Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ WO 94/17293 PCT/AU94/00028
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METHOD OF OPERATING AN INTERNAL COMBUSTION ENGINE
This invention relates to a method of operating an internal
combustion engine in order to produce high exhaust gas temperatures and is
particularly useful for internal combustion engines incorporating a catalytic
treatment means in the exhaust system for treatment of the exhaust gases to
reduce undesirable contaminants therein.
Although catalytic treatment of gases to reduce the level of undesirable
emissions therein is effective, the catalytic material or catalyst of the
catalytic
treatment means has a minimum operating temperature (generally referred to as
the "light off" temperature and conventionally taken as the temperature at
which
the catalyst is 50 % efficient), and is relatively ineffective until this
required
operating temperature has been reached. Thus, during such periods,
increased levels of undesirable emissions are likely to issue from the exhaust
system. Normally, at engine startup, particularly after a period of non-
operation,
the catalytic material is below its light-off temperature and in order to
reduce the
time, and therefore the amount of emissions output until light-off of the
catalyst, it
may be desirable to raise the temperature of the exhaust gases delivered from
the combustion chamber(s) of the engine to the exhaust system. However, at
startup the engine typically will operate at a relatively low load and speed,
such
as is termed "engine idle", and therefore the amount of fuel being delivered
to
the engine is comparatively small and hence, only a relatively small amount of
heat is available for raising the temperature of the exhaust gases and hence
the
temperature of the catalytic material to its "light-off" temperature.
Further, after catalyst light-off, when the engine is allowed to idle or
operate at a low load condition for a significant period of time, particularly
in low
ambient temperature conditions, the exhaust gas temperature may drop to a
value that is insufficient to maintain the catalytic material in the light-off
condition
and thus it will become ineffective in the treatment of contaminants and
undesirable emissions in the exhaust gas.
There have been proposals to heat the catalytic material by means of an
afterburner device placed upstream of the catalytic treatment means. In such
an
arrangement, the afterburner device ignites the remaining combustible mixture
CA 02151595 2005-01-06
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within the exhaust gases to raise the temperatt,re of the catalytic material.
This
arrangement does however add significantly to the cost and complexity of the
engine installation.
It is therefore the object of the present invention to provide a method of
operating an internal combustion engine which will assist in maintaining high
exhaust gas temperatures and thus, where appropriate, achieve rapid light-off
of
the catalytic material in the exhaust system and maintain such a light-off
condition whilst the engine is operating.
With this object in view, there is provided a method of operating an internal
combustion engine comprising retarding the ignition of a gas/fuel mixture
within at
least one cylinder of the engine to after top dead centre (ATDC) in respect of
the
combustion cycle of said at least one cylinder of the engine and, while said
ignition
is so retarded, increasing the fuelling rate of said at least one cylinder to
a level
higher than that required when the engine is operating normally to thereby
assist in
increasing the exhaust gas temperature of the engine, the timing of the
introduction
of fuel into the at least one cylinder being maintained at before top dead
centre
(BTDC).
Conveniently, ignition can be retarded uip to about -30 BTDC (i.e 301
ATDC) and is preferably of the order of -20 BTDC (i.e 20 ATDC). The ignition
retardation may alternatively be variable, preferably between 15 ATDC to 300
ATDC in the case of a multi cylinder engine such as a three cylinder engine.
Preferably, the fuelling rate (measured in mg/cylinder/cycle) is greater than
50%
of the fuelling rate at maximum ioad, and more preferably is up to about 80%
of
the fuelling rate at maximum load. However, if desired, the fuelling rate can
be
in excess of 100% of the fuelling rate at maximum engine load. However, the
selected fuelling rate is conveniently the minimum rate which will ensure that
the
desired exhaust gas temperature is achieved. The fuel may be introduced to the
combustion chamber before top dead centre (BTDC) and most preferably at 60
to 80 BTDC in the case of a direct injected engine. It is however also
envisaged that the fuel be introduced to the cylinder after top dead centre
(ATDC) under certain conditions or situations.
This method may conveniently be used in an engine including a catalytic
treatment means provided in the exhaust system of the engine. A flame arrester
may be placed between an exhaust port of the engine and the catalytic
CA 02151595 2004-01-15
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treatment means. This prevents the catalytic material held within the
catalytic
treatment means from directly contacting any flame that may arise as a
resulting
of any still burning exhaust gases. Additional air may be introduced upstream
of
the catalytic treatment means. This additional air helps to promote the
catalytic
oxidation of the exhaust gases.
The method can be operated during cold start of the engine. Alternatively
or in addition, the method is operated when the temperature of the catalytic
material is sensed or determined to be below a required operating temperature.
The engine may conveniently be a two stroke intemal combustion engine.
The engine may preferably have piston operated exhaust ports.
In the case of an air control system as disclosed in Australian Patent No.
647381, the by-pass air control valve under the control of the engine
management
system may be fully opened whilst the main manually controlled throttle valve
remains in the closed position.
controlled throttle valve remains in the closed position.
The invention will be more readily understood from the following
description of an exemplary embodiment of the method of operating an internal
combustion engine according to the present invention as shown in the
accompanying drawings.
