Method and Apparatus for Measuring Engine Compression Ratio, Clearance Volume and Related Cylinder Parameters

The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a functional block diagram of a presently preferred embodiment of the apparatus in accordance with the invention coupled to an internal combustion engine;

FIG. 2 is an elevational fragmentary partially sectioned view on an enlarged scale as compared with FIG. 1 of a microwave/pressure probe in accordance with the invention;

FIG. 3 is a sectional view taken along the line 3--3 in FIG. 2;

FIG. 4 is a flow chart illustrating operation of the invention; and

FIGS. 5-7 are graphic illustrations (not to scale) useful in understanding operation of the invention.

The preferred embodiments of the method and apparatus of the present invention employ techniques and principles which are disclosed in detail in the U.S. Pat. No. to Scott E. Wilson No. 4,331,029. This patent is assigned to the assignee hereof, and the entirety thereof is incorporated herein by reference.

FIG. 1 illustrates a conventional V-6 gasoline internal combustion engine 10 which includes a distributor 12 coupled to a plurality of spark plugs 14. For measuring clearance volume and compression ratio in accordance with a preferred embodiment of the invention, engine 10 is mounted on a "cold test" stand (not shown) and has its crankshaft 16 coupled to a motor 18 so that the engine may be cycled without actual fuel ignition. One of the spark plugs 14 is removed from the cylinder block and a microwave/pressure probe 20 in accordance with the invention is threaded into the engine cylinder bore in place thereof.

Referring to FIGS. 2 and 3, probe 20 includes an outer metal sleeve 22 threaded at one end 24 so as to be adapted for reception into the vacated spark plug opening. A lip 26 radiates from the opposing end of sleeve 24 and has a circular recess 28 formed in the axial face thereof tangentially contiguous with and overlapping the central bore 30 of sleeve 22. A block 32 of insulating material such as plastic is mounted by the screws 33 on sleeve 22 overlying lip 26. Block 32 has an integral sleeve 34 telescopically received and extending through axial bore 30. The radially facing edge of sleeve 34 adjacent to lip 26 is flattened as at 36 in FIG. 3 so as to cooperate with the opposing wall of bore 30 to form a part-cylindrical axial slot 38. Slot 38 extends from the tip of sleeve 22 at end 24 to recess 28 and is of sufficient cross sectional area to transmit pressure variations accurately.

A length of coaxial cable 40 is snugly received within the central bore of sleeve 34. Cable 40 includes an outer insulation sheath 42 surrounding an outer conductor 44. A central or inner conductor 46 extends through cable 40 and is separated from outer conductor 44 by the insulation layer 48. Insulation layers 42,48 and outer conductor 44 terminate in assembly flush with the coplanar ends of sleeves 32,34 while the coax cable conductor 46 protrudes or extends therefrom. A protective cap 50 of Teflon or the like covers the tip of conductor 46 but does not cover the end of slot 38. A pressure transducer 52 such as a piezoelectric crystal is mounted within a pocket 54 in block 32 in open communication with recess 28. A pair of coax connectors 56,58 are mounted on block 32 and are respectively connected to coax cable 40 and pressure transducer 52. Thus, with probe 20 received into the cylinder block in place of a spark plug 14 (FIG. 1), pressure transducer 52 communicates with the cylinder interior through pocket 54, recess 28 and slot 38 to provide at connector 58 an electrical signal indicative of pressure variations within the test cylinder as the piston reciprocates therein.

Returning to FIG. 1, connector 58 on probe 20 is connected through a suitable pressure signal amplifier 60 to the process and control electronics 64. Likewise, probe connector 56 is connected through the microwave transceiver electronics 62 to process and control electronics 64. To monitor or track rotation of crankshaft 16, a magnetic pick-up 66 is positioned adjacent the starting ring gear 68 which is coupled to the engine crankshaft. Pick-up 66 is connected to a phase locked loop 70 to provide a periodic series of pulsed signals to electronics 64 at predetermined increments of crankshaft rotation, such as on the order of every 0.1.degree.. A suitable phase locked loop 70 is illustrated in FIG. 8 of the referenced Wilson patent. The referenced patent also discloses an optical transducer which may be removably coupled to the engine crankshaft to monitor crankshaft rotation in place of pick-up 66. Process and control electronics 64, which may comprise a suitably programmed conventional microprocessor, receives an input from an inductive pick-up 72 operatively coupled to the cable of the spark plug 14 replaced by probe 20. This input is used to distinguish the compression/power stroke of the associated piston from the exhaust/intake stroke. Process and control electronics 64 also includes conventional analog-to-digital conversion and sample-and-hold circuitry responsive to amplifier 60 and transceiver 62. Process and control electronics 64 provide an output to a suitable display device 74 for providing a numerical display of compression ratio C/R and clearance volume V.sub.TDC, and of the cylinder constants Y and k if desired.

