U.S. patent application number 16/141809 was filed with the patent office on 2019-03-14 for exhaust management strategies for opposed-piston, two-stroke engines.
This patent application is currently assigned to ACHATES POWER, INC.. The applicant listed for this patent is ACHATES POWER, INC.. Invention is credited to Christopher J. Kalebjian, Suramya D. Naik, Fabien G. Redon.
Application Number | 20190078479 16/141809 |
Document ID | / |
Family ID | 47843417 |
Filed Date | 2019-03-14 |
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United States Patent
Application |
20190078479 |
Kind Code |
A1 |
Kalebjian; Christopher J. ;
et al. |
March 14, 2019 |
EXHAUST MANAGEMENT STRATEGIES FOR OPPOSED-PISTON, TWO-STROKE
ENGINES
Abstract
Exhaust temperature management strategies for an opposed-piston,
two-stroke engine with EGR are based on control of a ratio of the
mass of fresh air and external EGR delivered to a cylinder to the
mass of the trapped charge (density of the delivered charge
multiplied by the trapped volume at port closing).
Inventors: |
Kalebjian; Christopher J.;
(San Diego, CA) ; Naik; Suramya D.; (Milpitas,
CA) ; Redon; Fabien G.; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ACHATES POWER, INC. |
San Diego |
CA |
US |
|
|
Assignee: |
ACHATES POWER, INC.
San Diego
CA
|
Family ID: |
47843417 |
Appl. No.: |
16/141809 |
Filed: |
September 25, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14378252 |
Aug 12, 2014 |
10119444 |
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PCT/US2013/026737 |
Feb 19, 2013 |
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16141809 |
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61634168 |
Feb 21, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F02M 26/23 20160201;
F02B 2075/025 20130101; F02B 37/04 20130101; Y02T 10/146 20130101;
F02D 41/064 20130101; F02B 29/0412 20130101; Y02T 10/12 20130101;
F01B 7/14 20130101; F02M 26/08 20160201; F01B 7/02 20130101; F02B
75/282 20130101; F02B 25/08 20130101; F02B 75/02 20130101; Y02T
10/144 20130101; F02B 33/44 20130101; F02M 26/15 20160201; F02B
29/0418 20130101; F01N 3/2006 20130101 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F02B 29/04 20060101 F02B029/04; F02B 75/28 20060101
F02B075/28; F02B 75/02 20060101 F02B075/02; F02B 33/44 20060101
F02B033/44; F02M 26/08 20160101 F02M026/08; F02B 25/08 20060101
F02B025/08; F01B 7/14 20060101 F01B007/14; F02D 41/06 20060101
F02D041/06; F02B 37/04 20060101 F02B037/04; F01B 7/02 20060101
F01B007/02 |
Claims
1. A two-stroke cycle internal combustion engine including at least
one cylinder with exhaust and intake ports, two pistons disposed in
a bore of the cylinder with end surfaces in opposition to each
other, a charge air channel to provide charge air to at least one
intake port of the engine, and an exhaust channel to receive
exhaust from at least one exhaust port of the engine, in which an
exhaust gas recirculation (EGR) loop has a loop input coupled to
the exhaust channel and a loop output coupled to the charge air
channel, a supercharger is operable to pump charge air in the
charge air channel, a backpressure device is operable to control
backpressure in the exhaust channel, and a control mechanization is
operable to increase a trapped temperature in the cylinder by
controlling one or more of the supercharger, the EGR loop, and the
backpressure device during a cold start condition of the engine or
a low-load condition of the engine.
2. The two-stroke cycle internal combustion engine of claim 1, in
which the charge air channel includes at least one charge air
cooler, wherein the loop output is coupled in series with the at
least one charge air cooler.
3. The two-stroke cycle internal combustion engine of claim 1, in
which the EGR loop includes a variable valve and the control
mechanization is operable to control a setting of the valve in
response to a trapped temperature of the cylinder.
4. The two-stroke cycle internal combustion engine of claim 1, in
which the supercharger is one of a single-speed supercharger, a
multispeed supercharger, and a variable-speed supercharger and the
control mechanization is operable to control the speed of the
supercharger in response to a trapped temperature of the
cylinder.
5. The two-stroke cycle internal combustion engine of claim 1, in
which the backpressure device includes a variable valve and the
control mechanization is operable to control a setting of the valve
in response to a trapped temperature of the cylinder.
6. The two-stroke cycle internal combustion engine of claim 1, in
which the backpressure device includes one of a variable valve, a
variable nozzle turbo mechanism, a multi-stage turbo mechanism, and
a compound turbo mechanism, and the control mechanization is
operable to control a setting of the backpressure device in
response to a trapped temperature of the cylinder.
7. The two-stroke cycle internal combustion engine of claim 1,
further including a turbo-charger with a charge air output coupled
to the charge air channel and a turbine input coupled to the
exhaust channel, and a back pressure valve in series with the
turbine output, in which the backpressure valve is settable to a
state causing a back pressure acting upon the exhaust channel.
8. The two-stroke cycle internal combustion engine of claim 7, in
which the turbo-charger includes a variable-geometry turbine.
9. The two-stroke cycle internal combustion engine of claim 1,
further including a turbo-charger with a charge air output coupled
to the charge air channel, a turbine input coupled to the exhaust
channel, a turbine output coupled to an exhaust output, and a back
pressure valve in series between the turbine output and the exhaust
output, in which the backpressure valve is settable to a state
causing a back pressure acting upon the exhaust channel.
