Engine speed indicator

An engine rpm sensor utilizes a gas filled lamp for connection to a circu including an engine ignition system and a spark plug. A light pipe is exposed to the lamp and couples an optical signal each time a spark plug is fired to a remote location for utilization in a display or indicating device.

BACKGROUND OF THE INVENTION 
This invention relates generally to devices for sensing the speed of 
internal combustion engines. More specifically it relates to devices for 
sensing the revolutions per unit time of internal combustion engines. 
Internal combustion engine speed sensors or tachometers are, of course, 
well known in the art. To the applicant's knowledge there are two methods 
in use at this time for this purpose. One uses a capacitive pickup while 
the other uses an inductive pickup electrically or magnetically coupled to 
the engine ignition system. In addition, it has been proposed to use 
various optical systems. One such system is shown in U.S. Pat. No. 
3,704,405 which contemplates the provision of a perforated disk on a 
speedometer cable with a light source on one side of the disk and a 
photocell on the other side. Likewise, U.S. Pat. No. 3,348,143 suggests 
generally the use of magnetic, piezoelectric or optical pickup devices. 
Both capacitive and inductive pickups or sensors, are susceptible in one 
degree or another, to unwanted and spurious signals which are strongly 
prevalent in a multilinear internal combustion engine. 
These spurious signals in turn are passed on or conducted into the test 
instrument as unwanted signals. In addition, any pickup which is connected 
to a remote measuring or indicating device by the use of a cable 
conducting electrical signals encounters the problem that the connecting 
cable itself, since it is a capacitive device, will pick up spurious 
signals emanating from the high tension ignition cables of the engine. 
It is an object of this invention to provide a novel apparatus for sensing 
signals representative of engine rpm and coupling those signals to a 
remote location for utilization by indicating devices. 
It is a further object of this invention to provide a novel engine rpm 
pickup apparatus which is not sensitive to extraneous signals which might 
otherwise cause false indications. 
A further object is to provide a means of conducting these signals to an 
indicating instrument without this conductor itself becoming sensitive to 
spurious and extraneous signals and thus passing them on to the indicating 
instrument. 
A still further object of this invention is to provide a novel engine rpm 
indicating device which does not require any means for selecting the 
number of cylinders in the engine, and, therefore, is relatively simple 
and more economical to manufacture. 
A further object of the invention is to provide a novel engine rpm sensing 
apparatus which, because it uses a minimum of components, is less 
susceptible to temperature variations and is, therefore, more accurate 
over a wide range of temperatures. 
A further object of the invention is to provide a true current operated 
device which is not susceptible to capacitive coupling to other high 
voltage conductors prevalent in an internal combustion engine. 
BRIEF SUMMARY OF THE INVENTION 
Briefly, one aspect of the invention is the provision of a light producing 
element to be connected in a circuit between a high voltage producing 
element of an internal combustion engine ignition system and a spark plug 
of the engine, and means for coupling a signal produced by that device to 
an indicating means.

