Engine analyzers

An engine analyzer, to analyze a parameter of a spark ignition internal combustion engine, has a separate probe for connection to each spark plug or lead feeding only that spark plug, so that engines in which ignition pulses pass to the plugs only along separate leads, can be analyzed.

This invention relates to engine analysers for spark ignition internal 
combustion engines. 
According to the invention there is provided an engine analyser including a 
purality of probes each adapted to derive an input signal for the analyser 
from a different spark plug of the engine or lead feeding that spark plug 
only, and in which there is provided at least one probe for each of said 
spark plugs or spark plug leads. 
Preferably there is provided an engine analyser in which the signal from 
the or each spark plug or lead is displayed on a display means and in 
which a synchronising signal for synchronising the display means to the 
engine operating cycle is obtained from a spark plug or spark plug lead. 
Conveniently for each spark plug or lead, one probe is a capacitive probe. 
According to a feature of the invention in the engine analyser one or more 
probes will emit a succession of pulses during use, some or all of the 
pulses being of either one polarity or of the opposite polarity and 
including means to process said pulses and to emit coincident pulses all 
of a predetermined polarity, irrespective of the polarity of the pulses 
from the probe. 
Preferably each probe feeds a positive peak store and a negative peak store 
connected in parallel and connected to feed a common input of a comparator 
of selected polarity, the output of the comparator being connected to the 
input of an invertor arranged to invert pulses passing through it only 
when fed by a pulse from the comparator, and a feed to the invertor from 
the probe in parallel with the peak store, whereby the invertor will pass 
pulses from the probe which are of opposite polarity to said selected 
polarity and will invert pulses from the probe which are of said selected 
polarity so that all the pulses emitted by the invertor will be of 
polarity opposite to said selected polarity. 
Conveniently comparator means are arranged to compare the magnitude of any 
pulses received simultaneously from the probes and to transmit only the 
pulse of greatest magnitude at that time. 
The pulses from the probes may be converted to the same predetermined 
polarity and fed through a parallel array of diodes, one for each probe, 
to a common point, whereby the diode passing the pulse of greatest 
magnitude will inhibit simultaneous conduction of the other diodes. 
According to another feature of the invention the pulse from one probe may 
be used as a timing or display initiation pulse. 
Preferably rejection means are arranged to inhibit passage of those pulses 
from said one probe which do not reach a predetermined magnitude. 
The predetermined magnitude may be a predetermined percentage of the 
magnitude of and less than the previous pulse. 
Said one probe may be an inductive probe.

FIG. 1 shows, in diagramatic form, part of the ignition system of a typical 
4-cylinder spark ignition engine. Current through the low-voltage winding 
(10) of the ignition coil (11) is interrupted at appropriate times by a 
contact-breaker (12) to induce high-voltage ignition pulses in the 
secondary winding (13) of the coil (11). These pulses are conducted by a 
"King" lead (14) to a distributor (15), in known way. Each of the 
cylinders of the engine is fitted with a sparking plug (16), (17), (18), 
(19), through leads (20), (21), (22), (23). 
By inspecting, comparing and possibly utilising the high-voltage pulses fed 
in turn to the sparking plugs (16), (17), (18), (19), the condition of the 
ignition system and of the rest of the engine can be deduced. FIG. 2 shows 
a typical "parade" of the voltages applied to the sparking plugs, in which 
voltage is shown vertically against time horizontally, typically on a 
cathode ray oscilloscope. Often, a comparison of the relative heights of 
the voltage pulses is more important than their absolute values. 
To achieve these known results it is known to reproduce the stream of 
pulses passing along the King lead (14), by means of a capacitive probe 
(24), clipped on the King lead (14). To synchronize the trace shown in 
FIG. 2, so that the first pulse is always, say, from No. 1 cylinder, it is 
known to take a synchronizing pulse from the lead (23) by means of another 
capacitive or inductive probe (25). 
