engine has been explored and resulted in a significant
reduction in
the COV (IMEP) and lowered heat transfer losses when operating on propane. The
heat regeneration element is formed to profile the upper portion of the
combustion
chamber, above the top surface of the piston. As a result, the heat
regeneration
element, which, in this example was fabricated from a single piece of
material, is
subjected to the flame of the combustion and attains a high operating
temperature.
These criteria provide a unique method of heat transfer to the air/fuel
mixture to
enhance flame kernel development and combustion of the remaining air/fuel
mixture to
improve COV (IMEP).
[103] The heat regeneration element provides heat transfer to the entire
air/fuel
mixture, particularly during the compression stroke, which at any instant
results in a
stratified temperature of the charge. The highest temperature of the air/fuel
charge is in
the immediate vicinity of the surface of the heat regeneration element facing
the
air/fuel charge, such as that shown in FIG. 9. This temperature conditioning
of the
air/fuel charge enhances the flame speed, as a function of the stratified
temperature.
Advantageously, the highest air/fuel charge temperature is in contact with the
spark
plug, at position C of FIG. 9.
[104] In lean mixtures, COV (IMEP) is influenced by several factors (i.e.,
mixture
preparation, swirl), a high rate of development of the flame kernel is
essential and can
be examined by simulation. In short, upon the spark event the kernel can be
rapidly
developed by the flame front velocity as a function of the initial high
air/fuel
temperature in the vicinity of the spark plug. This process can be fully
simulated based
on chemical characterization/lean burn performance of the methane. The rapid
development of the flame kernel provides the basis of the stability of the
flame front
for the remainder of the combustion event and consequential improvement in COV
(IMEP).
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[105] Combustion of the lean air/fuel charge is achieved at high rates of heat
release,
as evidenced by the rate of change of cylinder pressure. The high rate of heat
release is
enabled by the instantaneous air/fuel temperature which is the direct result
of heat
transfer from the heat retaining element during the compression stroke. For
example,
characterization of methane for flame speed shows that for an 80 C increase
in
temperature over the unmodified, stock engine configuration due to the heat
regeneration element the flame speed has been shown to increase by 50%.
Therefore,
in an unmodified, stock engine where an air/fuel charge has not been
conditioned, the
overall burn time during the power stroke will be longer (due to lower rates
of heat
release), which results in higher levels of heat transfer loss to the cylinder
wall and
head. Increasing the rate of heat release in a controlled method is a very
effective
aspect for improving the combustion process for methane fuelled engines. In
state-of-
the-art, high rate of heat release, bi-fuel, diesel combustion technology the
heat
transferred to the coolant has been shown to be reduced from 19% to 10% and
heat
transferred to work has been shown to be increased by 14%.
[106] The temperature stratification achieved with the heat regeneration
element is
particularly advantageous as the highest temperature air/fuel mixture is
utilized to
stabilize the initial flame kernel. Temperature stratification has not been
able to be
attained with increased compression ratio or allowing the stock head to
overheat. Both
of these approaches are undesirable for methane combustion, as they promote
uncontrolled compression ignition. In other words, instantaneous temperature
stratification is desirable and highly effective.
[107] Test and simulation data indicate that RHRE fundamentally alters the
combustion process in-cylinder, improving engine performance on multiple
fronts
without negative trade-offs. These improvements include, but are not limited
to:
dramatically reduced emissions, particularly NOx, even while improving engine
stability and fuel economy; increased fuel economy without sacrificing power;
higher
power ratings for engines while complying with emissions standards thus
reducing the
need for additional capacity; improved lean combustion process eliminating
detonations and misfires; reducing engine wear and maintenance costs; reducing
or
eliminating engine performance problems associated with changing natural gas
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composition; allowing retrofitting of existing Legacy integral engine
population at a
much lower cost than replacement (providing emissions than other emission-
reducing
solutions, significantly improving the long-term savings); and making existing
engine
designs, which may have been discontinued due to emissions non-compliance,
viable
again when equipped with RHRE technology.
[108] Still another advantage is that modifying an existing engine to operate
with the
benefit of the regenerative heat retainer can be accomplished without major
modification of the engine head and block elements. Typically, only the head
must be
modified in some minor respects to accommodate the regenerative heat retainer
while
preserving the original compression ratio or modifying the compression ratio
as
desired.
[109] While particular embodiments of a method of modifying and a resulting
modified combustion chamber in a reciprocating piston internal combustion
engine are
discussed above, it is to be understood that not necessarily all objects or
advantages
may be achieved in accordance with any particular embodiment. Thus, for
example,
those skilled in the art will recognize that the embodiments and examples may
be
embodied or carried out in a manner that achieves or optimizes one advantage
or group
of advantages as taught herein without necessarily achieving other objects or
advantages as may be taught or suggested herein.
[110] The skilled artisan will recognize the interchangeability of various
disclosed
features. In addition to the variations described herein, other known
equivalents for
each feature can be mixed and matched by one of ordinary skill in this art
arrive at the
disclosed method or resulting modified engine in accordance with principles of
the
present disclosure.
[111] Although the method and modified engine described herein are disclosed
in the
context of certain exemplary embodiments and examples, it therefore will be
understood by those skilled in the art that the present disclosure extends
beyond the
specifically disclosed embodiments to other alternative embodiments and/or
uses of the
disclosure and obvious modifications and equivalents thereof. Thus, it is
intended that
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the scope of the present disclosure herein disclosed should not be limited by
the
particular disclosed embodiments described above.