10. The two-stroke cycle internal combustion engine of claim 9, in
which the turbo-charger includes a variable-geometry turbine.
11. The two-stroke cycle internal combustion engine of claim 1, in
which a pair of pistons is disposed in a bore of the at least one
cylinder with end surfaces opposing.
12. The two-stroke cycle internal combustion engine of claim 1,
further including a recirculation loop having an inlet connected to
the charge air channel downstream of the supercharger, an outlet
connected to the charge air channel upstream of the supercharger,
and a variable valve between the inlet and the outlet, and, an air
cooler in series with the variable valve.
13. The two-stroke cycle internal combustion engine of claim 1, in
which the EGR loop is one of a high-pressure EGR loop and a
low-pressure EGR loop.
Description
BACKGROUND
[0001] The field is internal combustion engines. Particularly, the
field relates to two-stroke internal combustion engines. More
specifically, the field covers exhaust management strategies for
opposed-piston, two-stroke engines.
[0002] As seen in FIG. 1, an opposed-piston engine includes at
least one cylinder 10 with a bore 12 and longitudinally-displaced
exhaust and intake ports 14 and 16 machined or formed therein. Fuel
injector nozzles 17 are located in or adjacent injector ports that
open through the side of the cylinder, at or near the longitudinal
center of the cylinder. Two pistons 20, 22 are disposed in the bore
12 of the cylinder with their end surfaces 20e, 22e in opposition
to each other. For convenience, the piston 20 is referred as the
"exhaust" piston because of its proximity to the exhaust port 14;
and, the end of the cylinder wherein the exhaust port is formed is
referred to as the "exhaust end". Similarly, the piston 22 is
referred as the "intake" piston because of its proximity to the
intake port 16, and the corresponding end of the cylinder is the
"intake end".
[0003] Operation of an opposed-piston engine with one or more
cylinders such as the cylinder 10 is well understood. In this
regard, and with reference to FIG. 2, in response to combustion
occurring between the end surfaces 20e, 22e the opposed pistons
move away from respective top dead center (TDC) positions where
they are at their closest positions relative to one another in the
cylinder. While moving from TDC, the pistons keep their associated
ports closed until they approach respective bottom dead center
(BDC) positions in which they are furthest apart from each other.
The pistons may move in phase so that the exhaust and intake ports
14, 16 open and close in unison. Alternatively, one piston may lead
the other in phase, in which case the intake and exhaust ports have
different opening and closing times.
[0004] In many opposed-piston constructions, a phase offset is
introduced into the piston movements. With reference to FIG. 1, for
example, presuming the exhaust piston leads the intake piston, the
phase offset causes the pistons to move around their BDC positions
in a sequence in which the exhaust port 14 opens as the exhaust
piston 20 moves through BDC while the intake port 16 is still
closed so that combustion gasses start to flow out of the exhaust
port 14. As the pistons continue moving away from each other, the
intake port 16 opens while the exhaust port 14 is still open and a
charge of pressurized air ("charge air") is forced into the
cylinder 10, driving exhaust gasses out of the exhaust port 14. The
displacement of exhaust gas from the cylinder through the exhaust
port while admitting charge air through the intake port is referred
to as "scavenging". Because the charge air entering the cylinder
flows in the same direction as the outflow of exhaust gas (toward
the exhaust port), the scavenging process is referred to as
"uniflow scavenging".
[0005] As the pistons move through their BDC locations and reverse
direction, the exhaust port 14 is closed by the exhaust piston 20
and scavenging ceases. The intake port 16 remains open while the
intake piston 22 continues to move away from BDC. As the pistons
continue moving toward TDC (FIG. 2), the intake port 16 is closed
and the charge air in the cylinder is compressed between the end
surfaces 20e and 22e. Typically, the charge air is swirled as it
passes through the intake port 16 to promote scavenging while the
ports are open and, after the ports close, to mix the air with the
injected fuel. Fuel (which is, typically, diesel) is injected into
the cylinder by one or more high pressure injectors. With reference
to FIG. 1 as an example, the swirling air (or simply, "swirl") 30
has a generally helical motion that forms a vortex in the bore that
circulates around the longitudinal axis of the cylinder. As best
seen in FIG. 2, as the pistons advance toward their respective TDC
locations in the cylinder bore, fuel 40 is injected through the
nozzles 17 directly into the swirling charge air 30 in the bore 12,
between the end surfaces 20e, 22e of the pistons. The swirling
mixture of charge air and fuel is compressed in a combustion
chamber 32 defined between the end surfaces 20e and 22e when the
pistons 20 and 22 are near their respective TDC locations. When the
mixture reaches an ignition temperature, the fuel ignites in the
combustion chamber, driving the pistons apart toward their
respective BDC locations.
[0006] Each complete movement of a piston 20, 22 (TDC to BDC and
BDC to TDC) is a "stroke". Since one complete power cycle of the
engine occurs in two complete movements of a piston, the engine is
referred to as a "two-stroke engine" or a "two-stroke cycle
engine".
[0007] As illustrated in FIG. 2, fuel is directly injected through
the side of the cylinder ("direct side injection") into the
cylinder bore and the movement of the fuel interacts with the
residual swirling motion of the charge air in the bore. As the
engine operating level increases and the heat of combustion rises,
an increasing amount of nitrogen oxide (NOx) is produced. However,
increasingly stringent emission requirements indicate the need for
a significant degree of NOx reduction. One technique reduces NOx
emission by exhaust gas recirculation ("EGR"). EGR has been
incorporated into spark-ignited four-stroke engine constructions
and two-stroke, compression-ignition engines with a single piston
operating in each cylinder. EGR constructions for opposed-piston,
two-stroke engines are taught in the assignee's U.S. patent
application Ser. No. 13/068,679, filed May 16, 2011 and published
as US 2011/0289916 A1 on Dec. 1, 2011.