DETAILED DESCRIPTION 
Referring to FIG. 1, a preferred embodiment of the invention is constituted 
by a gas filled (neon, for instance) lamp 2 shunted by a resistor 4. As 
may be seen, when in use the lamp 2 is connected in series between the 
ignition or high voltage output of an ignition system 6 and a spark plug 
8. As is well known, the output of the ignition system is usually a high 
voltage cable connected from a distributor to a spark plug. For protection 
against dirt and to provide, if necessary, electromagnetic shielding, the 
lamp 2 and resistor 4 may be enclosed in a metallic can or housing 10, and 
leads 12 and 14 extend therefrom for connection as shown. Secured in the 
protecting housing 10, as illustrated by the line 16, is one end of a 
light pipe 18 so that its input end is exposed to any light produced by 
the firing of the neon bulb. By enclosing the light source and input end 
of the light pipe 18 is the housing 10 and a high percentage of the 
generated light impinges on it thereby increasing its strength of the 
transmitted optical signal. The light transmitted by the light pipe 18 is 
received by a phototransistor which in response to such light produces an 
electrical signal which may be utilized as described hereinafter. 
In the operation of the system the sensor is connected as shown, and each 
time a high voltage pulse is produced on the electrical cable from the 
engine distributor. When the spark plug fires, current will flow through 
the resistor 4 creating a voltage drop which will fire the neon lamp 2, 
and the light thereby produced will be conducted via the light pipe 18 to 
a phototransistor 20. It should be noted that in the typical internal 
combustion engine ignition system the firing voltages are relatively high; 
usually in excess of 5000 volts. There are neon lamps available which will 
fire or conduct in the 70 to 100 volt range, and, of course, when they 
conduct they offer practically negligible resistance to the flow of 
current, and consequently the voltage drop across them, either for firing 
or during conduction, is negligible when compared to the total voltage 
available. 
The electrical signal produced by the phototransistor may be utilized for a 
number of different purposes and for a number of different indicating 
devices. FIG. 2 illustrates generally one such device, namely, a 
tachometer. Here the output of the phototransistor is connected to a pulse 
shaping circuit 22 which is usually desirable as measuring instruments, 
and meters operate better and more consistently when their pulse inputs 
are uniform in shape. It may take any number of forms. The most convenient 
form would be that of a one shot multi-vibrator putting out a square wave 
in response to each output signal of the phototransistor 20. A meter 24 is 
provided which may be either of the analog or digital variety. Thus, if 
digital, it could count the outputs of the pulse shaper 22 in a given time 
interval and provide a visual indication of that number. If analog, the 
meter 24 will usually provide a DC signal proportional to the number of 
pulses per unit time and utilize a conventional meter movement driving a 
pointer showing the revolutions per unit time. 
By utilizing the proper circuitry this will operate with either a two or 
four cycle engine. 
As may be appreciated, an engine rpm sensor in accordance with the 
invention does not require any means for compensating for a variation in 
the number of cylinders in the engine inasmuch as each firing of a spark 
plug is sensed and processed as described. This eliminates the necessity 
for circuitry for such purposes resulting in a considerable 
simplification. 
FIG. 3 illustrates generally the manner in which an rpm sensor in 
accordance with the invention may be utilized with a timing light in order 
to adjust the ignition spark setting of an engine. In this case a pulse 
shaper 26 which may take the form of a differentiator in combination with 
an amplifier to produce sharp or steeply rising pulses is connected to the 
output of the phototransistor, and its output in turn is coupled by a 
transformer 28 to a conventional flash tube 30. A power supply 32 is 
connected to the pulse shaper circuit and the flash tube. In this 
arrangement the tube 30 flashes each time a spark plug is fired, providing 
a visual indication permitting adjustment of engine timing. 
If it is desired to use an oscilloscope presentation the output of the 
phototransistor is connected once again to a pulse shaper 34, and its 
ouput in turn is connected to a time base generating circuit 36 
constituting the input to the horizontal deflection circuit of an 
oscilloscope 38. The vertical deflection circuit of the oscilloscope is 
connected to a suitable pickup such as a capacitive pickup 40 which in 
turn is coupled to a high voltage lead from the ignition coil. 
FIG. 5 illustrates another embodiment of the invention wherein the sensor 
in accordance with the invention is connected in parallel with a spark 
plug 8 instead of in series as shown in FIG. 1. In this instance the neon 
lamp 2' is connected in series with a resistor 4' and, as before, mounted 
in a protective housing 10'. The light pipe 18 has its input or receiving 
end connected into the protector 10' as illustrated diagrammatically 16. 
Its output end is connected to a phototransistor 20 as before. The 
embodiment of FIG. 5 may be used with any one of the indicating devices 
shown in FIGS. 2, 3, or 4 in the same manner as the embodiment of FIG. 1. 
An important feature of the invention, and an important advantage gained 
thereby, is the utilization of the light pipe to couple the light output 
of the neon lamp to a remote location as, for instance, a portable or 
fixed instrument or other indicating device. However, it is believed that 
an aspect of the invention, namely, the use of a voltage responsive light 
generating device connected directly to the firing circuit of an 
individual engine spark plug may be utilized with certain advantages. 
Thus, in FIGS. 6 and 7 a neon lamp 2' may be connected in series or 
parallel, respectively, with a spark plug in the manner described 
previously. However, no light pipe is provided to couple the optical 
signal through a phototransistor, but rather a phototransistor 20' is 
included within a shielded protective housing 10' and is exposed directly 
to the light source. The output of the phototransistor is then coupled via 
a cable 44 through a desired measuring or indicating instrument. Obviously 
the advantages of light pipe coupling are not achieved in the apparatus 
disclosed in FIGS. 6 and 7. However, some of the remaining advantages of 
the invention are achieved. 
Modifications of the invention beyond those shown and described herein may 
be effected by those skilled in the art, and it is intended by the claims 
appended hereto to cover all such modifications as come within their 
scope.