Spark plug testing under dynamic load

A method of testing spark plugs in an engine is described. The engine is driven by its starter motor or other motor while ignition voltage is applied, the throttle is open and the fuel is cut off. This loads the spark plugs to reveal defects not otherwise apparent. Abnormally high or low plug peak voltages as well as abnormal arc durations are detected and counted and the counts are displayed. A microprocessor based measuring instrument coupled to capacitive pickups adjacent the spark plug wires and to the engine distributor evaluates the spark voltage and duration characteristics and displays the fault data.

FIELD OF THE INVENTION 
This invention relates to testing spark plugs and particularly to such 
testing under engine motoring conditions. 
BACKGROUND OF THE INVENTION 
It is desirable to test spark plugs for automotive engines as a quality 
control measure in spark plug manufacturing, to assure engine quality at 
the time of engine manufacture, and to diagnose engine problems on 
vehicles already in service. In each case it is helpful to run the test in 
a manner which resembles actual operating conditions without introducing 
unknown or difficult-to-control variables. 
In making a spark plug load test, the object is to detect any physical 
problems in the spark plug. It has long been recognized that spark plug 
tests must be run under load in order to reveal defects which cause 
problems under actual running conditions. Spark plug load refers to the 
electrical load or impedance that the spark plug presents to the high 
voltage ignition circuit. 
Spark plug performance is controlled by two basic conditions. One is the 
physical (electrical) condition of the spark plug gap. The second is the 
physical condition within the spark plug gap itself at the precise instant 
that the ignition voltage is applied. These conditions are as follows: 
Physical Condition of the Spark Plug Gap. 
1. Size of the gap. 
2. Condition of the electrodes: geometry and contamination. 
3. Resistance of the insulation. 
4. Cracks or deformities in the insulation. 
5. Open electrode path. 
Physical Conditions within the Spark Plug Gap. 
1. Gas pressure at the instant of ignition. This pressure is dependent upon 
ignition timing, engine load, compression ratio and engine RPM. 
2. The gas velocity passing through the spark plug gap at the instant of 
ignition. 
3. Engine fuel. The actual composition of the fuel being fed into the gap. 
4. The temperature of the gas. 
5. The fuel/ratio at the instant of ignition. 
In prior spark plug load testing it has been the practice to make 
measurements under actual engine running conditions with a constant engine 
load imposed by a dynamometer. To make the spark plug test as repeatable 
and therefore as valid as possible, it was necessary to control the 
condition of the fuel as much as possible. While it was possible to 
maintain a consistent fuel quality (in the manufacturing environment) the 
fuel/air mix and its velocity in the plug gap at the instant of applied 
ignition voltage always remained a problem. Because of those two 
variables, the spark plug load readings displayed a somewhat erratic 
pattern from reading to reading. To compensate for this lack of control, 
several sets of spark plug load readings were taken and then average prior 
to being displayed by the measuring instrument. 
Some other variable conditions can be controlled during testing. These are 
engine speed, timing and torque. By keeping these three conditions as 
constant as possible and by using an averaged spark plug load, an 
acceptable level of test result validity can be achieved. Still, this test 
has limited application since the expense of a dynamometer precludes its 
use in many places where spark plug testing is desired in the 
manufacturing process. 
Moreover, the prior test is wholly impractical for vehicle servicing at 
local garages. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method of testing 
spark plugs in an engine under dynamic load and requiring no dynamometer 
or other very expensive equipment. 
It is another object of the invention to provide such a method which has 
few sensitive control variables and which produces accurate results. 
It is another object of the invention to provide a method of testing spark 
plug load in engines installed on vehicles. 
The invention is carried out by a method of testing spark plugs under load 
while installed in an engine comprising the steps of; opening the throttle 
of the engine to wide open position, applying ignition voltage to the 
spark plugs, driving the engine by an external power source without 
applying fuel to the engine, sensing the voltage and/or spark duration 
developed in the spark plugs, and evaluating the voltage level and/or arc 
duration of each spark plug and comparing it to acceptable standards.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Motoring an engine to test spark plugs provides an easily implemented spark 
plug stress technique as an alternative to an engine running under its own 
power and operating under load. Such a simulation facilitates spark plug 
load tests without the use of an engine load absorption device in hot test 
stands as well as in a vehicle. 