Operation of the invention will be described in greater detail in connection with the flow diagram of FIG. 4 and the graphic illustrations of FIGS. 5 and 6. More specifically, FIG. 5 includes a first graph 80 which illustrates the relationship of cylinder volume V to crankshaft angle .theta. in the region around TDC, and a second graph 82 which illustrates the relationship of cylinder pressure P to crankshaft angle .theta. in the same region. It will be noted that cylinder volume V describes a symmetrical curve about a minimum clearance volume V.sub.TDC at the crankshaft angle .theta..sub.TDC associated with the piston top dead center position. Cylinder pressure P is likewise illustrated as a symmetrical curve about a peak at .theta..sub.TDC, although for some engines it is believed that the dynamics of gas behavior in a motored engine may place maximum cylinder pressure slightly ahead of the piston top dead center angle. Operation of the invention is not affected by location of peak pressure. Also, these curves may have a slight, predictable asymmetry due to engine design parameters such as wrist pin offset. The illustrations of FIG. 5, which are not to scale, cover an angular range during the compression power stroke of the piston in which all valves are closed. It is presently preferred to perform the method of the invention while the engine is cold motored by an external source 18 because changes in cylinder pressure (with all valves closed) may be related directly to changes in volume due to piston motion, as will be described.

With the spark plug 14 removed and the probe 20 inserted in place thereof, the engine is cold motored by the motor 18 and microwave signals are injected by transceiver electronics 62 and probe 20 into the cylinder volume or bore. As the piston reciprocates within the cylinder, the microwave signals exhibit a plurality of resonance peaks which are generally symmetrical around piston TDC. Process and control electronics is responsive to the microwave resonance signals directed thereto by transceiver electronics 62, and to the crankshaft rotation signals obtained from phase locked loop 70, for accurately identifying the crankshaft angle associated with piston TDC, i.e. .theta..sub.TDC. The process for locating .theta..sub.TDC is described in greater detail in the referenced Wilson patent, and reference is made thereto for a more detailed description of the preferred method for locating piston TDC in practice of the present invention. Although microwave-range radiation is specifically disclosed in Wilson and is presently preferred, it is envisioned that radiation in other frequency ranges, such as X-rays or ultrasonics, may be used in appropriate circumstances while applying the Wilson technique for identifying .theta..sub.TDC from the radiation return signal.

As a next step, three angles .theta..sub.1, .theta..sub.2 and .theta..sub.3 are selected at spaced intervals with respect to .theta..sub.TDC. In the illustration of FIG. 5, angles .theta..sub.1, .theta..sub.2, .theta..sub.3 and .theta..sub.TDC are at equally spaced intervals during the piston compression stroke. Cylinder pressure as indicated by probe 20 and pressure signal amplifier 60 is then measured at each angle .theta..sub.1, .theta..sub.2, .theta..sub.3 and .theta..sub.TDC to obtain corresponding pressure signals. P.sub.1, P.sub.2, P.sub.3 and P.sub.TDC which are stored in memory. It is presently considered preferable, although not necessary, to measure pressure on several successive compression strokes, and to utilize average pressures for further computations. As previously indicated, the changes in cylinder volume during the intervals between the selected angles .theta..sub.1, .theta..sub.2 and .theta..sub.3 may be accurately determined as a function of engine mechanical design parameters. These incremental displacement volumes V.sub.1-2 and V.sub.2-3 may be either precalculated for predetermined values of .theta..sub.1, .theta..sub.2 and .theta..sub.3 and stored in memory for later retrieval, or may be rapidly calculated in real time within process and control electronics 64 from design parameter operator inputs (not shown).

