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Claims  |
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I claim:
1. An exhaust gas recirculation system for a compression ignition engine,
comprising:
a first pressure sensor for sensing an absolute gas pressure in an intake
manifold of the engine;
a second pressure sensor for sensing an absolute gas pressure in an exhaust
manifold of the engine;
an engine speed sensor for detecting a rotational speed of the engine crank
shaft;
a fuelling rate sensor for detecting a fuelling rate for the engine;
an air charge temperature sensor for detecting a temperature of intake air
in the intake manifold of the engine;
a conduit for providing a fluid passage between the exhaust manifold and
the intake manifold, and an exhaust gas recirculation valve disposed in
the conduit to regulate a flow of exhaust gas from the exhaust manifold to
the intake manifold;
means for controlling the exhaust gas recirculation valve so that the flow
of the exhaust gas through the conduit from the exhaust manifold to the
intake manifold is regulated; and
an electronic controller for receiving signals from the first and second
pressure sensors, the engine speed sensor, the fuelling rate sensor and
the air charge temperature sensor, computing an optimal ratio of the
exhaust gas to be recirculated to intake air based on the signals received
and computing a valve position that permits the optimal ratio of exhaust
gas to intake air to flow through the conduit to the intake manifold, and
actuating the means for controlling the exhaust gas recirculation valve to
position the valve at the computed valve position to permit the optimal
ratio of exhaust gas to intake air to flow through the conduit from the
exhaust manifold to the intake manifold.
2. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 1, wherein the engine speed sensor is a Hall Effect
sensor which is attached to an output shaft of a fuel pump of the engine.
3. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 1, wherein the fuelling rate sensor is a high resolution
potentiometer that measures a pedal position of a fuel pedal for the
engine.
4. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 1, wherein the exhaust gas recirculation valve is a
mechanically controlled butterfly valve having a mechanical linkage for
moving the butterfly valve from a fully closed to a fully open position.
5. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 4, wherein the means for controlling the exhaust gas
recirculation valve is an electric stepper motor operably connected to the
mechanical linkage.
6. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 5, wherein the butterfly valve is associated with a high
resolution potentiometer for indicating a current position of the
butterfly valve with respect to the fully closed position.
7. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 6, wherein the electronic controller accumulates a count
based on an output signal of the high resolution potentiometer and the
count is used to determine the current position of the butterfly valve
with respect to the fully closed position.
8. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 1, wherein the compression ignition engine is a diesel
engine.
9. An exhaust gas recirculation system for a compression ignition engine as
claimed in claim 1, wherein the compression ignition engine is a dual fuel
engine.
10. An exhaust gas recirculation system for a compression ignition engine
as claimed in claim 9, wherein the compression ignition engine is a dual
fuel engine equipped to operate with diesel and natural gas as fuels.
11. An exhaust gas recirculation system for a compression ignition engine
as claimed in claim 1, wherein the compression ignition engine is a
multifuel engine.
12. An exhaust gas recirculation system for a compression ignition engine
as claimed in claim 10, wherein the compression ignition engine is a
multifuel engine equipped to operate with diesel, natural gas and hydrogen
as fuels.
13. A method of controlling exhaust gas recirculation in a compression
ignition engine, comprising:
a) sensing a fuelling rate for the engine;
b) sensing a rotational speed of the engine;
c) sensing an exhaust manifold absolute pressure and an intake manifold
absolute pressure of the engine and computing a pressure drop between the
exhaust manifold and the intake manifold;
d) determining a volumetric efficiency of gas flow through the engine as a
function of the rotational speed and the pressure drop;
e) determining a % EGR as a function of the rotational speed and the
fuelling rate for the engine;
f) determining a temperature of gases in the exhaust manifold as a function
of the rotational speed and the fuelling rate for the engine;
g) sensing an intake air temperature of air drawn into the intake manifold;
h) computing a fluid density of the exhaust gas based on an absolute
pressure in the exhaust manifold, a molar mass of the exhaust gas and the
temperature of the exhaust gas;
h) computing a volumetric flow of exhaust gas through the EGR valve;
i) deriving a variable based on the volumetric flow, the fluid density of
the exhaust gas and the pressure drop to locate a required EGR valve
position; and
j) moving the EGR valve to the required position.
14. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the volumetric efficiency
of gas flow through the engine is determined by a table look-up of data
empirically derived from dynamometer testing of the compression ignition
engine.
15. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the % EGR is determined by
a table look-up of data empirically derived from operational testing of
the compression ignition engine.
16. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the temperature of gas in
the exhaust manifold is determined by a table look-up of data empirically
derived from operational testing of the compression ignition engine.
17. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the required EGR valve
position is derived from a table look-up of valve positions as a function
of the variable.
18. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the method further
includes the steps of:
a) determining a current position of the EGR valve:
b) comparing the current position of the EGR valve with the required
position of the EGR valve;
c) deriving a difference between the current position and the required
position; and
d) actuating a means for controlling a position of the EGR valve to move
the EGR valve an equivalent of the derived difference between the current
and the required position.
19. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the compression ignition
engine is a diesel engine.
20. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the compression ignition
engine is a dual fuel engine.
21. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 20, wherein the compression ignition
engine is a dual fuel engine adapted to use diesel and natural gas as
fuels.
22. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 13, wherein the compression ignition
engine is a multifuel engine.
23. A method of controlling exhaust gas recirculation in a compression
ignition engine as claimed in claim 22, wherein the compression ignition
engine is a multifuel engine adapted to use diesel, natural gas and
hydrogen as fuels. |
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Claims |
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Description |
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TECHNICAL FIELD
The present invention relates to exhaust gas recirculation in internal
combustion engines as a pollution control strategy and, in particular, to
an exhaust gas recirculation system for compression ignition engines.
BACKGROUND OF THE INVENTION
In the late 1950's, it was determined that internal combustion engine
exhaust emissions were a significant contributor to the photochemical
smoke and smog that enshrouded industrialized cities around the world. The
damage to human health, animal life and the environment in general due to
the effects of exhaust emissions have been widely studied and thoroughly
documented. In order to reduce smog levels, governments have enacted
legislation to control pollution emissions at their sources, including
exhaust emissions from internal combustion engines.
Starting in the early 1960's, manufacturers of spark ignition (otto cycle)
gasoline fuelled engines began installing the first rudimentary pollution
abatement devices, some of which are still in use today. Over the years as
automobile use has proliferated, government legislation has become more
stringent with regard to otto cycle engine exhaust emission levels and the
vehicle manufacturers have responded with more complicated equipment on
their engines to comply with more stringent regulations. While the exhaust
emissions of otto cycle engines have been strictly regulated, governments
have largely ignored compression ignition engines (diesel engines) because
those engines are more fuel efficient and have relatively lower emission
levels than otto cycle engines without pollution control equipment.
Another contributing factor was probably the much smaller number of diesel
engines compared to the number of otto cycle engines in concurrent use.
As more was learned through research about the effects of exhaust emission
caused pollution, governments legislated stricter controls of a broader
range of such emissions. In the early 1980's, even diesel powered vehicle
emissions began to be scrutinized and diesel engine manufacturers were
forced to follow the suit of otto cycle powered vehicle manufacturers who
applied a variety of increasingly complex strategies and apparatus in
multi-pronged responses to government legislation regulating acceptable
exhaust emission levels.
In the mid 1970's, otto cycle engine manufacturers developed a strategy for
recycling a portion of the exhaust gases back into the intake manifold to
subject that portion of recirculated exhaust to combustion conditions in
order to control nitrogen oxides (NO.sub.x), carbon monoxide (CO), and
total hydrocarbon emissions (THC).
Exhaust gas recirculation (EGR), by the introduction of gases from the
exhaust into the combustion cycle, causes lower combustion chamber
temperatures, thus inhibiting the formation of NO.sub.x as well as
promoting the oxidation of some of the previously unburned hydrocarbons.
The control of the recirculation of exhaust gases is performed by EGR
valves which are widely used on otto cycle engines and, to a much less
extent, on diesel engines.
Until the early 1990's, EGR valves were pneumatically actuated and
controlled and thus were not capable of precise monitoring or rapid
response to varying engine speeds and loads. The pneumatic actuation and
control method also induces inaccuracies in valve positioning and delays
in response time due to changing barometric pressures in the surrounding
atmosphere. In the early 1990's, EGR valves controlled by microprocessor
based engine controllers using electric actuator motors were introduced
for otto cycle engines.
A system for controlling the amount of recirculated exhaust gas for a
diesel engine is also known and described by Ikeda in U.S. Pat. No.
