Coordinated control method for turbocharged diesel engines having exhaust gas recirculation

A method of controlling the airflow into a compression ignition engine having an EGR and a VGT is disclosed. The control strategy includes the steps of determining the engine speed and fueling rate and, retrieving desired values for the intake manifold pressure (P.sub.1 *) and compressor mass flow rate (W.sub.a *) as a function of the engine speed and fueling rate wherein the desired values P.sub.1 * and W.sub.a * correspond to desired values for the air-fuel ratio and burnt gas fraction at each engine operating point. The desired values are then compared against measured values for the intake pressure (P.sub.1) and mass airflow (W.sub.a) to generate an EGR valve position command and VGT guide vane position command as a function of the weighted sum of the differences between P.sub.1 and P.sub.1 *, and W.sub.a and W.sub.a *. These position commands are then applied to the EGR valve and turbocharger turbine guide vanes, respectively, to drive the EGR valve and VGT vanes to the respective desired positions.

TECHNICAL FIELD 
This invention relates to turbocharged compression ignition engines having 
exhaust gas recirculation systems and, more particularly, to methods of 
controlling the air/fuel ratio and amount of exhaust gas recirculation in 
diesel engines equipped with variable geometry turbochargers (VGT) and 
exhaust gas recirculation (EGR) systems. 
BACKGROUND OF THE INVENTION 
High performance, high speed diesel engines are often equipped with 
turbochargers to increase power density over a wider engine operating 
range, and EGR systems to reduce the production of NOx emissions. 
Turbochargers use a portion of the exhaust gas energy to increase the mass 
of the air charge delivered to the engine combustion chambers. The larger 
mass of air can be burned with a larger quantity of fuel, thereby 
resulting in increased power and torque as compared to naturally aspirated 
engines. 
A typical turbocharger consists of a compressor and turbine coupled by a 
common shaft. The exhaust gas drives the turbine which drives the 
compressor which, in turn, compresses ambient air and directs it into the 
intake manifold. Variable geometry turbochargers (VGT) allow the intake 
airflow to be optimized over a range of engine speeds. This is 
accomplished by changing the angle of the inlet guide vanes on the turbine 
stator. An optimal position for the inlet guide vanes is determined from a 
combination of desired torque response, fuel economy, and emissions 
requirements. 
EGR systems are used to reduce NOx emissions by increasing the dilution 
fraction (F.sub.1) in the intake manifold. EGR is typically accomplished 
with an EGR valve that connects the intake manifold and the exhaust 
manifold. In the cylinders, the recirculated exhaust gas acts as an inert 
gas, thus lowering the flame and in-cylinder gas temperature and, hence, 
decreasing the formation of NOx. On the other hand, the recirculated 
exhaust gas displaces fresh air and reduces the air-to-fuel ratio (AFR) of 
the in-cylinder mixture. 
Visible smoke can be avoided by maintaining the AFR sufficiently lean, 
while low NOx (emissions is achieved by keeping F.sub.1 sufficiently 
large. Consequently, the performance of an engine control strategy is 
evaluated in terms of its ability to regulate AFR and F.sub.1. Neither of 
these performance variables, however, is directly measured. Thus, 
conventional control schemes generate control signals for EGR and VGT 
actuators to enforce tracking of set points on measured 
variables--typically intake manifold pressure P.sub.1 (measured by a 
manifold absolute pressure (MAP) sensor) and compressor mass airflow 
W.sub.a (measured by a mass airflow sensor (MAF)). The desired set points 
are typically achieved by independently controlling the VGT to regulate 
P.sub.1 and the EGR to regulate W.sub.a. This can result in large actuator 
effort to enforce the tracking of the measured variables. Consequently, 
there exists a need for a robust engine control strategy having stable 
regulation of the AFR and F.sub.1 which coordinates the control of the EGR 
and VGT. 
DISCLOSURE OF THE INVENTION 
One object of the present invention is to provide an improved compression 
ignition engine control strategy. 
