U.S. patent application number 14/075323 was filed with the patent office on 2015-05-14 for cold-start strategies for opposed-piston 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 Fabien G. Redon.
Application Number | 20150128907 14/075323 |
Document ID | / |
Family ID | 51790905 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150128907 |
Kind Code |
A1 |
Redon; Fabien G. |
May 14, 2015 |
Cold-Start Strategies for Opposed-Piston Engines
Abstract
A strategy to cold-start an opposed-piston engine includes,
before injecting fuel, preventing air flow through the engine while
cranking the engine to heat air retained in the engine, followed by
controlling mass air flow through and fuel injection into a
cylinder of the engine according to cold-start schedules so as to
create and preserve heat for stable engine firing and transition to
an idling state of operation.
Inventors: |
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: |
51790905 |
Appl. No.: |
14/075323 |
Filed: |
November 8, 2013 |
Current U.S.
Class: |
123/46R |
Current CPC
Class: |
F02D 2400/04 20130101;
F01B 7/14 20130101; Y02T 10/40 20130101; F02D 41/064 20130101; F02D
41/0002 20130101; Y02T 10/42 20130101; F02B 75/12 20130101; F02B
75/28 20130101 |
Class at
Publication: |
123/46.R |
International
Class: |
F02B 75/12 20060101
F02B075/12 |
Claims
1. A method of operating an opposed-piston engine with one or more
cylinders, in which each cylinder has a piston-controlled exhaust
port coupled to an exhaust subsystem and a piston-controlled intake
port coupled to a charge air subsystem, a pair of pistons disposed
for opposing movement in a bore of the cylinder, and one or more
fuel injectors disposed for injecting fuel into the cylinder, the
method comprising: receiving a signal to start the engine;
detecting a cold-start condition, followed by: before fuel is
injected, limiting charge air flow through the cylinder and heating
air retained in the cylinder by cranking the engine; and then,
injecting fuel into a combustion space between opposing end
surfaces of pistons in the cylinder according to a cold-start
schedule; providing a progressively increasing flow of charge air
through the cylinder until an engine idling state is reached; and,
injecting fuel into the combustion space according to an idling
schedule.
2. The method of claim 1, in which the engine further includes an
EGR channel coupling the exhaust subsystem to the charge air
subsystem, and the step of receiving the start signal is preceded
by: receiving an engine shut-down signal, followed by:
pre-conditioning air includes blocking the flow of exhaust gas
through the EGR channel, followed by opening the exhaust subsystem
and the charge air subsystem.
3. The method of claim 1, further comprising blocking circulation
of liquid coolant to the cylinders or the pistons until the engine
idling state is reached.
4. The method of claim 1, in which limiting charge air flow through
the cylinders includes either the step of reducing or blocking gas
flow through the exhaust subsystem and the charge air subsystem, or
the steps of reducing or blocking gas flow through the exhaust
subsystem and providing charge air to the cylinder at a target
boost pressure.
5. The method of claim 4, in which the charge air subsystem
includes a supercharger having an input and an output, an intake
manifold coupled to the output of the supercharger and in fluid
communication with the intake port, and a recirculation path
coupling the output of the supercharger to the input of the
supercharger, and reducing or blocking gas flow through the charge
air subsystem includes increasing charge air flow through the
recirculation path.
6. The method of claim 5, in which the charge air subsystem further
includes a cooling device coupled in series between the output of
the supercharger and an input to the recirculation path, further
including, while cranking the engine, bypassing the cooling device
by coupling the output of the supercharger directly to the input of
the recirculation path until the idling state is reached.
7. The method of claim 4, in which the exhaust subsystem includes a
backpressure valve for adjusting gas flow through the exhaust
subsystem, and reducing or blocking gas flow through the exhaust
subsystem includes closing the backpressure valve.
8. The method of claim 7, in which the charge air subsystem
includes a supercharger having an input and an output, an intake
manifold coupled to the output of the supercharger and in fluid
communication with the intake port, and a recirculation path
coupling the output of the supercharger to the input of the
supercharger, and reducing or blocking gas flow through the charge
air subsystem includes increasing charge air flow through the
recirculation path.
9. The method of claim 8, in which providing an increasing flow of
charge air through the cylinder includes progressively opening the
backpressure valve and progressively reducing charge air flow
through the recirculation path, according to a cold start schedule
until the idling state is reached.
10. The method of claim 9, in which the engine further includes an
EGR channel coupling the exhaust subsystem to the charge air
subsystem, and the step of receiving the start signal is preceded
by: receiving an engine shut-down signal, followed by:
pre-conditioning air includes blocking the flow of exhaust gas
through the EGR channel, followed by opening the exhaust subsystem
and the charge air subsystem.
11. The method of claim 10, further comprising blocking circulation
of liquid coolant to the cylinders or the pistons until the engine
idling state is reached.
12. A cold-start system for an opposed-piston engine with one or
more cylinders, in which each cylinder has a piston-controlled
exhaust port coupled to an exhaust subsystem and a
piston-controlled intake port coupled to a charge air subsystem, a
pair of pistons disposed for opposing movement in a bore of the
cylinder, and one or more fuel injectors disposed for injecting
fuel into the cylinder, comprising: a starter motor operable to
crank the engine during start-up; a backpressure valve in the
exhaust subsystem; the charge air subsystem including a
supercharger having an input and an output, an intake manifold
coupled to the output of the supercharger and in fluid
communication with the intake port, and a recirculation path
coupling the output of the supercharger to the input of the
supercharger; and, an engine control mechanization operable to
detect a cold-start condition and to operate the starter motor, the
backpressure valve, the recirculation path, and the fuel injectors
in response to the cold-start condition by: closing the
backpressure valve and opening the recirculation path to reduce or
block charge air flow through the cylinder and cranking the engine
to heat air by compression in the cylinder before fuel is injected,
followed by: causing the fuel injectors to inject fuel into a
combustion space defined between opposing end surfaces of pistons
in the cylinder according to a cold-start schedule, and
progressively opening the backpressure valve and closing the
recirculation path to increase the flow of charge air through the
cylinder until an engine idling state is reached.