4,562,821 which issued Jan. 7, 1986. In this system, an electronic
controller senses engine speed, intake manifold pressure, fuelling rate,
engine coolant temperature and combustion flame brightness in order to
control exhaust gas recirculation in a diesel engine. The system has two
principal shortcomings. First, an expensive combustion flame brightness
detecting system is required by the strategy. The sensors are not readily
retrofit to existing engines because a special adaptation of the engine is
required. Furthermore, the system relies on a vacuum actuated EGR valve
which as noted above is slow to respond to rapidly changing engine
operating conditions.
Another approach to the reduction of diesel engine emissions has been the
development of dual fuel and multifuel systems for replacing a portion of
the diesel fuel normally burned in compression ignition engines with a
lighter, cleaner burning gaseous fuel such as natural gas. Extensive
research has shown that EGR's compatibility with dual/multifuel engines to
be quite different from otto cycle engines. In dual/multifuel engines,
optimal EGR can vary between 0% and more than 50%, and manifold pressure
differentials are quite low. It has therefore become apparent that an otto
cycle engine EGR system and strategy are unsuitable for use in compression
ignition engines in general, and in dual/multifuel compression ignition
engines in particular.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an EGR system for compression
ignition engines which enables accurate, real time control of the amount
of exhaust gas recirculated to the engine.
It is a further object of the invention to provide an EGR system which is
adapted to be retrofit to an existing compression ignition engine.
It is yet a further object of the invention to provide an EGR system which
is adapted to be included as original equipment on a compression ignition
engine without redesign of the engine architecture.
These and other objects are realized in an exhaust gas recirculation system
for a compression ignition engine, comprising:
a first pressure sensor for sensing an absolute gas pressure in an intake
manifold of the engine;
a second pressure sensor for sensing an absolute gas pressure in an exhaust
manifold of the engine;
an engine speed sensor for detecting a rotational speed of the engine;
a fuelling rate sensor for detecting a fuelling rate for the engine;
an air charge temperature sensor for detecting a temperature of intake air
in the intake manifold of the engine;
a conduit for providing a fluid passage between the exhaust manifold and
the intake manifold, and an exhaust gas recirculation valve disposed in
the conduit to regulate a flow of exhaust gas from the exhaust manifold to
the intake manifold;
means for controlling the exhaust gas recirculation valve so that the flow
of the exhaust gas through the conduit from the exhaust manifold to the
intake manifold is regulated; and
an electronic controller for receiving signals from the first and second
pressure sensors, the engine speed sensor, the fuelling rate sensor and
the air charge temperature sensor, computing an optimal ratio of exhaust
gas to intake air based on the signals received, deriving a valve position
that permits the optimal ratio of exhaust gas to intake air to flow
through the conduit to the intake manifold, and actuating the means for
controlling the exhaust gas recirculation valve to position the valve at
the derived valve position to permit the optimal ratio of exhaust gas to
intake air to flow through the conduit from the exhaust manifold to the
intake manifold.
In accordance with a further aspect of the invention, there is provided a
method for controlling exhaust gas recirculation in a compression ignition
engine wherein rather than controlling % EGR directly, the method uses the
mass flow rate of air as a function of engine speed and fuelling rate as
its basic control variable and assumes that the mass flow rate of air
should be roughly constant for a given engine speed/engine load
combination, and applies EGR to maintain that mass flow rate of air at
varying inlet and exhaust temperatures and pressures. In particular, there
is provided a method of controlling the exhaust gas recirculation in a
compression ignition engine comprising:
a) sensing a fuelling rate for the engine;
b) sensing a rotational speed of the engine;
c) sensing an exhaust manifold absolute pressure and an intake manifold
absolute pressure of the engine and computing a pressure drop between the
exhaust manifold and the intake manifold;
d) determining a volumetric efficiency of gas flow through the engine as a
function of the rotational speed and the pressure drop;
e) determining a % EGR as a function of the rotational speed and the
fuelling rate for the engine;
f) determining a temperature of gases in the exhaust manifold as a function
of the rotational speed and the fuelling rate for the engine;
g) sensing an intake air temperature of air drawn into the intake manifold;
h) computing a fluid density of the exhaust gases based on the absolute
pressure in the exhaust manifold, a molecular weight of the exhaust gases
and the temperature of the exhaust gas;
h) computing a volumetric flow of exhaust gas through the EGR valve;
i) deriving a variable based on the volumetric flow of exhaust gas through
the EGR valve, the fluid density of the exhaust gas and the pressure drop
to locate a required EGR valve position; and
j) moving the EGR valve to the required position.
