Tachometer suitable for use in engine maintenance

Pressure changes in the engine case of an automobile are sensed to compute and display the engine rpm irrespective of the types of engines. A pressure sensing element is mounted on the oil filler port in the engine cylinder head cover by means of a mounting member made of an elastic material.

BACKGROUND OF THE INVENTION 
The present invention relates to engine tachometers, and more particularly 
to a tachometer which is designed to measure the speed in revolutions per 
minute of a Diesel engine during the period of idle adjustment. 
A known type of idle adjusting tachometer is designed so that as will be 
the case with gasoline engine tachometers, the primary or secondary 
voltage of the ignition coil is detected and the engine rpm is measured 
from the detection signal. However, and this prior art device has the 
disadvantage of being not suitable for use in vehicles equipped with 
Diesel engines. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a tachometer suitable 
for engine adjusting purposes, which is adapted for installation on all 
types of engines including Diesel engines and gasoline engines whereby the 
engine rpm can be seen directly at the location of the tachometer and thus 
the driver can adjust the engine idling rpm while observing the display on 
the tachometer. 
The tachometer according to the present invention features the tachometer 
proper for sensing pressure changes in the engine case so as to compute 
and display the engine rpm. The tachometer is mounted on the oil filler 
port in the engine cylinder head cover (the place where the oil filler cap 
is fitted).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will now be described in greater detail with 
reference to the illustrated embodiment. 
Referring first to FIGS. 1A and 1B, numeral 10 shows an exemplary form of a 
cylinder head cover of an engine which is in the upper portion of the 
engine, 11 an oil filler port through which the engine oil is introduced, 
12 an oil filler cap which covers the oil filler port to hermetically seal 
the engine case, and 13 a schematic construction of the device proper of a 
tachometer, which is fixedly mounted by forcing it into the oil filler 
port so as to measure the engine rpm. 
FIG. 2 shows the manner in which the device proper 13 is mounted on the oil 
filler port 11, and the cylinder head cover 10 is formed with a threaded 
groove 101 for mounting the oil filler cap 12. Numeral 110 shows an engine 
case interior, 20 a housing forming the outer shell of the device proper 
13, 201 an elastic member made of rubber or the like for hermetically 
sealing the engine case and fixedly mounting the device proper 13 on the 
engine, and 202 a pressure inlet pipe for sensing pressure changes in the 
engine case whereby the output signal of a pressure sensor is supplied to 
an engine rpm signal computing circuit (not shown) which is incorporated 
within the housing 20. The engine rpm is indicated by a display unit 38 so 
as to be visible outwardly. Numeral 21 shows the direction from which the 
engine rpm display will be seen. Numeral 205 designates a lead wire 
protective member made of rubber, and 206 a lead wire. 
FIG. 3 is a block diagram showing the overall circuit construction of the 
tachometer according to the invention. In the Figure, numeral 31 
designates a voltage supply circuit having terminals 301 and 302 which are 
connected to a vehicle-mounted battery. In connecting the terminals 301 
and 302, they have no directional properties such as positive and negative 
properties and thus they may each be connected to the positive or negative 
terminal of the battery to permit operation of electric circuits and also 
to produce a constant supply voltage for supplying the circuits. Numeral 
32 designates a pressure sensor for sensing pressure changes in the engine 
case, 33 an amplifier circuit, 34 a multiplier circuit for multiplying the 
output of the amplifier circuit 34 for the computational purposes which 
will be described later, 35 a constant-frequency oscillator circuit, 36 a 
computing circuit for producing a periodic signal having a predetermined 
time width and generating a multiplication signal only during the time 
width, and 37 a counter/decoder circuit responsive to the output signal of 
the computing circuit to generate a signal for actuating the digital-type 
display unit 38. 
With the construction described above, the operation of the tachometer 
according to the invention will now be described with reference to FIGS. 4 
to 7. Referring first to FIG. 4 showing a wiring diagram of the voltage 
supply circuit 31, the terminals 301 and 302 are each connected to the 
positive or negative terminal of the battery. The terminals 301 and 302 
are connected to a rectifier circuit comprising four diodes 41 so that a 
line 410 always has a positive potential and a line 411 always has a 
negative potential irrespective of whether the polarities of the supply 
voltage to the terminals 301 and 302 are respectively positive and 
negative or vice versa. As a result, a constant voltage is produced from 
this voltage by means of a transistor 43, a resistor 44 and a Zener diode 
45. This constant voltage is delivered through terminals 401 and 402 to 
supply power to the electric circuits which will be described later. 