Another object is to generate set points for the EGR valve and VGT position 
which correspond to a desired AFR and F.sub.1. 
A further object is to provide a multivariable control scheme which 
coordinates the VGT and EGR actuators to achieve the desired set points. 
According to the present invention, the foregoing and other objects and 
advantages are obtained by a method of controlling the airflow into a 
compression ignition engine having an EGR and a VGT. The method includes 
the steps of determining the engine speed (N(t)) and fueling rate (W.sub.f 
(t)) and, based on these values, retrieving desired values for the intake 
manifold pressure (P.sub.1 *) and compressor mass flow rate (W.sub.a *). 
These desired values are then compared against measured values for the 
intake pressure (P.sub.1) and mass airflow (W.sub.a) to generating an EGR 
valve position command (X.sub.egr (t)) and VGT guide vane position command 
(X.sub.vgt (t)) as a function of the weighted sum of the difference 
between P.sub.1 and P.sub.1 *, and W.sub.a and W.sub.a *. These values are 
then applied to the EGR valve and turbocharger turbine guide vanes, 
respectively, to drive the EGR valve and VGT vanes to the respective 
desired positions. 
The present control method is advantageous in that, by coordinating EGR and 
VGT control, it achieves VGT and EGR actuator command signals which are 
smaller that those observed in conventional independent control schemes. 
Thus, actuator wear is reduced, and actuator saturation is less frequent 
than in conventional control schemes, reducing the necessity for complex 
saturation-recovery logic. The present controller is also simple to 
calibrate and tune, and results in reduced turbo-lag compared to 
conventional controllers. 
Other objects and advantages of the invention will become apparent upon 
reading the following detailed description and appended claims, and upon 
reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning first to FIG. 1, there is shown a simplified schematic diagram of a 
compression ignition engine system 10 equipped with an exhaust gas 
recirculation (EGR) system 12 and a variable geometry turbocharger (VGT) 
14. A representative engine block 16 is shown having four combustion 
chambers 18. Each of the combustion chambers 18 includes a 
direct-injection fuel injector 20. The duty cycle of the fuel injectors 20 
is determined by the engine control unit (ECU) 24 and transmitted along 
signal line 22. Air enters the combustion chambers 18 through the intake 
manifold 26, and combustion gases are exhausted through the exhaust 
manifold 28 in the direction of arrow 30. 
To reduce the level of NOx emissions, the engine is equipped with an EGR 
system 12. The EGR system 12 comprises a conduit 32 connecting the exhaust 
manifold 28 to the intake manifold 26. This allows a portion of the 
exhaust gases to be circulated from the exhaust manifold 28 to the intake 
manifold 26 in the direction of arrow 31. An EGR valve 34 reculates the 
amount of exhaust gas recirculated from the exhaust manifold 28. In the 
combustion chambers, the recirculated exhaust gas acts as an inert gas, 
thus lowering the flame and in-cylinder gas temperature and decreasing the 
formation of NOx. On the other hand, the recirculated exhaust gas 
displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder 
mixture. 
The turbocharger 14 uses exhaust gas energy to increase the mass of the air 
charge delivered to the engine combustion chambers 18. The exhaust gas 
flowing in the direction of arrow 30 drives the turbocharger 14. This 
larger mass of air can be burned with a larger quantity of fuel, resulting 
in more torque and power as compared to naturally aspirated, 
non-turbocharged engines. 
The turbocharger 14 consists of a compressor 36 and a turbine 38 coupled by 
a common shaft 40. The exhaust gas 30 drives the turbine 38 which drives 
the compressor 36 which, in turn, compresses ambient air 42 and directs it 
(arrow 43) into the intake manifold 26. The VGT 14 can be modified during 
engine operation by varying the turbine flow area and the angle at which 
the exhaust gas 30 is directed at the turbine blades. This is accomplished 
by changing the angle of the inlet guide vanes 44 on the turbine 38. The 
optimal position for the inlet guide vanes 44 is determined from the 
desired engine operating characteristics at various engine speeds. 
