US3035224A - Spark plug tester - Google Patents

Description

May 15, 1962 J. A. WHALEY ETAL 3,035,224

SPARK PLUG TESTER Filed Nov. 27, 1956 Z/ i? i 1% ff {i W iffy /a" j? 33 AW g 52' I! I 7 I g 7 IN VEN TORS A TTORNEY United States Patent Ofihce 3,035,224 Patented May 15, 1962 3,035,224 SPARK PLUG TESTER John A. Whaley and Willard R. Houser, Flint, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Nov. 27, 1956, Ser. No. 624,558 1 Claim. (Cl. 32416) This invention relates to apparatus for ascertaining conditions of a spark plug in a combustion chamber and more particularly for testing spark plugs of internal combustion engines for pre-ignition, spark plug fouling, and the like.

In studies of the operation of internal combustion engines, for servicing or evaluation, the duration or time period of the combustion in the operating cycle is a characteristic of particular conditions in the combustion chamber. For example, in operation under conditions of normal ignition combustion occurs late in the compression stroke as determined by the timing of the spark ignition. Under abnormal ignition conditions such as pre-ignition, combustion is caused to occur earlier in the compression stroke and the duration of combustion is correspondingly increased. 'lhere are, of course, other abnormal conditions in the combustion chamber, such as spark plug fouling, which are induced by various causes and result in disturbance or change of the combustion duration.

It has been found that the electrical resistance between spaced points in a combustion chamber is related, not only to the condition of the spark plug but also, to conditions attendant with combustion which cause a marked decrease of resistance. In accordance with this invention ameasure of combustion duration is obtained by sens ing' the time average value of electrical resistance between spaced points in the combustion chamber. Thus, a change of ignition conditions may be detected by the change of resistance. This is accomplished during engine operation by a meter circuit connected across spaced electrodes, suitably the spark plug electrodes, which develop an indication of the time average value of resistance therebetween. The meter circuit, including a source of voltage, preferably comprises means for blocking the ignition voltage from the meter circuit.

A more complete understanding of the invention may be had from the detailed description which follows taken with the accompanying drawing in which the single FIG- URE is a schematic diagram of the inventive system.

Referring now to the drawing, there is shown an illustrative embodiment of the invention in a pre-ignition meter for a spark ignited internal combustion engine. The system, in general, comprises an engine including a cylinder 11 provided with a conventional spark plug 12 to which is connected the usual ignition circuit 14 and the meter circuit 16.

The engine cylinder defines a combustion chamber 18 into which extends the spark plug 12 for developing the igniting spark in the usual manner. The spark plug 12 has an electrode 20 connected through the spark plug shell to ground and a center electrode 22 insulated from the shell and extending to the terminal 23. The ignition circuit 14 is suitably of conventional design and includes the vehicle battery 24 having one terminal connected to ground and the other terminal extending through the igni tion switch 26 to the ignition coil or transformer 30. The igmtion circuit extends through the primary winding 28 of the coil 30 to the movable contact of the periodic switch or breaker points 32, the other contact of which is connected to ground. The breaker points 32 are opened and closed periodically by the cam 34 which is driven synchronously with the engine. A condenser 36 is connected across and aflords protection for the breaker points 32 in the usual manner. The secondary winding 38 of the ignition coil 30 has one terminal connected to a terminal of the primary winding and its other terminal connected to the rotor contact 40 of a distributor 42. The fixed contact 44 of the distributor corresponding to the cylinder 11 is connected to the terminal 23 of spark plug 12. The distributor rotor contact 40 is driven in timed relation with the cam 34.

The meter circuit 16 is connected across the spark plug electrodes 20 and 22 and comprises in series connection a voltage source or battery 46, a meter 48, a blocking diode 50 and a current limiting resistor 60. The meter 48 is suitably a conventional moving coil type micro-ammeter. The diode 50 is suitably of the high vacuum type and includes a plate electrode 58 and a thermally emissive cathode 52 provided with a heater circuit including a voltage source 54 and a switch 56. The battery 46 has one terminal connected to ground and its other terminal connected through the meter 48 to the cathode 52 of the diode and the plate of the diode is connected through the resistor 60 to the terminal 23 of the spark plug. It is to be noted that the orientation of the diode 50 in the meter circuit is such that it will conduct meter circuit current from the battery 46 but will block any current flow in the meter circuit due to the ignition voltage. For this purpose the polarities of the diode 50 and the battery 46 are of the same sense and are in a sense opposite that of the ignition voltage appearing across the spark plug elec trodes 20 and 22. In the illustrative example, the battery 24 has its negative terminal connected to ground and the ignition coil 30 is arranged to develop an ignition volt- I age which is negative with respect to ground. Accor ingly, the diode 50 is oriented with its plate receiving the negative ignition voltage and the battery 46 has its positive terminal connected to ground to apply a negative voltage to the cathode of the diode.

