Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02549215 2006-06-01
TITLE
IMPROVED CARBURETED NATURAL GAS TURBO CHARGED ENGINE
INTRODUCTION
[0001] This invention relates to a control system for a
carbureted natural gas engine and, more particularly, to an
improved control system for air/fuel ratio and for the
governor of a natural gas carbureted turbo charged engine.
BACKGROUND OF THE INVENTION
[0002] Natural gas powered engines are used pervasively
for various applications and are particularly used in
association with gas compression and electric power
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generation. Many of these engines are generally smaller type
engines typically in the range of 200HP to 800HP. These
existing engines have certain disadvantages including
relatively high exhaust emissions which typically contain
nitrogen oxides also known as NOx. In many jurisdictions,
regulations place an upper limit on the nitrogen oxide
emissions; hence the engines require technology to limit and
control these emissions. These engines often are turbo
charged and waste gates used with the turbo charger on such
engines are typically controlled only by the turbo charger
compressor pressure and therefore serve only to limit the
maximum turbo charger pressure output. The costs of
operation of these engines together with the cost of
existing control systems are relatively high. It would be
advantageous to provide a control system for less cost and
which control system would increase engine efficiency,
reduce nitrogen oxides emissions and reduce engine exhaust
temperatures by providing increased or otherwise
appropriately controlled air with the fuel for increased
efficiency in combustion.
[0003] The replacement of the existing control systems on
natural gas engines is difficult. It would further be
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advantageous to install an improved control system on existing
engines relatively inexpensively in addition to supplying such
a control system on OEM engines.
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SUMMARY OF THE INVENTION
According to one aspect of the invention there is
provided a method of controlling a natural gas engine, said
method comprising the steps of opening and closing an air/fuel
throttle valve associated with a carburetor, opening and
closing a fuel valve upstream of said carburetor to vary the
supply of fuel from a fuel source to said carburetor, said
fuel valve having a non-linear fuel flow response as said fuel
valve is opened and closed, compensating for said non-linear
fluid flow response passing through said fuel valve as said
fuel valve is opened and closed and providing a PID controller
having an output modified by said compensation for said non-
linear fluid flow response, said non-linear flow response
compensation being a non-linear curve which is inverse to said
flow characteristics of said fuel valve.
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According to a further aspect of the invention there is
provided a control system for a natural gas engine, said
control system comprising an air/fuel throttle valve operable
to be associated with a carburetor, a fuel valve having non-
linear fuel flow response as said fuel valve is opened and
closed, said fuel valve being located upstream of said
carburetor and associated with the supply of fuel to said
carburetor from a fuel source, and a flow compensator to
compensate for said non-linear flow of fuel passing through
said fuel valve as said fuel valve is opened and closed, a PID
controller associated with said flow compensator and having an
output which is modified by said flow compensator, said flow
compensator compensating for said non-linear flow of fuel by
generating a curve which is inverse to said flow of fuel
passing through said fuel valve.
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BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012] Specific embodiments of the invention will now be
described, by way of example only, with the use of drawings in
which:
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[0013] Figure 1 is a diagrammatic schematic illustrating a
control system for a natural gas engine which utilises air
compressed by the turbo charger for waste gate control
according to the PRIOR ART;
[0014] Figure 2A is a diagrammatic view illustrating a
control system incorporated into a natural gas engine and
which particularly illustrates a butterfly type fuel valve
according to a first embodiment of the invention;
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[0015] Figure 2B is a diagrammatic flow chart
illustrating the proportional integral derivative (PID)
controller, a compensator and an actuator for the fuel valve
used in the control system of Figure 1A;
[0016] Figure 3A is a diagrammatic schematic illustrating
a control system installed on a natural gas engine according
to a further aspect of the invention and particularly
illustrating an oxygen sensor used for compensating and
controlling the fuel valve;
[0017] Figure 3B is a diagrammatic flow chart
illustrating a proportional integral derivative (PID)
controller with a compensating algorithm which generates the
actuator of the air/fuel throttle valve used with the
control system of Figure 2A;
[0018] Figure 3C is a diagrammatic flow chart
illustrating a waste gate PID used with the throttle
position sensor used for actuator control;
[0019] Figure 4 is a diagrammatic schematic illustrating
a waste gate control and bypass arrangement used when
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instrument gas may be present in the vicinity of the hot
exhaust gases emanating from the turbo charger according to
a further aspect of the invention;
[0020] Figure 5A is a diagrammatic schematic of an
alternative pressure control for the waste gate illustrating
control pressure applied above the diaphragm of the waste
gate; and
[0021] Figure 5B illustrates the non-linear exhaust
oxygen response for a butterfly type valve used for gas flow
control.
DESCRIPTION OF SPECIFIC EMBODIMENT
[0022] Referring now to the drawings, a natural gas
turbo charged carbureted engine is illustrated generally at
100 in Figure 1, with its turbo charger being generally
illustrated at 101 and its carburetor being generally shown
at 102. The natural gas fuel source 103 provides the natural
gas used as fuel which enters the engine 100 through
carburetor 102.
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[0023] The turbo charger 101 is powered by the hot
exhaust gases leaving the engine 100 through exhaust
manifold 104. The turbo charger 101 uses atmospheric air 110
which enters the turbo charger 101 through duct 111. The
air is compressed by the turbo charger 101 and leaves the
turbo charger 101 through duct 112 which duct 112 provides
the compressed air to the carburetor 102.