The present invention therefore provides a relatively simple,
electronically controlled recirculation system for compression ignition
engines which uses standard commercially available components to provide
an inexpensive pollution abatement device that may be retrofit to an
existing compression ignition engine or provided as original equipment on
new engines. The system includes an electronic engine controller, an
engine intake manifold absolute pressure sensor, an engine exhaust
manifold absolute pressure sensor, a fuelling rate sensor, an engine
rotational speed sensor, and an air charge temperature sensor. The
recirculation of exhaust gas is controlled by an electronically actuated
EGR valve which is preferably a butterfly valve having a valve position
sensor. The position of the valve is preferably controlled by an
electronic stepper motor to ensure accurate, rapid response to changing
engine loads and operating conditions.
As noted above, the invention also provides a novel method of controlling
exhaust gas recirculation in compression ignition engines whereby an
optimum % EGR is empirically derived under controlled test conditions so
that a balance between maximum thermal efficiency and minimum pollutant
emissions is achieved. The mass flow rate through the engine is then
computed using the definition of % EGR and an air density that compensates
for variations in ambient air temperature and barometric pressure. The
mass flow rate is used to calculate the flow of EGR through the EGR valve
so that the valve position can be adjusted to achieve the optimum % EGR.
The appropriate valve position is derived using a function which yields a
dimensionless number from the volumetric flow of exhaust gas through the
EGR valve, the density of exhaust gas and the pressure drop from the
exhaust manifold to the intake manifold. The dimensionless number is then
used to locate the appropriate valve position in a two-dimensional table.
The method of controlling the exhaust gas recirculation relies on the
assumptions that:
1) both air and exhaust gas behave as ideal gas in the temperature/pressure
ranges encountered in engine operation;
2) exhaust gas temperature for a specified engine speed/load condition
changes negligibly with changing intake air temperature or barometric
pressure;
3) volumetric efficiency is a function only of engine speed and pressure
differential between the intake and exhaust manifolds;
4) the exhaust gas composition is adequately represented by O.sub.2,
N.sub.2, CO.sub.2, and H.sub.2 O for calculating exhaust gas properties,
since the concentrations of all other exhaust gas components are
negligible; and
5) for multifuel engines, the influence of gaseous fuel on the molar mass
of mixed air and exhaust gas in the intake manifold is negligible.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the invention will now be explained by way of
example only and with reference to the following drawings, wherein:
FIG. 1 is a schematic diagram of a compression ignition engine equipped
with an exhaust gas recirculation system in accordance with the invention;
FIG. 2 is a partially cutaway side view of a preferred embodiment of an EGR
valve for use in the exhaust gas recirculation system for a compression
ignition engine shown in FIG. 1;
FIG. 3 is a logic diagram of the method of controlling the exhaust gas
recirculation in a compression ignition engine in accordance with the
invention;
FIG. 4 shows a table for determining volumetric efficiency as a function of
engine speed and pressure drop between the exhaust manifold and the intake
manifold of a compression ignition engine;
FIG. 5 shows a table for determining the % EGR as a function of engine
speed and fuelling rate for a compression ignition engine;
FIG. 6 shows a table for determining exhaust gas temperature as a function
of engine speed and fuelling rate in a compression ignition engine;
FIG. 7 shows a table for determining an EGR valve position as a function of
exhaust gas flow, exhaust gas density and pressure drop between the
exhaust manifold and the intake manifold of a compression ignition engine;
and
FIG. 8 shows a table for determining an EGR valve position as a function of
electronic engine controller clock counts based on the output of a
potentiometer for monitoring EGR valve position.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus and the method in accordance with the invention may be
described in terms of a physical layout and hardware components, control
logic, and calculation procedures. In order to facilitate a complete
understanding of the invention, the detailed description of the preferred
embodiment is organized accordingly.