The constants .gamma. and k for the specific cylinder under test and the volume V.sub.1 at crankshaft angle .theta..sub.1 are then identified. Two techniques for finding .GAMMA., k and V.sub.1 are presently preferred. In the first technique, initial volume Vl and the constants Y and k are obtained by solution of the following three equations for at least two (Y and k) and preferably three unknowns:

These equations are readily solved using conventional microprocessor techniques for V.sub.1, .gamma. and k from measured pressures P.sub.1, P.sub.2 and P.sub.3 and from incremental displacement volumes Vl-2 and V.sub.2-3 determined on the basis of engine design parameters.

Alternatively, and in accordance with the second technique the constant .gamma. and the volume V.sub.1 are found using an iterative "graphic" technique. First, equations (2)-(4) above are rearranged as follows: ##EQU1## Knowing the approximate or nominal value of V.sub.1 (and .gamma.) from the mechanical design, progressively varying values of V.sub.1 (or .gamma.) around nominal are substituted into equations (5)-(7), again using the measured pressures P.sub.1, P.sub.2, P.sub.3 and the computed or precomputed incremental displacement volumes V.sub.1-2 and V.sub.2-3.

Solutions of these three equations for .gamma. for various values of V.sub.1 (or for V.sub.1 for various values of .gamma.) yield the three curves 84,86 and 88 illustrated in FIG..6. These three curves intersect at a point 90 which corresponds to the actual volume V.sub.1 and the actual constant Y associated with the test cylinder. With .gamma., P.sub.1 and V.sub.1 in hand, the constant k is given by the equation:

It will be appreciated that the iterative "graphic" technique may be readily performed using conventional microprocessor techniques without resorting to a manual "graphic" solution. Likewise, the point 90 of intersection may be found using only two of the three equations (5), (6) and (7). However, the use of three equations is preferred for enhanced accuracy over the full range of tolerances in engine design and the reliability tolerances of cylinder pressure measurement. That is, it may be expected under actual operating conditions that the three lines 84,86,88 do not intersect at a single point. Use of a third graph line helps to identify an "average" intersection point 90 with greater accuracy. Returning to FIG. 4, with the constants k and .gamma. identified and the pressure P.sub.TDC at piston top dead center measured per the above, the clearance volume V.sub.TDC at piston top dead center may be obtained per the equation: ##EQU2## Compression ratio C/R may then be obtained per the equation: ##EQU3## As a final step in the process of FIG. 4, clearance volume V.sub.TDC and compression ratio C/R may be displayed to the test system operator. Likewise, in analytical or research applications for example, other parameters such as .gamma. and k for the engine and cylinder under test may be displayed if desired.

One potentially useful modification to the embodiments and techniques of the invention hereinabove discussed involves substitution of V.sub.TDC for V.sub.1 as the arbitrarily selected starting volume, which would eliminate both the extra computation step of equation (9) and the necessity of measuring pressure P.sub.1 at angle .theta..sub.1. Thus, equations (2)-(4) may take the form:

where P.sub.2, P.sub.3 and P.sub.TDC are (average) pressures measured at .theta..sub.2, .theta..sub.3 and .theta..sub.TDC in FIG. 5, and V.sub.3-TDC and V.sub.2-3 are incremental displacement volumes calculated or precalculated as previously described. Note that these equations may be directly solved for V.sub.TDC, as well as for .gamma. and k. The iterative "graphic" technique of equations (5)-(7) and FIG. 6 may likewise be modified by substituting V.sub.TDC for V.sub.1.