13. The cold-start system of claim 12, further including an EGR
channel coupling the exhaust subsystem to the charge air subsystem,
and wherein the engine control mechanization is operable to limit
the flow of exhaust gas through the EGR channel until the idling
state is reached.
14. The cold-start system of claim 13, further including one or
more of a variable geometry turbine in the exhaust subsystem, a
charge air cooler bypass path in the charge air subsystem, and an
EGR cooler bypass path in the EGR channel, and wherein the control
mechanization is further operable to perform one or more of:
configuring the variable geometry turbine to limit the flow of gas
until the idling state is reached; enabling charge air flow through
the charge air cooler bypass path until the idling state is
reached; and, enabling exhaust flow through the EGR cooler bypass
path until the idling state is reached.
15. A method of operating an opposed-piston engine with an exhaust
subsystem, a charge air subsystem, an EGR channel coupling the
exhaust subsystem to the charge air subsystem, and one or more
cylinders, in which each cylinder has a piston-controlled exhaust
port coupled to the exhaust subsystem and a piston-controlled
intake port coupled to the charge air subsystem, a pair of pistons
disposed for opposing movement in a bore of the cylinder, and one
or more fuel injectors disposed for injecting fuel into the
cylinder, the method comprising: receiving an engine start signal;
detecting a cold-start condition and responding to the cold-start
condition by: blocking the flow of exhaust gas through the EGR
channel; and then, before fuel is injected, limiting charge air
flow through a cylinder and heating conditioned air in the cylinder
by cranking the engine, followed by: ceasing cranking while
injecting a cold-start sequence of fuel pulses into the cylinder
and increasing the flow of charge air through the cylinder, until
an engine idling state is reached; and, injecting an idling
sequence of fuel pulses into the cylinder.
16. The method of claim 15, further including blocking circulation
of liquid coolant to the cylinders or the pistons until the engine
idling state is reached.
17. The method of claim 15, in which limiting charge air flow
through the cylinders includes the step of blocking exhaust flow
through the exhaust subsystem, and either the step of blocking
charge air flow through the charge air subsystem, or the step of
causing charge air to flow to the cylinder at a target boost
pressure.
18. The method of claim 17, in which the charge air subsystem
includes a supercharger having an input and an output, an intake
manifold coupled to the output of the supercharger and in fluid
communication with the intake port, and a recirculation path
coupling the output of the supercharger to the input of the
supercharger, and blocking charge air flow through the charge air
subsystem includes increasing charge air flow through the
recirculation path.
19. The method of claim 18, in which the exhaust subsystem includes
a backpressure valve for adjusting gas flow through the exhaust
subsystem, and blocking gas flow through the exhaust subsystem
includes closing the backpressure valve.
20. The method of claim 19, in which providing an increasing flow
of charge air through the cylinder includes progressively opening
the backpressure valve and progressively reducing charge air flow
through the recirculation path according to a cold-start schedule
until the idling state is reached.
21. A method of operating an opposed-piston engine with an exhaust
subsystem, a charge air subsystem, an EGR channel coupling the
exhaust subsystem to the charge air subsystem, and one or more
cylinders, in which each cylinder has a piston-controlled exhaust
port coupled to the exhaust subsystem and a piston-controlled
intake port coupled to the charge air subsystem, a pair of pistons
disposed for opposing movement in a bore of the cylinder, and one
or more fuel injectors disposed for injecting fuel into the
cylinder, the method comprising: during shut-down of the engine,
closing the EGR channel, and then ceasing fuel injection into the
cylinder and flushing exhaust products from the cylinder; followed
by, detecting a cold-start condition in response to an engine start
signal and responding to the cold-start condition by: closing the
EGR channel; followed by, before fuel is injected, closing the
exhaust subsystem and providing pressurized charge air to a
cylinder and heating the pressurized charge air in the cylinder by
cranking the engine, followed by: injecting a cold-start sequence
of fuel pulses into the cylinder while progressively increasing the
flow of exhaust gas through the exhaust subsystem and progressively
increasing the flow of charge air to the cylinder, until an engine
idling state is reached; and then, injecting an idling sequence of
fuel pulses into the cylinder.
Description
RELATED APPLICATIONS
[0001] This application contains subject matter related to the
subject matter of co-pending U.S. patent application Ser. No.
13/654,340 for "Fuel Injection Strategies in Opposed-Piston Engines
with Multiple Fuel Injectors", published as US 2013/0104848, and
co-pending U.S. patent application Ser. No. 14/039,856 for "EGR
Constructions for Opposed-Piston Engines", whose parent has issued
as U.S. Pat. No. 8,549,854. The subject matter of this application
is also related to that of PCT application US2013/026737 for
"Exhaust Management Strategies for Opposed-Piston, Two-Stroke
Engines".
BACKGROUND
[0002] The field of this disclosure relates to vehicle engines,
specifically to two-stroke cycle, opposed-piston engines
constructed for compression ignition.
