Internal-combustion engines – Engine speed regulator – Open loop condition responsive
Reexamination Certificate
2002-11-05
2004-10-19
Huynh, Hai (Department: 3747)
Internal-combustion engines
Engine speed regulator
Open loop condition responsive
C123S568210
Reexamination Certificate
active
06805095
ABSTRACT:
TECHNICAL FIELD
This invention relates to internal combustion engines, and more particularly to systems and methods for controlling and estimating cylinder air charge in direct injection internal combustion engines.
BACKGROUND
As is known in the art, cylinder air charge estimation and control are important in meeting performance requirements of advanced technology engines, such as direct injection spark ignition (DISI) engines. The objective of the air-charge control in lean-burn, spark-ignited engines is to operate an electronic throttle and the exhaust gas recirculation (EGR) valve in a manner so as to provide the desired gas flow to the engine and the desired burnt gas fraction in this flow for NO
x
reduction. A secondary objective of air-charge control is to provide updated estimates of the in-cylinder conditions, in terms of charge quantity and burnt gas fraction, as inputs to other Engine Management System (EMS) features, such as torque and fueling control. This function is referred to as charge estimation. The conventional approach to the cylinder air-charge control is open-loop whereby the desired mass flow rates through the EGR valve and throttle are calculated as functions of the desired burnt gas fraction in the intake manifold and of the desired gas flow into the cylinder using the orifice equation.
On a typical DISI engine, an electronic throttle (ETC) is used to control the inlet fresh air while the burnt gas is recirculated from the exhaust manifold back into the intake manifold through an exhaust gas recirculation (EGR) valve. Another valve, referred to as the swirl control valve (SCV), or other charge motion device, such as cam profile switching (CPS), is also present in the intake system to effect different swirl ratios and therefore to provide the mixture motion in the cylinder that optimizes the combustion process, especially in the stratified operation.
Difficulties in developing a simple and robust cylinder air charge controller for a DISI engine result from several special characteristics associated with the lean burn and stratified operation. First, when the engine operates lean, the flow through the EGR valve contains unburnt air that has not been consumed in the previous combustion event, and this, together with the intake manifold dynamics, adds difficulties in accurately estimating the in-cylinder burnt gas fraction. Second, accurate air flow estimates via the orifice equation are hard to obtain at high manifold pressures where a DISI engine may frequently operate. Third, the buildup of soot and other deposits in the EGR conduit and intake ports is more severe on DISI engines because of the stratified operation and high flow volume of EGR. The deposits change the valve flow and engine breathing characteristics and make the charge control system very susceptible to aging. Other factors, such as actuator imperfection due to friction and quantization, also contribute to complicating the problem.
As is known in the art, open-loop charge estimation and control have been used for a conventional PFI engine. For an open-loop charge estimation for a DISI engine, the estimates of flows through the throttle and the EGR valve are based on the orifice equation, the flow into the cylinders based on the speed-density equation, and the burnt gas fraction in the intake manifold based on the manifold dynamic model and mass balance of air and burnt gas. More particularly, the standard orifice equation applied to the throttle and EGR valve gives the following estimates for W
thr
(i.e., the fresh air flowing through the throttle) and W
egr
(i.e., the recirculated gas flowing through the EGR valve):
W
thr
=
f
(
p
amb
T
amb
,
P
i
P
amb
)
u
thr
(
θ
thr
)
(
1
)
W
egr
=
f
(
p
exh
T
exh
,
P
i
P
exh
)
u
egr
(
θ
egr
)
(
2
)
where f is a function (fcn) of upstream pressure, upstream temperature, and the pressure ratio across the throttle and EGR valve given by:
f
(
x
,
y
)
=
γ
1
2
(
2
γ
+
1
)
x
if
y
≤
0.528
f
(
x
,
y
)
=
xy
1
γ
{
2
γ
γ
+
1
[
1
-
y
γ
-
1
γ
]
}
if
y
>
0.528
and &ggr; is the ratio of specific heats (&ggr;=1.4), P
i
, P
exh
, P
amb
are the pressures in the intake manifold, exhaust manifold and at ambient conditions, respectively, T
amb
, T
exh
are the temperatures at ambient conditions and in the exhaust gas, respectively. The parameters u
thr
and u
egr
are effective flow areas for the throttle and EGR valve, respectively, as functions of the direct control commands: throttle angle &thgr;
thr
(0 degrees-90 degrees) and the percentage of opening of the EGR valve d
egr
(0-100%). These two functions depend on the geometric configuration of the throttle and EGR valves, respectively, and are identified from the experimental data. In calibrating these two functions, numerical values of u
thr
and u
egr
are first calculated from the engine mapping data using equations (1) and (2) for different throttle and EGR valve openings. Then standard regression techniques are applied to correlate u
thr
with &thgr;
thr
, u
egr
with d
egr
, respectively.
The standard, open-loop scheme of controlling the cylinder air charge and the burnt gas fraction in the intake system consists of three steps:
1. Given the desired exhaust air-to-fuel ratio r
exh,d
, the desired in-cylinder flow W
cyl,d
and the desired intake manifold burnt gas fraction F
in,d
, backtrack the desired EGR flow W
egr,d
and throttle flow W
thr,d
:
W
egr
,
d
=
F
i
n
d
W
cyl
,
d
(
1
+
r
exh
,
d
)
1
+
r
stoich
(
3
)
W
thr
,
d
=
W
cyl
,
d
-
W
egr
,
d
(
4
)
2. Determine the desired intake manifold pressure P
i,d
from the speed density equation (7) below for the given W
cyl,d
, &sgr; and N, where the setting &sgr; for the swirl control valve is usually determined from a pre-stored lookup table.
3. Invert the orifice flow representations and the effective flow area functions to determine the desired commands for the throttle and EGR valve effective flow areas
u
thr
,
d
=
W
thr
,
d
f
(
P
amb
T
amb
,
P
i
,
d
P
amb
)
(
5
)
u
egr
,
d
=
W
egr
,
d
f
(
P
exh
T
exh
,
P
i
,
d
P
exh
)
(
6
)
Then invert the throttle and EGR valve effective flow area functions to determine the desired commands for throttle and EGR valve positions &thgr;
thr
, d
egr
.
The open-loop charge estimation and control approach described above has the advantages of being simple, intuitive and well understood. Its fundamental drawback, however, is the lack of robustness. In particular, this open-loop scheme does not address the following issues that are especially important for DISI engine operation: Limitations and sensitivities of the orifice equation under the high intake manifold pressure conditions (pressure drop close to 1); Lack of on-board measurements for exhaust manifold pressure and temperature; and, Soot deposit buildup and its effects on the engine behavior. Another difficulty in using the orifice equation for flow estimation is its reliance on the knowledge of upstream pressure and temperature. Especially for the EGR valve, the upstream (i.e., exhaust) temperature and pressure vary in a wide range, and no on-board measurement is available for these variables on most production vehicles. Any error in the estimated exhaust temperature and pressure will further deteriorate the quality of flow estimation.
The soot deposit buildup in the intake system or in the EGR conduit is another major problem for a stratified DISI engine. It is largely due to the stratified combustion and high volume of EGR flow. It is very difficult, if not impossible, to predict the effects of the deposits on the effective flow area over time.
SUMMARY
In accordance with the present invention, a method is provided for controlling cylinder charge in a direct-injection, spark-ignition engine. The engine includes an intake manifold and an electronically controlled throttle (ETC) valve controlling air flow from the atmosphere to
Kolmanovsky Ilya V.
Sun Jing
Ford Global Technologies LLC
Huynh Hai
LandOfFree
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