Note: Descriptions are shown in the official language in which they were submitted.
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Method Of Controlling A Direct-Injection Gaseous-Fuelled Internal
Combustion Engine System With A Selective Catalytic Reduction Converter
Field of the Invention
[0001 ] The present invention relates to a method of controlling a direct-
injection
gaseous-fuelled internal combustion engine system with a selective catalytic
reduction converter to reduce emissions of NOx when the engine is idling.
Backaround
[0002] Presently, most over-the-road heavy vehicles are fuelled by gasoline or
diesel fuel. Because both gasoline and diesel-fuelled internal combustion
engines generate a significant amount of pollutants such as oxides of nitrogen
(NOx) and particulate matter (PM), engine manufacturers have been searching
for best ways to improve their engines to comply with the new government
regulatory standards which are becoming progressively more stringent with
respect to the allowed levels of pollutants in tailpipe emissions.
[0003] For diesel-cycle engines one approach that shows a significant
improvement in reducing the levels of pollutants in tailpipe emissions
involves
substituting a part or all the diesel fuel with cleaner burning gaseous fuels
such
as natural gas, pure methane, ethane, liquefied petroleum gas, lighter
flammable
hydrocarbon derivatives, hydrogen, and blends of such fuels. Gaseous fuels are
generally defined herein as fuels that are gaseous at atmospheric pressure and
zero degrees Celsius. Whereas liquid fuels such as diesel are injected at very
high pressures in order to atomize the fuel, gaseous fuels can be injected
into an
engine's combustion chamber at lower pressures because no extra energy is
required for fuel atomization. An advantage of using the diesel-cycle and
substituting a gaseous fuel for diesel fuel is this approach can preserve the
high
efficiency and high torque of the conventional diesel engines, while reducing
pollutant levels in tailpipe emissions.
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[0004] However, some modifications are required to a conventional diesel
engine
to allow gaseous fuels to be substituted for diesel fuel. In a conventional
diesel
engine, the heat produced by the mechanical compression of the fuel and air
mixture auto-ignites the liquid diesel fuel charge at or near the end of the
piston's
compression stroke. Other liquid fuels such as dimethyl ether, bio-diesel, and
kerosene will also auto-ignite at the temperatures and pressures within the
combustion chamber generated by the compression of the charge within the
combustion chamber. However, under the same temperature and pressure
conditions generated by the compression of the charge within the combustion
chamber, gaseous fuels such as natural gas will not reliably auto-ignite.
Therefore, in order to reliably burn a gaseous fuel in a conventional
compression
ignition engine with the same compression ratio as a diesel engine, an igniter
is
required within the combustion chamber to assist with ignition of the gaseous
fuel, such as a hot surface provided by a glow plug, a spark plug, or a fuel
injection valve for introducing a fuel that will reliably auto-ignite, acting
as a pilot
fuel. The pilot fuel can be a small quantity of diesel fuel, whereby the auto-
ignition of the diesel fuel triggers the ignition of gaseous fuel.
[0005] While gaseous fuels are generally cleaner burning than conventional
liquid fuels, tailpipe emissions from gaseous-fuelled engines can be further
improved to reduce the levels of NOx by applying a treatment called Selective
Catalytic Reduction ("SCR") to the gases exhausted from the engine. In an SCR
converter, ammonia is injected into the exhaust stream upstream of the SCR
catalyst as a reduction agent. The ability of ammonia as a reductant to
achieve a
significant reduction of NOx has been proven for stationary power applications
and therefore has been used in diesel-fuelled engines. Any form of ammonia
may be used, such as urea, aqueous, gaseous or liquid ammonia. Using an
SCR converter, the SCR catalyst facilitates the reaction between ammonia and
NOx to produce water and nitrogen gas.
[0006] However, the applicants have found that combining an SCR converter with
a gaseous-fuelled engine did not always achieve the same NOx conversion
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rates. Under some conditions, especially when the engine is idling, it was
found
that the temperature of the exhaust gas exiting the combustion chamber was
significantly lower than the temperatures normally found under higher speed
engine operation . To maintain a high NOx conversion rate it was determined
that the temperature of the catalytic bed in the SCR converter is preferably
above
a predetermined temperature which can vary depending upon the composition of
the catalyst. Generally, if the temperature of the exhaust gas exiting the
combustion chamber is maintained above 200 degrees Celsius, acceptable NOx
conversion rates are achieved.
[0007] For conventional diesel engines there are many known approaches for
increasing the exhaust gas temperature, but there are particular
characteristics of
gaseous-fuelled engines that prevent the simple transfer of these approaches.
For example, some approaches result in unburned fuel being introduced into the
exhaust stream, and gaseous fuels, such as natural gas, which consists mostly
of light hydrocarbons (methane in particular), do not readily oxidize in SCR
converters, especially at lower temperatures, and can become deposited on the
SCR catalyst.
[0008] Therefore there are special considerations that need to be taken into
account to develop a successful engine system that uses a gaseous fuel and a
SCR converter for reducing levels of NOx in the tailpipe emissions.