[0003] Ambient environmental conditions can affect combustion
behavior of a compression-ignition engine. For example, a
compression-ignition engine installed in a vehicle frequently
requires assistance when starting cold in low temperature
conditions. In this regard, effective combustion is dependent on
achieving a high temperature by compression of air in the bore of a
cylinder. However, under cold ambient and engine conditions the air
drawn into the engine may be too cold to support combustion when
the engine is started. Further, as the engine is cranked, the heat
of compression is drawn away from the combustion chamber by the
cold engine parts. In some cases, for example during winter, a
minimally-equipped compression-ignition engine may require long
periods of cranking in order to raise the temperature of the
combustion chamber elements to a level that supports ignition,
effective combustion, and minimal pollution.
[0004] A compression-ignition engine may be equipped with one or
more of glow plugs, block heaters, intake heaters, ether injection,
and/or other aids to facilitate start-up under cold conditions.
However, these solutions bring added complexity and expense to the
construction and operation of such an engine.
[0005] An opposed-piston engine is a type of compression-ignition
engine in which effective combustion is dependent on achieving a
high temperature by compression of air between adjacent end
surfaces of a pair of pistons disposed for opposed movement in the
bore of a cylinder. Advantages in engine architecture and
two-stroke operation enable opposed-piston engines to deliver
superior fuel, weight, and volume efficiencies when compared with
conventional single-piston compression-ignition engines. A
two-stroke cycle, opposed-piston engine must be able to start
quickly and operate effectively in cold ambient conditions, with as
little derogation of its inherent advantages as possible. It is
therefore desirable to provide a strategy for starting a
compression-ignition, opposed-piston engine under cold conditions
while minimizing any added complexity and expense.
SUMMARY
[0006] The desired objective and other advantages are achieved by
way of methods and systems that implement and execute strategies
for starting a compression-ignition, opposed-piston engine in cold
conditions. In this regard, a "cold start strategy" is a series of
steps or procedures taken to fire up an engine in response to
detection of thermal conditions indicating possible difficulty in
starting. In this description, a cold-start strategy includes
actions to be taken by specific components or elements of the
engine under governance of a control mechanization to start an
engine under cold conditions. A cold start strategy includes
controlling mass air flow through, and fuel injection into, a
cylinder of the engine according to cold-start schedules that
create and preserve heat for successful engine firing and
transition to an idling state.
[0007] A cold-start strategy is embodied in a method of operating
an opposed-piston engine with one or more cylinders, in which each
cylinder has an exhaust port coupled to an exhaust subsystem and an
intake port coupled to a charge air subsystem, a pair of pistons
disposed for opposing movement in a bore of the cylinder, and one
or more fuel injectors disposed for injecting fuel into the
cylinder. The method includes generating an engine start signal. If
a cold-start condition is detected following the start signal, the
engine is cranked before fuel is injected and charge air flow
through the cylinder is reduced, or blocked, while air in a
cylinder is continually heated by compression while cranking the
engine. Then fuel is injected into cylinder space between opposing
end surfaces of pistons in the cylinder according to a cold-start
schedule and the flow of charge air through the cylinder is
increased until an idling state of engine operation is reached.
When the idling state is reached, combustion control is passed to
an idle governor.
[0008] In some aspects, the engine may be prepared for an
anticipated cold start by conditioning air in a cylinder during
shut-down of the engine. In this regard, conditioned air is air
with little or no exhaust products remaining that is retained in
the engine at the end of shut down.
[0009] A cold-start strategy is enabled by a cold-start system for
an opposed-piston engine with one or more cylinders, in which each
cylinder has an exhaust port coupled to an exhaust subsystem and an
intake port coupled to a charge air subsystem, a pair of pistons
disposed for opposing movement in a bore of the cylinder, and one
or more fuel injectors disposed for injecting fuel into the
cylinder. The cold-start system includes a starter motor operable
to crank the engine during start-up, a backpressure valve in the
exhaust subsystem, and a supercharger in the charge air subsystem
having an input and an output. An intake manifold in the charge air
subsystem is coupled to the output of the supercharger and is in
fluid communication with the intake port, and a recirculation path
in the charge air subsystem couples the output of the supercharger
to the input of the supercharger. An engine control mechanization
is operable to detect a cold-start condition and to operate the
starter, the backpressure valve, the recirculation path, and the
fuel injectors in response to the cold-start condition by closing
the backpressure valve and opening the recirculation path to reduce
charge air flow through the cylinder and cranking the engine to
heat air by compression in the cylinder before fuel is injected.
The engine control mechanization is further operable to cause the
fuel injectors to inject fuel into a combustion chamber defined
between opposing end surfaces of pistons in the cylinder according
to a cold-start schedule, and to progressively open the
backpressure valve and close the recirculation path to increase the
flow of charge air through the cylinder until an engine idling
state is reached.
[0010] A cold-start strategy is embodied in a method of operating
an opposed-piston engine with an exhaust subsystem, a charge air
subsystem, an EGR channel coupling the exhaust subsystem to the
charge air subsystem, and one or more cylinders, in which each
cylinder has an exhaust port coupled to the exhaust subsystem and
an intake port coupled to the charge air subsystem, a pair of
pistons disposed for opposing movement in a bore of the cylinder,
and one or more fuel injectors disposed for injecting fuel into the
cylinder. The method includes, during shut down of the engine,
closing the EGR channel, and then ceasing fuel injection into the
cylinder and flushing charge air containing exhaust products from
the cylinder. Then when a cold-start condition is detected during
engine start-up the engine is operated by, before fuel is injected,
reducing, or blocking, charge air flow through a cylinder while
compression-heating conditioned air in the cylinder by cranking the
engine. A cold-start sequence of fuel pulses is injected into the
cylinder while the flow of charge air through the cylinder is
increased until an engine idling state is reached. When the idling
state is reached, an idling sequence of fuel pulses is injected
into the cylinder
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The below-described drawings are intended to illustrate
examples discussed in the following description. The drawings
include schematic drawings that represent elements of a
compression-ignition, opposed-piston engine by means of
well-understood and widely-employed symbols.
