[0001] This invention relates to methods for controlling the supply of fuel to an internal
combustion engine.
[0002] There are two types of systems for controlling electrically the amount of fuel metered
to an internal combustion engine. One of these is the mass air flow system, in which
the volume or mass of air flowing into an engine is actually measured and the fuel
is metered accordingly. The other system, speed-density, uses engine speed and the
engine intake manifold absolute pressure to determine indirectly the amount of air
entering an engine. In both types of electronic fuel control systems, the appropriate
quantity of fuel is metered with a suitable fuel control apparatus. This apparatus
typically has been a plurality of electromagnetic fuel injectors intermittently operated
to deliver fuel into the intake manifold upstream of the usually provided intake valves.
[0003] In the speed-density fuel control system described in U.S. patent 4,086,884, the
fuel control system employs a digital computer to calculate the amount of fuel required
by the engine. The calculation is done respectively to permit the fuel supply to be
adjusted sufficiently often so that adequately precise control of fuel is achieved
on a real-time basis. The computer preferably controls fuel in an interactive manner,
that is, fuel supply, ignition timing and exhaust gas recirculation all are controlled
simultaneoulsy as interdependent output variables.
[0004] U.S. patent 3,969,614 describes an interactive engine control system. In such a digital
computer engine control system, an output variable, such as ignition timing, is taken
into account in the determination of another output variable, such as the time and
duration of injection in an intermittent-type fuel injection system. (If the injection
is continuous, of course, determination of the usual points in the engine cycle at
which injection is to be initiated is unnecessary.)
[0005] The speed-density fuel injection system described in U.S. patent 4,086,884 requires
that the volumetric efficiency of the engine be used, directly or indirectly, in the
calculation of the quantity of fuel to be supplied to the engine. Unfortunately, the
volumetric efficiency is a function of several parameters including engine speed and
engine load. This means that these changing factors have had to be taken into account
in the calculation of the quantity of fuel to be metered to the engine to satisfy
the oxygen content of the intake mixture that actually enters the engine's combustion
chambers. The desired fuel amount at any given time may, of course, be selected to
provide a rich, a stoichiometric or a lean air/fuel mixture as may be required for
engine operation in an open or closed-loop mode of engine operation.
[0006] According to the present invention, there is provided a method for controlling the
supply of fuel to an internal combustion engine having an intake conduit and an exhaust
conduit, the method comprising the steps of:
(a) determining the ratio of the engine's intake conduit pressure to its exhaust conduit
pressure or vice versa;
(b) using the determined ratio to determine the volumetric efficiency of the engine,
the volumetric efficiency being determined with respect to the flow of gases into
at least one combustion chamber of the engine; and
(c) metering fuel to the engine in a quantity based upon such determined volumetric
efficiency.
[0007] The method of the invention improves fuel control in an internal combustion engine
by providing for the computer calculation of an engine's current volumetric efficiency.
The volumetric efficiency varies as a function of engine operating parameters, such
as engine load, engine speed and other less significant variables.
[0008] Preferably, the method of the invention comprises the steps of determining the ratio
of the pressure in an engine's intake manifold to the absolute pressure of the products
of combustion in a passage through which the products of combustion pass after leaving
the engine's combustion chamber or chambers.
[0009] This ratio of intake mixture and exhaust gas absolute pressures, or the inverse ratio,
is combined mathematically with a second factor, which may be related to the engine
speed, represmtative of forces acting upon the intake mixture as it flows toward the
combustion chambers. The combined ratio and second factor are used to determine the
volumetric efficiency of the engine with respect to the flow of gases into at least
one combustion chamber thereof. This real-time volumetric efficiency then may be used
to determine the amount of fuel metered to the engine.
[0010] The method of the invention is of value as compared to the prior art because of the
simplicity and accuracy with which an engine's current volumetric efficiency can be
determined. The ratio of the intake mixture and exhaust gas pressures is easily determined
with the use of sensors typically found on engines having speed-density fuel control
systems. Also, the engine speed is a variable that is readily available on a continuous
basis in electronic engine control systems. The prior art speed-density systems, in
contrast, have required the use of many time-consuming calculations, either digital
or analog or both, based upon approximations of engine characteristics and design
features. The system described in Moon et al patent 4,086,884 mentioned above avoided
this. The volumetric efficiency was treated as a function of temperature and pressure
conditions in the intake manifold at the time the quantity of fuel to be delivered
to the engine, i.e., the injector pulse width, was being calculated.