[0024] The hot gases passing to the turbo charger 101
from exhaust manifold 104 leave the turbo charger 101 and
are exhausted to the atmosphere through exhaust duct 113. A
waste gate generally illustrated at 114 may be used to
reduce the volume of hot engine gases entering the turbo
charger 101 by bypassing a portion of the hot engine gases
from passing through turbo charger 101.
[0025] An air/fuel throttle valve 120 is associated with
the carburetor 102. Throttle valve 120 is generally in the
form of a butterfly valve and is conveniently operated by a
governor 121 which runs off the engine rpm. If the engines
rpm falls, the governor 121 instructs the throttle valve 120
to open to a greater position thereby admitting more air-
fuel mixture to engine 100 and if the engine rpm increases,
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the governor 121 instructs the throttle valve 120 to move to
a more closed position thereby restricting air-fuel flow to
the engine 100. This is known technology and part of the
prior art as indicated in Figure 1.
[0026] Referring to Figure 2A, a fuel flow valve 124 is
used to admit the natural gas fuel to the carburetor 102. An
oxygen sensor (UEGO) 122 is mounted so as to measure the
oxygen in the exhaust duct 113 and a controller 123 measures
the output of the oxygen sensor 122 and a user set point
which is close to or at the optimum oxygen/fuel ratio. The
controller 123 will accordingly provide a change in position
for the fuel flow valve 124 to either admit more or less
fuel to the carburetor 102 based on the optimum value of
oxygen in the exhaust as measured by the oxygen sensor 122.
[0027] The fuel flow valve 124 conveniently takes the
form of a butterfly type valve and is controlled by actuator
130. Butterfly valves are useful since they are simple in
operation and inexpensive. Reference is made to Figure 5B
where PID output is a function of the percentage of oxygen
in the exhaust. However, there are non-linearities
associated with butterfly type valves as it is apparent that
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such valves have non-linear flow changes as the valve opens
and closes. This non-linear valve characteristic output
makes it difficult to adjust the controller to give a fast
response for all outputs. To enhance the operation of the
valve 124, a proportional integral derivative (PID)
controller 131 (Figure 2B) is located upstream from the
actuator 130 and a flow compensation algorithm is
incorporated in a compensator 132 located between the
controller 131 and the actuator 130.
[0028] For the same reasons described above in
association with fuel flow valve 124, the air/fuel throttle
valve 120 (Figure 2A) suffers from non-linear fuel flow when
the throttle valve 120 is opened and closed. When the
governor 121 (Figure 2A) is of the mechanical type, a
throttle position sensor 133 is operably connected to the
throttle valve 120 in order that the angle position of the
throttle valve 120 can be measured and recorded. A second
PID controller 134 obtains this position of the throttle
valve 120 from throttle position sensor 133 and determines
the difference between such position and an optimum and
predetermined set point as entered into the controller 134.
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[0029] The output of the controller 134 will then reflect
this difference. The controller 134 is operably connected to
the waste gate 101 by way of a pressure transducer 140. The
controller 134 and pressure transducer 140 will thereby open
and close the waste gate valve 101 thereby increasing or
decreasing the pressure of the compressed air leaving turbo
charger 101 and entering carburetor 102 through duct 112.
Thus, the air/fuel throttle valve 120 is under the direction
of the controller 134 which allows the governor 121 to
operate in a more limited range for which the governor 121
may be more precisely turned.
[0030] A further aspect of the invention relates to
engines which utilise a governor 142 which is electronically
controlled as opposed to being mechanically controlled and
reference is made to Figure 3B where the waste gate control
system described in association with Figure 2A and which is
associated with
throttle position sensor 133 is not required. A PID
controller 160 operably connected with the engine speed
sensor 161 will change the output of the controller 160. A
flow compensation algorithm is incorporated in a compensator
143 and will correct for the inherent non-linear flow
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characteristics of the throttle valve 120 which will allow
optimum tuning of the controller 160 and
therefore the throttle valve 120. The compensator 143
effectively inverses the non-linear action of the air/fuel
throttle valve 120 so that a substantially linear response
is similar to the oxygen-PID output curve illustrated in
Figure 5B.
[0031] A further aspect of the invention relates to the
use of instrument gas being used for pneumatic devices
instead of instrument air. Instrument gas is commonly
pressurized natural gas and it is desirable to isolate this
gas from the hot exhaust gases emanating from the turbo
charger 101. Reference is made to Figure 4 where a pneumatic
relay 143 is used. The relay 143 is controlled by the
instrument gas acting on the pressure transducer 144. If it
desired to open the waste gate 114, relay 143 will be
activated by pressure transducer 144 which will allow the
compressed air in duct 112 to travel through line 150 to
relay 143 and thence to the waste gate 114
where it will open the waste gate 114 and allow a portion of
the exhaust in the exhaust manifold 104 to escape directly
to the exhaust stack 113 without driving the turbo charger
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101 so that the speed of the turbo charger 101 will be
reduced which will, in turn, reduce the pressure of the
compressed air in duct 112.
[0032] In the event of failure of the control system,
apparatus may conveniently be used to return the waste gate
control to that of the original system.
[0033] Reference is again made to Figure 4 where default
equipment is added to the circuit by way of a solenoid valve
151 which opens in the event of a control system failure. A
pressure regulator 152 may also be provided to reduce the
pressure in line 150 if required.
Opening the solenoid valve 151 will allow the compressed air
in duct 112 and line 150 to be applied to the waste gate 114
directly.
[0034] Many modifications will readily occur to those
skilled in the art to which the invention relates and the
specific embodiments herein described should be taken as
illustrative of the invention only and not as limiting its
scope as defined in accordance with the accompanying claims.