In the drawings:
Figure 1 is a graph showing the cylinder pressure-crankangle
characteristics for a typical direct injected two-stroke internal combustion
engine;
and
Figure 2 is a graph showing the cylinder pressure-crankangle
characteristics for a direct injected two-stroke internal combustion engine
operated according to the method of the present invention.
The method according to the invention can be used on a two stroke
internal combustion engine having piston controlled exhaust ports and the
invention will be described in relation to this exemplary application.
Referring initially to Figure 1, in a typical direct injected two-stroke
internal
combustion engine, the fuel is introduced to the cylinder at approximately 60
before top dead centre (BTDC) with ignition within the cylinder occurring
prior to
WO 94/17293 PCT/AU94/00028 -
21515 9 5 4
top dead centre at approximately 350 BTDC. The solid curve of the graph of
Figure 1 shows the cylinder pressure crankangle characteristics where ignition
has occurred. The dashed curve shows the situation where ignition does not
occur.
In the method according to the invention and as shown in an exemplary
embodiment in Figure 2, while the fuel is introduced to the cylinder at
between
60 and 80 BTDC, the ignition within the cylinder is retarded and occurs at
up to
about -30 BTDC, ie. 30 after top dead centre (ATDC). The curve of the graph
of Figure 2 shows the cylinder pressure crankangle characteristics where the
fuel is introduced to the cylinder at 60 BTDC and ignition thereof occurs at -
20
BTDC.
The fuelling rate can also be varied such that it is greater than 50% of the
fuelling rate at maximum load, and preferably up to about 80% of the fuelling
rate at maximum load.
The method according to the present invention can be varied depending
on the number of cylinders of the engine. For example, in a three cylinder
engine where only one or two cylinders are to be operated in accordance with
said method, it is preferred that the fuelling rate thereto be kept constant
and that
the ignition be retarded a fixed amount, typically about 20 ATDC, for the
cylinder(s) operating under ignition retard/high fuelling rate conditions.
Under
such operation, the other cylinder(s) still operates under normal conditions
and
the operation thereof may be such as to compensate for the temporary loss in
torque while the other cylinder(s) operate(s) under said conditions. The
cylinder(s) operating under normal conditions may also regulate the engine
idle
speed.
By comparison, in a three cylinder engine where all cylinders are
operating in accordance with said method it is preferred that a high fuelling
rate
be fixed for all of the cylinders and that all of the cylinders operate with
retarded
ignition. The degree of retardation for each of the cylinders may conveniently
be
the same during at least one combustion cycle of one light-off period.
Further, it is preferable that the degree of ignition retard of the cylinders
during the one light-off period be altered from cycle to cycle, typically
between
' WO 94/17293 PCT/AU94/00028
ATDC to 301 ATDC, to control the engine idle speed. However, it is also
envisaged that the degree of ignition retard could alternatively or in
addition be
controlled to differ between respective cylinders during the one light-off
period.
By virtue primarily of the retarded ignition and also to a lesser extent the
5 high fuelling rate, the overall thermal efficiency (i.e the efficiency of
conversion of
energy provided by combustion of the fuel into useful work) is quite low. Thus
there is a high level of thermal energy available to heat the catalytic
treatment
means provided for treatment of the exhaust gases. In addition, extra heat is
released to the engine coolant thereby rapidly increasing the coolant
10 temperature which results in a lower engine output of HC emissions thereby
reducing the dependence on the catalytic treatment means to maintain the
required HC emissions level.
The combustion preferably occurs under rich conditions with the overall
air/fuel ratio being close to the stoichiometric ratio. Because of the
inefficient
15 combustion conditions, gases with lower oxidation temperatures such as H
and
CO will be produced. These gases can react with the catalytic material to
increase its temperature and therefore aid the catalytic material in achieving
its
light-off temperature.
If desired, additional oxygen containing gas, such as air, may be
introduced upstream of the catalytic treatment means provided in the exhaust
system of the engine, for example, by use of an air pump, thus ensuring the
introduction of excess oxygen to the exhaust system enabling catalytic
conversion of any contaminants in the exhaust gas. In many cases, it will be
desirable that the throttle or air control means for the air supply to the
combustion chamber with retarded ignition be set at a "wide open" or near
"wide
open" value such as to maximise air supply to that combustion chamber, thus
allowing higher fuelling rates to be used. However, in the case that the air
control system serves more than one cylinder, then the air flow rate must be
established such that the combustion efficiency of the combustion chamber(s)
without retarded ignition is not adversely affected.
It has been found that maintaining the retarded ignition and high fuelling
rate conditions for a "light-off" period only of the order of 30 seconds from
engine
WO 94/17293 PCT/AU94/00028 -
2151595 6
startup is sufficient to bring the catalytic treatment means up to temperature
to
establish light-off of the catalytic material in the treatment means. In some
cases, the level of thermal energy available is even greater thus shortening
the
above period to 5 seconds or so. Furthermore, and particularly in the case of
two stroke engines, there may be insufficient time between the commencement
of ignition and the opening of the exhaust port for all of the fuel to be
combusted
within the combustion chamber. Thus, combustion may continue as the
combustion gases flow from the combustion chamber into the exhaust system.