[0011] A very significant feature of the invention is that the real-time determination of
volumetric efficiency takes into account the effects of changes in altitude on the
characteristics of an engine's operation.
[0012] The prior art calculation of the quantity of fuel to be supplied to an engine employing
a speed-density fuel control system, whether accomplished with analog electronic circuitry
of with a digital computer and associated software or a combination of these, has
been based primarily on the speed of the engine and the intake manifold pressure at
the time the calculation is made. In these prior art control systems for spark-ignition
internal combustion engines, the other parameters of engine operation have been regardes
as being of substantially lesser significance. The other parameters are less variable,
generally speaking, and consequently can be treated as environmental conditions that
should be taken into account for purposes of accuracy and calibration. The more extreme
modes of engine operation, such as occur during engine cranking at start, cold-engine
warm-up and wide-open throttle, usually have been treated as situations requiring
separate control provisions. Because catalysts of the three-way type now are used
extensively in automotive engines and because exhaust gas recirculation makes the
oxygen content of the intake mixture less predictable under all conditions of engine
operation, the use of engine speed and intake manifold pressure alone to determine
the quantity of fuel to be supplied to an engine no longer is satisfactory, whether
or not the density of the intake mixture is taken into account.
[0013] The system disclosed in US patent 4
t086,884 was intended to improve the speed-density fuel control system by taking into
account the effect of exhaust gas recirculation on the amount of fuel required by
an engine. This much improved system also was designed to allow the slowly varying
parameters of engine operation, such as volumetric efficiency, to be updated less
frequently than the more rapidly varying parameters, such as intake manifold pressure
and the quantity of recirculated exhaust gas. The method of the present invention
carries the development of electronic fuel metering an additional step by providing
an effective way to allow an engine's volumetric efficiency to be monitored on a real-time
basis.
[0014] The volumetric efficiency of the engine can be of great significance where precise
control of the air/fuel ratio of the mixture supplied to an engine is required. If
fuel economy, engine performance and exhaust emissions are of concern, air/fuel mixtures
must be precisely controlled over a range of rich, stoichiometric and lean air/fuel
ratios. The volumetric efficiency of an engine is the volume of such combustion chamber
or chambers of the engine; the volume of gaseous material entering the engine is referenced
to a selected temperature and pressure and in effect is a mass flow. This definition
is useful here in that it indicates that volumetric efficiency, for an engine of fixed
displacement, is dependent only upon the volume of gaseous material that enters the
combustion chamber or chambers of the engine. Necessarily, this volume is not the
same as the volume exhausted because additional gases are formed during combustion.
[0015] Volumetric efficiency of an engine in the past has been determined primarily from
the intake manifold absolute pressure and the engine speed based upon accumulated
engine dynamometer data for a given engine and exhaust system design. Every variation
in intake manifold pressure changes the volumetric efficiency; intake manifold pressure
is a function of both engine speed and engine load, as well as the density of the
gaseous mixture in the manifold.
[0016] The present invention is based on the appreciation that the volumetric efficiency,
regardless of engine operation in geographical locations of widely varying altitudes,
is related to the ratio of the intake manifold absolute pressure and the engine exhaust
system absolute pressure immediately downstream of the combustion chamber. The relationship
is almost hyperbolic. If the ratio is inverted, it is almost linear. Otherwise stated,
the ratio of intake manifold absolute pressure to the absolute pressure in the engine's
exhaust conduit, when combined with a second factor, can be used to determine volumetric
efficiency. The second factor represents the frictional and inertial forces that are
resisting the flow of the gaseous intake mixture entering the combustion chamber or
chambers of the engine.