All of the engine systems, including the EGR 12, VGT 14 and fuel injectors 
20 are controlled by the ECU. For example, signal 46 from the ECU 24 
regulates the EGR valve position, and signal 48 regulates the position of 
the VGT guide vanes 44. 
In the ECU 24, the command signals 46, 48 to the EGR 12 and VGT 14 
actuators are calculated from measured variables and engine operating 
parameters by means of a control algorithm. Sensors and calibratable 
lookup tables residing in ECU memory provide the ECU 24 with engine 
operating information. For example, an intake manifold pressure (MAP) 
sensor 50 provides a signal (P.sub.1) 52 to the ECU indicative of the 
pressure in the intake manifold 26. Likewise, exhaust manifold pressure 
(EXMP) sensor 54 provides a signal (P.sub.2) 56 to the ECU 24 indicative 
of the pressure in the exhaust manifold 28. Further, an intake manifold 
temperature sensor 58 provides a signal (T.sub.m) 60 to the ECU 24 
indicative of the intake manifold temperature. A mass airflow (MAF) sensor 
64 also provides a signal (W.sub.a) 66 indicative of the compressor mass 
airflow to the ECU 24. 
Additional sensory inputs can also be received by the ECU along signal line 
62 such as engine coolant temperature, engine speed, and throttle 
position. Additional operator inputs 68 are received along signal 70 such 
as the accelerator pedal position or other fueling request input. 
The engine control methods described herein apply to all turbocharged 
compression ignition engines equipped with EGR systems, regardless of the 
type of fuel used. Thus, it is to be understood that references to diesel 
engines are equally applicable to other compression ignition engines as 
well. In addition, throughout the specification, the following notations 
are used in describing measured or calculated variables: 
______________________________________ 
N engine speed (RPM) 
P.sub.1 intake manifold pressure (MAP) (kPa) 
P.sub.2 exhaust manifold pressure (EXMP) (kPa) 
P.sub.a ambient (barometric) pressure (kPa) 
W.sub.a compressor mass flow rate (MAF) (kg/s) 
W.sub.egr EGR mass flow rate (kg/s) 
W.sub.f fuel mass flow rate (kg/h) 
F.sub.1 * desired intake burnt gas fraction 
AFR* desired air/fuel ratio 
AF.sub.s stoichiometric air/fuel ratio (14.6 for 
diesel fuel) 
X.sub.egr EGR valve position 
X.sub.vgt VGT actuator position 
______________________________________ 
The disclosed engine control method can be implemented in a modular fashion 
with existing fuel limiting schemes as shown in FIG. 2. Accordingly, the 
engine control system has four major components: (1) control block 200 
generates desired set points for the compressor mass flow rate (W.sub.a *) 
and intake manifold pressure (P.sub.1 *); (2) control block 202 is the 
feedback controller to achieve the desired set points for W.sub.a and 
P.sub.1 ; (3) the plant or engine is represented by block 204; and (4) 
block 206 represents conventional fuel limiting schemes which may include 
an air density limiter and slew rate limiter. This invention relates 
primarily to the set point generator 200 and the controller 202, and their 
implementation in an engine control strategy. 
Control block 200 receives as inputs the engine speed (N), requested 
fueling rate (W.sub.f) and generates set points for the compressor flow 
rate (W.sub.a *) and intake manifold pressure (P.sub.1 *) in order to 
achieve the desired AFR (AFR*) and dilution fraction (F.sub.1 *). These 
desired values are obtained by optimizing the steady-state fuel 
consumption and emissions based on the engine mapping data. Specifically, 
a two-dimensional grid of engine speed values and fueling rate values is 
created. For each grid point, an optimal EGR valve position (X.sub.egr) 
and VGT actuator position (X.sub.vgt) is developed with reference to 
F.sub.1, smoke production, and P.sub.1. 