In such a case, it may be beneficial to place a flameshield upstream of the
catalytic treatment means to protect it from contact with any flames. Where
necessary, and as alluded to hereinbefore, excess air may be introduced to the
exhaust system to promote the catalytic oxidation of the exhaust gases.
Further, it is to be understood that the high degree of retarding of the
ignition resulting in the relatively short period between ignition and exhaust
and
the high fuelling rate may only produce a relatively low torque output.
Accordingly, at engine idle, the high fuelling rate does not result in the
engine
revving at a speed significantly different to the normal engine idle speed. In
this
regard, it may be preferable that in a multi-cylinder engine, only one or some
of
the cylinders are subjected to the highly retarded ignition and high fuelling
rate
thereby enabling the remaining cylinder(s) to provide the necessary control of
engine idle speed as aluded to hereinbefore.
It may be advantageous to "rotate" the cylinders such that each cylinder
sequentially operates under the retarded ignition and high fuelling rate
conditions. This rotation between cylinders may occur within a single light-
off
period. Alternatively, a different cylinder could be used for consecutive
light-off
periods. This helps to ensure that each cylinder is subjected to similar
temperature and/or carbon formation conditions.
The reduction in the time for the catalyst to reach its light-off temperature
achieved by the use of the present invention also enables the catalytic
treatment
means to be located a greater distance downstream from the engine exhaust
port than may otherwise be possible, thereby improving the durability of the
catalytic treatment means.
WO 94/17293 21 ~ 15 9 5 PCT/AU94/00028
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It will be appreciated that where an engine start-up occurs after only a
short period of time after shut-down of the engine, the catalytic treatment
means
may still be at a sufficiently high temperature to immediately light-off on
restarting the engine and hence it may be undesirable to further heat the
catalytic treatment means by way of the present invention. However, this
condition can be determined by appropriate sensing of other engine parameters
such as the temperature of the engine in general, cooling water temperature or
the temperature of the exhaust system in the vicinity of the catalytic
treatment
means. Accordingly, sensing of these and/or other engine parameters may be
effected and the specific ignition retarding and high fuelling rate conditions
only
implemented if for example, the sensed temperature condition of the engine
and/or exhaust system indicates that the temperature of the catalytic
treatment
means is at a level which would necessitate assistance in achieving prompt
light-off thereof.
Further, where the engine is left idling for a considerable time, and
particularly in low ambient temperature conditions, the exhaust system and
particularly the catalytic treatment means may fall in temperature to a level
at
which the catalytic treatment means is below the light-off temperature.
Similarly,
appropriate sensors can be provided to detect this condition and the engine
management system can be arranged to respond to the sensing of such
conditions to implement the ignition retard and high fuelling rate conditions
to
restore or maintain the catalytic treatment means in an acceptable operational
condition.
When the appropriate sensor or sensors detect that the engine
parameter, for example the exhaust system temperature, is again above the
acceptable value, the engine management system may then cease to effect the
ignition retard and high fuelling rate conditions and return the cylinder to
normal
ignition timing and fuelling rates. Where more than one cylinder has been
operating with retarded ignition and a high fuelling rate, the return to
normal
operation is preferably sequential, that is, one cylinder at a time is
returned to
normal operation and stabilised. This is particularly useful in the case where
switching from a high to a lower fuelling rate causes fuel "hangup", that is,
WO 94/17293 PCT/AU94/00028
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21515~5
retention of fuel within the means used to deliver fuel to the cylinder, in
which
case the transient response of the engine may not be precise. However, if
multiple cylinders are operating with retarded ignition and a high fuelling
rate,
they may be returned to normal operation simultaneously if there is a
substantial
increase in operator demand. The method of operating the engine according to
the present invention can therefore be initiated, both during cold start of
the
engine, and when the temperature of catalytic material is sensed or determined
to be below the required light-off operating temperature any time during the
running of the engine.
While the method of the present invention is particularly suitable for two-
stroke engines, preferably those engines which are directly injected, the
invention is not limited in its applicability to such engines. In a two-stroke
three
cylinder 1.2 litre direct injected engine, the anticipated fuel per cycle at
normal
engine idle is 3 mg/cylinder/cycle whereas when retarded ignition and a high
fuelling rate is enabled in accordance with the method of the present
invention,
the increased fuelling rate may be as high as 18 to 25 mg/cylinder/cycle, i.e
85
% to 115 % of the fuelling rate at maximum engine load.
In an air-assisted direct injected 2-stroke engine as described, for
example, in the applicant's US Patent No. 4693224, it may be convenient to
control the speed of the engine by controlling the fuelling rate to the
cylinders
that are running under normal settings and by controlling the ignition timing
of
those cylinders that are operating with retarded ignition and increased
fuelling
rate according to the invention as described herein.
It will be appreciated that the invention is particularly beneficial for
bringing the engine catalytic treatment means rapidly to its light-off
temperature,
and few if any additional components are required. This results in little to
no
additional costs to the piece price of the engine.