[0008] However, while opposed-piston two-cycle engines can now be
equipped with EGR to reduce NOx emissions, new control strategies
are needed to manage tailpipe emissions (exhaust) in response to
varying engine operating conditions such as cold start, low load,
and low ambient temperature.
[0009] In this regard, diesel engines can be equipped with
after-treatment systems that subject exhaust gasses to catalytic
processes that convert HC, CO and NOx. The catalytic materials must
be heated to a "light-off" level in order to function.
After-treatment systems may also include a diesel particulate
filter (DPF) that filters soot from the exhaust gasses. Thermal
energy must be delivered to the filtration material in order for it
to achieve a temperature level at which it properly regenerates. In
both cases, heat is obtained from the exhaust gasses themselves.
However, when the engine is initially turned on from an ambient
non-operating thermal state (referred to as a "cold start"), the
exhaust gasses are insufficiently hot to activate the catalytic
materials and/or to regenerate the DPF. Presently, the majority of
tailpipe emissions during certification as well as under real-world
driving conditions occur just after starting the engine, before the
catalyst and filtration materials reach operating temperatures. In
many applications, greater than 50% of the tailpipe emissions on a
diesel FTP-75 test occur in the cold start phase (Lambert, C.,
2006. "Advanced CIDI Emission Control System Development", Report
for the U.S. Department of Energy, DE-FC26-01NT41103, Jun. 30,
2006). In fact, with a well designed after-treatment system, it can
be shown that greater than 50% of the tailpipe emissions occur
during the first 200 seconds of the test. Thus, the time from
initial engine start-up to when the catalyst and filtration
materials have enough heat to perform adequately needs to
decrease.
[0010] Accordingly, it is desirable to operate an opposed-piston
two-cycle engine during cold start by rapidly raising the exhaust
gas temperature in order to achieve early activation of
after-treatment materials.
[0011] Another characteristic of diesel cold start is combustion
instability in which successful combustion is punctuated by
complete or partial misfires as the engine accelerates to an idling
speed. Therefore, in some aspects, it is desirable that a strategy
for elevating exhaust temperatures during cold start of an
opposed-piston two-stroke engine also provide for combustion
stability.
[0012] In other words, it is desirable to provide high exhaust
temperatures in the exhaust gasses of an EGR-equipped,
opposed-piston two-stroke engine under varying operating conditions
while maintaining the engine's combustion stability.
SUMMARY
[0013] A strategy to achieve these objectives is based upon
management of the gas temperature in the cylinder just after port
closing (PC) so as to control the exhaust temperature in a way that
enables significant heat to be driven into the exhaust system while
maintaining combustion stability.
[0014] In some aspects, when the exhaust temperature is to be
elevated, the engine is operated with a low modified air delivery
ratio which yields increased internal residuals in the cylinder
after port closure, which in turn increases the temperature of the
air charge trapped (retained) in the cylinder. This higher trapped
air charge temperature yields a higher exhaust temperature.
[0015] Preferably, a modified air delivery ratio represents a
normalized mass flow rate into a cylinder. In this regard, a
modified delivery ratio of 1 means that the amount of air delivered
through the intake port is equal to the amount of mass that is
inside the cylinder.
[0016] The elevated trapped temperature enables faster light-off of
after-treatment catalysts during cold start, low load operation,
and low ambient temperatures, catalyst temperatures high enough for
light-off, and exhaust temperatures high enough to regenerate a
diesel particulate filter under varying driving conditions,
including idle and light loads.
[0017] In the low modified air delivery ratio operating mode, an
intake manifold pressure is maintained at a level high enough to
achieve an air-fuel ratio adequate for combustion stability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a side sectional partially schematic drawing of a
cylinder of a prior art opposed-piston engine with opposed pistons
near respective bottom dead center locations, and is appropriately
labeled "Prior Art".
[0019] FIG. 2 is a side sectional partially schematic drawing of
the cylinder of FIG. 1 with the opposed pistons near respective top
dead center locations where end surfaces of the pistons define a
combustion chamber, and is appropriately labeled "Prior Art".
[0020] FIG. 3 is a conceptual schematic diagram of an
opposed-piston two-cycle engine in which aspects of an air
management system with EGR are illustrated.
[0021] FIG. 4 is a conceptual schematic drawing illustrating a
preferred construction for EGR in an opposed-piston two-cycle
engine.
[0022] FIG. 5 is a graphical representation of varying a modified
air delivery ratio.
[0023] FIG. 6A is a flow diagram representing a control loop for
managing trapped air charge temperature based upon control variable
values.
[0024] FIG. 6B is a graphical representation showing temperature
profiles of a trapped air charge while in a cylinder during a
normal operating condition and during a warm-up condition.
[0025] FIG. 6C is a graphical representation of a relationship
between trapped temperature and exhaust gas temperature.
[0026] FIGS. 7A, 7B, 7C, and 7D are conceptual schematic drawings
illustrating alternate constructions for cooling charge air in a
supercharger recirculation loop.
[0027] FIG. 8 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a first arrangement of
after-treatment components.
[0028] FIG. 9 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a second arrangement of
after-treatment components.
[0029] FIG. 10 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a third arrangement of
after-treatment components.