Another modification of potential interest involves measurement of total displacement volume V.sub.DP rather than, or in addition to, calculation or precalculation thereof as previously described. In this modification, assuming that all valves are closed at piston bottom dead center, total displacement volume is measured as a function of the equation:

where V.sub.BDC is cylinder volume at piston bottom dead center. To obtain V.sub.BDC, the crankshaft angle associated with piston BDC is first obtained. In theory, this angle, .theta..sub.BDC, is equal to .theta..sub.TDC +180.degree. . In actual practice, however, factors such as wrist pin offset, etc. are such that 0 is slightly displaced from .theta.TDC+180.degree. . This displacement can be calculated from engine design parameters. When 8BDC has been identified, a corresponding (average) P.sub.BDC is obtained, V.sub.BDC is determined from the equation: ##EQU4##

Compression ratio may then be obtained as: ##EQU5## Comparison of the compression ratio measurement made as described above with a measurement obtained as previously described may be used to detect engine faults or compression ratio measurement error caused, for example, by unsealed piston rings.

It is also possible, within the scope of the invention, to instrument two different cylinders for microwave and pressure measurement, or indeed to instrument all cylinders for pressure but only one cylinder for microwave where compression ratio, clearance volume, .gamma. or k are to be measured at all cylinders, for example. In these cases, .theta..sub.TDC for each cylinder would be determined by adding an appropriate angular increment to the .theta..sub.TDC measured for the microwave-instrumented cylinder. In a similar manner, two different cylinders could be instrumented for microwave and pressure measurements when the use of specialized pressure transducers, such as a flush-mounted pressure transducer, is desirable. Such flush-mounting may improve accuracy in the measurement of pressure variations, and thus overall accuracy of cylinder parameter measurement, under some conditions.

Thus, the method of the invention to the extent thus far described contemplates identification of crankangle .theta..sub.TDC at piston top dead center. Most preferably, this is accomplished using the microwave techniques disclosed in the referenced Wilson patent. P.sub.a,P.sub.b,P.sub.c are measured at predetermined angles .theta..sub.a, .theta..sub.b and .theta..sub.c incrementally spaced from .theta..sub.TDC. Cylinder pressure P.sub.TDC is then measured at the TDC crankangle .theta..sub.TDC. (One of the positions .theta..sub.a, .theta..sub.b or .theta..sub.c may be .theta..sub.TDC. ) Most preferably, these incremental angles are equal, and the pressure measurements are taken when all valves are closed during the piston compression stroke. Incremental displacement volumes V.sub.a-b and V.sub.b-c are calculated or, more preferably, precalculated for the type of engine under test from nominal engine design parameters.

With the pressure readings P.sub.a,P.sub.b,P.sub.c and the displacement volumes V.sub.a-b,V.sub.b-c in hand, equation (1) may be expressed in the form of three equations, which may then be solved for the two unknown constants .gamma. and k, and also for the initial "reference" volume V.sub.a, which may be V.sub.TDC. Where V.sub.a is not V.sub.TDC, and knowing .gamma. and k for the cylinder under test, clearance volume V.sub.TDC may be determined using equation (1) and P.sub.TDC. A second technique for identifying the unknowns .gamma., k and V.sub.a is an iterative "graphic" solution of two dimensions to locate the point of "graphic" intersection. This point of intersection corresponds to the actual solution, and yields the desired unknowns. Most preferably, either technique is employed in real time using a suitably programmed conventional digital microprocessor.

Thus, one aspect of the method of the invention contemplates measurement of .gamma., k or clearance volume V.sub.TDC as previously described. Another aspect of the method contemplates determination of compression ratio (C/R) for the cylinder under test as a function of the fraction (V.sub.TDC +V.sub.DP)/V.sub.TDC where VDP is total piston displacement volume. Total displacement volume is obtained either as a calculated or precalculated function of engine design parameters, or by measuring the volume V.sub.BDC at piston bottom dead center if all valves are closed, in accordance with equation (15).

