Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CHAMFERED PISTON
Field of the Invention
[0001] The present application relates to a piston in an internal
combustion engine
operating in a premixed combustion mode.
Background of the Invention
[0002] Compression ignition engines operating on diesel are designed to
introduce
the fuel directly into combustion chambers later in the compression stroke
using a high
pressure injection system. The fuel forms a stratified charge and burns in a
diffusion
combustion mode. When converting compression ignition engines from being
fuelled
with diesel to being fuelled with a gaseous fuel, such as natural gas, the
gaseous fuel is
typically introduced upstream of intake valves associated with respective
engine
cylinders by a low pressure injection system. The fuel is mixed with intake
air, and in
some cases exhaust gases from an exhaust gas recirculation (EGR) system, to
form a
combustible mixture. The mixture is later ignited in the engine cylinders by
an ignition
source, which can be combustion of a pilot fuel or a positive ignition source
such as a
spark plug, and combustion proceeds in a premixed combustion mode.
[0003] A problem with the amount of unburned hydrocarbons arises when using
the
piston designed for the compression ignition engine operating on diesel, the
"diesel
piston", for premixed combustion of natural gas, which it is not designed or
optimized
for. With reference to FIG. 1, diesel piston 10 has a large top-land volume,
which is
defined herein to be the volume in crevice 20 between the diesel piston and
liner 30
associated with cylinder wall 40, above piston ring 45 disposed in groove 50
and plane 15
of the top-land of the piston. The premixed air-fuel charge penetrates into
crevice 20,
unlike the stratified charge that forms when fuelled with diesel which is
injected late in
the compression stroke. It is difficult for the flame front of the premixed
flame resulting
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from combusting the gaseous fuel to reach deep into crevice 20 due to
excessive heat loss
to diesel piston 10 and liner 30. As a result, compression ignition engines
operating in a
premixed combustion mode often see high unburned hydrocarbon emissions due to
fuel
trapped in crevice 20.
[0004] Previously, to reduce the top-land volume the radial distance from
piston 10 to
liner 30 was reduced, and piston ring 50 was raised to decrease the depth of
crevice 20.
This led to issues with piston thermal management and increased the chance of
piston
failure. In addition, replacing the existing diesel piston 10 with a new
piston increased the
cost of conversion.
[0005] The state of the art is lacking in techniques for reducing unburned
hydrocarbon emissions in compression ignition engines operating in a premixed
combustion mode. The present method and apparatus provides a technique for
improving
these emissions.
Summary of the Invention
[0006] An improved piston for reciprocation in a cylinder bore along a
longitudinal
axis thereof in an internal combustion engine operating in a premixed
combustion mode,
the piston comprises a top surface partially defining a combustion chamber; an
outer
surface facing the cylinder bore; and an annular groove in the outer surface
extending
around the longitudinal axis; wherein the piston has a chamfered edge
extending from the
top surface to the outer surface spaced apart from the annular groove, wherein
a chamfer
angle is selected to reduce unburned hydrocarbons compared to when the piston
is not
chamfered.
[0007] An improved piston manufactured for an internal combustion engine
designed
to operate in a diffusion combustion mode, modified for operation in a
premixed
combustion mode, the piston for reciprocation in a cylinder bore along a
longitudinal axis
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thereof, the piston comprises a top surface partially defining a combustion
chamber; an
outer surface facing by the cylinder bore; and an annular groove in the outer
surface
extending around the longitudinal axis; wherein the piston has a chamfered
edge
extending from the top surface to the outer surface spaced apart from the
annular groove,
wherein a chamfer angle is selected to reduce unburned hydrocarbons compared
to when
the piston is not chamfered.
[0008] A gaseous fuel is the fuel that burns in the premixed combustion mode.
In a
preferred embodiment, the internal combustion engine is a dual fuel engine and
the
gaseous fuel is ignited by a pilot fuel. The dual fuel engine has a
compression ratio of at
least 15:1. In a preferred embodiment, the mass of unburned gaseous fuel is
reduced by at
least 30%. A chamfer depth is between a range of 5 mm and 7mm, and in a
preferred
embodiment the chamfer depth is approximately 6 mm. The chamfer angle is
between a
range of 30 and 60 , and in a preferred embodiment the angle is approximately
45 . In
another preferred embodiment, after the chamfer angle is selected, the chamfer
depth is
selected such that the compression ratio is at least 15:1 and the chamfer does
not intersect
a piston ring groove. The piston further comprises at least one of an
intersection between
a chamfer surface and the top surface is rounded to reduce thermal load; and
an
intersection between the chamfer surface and the outer surface is rounded to
reduce
thermal load. A residual volume is less than 2.5%, and preferably less than
1%. A
compression ratio when operating in the diffusion combustion mode can be
reduced when
operating in the premixed combustion mode.