The EGR burnt gas fraction (F.sub.1) in the intake manifold is calculated 
as follows: 
EQU F.sub.1 =W.sub.f ((1+1/AF.sub.s)/(W.sub.f +W.sub.a)) (W.sub.egr /(W.sub.a 
+W.sub.egr)) (1) 
where AF.sub.s is the stoichiometric air/fuel ratio, which equals 
approximately 1/14.6 for diesel fuel. W.sub.a is measured by MAF sensor 64 
of FIG. 1, and W.sub.egr can be calculated from measurements of the intake 
manifold CO.sub.2 concentration, exhaust manifold CO.sub.2 concentration, 
and W.sub.a. Measurements for W.sub.f, W.sub.a, smoke, and CO.sub.2 
concentrations are typically recorded during engine dynamometer testing 
and mapping, and can be gathered by any known method. 
When developing values for X.sub.egr and X.sub.vgt, F.sub.1 is preferably 
maximized for NOx reduction, smoke is preferably kept below an acceptable 
level, and the intake manifold pressure, P.sub.1, is preferably limited to 
a maximum value for overboost protection and fuel economy. 
Alternatively, X.sub.egr and X.sub.vgt can be developed based on the 
maximized weighted sum of engine brake torque and F.sub.1 at each engine 
operating point. In such a case, the weights of the summing function are 
experimentally selected to achieve the desired performance tradeoff 
between NOx production and fuel consumption. 
The optimized values for the performance variables F.sub.1 and AFR can be 
expressed as: 
##EQU1## 
These values are used in the controller calibration stage to aid in 
defining optimum values for the measured variables. 
The optimized values for the EGR position and VGT position can be expressed 
as: 
##EQU2## 
And the corresponding values of the measured outputs can be expressed as: 
##EQU3## 
Accordingly, two-dimensional lookup tables are obtained for z*, u*, and y* 
at each engine operating point based on the engine speed and fueling rate. 
If the values of the engine speed or fueling rate do not coincide with one 
of the grid values, linear interpolation between grid values can be used 
to obtain the corresponding values for z*, u*, and y*. 
Given the desired set points W.sub.a * and P.sub.1 * from the set point 
generator 200 which correspond to z*, the feedback controller 202 
calculates desired values for the EGR valve position and VGT actuator 
position to achieve the desired compressor flow rate and intake pressure. 
Referring to FIG. 3, at node 312, a compressor flow error term is 
established which is equal to the difference between the actual (measured) 
and desired compressor mass flow rates (W.sub.a -W.sub.a *). Similarly, at 
node 314, an intake pressure error term is generated which is equal to the 
difference between the actual and desired intake manifold pressures 
(P.sub.1 -p.sub.1 *). The control signals for EGR valve position and VGT 
actuator position are scheduled based on the optimal combinations of the 
deviation from the desired compressor mass air flow and intake manifold 
pressure in box 316. These combinations are optimized based on the 
performance variables, F.sub.1 and AFR, not the measured variables. 
The VGT actuator and EGR valve commands are coordinated by identifying the 
steady-state gains of each actuator at the optimal set points, X.sub.egr 
and X.sub.vgt. These gains are arranged in a 2.times.2 DC-gain matrix 
P.sub.y shown by block 318. 
To obtain values for the gain matrix P.sub.yij, for each fueling rate and 
engine speed, the EGR valve position and VGT actuator position, u.sub.j, 
are perturbed small amount around the optimal value u.sub.j * such as 
within 5% of the value of u.sub.j * (.DELTA.u.sub.j =u.sub.j -u.sub.j *=5% 
u.sub.j *). The resulting steady-state difference .DELTA.y.sub.i =y.sub.i 
-y.sub.i * is then measured, and values for the steady-state gain matrix 
P.sub.y are calculated as follows: 
EQU P.sub.yij =.DELTA.y.sub.i /.DELTA.u.sub.j (2) 
The procedure is repeated for all values of i=1,2 and j=1,2 and for all 
values of engine speed and fueling rate from the grid. The values of the 
entries for P.sub.yij are stored in ECU memory in a two-dimensional table. 