Probes conventionally employed for measuring cylinder pressure provide what may be envisioned as the varying or differential portion of the pressure signal. In applying the specific techniques heretofore discussed, this varying pressure signal must be added to a reference level to obtain an absolute pressure measurement P.sub.a, P.sub.b, etc. See Lancaster et al, "Measurement and Analysis of Engine Pressure Data", SAE Paper No. 750026 (1975) at page 5 or Brown, "Methods for Evaluating Requirements and Errors in Cylinder Pressure Measurement"SAE Paper No. 670008 (1967) at pages 66-67. Regardless of whether a given probe provides differential or absolute pressure measurements, the principles of the invention hereinbefore discussed may be implemented equally as well by employing only the pressure differential portion of the signal and discarding any absolute pressure information, thereby avoiding the need to measure a reference level or rely upon a precalibrated indication of reference level. More specifically, all absolute pressure measurements may be envisioned as the sum of an unknown reference level P.sub.o and a differential pressure .DELTA.P.sub.n, which may be the difference between the sinusoidal probe pressure signal at angle .theta..sub.n and the pressure signal at BDC following the intake stroke, for example. Pressure measurements are taken at four points .theta..sub.a, .theta..sub.b, .theta..sub.c, .theta..sub.d, yielding four pressure differential signals .DELTA.P.sub.a, .DELTA.P.sub.b, .DELTA.P.sub.c and .DELTA.P.sub.d. Using equation (1) to establish four independent equations analogous to equations (2) through (4), each of the unknowns .gamma., k, V.sub.1 and P.sub.o may be obtained employing either mathematical or iterative techniques analogous to those previously discussed.

It will be appreciated from the graph 82 in FIG. 5 that resolution of pressure versus crank angle is at a minimum at TDC. Moreover, as previously noted, the dynamics of gas behavior at or near TDC may cause cylinder behavior to depart somewhat from theoretical equation (1). Cylinder leakage, if any, would normally maximize around TDC. To overcome these and other difficulties, another important modification to the implementations of the invention heretofore discussed obtains improved accuracy by measurin V.sub.TDC without measurin P.sub.TDC.

FIG. 7 is a graphic illustration (not to scale) which relates piston volume V to various crank angles .theta..sub.n and complements FIG. 5 heretofore discussed. Volume V.sub.1 is first determined per the previous discussions by measuring pressures P.sub.1, P.sub.2, P.sub.3 at angles .theta..sub.1, .theta..sub.2, .theta..sub.3, determining differential volumes V.sub.1-2 and V.sub.2-3 from engine design geometry, and mathematically implementing equations (2)-(4) or iteratively implementing equations (5)-(7). Instead of employing equation (9) to find V.sub.TDC from P.sub.TDC, however, engine design geometry is again referenced to find differential volume V.sub.1-TDC between crank angles .theta..sub.1 and .theta..sub.TDC. Volume V.sub.TDC is then found as:

Constants k and .gamma. may be determined as before. In addition to avoiding difficulties associated with measuring P.sub.TDC, this aspect of the invention permits V.sub.TDC to be measured during actual engine operation, where P.sub.TDC would be greatly affected by combustion, by obtaining all necessary pressure data well ahead of TDC before combustion is initiated.

The foregoing aspect of the invention leads to yet another application for measuring or quantifying cylinder leakage during the compression stroke. That is, clearance volume V.sub.TDC and constants .gamma. and k are first determined using the method set forth immediately above which does not require measurement of P.sub.TDC. A theoretical or predicted pressure at TDC is then identified as a function of the equation: ##EQU6## Actual cylinder pressure P.sub.TDC is then measured at .theta..sub.TDC, and may be averaged over several cycles. The difference P'.sub.TDC -P.sub.TDC is thus a measure of pressure leakage, which may be due to improperly installed or defective piston rings, etc. It will be appreciated, of course, that this method of quantifying cylinder leakage is best carried out while cold motoring the engine. However, the principle of this method may also be applied during actual engine operation. Evaluation of predicted versus actual pressure is performed at a crankshaft position somewhat prior to start of combustion. Cylinder volume at this crankshaft position is determined as a function of volume V.sub.1 from equation (17) and engine design parameters. Regardless of which crankshaft position is used, pressure readings for determination of cylinder volume, k and .gamma. are taken at an early portion of the compression cycle after the intake valve has closed but before substantial compression (and possible leakage) has taken place.

