The provision of an instantaneously correct fuel/air mixture to an internal combustion engine over a full range of speeds and loads has for years occupied countless engineers, technicians and inventors. Accordingly, the charge forming apparatus have progressed over the years from crude mixing devices to complex, sophisticated carburetion and fuel injection systems. Nonetheless, it is recognized that to date no charge forming system has been implemented which will provide the ideal fuel/air mixture to an internal combustion engine over a complete operating range.
Of the various approaches to charge forming systems, two basic lines of attack may be discerned. With one approach, air inflow is controlled by a throttle or the like and fuel is caused to be entrained with the passing air, in response to the air flow characteristics. The metering of the mixing apparatus is such that a relatively constant, predictable mixture results over the anticipated air flow range. This is the basic rule of operation of the carburetor, which is of course the predominant charge forming mechanism used with internal combustion engines. The same thesis, however, has also been implemented in many forms of fuel injection apparatus.
This approach may be referred to as a "programmed" approach inasmuch as the charge forming system is "programmed" or adjusted to respond to certain stimuli to effect a predetermined fuel/air ratio. The stimuli applied to such systems may include temperature, pressure, engine speed, and the like. Conventionally, such systems make use of not one but a plurality of stimuli in order to more accurately react to inferred engine operating conditions. Charge forming systems of this type thus react to given types of stimuli in a predetermined manner, effecting a fuel/air ratio which is assumed to be the "correct" one for the engine. In theory this approach should work well, assuming that a great number of preprogrammed responses are available to correspond to the almost limitless combinations of stimuli which occur over the full range of operation for most internal combustion engines, especially those used in vehicles.
In recent years the increased recognition of environmental degradation caused by internal combustion engine exhaust, along with the need for economical operation, has caused renewed interest in more efficient and sophisticated charge forming control systems. Generally, the approach taken in developing more responsive charge control systems has been to make use of the most modern and sophisticated technology, particularly in the field of electronics, to accommodate the functional complexities which are inherent in the "programmed" type of system. That is, modern technology is used to implement a more complicated "program". Accordingly, control systems which are in effect small analog or digital computers are created which can "tailor" a fuel/air ratio to a given operating condition, as evidenced by various stimuli. The stimuli or sensed parameters include exhaust gas temperature and composition, engine temperature, combustion pressure, inlet manifold and/or venturi vacuum, engine speed, along with many other operating parameters. However, inasmuch as all such programmed systems provide a fixed, predetermined response to a given set of stimuli, they cannot be flexible enough to find the true optimum fuel/air ratio for all operating conditions, but rather provide mixture ratios which are only assumed to be correct given certain stimuli.
Accordingly, it will be appreciated that it would be desirable to provide an improved charge forming system which consistently achieves the actual optimum fuel/air ratio for any given operating condition.
When the constraints of exhaust emission control, fuel consumption, and vehicle drivability are considered, the optimum engine fuel/air mixture for an internal combustion engine is one which is in the vicinity of the lean limit. This "optimum mixture" normally changes significantly with changes in fuel chemistry, ambient conditions and engine operating conditions. Accordingly, modern programmed charge forming systems, despite exceeding complexity, are nevertheless systems unable to respond to all such variables.
The second basic approach to provision of a fuel/air charge to a vehicle engine recognizes that a typical vehicle internal combustion engine is subjected to extreme variations in speed and load, and that these variations occur rapidly. The ideal control system, which would seek out the optimum mixture continually, must therefore be able to respond to changes in speed and load very rapidly, to avoid the delivery of an off-optimum mixture to the engine during transient operation. Moreover, it would be desirable to provide a control system capable of compensating for these variables, as well as others such as differing fuels and the like without modification. The prior art shows several "adaptive" mixture controllers which are stated to seek out the lean limit mixture, but their slowness of response to transient engine conditions has prevented their use in vehicles. The charge forming system of the invention is likewise "adaptive" in that it continuously monitors engine output characteristics and adjusts the charge accordingly.