[0017] All of the gaseous mixture entering the engine's intake system and flowing toward
the engine's combustion chamber or chambers travels through the engine's intake conduit
or manifold before passing through the respective intake valves and into the corresponding
combustion chambers. There is resistance to this flow in the form of frictional and
inertial forces. The frictional forces are the result of the interaction of the fluids
entering the combustion chambers with the intake conduit and the intake valves.
[0018] Volumetric efficiency of an engine is a measure of the quantity of gaseous material
inducted into a combustion chamber or chambers. Accurate determination of the volumetric
efficiency makes possible delivery of exactly the right amount of fuel to the combustion
chambers to satisfy the requirements of the air or oxygen in the combustion chambers.
In ohter words, exact knowledge of an engine's volumetric efficiency throughout the
operation of the engine allows the proper amount of fuel for the oxygen entering the
combustion chamber or chambers during each cycle of the engine to be calculated and
delivered.
[0019] The pressure ratio of the engine can be expressed by a pneumonic suitable for use
in computer programming. Thus, it may be represented as PIOPE, which means intake
conduit absolute pressure, over or divided by exhaust conduit absolute pressure.
[0020] The pressure ratio also can be represented pnemonically in other ways. For example,
the pressure ratio may be written as PEOPI, meaning exhaust pressure over or divided
by intake pressure; the PEOPI is a pressure ratio, as is PIOPE. Volumetric efficiency
VEFF preferably is related to PEOPI as follows:
VEFF = ( ( PEOPI ) (K1) + K2) (second factor).
[0021] In this equation, K1 and K
2 are constants. The second factor represents the frictional and inertial forces acting
on the air, or air and exhaust gas, or air, exhaust gas and fuel mixture moving within
the intake conduit toward the intake valves and combustion chambers.
[0022] Whatever the pnenonic representation in the digital computer computation of volumetric
efficiency or its equivalents, the significant factor is the use of the PIOPE or PEOPI
ratio of absolute pressures. These pressures in ratio and when combined with a second
factor provide direct and accurate indications of current or real-time engine volumetric
efficiency, i.e., volumetric efficiency as of the time the absolute pressures are
determined. (This, of course, assumes the intake and exhaust conduit pressures are
measured or determined at the same or insignificantly different times). The second
factor mentioned above is representative of the dyanamic forces of friction and inertia
that act upon, and tend to retard the flow of, the gaseous mixture in the engine's
intake conduit; these forces are proportional to engine speed and other engine operating
parameters of lesser significance. The second factor, and also the constants K and
K
2 above, can be determined by multiple regression analysis of data obtained by testing
a particular engine design on an engine dynamometer. This method for determining the
second factor typically results in the second factor being defined by a quadratic
equation, having known constants K
39 K
4 and K
5, as follows:-
second factor = K
3 + (K
4) (engine RFM) + (K
5)(engine RPM
2).
[0023] A particularly suitable method for determining volumetric efficiency on a real-time
basis is with the aid of values placed in computer memory in tabular form as a function
of PIOPE and engine speed. The PIOPE and engine speed may be represented as binary
numbers used to obtain access to a value or values of volumetric efficiency retained
in computer memory. Well known techniques preferably are employed to interpolate between
volumetric efficiency values stored in the memory; four-point interpolation is most
accurate. The accessed volumetric efficiency value then can be used in a computer
program for determining required fuel delivery. An example of a suitable equation
for use in calculating fuel injection pulse width using the engine's volumetric efficiency,
in a speed-density system, is given in Moon et al patent 4,086,884. Engine period
and PEOPI, or some other suitable combination of pressure ratio with a second factor
that together reflect the engine's current operational volumetric efficiency, can
be used to obtain the fuel delivery required for such current volumetric efficiency.