Summary of the Invention
[0009] A control method is provided for an internal combustion engine
comprising
a combustion chamber defined by a cylinder and a piston reciprocable within
the
cylinder, the piston being connected to a crankshaft that rotates when the
piston
reciprocates, and an injector for injecting a gaseous fuel directly into said
combustion chamber. The exhaust gas exiting from said combustion chamber is
received in a selective catalyst reduction (SCR) converter that is operative
to
reduce levels of NOx in said exhaust gas by converting NOx into nitrogen and
water.
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[0010] The method comprises the steps of detecting at least one engine
parameter indicative of when said internal combustion engine is idling and, in
an
engine cycle, when determining that said internal combustion engine is idling
timing the injection of a first quantity of fuel to begin injection when said
piston is
near top dead centre and controlling temperature of exhaust gas exiting the
combustion chamber to be above a predetermined temperature that is defined by
an operating temperature range that achieves a desired conversion efficiency
for
the selective catalytic reduction converter. The temperature of the exhaust
gas is
controlled by timing the beginning of injection of the gaseous fuel to be
after the
injection the first quantity of fuel, and increasing exhaust gas temperature
by
increasing a delay in timing for injecting the gaseous fuel, while limiting
the delay
to keep the concentration of unburned fuel exiting said combustion chamber
below a predetermined concentration.
[0011 ] One parameter indicative of when the engine is idling can be the
engine's
speed. Another parameter indicative of when the engine is idling can be a
total
fuelling amount. Also, a controller could read the values of both these
parameters
from a two-axis map to determine when the engine is idling.
[0012] In some embodiments of the present method, the fuel injector injects
the
first quantity of fuel in a plurality of pulses introduced sequentially into
said
combustion chamber. Each one of the pulses can have the same duration, or
they can be different in duration.
[0013] If a controller determines from at least one engine parameter, for
example
the engine speed or total fuelling amount, that the engine has transitioned
from
idling to load, it gradually advances over a predetermined time both timing
for
beginning injection of the first quantity of fuel and timing for beginning
injection of
the gaseous fuel until each begins before the piston is at top dead centre
with
each timing predetermined based on engine speed and respective commanded
quantities of fuel based on total fuel energy required by engine load.
[0014] If the first quantity of fuel is injected into the combustion chamber
in a
plurality of pulses and the controller determines that the engine has
transitioned
from idling to load, it gradually advances over a predetermined time both
timing
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for beginning injection of each one of the plurality of pulses and timing for
beginning injection of the gaseous fuel, and decreasing elapsed time between
the plurality of pulses, until at the end of said predetermined time, the
plurality of
pulses has merged into a single pulse and until the timing for beginning
injection
of the first quantity of fuel and said gaseous fuel is advanced to occur
before said
piston is at top dead centre and the injection timings are predetermined based
on
engine speed and respective commanded quantities of fuel based on total fuel
energy required by engine load.
[0015] In a preferred embodiment, when determining from the engine speed and
total fuelling amount that the engine is idling, the method comprises the step
of
timing an injection of a first quantity of fuel in two pulses to begin
injection of a
first pulse when said piston is near top dead centre and to begin injection of
a
second pulse after ending the injection of the first pulse. The controller
controls
the temperature of exhaust gas exiting said combustion chamber to be above a
predetermined temperature that is defined by an operating temperature range
that achieves a desired conversion efficiency for said selective catalytic
reduction
converter by timing beginning of an injection of said gaseous fuel directly
into the
combustion chamber to be after timing for injection of said two pulses, and
increasing exhaust gas temperature by increasing a delay in timing for
injecting
the gaseous fuel, while limiting the delay to keep concentration of unburned
fuel
exiting the combustion chamber below a predetermined concentration. The
controller adjusts the timing for beginning injection of the second pulse to
be
generally near beginning of the gaseous fuel injection. The second pulse can
have the same duration as the first pulse or they can be different in
duration.
[0016] When the first quantity of fuel is injected in two pulses and the
controller
determines from said engine speed and total fuelling amount that the engine
has
transitioned from idling to load, it gradually advances over a predetermined
time
both timing for beginning injection of each one of two pulses and timing for
beginning injection of the gaseous fuel, decreases elapsed time between the
end
of the first pulse and the beginning of the gaseous fuel injection and
decreases
the amount of fuel injected in the second pulse, until at the end of the
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predetermined time a single pulse is injected into the combustion chamber and
until the timing for beginning injection of the single pulse and the gaseous
fuel is
before the piston is at top dead centre with each timing predetermined based
on
engine speed and respective commanded quantities of fuel based on total fuel
energy required by engine load.
