Method for improving fuel control in an internal combustion engine

A method for improving fuel control in an internal combustion engine employs a pressure ratio to determine the engine's actual volumetric efficiency continuously as the engine is operating. The air/fuel ratio of the engine's combustible mixture is affected by the volumetric efficiency. The volumetric efficiency varies during engine operation. Determination of the actual volumetric efficiency on a real-time basis allows greater accuracy in fuel metering. The pressure ratio used in the determination of the volumetric efficiency is the ratio of the intake and exhaust pressures or the inverse of this ratio. The ratio of pressures is combined with a second factor representative of the forces acting upon the air or air/fuel mixture inducted into the engine. The pressure ratio may be obtained without actual measurement of the pressure in the engine's exhaust conduit.

This invention is related to commonly-assigned U.S. Pat. No. 3,969,614 to 
Moyer et al. issued July 13, 1976 and entitled "Method and Apparatus for 
Engine Control" and to commonly-assigned U.S. Pat. No. 4,086,884 to Moon 
et al. issued May 2, 1978 and entitled "Method and Apparatus for 
Controlling the Amount of Fuel Metered into an Internal Combustion 
Engine". 
BACKGROUND 
This invention relates to a method for improving fuel control in an 
internal combustion engine. More particularly, it relates to a method for 
improving the manner in which fuel is metered in an internal combustion 
engine fuel control system of the speed-density type. 
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. 
In the speed-density fuel control system described in the aforementioned 
U.S. Pat. No. 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 repetitively 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 simultaneously as interdependent output 
variables. The aforementioned U.S. Pat. No. 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.) The speed-density fuel injection system described in U.S. 
Pat. No. 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. 
A system using "speed-density" means that the system measures engine speed 
and intake charge density and a predetermined value for volumetric 
efficiency to calculate air flow. Based on dynamometer data for a 
particular 5 liter engine, the mathematical expression for this is: 
##EQU1## 
where: MAP=Manifold Absolute Pressure from MAP sensor 
RPM=Engine Speed--from CP sensor 
MCT=Manifold Charge Temperature (used for density correction) from MCT 
sensor. 
VEFF=Volumetric Efficiency for the engine from a look up table stored in 
the ECU. The value taken for VEFF is dependent on RPM and MAP. 
To get the airmass from the above expression EGR mass must be subtracted. 
In an ideal engine having an efficiency, VEFF, of 1 and a more generalized 
proportionality constant, K, the equation reduces to 
EQU Total Mass Flow=(K)(MAP)(RPM)/MCT. (2) 
This is a simple expression for the calculation of air mass flow using the 
speed-density approach That is, the speed is relating to engine RPM and 
the air density is related to pressure (MAP) and temperature (MCT). The 
proportionality constant K typically takes into account such factors as 
the volume of the engine, the number of cylinders filled per revolution 
and the units of the calculation. 
The improved method of the present invention permits the volumetric 
efficiency of an engine having a speed-density electronic fuel control 
system to be determined much more accurately, under varying engine 
operating conditions, than is the case in the prior art systems. As a 
result, much more accurate fuel control is made possible and desired fuel 
economy and emission control benefits may be realized under certain 
circumstances. 
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. Specifically, the improved 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. 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, representative of forces acting upon the intake mixture 
as it flow 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. 
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. U.S. Pat. No. 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. 
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.

DETAILED DESCRIPTION 
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 or 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 
regarded 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 
denisty of the intake mixture is taken into account. 
The system disclosed in Moon et al. U.S. Pat. No. 4,086,884 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. 
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 gaseous material that enters the combustion 
chamber or chambers of an engine divided by the displacement 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. 
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. 
The inventors have found that 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. 
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. 
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 other 
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. 
The pressure ratio of the engine can be expressed by a pnemonic 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. 
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: 
EQU VEFF=[(PEOPI)(K.sub.1)+K.sub.2 ](second factor). (3) 
In this equation, K.sub.1 and K.sub.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. 
Whatever the pnemonic 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 dynamic 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 
paratmeters of lesser significance. The second factor, and also the 
constants K.sub.1 and K.sub.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.sub.3, K.sub.4 and K.sub.5, as 
follows: 
EQU second factor=K.sub.3 +(K.sub.4)(engine RPM)+(K.sub.5)(engine RPM.sup.2). 
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. U.S. Pat. No. 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. 
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 a 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 subtracted as taught in the Moon et al. U.S. Pat. No. 
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 exhaust 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 calculation is 
repeated continually during engine operation. 
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. 
For the sake of clarity, the following example of a calculation is given: 
(a) measure intake conduit pressure value (MAP), engine speed (RPM) and 
manifold charge temperature (TMAN); 
(b) calculate ideal cycle gas flow assuming an ideal volumetric efficiency 
(Nv) of 1.0. In the following equation, BASMD is the basic gas flow and 
K.sub.1 is a constant intake BASEMD=(K.sub.1)(MAP)(RPM)/(TMAN); 
(c) calculate the exhaust gage pressure (PEXH) (i.e., the pressure above 
atmospheric pressure) using a previously calculated airflow (AM): 
PEXH=(K.sub.2)(AM.sup.2), wherein K.sub.2 is a constant and AM=0.0 is used 
for first calculation; 
(d) determine the value of Barometric Absolute Pressure (BAP) from a 
sensor; 
(e) calculate the ratio of intake absolute pressure over the exhaust 
absolute pressure PIOPE, using PIOPE=MAP/(PEXH+BAP); 
(f) determine the value of the volumetric efficiency (Nv or VEFF) as a 
function of PIOPE and RPM (table look up or the equation numbered (3) 
above); 
(g) calculate the actual gas flow, AMPEM, into the engine which is equal to 
BASEMD times the volumetric efficiency (VEFF),or AMPEM=BASEMD.times.VEFF; 
(h) calculate the EGR mass flow (EM) from the EGR pintle position (EGR POS) 
i.e., EM=(K.sub.3)(EGR POS)-(K.sub.4)(EGR POS.sup.2) wherein K.sub.3 and 
K.sub.4 are constants; 
(i) calculate the actual air flow AM=AMPEM=EM; 
(j) use the last calculated AM value to calculate PEXH in the next 
calculation loop; 
(k) meter fuel proportional to the last calculated AM.