[0030] FIG. 11 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a fourth arrangement of
after-treatment components.
[0031] FIG. 12 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a fifth arrangement of
after-treatment components.
[0032] FIG. 13 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a high-pressure EGR loop
construction in combination with a sixth arrangement of
after-treatment components.
[0033] FIG. 14 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a low-pressure EGR loop
construction in combination with a seventh arrangement of
after-treatment components.
[0034] FIG. 15 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a low-pressure EGR loop
construction in combination with an eighth arrangement of
after-treatment components.
[0035] FIG. 16 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a low-pressure EGR loop
construction in combination with a ninth arrangement of
after-treatment components.
[0036] FIG. 17 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a low-pressure EGR loop
construction in combination with a tenth arrangement of
after-treatment components.
[0037] FIG. 18 is a conceptual schematic drawing illustrating an
opposed-piston two cycle engine with a low-pressure EGR loop
construction in combination with an eleventh arrangement of
after-treatment components.
[0038] FIGS. 19, 20, and 21 are conceptual schematic drawings
illustrating an opposed-piston two cycle engine with a low-pressure
EGR loop construction in combination with twelfth, thirteenth, and
fourteenth arrangements of after-treatment components.
DETAILED DESCRIPTION
[0039] The constructions described in this specification are
presented in an explanatory context that includes an opposed-piston
two-cycle engine having at least one cylinder in which a pair of
pistons is disposed with their end surfaces in opposition. Each
cylinder includes intake and exhaust ports formed or machined in a
sidewall thereof. This explanatory context is intended to provide a
basis for understanding various exhaust management strategies by
way of illustrative examples.
[0040] In FIG. 3, an opposed-piston, two-cycle engine 49
(hereinafter referred to as "the opposed-piston engine") has at
least one cylinder 50. For example, the opposed-piston engine may
have one cylinder, two cylinders, three cylinders, or four or more
cylinders. Each cylinder 50 has a bore 52 and exhaust and intake
ports 54 and 56 formed or machined in respective ends thereof. The
exhaust and intake ports 54 and 56 each include one or more
circumferential arrays of openings in which adjacent openings are
separated by a solid bridge. (In some descriptions, each opening is
referred to as a "port"; however, the construction of a
circumferential array of such "ports" is no different than the port
constructions shown in FIG. 3.) Exhaust and intake pistons 60 and
62 are slidably disposed in each cylinder bore 52 with their end
surfaces 61 and 63 opposing one another. The exhaust pistons 60 are
coupled to a crankshaft 71, the intake pistons are coupled to the
crankshaft 72.
[0041] When the pistons 60 and 62 of a cylinder 50 are at or near
their TDC positions, a combustion chamber is defined in the bore 52
between the end surfaces 61 and 63 of the pistons. Fuel is injected
directly into the combustion chamber through at least one fuel
injector nozzle 100 positioned in an opening through the sidewall
of a cylinder 50.
[0042] With further reference to FIG. 3, the engine 49 includes an
air management system 51 that manages the transport of charge air
provided to, and exhaust gas produced by, the engine 49. A
representative air management system construction includes a charge
air subsystem and an exhaust subsystem. In the air management
system 51, the charge air subsystem includes a charge air source
that receives intake air and processes it into charge air, a charge
air channel coupled to the charge air source through which charge
air is transported to the at least one intake port of the engine,
and at least one air cooler in the charge air channel that is
coupled to receive and cool the charge air (which may include
exhaust gasses) before delivery to the intake port or ports of the
engine. Such a cooler can comprise an air-to-liquid and/or an
air-to-air device, or another cooling device. Hereinafter, such a
cooler is denoted as a "charge air cooler". The charge air
subsystem also includes a supercharger that pumps charge air in the
charge air channel to intake ports of the engine. The exhaust
subsystem includes an exhaust channel that transports exhaust
products from exhaust ports of the engine to an exhaust pipe.
[0043] As per FIG. 3, the preferred charge air subsystem includes a
supercharger 110, which can be driven by an electrical motor, or by
a gear, chain, or belt apparatus coupled to a crankshaft. The
supercharger 110 can be a single-speed or multiple-speed device, or
a fully variable-speed device. In some aspects, the air management
system 51 includes a turbo-charger 120 with a turbine 121 and a
compressor 122 that rotate on a common shaft 123. The turbine 121
is coupled to the exhaust subsystem and the compressor 122 is
coupled to the charge air subsystem. The turbine 121 can be a
fixed-geometry or a variable-geometry device. The turbo-charger 120
extracts energy from exhaust gas that exits the exhaust ports 54
and flows into the exhaust channel 124 directly from the exhaust
ports 54, or from an exhaust manifold 125. In this regard, the
turbine 121 is rotated by exhaust gas passing through it. This
rotates the compressor 122, causing it to generate charge air by
compressing intake air. The charge air output by the compressor 122
flows through a conduit 126 to a charge air cooler 127, whence it
is pumped by the supercharger 110 to the intake ports. Air
compressed by the supercharger 110 is output from the supercharger
through a charge air cooler 129 to an intake manifold 130. The
intake ports 56 receive charge air pumped by the supercharger 110,
through the intake manifold 130. Preferably, but not necessarily,
in multi-cylinder opposed-piston engines, the intake manifold 130
is constituted of an intake plenum that communicates with the
intake ports 56 of all cylinders 50.
[0044] Exhaust gas management: The air management construction
enables control of NOx emissions by recirculating exhaust gas
through the one or more ported cylinders of the opposed-piston
engine. The recirculated exhaust gas is mixed with charge air to
lower peak combustion temperatures, which lowers NOx emissions.