For example, U.S. Pat. No. 2,628,606 to Draper and Li discloses the use of a closed loop control system, for oscillating or "dithering" the air/fuel mixture supplied to an engine about a set point. The engine output is observed, and the mixture controlled in the direction of increasing output. This "dithering" technique has been successfully applied to control of the spark timing, but has never been successfully demonstrated for dynamic control of the air/fuel mixture. The explanation for this shortcoming lies in the response times of the individual elements in a mixture control loop. In the case of Draper and Li, the minimum timed required to complete one dither-cycle is determined by the maximum rate of response of the engine to a change of mixture. It has been demonstrated experimentally that the maximum dither frequency which produces a measurable effect is approximately 5 cycles per second. At mixture dither frequencies above that value, the engine is unable to respond. However, at a dither rate of 5 cycles per second, information about the power output can only be computed every 200 milliseconds, which is far too slow to track transient engine operation.
Taplin et al were granted U.S. Pat. No. 3,789,816 for an improved mixture control system which adaptively leans the mixture until the engine becomes rough. The Taplin system monitors the absolute value of instantaneous crankshaft acceleration, and as long as the magnitude of acceleration/deceleration remains below a pre-determined threshold amplitude, the mixture is biased continuously leaner. Effectively, roughness is compared continuously with a threshold which represents an "acceptable drivability" level; when roughness exceeds the preset reference, the mixture is enriched.
As in the case of the Draper and Li system, the Taplin roughness governor is workable under steady state conditions, but suffers from slowness of response. This slowness is an indirect result of the basic control philosphy: that is, use of the preset level of roughness for two purposes. The roughness threshold defined in Taplin et al is used both as a standard for comparison, and as the desired level of roughness which the servo loop is designed to produce. That is, the aimed-at point is also the point of comparison. Moreover, all differences from the desired point are treated equally; in other words, the Taplin patent describes a control system which seeks out an optimum mixture, without regard for how near or far the actual mixture is from optimum. Therefore, in order that the Taplin control system can avoid "overshooting" its prescribed roughness level, it must approach the thus-defined lean limit very slowly. Since the Taplin mixture controller does not "know" whether the desired operating point is near or far from the actual operating point, it must always travel in the lean direction at the same rate, limited by the servo system "overshoot" characteristics. It will be appreciated that a finite amount of time is required for an altered air/fuel mixture to be drawn into the intake manifold, drawn into the combustion chamber, be compressed, ignited, and expanded. This time delay is RPM-dependent, approximately equal to one complete engine cycle, or 1/RPM. If the mixture controller of Taplin sees roughness which is less than the preset threshold, it causes the mixture to be leaned at some prescribed rate; if roughness is detected which is above the threshold, the mixture entering the engine intake is enriched. Thus, the roughness signal which initiates enrichment occurs approximately one revolution after the "too lean" mixture was supplied by the control system. During the period between the ingestion of the "too lean" mixture by the engine and the resulting roughness at the output, the control element will have continued to bias the mixture leaner at its pre-selected rate; the greater the rate of mixture change, the greater the overshoot. It is for this reason that a control system such as described by Taplin must have a limited rate of change of mixture. This limitation directly affects the transient response of the whole control loop. It is admitted by Taplin et al that their system is only workable under steady state conditions:
It is therefore another object of the present invention to provide a closed loop fuel control system that normally maintains as lean as air fuel [sic] as possible so as to just follow the threshold of unacceptable engine roughness during steady state operation and that permits a different control of air fuel ratio to be effected in the presence of other modes of operation. PA1 . . a continuous measurement of the combustion chamber pressure would not be useful for a stable control of the fuel-air mixture and hence the operational behavior of an internal combustion engine.
Additionally, Taplin et al failed to recognize the fact that the positive and negative components of roughness indicate different phenomena. Instead the Taplin system full-wave-rectifies the roughenss signal, weighing positive and negative roughness components equally, and richening in response to both.