However, the type of ignition system described so far is not always used 
and FIG. 3 shows, again in diagramatic form, part of the ignition system 
commonly used on 2-cylinder engines, for example in some motor cars and on 
motor cycles. For engines having 4 or 6 cylinders, the circuit of FIG. 3 
is duplicated or triplicated. For engines having more cylinders one 
circuit is added for each extra pair of cylinders. Although FIG. 3 shows 
the coil (11) and contact-breaker (12), there is no distributor (15). The 
secondary winding (13) is connected at each end with a high-voltage lead 
(30), (31), leading to respective sparking plugs (32), (33). Whereas in 
FIG. 1 the contact-breaker (12) operates normally at half engine rotation 
speed, in FIG. 3 it operates at engine rotation speed, so that each time a 
spark is produced at the correct time for ignition of a charge in one of 
the cylinders there will also be produced, at the same instant, a spark in 
the other cylinder at the end of the exhaust stroke and beginning of the 
induction stroke, on a 4-stroke cycle engine. The spark required for 
ignition is designated the "real" spark and the coincident spark in the 
other cylinder is called the "wasted" spark. It will be seen that the 
pulses in lead (31) will always be positive relative to earth and the 
pulses in lead (30) will always be negative relative to earth, or vice 
versa, depending on the polarity of the lead (34). 
It will be seen that in this type of ignition system there is no King lead 
(14) along which all the ignition pulses pass. Therefore, it is necessary 
to affix a separate capacitive probe to each lead (30), (31), and in some 
way to combine the signals from both leads (30), (31), to generate a 
parade of the type shown in FIG. 2. 
FIG. 4 shows a plot of voltage against time of the voltage in lead (31). It 
will be seen that due to the lower pressure in the cylinder during the 
wasted spark, the peak voltages of the wasted sparks (35), are lower than 
those of the real sparks (36). 
FIG. 5 shows the voltage pattern in the lead (30), starting at the same 
time as FIG. 4. It will be seen that the pulses are all negative relative 
to earth in FIG. 5, whereas in FIG. 4 they are all positive. It will also 
be seen that in FIG. 5 each wasted spark coincides with a real spark of 
FIG. 4. 
The first requirement is to invert the pulses of either FIG. 4 or FIG. 5, 
so that they can all be displayed in the same sense and thus be directly 
compared, as in FIG. 2. As mentioned above, for each lead of the 2 or more 
cylinders there is provided a preferably capacitive probe (40), in FIG. 6. 
For each probe (40) there is provided a circuit of the type shown in FIG. 
6, comprising a buffer amplifier (41), feeding a positive peak store (42) 
and a negative peak store (43), arranged in parallel. The outputs of the 
peak stores (42), (43), combine through resistors (44), (45), to feed the 
positive input of a comparator (46), which is arranged to emit a signal 
only when the input peaks are positive. 
The output signal from the buffer amplifer (41) feeds, through a scaling 
potentiometer (47), to the input of an inverting amplifer (48), which only 
inverts the pulses when it is fed by a signal from the comparator (46). 
Therefore, when the pulses are negative the comparator (46) emits no 
signal, so that the pulses pass straight through the invertor (48). On the 
other hand, when the pulses are positive, the comparator (46) will emit a 
signal which will cause the invertor (48) to invert the positive input 
pulses and emit negative output pulses. 
It will be seen that the outputs of all the invertors (48) will always be 
negative pulses, which could be super-imposed on a time base to give a 
parade of the pulses to all the cylinders, as in FIG. 2. However, if the 
pulses in FIG. 4 are inverted and super-imposed on the pulses of FIG. 5, 
the pulse of each real spark will be added to the pulse of a wasted spark 
at the same time. FIG. 7 shows a circuit for eliminating the wasted spark 
pulses, so that only the real spark pulses constitute the parade. In FIG. 
7 each output (49) from each invertor (48) feeds through a diode (50), 
before joining at the input of a further amplifer (51). Thus, a real spark 
pulse passing through one of the diodes (50) will inhibit conduction of 
any diode (50) which is fed with a wasted spark pulse. The amplifer (51) 
will, therefore, only emit the real spark pulses, to constitute the 
vertical voltages of the parade. 
The horizontal sweep voltages are generated within the cathode ray 
equipment, in the usual way, but are triggered from No. 1 cylinder. For 
synchronization purposes the wave form of the current in the lead (31) is 
more reliable than the voltage wave form. Therefore, whereas a capacitive 
probe is generally used to derive the voltage pattern, an extra probe (53) 
in FIG. 8, of inductive type, is attached to the No. 1 cylinder lead (31). 
