Patent Application
20090259382
The present invention relates generally to a method for low and high indicated mean effective pressure (IMEP) cylinder identification to enable fuel/spark or other control for cylinder balancing.
A misfire condition in an internal combustion engine results from either a lack of combustion of the air/fuel mixture, sometimes called a total misfire, or an instability during combustion, sometimes referred to as a partial misfire. In such case, torque production attributable to the misfiring cylinder decreases, due to, among other things, a reduced level of combustion (i.e., manifested by a reduced Indicated Mean Effective Pressure (IMEP)). Additionally, un-combusted fuel enters the exhaust system, which is undesirable. Because of the possible impact on the ability to meet certain emission requirements, engine misfire detection is routinely provided on automotive vehicles. Most common approaches use various engine speed derivatives (e.g., crankshaft acceleration) to detect fluctuations attributable to one or more cylinders, and thus to detect misfire and to identify what cylinder or cylinders have misfired. Accordingly, most internal combustion engine systems already have such engine speed derivative data stored and available by virtue of the need to detect misfire.
While cylinder imbalance may be the result of misfire in a particular cylinder, there is also recognized an inherent cylinder-to-cylinder IMEP variation attributed to manufacturing and durability variations in the base engine and engine control hardware. Whatever the source, a level of cylinder imbalance can be measured by a so-called COVIMEP parameter (i.e., Covariance of Indicated Mean Effective Pressure), as seen by reference to co-pending U.S. application Ser. No. 11/973,099 filed Oct. 5, 2007 entitled “METHOD FOR DETERMINATION OF COVARIANCE OF INDICATED MEAN EFFECTIVE PRESSURE FROM CRANKSHAFT MISFIRE ACCELERATION” (DP-315834), assigned to the common assignee of the present invention and hereby incorporated by reference. U.S. application Ser. No. 11/973,099 in turn teaches a method for inferring COVIMEP from various misfire-originated engine speed derivatives. However, to effect improvement in the COVIMEP performance, it is desirable to identify which cylinder is the weakest (lowest IMEP) and which is the strongest (highest IMEP) so that one or more various control actions can be taken to reduce the variation or imbalance between the cylinders.
There is therefore a need for a system and method for low and high IMEP cylinder identification so as to allow for cylinder balancing.
One advantage of the invention is that enables control action by an engine controller or the like so as to reduce cylinder torque imbalance. The invention, in a preferred embodiment, takes advantage of the fact that engine speed derivative data (e.g., crankshaft speed or acceleration fluctuation data), used in the invention, is already available in most internal combustion engine systems by virtue of the need to detect misfire, as described in the Background. A method for operating a multi-cylinder internal combustion engine system includes a number of steps. The first step involves providing an input array including an engine speed derivative for each cylinder of the engine. As used herein, engine speed derivative simply means a value derived from engine speed indicative data, and is not meant to be limited to only the first order mathematical derivative of engine speed (i.e., acceleration), although the term engine speed derivative includes this meaning. Engine speed derivative thus also includes not only the second order mathematical derivative (i.e., jerk acceleration), but also could include still higher order mathematical derivatives as well, as well as other parameter values derived from engine speed data. Next, identifying (i) a first one of the cylinders that has the lowest Indicated Mean Effect Pressure (IMEP) (“weakest” cylinder), and (ii) a second one of the cylinders that has the highest IMEP (“strongest” cylinder), all based on the information in the input array. The next step involves determining a delta parameter indicative of a difference between the engine speed derivative values for the first and second cylinders. This is significant since the “strongest” cylinder usually follows the “weakest” cylinder in the firing order, since, by comparison to a “weak” cylinder, the recovery back to “normal” is perceived as decisive acceleration, thus, even a normal cylinder will be perceived as strong. This is referred to herein as the shadow effect. The final step involves, in a preferred embodiment, controlling the torque of the first, lowest IMEP (“weakest”) cylinder based on the delta parameter so as to reduce the difference between the weakest and strongest cylinders. In a further, preferred embodiment, the control action is continued until it is no longer the “weakest” cylinder. Then, any remaining “weak” cylinders are adjusted through control action. The “weak” cylinders are preferably adjusted first because a weak cylinder creates the perception of exceptionally good performance for the cylinder which follows in the firing order as noted above. Preferably, the crankshaft positions are corrected for tooth machining errors before calculating the engine speed derivatives. Other features, aspects and advantages will become apparent in light of the description to follow.
The present invention will now be described by way of example, with reference to the accompanying drawings.
Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,
The engine 12 includes a plurality of cylinders, illustrated in exemplary fashion as a V-type, six (6) cylinder engine where the cylinders are designated 161, 162, 163, . . . 166. In one arrangement, for example, the firing order may be designated as cylinders numbers 2-3-4-5-6-1. Of course, other numbering schemes and/or firing orders are possible. Moreover, the present invention is not limited to any particular number of cylinders, i.e., a six cylinder engine as shown is exemplary only, and the invention may be applicable, for example, to a four-cylinder engine or an eight-cylinder engine.
The basic arrangement of the engine 12 is known in the art, and will not be repeated exhaustively herein in detail. However, it should be understood that each cylinder 161, 162, 163, . . . 166 is equipped with a corresponding piston (not shown), which is connected to a common crankshaft 18, as shown by the dashed-line in
In recent years, a commonly employed target wheel is one variant known as a 58× target wheel (i.e., 60-2; 58 teeth spaced around the wheel, spaced as though there were 60 evenly spaced teeth but with two teeth missing). In the illustrated embodiment, the target wheel 20 may be the 58× form target wheel known in the art. This form of a target wheel 20 provides a rising edge in the output signal every 6 degrees, with the exception of the 2-tooth gap, which as known is used as a reference. A speed-based signal, for example, can be formed by determining the speed, or a representative signal, every 6 degrees or multiples of 6 degrees as typically only one edge is used.
The ECM 14 may include a control unit 34 configured with a low/high IMEP cylinder identification capability sufficient for torque control action suitable for cylinder balancing as described herein. The ECM 14 may be characterized by general computing capability, memory storage, input/output (interface) capabilities and the like, all as known in the art. The ECM 14 is configured generally to receive a plurality of input signals representing various operating parameters associated with engine 12, with three such inputs being shown, namely, crankshaft sensor output signal 28, MAP output signal 32 and CAM signal 33. The ECM 14 is configured with various control strategies for producing needed output signals, such as fuel delivery control signals (for fuel injectors—not shown), all in order to control the combustion events, as well as spark timing signals (for respective spark plugs—not shown). In this regard, the ECM 14 may be programmed in accordance with conventional, known air/fuel control strategies and spark timing strategies.
An input array 36 is shown in block form in
The engine speed derivatives 38 preferably comprises an array of values 461, 462, . . . 46n representing an engine speed derivative associated with a respective one of the cylinders. For example, value 461 is associated with cylinder #1, value 462 is associated with cylinder 2, and so on with value 46n being associated with the last cylinder #n, where n is the total number of cylinders in the engine. As known, while the engine (e.g., crankshaft) will experience a normal, expected amount of acceleration for a normal combustion event in a particular cylinder, the engine, conversely, will experience an abnormal, unexpected deceleration when a partial or total misfire occurs in that cylinder. Alternatively, even during “normal” combustion, manufacturing variations or variations due to wear or passage of time can result in differences in combustion (IMEP) and the resulting acceleration. As described in the Background, conventional misfire detection systems are configured to look for such fluctuations and accordingly are configured to generate various engine speed derivative values for that purpose. Whether or not there is sufficient combustion failure/instability to warrant a “misfire” detection, such engine speed derivative data is nonetheless indicative of the underlying torque production attributable to each cylinder (and by extension the IMEP associated with each cylinder).
In one embodiment, the engine speed derivatives 461, 462, . . . 46n may comprise a respective engine speed first mathematical derivative variation (acceleration) attributable to that cylinder (i.e., either firing or misfiring). In a preferred embodiment, however, the engine speed derivatives 461, 462, . . . 46n may comprise second mathematical derivatives of engine speed, or, a mathematical derivative of an acceleration value (i.e., jerk acceleration) attributable to that cylinder. It is well known how to determine variations in engine speed (and derivatives thereof), particularly contribution attributed to each cylinder, using time markers and its location information received from crankshaft position sensor 22 and camshaft position sensor 31. The engine speed derivatives are produced in a crankshaft timing window which optimizes the match between the cylinder pressure (IMEP) and the resulting crankshaft acceleration.
The cylinder ordering described above is the firing order, not the cylinder number as that term is understood in the art. For example, the first value in the array 36, with a textual name of cylinder #1 and a value 461, is the first cylinder in the firing order. In this example, however, cylinder #1 may be cylinder number 2 in an engine where the firing order is 2-3-4-5-6-1. It is contemplated that in typical embodiments, a misfire detection system already resident in the ECM 14 will have populated the values 461, 462, . . . 46n in the array 36 during the course of performing its function of misfire detection. Consistent with typical misfire detection systems, preferably, the constituent values 461, 462, . . . 46n of the input array 36 are updated once each combustion event. In other words, the engine speed derivatives are produced in a crankshaft timing window, thus, the identified weakest cylinder will be subject of the controller's 34 action at the end of each individual combustion cycle. Also, it should be understood that the ECM 14 may be configured to produce such engine speed derivative values independent of any misfire detection system.