In U.S. Pat. Nos. 4,099,493 and 4,161,162 as well as in abandoned application Ser. No. 597,404 Latsch et al aim at the same objective (controlling the mixture at the lean limit) but use a different strategy. Instead of monitoring the instantaneous value of a continuous signal, the Latsch schemes all sample an indication of roughness at discrete, non-continuous intervals. The period length is defined in application Ser. No. 597,404 thus: " . . . the phase angle is measured after each or after several operating cycles of the engine." Other methods for achieving the same goal are disclosed in Latsch et al U.S. Pat. No. 4,161,162. Each of the Latsch methods involves the sampling of discrete portions of the "roughness signal", and performing a comparison either with a synthetic "ideal signal" or between two or more subsequent samples. Such comparisons cannot be made until the end of a discrete sample period. In other words, the sampling is discontinuous, and the information gathered by the process is incomplete. Therefore, no decision can be made by the comparison circuit until the end of the sample period--introducing time delay.
In any engine, the instantaneous change in angular velocity at the crankshaft is attributable to the sum of the negative and positive work being done by the individual cylinders. The interaction of combustion, engine pumping, vibration, and driver-induced accelerations are manifested in the net work output by the crankshaft and its angular acceleration and deceleration. While this instantaneous work output can be related to selected angular locations of the crankshaft corresponding to the individual power-producing piston strokes, it is the net interaction of all forces on the crankshaft which influences "roughness". The work done by the pistons on the crankshaft is applied in specific angular intervals. However, when the work applied by the pistons becomes non-uniform, many modes of crankshaft acceleration/deceleration are excited, and the non-uniformity of combustion may be detected in a relatively short time frame, without waiting for the entire engine cycle to be completed. This signal is a continuous one, and is not preeminent in any selected angular region of the crankshaft.
In both of the Latsch et al patents as well as in their abandoned application, reference is made to the distinction between continuous and discrete sampling of "roughness"; Latsch et al teach away from continuous monitoring. Quoting from U.S. Pat. No. 4,161,162:
The comparison of two subsequent samples as performed by Latsch et al also leaves open the possibility of comparing two "rough" samples to one another, and interpreting them as "similar" and therefore satisfactory.
Moreover, it is noted that Latsch et al use the terms "combustion chamber pressure" and "crankshaft acceleration" interchangeably--the implication being that either could be usefully monitored.
The same arguments which relate the the slowness of response of the Taplin method apply also to the Latsch methods; a delay is deliberately introduced into the servo loop. Any amount of delay--however small--limits the system response speed. According to Latsch et al, the delay is equal to approximately 180.degree. of crankshaft rotation, which it a very significant limitation on response time. (180.degree. corresponds to between 5 and 50 milliseconds, depending on RPM.)
The Latsch patents did correct one of the deficiencies of the Taplin apparatus by limiting the signals of interest to negative crankshaft accelerations only.
The final reference is Frobenius U.S. Pat. No. 4,104,990. The scheme shown there samples the RPM in two distinct angular regions of the crankshaft, and compares the two samples, so as to detect an angular deceleration beyond a threshold amplitude. This arrangement combines the least desirable features of both Taplin with those of Latsch.
As in Latsch, the information gathered is discontinuous, introducing measurement delay.
As in Taplin, the "roughness" cannot be detected until an unacceptable roughness level has already been exceeded. FIG. 3 of the Frobenius patent shows that the "lean limit" signal is almost totally absent when the air number is richer than the lean limit. At the prescribed "lean limit" mixture, the signal used to denote "roughness" increases markedly. This signal characteristic is similar to that of Taplin, in that the lean limit is reached without warning. As with Taplin, the Frobenius mixture controller must therefore approach the lean limit slowly, for fear of overshooting its mark.
Therefore, it is an object of this invention to provide a mixture control system which overcomes the transient response limitations inherent in the prior art.
It is a further object of the invention to provide an automatically controlled adaptive fuel metering system which compensates for variables which are not sensed directly, such as fuel chemistry, ambient conditions, and engine operating parameters.
It is another object of the invention to provide a closed loop, lean limit control apparatus which is less complex and more reliable than the prior art.
It is a further object of the present invention to provide an improved charge forming system for optimizing the mixture ingested by an internal combustion engine over a broad operating range.
It is another object of the present invention to provide an improved charge forming system which does not provide a predetermined or programmed mixture to an internal combustion engine.
It is a further object of the invention to provide a charge forming system for an internal combustion engine which effects a marked reduction in the output of pollutants by the engine.