The pulses from the probe (53) are fed through a buffer amplifer (54) and 
through a bi-stable device (55). The positive edge of the bi-stable output 
is used to initiate the horizontal sweep. 
FIG. 9 shows that without a synchronising pulse applied to the control 
input (56) of the bi-stable device (55), two timing sequences are 
possible, one with the trace synchronised to the real spark pulses (57) 
and the other synchronised to the wasted spark pulses (58). 
FIG. 10 shows a synchronisation circuit which is fed from the capacitive 
probe (40) on the lead to No. 1 sparking plug only. The amplifer (41) and 
peak stores (42) and (43), are those used in FIG. 6. Further outputs from 
the stores (42), (43) are fed through voltage-dividing resistors, as shown 
in FIG. 10, to feed 2 further comparators (60), (61). The output signal 
from the comparator (46) is fed along line (62) to the comparator (61), 
and also through an invertor (63) to the comparator (60). The signal in 
line (62) is used to inhibit the comparator (61), when the input signal is 
positive. The inverted signal from invertor (63) is used to inhibit the 
comparator (60) when the input signal is negative. 
If the signal from the probe (40) comprises a train of positive pulses, as 
in FIG. 4, the positive peak detector (42) will hold to the maximum value 
of the real spark. 80% of this value is applied to the comparator (60) as 
a reference. Thus only real spark signals will give synchronisation pulses 
along line (56), since wasted spark voltages will generally be below the 
reference amplitude. The actual value of the percentage selected depends 
on the relative values of the real and wasted sparks. 
The real spark synchronizing pulses can be used for other purposes e.g. 
measuring spark advance angle by comparison with a crankshaft position 
generated pulse. 
At low engine speeds, i.e. less than 2,000 r.p.m., the real spark is 
significantly greater than the wasted spark. Above this speed real and 
wasted sparks may become of comparable amplitude under certain engine 
conditions. 
To ensure that this is not a problem the synchronisation circuit is only 
allowed to operate below engine speeds of 2,000 r.p.m., for example by 
using a tachometer range change relay (unshown) as a switch. 
The analyser described above may be adapted for use on distributorless 
engine ignition equipment, such as certain transistorised types, where the 
circuit is enclosed in a box of which the only output comprises the leads 
to the spark plugs, i.e. there is no single output lead along which all of 
the ignitition pulses pass. 
Although the invention has been described with reference to a four stroke 
engine it is equally useful for analysing the performance of 
multi-cylinder two stroke cycle engines. It is particularly useful for 
analysing the kind in which an ignition spark occurs twice at each spark 
plug at each revolution of the engine. In each cylinder one of the sparks 
occurs near top dead centre and the next spark occurs near bottom dead 
centre, so that the latter alternate sparks occur near the end of the 
power stroke. The analyser described above for a four stroke cycle engine 
is thus able to display all of the voltage signals intended to ignite 
mixture in the cylinders but not to display voltage signals associated 
with the said alternate sparks, which occur near the end of the power 
strokes. 
Instead of displaying the magnitude of the voltage pulses on a cathode ray 
oscillosope, the information can be presented in any other appropriate 
manner, for example on one or more meters, a visual display unit, 
histogram display, digital display, etc. 
The construction of the capacitive probe (24), inductive probe (25), buffer 
amplifier (41), peak stores (42), (43), comparators (46), (60) and (61), 
invertors (48) and (63), diode (50), amplifier (51), bi-stable device (55) 
are all well known to those skilled in the art and need not be described 
further. 
Instead of displaying the ignition voltage traces of two or more cylinders 
at the same time on the oscilloscope it is occasionally required to 
display the whole voltage trace of a selected cylinder over the whole of 
its working cycle and then to be able to switch the analyser to display 
whole voltage trace of a different cylinder for comparison. The analyser 
described above enables such a mode of working with the advantage that all 
of the successive displays are presented in the same polarity. 
Although the use of the extra probe (53) has been described as providing 
more reliable synchronization, in some circumstances the signal from one 
of the capacitive probes (40), or from an extra capacitive probe (53) can 
be utilised.