Referring now to FIG. 5, the pressure sensor 32 includes a pressure sensing 
element 511 consisting for example of a known type of semiconductor 
pressure sensing element whose output voltage varies with pressure changes 
and thus the pressure sensing element 511 generates at its output a 
voltage signal as shown in (a) of FIG. 7 (in the case of a four-cylinder 
engine, for example, two pressure changes occur for every engine 
crankshaft revolution and two periods of the voltage signal represent one 
complete engine revolution). This signal is amplified by an amplifier 513 
of the amplifier circuit 33 through a capacitor 512 and the signal shown 
in (b) of FIG. 7 appears at the output of the amplifier 513. This signal 
is applied to the input portion of a multiplying element 514 of the 
multiplier circuit 34. In this embodiment, the multiplying element 514 is 
of the same type as the RCA COS/MOS CD4046A. A counter 515, a NAND gate 
516, an inverter 517 and a D-type flip-flop 518 produce one signal pulse 
at the output Q of the D-type flip-flop 518 each time the number of pulses 
applied to the input of the counter 515 reaches 60. As a result, if the 
multiplying element 514 has its terminal VCO OUT connected to the input 
.phi. of the counter 515 and its terminal COMATOR IN connected to the 
output terminal Q of the D-type flip-flop 518 and if resistors 520, 521 
and 522 and capacitors 523 and 524 are connected to the multiplying 
element 514, a frequency which is 60 times the frequency at the terminal 
SIGNAL IN is generated at the output terminal VCO OUT of the multiplying 
device 514. In other words, if the engine speed is 600 rpm, then a pulse 
signal of (600.times.2)/60=20 Hz is applied to the terminal SIGNAL IN of 
the multiplying element 514 and a pulse signal of 1200 (=20.times.60) Hz 
is generated at its output terminal VCO OUT. 
On the other hand, the oscillator circuit 35 generates an oscillation pulse 
signal of a constant frequency and the pulses are counted by a counter 531 
which in turn generates at its output terminal Qn a pulse signal of 10 Hz 
as shown in (c) of FIG. 7. This 10 Hz signal is applied to the input of a 
counter 532 which in turn generates the signals shown in (d), (e), (f) and 
(g) of FIG. 7 at its output terminals "9", "7", "8" and CARRY OUT. The 
operation of the counter 532 is the same as the RCA COS/MOS Decade 
Counter/Divider CD4017. The CARRY OUT signal is applied to one input of a 
NAND gate 538 whose other input receives the previously mentioned 
multiplication signal shown in (b) of FIG. 7. Thus, the NAND gate 538 is 
opened for a time interval T.sub.05 corresponding to the five clock pulses 
shown in (c) of FIG. 7 ((1/10).times.5=0.5 sec in this embodiment) and 
thus the NAND gate 538 generates at its output the multiplication signal 
only during the time interval T.sub.05 as shown in (i) of FIG. 7. In other 
words, if the engine speed is 600 rpm as mentioned previously, 
1200.times.0.5=600 pulses will be present in the time interval T.sub.05 
since the multiplication signal has a frequency of 1200 Hz. In the case of 
this embodiment, as many pulses as the engine rpm will appear during the 
time interval T.sub.05. 
A resistor 552, a capacitor 550 and an inverter 541 are provided to reset 
the electric circuits to the initial states upon application of the supply 
voltage, so that only at the instant that the supply voltage is applied, a 
"1" signal is generated at the output of the inverter gate 541 and the 
counters 531, 532, etc., are reset to the initial states. As a result, the 
signal at the output Q.sub.1 of a counter 533 changes from "0" to "1" at 
the expiration of one second after the application of the supply voltage 
and the signal is applied to its ENABLE terminal through an inverter gate 
534, thus maintaining the output of the inverter gate 534 in the "0" 
signal state. The fact that the output signal of the inverter gate 534 
goes to "1" only for one second after the application of the supply 
voltage, has the effect of preventing any erroneous display by the display 
unit 38 which will be described later. Also the output signal of the 
inverter gate 534 is applied to the data input of a D-type flip-flop 535 
whose clock input receives the previously mentioned engine rpm signal (the 
amplifier output signal). The D-type flip-flop 535 and the following 
D-type flip-flop 536 are provided to determine whether the engine rpm 
signal has been generated so that if it is, a "1" signal is generated at 
the output Q of the flip-flop 536 and NAND gates 539 and 540 are opened. 