In operation of the inventive system, with the ignition switch 26 closed and the engine running, the ignition Volt-- age is applied to the spark plug 12 once each cycle late in the compression stroke of the piston in cylinder 10. This is accomplished in the usual manner by the alternate closing and opening of the breaker points 32 which energizes and dc-energizes the primary winding 28 to cause a high voltage to be induced in the secondary Winding 36 and applied through the distributor 42 to the terminal 23 of the spark plug 12 by the synchronized rotation of the cam 34 and the distributor rotor 40 by the engine. This ignition voltage, of negative polarity, is also applied through the resistor 60 to the plate 58 of the blocking diode 50. Due to the relative polarity of the ignition volt-age and the diode 50, no current is caused to how in the meter circuit 16.

When the engine is operating under normal ignition conditions, the meter circuit 16 is conditioned for operation by closing the switch 56 in the heater circuit to render the cathode '52 emissive. Accordingly, the battery 46 will cause a current to flow in the meter circuit from the positive terminal of the battery through ground to the electrode 20 and thence across the spark plug gap to the electrode 22. The circuit is completed from the spark plug terminal 23 through the resistor 60, the diode 50, and the meter 48 to the negative terminal of the battery. The spark plug gap between electrodes 20 and 22, in air or a gaseous dielectric at low temperature, presents a very high electrical resistance. However when combustion occurs, accompanied by a flame front and very high temper-atures', the resistance of the gap is substantially reduced. Accordingly, an appreciable amount of current may flow in the meter circuit under the influence of the battery 46. Under normal conditions of ignition; that is, when combustion does not precede the occurrence of the ignition voltage, the meter circuit current has a value dependent upon the magnitude of the voltage in the meter circuit and the resistance presented by the spark plug gap. As a typical value, the voltage of battery 46 may be 270 volts. The meter 48 responds to a time average value of the current flow in the meter circuit and averages the high value of current during combustion with the low value of current during the remainder of the cycle. A typical range of values which have been found to correspond to normal ignition conditions is of the order of to 25 micro-amperes.

When the combustion conditions change in the cylinder, as by the occurrence of pre-ignition, the flame front and high temperature occurs earlier in the compression stroke. Accordingly, the resistance of the spark plug gap assumes a low value for a longer interval during each operating cycle. Thus the period of high current fiow in the meter circuit is increased and the time average value, as indicated by the meter 48, is abruptly increased. A typical range of current values in the meter circuit corresponding to pre-ignition conditions is of the order of 50 to 100 micro-amperes. Thus, the occurrence of p-re-ignition or the increased duration of combustion during the engine cycle may be readily detected by the abrupt change of the average value of current flow in the meter circuit.

Although the description of this invention has been given with respect to a particular embodiment, it is not .to be construed in a limiting sense. Many variations and modifications within the spirit and scope of the invention will now occur to those skilled in the art. For a definition of the invention reference is made to the appended claim.

We claim:

-Apparatus for detecting preignition and like conditions in operation of a spark ignited combustion engine comprising, an internal combustion engine with a spark plug having a pair of electrodes disposed in a combustion chamber, an engine ignition circuit connected across the electrodes of the spark plug to apply a sparking voltage of a given polarity across the electrodes, a measuring circuit comprising a first direct voltage source, an average current indicating device, a current limiting resistor and a unidirectional conductive device connected in series across the electrodes of the spark plug, the unidirectional conductive device being connected with a polarity opposite the sparking voltage to block current flow from the ignition system through the current indicating device, the unidirectional conductive device being connected with the same polarity as said first direct voltage source whereby said first direct voltage source causes a low value of current flow between the spark plug electrodes when the resistance therebetween is of high value prior to combustion in the chamber and causes a high value of current flow between the spark plug electrodes during combustion in the chamber when the resistance therebetween is of low value so that the time average value of the currents, as indicated by the current indicating device, increases when the resistance between the electrodes is decreased, by preignition or like conditions, prior to the occurrence of the sparking voltage from the ignition circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,254,080 McCarty Aug. 26, 1941 2,254,175 Eltgroth Aug. 26, 1941 2,324,458 Peters July 13, 1943 2,425,321 Horton Aug. 12, 1947 2,543,141 Vichnievsky Feb. 27, 1951 2,648,818 Cohen Aug. 11, 1953 2,695,987 McCollom et al. Nov. 30, 1954 2,762,977 Krueger Sept. 11, 1956 2,774,063 Penn Dec. 11, 1956 2,820,945 Marsden Jan. 21, 1958 2,888,822 Burhans June 2, 1959 2,908,859 Moehring Oct. 13, 1959 OTHER REFERENCES Article: Note Techniques No. 27, R. Vichnievsky; Methodes de Determination de Lintensite de la Detonation; G.R.A.; Paris 3, Rue Leon-Bonnat; 1946.

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