[0017] In preferred embodiments, for example for a 15 liter direct-injection
natural
gas internal combustion engine ignited by a diesel fuel, the delay in timing
for
injecting the gaseous fuel, measured in degrees of crank angle rotation can be
between 14 and 25 degrees after top dead centre. In such embodiments the
controller can end injection of the first quantity of fuel at a timing when
said
crankshaft angle of rotation is within 1 degree before or after of beginning
the
injection of the natural gas so that the ignition of the diesel fuel can warm
up the
combustion chamber and thereby transfer the heat to the natural gas injected
into the combustion chamber after the diesel. The beginning of the injection
of
the first quantity of fuel can start when said crankshaft is positioned
between 2
crank angle degrees before top dead centre and 5 crank angle degrees after top
dead centre.
[0018] For many engines, the predetermined concentration of unburned fuel
exiting said combustion chamber that is acceptable for an efficient operation
of
the selective catalytic reduction converter is 1000 ppm. In preferred
embodiments the concentration of unburned fuel in the exhaust can be below
between 200 and 300 ppm.
[0019] Since selective catalytic reduction converters require a temperature of
at
least 200 degrees Celsius to operate efficiently, the predetermined
temperature
of the exhaust gas exiting the combustion chamber according to the present
method is generally equal to or higher than 200 degrees Celsius.
[0020] The gaseous fuel injected directly into the combustion chamber is
selected
from the group consisting of natural gas, methane, propane, butane, hydrogen,
and mixtures thereof.
[0021 ] When the first quantity of fuel injected into the combustion chamber
is a
fuel that is the same as the gaseous fuel, the internal combustion engine
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comprises an igniter disposed within said combustion chamber for igniting said
fuel, for example a glow plug, a spark plug or a hot surface.
[0022] In other embodiments, the first quantity of fuel injected into the
combustion
chamber is a fuel that auto-ignites in the combustion chamber, for example a
fuel
selected from the group consisting of diesel fuel, dimethylether, bio-diesel,
and
kerosene.
Brief Description of the Drawinas
[0023] Figure 1 is a schematic view of a direct-injection gaseous-fuelled
internal
combustion engine system comprising an exhaust after-treatment subsystem and
an exhaust gas recirculation loop;
[0024] Figure 2 is a schematic representation of the fuel injection timings
for a
gaseous-fuelled internal combustion engine when the engine is operating at
idle,
mid speed or higher speeds according to a conventional method of fuel
injection
control known in the prior art;
[0025] Figure 3 is a schematic representation of the fuel injection timings
when
the engine operates at idle according to the present method whereby the
exhaust
gas temperature is increased and maintained at the required temperature for
improving NOx conversion rates in the SCR converter;
[0026] Figure 4 is an illustration of the fuel injection timing zones for an
engine
test cycle when the present fuel injection control strategy was applied and it
also
illustrates the SCR catalyst bed temperature recorded during testing, and
[0027] Figure 5 is an illustration of the engine's speed-fuelling map that
shows
the range of engine speed and total fuelling amount where the present fuel
injection strategy can be applied.
Detailed Description
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[0028] Figure 1 shows a schematic view of a direct-injection gaseous-fuelled
internal combustion engine system comprising an exhaust after-treatment
subsystem and an exhaust gas recirculation loop. Herein "direct-injection" is
used to refer to the injection of fuel directly into the combustion chamber of
an
internal combustion engine, which is an approach that is technically distinct
from
engines that inject fuel into an engine's intake manifold or into the intake
ports on
the manifold side of the engine's intake valves. With direct-injection engines
the
fuel can be injected later in the engine cycle, thereby avoiding fuelling and
compression ratio limitations associated with avoiding engine knock ("pre-
mature
detonation of the fuel"). Conversely, this generally allows direct-injection
engines
to employ higher compression ratios, and achieve higher efficiencies and power
outputs compared to other engines with the same displacement. The disclosed
method can be used with engines that inject gaseous fuel directly into the
combustion chamber through an injector. The gaseous fuel can be ignited by an
ignition means which can be a spark plug, a glow plug, a hot surface or a
pilot
fuel that auto-ignites inside the combustion chamber. When gaseous fuel
ignition
is assisted by a pilot fuel, the pilot fuel is preferably introduced directly
into the
combustion chamber by a separate injector. In some embodiments the gaseous
fuel injector and the pilot fuel injector are integrated into a single
assembly, but
with separate passages for the gaseous and pilot fuels so that the two
injectors
are independently operable to separately inject each fuel at different times.
The
schematic view shown in Figure 1 is not to scale, with some parts shown larger
relative to the other parts to better illustrate their function. The disclosed
direct-
injection internal combustion engine has at least one cylinder, a piston being
reciprocable within the cylinder in known fashion, and having a crankshaft
connected to the piston which is rotatable by the reciprocal movement of the
piston within the cylinder. In this disclosure, the fuel injection into the
combustion
chamber is described with reference to crank angle degrees before or after top
dead centre (TDC) which represent the position of the crankshaft relative its
position when the piston is at top dead centre (TDC). The piston is at TDC
when
it has reached the end of a compression stroke and is about to begin an
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expansion stroke, more specifically when the piston is closest to the cylinder
head.