[0012] FIG. 1 is a schematic diagram of a prior art two-stroke
cycle, opposed-piston engine constructed for compression ignition,
and is appropriately labeled "Prior Art".
[0013] FIG. 2 is a schematic diagram showing details of an air
handling system for a compression-ignition, opposed-piston engine
according to this disclosure.
[0014] FIG. 3 is a schematic diagram showing details of a fuel
injection system for a compression-ignition, opposed-piston engine
according to this disclosure.
[0015] FIG. 4 is a schematic diagram illustrating a
compression-ignition, opposed-piston engine equipped with a
cold-start system according to this disclosure.
[0016] FIG. 5 is a flow diagram illustrating cold-start procedures
implemented by operation of a cold-start system according to FIG.
4.
[0017] FIG. 6 is a graph showing an example of fuel delivery during
a shut-down phase of a compression-ignition, opposed-piston
engine.
[0018] FIG. 7 is a graph showing an example of fuel delivery while
cold-starting a compression-ignition, opposed-piston engine
[0019] FIG. 8 is a schematic diagram illustrating optional
additional elements for a cold-start system according to the this
disclosure.
DETAILED DESCRIPTION
[0020] A compression-ignition engine for a vehicle is an internal
combustion engine in which the heat of compressed air ignites fuel
injected into, and mixed with, the air as it is compressed. A
two-stroke cycle engine is a type of compression-ignition engine
that completes a power cycle with a single complete rotation of a
crankshaft and two strokes of a piston connected to the crankshaft.
An opposed-piston engine is a two-stroke cycle,
compression-ignition, internal combustion engine in which two
pistons are disposed in opposition in the bore of a cylinder for
reciprocating movement in opposing directions. The cylinder has
longitudinally-spaced inlet and exhaust ports that are located near
respective ends of the cylinder. Each of the opposed pistons
controls one of the ports, opening the port as it moves to a bottom
center (BC) location, and closing the port as it moves from BC
toward a top center (TC) location. One of the ports provides
passage for the products of combustion out of the bore, the other
serves to admit charge air into the bore; these are respectively
termed the "exhaust" and "intake" ports. In a uniflow-scavenged
opposed-piston engine, charge air enters a cylinder through its
intake port as exhaust gas flows out of its exhaust port, thus gas
flows through the cylinder in a single direction ("uniflow")--from
intake port to exhaust port.
[0021] In this disclosure, "fuel" is any fuel that may be ignited
by compression of air in an opposed-piston engine. The fuel may be
a relatively homogeneous composition, a blend, a mixture of fuels,
or separate injections of different fuels. For example, the fuel
may be a gaseous fuel, a liquid fuel, or any other fuel ignitable
by compression ignition. In some aspects, fuel may be injected into
compressed air in a combustion chamber when opposed pistons are at
or near TC locations. In other aspects, injection may occur earlier
in the compression stroke, soon after port closure. The air is
preferably pressurized ambient air; however, it may include other
components such as exhaust gases or other diluents. In any such
case, the air is referred to as "charge air."
[0022] FIG. 1 illustrates a prior-art two-stroke cycle,
compression-ignition, opposed-piston engine 10 as would be used,
for example, in a vehicle. The engine 10 has at least one ported
cylinder 50. For example, the engine may have one ported cylinder,
two ported cylinders, three ported cylinders, or four or more
ported cylinders. Each ported cylinder 50 has a bore 52 and
longitudinally-spaced exhaust and intake ports 54 and 56 formed or
machined in respective ends of a cylinder wall. Each of the exhaust
and intake ports 54 and 56 includes 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. 1. Pistons 60 and 62 are slidably disposed in the bore 52 with
their end surfaces 61 and 63 opposing one another. The piston 60
controls the exhaust port 54, and the piston 62 controls the intake
port 56. In the example shown, the engine 10 further includes at
least one crankshaft; preferably, the engine includes two
crankshafts 71 and 72. In the example shown, the exhaust pistons 60
of the engine are coupled to the crankshaft 71, and the intake
pistons 62 of the engine are coupled to the crankshaft 72.
[0023] As the pistons 60 and 62 near TC, a combustion chamber is
defined in the bore 52 between the end surfaces 61 and 63 of the
pistons. Combustion timing is frequently referenced to the point in
a compression cycle where minimum combustion chamber volume occurs;
this point is referred to as "minimum volume." Fuel is injected
directly into cylinder space located between the end surfaces 61
and 63. In some instances injection occurs at or near minimum
volume; in other instances, injection may occur before minimum
volume. Fuel is injected through at least one fuel injector nozzle
70 positioned in an opening through the sidewall of a cylinder 50;
preferably, the engine includes two fuel injector nozzles 70. The
fuel mixes with charge air admitted into the bore through the
intake port 56. As the air-fuel mixture is compressed between the
end surfaces the compressed air reaches a temperature that causes
the fuel to ignite. Combustion follows.
[0024] With further reference to FIG. 1, the engine 10 includes an
air handling system 80 that manages the transport of charge air
provided to, and exhaust gas produced by, the engine 10. A
representative air handling system construction includes a charge
air subsystem and an exhaust subsystem. In the air handling system
80, a charge air source receives fresh air and processes it into
charge air. The charge air subsystem receives the charge air and
transports it to the at least one intake port of the engine. The
exhaust subsystem transports exhaust products from exhaust ports of
the engine for delivery to other exhaust components.