Physical Layout and Hardware Components
FIG. 1 shows a schematic diagram of a compression ignition engine 20
equipped with an exhaust gas recirculation system in accordance with the
invention. The compression ignition engine 20 may be a diesel engine or a
multifuel engine such as a diesel/natural gas engine, the construction of
which is known in the art and is accordingly not explained in this
document. The compression ignition engine 20 is equipped with an intake
manifold 22 for supplying combustion air to the engine cylinders (not
illustrated) and an exhaust manifold 24 for exhausting combustion gas from
the cylinders of the compression ignition engine 20. The exhaust gas
recirculation system (hereinafter referred to as an EGR system)is
controlled by an electronic controller 26. The electronic controller 26
may be an electronic engine controller for controlling other operations of
the compression ignition engine 20 as well as controlling the EGR system,
or it may be an electronic controller dedicated to the function of
controlling the EGR system in accordance with the invention. The
electronic controller 26 is any one of a number of dedicated processors
commercially available for engine control systems.
A primary function of the electronic engine, controller is to control the
position of an EGR valve 28 which is located within an exhaust
recirculation conduit 30 that interconnects the exhaust manifold 24 and
the intake manifold 22. The EGR valve 28 will be explained in more detail
with relation to FIG. 2. The EGR valve 28 includes a control linkage 36
which is connected to a motor linkage 34 that is pivotally connected to an
electric stepper motor 32 that controls the rotational position of the EGR
valve 28. In order to provide effective and optimal control of the EGR
valve 28, a number of sensors are required for monitoring the operating
conditions of the compression ignition engine 20. Those sensors include an
intake manifold absolute pressure sensor 38 located in the intake manifold
22 and an exhaust manifold absolute pressure sensor 40 located in the
exhaust manifold 24. If the compression ignition engine 20 is a multifuel
engine, the system may include a fuel mode selector switch 42 for
switching the engine from diesel only to a multifuel mode, for example,
such as a diesel/natural gas fuel combination. The system also includes a
fuelling rate sensor 44, typically a high resolution potentiometer which
monitors the position of a fuel pedal, or some equivalent such as a
throttle position sensor. The engine is also equipped with an engine
rotation speed sensor 46 (hereinafter referred to as RPM sensor 46) for
determining the rotational speed of the engine crank shaft. The RPM sensor
46 is preferably a Hall Effect sensor which may be attached to the diesel
fuel injection pump of the compression ignition engine 20, a flywheel, or
an output end of the crank shaft, as desired. The location of the RPM
sensor 46 is not important so long as it provides a reliable indication of
the rotational speed of the engine's crank shaft. The system also includes
an air charge temperature sensor 48 which measures the temperature of
combustion air drawn into the intake manifold 22. The exhaust system of
the compression ignition engine 20 is also typically equipped with a
catalytic converter 52 but the catalytic converter is ancillary to the
exhaust gas recirculation system in accordance with the invention.
FIG. 2 shows a side elevational view of a partial cross-section through a
preferred construction for the EGR valve 28 for use in the exhaust gas
recirculation system in accordance with the invention. The EGR valve 28
includes a central passage 54 having a diameter preferably equal to or
larger than the diameter of the exhaust recirculation conduit 30 (see FIG.
1). The central passage 54 may be closed by a butterfly valve 56 which is
rotationally positioned by a valve shaft 58 that is connected on its lower
end to the EGR valve linkage 34 and on its upper end to a high resolution
potentiometer 60 of a type well known in the automotive industry. The high
resolution potentiometer 60 is used to determine a rotational position of
the butterfly valve 56 in a way that will be explained in detail below.
The EGR valve 28 is preferably an electronically controlled butterfly
valve because that construction provides a precise, rapid control response
to changing combustion conditions in the compression ignition engine 20,
as will also be explained in detail below.
Control Logic
FIG. 3 shows a flow diagram of the general control logic executed by the
electronic engine controller 26 to control the position of the EGR valve
28 and thereby to control the amount of exhaust gas recirculated from the
exhaust manifold 24 to the intake manifold 22. The controller executes a
program loop which begins with determining a commanded fuelling rate from
the fuelling rate sensor 44 and ends with orienting the butterfly valve 56
of the EGR valve 28 to a required orientation for providing an optimum
exhaust gas recirculation to the intake manifold 22. The control logic
will now be explained in detail.
As shown in FIG. 3, the process begins with a step 62 in which a commanded
fuelling rate for the compression ignition engine 20 is determined by
analyzing input signals from the fuelling rate sensor 44.