Linear interpolation is used to determine values of P.sub.yij for engine 
speed values and fueling rate values that differ from the values of the 
grid points. Thus, the DC-gain matrix used to convert actuator position 
deviations for EGR and VGT to steady-state measurement deviations for MAF 
and MAP yields: 
##EQU4## 
or 
EQU .DELTA.y=P.sub.y .DELTA.u (4) 
Where .DELTA.W.sub.a, .DELTA.P.sub.1, .DELTA.X.sub.egr, .DELTA.X.sub.vgt, 
.DELTA.y, and .DELTA.u correspond to deviations of the respective 
variables from the nominal values. 
Similarly, FIG. 4 shows the DC-gain matrix P.sub.z 400 which is developed 
to define the steady-state gains from the actuator positions for the EGR 
and VGT to the performance variables F.sub.1 and AFR. The values for the 
matrix P.sub.z are calculated using a formula similar to equation (2): 
P.sub.zij =.DELTA.z.sub.i /.DELTA.u.sub.j from the measurements of the 
steady-state difference .DELTA.z.sub.i =z.sub.i -z.sub.i * resulting from 
the perturbation .DELTA.u.sub.j =u.sub.j -u.sub.j *. Accordingly, the 
DC-gain matrix used to convert actuator signals for EGR and VGT to 
steady-state measurements for the performance variables can be represented 
as follows: 
##EQU5## 
or 
EQU .DELTA.z=P.sub.z .DELTA.u (6) 
Where .DELTA.F.sub.1, .DELTA.AFR, .DELTA.X.sub.egr, .DELTA.X.sub.vgt, 
.DELTA.z, and .DELTA.u correspond to the deviations of the respective 
variables from the nominal values. 
Referring again to FIG. 3, it is apparent that coordination of the EGR 
valve and VGT actuator requires calibration of all four variables, 
c.sub.11, c.sub.12, c.sub.21, and c.sub.22. These variables are calibrated 
assuming that, at the optimum set points for F.sub.1 and AFR, the 
performance variables are almost dependent. Specifically, increasing 
F.sub.1, decreases AFR and vice versa. As a result, the coordination 
scheme of box 316 can be simplified to the controller shown in FIG. 5 as 
box 516. Thus, P.sub.z is decomposed to generate gains g.sub.1 and g.sub.2 
and, rather than using two integrators on two signals, a single integrator 
and weighted sum is used for the controller implementation. Accordingly, 
the EGR and VGT are coupled through the gains g.sub.1 and g.sub.2, and the 
controller 516 enforces tracking of a weighted sum of the measurement 
errors through the gains h.sub.1 and h.sub.2. Preferrably, g.sub.1, 
g.sub.2, h.sub.1 and h.sub.2 are selected to maximize the effect of the 
actuators on AFR and F.sub.1 along a given direction. This is done 
because, in this embodiment, the actuators do not have the authority to 
manipulate AFR and F.sub.1 independently. The gains g.sub.1 and g.sub.2 
are calculated as follows: 
if P.sub.z11.sup.2 +P.sub.z21.sup.2 &lt;P.sub.z12.sup.2 +Pz.sub.z22.sup.2 
then 
EQU .alpha.=(1+.vertline.(P.sub.z11 P.sub.z12 +P.sub.z21 
P.sub.z22)/(P.sub.z12.sup.2 +P.sub.z22.sup.2).vertline./.sup.2).sup.1/2 
EQU g.sub.1 =(1/.alpha.)((P.sub.z11 P.sub.z12 +P.sub.z21 
P.sub.z22)/(P.sub.z12.sup.2 +P.sub.z22.sup.2)) (7) 
EQU g.sub.2 =(1/.alpha.) (8) 
else 
EQU .alpha.=(1+.vertline.(P.sub.z11 P.sub.z12 +P.sub.z21 
P.sub.z22)/(P.sub.z11.sup.2 +P.sub.z21.sup.2).vertline..sup.2).sup.1/2 