[0025] The air handling system 80 includes a turbocharger 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
turbocharger 120 extracts energy from exhaust gas that exits the
exhaust ports 54 and flows into an exhaust channel 124 directly
from the exhaust ports 54, or from an exhaust manifold assembly 125
that collects exhaust gasses output through the exhaust ports 54.
In this regard, the turbine 121 is rotated by exhaust gas passing
through it into an exhaust outlet 128. This rotates the compressor
122, causing it to generate charge air by compressing fresh air.
The charge air subsystem includes a supercharger 110 and an intake
manifold 130. The charge air subsystem further includes at least
one air cooler coupled to receive and cool the charge air before
delivery to the intake port or ports of the engine. The charge air
output by the compressor 122 flows through a charge air channel 126
to a cooler 127, whence it is pumped by the supercharger 110 to the
intake ports. Charge air compressed by the supercharger 110 is
output to an intake manifold 130. The intake ports 56 receive
charge air pumped by the supercharger 110, through the intake
manifold 130. Preferably, 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. A second
cooler 129 may be provided between the output of the supercharger
110 and the input to the intake manifold 130.
[0026] In some aspects, the air handling system 80 may be
constructed to reduce NOx emissions produced by combustion by
recirculating exhaust gas through the ported cylinders of the
engine. The recirculated exhaust gas is mixed with charge air to
lower peak combustion temperatures, which reduces production of
NOx. This process is referred to as exhaust gas recirculation
("EGR"). The EGR construction shown obtains a portion of the
exhaust gasses flowing from the port 54 during scavenging and
transports them via an EGR channel 131 external to the cylinder
into the incoming stream of fresh intake air in the charge air
subsystem. The recirculated exhaust gas flows through the EGR
channel 131 under the control of a valve 138 (this valve may also
be referred to as the "EGR valve").
[0027] The implementation of a cold-start strategy for an
opposed-piston engine such as that shown in FIG. 1 involves a
number of engine systems, including air handling and fuel injection
systems, which are used to execute the strategy, as well as thermal
management systems, which provide data used to guide the execution
of the strategy and which can also contribute to execution.
[0028] Using the engine of FIG. 1 as a basis, FIG. 2 shows
modifications and additions with which the air handling system 80
may be configured to implement a cold-start strategy according to
this specification. In this regard, the charge air subsystem
provides inlet air to the compressor 122 via an air filter 150. As
the compressor is rotated, the compressed inlet air flows through
the cooler 127 into the inlet 151 of the supercharger 110. Air
pumped by the supercharger 110 flows through the supercharger's
outlet 152 into the intake manifold 130. Pressurized charge air is
delivered from the intake manifold 130 to the intake ports of the
cylinders 50, which are supported in an engine block 160. In some
instances, although not necessarily, the second cooler 129 is
provided in the charge air subsystem, in series between the output
of the supercharger 110 and the intake manifold 130. In other
instances, there may be no second cooler 129 in the charge air
subsystem.
[0029] Exhaust gasses from the exhaust ports of the cylinders 50
flow from the exhaust manifold assembly 125 into the inlet of the
turbine 121, and from the turbine's outlet into the exhaust outlet
channel 128. In some instances, one or more after-treatment devices
162 are provided in the exhaust channel 128. Exhaust is
recirculated through the EGR channel 131, under control of the EGR
valve 138. The EGR channel 131 is coupled to the charge air
subsystem via the EGR mixer 163. In some instances, although not
necessarily, an EGR cooler 164 is provided in the EGR channel 131,
in series between the EGR valve 138 and the EGR mixer 163. In other
instances, there may be no cooler in the EGR channel 131.
[0030] With further reference to FIG. 2, the air handling system 80
is equipped for control of gas flow at separate control points in
the charge air and exhaust subsystems. In the charge air subsystem,
charge air flow and boost pressure are controlled by operation of a
recirculation path 165 coupling the output 152 of the supercharger
to the supercharger's input 151. The recirculation path 165
includes a valve (the "recirculation valve") 166 that governs the
flow of charge air into, and thus the pressure in, the intake
manifold 130. A valve (the "backpressure valve") 170 in the exhaust
outlet 128 governs the flow of exhaust out of, and thus the
backpressure in, the exhaust subsystem. As per FIG. 2, the
backpressure valve is positioned in the exhaust outlet 128, between
the output of the turbine 121 and the after-treatment devices
162.
[0031] In some instances, additional control of gas flow (and
pressure) is provided by way of a variable speed supercharger
and/or a variable-geometry turbine. Thus, in some aspects the
supercharger 110 is coupled by a drive mechanism (not shown) to a
crankshaft, or another rotating element of the engine, to be driven
thereby. The drive mechanism can comprise a stepwise transmission,
or continuously variable transmission (CVT), device, in which
cases, charge air flow, and boost pressure, may be varied by
varying the speed of the supercharger 110 in response to a speed
control signal provided to the drive mechanism. In other instances,
the supercharger may be a single-speed device. In other aspects,
the turbine 121 may be a variable-geometry device having an
effective aspect ratio that may be varied in response to changing
speeds and loads of the engine.
[0032] FIG. 3 shows a fuel injection system that may be configured
to contribute to the execution of a cold-start strategy according
to this specification. The fuel injection system 180 delivers fuel
to each cylinder 50 by injection into the cylinder. Preferably,
each cylinder 50 is provided with multiple fuel injectors 70
mounted for direct injection into cylinder space between the end
surfaces of the pistons. For example, each cylinder 50 has two fuel
injectors 70. Preferably, fuel is fed to the fuel injectors 70 from
a fuel source 182 that includes a rail/accumulator mechanism to
which fuel is pumped by a fuel pump 183. A fuel return manifold 184
collects fuel from the fuel injectors 70 and the fuel source 182
for return to a reservoir from which the fuel is pumped. Although
FIG. 3 shows the fuel injectors 70 of each cylinder disposed at an
angle of less than 180.degree., this is merely a schematic
representation and is not intended to be limiting with respect to
the locations of the injectors or the directions of the sprays that
they inject. In one preferred configuration, the injector nozzles
are disposed for injecting fuel sprays in diametrically opposing
directions along a common axis. Preferably, each fuel injector 70
includes or is associated with an electrically-operated actuator
(such as a solenoid) that operates the injector. Preferably, the
actuator is controlled by a respective drive signal produced by an
electronic multichannel injector driver 186.