On the contrary, if no engine rpm signal is present, a "0" signal is 
generated at the output Q of the flip-flop 536 and the NAND gates 539 and 
540 are closed. The purpose of this arrangement is to prevent any 
erroneous display when no engine rpm signal is present. 
The above-described operation results in the generation at a terminal 501 
of a pulse signal having a constant frequency (about 1 KHz in this 
embodiment), at a terminal 502 of a pulse signal superposed by an engine 
rpm signal as shown in (i) of FIG. 7, at a terminal 503 of a reset signal 
which goes to "1" at one-second intervals as shown in (d) of FIG. 7, at a 
terminal 504 of the storage signal shown in (e) of FIG. 7 and at a 
terminal 505 of a signal which goes to "1" only for one second after the 
application of the supply voltage. These signals are applied to the 
counter/decoder circuit 37 shown in FIG. 6. 
In FIG. 6, the terminals 401 and 402 are the positive and negative supply 
terminals. Although the terminals are shown not wired, they are connected 
to the associated electronic components. The counter/decoder circuit 37 
includes a counter 711 which operates in the same manner as the Toshiba 
C.sup.2 MOS TC 5051P 4-digit decade counter with blanking control; the 
counts for the respective digit positions are sequentially delivered to 
the BCD outputs in a dynamic manner beginning with the most-significant 
digit position. 
Assuming now that the engine speed is 1600 rpm, the number of the engine 
rpm signal pulses shown in (i) of FIG. 7 and appearing at the terminal 502 
amounts to 1600 pulses during the time interval T.sub.05 as mentioned 
previously. In response to the SCAN IN input signal arriving at the 
terminal 501, the output Q.sub.1 of the counter 711 first goes to "1" and 
thus "1", "0", "0" and "0" signals respectively appear at its BCD outputs 
A, B, C and D representing a decimal digit "1". When the output Q.sub.1 
goes to "0" and the output Q.sub.2 goes to "1", "0", "1", "1" and "0" 
signals respectively appear at the outputs A, B, C and D representing a 
decimal digit "6". Then when the output Q.sub.3 goes to " 1", a "0" signal 
appears at each of the outputs A, B, C and D representing a digit "0", and 
when the output Q.sub.4 goes to "1", a "0" signal appears at each of the 
outputs A, B, C and D representing a decimal digit "0". The output signals 
from the outputs Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 are respectively 
applied to NAND gates 713, 714, 715 and 716 each serving as a NOT gate and 
operated by a large drive current; the outputs of these NAND gates drive 
the display unit 38. The RCA CD40107B may be used satisfactorily for each 
of these NAND gates. 
On the other hand, the BCD outputs are applied to the corresponding inputs 
of a decoder 712 (e.g., the Toshiba C.sup.2 MOS TC 5022BP BCD-to-7-Segment 
Decoder/Driver) and a 7-segment display drive signal corresponding to the 
BCD input signal appears as its outputs a to g. Thus each of four 
7-segment light-emitting displays 720 of the display unit 38 is lit by the 
corresponding segment signal, thus displaying a decimal number "1600" as 
shown in the Figure. 
While, in the embodiment described above, the power supply is adapted to be 
connected to the battery installed in a vehicle by a lead wire, a dry cell 
or the like may be incorporated in the tachometer to thereby eliminate the 
use of any lead wire. 
Further, while the tachometer according to the embodiment is adapted to be 
forced into and fixedly mounted on the oil filler port in the engine 
cylinder head cover by utilizing the resiliency of rubber, the invention 
is not intended to be limited thereto and the tachometer may be fixedly 
mounted by means of a screw or the like. 
Further, while the engine rpm is computed by multiplying the signal 
generated by detecting pressure changes, a pulse signal having a high 
frequency (e.g., 1 MHz) may be superposed on the signal shown in (b) of 
FIG. 7 so as to count the number of pulses appearing during the "1" period 
.