Process for controlling the exhaust gas recirculation system of an internal combustion engine

Process for controlling the exhaust gas recirculation system of an internal combustion engine 1 of the type comprising a pipe (4) assuring the connection between the exhaust manifold (3) and the intake manifold (2), said pipe being equipped with an E.G.R. valve (5) controlled from the electronic system (6) for engine control to assure the recirculation of a given rate of exhaust gas, characterized in that said engine control system is equipped with means for calculating the values Tm.sub.-- i of a representative magnitude of the torque produced by each of the combustions of the engine and in that the opening of the E.G.R. valve is controlled, for given operating conditions of the engine, as a function of said values Tm.sub.-- i.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for controlling the system for 
recirculation of the exhaust gases at the intake of an internal combustion 
engine that equips in particular a motor vehicle. 
2. Discussion of the Background 
The standards relating to pollution and the consumption of the internal 
combustion engines that equip motor vehicles, become more severe each day 
in all industrialized countries. The automobile industry is therefore 
today busy finding technical solutions to meet these obligations and this, 
without restricting too much the performances of the engines or adversely 
affecting their cost. 
A known technique for pollution control of internal combustion engines 
consists in performing the recirculation of the exhaust gases to reduce 
the emissions of pollutants and more particularly nitric oxides. This 
technique, called E.G.R. (Exhaust Gas Recirculation), consists in 
reinjecting a portion of the exhaust gases into the intake manifold of the 
engine, which has the effect of reducing the amount of gas to be burned 
and consequently of lowering the combustion temperature, hence a reduction 
of the production of NOx's or nitric oxides. 
The device most often adopted for use is that of the external 
recirculation, as opposed to the internal recirculation caused by the 
crossing of the intake and exhaust pipes. To do this, a connecting duct 
connects the exhaust manifold and the intake manifold, the exhaust gases 
are then sucked through this duct by the natural partial vacuum of the 
engine and reintroduced at the intake into the flow of combustion air. The 
duct is equipped with a valve that makes it possible to control the amount 
of exhaust gases reintroduced at the intake depending on the operating 
conditions of the engine and in particular depending on the level of power 
required of the engine. To do this, the valve, generally called an E.G.R. 
valve, is directly controlled by the electronic system for engine control 
that adjusts the recirculation rates depending on the operating phases of 
the engine. 
This E.G.R. valve, however, exhibits the drawback of being subject to 
losses of adjustment that can cause malfunctioning of the engine and 
therefore increased emissions of polluting substances. Actually, the 
E.G.R. valve is subjected to the attack of the highly corrosive exhaust 
gases and its gradual fouling tends to change the characteristics of the 
flow of the exhaust gases for the same instruction issued by the engine 
control system. 
In the context of future antipollution regulations such as the U.S. 
standard OBD2 (On Board Diagnostic Level 2), it is provided that the 
electronic systems for engine control that equip the particular vehicles 
are induced to diagnose and/or correct automatically certain engine faults 
which have an impact on the emissions of pollutants. 
Thus, it is provided to install in future electronic systems for engine 
control, devices that are capable of detecting, while driving, faults 
relating in particular to the operation of the E.G.R. system. These faults 
will cause, when there is a failure of an element acting on the level of 
pollution, either the activation of degraded modes of operation and/or the 
lighting of a warning light on the instrument panel informing the driver 
(not very harsh regulations), or will require that corrective measures be 
taken or that the vehicle be stopped (strict regulations). 
Automobile designers and outfitters have therefore developed a certain 
number of techniques for controlling the operation of the E.G.R. valve. 
By way of example, it is possible to cite the use of Venturi tubes on the 
pipe coming out of the E.G.R. valve, of means for controlling the movement 
of the needle of the valve, or further, of temperature probes, etc. 
It is also possible to cite the control process described in patent 
application No. FR-A-2,674,574, characterized in that the gas intake load 
in the engine is measured upstream and downstream from the outlet of the 
recycling pipe, before and after the control on opening or on closing of 
the valve, the load measurements upstream from the outlet are compared 
first to be assured of a stabilized operation of the engine, then if such 
is the case, the engine load measurements downstream from the outlet of 
the recycling pipe are compared before and after controlling the valve, to 
determine a proper operation of the valve in case of difference between 
these measurements and a malfunctioning of it in case of equality between 
these measurements. 
This device makes it possible to detect the nonoperation of the E.G.R. 
valve but not a degraded operation, further this device like all devices 
proposed in the prior art, exhibits a certain number of drawbacks at the 
level of the complexity of its structure, its difficult installation in 
the engine and the reliability itself of the means added to control the 
operation of the E.G.R. valve. 
SUMMARY OF THE INVENTION 
The purpose of the invention is therefore to eliminate the drawbacks of the 
prior art by proposing a process for controlling the recirculation system 
that makes possible a diagnosis of the E.G.R. valve and that makes it 
possible, consequently, to adapt the control of the engine control system 
to take into account the possible deviation of the operation of the valve, 
thanks to a closed-loop correction. 