[0033] As seen in FIG. 4, an engine control unit (ECU) 200 may be
configured to control charge air flow and the amount of exhaust gas
mixed with the pressurized charge air in response to specified
engine operating conditions by automatically operating the valves
138, 166, and 170 (and, possibly other valves), the supercharger
110, if a multi-speed or variable speed device is used, and the
turbo-charger 121, if a variable-geometry device is used. Of
course, operation of valves 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 state in which it is open (to some setting
controlled by the ECU 200) to allow gas to flow through it, and a
state in which it is closed to block gas from flowing through. In
some aspects, the ECU 200 may be configured to control and operate
the air handling system in a manner based on the disclosure in
related U.S. patent application Ser. No. 14/039,856.
[0034] As per FIGS. 3 and 4, the ECU 200 may be configured to
control a fuel injection mechanization according to fuel injection
control schedules in response to measured parameter values obtained
from engine sensors. These control schedules cause the generation
of output control signals that are coupled to the reservoir 182,
the pump 183, and the injector driver 186. In response to the
control signals, the injector driver 186 generates drive signals on
separate dedicated channels to operate the fuel injectors 50. In
some aspects, the ECU 200 may be configured to control and operate
the fuel injection system 180 in a manner based on the disclosure
in related U.S. patent application Ser. No. 13/654,340.
Cold Start Strategies for an Opposed-Piston Engine
[0035] With reference to FIG. 4, a cold-start system for a
two-stroke cycle opposed-piston engine with one or more cylinders
that is configured to operate the engine's air handling, fuel, and
thermal management systems includes the ECU 200, a starter motor
202, the valves 138, 166, and 170, and fuel injectors 70. In some
aspects, the cold start system further includes a variable speed
supercharger 110 and/or variable geometry turbine 121.
[0036] The air handling system includes a set of sensors that
indicate current conditions relating to gas flow and combustion;
other engine sensors indicate current conditions relating to
ambient conditions, engine speed, and thermal management of engine
cooling and lubrication systems. A subset of sensors, including air
handling, thermal management, and other sensors provide
environmental and engine data to the ECU 200 that is used by the
ECU to control the execution of a cold-start strategy. As seen in
FIG. 4, this set includes an ambient temperature sensor 210, an
ambient pressure sensor 212, an inlet temperature sensor 214, an
intake temperature sensor 216, a cylinder coolant temperature
sensor 218, and an oil temperature sensor 220. The sensors 210 and
212 are external to the engine and sense, respectively, thermal and
atmospheric conditions of the ambient environment. As per FIG. 2,
the inlet temperature sensor 214 is placed upstream of the filter
150 in order to sense the thermal condition of inlet air flowing
into the air handling system. As seen in FIG. 2, the intake
temperature sensor 216 is located in the intake manifold 130, where
it gauges the temperature of the charge air entering the cylinder
intake ports. Temperature data reported by the sensors 218 and 220
indicate the thermal state of the engine itself.
[0037] Using environmental data provided by these (and, possibly,
other) sensors, the ECU 200 is enabled by programming to detect a
cold start condition (C_S) as a function of a set of engine
operating parameters. These parameters include ambient temperature
(t.sub.a), ambient pressure (p.sub.a), inlet temperature
(t.sub.il), intake temperature (t.sub.ik), coolant temperature
(t.sub.c), and oil temperature (t.sub.o) (and, possibly, others).
In other words: C_S=f(t.sub.a, p.sub.a, t.sub.il, t.sub.ik,
t.sub.c, t.sub.o, . . . ). These parameters are useful during
engine start-up to distinguish circumstances in which a cold-start
strategy is not needed from those in which a cold-start strategy is
very desirable. For example, a cold-start strategy may not be
needed to start a recently-stopped, and still warm, engine on a
cold day. In another example, a cold start strategy is almost
certainly indicated for starting an engine on a cold winter morning
after the engine has been stopped for a long period (overnight, for
example) in subzero ambient conditions. However, it may be the case
that overnight shut-down of the same engine during a warm Indian
summer period will not lower the internal temperatures of the
engine to a level that necessitates a cold-strategy start-up of an
engine on a cool fall morning.
[0038] FIG. 5 illustrates an example of a method by which
cold-start procedures are performed using elements of the
cold-start system of FIG. 4 in operating a two-stroke cycle,
compression-ignition, opposed-piston engine. Referring now to both
figures, presume that the engine is installed in a vehicle equipped
with an engine start/stop mechanism such as an electro-mechanical
or a fully electrical key system capable of providing an engine
Start/Stop signal to the ECU 200. Presume that the engine is
running when a Stop signal is received at step 300. At this point,
the ECU 200 initiates a shut-down sequence. In some aspects, but
not necessarily, the shut-down sequence may include a process that
preconditions air retained in the engine when it stops.