This process is referred to as exhaust gas recirculation ("EGR").
An EGR construction can utilize exhaust gasses transported in an
EGR channel external to the cylinder into the incoming stream of
fresh intake air as per the valve-controlled recirculation channel
131 in FIG. 3. Alternatively, or additionally, an EGR process can
utilize residual exhaust gasses that are trapped (retained) in the
cylinders 50 when the exhaust and intake ports are closed and
scavenging ceases. In the case of external EGR, the exhaust gas is
pumped into the inflowing stream of air. A source of pressure in
communication with the EGR channel creates a pressure differential
that causes exhaust gas to flow through the EGR channel into the
charge air subsystem. In some aspects, a virtual pump exists when
the exhaust gas to be recirculated is obtained from a source
guaranteed to be at a higher pressure than the point where it is
fed into the intake stream of charge air. In other aspects, an
active pump, such as the supercharger 110, is used to pump the
exhaust gas to be recirculated into the charge air that the
supercharger is pumping to the intake ports. In these aspects, use
of a supercharger provides an additional variable for controlling
EGR operations in an opposed-piston engine. In some aspects
recirculated exhaust gas is cooled by way of one or more EGR
coolers, which can comprise air-to-liquid and/or air-to-air
devices. In other aspects, recirculated exhaust gas is cooled by
one or more charge air coolers alone or in combination with one or
more EGR coolers.
[0045] EGR loop construction: The opposed-piston engine 49 seen in
FIG. 3 is equipped with an EGR loop that channels exhaust gas from
the exhaust subsystem into the charge air subsystem. An example of
a specific EGR loop construction (which is not intended to be
limiting) is a high pressure configuration illustrated in FIG. 4.
In this regard, a high pressure EGR loop circulates exhaust gas
obtained from a source upstream of the turbine 121 to a mixing
point downstream of the compressor 122. In this EGR loop the
conduit 131 and an EGR valve 138 shunt a portion of the exhaust gas
from the exhaust manifold 125 to be mixed with charge air output by
the compressor 122 into the conduit 126. If no exhaust/air mixing
is required the valve 138 is fully shut and charge air with no
exhaust gas is delivered to the cylinders. As the valve 138 is
increasingly opened, an increasing amount of exhaust gas is mixed
into the charge air. This loop subjects the exhaust gas to the
cooling effects of the two coolers 127 and 129. If less cooling is
merited, the exhaust gas portion can be shunted around the cooler
127 to the input of the supercharger 110. This alternative subjects
the exhaust gas portion to cooling by only the charge air cooler
129. A dedicated EGR cooler that cools only exhaust gas can be
incorporated into the conduit 131, in series with the valve 138, or
in series with the output port of the valve 138 and the input to
the supercharger 110.
[0046] EGR using retained exhaust gas: Another mode of mixing
exhaust gas with charge air is provided in the uniflow-scavenged
opposed-piston engine 49 by trapping (or retaining) a residual
amount of exhaust gas in a cylinder after the exhaust and intake
ports close and scavenging ceases. The residual exhaust gas can be
used to adjust the initial conditions for combustion to a point
advantageous for reducing NOx emissions. Depending on the
configuration of the turbo-machinery, at low and medium engine
speeds and loads, a uniflow-scavenged opposed-piston engine may
exhibit incomplete scavenging. Since the residual exhaust gas
inside the cylinder is hot, the resulting temperature of the new
charge of air may be substantially elevated; therefore this method
is well-suited for reducing NOx under partial engine load
conditions. Further, when combustion occurs, the heightened air
charge temperature results in an increased exhaust gas temperature,
which is useful for light-up of after-treatment catalysts and
regeneration of diesel particulate filter materials.
[0047] The amount of charge air that is fed into a cylinder each
cycle can be used to alter the amount of residual exhaust gas left
in the cylinder. In this regard, adjusting the amount of charge air
that is fed into the cylinder in any given cycle of operation can
be used to "tune" the amount of exhaust gas retained in the
cylinder for the next combustion occurrence. In one aspect of
retained exhaust gas EGR, seen in FIG. 4, a recirculation loop 148
including a valve 139 is placed in parallel with the supercharger
110. The valve 139 is operated to control the amount of charge air
pumped into a cylinder by the supercharger 110. Setting the amount
of charge air pumped allows control of the amount of exhaust gas
scavenged, and, consequently, the amount of exhaust gas retained in
any cylinder following scavenging. In this regard, if a high intake
manifold pressure is desired the valve 139 is fully shut and charge
air is delivered to the engine at a high rate. As the valve 139 is
increasingly opened, an increasing amount of charge air pumped by
the supercharger 110 is returned (recirculated) to the inlet of the
supercharger, which proportionately reduces the amount of charge
air delivered to the engine. Thus, the charge air/fuel ratio is
reduced and the amount of exhaust gas retained in any cylinder is
increased. Among the benefits realized by this aspect of retained
exhaust gas EGR are NOx reduction, reduction of the pumping load
imposed on the engine by the supercharger 110, and an increased
temperature due to the combustion gasses trapped in the
cylinder.