As shown in FIG. 1, an engine 10 has standard equipment including a 
throttle 12 in the induction passage, an ignition system 14 which includes 
spark plugs 16 and a distributor 18, and a starter 20. The fuel system is 
not shown. 
Instrumentation for carrying out the method of the invention is not new, 
per se, since the basic components have been used with the prior art 
method. The special test equipment requires an instrument 22 for analyzing 
the spark voltage signals and giving an output indication of defective 
operation. The preferred instrument for this purpose is a Motorola 6800 
microprocessor based measuring and control device commerically available 
from Balance Engineering Corp., Troy, Mich., as CBI-55. The unit is 
equipped with an input board with circuits to sample and hold the peak 
voltage of each spark pulse and to perform an analog-to-digital conversion 
for inputting to the digital circuits of the CBI unit. 
Several inputs are coupled to the CBI unit 22. A capacitive pickup 24 for 
sensing the spark plug voltage comprises a metal ring or a plastic ring 
with an embedded conductor which fits atop the distributor 26 adjacent the 
spark plug wires 28. The pickup 24 develops a signal corresponding in 
magnitude to each spark voltage pulse. A second pickup 26 is attached to 
the number one spark plug wire to obtain a signal indicating when that 
spark plug fires to allow the subsequent spark signals to be correlated to 
specific spark plugs. A tachometer signal produced by the distributor 18 
is coupled to the CBI unit via conductor 30. The tachometer signal 
contains information defining the duration of each spark pulse. 
FIG. 2 depicts the CBI circuit for processing the signal from the pickup 
24. The capacitive pickup 24 adjacent the boot of each spark plug wire 28 
is shown schematically as a separate pickup for each wire. A single 
conductor 32 leads to a voltage divider or scaling circuit 34 which is 
coupled to a peak hold circuit 36 on the input board to change the 
momentary high voltage pulse 32' to a wider signal 34' of the same 
amplitude. An A/D converter 38 supplies an equivalent value to the 
microprocessor circuit which detects the faults, counts the faults and 
normalizes the the count value, and alternately displays the normalized 
high and low fault count values. 
FIG. 3 depicts the spark duration processing circuit. The line 30 carries 
the tachometer signal from the distributor 18 to the input board of the 
CBI 22. That board provides a scaling circuit 40 and a spark duration 
processor 42 incorporating a flip-flop which responds to the tachometer 
signal 30' to produce a square wave pulse 42' having a width equal to the 
pulse duration. A programmable timer 44 digitizes the pulse width and 
furnishes that digital signal to the digital circuitry of the CBI unit 22 
which detects duration faults, counts the faults and normalizes the count 
value, and alternately displays the normalized high and low fault values. 
As shown in FIG. 4, the display of the CBI unit 22 separately displays the 
peak voltage faults and the duration faults. Left and right groups of 
indicators 50 and 52 give voltage fault and duration fault information 
respectively. Each group of the display has a single digit numeric 
indicator 56 for each cylinder and is divided into two banks. The right 
bank displays (for a V-8 engine) cylinders 1, 3, 5 and 7 while the left 
bank displays the remainder. In each group a central pair of LED's 54 
indicate whether high or low indications are currently on display. 
The CBI unit is programmed to continually switch between the high and low 
impedance readings. The high peak voltage fault count and the short 
duration fault count are shown simultaneously and after a few seconds, say 
about four seconds, the display changes to show the low peak voltage fault 
count and the long duration fault count. In this way the high and low 
impedance gaps are segregated and the correlating high or low count rates 
are displayed simultaneously. 
In operation, the engine is motored or driven externally in its normal 
rotation direction. The driving source can be any device such as a 
dynamometer, test stand starter or the engine starter. When the engine 
starter is used, the rotation speed is usually about 200 RPM. This is an 
excellent speed for the test since it yields the correct range of spark 
plug loading when the test is set up according to the following 
description. 