[0039] Thus, in response to the Stop signal, the charge air in the
cylinder 50 may be optionally preconditioned to prepare for a
subsequent start; in this regard, the charge air may be brought to
a desired level of purity by reduction or elimination of exhaust
products that it may otherwise contain when the engine stops. In
step 302, if the preconditioning option has been elected, the ECU
200 initiates the following actions during engine shut down: first,
it closes the EGR valve 138 to block the flow of exhaust gas
through the EGR channel and into the charge air subsystem, and then
it shuts down delivery of fuel to, and operation of, the injectors
70. Next, the ECU 200 closes the supercharger bypass valve 166 and
opens the exhaust backpressure valve 170. If the turbine 122 is a
variable geometry device, the ECU fully opens the turbine to
maximize exhaust flow therethrough. This maximizes the flow of
charge air into and through the cylinder 50, while simultaneously
flushing exhaust gas from the cylinder. The result is that the air
remaining in the cylinders of the engine is undiluted with exhaust
and produces a more nearly stoichiometric result when mixed with
fuel during a subsequent start.
[0040] On receipt of a Start signal following shut down (step 304),
the ECU 200 at step 306 surveys the cold-start parameters (t.sub.a,
p.sub.a, t.sub.il, t.sub.ik, t.sub.c, t.sub.o, . . . ) and decides
whether or not to perform a cold-start procedure. If not, the ECU
200 uses normal start procedures at step 308 to start the engine,
which places the engine in an idle state of operation at step 310.
Otherwise, the engine is assumed to be in a cold-start state and a
cold-start procedure is commenced at step 312.
[0041] At step 312, before causing fuel to be injected, the ECU 200
initiates engine cranking by activating a starter motor 202. Engine
speed and a count of revolutions are determined by the ECU 200
based on crank angle (CA) data reported by an engine speed sensor
203 that monitors movement of the crankshaft 71. At step 314,
actions are taken to heat up the charge air before fuel is injected
and while the engine is being cranked by the starter motor 202. In
this regard, the ECU 200 closes the EGR valve 138 and the
back-pressure valve 170 (and the turbine 121, if a variable
geometry device is used) and sets the recirculation valve 166 to
limit the flow of charge air through the engine, while operating
the starter motor 202 to crank the engine, without injecting fuel,
for a specified number of revolutions of the crankshaft 71, as
determined by the ECU 200. In limiting the flow of charge air, the
recirculation valve 166 may be fully open so that, with the
backpressure valve 170 closed, charge air flow through the
cylinders is minimized, if not blocked. In some alternative
aspects, the ECU 200 may be configured to partially close the
supercharger bypass valve 166 so as to achieve a pre-injection
boost pressure target inside the intake manifold 130. In these
aspects, the supercharger will not only cause the charge air to
heat up in the compression process but will also provide a higher
starting pressure for the compression of retained air, which will
lead to an even higher temperature inside the cylinder. In either
case, engine cranking causes the pistons to reciprocate, thereby
heating the charge air by a sequence of unfueled compressions. In
some instances, the ECU 200 may also set control of the cooling
system 205 to block the flow of liquid coolant to the cylinders
(cooled by water, for example) and/or the pistons (cooled by oil,
for example), which prevents the heat resulting from compression
being conducted away from the cylinder and pistons. This process
allows for a build-up in the combustion wall temperatures and a
build-up in the temperature of the charge air retained in the
cylinders.
[0042] After the specified number of crankshaft revolutions is
counted, the ECU 200 shuts off the starter motor 202 while
initiating combustion at step 316. The ECU triggers the first fuel
injection by the injectors 70 at a pre-determined timing based on
the specified number of engine revolutions. The first fuel
injection is defined by a cold-start schedule defining a split
injection method comprising one or more main injections and a
sequence of sub-injections, at a pre-determined injection pressure
and predetermined quantities. For example the main injection timing
could be set to 10 degrees CA before minimum volume, with two pilot
injections preceding it by 10 degrees CA and the fuel pressure set
to 600 Bar. The fuel quantity injected at each combustion event in
each cylinder is higher than an idle quantity so as to ensure that
the engine speed quickly approaches an idle speed set point.
[0043] At step 318, the ECU 200 may be configured to stabilize
combustion by way of an idle governor 209, a routine included in
the programming of the ECU 200 that regulates engine speed. In this
regard, the idle governor 209 causes the fuel quantity injected via
the injectors 70 to achieve and maintain a predetermined idle
speed. During stabilization, injection timing is retarded at step
318 more so than during the initial combustion events at step 316.
For example, the main injection may now occur at 5 degrees CA
before minimum volume while the pilot injections continue to occur
at 10 degrees CA before the main injection. In these instances, the
rail pressure may be reduced to 400 Bar for example to minimize
combustion noise. This happens while the ECU 200, using a
pre-determined mass flow schedule, causes the back-pressure valve
170 to progressively move to a more open position and the
supercharger bypass valve 166 to progressively move to a more
closed position, so as to replenish the combustion chamber with the
enough fresh charge air to guarantee combustion stability. In some
aspects, the ECU 200 may be configured to transition control of the
air handling system to a closed-loop idle governor process that
targets specific boost pressure, air mass flow, and EGR rate
targets by automatically controlling the supercharger bypass valve
166, exhaust back-pressure valve 170 (and/or, the VGT 121, if
equipped), and the EGR valve 138 positions. Finally, in step 310,
the ECU 200 may be configured to activate the cooling system 205 so
as to manage warm-up of the cylinders, pistons, and other engine
components.
[0044] An example of cold-start scheduling of air flow through, and
delivery of fuel to, the engine during a cold-start procedure is
presented in Table I.