[0029] Referring to Figure 1, internal combustion engine system 100 shows an
illustrative embodiment of a direct-injection gaseous-fuelled engine that uses
a
pilot fuel, for example diesel fuel, to assist in igniting the gaseous fuel
injected
into a combustion chamber. Internal combustion engine system 100 generally
comprises engine 110, diesel fuel delivery subsystem 112, gaseous fuel
delivery
subsystem 114, and controller 116. The engine system further comprises air
intake line 118 and exhaust gas line 120. Some of the exhaust gas exiting the
engine in direction 122 is directed through exhaust gas recirculation loop 124
in
direction 126 and through valve 128 into air intake line 118 where it is mixed
with
intake air flowing through air intake line 118. The mix of fresh intake air
and
recirculated exhaust gas is delivered to the intake ports of engine 110 in the
direction shown by arrow 130. The exhaust gas exiting engine 110, which is not
recirculated, flows in direction shown by arrow 132 through turbocharger 138
and
on to line 134 which is connected to exhaust gas after-treatment subsystem 140
and from there the exhaust gas is released into the atmosphere through the
tailpipe. Turbocharger 138 preferably has a variable geometry as known in to
those skilled in technology but the disclosed method can also be applied to
engines without a turbocharger.
[0030] Exhaust gas after-treatment subsystem 140 comprises selective catalytic
reduction (SCR) converter 142 and urea injection system 144. Exhaust gas after-
treatment subsystem 140 can also comprise particulate filter 146 (known as a
"DPF") and diesel fuel injector 148. DPF 146 may comprise a diesel oxidation
catalyst device 149 for oxidizing the hydrocarbons and carbon monoxide within
the exhaust gas.
[0031 ] Controller 116 can be integrated into a vehicle controller or it can
be a
separate controller that communicates with the vehicle controller. Controller
116
controls diesel fuel injection delivery subsystem 112, gaseous fuel delivery
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subsystem 114 and exhaust gas after-treatment subsystem 140 based on the
detected engine operation state. Controller 116 receives information about at
least one engine parameter indicative of the engine operation state, such as
the
engine speed and total fuelling amount. Based on the engine maps stored in its
memory, controller 116 can determine when the engine is idling as further
explained below in relation to Figures 4 and 5.
[0032] Figure 2 illustrates a schematic representation of the fuel injection
timings
for a direct-injection gaseous-fuelled internal combustion engine similar to
the
one illustrated in Figure 1, when the engine is operating at idle (1), mid-
speed (2)
or higher speeds (3). The approach to timing the start of injection is adapted
from the known approach used for injecting diesel into diesel-fuelled engines,
except that instead of injecting just diesel fuel, an initial pulse of diesel
pilot fuel is
injected followed by a larger pulse of gaseous fuel. Herein "idle" is used to
refer
to the state of an engine operating at low speeds (typically around 700 rpm
for
compression ignition engines, but idle speed can vary depending upon engine
design) when the only load served by the engine is generated by friction and
parasitic loads. When the engine operates at idle (1) controller 116 controls
fuel
delivery subsystems 112 and 114 to inject a quantity of diesel fuel 210
directly
into the combustion chamber of the engine before top dead centre and to inject
a
quantity of gaseous fuel 220 directly into the combustion chamber shortly
after
the diesel fuel injection and near top dead centre. The timing for injecting
the
fuel and the fuel quantity is optimized to maintain a predetermined engine
idling
speed when no productive loads are served by the engine and the only load is
generated by friction and parasitic loads. During the engine's mid-speed
operation (2), when the engine speed increases compared to when the engine
operates at idle, the timing of diesel fuel injection 230 and of gaseous fuel
injection 240 occur earlier in the engine cycle relative to the top dead
centre
when compared to the timing of fuel injection at idle. When the engine speed
increases further to higher speeds, as shown in example (3) of Figure 2, the
timing of diesel fuel injection 250 and of gaseous fuel injection 260 occurs
even
earlier in the engine cycle relative to the top dead centre.
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[0033] In the prior art method described above, like with diesel-fuelled
engines, in
all operation modes diesel fuel is injected before top dead centre and the
start of
the gaseous fuel injection generally occurs near or before top dead centre. As
engine speed increases, the start of diesel fuel injection and of gaseous fuel
injection occurs earlier in the engine cycle. For example, for a 15 liter
direct-
injection gaseous-fuelled engine using diesel pilot ignition, the earliest
timing of
diesel fuel injection can be between 10 to 20 degrees crank angle before TDC
when the engine operates at high speeds. In the prior art method illustrated
in
Figure 2, diesel fuel is injected into the combustion chamber near the start
of the
gaseous fuel injection so that when the diesel pilot fuel ignites, it
generates
enough heat for heating the combustion chamber and effectively igniting the
gaseous fuel that is introduced into the combustion chamber sequentially after
the pilot fuel.