[0048] An increase in the pressure felt by exhaust gas flowing to
the turbine 121 ("backpressure") can also be used to alter the
amount of residual exhaust gas left in the cylinder. In this
regard, adjusting the amount of backpressure in any given cycle of
operation can be used to "tune" the amount of residual exhaust gas
for the next combustion occurrence. Therefore, in another aspect of
retained exhaust gas EGR, seen in FIG. 4, a variable valve 140 is
placed in series with exhaust gas output from the exhaust manifold
125. The setting of the valve 140 directly influences the
backpressure felt upstream of the valve and, consequently, the
amount of exhaust gas retained in any cylinder after scavenging. In
FIG. 4, the valve 140 is placed in series with the output of the
turbine 121, upstream of the tailpipe 128. In this case, any
backpressure resulting from the setting of the valve is distributed
over all cylinders of the engine. In an alternative aspect, an
equivalent valve (not seen) can be placed in series between the
input to the turbine 121 and the exhaust manifold 125. In yet
another alternative aspect, the equivalent valve can be placed in
series with an exhaust manifold or exhaust port of each of a
plurality of cylinders.
[0049] Turbine bypass construction: As seen in FIG. 4 of related
U.S. patent application Ser. No. 13/068,679, filed May 16, 2011 and
published as US 2011/0289916 A1 on Dec. 1, 2011, a bypass conduit
loop 143 including a valve 144 can be placed in parallel with the
turbine 121. The valve 144 can be operated to control the amount of
exhaust gas flowing from the engine into the turbine 121. Setting
the valve 144 to bypass the turbine 121 allows exhaust energy to be
dumped into the exhaust pipe 128 without operating the turbine 121
and compressor 122. This keeps the exhaust gas at a higher
temperature level and can increase after-treatment conversion
efficiencies (for catalytic devices and particulate filters) at
engine warm-up during partial engine load conditions such as from a
cold start. Further, setting the valve 144 to bypass the turbine
121 during engine operation under partial engine load conditions
reduces turbo-charger operation, and allows more exhaust gas to be
driven over the supercharger 110 (via valve 138, for example) while
also delivering exhaust gas at a higher temperature to increase
after-treatment conversion efficiencies. Another construction for
varying the amount of exhaust gas flowing from the engine into the
turbine 121 includes a turbine with a variable geometry
construction to control pressure in the exhaust conduit 124,
upstream of the valve 144. Using a variable geometry turbine (VGT)
instead of a fixed geometry turbine does not necessarily eliminate
the need for a turbine bypass valve such as the valve 144. A VGT
has only a limited mass flow range where it works at acceptable
efficiencies. Outside this range, a turbine bypass valve can be
used to control the mass flow and intake pressure of the engine
49.
[0050] Optionally, an intake throttle valve 141 and a turbine
bypass valve can be included in the air management system for high
precision control of the ratio of recirculated exhaust gas to fresh
air.
[0051] Exhaust configuration and control: The EGR constructions
illustrated in FIG. 4 are combined in the air management system of
the opposed piston engine described and illustrated herein so as to
trap (retain) uncooled exhaust gas in a cylinder following port
closure where it is combined or mixed with charge air including
recirculated exhaust gas that is cooled and provided to the
cylinder prior to port closing. The relative amounts of retained
and recirculated exhaust gas are varied in order to precisely
control the EGR rate and temperature. This enables operation of the
air management system of the opposed piston engine in such a way as
to allow significant heat to be expelled into the exhaust
subsystem. In this regard, a modified air delivery ratio
A*=m.sub.del/p.sub.delV.sub.tr is the ratio of the mass of the
delivered air m.sub.del (fresh air and external EGR delivered to a
cylinder) divided by the mass of the trapped charge .rho..sub.del
V.sub.tr (density of the delivered charge multiplied by the trapped
volume at port closing). FIG. 5 represents the amount of cooled
fresh charge (indicated by the long arrows) in a combustion chamber
compared to the amount of hot residual charge (indicated by the
short arrows). As the modified air delivery ratio is increased, the
percentage of fresh charge trapped in the cylinder is increased and
the quantity of residual charge from the previous cycle
decreases.
[0052] As per FIG. 5, running the engine at low modified air
delivery ratios yields significant internal residuals, which in
turn increases the trapped temperature of the charge. In this
operating mode, a sufficiently high intake manifold pressure is
required to achieve an adequate air-fuel ratio and good combustion
stability during light load operation, low ambient temperature
operation and extreme cold starts. In addition, temperatures can be
further enhanced when low modified delivery ratios and high intake
manifold pressure are coupled with a late-injection strategy. This
creates a significant temperature rise in the exhaust for catalyst
light-off and management, yielding good conversion efficiency for
after-treatment performance. This approach also supports in-use
rate diagnostic requirements for the after-treatment as well as the
temperature required to regenerate the diesel particulate filter
under all driving conditions, including idle and light loads.
[0053] For this strategy to be effective, it is desirable to
control the modified air delivery ratio and, therefore, the
composition of the trapped charge. The modified air delivery ratio
is primarily a function of the scavenging characteristics of the
engine, the intake manifold conditions and the exhaust
backpressure. The intake manifold conditions, in turn, depend on
the speed of the supercharger 110, the settings of the
recirculation valve 139, and the rate of cooled external EGR
delivered into the charge air stream as determined by the settings
of the EGR valve 138. The backpressure conditions are controlled by
the settings of the backpressure valve 140; additionally or
alternatively, backpressure can be controlled by a variable nozzle
turbo mechanism, by multi-stage turbo-charging, or by
turbo-compounding.
[0054] During low load operations as well as low ambient
temperatures, the temperature of the trapped charge in the cylinder
can be significantly increased by running a low modified delivery
ratio. This will result in an increased exhaust temperature as well
as improved combustion stability. Maintaining a low delivery ratio
will also decrease the pumping work which will help BSFC/fuel
economy.
