Electronic ignition device for internal combustion engines

An electronic ignition device for internal combustion engines is disclosed. The disclosed device employs a Hall sensor along with a Permanent Magnet (PM) mounted on a moving part of an engine to establish a reference point from which an Advance Firing Angle (AFA) is calculated and applied to the firing of a spark plug. The signal generated by the PM passing the active area of the Hall sensor is called the Master Reference Signal (MRS). The invention senses the speed of the engine and based on the speed of the engine sets an AFA for the firing of the spark plug from the set reference point. As the speed of the engine changes the AFA at the which the spark plug fires at is also changed by the invention. The invention also has a Regulated Supply Voltage Booster that boosts the supply voltage to a higher voltage for charging a capacitor that is used in capacitive discharge firing of a spark plug. This device also contains a means for the user of the invention to change the AFA of the firing of a spark plug while the invention and engine is running so that maximum performance of the engine may be obtained under different engine load conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a device electronic in nature with no moving 
parts that accomplishes the ignition of a fuel air mixture in an internal 
combustion engine, and in particular to a device that determines the 
correct time to fire the fuel air mixture so that a predetermined and set 
advance firing angle (AFA) may be applied to the firing of a spark plug 
which is dependent on the speed of the engine. The correct time for firing 
the spark plug at the desired AFA as well as the determination of the 
speed of the engine is determined with each firing cycle of the cylinder 
and this information is used on the next firing cycle of the cylinder. The 
invention accomplishes the above with the use of hardware that is solely 
dedicated to performing the above tasks as opposed to units that use a 
microprocessor and associated software. 
2. Prior Art 
Ignition devices for the firing of a spark plug in an internal combustion 
engine are well know in prior art and are widely used. This invention 
seeks to provide a device which has a regulated spark plug firing voltage 
which is independent of the speed of the engine so as to improve the 
reliability of the firing of the spark plug. This invention seeks to 
improve the speed pick up or acceleration of an internal combustion engine 
that utilizes electronic ignition devices that utilize speed averaging 
techniques for the determining of the speed of the engine and for the 
setting of the AFA at which the spark plug is fired. The improvement is 
accomplished by determining the speed of the engine and the AFA to be 
applied to the firing of the spark plug with each fuel air compression 
stroke of the cylinder, and applying this to the next firing cycle of the 
cylinder. This thus affords a much faster speed pick up of an engine. 
This invention seeks to provide an improvement over ignition devices that 
use a mechanical means of changing the AFA at which the spark plug is 
fired. A mechanical means of changing the AFA of the spark plug firing 
normally means moving a sensor such as a Hall sensor with relationship to 
a moving permanent magnet (PM) which is normally mounted to the rotating 
shaft of an engine. Movement of the sensor is typically done by 
mechanically coupling it to the throttle of the engine. Having a device 
such as this invention that has a single nonmoving sensor such as a Hall 
sensor for the sensing of a passing reference point in the engine's firing 
cycle means a greater ease of installation and reliability as there are no 
moving parts which may wear out or break. A further advantage of this 
invention is that it allows the user of the invention to change the AFA at 
which the spark plug is fired while the engine is running at full throttle 
or above a reference speed called the idle Threshold Speed (ITS) of the 
engine by the adjusting of a potentiometer. The AFA of the spark plug is 
set to 0 degrees below the ITS and it is independent of the above said 
potentiometer. This allows for a much smoother idle of the engine. With an 
ignition device that uses a mechanical means of changing the AFA of the 
spark plug, changes that are made to the AFA of the spark plug firing at 
the full throttle setting of the engine have a high probability of also 
affecting the AFA of the spark plug firing at the low throttle position of 
the engine as well. If the same AFA firing of the spark plug is to be 
maintained at the low throttle setting when the high throttle setting of 
the AFA of the spark plug firing has been changed, then fundamental 
changes to the movement of the Hall sensor will have to be made which may 
prove difficult. 