Yet another application of the invention contemplates determination of the effective or equivalent angle of valve operation--i.e. the crankshaft angle during the intake/ compression cycle at which the intake valve closes, and/or the crank angle during the power/exhaust cycle at which the exhaust valve opens. Simply stated, this is accomplished by first determining k, .gamma. and V.sub.TDC per any of the foregoing techniques. Due to potential heat loss from the compressed gas to the surrounding structure, k and .gamma. are computed for the compression stroke and recomputed for the power stroke. (Of course, all pressure readings for determining Y, k and V.sub.TDC must be taken at crank angles where all valves are known to be closed.) V.sub.TDC and engine design geometry yield volume as a function of crank angle - i.e., V(.theta.). Pressure readings are then taken (during cold motoring) through the intake/compression and power/exhaust cycles, and are compared with predicted pressures per equation (1) and V(.theta.) to locate those angles .theta..sub.V at which equation (1) is no longer valid, which means that the cylinder is no longer a "closed" cylinder. During the power/exhaust cycle, this angle .theta..sub.V is the effective angle of exhaust valve opening. During the intake/compression cycle, this angle is the effective angle of intake valve closure.

Most conventionally, compression ratio is defined per equation (16) as a simple volume ratio. However, in much of the literature on two-stroke engines, compression ratio is defined with reference to cylinder volume at intake port closure rather than V.sub.BDC in an attempt to accommodate substantial piston "compression" travel with the intake port open. See Taylor, The Internal-Combustion Enqine in Theory and Practice, MIT Press, 2nd Ed. (1966), Vol. 1, p. 216. The same may be said for specialized four stroke engines wherein the intake valve remains open well after BDC. For such engines, effective compression ratio can be determined in accordance with yet another implementation of the invention.

Applying equations (1) and (16), compression ratio C/R can be expressed as: ##EQU7## However, in those applications where the angle .theta..sub.V of intake valve closure is well after BDC, effective compression ratio C/R' may be defined as: ##EQU8## Thus, after determining .gamma., k and .theta..sub.V per the foregoing discussion, volumes V.sub..theta..sbsb.V and V.sub.TDC may be identified and effective compression ratio C/R' identified per equation (20), or pressures P.sub.TDC and P.sub..theta..sbsb.V may be measured at TDC and .theta..sub.V respectively and effective compression ratio C/R determined per equation (21).

An analogous application of the invention to that discussed immediately above contemplates determination of effective intake pressure. This aspect of the invention finds particular utility with respect to supercharged engines where a compressor or the like is employed to raise intake pressure above ambient. Solving equation (19) for intake pressure at BDC: ##EQU9## Compression ratio (V.sub.BDC /V.sub.TDC), constant .gamma. and pressure P.sub.TDC may be determined or measured per the foregoing discussion, and equation (22) then solved for effective intake pressure. Alternatively, equation (22) may be expressed as: ##EQU10## where pressure P.sub.a and volume V.sub.a correspond to crankangle .theta..sub.a, and volume V.sub..theta..sbsb.V to measured or nominal intake valve closure angle .theta..sub.V.

The invention is applicable to the various aspects of engine research, design, production, diagnosis, operation and control. For example, in the production of engine heads, this invention may be employed to measure the volumes of the cavities that correspond to the individual cylinders. The engine head, with valve and spark plug holes sealed, may be affixed temporarily to a permanent fixture that resembles a cold motored engine without a head. This fixture may be constructed so as to allow adequate cylinder height for incorporation of a valve and a pressure sensor for each cylinder. During motoring of the reciprocating pistons within the cylinders of the fixture, the volumes of the individual cavities can be derived by use of the invention as described hereinabove. Machining operations may then be employed to modify these cavity volumes, if required. Another example of the application of the invention is found in the field of engine operation and control. In this application, the engine may be instrumented so as to provide continuous information about crankshaft position. Each cylinder may be instrumented with a pressure sensor, such as that described in Randall and Powell, "A Cylinder Pressure Sensor for Spark Advance Control and Knock Detection", SAE Paper No. 790139 (1979). The invention may then be employed to monitor such factors as cylinder volume change caused by either engine component wear or the accumulation of carbon deposits, and cylinder leakage caused, for example, by engine component wear. Information gained in this manner can be used by the engine control system to further optimize engine operation.

The invention also contemplates apparatus for determining cylinder thermodynamic constants k and .gamma., cylinder clearance volume, compression ratio, cylinder leakage, valve operation, effective compression ratio and/or effective intake pressure in accordance with the foregoing method, including in particular a dual microwave/pressure probe adapted to communicate with the cylinder bore.