[0034] When an igniter is employed, such as, for example, a glow plug or other
hot surface, or a spark plug, a pilot fuel is not needed. Nevertheless, in
some
embodiments the same fuelling strategy can be used if the igniter is employed
to
ignite a pilot quantity of gaseous fuel which in turn ignites the main
quantity of
gaseous fuel injected in respective pulses 220, 240 and 260.
[0035] The presently disclosed improved method of controlling fuel injection
when
the engine operates at idle is illustrated in Figure 3 which shows the fuel
injection
timings for four different embodiments A to D, all relating to fuel injection
strategies for when a gaseous-fuelled direct-injection internal combustion
engine
is idling. A person skilled in the technology would be able to understand,
based
on the embodiments illustrated in Figure 3 and described in more detail below,
that the present method can be implemented on a gaseous-fuelled internal
combustion engine that employs a pilot fuel such as diesel for assisting
ignition
or engines that employ other means of assisting ignition, such as, for example
a
glow plug or another hot surface, or a spark plug. In such cases gaseous fuel
pulses can replace diesel pulses in the injections described below while still
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following the injection patterns illustrated in embodiments A through D of
Figure
3.
[0036] In all embodiments of the presently disclosed method, gaseous fuel
injection starts later in the engine cycle compared to the gaseous fuel
injection
timing practiced in the prior art conventional methods that followed
conventional
approaches employed by diesel-fuelled engines, as illustrated in Figure 2. By
delaying the injection of the gaseous fuel, more heat is transferred to the
exhaust
gas exiting the combustion chamber and this heat is carried into the SCR
catalyst
bed, helping to maintain it at a higher temperature when the engine is idling.
In
embodiments in which a pilot fuel is employed as the means for assisting
ignition,
because of the relatively low energy requirements for sustaining the engine
speed when idling, most of the energy needed to sustain idling is provided by
the
combustion of the pilot fuel, and much of the energy from burning the gaseous
fuel is converted into heat. In embodiment A of the presently disclosed method
the timing for injecting first quantity of fuel 310 directly into the
combustion
chamber is set to begin when the piston is near top dead centre, and the
combustion of this fuel contributes mostly to overcoming friction and
parasitic
loads to sustain engine idling speed. The injection of first quantity of fuel
310 can
end near the timing for beginning gaseous fuel injection 312. Gaseous fuel is
injected directly into the combustion chamber and gaseous fuel injection 312
begins sequentially after the timing of the injection of the first quantity of
fuel 310
and after top dead centre. With this approach, the timing for the combustion
of
the gaseous fuel is adjusted to increase the exhaust gas temperature to at
least
200 degrees Celsius, which maintains the temperature inside SCR converter 142
at an operating range that improves NOx conversion efficiency. That is, the
temperature of the exhaust gas exiting the combustion chamber is increased by
increasing a delay in timing for beginning injection of gaseous fuel injection
312,
but unlike with conventional diesel engines, for gaseous-fuelled engines the
length of this delay is limited to keep concentration of unburned fuel exiting
the
combustion chamber below a predetermined concentration, generally below
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1000 ppm and, in preferred embodiments, below a concentration of between 200
and 300 ppm.
[0037] In preferred embodiments, injection of first quantity of fuel 310 ends
within
1 degree crank angle before or after of timing for beginning gaseous fuel
injection
312 so that diesel fuel can effectively ignite the gas. Injection of first
quantity of
fuel can begin when the crankshaft is positioned between about 2 crank angle
degrees before TDC and about 5 crank angle degrees after TDC.
[0038] When an engine is idling, and diesel fuel is employed as a pilot to
ignite
the gaseous fuel, diesel fuel is injected as the first quantity of fuel 310
and the
combustion of the diesel fuel injected near TDC does most of the work to
overcome friction and parasitic loads to sustain a predetermined engine idling
speed and the combustion of the portion of diesel fuel injected near the
beginning of gaseous fuel injection 312 helps to ensure ignition of the
gaseous
fuel. In this method any fuel that auto-ignites within the combustion chamber
can
be injected as the first quantity of fuel 310. Such a fuel can be selected
from the
group consisting of dimethylether, bio-diesel and kerosene. Even if throughout
the present disclosure diesel fuel is referred to as a pilot fuel a person
skilled in
the art would be able to understand that the diesel fuel injected as the first
quantity of fuel 310 serves both for igniting the gaseous fuel and for
sustaining a
predetermined engine idling speed as explained above.
[0039] When the engine uses an igniter other than a pilot fuel, to ensure
ignition
of the gaseous fuel, such as a glow plug or other hot surface, or a spark
plug, the
first quantity of fuel 310, can be the gaseous fuel and the gaseous fuel
injected
near top dead centre is combusted to do work and overcome friction and any
parasitic loads, and thereby sustain a predetermined engine idling speed. The
gaseous fuel injected in the later portion of the first quantity of fuel 310
burns to
help ignite gaseous fuel injection 312.