[0009] An improved method of modifying a piston designed for an internal
combustion engine operating in a diffusion combustion mode to operating in a
premixed
combustion method, the piston comprising a top surface partially defining a
combustion
chamber and an outer surface facing a cylinder bore of the internal combustion
engine,
the method comprising chamfering an edge of the piston defined by an
intersection
between the top surface and the outer surface, wherein a chamfer angle is
selected to
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reduce unburned hydrocarbons compared to when the piston is not chamfered. The
method further comprising selecting a chamfer depth such that a compression
ratio is at
least 15:1, the chamfer angle and the chamfer depth cooperating to reduce
unburned
hydrocarbon emissions compared to when the piston is not chamfered.
Brief Description of the Drawings
[0010] FIG. 1 is a schematic view of a prior art piston for a compression
ignition
engine operating on diesel fuel.
[0011] FIG. 2 is a schematic view of a piston for a compression ignition
engine
operating on a gaseous fuel in a premixed combustion mode according to one
embodiment.
[0012] FIG. 3 is a cross-sectional view of the piston of FIG. 2.
[0013] FIG. 4 is a chart view tabulating emission results from computational
fluid
dynamic simulations employing the piston of FIG. 2 having a variety of
chamfers.
Detailed Description of Preferred Embodiment(s)
[0014] Referring to FIGS. 2 and 3, piston 12 is an embodiment of piston 10
modified
for operation in an internal combustion engine consuming a gaseous fuel in a
premixed
combustion mode. Piston 12 reciprocates along longitudinal axis 70 of a
cylinder bore of
the internal combustion engine defined by liner 30 and cylinder wall 40.
Combustion
chamber 100 is defined partially by top surface 90 of piston 12, liner 30 and
a cylinder
head (not shown). Piston 12 comprises piston bowl 95 (seen in FIG. 3) that
further
defines combustion chamber 100. Chamfer 80 is formed at the intersection of
top surface
90 and outer surface 110 of the piston, by removing edge 60 as seen in FIG. 1.
Edge 82 is
annular around longitudinal axis 70 and is defined by the intersection between
top surface
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90 and chamfer surface 86. Edge 84 is similarly annular and defined by the
intersection
between outer surface 110 and chamfer surface 86. Chamfer 80 is defined by
chamfer
angle 0 and chamfer depth h. Chamfer angle 0 is the angle between top surface
90 and
chamfer surface 86. Chamfer depth h is the axial distance along longitudinal
axis 70
between top surface 90 and edge 84. For a particular chamfer angle 0, chamfer
depth h is
selected such that chamfer 80 does not intersect piston ring groove 50, and,
in
combination with the chamfer angle, such that unburned hydrocarbon emissions
are
reduced. Volume 25 is defined as a residual volume between piston ring 45,
outer surface
110, liner 30 and plane 35, where the plane is horizontal to the piston ring
and intersects
chamfer edge 84. It is desirable to constrain the size of volume 25 such that
the flame
front can penetrate this volume, and if the flame front cannot penetrate this
volume to
reduce the amount of unburned hydrocarbons. Preferably, volume 25 is less than
2.5% of
combustion chamber volume when piston 12 is at top dead center, and more
preferably
less than 1%. Relatively speaking, typical top-land volumes for unchamfered
piston 10
shown in FIG. 1 can be greater than 4% of the combustion chamber volume when
the
piston is at the top dead center position.