A further advantage of this invention over that of an ignition device that 
has an AFA that is mechanically controlled is that it ensures that the AFA 
firing of the spark plug will not be advanced too rapidly or by not too 
much for the given current speed of the engine as the AFA applied to the 
firing of the spark plug is dependent on the speed of the engine. Rapid 
advances of AFA to the firing of the spark plug can cause too early a 
firing of the fuel air mixture and this can cause the engine speed to be 
slowed and in severe cases it can cause the engine to be stopped as well 
as cause excessive vibration of the engine to occur. 
Furthermore this invention seeks to provide a greater safety aspect for the 
user of internal combustion engines that are going to be hand started, 
such as those engines employed in chain saw applications or in model 
aircraft applications. An engine whose AFA firing of the spark plug is 
controlled by mechanical coupling to the throttle may be started with the 
AFA firing of the spark plug at greater than 0 degrees of AFA. If during 
starting of an engine with this type of mechanical AFA control, the user 
does not put enough energy into rotating the crank shaft of the engine on 
the fuel air compression stroke of the engine to overcome the additional 
pressure of the firing fuel air mixture so that the piston goes beyond top 
dead center of the compression stroke of the engine, the engine will 
backfire with a strong possibility of hurting the person starting the 
engine. This is especially true of engines that are employed in model 
aircraft applications as the user is probably hand flipping the propeller 
and a backfire may hurt the user's hand, and in severe cases may cut it 
and or remove it. The propeller may even come off as often does happen and 
strike the user or bystanders. 
Furthermore this invention seeks to provide an advantage over ignition 
units for internal combustion engines that are computer controlled through 
software. The advantages are that this invention causes less 
electromagnetic radiation to be emitted thus resulting in less radio 
interference which is desirable in the operation of radio control model 
aircraft. Less electromagnetic radiation emissions is a result of having 
the circuit elements of the invention transition once per compression 
stroke of the fuel air mixture of the cylinder whereas computer controlled 
ignition units have internal signals changing many times per compression 
stroke of the fuel air mixture of the cylinder. A further advantage of 
having a dedicated piece of electrical hardware to control the AFA firing 
of the spark plug is that it will be able to perform this function much 
faster than a software controlled ignition unit and with a greater AFA 
accuracy possible at the higher engine speeds. A further object of this 
invention is to provide an advantage over those units that are commonly 
known as 2 fixed sensor or 2 fixed position ignition devices where the AFA 
applied to the firing of the spark plug is switched between these 2 
points. The main advantage of this invention over the just said ignition 
device is that the top engine speed AFA firing of the spark plug is 
electronically changeable by mean of a potentiometer as opposed to being 
fixed. To change the AFA of the firing of a spark plug of a 2 sensor 
position ignition device one has to mechanically alter the relative 
position of the high speed AFA sensor. This may prove difficult to do if 
the engine is running due to safety aspects. Engines normally used in 
model airplane applications have the sensors placed on the shaft of the 
engine in close proximity to the propeller therefore making adjustments 
difficult if the propeller is rotating. Also with a 2 position ignition 
device there are only 2 set points for the AFA of the firing of the spark 
plug. The disclosed invention changes the AFA firing of the spark plug 
depending on the speed of the engine, exact details of how this is 
accomplished is covered in the detailed description of the invention, 
SUMMARY OF THE INVENTION 
In accordance with this invention I provide a device which I denote as an 
electronically controlled ignition device for internal combustion engines. 
The ignition device senses the relative position of an engine's fuel air 
compression stroke through the use of a Hall sensor and a PM. The PM is 
normally placed on the shaft of the engine or other moving part such that 
the Hall sensor produces a rising edge signal when the piston is in the 
compression stroke of the fuel air mixture and the piston is at or near 
the top dead center of the stoke. The user physically positions the Hall 
sensor such that a rising edge is generated where the user wants the 0 
degree AFA firing of the spark plug to occur and this point becomes the 
reference point from which the invention then advances the firing of the 
spark plug. The AFA applied to the firing of the spark plug is dependent 
on the speed of the engine. If the speed of the engine is below a speed 
called the ITS, then 0 degrees of AFA will be applied to the firing of the 
spark plug. The user of the invention by use of a potentiometer in the 
invention may change the AFA at which the spark plug fires while the 
engine is running if the engine speed is above the ITS. 