[0040] With reference still to Figure 3, in embodiment B of the presently
disclosed
method, the injection of the first quantity of fuel is divided into two pulses
such
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that a first pulse 314 is injected into the combustion chamber near top dead
centre and second fuel pulse 316 is injected into the combustion chamber near
the start of gaseous fuel injection 318. In this embodiment the injection of
first
pulse 314 may start at a crank angle of between 2 degrees before TDC and 5
degrees after TDC. Gaseous fuel injection 318 starts later in the engine cycle
at
a crank angle determined through experimental tests to increase the
temperature
of the exhaust gas to the operating temperature range for the SCR converter,
generally above 200 degrees Celsius. Higher temperatures in the SCR converter
are generally associated with higher conversion efficiencies and like in all
embodiments, longer delays in the timing for beginning gaseous fuel injection
318 result in higher temperatures for the exhaust gas exiting the combustion
chamber and with the disclosed method temperatures higher than 200 degrees
Celsius can be achieved as long as the delay is not so long as to result in
the
unburned fuel concentration in the exhaust gas exceeding a predetermined
level,
generally, 1000 ppm, or in preferred embodiments 200ppm or 300 ppm.
[0041 ] Combustion of first fuel pulse 314 serves to overcome friction and
satisfy
parasitic loads to sustain a predetermined engine idling speed and combustion
of
second fuel pulse 316, injected near the beginning of gaseous fuel injection
314,
contributes to the ignition of gaseous fuel injection 318. The end of second
pulse
316 generally occurs within 1-degree crank angle before or after the beginning
of
fuel injection 318 such that the ignition of fuel injected in pulse 316 can
effectively
heat the combustion chamber and contribute to the gaseous fuel ignition.
[0042] In alternate embodiments C and D of the presently disclosed method,
injection of the first quantity of fuel is divided into a plurality of pulses,
by injecting
respective first pulses 320 and 330, into the combustion chamber near TDC,
followed by one or more respective pulses 322, 332 and 334, as shown in Figure
3, injected sequentially into the combustion chamber before a last respective
fuel
pulse 324 and 336, which is injected into the combustion chamber shortly
before
respective gaseous fuel injections 326 and 338, so that the end of respective
pulses 324 and 336, is near the start of respective gaseous fuel injections
326
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and 338. The end of respective last pulses 324 and 336 generally occurs within
1-degree crank angle before or after the start of respective gaseous fuel
injection
326 and 338, as illustrated in Figure 3. Pulses 320, 322 and 324 and
respectively 330, 332, 334 and 336 can each be of the same duration or can
vary
in duration. The preferred duration of each pulse and the separation time
between these pulses can depend upon the size of engine, the type of fuel used
and the engine's desired operating characteristics, but these parameters can
be
determined empirically by known calibration methods. Once an engine has been
calibrated, the calibrated parameters can be entered into look up tables or
multi-
dimensional maps which are then stored in the memory of controller 116 to be
used for controlling the fuel injection strategy for all engines made with the
same
design.
[0043] The quantity of fuel injected in first injection 310 and in second
injection
312 in embodiment A are each controllable by controller 116 in response to the
engine speed or any existent load communicated from the vehicle controller.
Similarly, when a plurality of pulses are injected into the combustion chamber
as
described in embodiments B, C or D of the present method, the quantity of fuel
injected into the combustion chamber in the first pulse (for example, pulse
314,
320 or 330) and the quantity of gaseous fuel (injected for example in
injections
318, 326 and 328) are controllable by controller 116 in response to the engine
speed or any current engine load communicated directly to controller 116 or
indirectly through a vehicle controller. Such variations of engine speed and
load
are generally minimal due to the fact that during idling the engine speed
stays
low, around 700 ppm, and the engine load is negligible considering that the
engine only needs to overcome friction and parasitic loads, which are
generally
very small relative to the engine's maximum load output.
[0044] When gaseous-fuelled internal combustion engine systems comprising a
SCR converter, as illustrated in Figure 1, operate at idle according to the
prior art
method illustrated in Figure 2, the temperature of the exhaust gas is not high
enough to maintain the SCR catalyst bed temperature above 200 degrees
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Celsius which results in a reduced conversion efficiency for the SCR
converter.
The presently disclosed method illustrated in Figure 3 differs from the
conventional method of fuel injection used for diesel and direct-injection
gaseous-fuelled internal combustion engines at idle in that the start of fuel
injection occurs later in the engine cycle. More specifically the injection of
the
first quantity of fuel starts near top dead centre and the injection of
gaseous fuel
starts after top dead centre, when the piston is on its expansion stroke
within the
cylinder. The first quantity of fuel can be a fuel that reliably auto-ignites,
such as
diesel fuel or it can be the gaseous fuel, if the engine is equipped with an
igniter
in the combustion chamber to assist with ignition of the gaseous fuel. For
example, the igniter can be a glow plug or other hot surface device, or a
spark
plug. By delaying injection of fuel compared to prior art gaseous-fuelled
engines,
the late combustion of the gaseous fuel generates less work on the piston and
results in more heat being transferred to the exhaust gas stream. Some of the
heat in the exhaust gases is transferred to the SCR converter in the after-
treatment system and this helps to keep the temperature of the SCR catalyst
bed
above a predetermined temperature that results in more efficient NOx
conversion
rates; this predetermined temperature can vary depending upon the catalyst
composition, but using known catalyst compositions this predetermined
temperature has been found to be generally around 200 degrees Celsius.