With continued attention to
Step 48 involves producing a respective engine speed derivative value attributable to each cylinder. This has been described above. The method proceeds to step 50.
Step 50 involves identifying the “weakest” (lowest IMEP) and “strongest” (highest IMEP) cylinders based on the engine speed derivative values. In a preferred embodiment, the “weakest” cylinder is identified by the cycle average of
MAX (CYL#1, CYL#2, CYL#3, . . . , CYL#n) over N cycles (the maximum jerk acceleration indicates here the recovery from weak combustion to normal combustion), where N equals the number of cycles (and is equal to or larger than 1) used in the running average, where CYL#1, CYL#2, . . . , CYL#n correspond to the engine speed derivative values 461, 462, . . . 46n, specifically corresponding to a time period between crankshaft reference points for the cylinders in the engine, in firing order, and where n is the number of cylinders. Likewise, in the preferred embodiment, the “strongest” cylinder is identified by the cycle average of MIN(CYL#1, CYL#2, CYL#3, . . . , CYL#n) over N cycles, where N equals the number of cycles used in the running average, where CYL#1, CYL#2, . . . , CYL#n correspond to the engine speed derivative values 461, 462, . . . 46n, specifically corresponding to a time period between crankshaft reference points for the cylinders in the engine, in firing order, and where n is the number of cylinders. In a constructed embodiment, the “weakest” and “strongest” cylinders have been observed to emerge on a consistent basis after a predetermined number of combustion cycles for a given engine speed (rpm), typically, at the controller's action initiation, between about 10 and 30 cycles at idle, sampled at 3× per crankshaft rotation. Once the controller's action is initiated, at steady state engine conditions, one combustion cycle suffice for the update of the identification of the weakest cylinder.
It should be understood that in embodiments where some other engine speed derivatives are utilized, the “weakest” cylinder may be determined as a MIN function and the “strongest” cylinder may be determined as a MAX function of such engine speed derivatives. Other variations are possible. The method then proceeds to step 52, which is not necessary but improves the gain of the control loop.
Step 52 involves determining a delta parameter indicative of a difference between the “weakest” cylinder and the “strongest” cylinder. In one embodiment, the delta parameter is determined as follows:
Delta=CYL#weakest−CYL#strongest,
Where=CYL#weakest is the engine speed derivative (e.g., fluctuation is time period, fluctuation in crankshaft acceleration, etc.) for the identified “weakest” cylinder; and
CYL#stongest, is the engine speed derivative (e.g., fluctuation in crankshaft angular speed, fluctuation in crankshaft acceleration, etc.) for the identified “strongest” cylinder.
In one embodiment, the delta parameter is calculated as a function of not only (i) crankshaft acceleration, the first mathematical derivative of speed, but also (ii) the mathematical derivative of acceleration (i.e., jerk acceleration). Also, in constructed embodiments, the crankshaft positions are corrected for tooth errors before calculating these values. Note that the way in which the delta parameter is computed takes advantage of the stronger cylinder shadow effect described above for providing an improved signal. Therefore, as was stated earlier, is a desirable but not necessary step in the detection of the weakest cylinder. The method then proceeds to step 54.
In step 54, the method involves controlling the torque attributable to either one of the “weakest” or “strongest” cylinder (preferably the “weakest” cylinder—more below) based on the delta parameter so as to reduce a cylinder torque imbalance.
With reference to
Once the delta parameter is calculated, in a preferred embodiment, control action is initially taken with respect to the identified “weakest” cylinder. With reference to
In an alternate embodiment, control action is not immediately taken after the “weakest” and “strongest” cylinders have been identified, but is rather deferred. The results from a number of combustion cycles are stored in a data buffer or the like. Then, after control action is taken, based on the accumulated data in the data buffer (e.g., the average of the individual “delta” parameter values). This embodiment may result in less aggressive control action due to the averaging.”
Misfire indicators (i.e., engine speed derivative values) for the cylinders during “normal” operation (not shown) may be relatively closely clustered, unlike
The control unit 34 is configured with a low/high IMEP cylinder identification function, suitable for use in control action to effect cylinder balancing, as described herein. It should be understood that the functional and other descriptions and accompanying illustrations contained herein will enable one of ordinary skill in the art to practice the inventions herein without undue experimentation. It is contemplated that the invention will preferably be practiced through programmed operation (i.e., execution of software computer programs) of the control unit 34.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.
