Measuring and testing – Simulating operating condition – Marine
Reexamination Certificate
2000-05-18
2001-05-22
McCall, Eric S. (Department: 2855)
Measuring and testing
Simulating operating condition
Marine
Reexamination Certificate
active
06234012
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an air/fuel ratio control system for an internal combustion engine where air flow and exhaust gas recirculation flow are calculated from pressure sensors.
BACKGROUND OF THE INVENTION
Engine control systems often determine the amount of fuel to inject by measuring a manifold pressure, along with other engine operating conditions. This method is often referred to by those skilled in the art as the speed density method. In this method, a mean value model of engine operation is constructed, where an average manifold pressure at a given speed results in a certain air flow into the cylinder. In this type of system, measurement of the manifold pressure is critical for proper prediction of the air flow into the cylinder and thus for proper air/fuel ratio control.
One approach for calculating a value of the manifold pressure to use in the speed density approach is to sample the manifold pressure sensor when the piston is at top dead center, bottom dead center, and two other points equally spaced from dead center positions. For example, the two other equally spaced samples could be 60 degrees after dead center. Thus, four samples per revolution of the crankshaft are used that are not necessarily equally spaced. Then, an average of the last two and the current value of the manifold pressure is taken to obtain the averaged value of the manifold pressure used in the speed density method. Such a system is disclosed in U.S. Pat. No. 5,497,329.
The inventors herein have recognized numerous disadvantages with the above approach. For example, the sampling scheme described above will produce a constant bias unless the two equally spaced samples occur in the proper location. In particular, the resulting averaged manifold pressure will be consistently offset from the true average. This results in an error in the prediction of air flow at a steady state operating condition. Another disadvantage, for example, is that the resulting averaged manifold pressure will still contain oscillations that will cause cyclic errors in prediction of air flow at a steady state operating condition. These cyclic errors may cause reduced efficiency in controlling regulated emissions.
Also, engine control systems relying on a manifold pressure sensor to determine fresh charge entering the engine must be able to measure flow of exhaust gas recirculation to accurately control the exhaust air/fuel ratio. Previous systems have used a differential pressure measurement across an orifice to infer a flow of exhaust gas. Traditionally, the orifice is located upstream of the exhaust gas recirculation flow control valve. Thus, the pressure measurements are shielded from the intake manifold pressure pulsations; however, the pressure measurements are not shielded from the exhaust pressure pulsations. In the traditional system, the high frequency pressure pulsations present in the pressure measurements are reduced by using a conventional low pass filter. Such a system is disclosed in U.S. Pat. No. 5,613,479.
The inventors herein have recognized a significant opportunity to reduce total system cost by relocating the orifice downstream of the exhaust gas recirculation flow control valve but before the intake manifold. Thus, the manifold pressure sensor can be used to measure the pressure downstream of the orifice and a single absolute pressure sensor can be used to measure the pressure upstream of the orifice. This creates the needed differential pressure to measure exhaust gas recirculation flow.
The inventors herein have recognized numerous disadvantages with the above approach. For example, the manifold pressure sensor is sensitive to pressure fluctuations in the manifold and the upstream exhaust pressure sensor is sensitive to pressure fluctuations in the exhaust pressure. Since these fluctuations are out of phase with one another, a significant error is created in the difference between the two. Another disadvantage is the need for a conventional low pass filter to reduce these oscillations, where the conventional low pass filter is known to hinder transient performance.
SUMMARY OF THE INVENTION
An object of the invention claimed herein is to provide a method to more accurately calculate the fresh air entering a cylinder of an engine.
The above object is achieved, and disadvantages of prior approaches overcome, by a method for calculating air flow in an internal combustion engine. The method comprises sensing an engine speed of the engine, synchronously sampling a first pressure sensor with a frequency proportional to a firing frequency of the engine, filtering said synchronously sampled first pressure with a filter to remove oscillations at frequencies proportional to said firing frequency, and calculating a mass of gas entering a cylinder of the engine responsive to said first filtered pressure and said engine speed.
By sampling the pressure waveform synchronously at a rate proportional to the firing frequency of the engine and sampling with the proper proportion, the pressure pulsations caused by the firing orders of the engine can be removed. This leaves the proper value of pressure, which represents the mean value, for calculating air flow entering the cylinder. Thus, by properly selecting the location of sample points occurring at a frequency proportional to the firing frequency of the engine, the inventors have obtained an unexpected benefit that not only eliminates all fluctuations in the filtered pressure, but also removes any constant bias.
In another aspect of the present invention both upstream and downstream pressures are sensed by sampling both the pressure waveforms synchronously at a rate proportional to the firing frequency of the engine and sampling with the proper proportion. Thus, the pressure pulsations caused by the firing orders of the engine can be removed. This leaves the proper constant value of differential pressure, which represents the average value, for calculating exhaust gas recirculation flow entering the manifold.
An advantage of the above aspect of the invention is that the response to transients in the mean pressure value is greatly improved.
Another advantage of the above aspect of the invention is improved emission control.
Another advantage of the above aspect of the invention is that the accuracy of the air charge calculation is increased.
Still another advantage of the above aspect of the invention is decreased system cost.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.
REFERENCES:
patent: 4553518 (1985-11-01), Takao et al.
patent: 5483938 (1996-01-01), Oshima et al.
patent: 5497329 (1996-03-01), Tang
patent: 5613479 (1997-03-01), Gates et al.
Kotwicki Allan Joseph
Lewis Donald J.
Pursifull Ross Dykstra
Russell John David
Ford Motor Company
McCall Eric S.
Russell John D.
LandOfFree
Air/fuel ratio control system does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Air/fuel ratio control system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Air/fuel ratio control system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2516583