[0045] It is important to note that gaseous-fuelled engines and conventional
diesel-fuelled engines which are not fuelled with any gaseous fuel, have
distinct
differences, which prevent methods used by conventional diesel-fuelled engines
from being directly applied to gaseous-fuelled engines. With a conventional
diesel engine, the unburned diesel fuel exiting the combustion chamber which
reaches the after-treatment subsystem is oxidized on the oxidation catalysts
of
the exhaust gas after-treatment subsystem generating heat which further
increases the temperature of the exhaust gas. Accordingly, the presence of
excessive unburned diesel fuel in the exhaust gas exiting the combustion
chamber does not result in any adverse effect on the aftertreatment subsystem
and may even be beneficial by raising the temperature in the aftertreatment
CA 02698342 2010-04-20
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subsystem. This is different from gaseous-fuelled engines, where unburned fuel
which consists mainly of lighter hydrocarbons such as methane, which does not
oxidize easily on the after-treatment catalysts especially at lower
temperatures
and can adversely affect the performance of aftertreatment subsystems by
getting deposited therein or by being released unburned in to the atmosphere
through the tailpipe. Therefore, in a gaseous-fuelled internal combustion
engine
system it is preferable to avoid expelling unburned fuel from the combustion
chamber. The presently disclosed method teaches delaying the injection of
gaseous fuel to begin later in the expansion stroke, while limiting this delay
in
order to combust substantially all of the gaseous fuel within the combustion
chamber. The timing for beginning injection of the gaseous fuel can be
determined empirically by known engine calibration methods, and can depend on
various factors such as the engine size and type of fuel. Such calibration
methods were used with a Westport GX 15 liter engine fuelled with natural gas
to
produce experimental data that showed that a preferred timing for the gaseous-
fuel injection occurs after the injection of a diesel pilot fuel, when the
crankshaft
is at a crank angle between 14 and 25 degrees after TDC.
[0046] The presently disclosed method of controlling fuel injection timing to
maintain the SCR catalyst bed temperature above 200 degrees Celsius can be
complemented by a preferred air handling strategy. Referring once again to the
embodiment shown in Figure 1, when controller 116 controls valve 128 and
turbocharger 138 to reduce the cross-section through which the exhaust gas
flows, the result is a reduction in the turbocharger efficiency which causes
an
increase of the pumping work of the engine to maintain a desired power. To
generate more pumping work, the fuelling amount supplied to the engine has to
increase. As a consequence, more fuel is combusted within the combustion
chamber generating more heat which is partially transferred to the exhaust gas
exiting the combustion chamber and thereby increasing its temperature which is
in turn transferred to the catalyst bed in the after-treatment subsystem.
CA 02698342 2010-04-20
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[0047] A Westport GX 15 liter engine fuelled with natural gas was employed to
validate the disclosed method and to compare the engine's emissions against
government regulations. Figure 4 shows a plot of the engine speed shown by
line
410 and torque output shown by line 420. The hatched areas show when the
engine was idling. The presently disclosed method was applied in zones 430,
when engine speed 410 and torque output 420 had low values.
[0048] To demonstrate the effect of the presently disclosed method on the SCR
catalyst bed temperature, line 440 is plotted beneath the engine speed and
torque curves. Line 440 is a plot of the SCR catalyst bed temperature during
the
engine test corresponding to the torque and speed curves, when the presently
disclosed injection strategy was implemented in zones 430. Line 440 shows that
a short time after the beginning of the test (around 25 seconds) the SCR bed
temperature rose to a temperature above 200 degrees Celsius, and after this
initial rise in the SCR bed temperature, the temperature was maintained above
200 degrees Celsius for the entire test, including during the subsequent times
when the engine was operating at idle (zones 430). Line 450 is a plot of the
SCR
catalyst bed temperature during a test performed under the same conditions on
the same engine when the conventional method of fuel injection illustrated in
Figure 2 was applied, showing that the SCR catalyst bed remained at a
significantly lower temperature for an extended time, which resulted in
significantly higher levels of NOx in the tailpipe emissions.
[0049] Figure 5 illustrates a speed-fuelling map for a Westport GX 15 liter
gaseous-fuelled direct injection engine where the full load fuelling curve is
identified by reference numeral 510. This map also shows a zone 520 that
represents the zone on the map where the engine is considered to operate at
idle
and where the presently disclosed method of fuel injection control could be
used.
The predetermined range of engine speed and total fuelling amount of zone 520
where the engine is considered to operate at "idle" can vary from one engine
to
another depending on the engine size and type. Generally most engines are
considered to operate at "idle" when the engine speed is around 700 rpm and
the
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values of total fuelling are in a range at the lowest end of the scale on the
map.