[0055] During extreme cold start conditions low modified delivery
ratios can be used on the first few cycles of the engine cranking,
which would keep much, if not most, of the same charge in the
cylinder. This would heat up the charge as well as the combustion
chamber before even introducing any fuel to burn. This would
greatly improve the starting and robustness to early misfires.
[0056] During DPF regeneration at idle or low loads leveraging a
low delivery ratio yielding higher exhaust temperatures would
enable a diesel oxidation catalyst to be above the required
light-up temperature to convert hydrocarbons. The hydrocarbons
would generate an exothermic reaction to yield the high
temperatures required to burn the soot loaded in the DPF.
[0057] For example, the following ranges have been used for the
identified conditions:
TABLE-US-00001 Condition Range Cold start 0.4 .ltoreq. * .ltoreq.
0.5 Low load operation 0.48 .ltoreq. * .ltoreq. 0.6 Normal, High
load operation 0.5 .ltoreq. * .ltoreq. 0.85 Full load, low speed
operation 0.7 .ltoreq. * .ltoreq. 1.0
[0058] As per FIG. 4, a process to operate an opposed-piston
two-stroke engine with an exhaust subsystem that includes EGR loop
and retained exhaust constructions, under control of a modified
delivery ratio, is executed by an ECU 149. The ECU maintains and
changes the modified delivery ratio in response to specified engine
operating conditions by automatically operating the valves 138,
139, and 140, the supercharger 110, if a multi-speed or variable
speed device is used, and the turbo-charger 120, if a
variable-geometry device is used. Of course, operation of valves,
throttles, and associated elements used for EGR can include any one
or more of electrical, pneumatic, mechanical, and hydraulic
actuating operations. For fast, precise automatic operation, it is
preferred that the valves be high-speed, computer-controlled
devices with continuously-variable settings. Each valve has a first
state in which it is open (to some setting controlled by the ECU
149) to allow gas to flow through it, and a second state in which
it is closed to block gas from flowing through it.
[0059] Preferably an integrated exhaust control process including a
modified delivery ratio routine automatically operates a
multifunction EGR system incorporating EGR loop and retained
exhaust constructions described and illustrated herein based upon
one or more parameters relating to recirculated exhaust gas and to
a mixture of recirculated exhaust gas and charge air. Parameter
values are determined by a combination of one or more of sensors,
calculations, and table lookup so as to manage the values of
individual parameters and one or more ratios of EGR and mixture
parameters in one or more cylinders.
[0060] Exhaust management: Referring now to FIG. 4, the combination
of the turbocharger compressor 122 and the supercharger 110 needs
to provide enough air to produce stable combustion at a low
air/fuel ratio. At the same time, elevated backpressure must be
applied to the exhaust subsystem. The result of high intake
manifold boost pressure and exhaust backpressure will yield
adequate air/fuel ratio levels with low engine delta pressure
(pressure difference) and significant residual gases trapped in the
cylinder for combustion.
[0061] These results can be achieved by a control mechanization
implemented by ECU programming. Operation of the control
mechanization enables the execution of an exhaust management
strategy by controlling trapped air/fuel ratio ("trapped .lamda."),
trapped burn fraction ("trapped BF"), and trapped temperature.
[0062] The ECU 149 reads all the available engine sensors. Based on
the sensor values the current engine torque demand (load) and RPM
are determined. This information is fed into a
desired-trapped-cylinder-condition module. In this module the
desired trapped .lamda. at port closing and trapped BF at port
closing are determined based on look up tables (maps) that are
indexed by engine torque and RPM to meet the desired performance,
emissions and temperature goals. These maps are pre-filled based on
engine dynamometer testing and stored in the ECU 149.
[0063] An estimate is made of actual trapped .lamda., actual
trapped BF and actual trapped temperature. Based on the routine
shown in FIG. 6A, the actual and desired trapped conditions are
compared. If the absolute value of each difference is greater than
a threshold, the routine adjusts accordingly to get this difference
within an acceptable limit. The adjustment in fresh air flow or
intake manifold pressure is carried out by controlling a
supercharger actuator output. For example, if the supercharger is
driven by continuously variable drive then the routine varies the
drive ratio. If the supercharger is driven by fixed drive ratio
then the routine varies the setting of the recirculation valve 139.
Control of the trapped BF is enabled by varying EGR flow rate using
the EGR valve 138 to minimize the error between the actual trapped
BF and desired BF. Once the trapped .lamda. and trapped BF errors
have been minimized, the routine then compares actual trapped
temperature to a predefined value. This predefined value of trapped
temperature is determined based on engine dynamometer testing and
stored in the ECU. If the actual trapped temperature turns out to
be significantly different than this predefined temperature then
the strategy adjusts the back pressure (for example, by the
backpressure valve 140) and loops back to recalculate the desired
trapped .lamda. and desired trapped BF. The routine then readjusts
the supercharger output and EGR valve position. This is repeated
continuously as the engine transitions from one operating point
(engine load and speed) to another operating point (engine load and
speed).
[0064] The differences between a normal operating condition and
this warm-up operating condition are illustrated in FIG. 6B which
shows the temperature profile of the charge while in the cylinder.
As per FIG. 6B, the warm-up mode strategy provides lower peak
temperatures while increasing the starting and ending temperatures.
A relationship between the trapped temperature and the exhaust
temperature measured for a single cylinder test engine is shown in
FIG. 6C.