The device contains a Regulated Supply Voltage Booster that boosts the 
supply voltage to a higher voltage so as to charge a capacitor to be 
called a Capacitive Discharge Capacitor (CDC) that is used in the 
capacitive discharge firing circuit of a spark plug. The invention 
contains a circuit that senses the voltage present across the CDC and if 
it is beyond a set threshold voltage it stops further build up of voltage 
across the CDC. This is advantageous as it provides for a regulated firing 
voltage of the spark plug.

DETAILED DESCRIPTION OF THE INVENTION 
The following detailed description of this invention is meant to be 
illustrative only and not limiting. Other embodiments of this invention 
will be obvious to those skilled in the art in view of the following 
disclosure. Numerous specific details are set forth, such as circuit 
configurations, timing diagrams and the like, in order to provide a 
thorough understanding of the invention. It will be obvious to one skilled 
in the art that the present invention may be practiced without these 
specific details. In other instances, well-known circuit details and steps 
are not described in detail so as not to obscure the invention. 
An embodiment of the invention is contained in FIGS. 1, 2, 3 taken as a 
whole. Also it should be noted that other figures may replace those listed 
above and this will be noted where appropriate. 
Now referring to FIG. 2: 
The reference voltage REF1 is constructed by use of the resistors RF1 and 
RF2. The top node of a resistor RF1 is connected to the positive power 
supply terminal as indicated by the symbol. The reference voltage REF1 is 
constructed such that it is equal to approximately 1/2 of the power supply 
voltage or to suit the electrical components that it is acting as a 
reference to. 
Now referring to FIG. 1: 
A Hall sensor 1 along with a permanent magnet (PM) 2 acts as a relative 
position indicator portion of the invention. It produces a down going 
pulse as referred to by reference numeral 3 every time a PM passes 
sufficiently close to the active area of the Hall sensor. The PM is 
positioned on a moving portion of the engine such as a rotating shaft such 
that the Hall sensor produces a downward going voltage pulse every time 
the PM passes the active area of the Hall sensor. 
The signal output from the Hall sensor is called the Master Reference 
Signal (MRS), it is buffered and called CLK. The beginning and end of the 
invention's timing cycle is marked by the rising edge of the MRS which 
causes the capacitor C1 to be discharged to ground potential, how this is 
achieved is explained later. Just prior to the discharge of capacitor C1, 
C1 has reached its maximum voltage for the current period of the MRS, and 
this maximum voltage attained is dependent entirely on the length of the 
period of the MRS. 
The component values chosen for use in the Main Timing Element (MTE) which 
is composed of circuit elements resistor R1 and capacitor C1 is dependent 
on the period of the MRS. The shorter the period of the MRS the lower the 
voltage which capacitor C1 attains at any point in time for any set of 
component values that make up the MTE. A higher VOD signal voltage value 
may be obtained at any point in the timing cycle if the value of capacitor 
C1 is lowered, or if the value of resistor R1 is lowered, or both. A 
higher voltage rise time of VOD may be desired if the period of the MRS is 
such that the voltage which signal VOD attains is not sufficient for 
proper operation of the electrical components that it supplies a signal 
to. 
The voltage across capacitor C1 is buffered by operational amplifier A1. 