In step 64, the electronic controller 26 computes the engine rotational
speed by analyzing output signals from an RPM sensor 46, preferably a Hall
Effect sensor (not illustrated) attached to a diesel fuel injection pump
of the engine (also not illustrated) which runs at one-half the engine
speed for a 4 stroke engine. In step 66, the electronic engine controller
26 reads the input signals from the exhaust manifold absolute pressure
sensor 40 and the intake manifold absolute pressure sensor 38 and computes
a pressure drop (.DELTA.P) in accordance with the following formula:
##EQU1##
In step 68, the results of steps 62-66 are used to determine the volumetric
efficiency, % EGR, and exhaust gas temperature from tables of data
empirically derived from engine testing using a compression ignition
engine monitored with a dynamometer in a manner well known in the art.
FIG. 4 shows the structure of a table used to retrieve the volumetric
efficiency of the engine as a function of engine speed and the pressure
drop (.DELTA.P). The table in accordance with the preferred embodiment is
arranged in rows of respectively assigned values for .DELTA.P and columns
of respectively assigned values for RPM. The values respectively assigned
to each row and each column of FIG. 4 may be incremental, but are not
necessarily expressed in even increments. The assigned value will depend
on the particular engine model to be equipped with EGR and may be
clustered around particular engine speeds for finer resolution and better
control in a particular operating speed range in order to meet a
particular emissions standard. It should also be understood that the size
of the table of FIG. 4, as shown, is illustrative only, the actual size of
table required being dictated by the performance characteristics to be
achieved. The data in FIG. 4 is empirically derived based on a
mathematical definition of the volumetric efficiency of the engine, given
by the formula:
##EQU2##
wherein .eta..sub.vol is the volumetric efficiency of the engine,
Q.sub.tot is the total volumetric flow through the intake manifold in
liters per second, RPM is the crank shaft rotational speed of the engine
and V.sub.disp is the engine displacement volume in liters. Because there
is only one intake stroke for each two crank shaft revolutions in a four
cycle compression ignition engine, the number of intake strokes is divided
by 2.
Given this definition and the fact that the mixed temperature of air,
exhaust gas and gaseous fuels in a multifuel engine cannot be measured
accurately, the data used to complete the table shown in FIG. 4 is
preferably obtained from an engine operating in diesel only mode without
EGR. When the engine is operating in diesel only mode on a test stand,
Q.sub.tot =Q.sub.air and Q.sub.air is measured with a gas flow meter. RPM
is measured and the equation is solved for .eta..sub.vol to obtain the
volumetric efficiency for each cell in the FIG. 4.
The % EGR is derived from the table shown in FIG. 5 where % EGR is
expressed as a function of engine rotational speed and fuelling rate. The
data in this table is likewise preferably empirically derived from
dynamometer testing of a test engine, based on the mathematical definition
of % EGR expressed by the formula:
##EQU3##
where Q.sub.air is the volumetric flow of air into the engine at a given
air temperature and intake manifold pressure, Q.sub.gas is the volumetric
flow of gaseous fuel into the engine if the compression ignition engine 20
is a multifuel engine, and Q.sub.tot is the total volumetric flow through
the intake manifold.
With the engine running on a test stand, the EGR valve 28 is positioned so
that a balance between maximum thermal efficiency and minimum pollutant
emissions is achieved. Q.sub.gas can be determined from the commanded fuel
rate, Q.sub.tot is known and can be derived from FIG. 4, and Q.sub.air can
be measured using a gas flow meter. % EGR is then calculated to complete
the data in FIG. 5.
Exhaust gas temperature is derived from the table shown in FIG. 6 where
exhaust gas temperature is expressed as a function of engine rotational
speed and fuelling rate. The temperatures are expressed in .degree.C and
are empirically derived on test engines from actual temperature
measurements after a desired % EGR at a given RPM and fuelling rate has
been established. The temperatures derived from the tables shown in FIG. 6
must be converted to degrees Kelvin before being used in calculations for
computing fluid density of the exhaust gas and volumetric flow of exhaust
gas as will be explained below.
All incremental intervals in the tables shown in FIGS. 4-6 and all data in
the tables are engine model specific and are empirically derived during
dynamometer test runs on each particular model. Furthermore, the
electronic controller 26 is programmed to perform two dimensional linear
interpolation where sensor values fall between discrete values loaded in
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