[0065] Alternate supercharger recirculation embodiments: A
supercharger 110 is used to create a positive pressure differential
across the opposed-piston engine 49 in order to drive the charge
air flow and EGR flow. When operating the supercharger 110 with the
recirculation valve 139 as required to limit the pressure ratio
between the intake and exhaust manifolds, the temperature of the
recirculation flow from the outlet of the supercharger is increased
compared to the fresh air charge flow into the inlet of the
supercharger. The higher temperature gas then mixes with the inlet
charge air flow and increases the temperature of the airflow into
the supercharger. In some aspects, this bypass flow is cooled in
order to keep the supercharger from experiencing temperatures above
its hardware limitations under certain desired conditions. In this
regard, the recirculation loop 148 has a construction with an inlet
connected to the charge air channel downstream of the supercharger
110, an outlet connected to the charge air channel upstream of the
supercharger 110, and a variable valve 139 between the inlet and
the outlet. An air cooler between the inlet and the outlet, in
series with the valve 139 cools the charge air recirculated from
the outlet to the inlet of the supercharger 110. For example, in
FIG. 7A, the output of the recirculation loop 148 is fed into the
charge air stream upstream of the cooler 127. In FIG. 7B, the input
to the valve 139 is fed from the charge air stream downstream of
the cooler 129. In FIG. 7C, the input to the valve 139 is fed from
the charge air stream downstream of the cooler 129 and the output
of the recirculation loop 148 is fed into the charge air stream
upstream of the cooler 127. In FIG. 7D, a cooler 150 specific to
the valve 139 is placed in the recirculation loop 148 either
upstream or downstream of the valve 139.
[0066] EGR and after-treatment combinations: The EGR construction
of FIG. 4 includes a high pressure EGR loop. Alternatively, a low
pressure EGR loop can be used to the same effect. In this regard, a
low pressure EGR loop circulates exhaust gas obtained from a source
downstream of the turbine 121 to a mixing point upstream of the
compressor 122. In addition various arrangements of after-treatment
components can be combined with high- and low-pressure EGR loop
constructions for an opposed-piston two-stroke engine operated in
the manner described above. The after-treatment components include
a diesel oxidation catalyst device (DOC), a selective catalytic
reduction device (SCR), and a diesel particulate filter device
(DFR).
[0067] FIG. 8 depicts an exhaust subsystem construction including a
high-pressure EGR loop and a DOC and DPF located in the exhaust
stream prior to the turbine. This construction allows for higher
exhaust temperature going through the DOC and DPF, which can
improve the soot regeneration in the DPF and improves the light-off
time of the DOC. The higher pressure before the turbine also
increases the density of the exhaust flow and minimizes the
backpressure caused by the after-treatment, in this case DOC and
DPF.
[0068] FIG. 9 depicts an exhaust subsystem construction including a
high-pressure EGR loop and a DOC, an SCR and a DPF located upstream
of the turbine.
[0069] FIG. 10 is similar to FIG. 9 except the SCR is positioned
after the DOC and DPF.
[0070] FIG. 11 depicts an exhaust subsystem construction including
a high-pressure EGR loop and a DOC and DPF located upstream of the
EGR pick-up location. This is a different system than the one from
FIG. 8 because in this case all the exhaust flow goes through the
DOC-DPF, and the EGR, which is taken downstream, is clean of soot,
hydrocarbons and CO, which are removed by the DOC and DPF. This may
provide a favorable condition for the air charge subsystem which
would not be contaminated by soot and unburned hydrocarbon.
[0071] FIGS. 12 and 13 depict an exhaust subsystem construction
similar to the one from FIG. 11 but where both sets of
after-treatment devices are located in various orders before the
EGR pick-up point. Note that in the case of FIG. 12 where the SCR
is located before the DPF that an additional DOC is needed before
the SCR to control hydrocarbon and carbon monoxide.
[0072] FIG. 14 depicts an exhaust subsystem construction including
a low-pressure EGR loop with a dedicated EGR cooler. This permits
different choices of turbocharger and supercharger sizes that may
be more favorable for some applications. Note the addition of a
backpressure valve upstream of the compressor inlet that is used to
increase the driving pressure for the EGR.
[0073] FIG. 15 depicts an exhaust subsystem construction similar to
the one in FIG. 14 but with the EGR valve installed downstream of
the EGR cooler.
[0074] FIG. 16 depicts an exhaust subsystem construction including
a low-pressure EGR loop with a dedicated EGR cooler where both the
DOC-DPF and SCR are located upstream of the turbine.
[0075] FIG. 17 depicts an exhaust subsystem construction similar to
FIG. 16 but with the SCR and DPF switched and a DOC added before
the SCR.
[0076] FIG. 18 depicts an exhaust subsystem construction including
a low-pressure EGR loop with a dedicated EGR cooler and an input
taken from after the SCR which is located post turbine.
[0077] FIGS. 19, 20, and 21 depicts an exhaust subsystem
construction including a low-pressure EGR loop with a dedicated EGR
cooler where the after-treatment devices are located after the
turbine. Each case shows a different combination of after-treatment
placement and pick-up point for the EGR. Note that the EGR loop is
always sampled downstream of the DPF. This is needed to protect the
compressor wheel from soot.
[0078] Although exhaust management strategies have been described
with reference to an opposed engine with two crankshafts, it should
be understood that these constructions can be applied to
opposed-piston engines with one or more crankshafts. Moreover,
various aspects of these strategies can be applied to
opposed-piston engines with ported cylinders disposed in
opposition, and/or on either side of one or more crankshafts.
Accordingly, the protection afforded to these constructions is
limited only by the following claims.
* * * * *