[0024] In the determination of the absolute pressure ratio, it is not necessary to actually
measure the absolute pressure in the exhaust conduit of the engine. The intake manifold
absolute pressure is a quantity that is routinely used and available in known speed-density
fuel injection systems for spark-ignition internal combustion engines. The ambient
or barometric pressure also is available in such systems. The engine's combustion
chamber displacement is a constant equal to the current mass flow of gases into the
engine divided by the volumetric efficiency of the engine as calculated on the last
cycle of the engine. (It should be noted that the exhaust conduit back pressure also
is very much related to the mass flow of gases into the engine's combustion chamber
or chambers immediately before it is exhausted to produce the exhaust pressure. This
is a factor in determining the volumetric efficiency for the next succeeding engine
cycle.) The mass gas flow into the engine or volumetric efficiency for a preceding
cycle may, therefore, be used to determine the volumetric efficiency for succeeding
cycle. To do this, the displacement of the engine's combustion chambers may be divided
by the volumetric efficiency last determined to yield a number approximately equal
to the actual gas flow through the engine per complete engine cycle. If then this
number is multiplied by the number of engine cycles per unit time (usually RPM/2),
the gas flow rate through the engine is found. This flow rate may include recirculated
exhaust gas and the amount of its contribution to the gas flow rate may be substracted
as taught in the Moon et al patent 4,086,884
- The exhaust conduit gage pressure is a simple quadratic function of engine air mass
flow rate, that is, exhaust conduit gage pressure is equal to a constant times the
square of the air mass flow rate. The absolute value of the achaust pressure is the
gage pressure plus the known or sensed barometric pressure. Following this, the ratio
PIOPE or PEOPI can be obtained with the use of the most recently available intake
manifold absolute pressure and the calculated exhaust conduit absolute pressure. The
ratio then is used, in combination with the aforementioned second factor representing
frictional and inertial forces, to produce a new engine volumetric efficiency value.
The calaculation is repeated continually during engine operation.
[0025] If it is desired to use the digital computer program and memory for more than one
engine or vehicle system without changing the volumetric efficiency table that is
selected, this can be accomplished by the use of scaling factors and terms in the
basic equation that relates mass air flow into the engine's combustion chambers to
the exhaust system gage pressure. For this purpose, the exhaust system gage pressure
may be regarded as a term that is equal to the sum of a constant and two or more other
terms each having air mass flow as a factor with a coefficient that is selected for
the particular engine or vehicle system in question.
1. A method for controlling the supply of fuel to an internal combustion engine having
an intake conduit and an exhaust conduit, the method comprising the steps of:
(a) determining the ratio of the engine's intake conduit pressure to its exhaust conduit
pressure or vice versa;
(b) using the determined ratio to determine the volumetric efficiency of the engine,
the volumetric efficiency being determined with respect to the flow of gases into
at least one combustion chamber of the engine; and
(c) metering fuel to the engine in a quantity based upon such determined volumetric
efficiency.
2. A method according to Claim 1 wherein the exhaust conduit pressure is measured.
3. A method according to Claim 1 wherein the intake conduit pressure is measured.
4. A method according to Claims 1 or 3 wherein the exhaust conduit pressure is calculated.
5. A method according to Claims 1, 3 or 4 wherein the exhaust conduit pressure is
calculated with the use of the intake conduit pressure.
6. A method according to Claims 1, 3 or 4 wherein the exhaust conduit pressure is calculated using both the intake
conduit pressure and a volumetric efficiency valve.
7. A method according to Claim 6 wherein the value of the engine's volumetric efficiency
used to calculate the exhaust conduit pressure is a value previously calculated in
accordance with step (b) in Claim 1.
8. A method according to any one of Claims 1 to 7 wherein the sequence of steps recited therein is repeated, the volumetric efficiency
determined in step (b) of the recited sequence being employed in a succeeding determination
of the ratio of intake conduit pressure to exhaust conduit pressure or vice versa.
9. A method according to any one of Claims 1 to 8 wherein the volumetric efficiency
is contained in a table stored in the memory of a digital computer.
10. A method according to any one of Claims 1 to 9 wherein the pressure ratio and
a second factor are combined to obtain the volumetric efficiency determined in accordance
with step (b) in Claim 1.
11. A method according to any one of Claims 1 to 10 wherein the pressure ratio with
a second factor representing the frictional and inertial forces acting upon the mixture
of gases flowing through the engine's intake conduit are used to obtain the volumetric
efficiency determined in accordance with step (b) in Claim 1.