Operational amplifier A1 can only place charge onto the capacitor C2 to be 
called the Sample Voltage Capacitor (SVC) through the diode D6. Charge is 
removed from the SVC by use of a resistor R2 to be called the Sample Bleed 
Resistor (SBR) which is connected in parallel across the SVC. The SVC then 
tends to act as a storage element for the maximum voltage that was 
achieved by capacitor C1 during the period of the MRS. The time constant 
created by the SVC C2 and the SBR R2 is much longer than that of the 
period of the MRS. 
For long periods of the MRS the values of AFA that the circuits of FIGS. 3 
or 5 generates will have to be adjusted downwards to take into account the 
voltage drop in the SVC C2 due to the SBR R2 which causes a higher AFA 
firing of the spark plug to take place. The lowering of the value of the 
SBR will increase the responsiveness of the VOA signal to rapid decreases 
in the period of the MRS. The circuits involved in the placing and holding 
of a voltage on the SVC is called a Sample and Hold Circuit. 
The rising edge of the MRS causes a positive voltage pulse to be generated 
at the noninverting input of voltage comparator A4 and at the signal port 
of VMR. Circuit elements capacitor C3, resistor R3 and diode D1 are 
configured as shown and is called in this invention an Edge to Pulse 
Conversion Circuit of positive edge sensitivity. The purpose of diode D1 
is to ensure that negative under shoot voltages below ground potential are 
shorted to ground potential so as not to damage the inputs of other 
circuit elements in the invention. 
The duration of the positive pulse that goes to the noninverting input of 
the voltage comparator A4 is dependent on the values of capacitor C3 and 
resistor R3. Similarly the positive voltage pulse which makes up the 
signal VMR is also constructed. 
When the output of operational amplifier A4 goes to maximum voltage it 
causes the transistor T1 to be turned on and this causes the capacitor C1 
to be discharged. Transistor T1 should be on long enough only to allow the 
full discharge of the capacitor C1. With this circuit configuration the 
beginning and end of the period of the MRS is marked by successive 
discharges of the capacitor C1 which is marked by the rising edge of the 
MRS. The circuit elements and configuration which causes the discharge of 
capacitor C1 to take place when the rising edge of the MRS occurs is to be 
called the Cyclic Discharge Circuit of this circuit configuration. The 
value of the voltage present across the SVC C2 is a direct function of the 
period of the MRS. The circuit elements and configuration employed in the 
generation of the voltage across the SVC is called the Period 
Determination Circuit. 
The circuit of FIG. 1 may be replaced by that of the circuit contained in 
FIG. 4. The difference between the just said 2 circuits is that diode D6 
of FIG. 1 is replaced by transistor T3 of FIG. 4 and its associated 
control circuitry, and a change in the control circuitry of transistor T1 
of FIG. 4; all other aspects of operation remain the same, and sections of 
the circuit which are identical have retained their same identifiers. 
Newly introduced circuit configurations and elements have been given new 
identifiers. 
Now referring to FIG. 4: 
Transistor T3 is a MOSFET. In this invention it is called a Pass Gate 
Transistor (PGT). When the Gate of the PGT is taken to the power supply 
voltage it can conduct current in both directions and can be equated to a 
copper wire connecting 2 nodes in a circuit. When the PGT is on, the 
voltage present at the output of operational amplifier A1 is placed across 
the SVC C2. The Gate of the PGT receives a positive voltage pulse, which 
turns it on, which is called the Pass Gate Turn On Signal (PGTOS), from 
the output of operational amplifier A4. The PGTOS is generated from the 
rising edge of the MRS and its duration and construction is dependent on 
the component values of capacitor C3 and resistor R3. Capacitor C3 and 
resistor R3 are configured as an Edge to Pulse Conversion Circuit that has 
positive edge sensitivity. Diode D1 acts to short out to ground potential 
negative under shoot voltages. 