100151 In operation, gaseous fuel and intake air are introduced into
combustion
chamber 100 where they form a homogenous, premixed air-fuel charge, which may
include recirculated EGR gas. Prior to ignition, the premixed air-fuel charge
extends into
crevice 21. Chamfer 80 opens up the top-land volume and allows the flame front
of the
premixed flame to travel deeper into crevice 21 to better consume the fuel,
thereby
reducing unburned hydrocarbon emissions. When converting a diesel engine that
operates
in diffusion combustion mode to operate in premixed combustion mode, diesel
piston 10
of FIG. 1 can be modified by forming chamfer 80 (a relatively inexpensive
modification),
avoiding the necessity to replace the piston with another one (a relatively
expensive
modification). In this manner all the other performance characteristics
associated with
piston 10 remain. Chamfer 80 increases the volume of the combustion chamber,
since
material is removed from the piston, reducing the compression ratio of the
engine.
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Reducing the compression ratio is undesirable for conventional diesel engines
because
they are designed for high compression ratios to facilitate compression
ignition and there
is no problem with fuel being trapped in the crevice because late cycle
injection forms a
stratified charge for a diffusion combustion mode. However, reducing the
compression
ratio is often desirable when running a compression ignition engine in
premixed
combustion mode to reduce the likelihood of premature ignition and engine
knock. An
example of such an engine is a dual fuel engine that employs a gaseous fuel as
a main
fuel, which forms a premixed air-fuel charge in the combustion chamber that is
ignited by
a pilot fuel, such as diesel. In this disclosure, premature ignition is the
ignition of the air-
fuel charge before a predetermined ignition event, and engine knock occurs
after the
predetermined ignition event and is the ignition of portions of the air-fuel
charge prior to
ignition by the advancing flame front of the premixed flame. It is preferred
that the
compression ratio remains at least 15:1 for the dual fuel engine such that the
pilot fuel
can be compression ignited, especially when the engine is starting.
100161 A series of computational fluid dynamic (CFD) simulations were
performed to
investigate the effectiveness of the chamfered piston. A variety of chamfers
were tested,
each defined by a unique chamfer angle 0 and depth h, to determine the impact
on
emissions. Each simulation employed a unique chamfer and entailed combustion
of a
premixed air-fuel charge in a combustion chamber and the measurement of the
resulting
emissions. The emission results for all the simulations are graphed in FIG.4.
Baseline
simulation 200 employed the unchamfered piston 10 from FIG. 1, and simulations
201
through 209 employed piston 12 with various chamfer angles 0 and depths h as
tabulated
in Table. 1. In FIG. 4, for each simulation 200 through 209, the left hand
bars are the
nitrous oxide (N0x) emissions, the middle bars are the carbon monoxide (CO)
emissions
and the right hand bars are the unburned hydrocarbon (CH4) emissions.
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Simulation Chamfer Chamfer Angle
Depth (mm) (degrees)
200 0 0
201 3 20
202 3 30
203 3 45
204 6 20
205 6 30
206 6 45
207 9 20
208 9 30
209 9 45
Table 1
100171 A surprising result of the simulations was that not all chamfer angles
and
depths resulted in improved unburned hydrocarbon (CH4) emissions compared to
baseline test 200. In simulations 201 and 205 the unburned hydrocarbon (CH4)
emissions
actually increased (got worse). This illustrates the unpredictable behavior of
the
combustion environment in general, and the difficulty in predicting the
behavior of the
flame front in crevice 21 in particular, and the significance of optimizing
chamfer angle
and chamfer depth for achieving increased reduction in unburned hydrocarbon
emissions.
In simulations 201 through 209, the nitrous oxide (N0x) emissions increased
slightly, but
within acceptable limits, and the carbon monoxide (CO) emissions decreased
compared
to baseline test 200. The chamfer employed in simulation 206 has been
identified as an
exemplary modification to piston 210, where unburned hydrocarbon (CH4)
emissions are
reduced and the compression ratio has been decreased enough to reduce the
likelihood of
premature ignition and engine knock, but not to significantly decrease the
volumetric
efficiency of the engine. Fixing chamber angle 0 at a preferred value, such as
45 ,
chamfer depth h can be selected such that the compression ratio is at least
15: 1 and such
that chamfer 80 does not intersect piston ring groove 50. A thermal analysis
of the pistons
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in simulations 201 through 209 revealed that edges 82 and 84 of chamfer 80
concentrate
heat, and to reduce such thermal loading these edges can be rounded.
100181 While particular elements, embodiments and applications of the present
invention have been shown and described, it will be understood, that the
invention is not
limited thereto since modifications can be made by those skilled in the art
without
departing from the scope of the present disclosure, particularly in light of
the foregoing
teachings.