Furthermore, the process according to the invention offers a cost/benefit 
ratio that is much more favorable than the processes of the prior art. 
Actually, it uses no specific equipment and requires only a slight 
software excess cost. 
The process for controlling the exhaust gas recirculation system of an 
internal combustion engine according to the invention, relates to a 
recirculation system of the type comprising a pipe assuring the connection 
between the exhaust manifold and the intake manifold, the pipe being 
equipped with an E.G.R. valve controlled from the electronic system for 
engine control to assure the recirculation of a given rate of exhaust gas. 
According to the invention, the process is characterized in that the engine 
control system is equipped with means for calculating the values Tm.sub.-- 
i of a representative magnitude of the torque produced by each of the 
combustions of the engine and in that the opening of the E.G.R. valve is 
controlled, for given operating conditions of the engine, as a function of 
said values Tm.sub.-- i. 
According to another characteristic of the invention, the process for 
controlling the exhaust gas recirculation system comprises the following 
operations: 
control the E.G.R. valve from the operating conditions of the engine to 
obtain a predetermined exhaust gas recirculation rate R; 
then calculate from each representative value Tm.sub.-- i of the torque, a 
representative statistical magnitude ATm.sub.-- i of the average of the 
values Tm.sub.-- i; 
calculate the deviation DTm.sub.-- i of the value Tm.sub.-- i from the 
average ATm.sub.-- i; 
associate with each value DTm.sub.-- i, and depending on the operating 
conditions of the engine, a value Ra.sub.-- i corresponding to the rate of 
exhaust gas actually recirculated; 
compare the value Ra.sub.-- i with the value R of the rate of recirculation 
initially controlled and deduce from it, in case of difference, a 
malfunction of the E.G.R. valve. 
According to another characteristic of the process that is the object of 
the invention, depending on the value Ra.sub.-- i obtained, the E.G.R. 
valve is controlled on opening and on closing so as to cause the actual 
rate Ra of recirculated exhaust gas to converge on the rate R. 
According to another characteristic of the process that is the object of 
the invention, the values ATm.sub.-- i of said statistical magnitude are 
calculated from the following formula: 
EQU ATm.sub.-- i=ATm.sub.-- i-1+((Tm.sub.-- i-(ATm.sub.-- i-1))/F1 
where F1 is a filtering constant and where ATm.sub.-- 0=0. 
According to another characteristic of the process that is the object of 
the invention, the value DTm.sub.-- i is defined as being the standard 
deviation between Tm.sub.-- i and ATm.sub.-- i. 
According to another characteristic of the process that is the object of 
the invention, the values Ra.sub.-- i of the actual rate of recirculation 
of the exhaust gases at the intake are deduced from a specific mapping 
contained in the engine control system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 therefore describes an internal combustion engine using the process 
for controlling the exhaust gas recirculation system. Only the elements 
necessary for the understanding of the invention have been shown. 
The internal combustion engine shown, for example of the four-stroke and 
four-cylinder in-line type, referenced 1, is equipped with a system for 
outer recirculation of the exhaust gases. A connecting pipe 4 assures the 
recirculation of the exhaust gases between pipe 3 for exhaust of the burnt 
gases and manifold 2 for intake of the combustion gases into the cylinders 
of the engine, the direction of circulation of the gases is represented by 
arrows. 
An E.G.R. valve 5 equips connecting pipe 4 so as to control the amount of 
exhaust gas admitted for recirculation. This valve 5 is controlled by 
electronic system 6 for engine control, the latter controlling the opening 
of valve 5 to obtain a predetermined rate of exhaust gases present in the 
combustion gases depending on the operating conditions of the engine. 
Engine control system 6 comprises in a standard way a computer comprising a 
CPU, a random access memory (RAM), a read only memory (ROM), analog to 
digital (A/D) converters, and different input and output interfaces. It 
receives different input signals relating to the operation of the engine 
(load, speed, water temperature, manifold pressure, etc.), and peripheral 
elements, performs operations and generates output signals destined in 
particular for the fuel injectors and for the sparkplugs and therefore for 
E.G.R. valve 5. Thus, a predetermined mapping for the test stand is 
installed in the memory of the engine control system so that the latter 
determines for each engine operating condition a signal for controlling 
the E.G.R. valve corresponding to the desired recirculation rate. 
According to the process that is the object of the invention, among the 
calculations performed by the engine control system appears the one of a 
magnitude Tm deduced from the rotation of the drive shaft and 
representative of the gas torque produced by each of the combustions of 
the engine. This value is deduced from the observation of the rotation of 
a measuring ring gear 7, integral with the flywheel of the engine, for 
example the standard type ring gear mounted on electronic ignition 
engines. 
Associated with ring gear 7 is a stationary sensor 8, for example 
variable-reluctance, which delivers an alternating signal with a frequency 
proportional to the speed of advance of the teeth of the ring gear, i.e. 
representative of the instantaneous speed of the flywheel. The passage of 
this alternating signal, with a frequency proportional to the speed of 
advance of the teeth of the ring gear, at the representative value Tm of 
the gas torque produced by each combustion of the gas mixture in the 
cylinders of the engine, is performed by a process detailed in the French 
patent applications filed by the applicant under numbers 91/11273 and 
91/11274, relating to a "process and device for measuring the torque of an 
internal combustion heat engine." 