The falling edge of the PGTOS generates a subsequent positive voltage pulse 
at the output of voltage comparator A8, to be called the Master Discharge 
Signal (MDS), which turns on transistor T1 and which causes discharge of 
capacitor C1 to take place. The duration and construction of the MDS is 
through the use of components resistor R9, capacitor C7 and voltage 
comparator A8. Diode D8, resistor R9 and capacitor C7 are configured as an 
Edge to Pulse Conversion Circuit that has negative edge sensitivity and 
voltage comparator A8 is configured as an inverter. Diode D8 is present to 
short out over shoot voltage pulses to the voltage power supply. The 
circuit elements and configuration which causes the discharge of capacitor 
C1 to take place when the rising edge of the MRS occurs is to be called 
the Cyclic Discharge Circuit of this circuit configuration. 
Now referring to FIG. 3: 
Signal CLK drives the common clock line of multiple D-type flip-flops F-1 
through F-N. Any number of D-type flip-flops may be used depending on the 
number of AFA steps that may be desired. The D-type flip-flops through F-N 
are positive clock edge triggered. The D-type flip-flops on power up have 
the Q outputs set to a logical level of 0. The voltage of signal VOA is 
fed to the top of a resistor divider ladder composed of the resistors S-1 
through S-N. The voltage that is seen at the inverting inputs of the 
operational amplifiers O-1 through O-N is dependent on the value of the 
resistors present in the resistor divider ladder S-1 through S-N as shown. 
The values of the resistors S-1 through S-N are selected such that they, 
in conjunction with reference voltage REF 1, cause each of the outputs of 
the operational amplifiers O-1 through O-N each in their turn to change 
state when voltage of signal VOA causes the inverting input of the voltage 
comparator in observation to pass beyond the voltage at the noninverting 
input. With this circuit configuration each of the outputs of the voltage 
comparators will transition state as the voltage of signal VOA passes 
through a unique voltage level specific to that voltage comparator. The 
triggering point of each of the operational amplifiers is set through the 
effective resistor divider ladder composed of the total resistance above 
and below the node at the inverting input of each of the voltage 
comparators in conjunction with the reference voltage REF1 set at the 
noninverting input of the operational amplifier. 
The outputs of each of the operational amplifiers O-1 through O-N is fed to 
the D inputs of the corresponding D-type flip-flops F-1 through F-N as is 
shown. The D-type flip-flops sample and store the outputs of the voltage 
comparators at the rising edge of the signal CLK. Each of the Q outputs of 
the D-type flip-flops F-1 through F-N is connected to the Gate of the 
corresponding MOSFET transistors P-1 through P-N. The Source of each of 
the transistors P-1 through P-N is connected to ground potential. As each 
of the respective transistor P-1 through P-N turn on in their turn as a 
result of the voltage VOA dropping, the portion of VOA that makes up VOC 
is also caused to drop. The value of VOC in comparison to VOA is what 
determines the AFA which is to be applied to the firing of the spark plug 
as is explained later in reference to FIG. 2. 
Signal IDL becomes a logic level 1 when the period of the MRS falls below a 
certain set point which is indicated by a specific voltage level of the 
signal VOA. The user of the invention may also adjust the value of VOC as 
a portion of VOA while the invention is in operational use by adjusting 
the potentiometer RP. This allows the user to change the AFA at which the 
spark plug fires for a given period of the MRS if the signal IDL is of a 
logical value of 1. The output VOC is a result of the described circuit 
configuration called the Step Advance Set Circuit. The described Step 
Advance Set Circuit changes the effective AFA at which the spark plug is 
fired in steps as opposed to a continuous smooth function. 
The circuit of FIG. 3 of the invention may be replaced with the circuits 
contained in FIG. 5 and FIG. 6. Now referring to FIG. 5. Voltage VOA 
through a negative amplification factor is applied to the Gate of 
transistor T4. The point at which polarity reversal takes place is about 
the voltage which is at the noninverting input of operational amplifier 
OG, this voltage is known as a Virtual Ground. The Virtual Ground may be 
set by the potentiometer RM. In this circuit configuration transistor T4 
is used as a voltage controlled resistor. The transfer characteristics of 
the transistor T4 is such that as the Gate to Source voltage of transistor 
T4 increases, the current that it is capable of conducting also increases. 