TABLE-US-00001 TABLE I Cold-Start Scheduling of Air Flow and Fuel
Delivery Steps 312 Idling Step 302 and 314 Step 316 Step 318 State
Fuel quantity 0 0 35 20 15+ (mg/injection) Engine Speed Decreasing
100-300 100-300 800 800+ (RPM) to 0 Supercharger Closed Open
Controlled to Controlled to Controlled to Bypass 166 meet a boost
meet a boost meet a boost target target target EGR Valve 138 Closed
Closed Controlled to Controlled to Controlled to meet an meet an
meet an EGR target EGR target EGR target Backpressure Open Closed
Controlled to Controlled to Controlled to Valve 170 (or meet an air
meet an air meet an air VGT) flow target flow target flow target
Fuel Injection Decreasing 600 600 400 400+ Pressure (Bar) to 0 Main
injection No fuel No fuel 10 5 20-0 timing.sup.1 injection
injection Pilot No fuel No fuel 10 10 5-20 Injection(s).sup.2
injection injection .sup.1in degrees CA before minimum volume
.sup.2in degrees CA before main injection
[0045] In FIG. 6, engine speed and the rate of fuel delivered to a
cylinder during engine shut-down are plotted against time. With
reference to FIGS. 4, 5, and 6, the ECU 200 may be configured to
cause fuel injection to stop at 320 when engine shut-down begins.
When the rate of injection declines to zero at 322, engine speed
begins to drop sharply at 323, finally ceasing at 325. With no
combustion taking place, no exhaust or boost air being
recirculated, and the exhaust subsystem fully open during the short
time it takes for the engine to cease turning, exhaust-free charge
air is pumped by the supercharger 110 to the intake manifold 139,
and flows from there to the cylinders. The result is a cleansing
flow of charge air through the cylinders.
[0046] In FIG. 7, engine speed and the rate of fuel delivered to a
cylinder during engine shut-down are plotted against time. With
reference to FIGS. 4, 5, and 7, the ECU 200 may be configured to
cause the cold-start system elements to perform cold-start
procedures that quickly and effectively elevate combustion
temperatures to a level that results in stable combustion and low
emissions. Desirably, the fast advance to high combustion
temperature ensures elevated exhaust temperature as is needed to
quickly light off after-treatment catalyst devices. Thus, at 330,
the ECU 200 causes the starter motor 202 to begin cranking the
engine. At 332, after a predetermined number of cranks increase
engine speed to a predetermined level, and while no fuel is
injected, the ECU 200 causes the valves 138 and 170 to close and
the valve 166 to open, thereby minimizing airflow, which causes
heating of air retained in the cylinders 50 in the portion of the
graph labeled "Charge Heating". In the "Initial Combustion" section
of the graph, at 334, the ECU 200 causes fuel to be injected at a
rapidly increasing rate according to a cold-start split fuel
injection schedule that is followed for all cylinders, which
initiates and rapidly intensifies combustion until an injection
peak is reached at 336. This causes the engine speed to ramp
upwardly to a peak at 337. The ECU 200 may be configured to cause a
transition to stabilization of combustion ("Stabilization") by
progressively reducing the rate of fuel injection from 336 to 338
where a maximum point in an injection rate range in an idling
schedule is reached. From 338, the ECU may be configured to reduce
the rate of fuel injection according to an idling schedule so as to
reach (or maintain) an engine idling speed target, and the graph
shows engine speed falling from the peak at 337, to an idling speed
of about 800 RPM. As per Table I, from initialization, through
stabilization to idling, the ECU 200 may be configured to control
the valves 170 and 166 so as to continually replenish the
combustion chamber with enough fresh charge air to achieve and
maintain combustion stability. As per Table I, from initialization,
through stabilization to idling, the ECU 200 may be configured to
control the EGR valve 138 so as to recirculate exhaust products to
the charge air subsystem in enough quantity to achieve and maintain
low levels of NOx emissions.
[0047] Those skilled in the art will realize that the cold-start
schedules illustrated in Table I and the graphs of FIGS. 6 and 7
are for illustration only and are not limiting.
[0048] FIG. 8 illustrates additional options that may be
incorporated into the cold-start system of FIG. 4. In this regard,
the cold-start system may include one or more of a charge air
cooler bypass path in the charge air subsystem and an EGR cooler
bypass path in the EGR channel (if an EGR cooler is provided). In
some aspects there may be a bypass path 350 for the cooler 127
and/or a bypass path 352 for the cooler 129. If provided, the
bypass path 350 is opened or closed by a three-way valve 351; if
provided, the bypass path 352 is opened or closed by a three-way
valve 353. The ECU 200 may be configured to control the position of
either or both of the three-way valves 351 and 353 so as to open or
close the cooler bypass path that it controls. When open, a cooler
bypass path conducts charge air through a cooler; when closed the
bypass path conducts charge air around a cooler. Thus, the ECU 200
may be further configured to suppress cooling of charge air
beginning at step 314 (FIG. 5) by enabling charge air flow through
either or both charge air cooler bypass paths 350 and 352 until the
warm up phase of the idling state is reached.
[0049] As per FIG. 8, in some aspects, if the EGR channel includes
the EGR cooler 164, there may be a bypass path 356 for the cooler
164. If provided, the bypass path 356 is opened or closed by a
three-way valve 357. The ECU 200 may be configured to control the
position of the three-way valve 357 so as to open or close the
cooler bypass path 356. When open, the cooler bypass path 356
conducts exhaust gas through the cooler 164; when closed the bypass
path 356 conducts exhaust gas around the cooler 164. Thus the ECU
200 may be further configured to suppress cooling of recirculated
exhaust gas by enabling exhaust flow through the EGR cooler bypass
path 356 beginning at step 316 (FIG. 5) until the idling state is
reached.
[0050] The cold-start strategy embodiments that are described
herein, and the systems and procedures with which they are
implemented, are illustrative and are not intended to be
limiting.
* * * * *