The object of the process for controlling the exhaust gas recirculation 
system according to the invention, is to use this gas torque information 
already used by the engine control system for other applications such as 
cylinder recognition or detection of combustion misfires, to control the 
exhaust gas recirculation system and particularly the operation of the 
E.G.R. valve. 
This use is made possible thanks to the following finding illustrated by 
the graph of FIG. 3. The applicant has observed that under given engine 
operating conditions, the instabilities of the engine (i.e., the 
variations of the characteristics of the combustions and therefore the 
variations of the gas torque) are directly connected to the amount of 
recirculated exhaust gas. It is thus possible, knowing the level of 
instability of the engine, to deduce the E.G.R. rate. Now, the 
instabilities of the engine are directly quantifiable from the knowledge 
of the gas torque of the engine and more specifically, of the standard 
deviation of the latter. 
Thus, by following the level of the instabilities of the engine, through 
the variations of the gas torque, it is possible to follow the rate of 
recirculation of the exhaust gases, all things being equal moreover, and 
therefore to diagnose the operation of E.G.R. valve 5 by comparing the 
actual rate of recirculated gas and the instruction rate and to deduce 
from it a system for controlling this valve 5 in a closed loop. 
The process for controlling the gas recirculation system used by the engine 
control system therefore comprises the following steps, illustrated by the 
flowchart of FIG. 2. 
Engine control system 6 controls E.G.R. valve 5, from the engine operating 
conditions, to generate a predetermined exhaust gas recirculation rate R. 
This value R of the E.G.R. rate is read into a table or mapping initially 
stored in the engine control system. The control of the E.G.R. valve is 
performed from an opening signal S corresponding to the instruction value 
R from a table of appropriate and determined correspondence. 
The successive values of the gas torque Tm, produced for each combustion by 
the appropriate calculating means from the analysis of the rotation of the 
target 7, then are the object of a specific processing making it possible 
to control the actual rate of recirculated exhaust gas. 
From each representative value Tm.sub.-- i of the gas torque caused by the 
i.sup.th combustion, a representative statistical magnitude ATm.sub.-- i 
of the average of the values Tm.sub.-- i over a given horizon of 
combustions. This average value is traditionally obtained by a numeric 
filtering of the first order: the values ATm.sub.-- i are calculated from 
the following formula: 
EQU ATm.sub.-- i=ATm.sub.-- i-1+((Tm.sub.-- i-(ATm.sub.-- i-1))/F1) 
where F1 is a filtering constant and where ATm.sub.-- 0=0. 
Then the deviation DTm.sub.-- i is calculated of the value Tm.sub.-- i from 
the average ATm.sub.-- i thus obtained. This deviation is calculated 
preferably with the formula of the standard deviation: 
EQU DTm.sub.-- i=([(ATm.sub.-- i).sup.2 -(tm.sub.-- i).sup.2 ]).sup.1/2 
Then there is associated with each value DTm.sub.-- i, and depending on the 
engine operating conditions, a value Ra.sub.-- i corresponding to the rate 
of exhaust gas actually recirculated. This value is read into a table or 
mapping initially stored in the engine control system. This table is 
developed from measurements made on the test stand and reproduced in FIG. 
3. 
It is then enough to compare the value Ra.sub.-- i with the value R of the 
recirculation rate initially controlled to deduce from it, in case of 
difference, a malfunction of the E.G.R. valve. 
The engine control system then controls the E.G.R. valve by modifying the 
value of the opening signal S corresponding to the desired rate R. When 
the rate Ra has converged on the rate R, the signal for opening 
corresponding valve 5 is then stored and replaces the previously contained 
value. Thus, when the engine will again be under the same operating 
conditions, the corrections eliminating the operating deviations of valve 
5, are immediately operational. 
Of course, the invention is in no way limited to the embodiment described 
and illustrated which has been given only by way of example. 
On the other hand, the invention comprises all the technical equivalents of 
the means described as well as their combinations if they are made 
according to its spirit. 
This regulation of the E.G.R. rate can be performed in various forms: 
either with analog electronic components for which the adders, comparators 
and other filters are made with operational amplifiers; 
or with digital electronic components that would perform the hard-wired 
logic function; 
or by an algorithm for processing the signal installed in the form of a 
software module forming a software system for engine control that causes 
the microcontroller of an electronic computer to operate. 
or further, by a specific (custom) chip whose hardware and software 
resources will have been optimized to perform the functions that are the 
object of the invention: a chip that is or is not microprogrammable, 
packaged separately or else all or part of a coprocessor installed in a 
microcontroller or microprocessor, etc. 
Also, the invention comprises all the technical equivalents applied to an 
internal combustion engine regardless of its combustion cycle (2 cycle, 4 
cycle), the fuel used Diesel or gasoline or further, the number of its 
cylinders.