The air intake throttle valve is placed in its full open position. This 
position is preferred because it can be easily repeated as a test 
condition. The engine fuel is turned off and the ignition system is turned 
on. The ignition power supply or battery should be held at the normal 12 
volt level. When the engine is motored under these conditions the cylinder 
air pressure can be very high and the spark gap impedance will be 
correspondingly high and sufficient to test the spark plug. That is, the 
spark plug gap breakdown or ionization voltage will be high at high 
pressures thus allowing the spark plug voltage to become sufficiently high 
to reveal spark plug defects. If, on the other hand, the voltage becomes 
too high, then arcing externally of the engine such as between the spark 
plug wires and the engine block, can occur. This has been observed at high 
engine speeds when high cylinder pressures were obtained and the ignition 
voltage was applied near the peak of the pressure. This condition should 
be avoided by running the engine at a slower speed or selecting ignition 
timing so that the spark plugs will fire when the spark plug impedance is 
low enough to avoid the external arcing. The test conditions (throttle 
setting, RPM and timing) should be repeatable since they affect the peak 
spark plug ionization voltage and the spark plug arc duration time. These 
are the two parameters which are monitored. 
Motoring under the above described conditions simulates a loaded engine 
running under its own power in that the spark plug impedance is raised to 
a similar or higher value than the spark plug impedance of a loaded 
engine. Comparison tests have shown that a given engine when run with fuel 
at 800 RPM with a load of 150 ft.lbs. applied by a dynamometer yielded an 
ignition voltage level of 18 KV. The same engine motored in accordance 
with the method of the invention at 220 RPM yielded an ignition voltage 
level of 20 KV. Such testing will produce greater repeatability of spark 
plug arc conditions than an engine run under load. One reason for this is 
that the low RPM operation will produce lower air or gas swirl through the 
spark plug gap. 
The CBI unit 22 receives the spark plug voltage signals and the tachometer 
signal and makes a comparison with preset peak voltage and arc duration 
values or value ranges. For a given air pressure in the gap, the 
ionization voltage of a good spark plug will fall within a range and any 
events above or below that range are counted and displayed. Generally, a 
spark plug which exhibits a high peak voltage has a high impedance and 
will thus impose a short time constant on the ignition circuit. The arc 
duration for such a spark plug will be relatively short. Similarly, a low 
peak voltage usually corresponds to a long arc duration. The CBI unit 22 
calculates the duration from the tachometer signal on line 30, compares 
that to a preset range, and indicates events above or below that range as 
faults. While the display of both duration and voltage faults may seem to 
be redundant, practical experience has shown that in some cases one will 
reveal a defective spark plug better than the other. There is another 
advantage not related to spark plug condition but which is important to 
the test. If the test setup has been properly calibrated, a high number of 
high voltage faults will be accompanied by a high number of short duration 
faults. If that correlation does not occur, the system should be 
recalibrated. Another possible cause of poor correlation is a result of 
defective spark plug wires or bad spark plug wire connections. Thus, in a 
sense, the system is self-diagnostic since the displayed indications 
reveal the problems which might cause misleading spark plug defect 
indications. 
Calibration of the test system requires operating the test at a desired 
engine RPM and timing angle with known good spark plugs and adjusting the 
acceptable ranges on the CBI unit to the smallest ranges which yield no 
defect indications. 
The method of counting faults is as follows: 
1. The voltage of an individual spark is measured. 
2. The measured voltage is compared with the preset acceptable voltage 
range. 
3. The CBI unit internally records the measurement as an acceptable, high 
or low reading. High and low readings are labeled as faults. 
4. The CBI unit accumulates the fault counts occurring in a selected set of 
consecutive voltage measurements for each spark plug and displays the 
results on a scale of 0 to 9. Depending upon engine RPM and display time, 
a convenient set to avoid missing and faults would be 9 measurements for a 
speed up to 260 RPM, 18 measurements for a range of 260 RPM to 520 RPM, or 
36 measurements for a range of 520 RPM to 1040 RPM. For the purpose of 
normalizing the results for display on the same scale, the fault counts 
for sets of 18 and 36 are divided by 2 and 4, respectively. 
5. If all measurements are acceptable the CBI unit will display a fault 
count of zero. If any faults exist, the total or normalized number of each 
type of fault (high or low) is then displayed as the high or low fault 
count for the respective cylinder. 
The engine must be motored a few seconds to acquire the required number of 
counts. The method of the invention provides such good repeatability, 
however, that the test period can be reduced to accumulate only a set of 9 
or at most 18 measurements, thus dispensing with any long term averaging 
and minimizing the motoring time. 
It will thus be seen that the method of the invention greatly facilitates 
engine testing of spark plugs with a speed, simplicity and accuracy not 
previously achieved, yet requiring no expensive loading device. This 
facilitates accurate, inexpensive spark plug testing in manufacturing 
facilities as well as in vehicle service garages.