When the engine is idling the load on the engine is mainly caused by friction
and
parasitic loads. Examples of parasitic loads include, belt driven auxiliary
equipment such as fuel pumps, pumps for engine cooling systems and hydraulic
systems, alternators for producing electrical energy, air conditioning, and
refrigeration units.
[0050] Another zone on the map is zone 540 where the engine is operating at
load. For a truck engine, zone 540 represents the load when the engine is
working to propel the truck. In zone 540 on the map fuel injection into the
combustion chamber can be controlled according to conventional methods
known in the prior art and illustrated in the examples of mid-range speed (2)
and
higher speed (3) in Figure 2. This is because at higher loads, when more fuel
is
being combusted, with the fuelling methods shown in embodiments (2) and (3) in
Figure 2, the temperature of the exhaust gas exiting the combustion chamber is
above 200 degrees Celsius without needing to delay combustion of the gaseous
fuel to elevate exhaust gas temperatures.
[0051 ] For an engine used to power a vehicle, a speed-fuelling map as the one
illustrated in Figure 5 can be stored in the vehicle controller's memory. The
vehicle controller monitors the engine speed and other parameters indicative
of
the engine condition, as for example the total fuelling amount, and
communicates
the values of these parameters to controller 116 which controls the fuel
injection
into the combustion chamber. When the detected engine speed and total fuelling
amount is within zone 520 on the map, controller 116 controls the fuel
injection
according to the present method. When transitioning from engine idle zone 520
to load zone 540, more specifically when the engine speed and the total
fuelling
amount are within the boundaries of zone 530, the engine operates in a
transition
mode described below.
[0052] If at idle fuel injection is controlled according to embodiment A shown
in
Figure 3, during the transition mode the method comprises gradually advancing
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the timing for beginning injection of first quantity of fuel 310 and the
timing for
beginning injection of gaseous fuel 312 over a predetermined time until each
begins when the crankshaft reaches a crank angle before top dead centre which
is determined based on the engine speed according to a conventional method
characteristic to the engine operation at load. The method further comprises
over the same period of time, controlling the quantity of fuel injected into
said
combustion chamber during injection of first quantity of fuel 310 and during
injection of gaseous fuel 312 until the quantity of fuel injected in each
injection is
commanded based on the engine load according to a conventional method
characteristic to the engine operation at load.
[0053] If at idle fuel injection is controlled according to embodiment B shown
in
Figure 3 and controller 116 determines that the engine has transitioned from
idling to load, more specifically when the engine starts to operate in zone
530 on
the speed-fuelling map, the amount of fuel injected in second pulse 316 and
the
separation time between the end of first pulse 314 and fuel injection 318 are
gradually decreased over a predetermined time until the amount of fuel
injected
in the second pulse 316 is close to zero, and the separation time between
first
fuel pulse 314 and fuel injection 318 reaches a value that corresponds to the
separation time between the first fuel injection and the second fuel injection
determined according to a conventional method of controlling the fuel
injection at
load. Over the same period of time controller 116 gradually advances the
timing
of first pulse 314, second pulse 316 and of fuel injection 318 to ensure a
smooth
transition to zone 540.
[0054] Similarly, if at idle fuel injection is controlled according to
embodiment C or
D shown in Figure 3 and controller 116 determines that the engine has
transitioned from idling to load, more specifically when the engine starts to
operate in zone 530 on the speed-fuelling map, the timing for beginning
injection
of each fuel pulse injected into the combustion chamber, for example, pulses
320, 322 and 324 and, respectively 330, 332, 334 and 336 and the timing for
beginning injection of gaseous fuel 326, and respectively 338, are gradually
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advanced over a predetermined time to an earlier timing in the engine cycle
which corresponds to the fuel injection timing determined according to a
conventional method of controlling fuel injection at load. Over the same
period of
time controller 116 controls the fuel injector to gradually decrease the
number of
pulses and the amount of fuel injected in fuel pulses 322 and 324 and
respectively in fuel pulses 332, 334 and 336 while decreasing the separation
time
between the end of first fuel pulse 320 and the start of fuel injection 326
and
respectively between the end of first fuel pulse 330 and gaseous fuel
injection
338 until a single first pulse is injected into the combustion chamber before
and
near the beginning of the gaseous fuel injection and the quantities of fuel
injected
in the first injection and in the gaseous fuel injection have the values that
correspond to the quantities of fuel determined according to a conventional
method of controlling fuel injection at load.
[0055] As described above it is in this transition zone 530 illustrated on the
speed-fuelling map that the injection control strategy changes from the
present
method to an injection control strategy similar to those presented in Figure
2,
examples (2) and (3).
[0056] The present invention has been described with regard to several
illustrative embodiments. However, it will be apparent to persons skilled in
the
art that a number of variations and modifications can be made without
departing
from the scope of the invention as defined in the claims.
