Internal-combustion engines – Charge forming device – Exhaust gas used with the combustible mixture
Reexamination Certificate
2000-06-30
2001-02-06
Wolfe, Willis R. (Department: 3747)
Internal-combustion engines
Charge forming device
Exhaust gas used with the combustible mixture
C073S861610
Reexamination Certificate
active
06182644
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system and method to measure and control gas flow using pressure measurements upstream and downstream of an orifice, and in particular to measurement of exhaust gas recirculation flow with a flow control valve upstream of the orifice.
BACKGROUND OF THE INVENTION
Engine control systems require accurate control of exhaust gas recirculation (EGR) for controlling regulated emissions and achieving fuel economy improvements. One type of exhaust gas recirculation system externally recirculates the exhaust gas from the exhaust manifold to the intake manifold with a flow control valve placed in the flow path between the exhaust manifold and the intake manifold. Typically, the valve is pneumatically operated and controlled by an electronic engine controller.
One approach to controlling exhaust gas recirculation flow is to use a feedback variable to assure that the actual exhaust gas recirculation flow converges to the desired exhaust gas recirculation flow. One method is to use a differential pressure measured across an orifice in the exhaust flow path upstream of the flow control valve. Then, the differential pressure can be used to infer the actual exhaust gas recirculation flow. The differential pressure measurement provides adequate correlation to exhaust flow because the exhaust pressure varies only slightly in the region where EGR is utilized. Further temperature effects can be accounted for because the upstream exhaust manifold temperature can be correlated to engine operating conditions or ignored due to relatively small variations. Finally, an error between the actual and desired exhaust gas recirculation flow is used to create a control signal that is sent to the flow control valve. Thus, the system can compensate for the effects of engine and component aging, as well as other errors in the system. Such a system is disclosed in U.S. Pat. No. 5,190,017.
The inventors herein have recognized a disadvantage with the above system when the orifice is placed downstream of the valve. In this configuration, flow from the exhaust travels first through a flow control valve and then through the orifice before entering the intake manifold. In this case, the pressure upstream of the orifice (downstream of the valve) varies widely and the assumptions made regarding differential pressure and flow are no longer valid. Also, the temperature upstream of the orifice (downstream of the valve) is no longer correlated directly to engine operating conditions due to the flow expansion in the valve. Thus, there is a significant measurement error when using a differential pressure measurement with a downstream orifice.
One approach to more accurately measure flow is to measure absolute pressure upstream of the orifice, pressure differential across the orifice, and temperature upstream of the orifice. In this way, a correlation between the pressures and temperature can be used to measure the exhaust flow where the pressure and temperatures are widely varying. Alternatively, this approach can be used with the flow control valve where pressure upstream of the valve, pressure differential across the valve, temperature upstream of the valve, and valve area are used to measure flow. Such a system is disclosed in U.S. Pat. No. 4,406,161.
The inventors have recognized a disadvantage with the above approach. The approach requires that upstream temperature be known. Thus, a sensor is needed which adds additional cost and is unacceptable. Further, exhaust manifold temperature estimates based on engine operating conditions inaccurately represent the temperature downstream of a flow control valve. Further, with application of prior art approaches to the flow control valve, valve position, or area, must be measured, adding additional cost.
SUMMARY OF THE INVENTION
An object of the invention claimed herein is to provide an exhaust gas recirculation measurement system and method for an exhaust gas recirculation system having an upstream flow control valve and a downstream measuring orifice.
The above object is achieved, and problems of prior approaches overcome, by a flow measurement system for measuring exhaust gas flow from an exhaust manifold of an internal combustion engine to an intake manifold of the engine. The system comprises a flow control valve having a variable orifice positioned in an exhaust gas recirculation path between the exhaust manifold and intake manifold of the engine, a fixed orifice area located in said path and downstream of said valve, and a computer for measuring a first pressure downstream of said measuring orifice, measuring a differential pressure across said measuring orifice, and calculating a mass flow based on said first pressure and said differential pressure.
By using a combination of the differential pressure across the orifice and a correction as a function of an absolute pressure downstream of the orifice, wherein the orifice is downstream of a flow control valve, a measurement of flow is obtained that gives acceptable performance and avoids discrepancies of prior approaches. Stated another way, an approximation using pressure differential across the orifice and absolute pressure downstream of the orifice accurately measures flow. Temperature measurement upstream of the orifice (downstream of the flow control valve) is inherently included. This measurement method is justified for the special case of flow through an orifice located downstream of a valve, wherein flow originates from an exhaust manifold of an internal combustion engine an exits to an intake manifold of the engine.
This embodiment, herein referred to as the first embodiment, uses the product of pressure differential across the orifice and a correction factor, where the correction factor is related to the pressure downstream of the orifice.
An advantage of the above aspect of the invention is that more accurate feedback control of EGR is obtained.
Another advantage of the above aspect of the invention is that the more accurate feedback control quality yields better fuel economy and driveability.
Yet another advantage of the above aspect of the invention is that the consistent feedback control quality yields lower emissions.
Still another advantage of the above aspect of the invention is that the pressure measurement downstream of the measuring orifice serves the dual purpose of forming a correction factor for the EGR flow and measuring manifold pressure for other uses.
In another aspect of the invention, a second correction is used to further improve the measurement system by a method for measuring flow from an engine exhaust to and engine intake wherein the flow passes through a flow control valve and then a fixed area measuring orifice. The method comprises measuring a pressure difference across the measuring orifice, measuring a pressure downstream of the measuring orifice representative of manifold pressure, calculating a pressure and temperature correction based on said downstream pressure and said differential pressure, and calculating a flow based on said downstream pressure, said differential pressure, and said correction.
In the configuration where the flow control valve is placed upstream and the measuring orifice is placed downstream, both between in the exhaust manifold and intake manifold, a further approximation using manifold pressure and pressure drop across the measuring orifice can be found to include both pressure and temperature effects due to compressible flow expansion through the valve. This embodiment, herein referred to as the second embodiment, obtains further improved accuracy over the first embodiment previously described herein with no additional sensors. In the second embodiment, there is an additional correction factor used that is a function of both pressure difference across the orifice and pressure downstream of the orifice. Again, temperature measurement upstream of the orifice (downstream of the flow control valve) is unnecessary.
An advantage of the above aspect of the invention is that more accurate feedback control of EGR
Kotwicki Allan Joseph
Russell John David
Ford Global Technologies Inc.
Russell John D.
Wolfe Willis R.
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