EQU g.sub.1 =(1/.alpha.) (9) 
EQU g.sub.2 =(1/.alpha.)((P.sub.z11 P.sub.z12 +P.sub.z21 
P.sub.z22)/(P.sub.z11.sup.2 +P.sub.z21.sup.2)) (10) 
The gains h.sub.1 and h.sub.2 are calculated as follows: 
let e.sub.1 =P.sub.z11 g.sub.1 +P.sub.z12 g.sub.2 
e.sub.2 =P.sub.z21 g.sub.1 +P.sub.z22 g.sub.2 
and DTP.sub.y =P.sub.y11 P.sub.y22 -P.sub.y12 P.sub.y21 
then values for h.sub.1 and h.sub.2 can be defined in terms of h.sub.1 and 
h.sub.2 : 
EQU h.sub.1 =(1/DTP.sub.y)[e.sub.1 (P.sub.z11 P.sub.y22 -P.sub.z12 
P.sub.y21)+e.sub.2 (P .sub.z21 P.sub.y22 -P.sub.z22 P.sub.y21)] 
EQU h.sub.2 =(1/DTP.sub.y)[e.sub.1 (-P.sub.z11 P.sub.y12 +P.sub.z12 P 
.sub.y11)+e.sub.2 (-P.sub.z21 P.sub.y12 +Pz.sub.22 P.sub.y11)] 
resulting in: 
EQU h.sub.1 =h.sub.1 /(h.sub.1 .sup.2 +h.sub.2 .sup.2).sup.1/2 (11) 
EQU h.sub.2 =h.sub.2 /(h.sub.1 .sup.2 +h.sub.2 .sup.2).sup.1/2 (12) 
The gains g.sub.1, g.sub.2, h.sub.1, and h.sub.2 are stored in lookup 
tables in ECU memory and are used to gain-schedule the controller across 
the entire engine operating range. The controller box 502 in FIG. 5 is 
implemented using a proportional (k.sub.p) plus integral (k.sub.i) 
controller and is adjusted to meet the desired transient characteristics 
of the engine response based on the EGR valve and VGT actuator 
characteristics. Thus, k.sub.p and k.sub.i are constant values for all 
engine operating states whereas g.sub.1, g.sub.2, h.sub.1, and h.sub.2 
vary based on the engine operating state because of P.sub.z and P.sub.y. 
Alternatively, k.sub.p and k.sub.i could be replaced by any dynamic 
controller which provides a zero steady-state error. 
In order to implement the controller scheme in the digital ECU, for each 
time instant (t), the weighted sum of the measurement errors h.sub.1 and 
h.sub.2, is used to generate position commands for the EGR valve and VGT 
actuator as follows: 
EQU X.sub.c (t+1)=X.sub.c (t)+.delta.t[h.sub.1 (W.sub.a -W.sub.a *)+h.sub.2 
(P.sub.1 +P.sub.1 *)] (13) 
EQU X.sub.egr (t)=X.sub.egr *(t)+g.sub.1 (k.sub.i X.sub.c (t)+k.sub.p h.sub.1 
((W.sub.a -W.sub.a *)+h.sub.2 (P.sub.1 -P.sub.1 *))) (14) 
EQU X.sub.vgt (t)=X.sub.vgt *(t)+g.sub.2 (k.sub.i X.sub.c (t)+k.sub.p h.sub.1 
((W.sub.a -W.sub.a *)+h.sub.2 (P.sub.1 -P.sub.1 *))) (15) 
Where X.sub.c (t) is the integrator state of the proportional plus integral 
controller. These commanded values are then applied to the actuator 
drivers of the ECR and VGT along signal lines 46 and 48, respectively of 
FIG. 1. 
While the invention has been described in connection with one or more 
embodiments, it will be understood that the invention is not limited to 
those embodiments. For example, instead of measuring the compressor mass 
airflow (W.sub.a), it can be estimated from the measured intake and 
exhaust manifold pressures and intake manifold temperature signals. The 
structure of the control algorithm would remain the same, except that an 
estimate of the compressor mass airflow is used instead of its measured 
value. Accordingly, the invention covers all alternatives, modifications, 
and equivalents, as may be included within the spirit and scope of the 
appended claims.