The effect of this is that transistor T4 may be used as voltage controlled 
resistor whose resistance drops as the Gate to Source voltage increases. 
The voltage at the Gate of transistor T4 increase as the voltage VOA drops 
and this causes the effective resistance of the transistor T4 to drop 
which in turn causes the voltage VOC to be a lower fraction of the voltage 
value of VOA. With this circuit configuration it is possible to have a 
continuous as opposed to a step-wise relationship between the voltage VOA 
and VOC. The potentiometer RPB enables the user to change the portion of 
the voltage VOA that makes up the signal VOC and thus change the AFA that 
is applied to the firing of the spark plug for a given period of the MRS 
if the IDL signal is at a logical value of 1. The output VOC is a result 
of the described circuit configuration called Continuous Advance Set 
Circuit. The described Continuous Advance Set Circuit changes the 
effective AFA at which the spark plug is fired in a continuous function as 
opposed to a step function. 
Now referring to FIG. 6: 
The voltage VOD is buffered by operational amplifier OB. By use of the 
potentiometer RI the voltage value of VOD which causes the output of the 
voltage comparator OI to change state can be set. The output of the D-type 
flip-flop FI is updated on the rising edge of the signal CLK. If the 
signal IDL is of a logical value 1 then the period of the MRS is below 
that of the set value. 
Now referring to FIG. 2: 
Voltage comparator A2 and its function is called the Spark Plug Firing Time 
Calculator (SPFTC) The output of SPFTC is called the AFA Signal (AFAS). 
The rising edge of the AFAS marks the point in time where the spark plug 
will be fired at if the signal IDL is of a logical value of 1. The time 
difference between the rising edge of the AFAS and the rising edge of the 
MRS is the AFA at which the spark plug is fired. 
The AFAS is generated by having the voltage VOC feeding the inverting input 
of the SPFTC. The noninverting input of SPFTC A2 gets fed to it voltage 
VOD. When voltage VOD goes beyond the voltage of VOC the output of the 
SPFTC goes to a logical level of 1 indicating that the spark plug should 
be fired if the signal IDL is at a logical level of 1. The voltage value 
of VOC is a fraction of the voltage value of VOA. 
With this circuit implementation the actual point in time where the spark 
plug is fired is dependent on the period of the previous cycle and also on 
the value of the voltage of the signal VOC. The lower the fraction of VOA 
that VOC is, the higher will be the AFA that will be applied to the firing 
of the spark plug. The AFAS which is the output of the SPFTC A2 is 
calculated with each period of the MRS. 
If the IDL signal is of a logical value of 1 then it will allow the AFAS 
from the SPFTC A2 to propagate through the logic gates G1 and G2 and cause 
the Master Firing Signal (MFS) 4 to have a rising edge occurring which is 
in advance of the rising edge of the MRS. If the signal IDL is at a logic 
state of 0 then it will allow the signal VRM to cause a rising edge of the 
MFS which is coincident with the rising edge of the MRS thus causing no 
advance to be applied to the firing of the spark plug. The circuitry used 
to generate the rising edge of the MFS such that it is dependent on the 
state of the signal IDL is called the Capacitive Discharge Control 
Circuit. 
The rising edge of the MFS 4 is converted to a positive going voltage pulse 
signal which is called the SCR Firing Signal (SCR-F-S) 5 whose width is 
controlled by the value of the components capacitor C4 and resistor R4. 
The width of the SCR-F-S should be such that it assures the firing of the 
SCR S1. The circuit that generates the SCR-F-S from the rising edge of the 
MFS is called the Spark Plug Firing Control Logic. 
The Oscillator Gate Off Signal (OGOS) 7 is an elongated version of signal 
SCR-F-S 5. The OGOS is constructed such that it is active for the duration 
of the SCR-F-S and also for a period of time longer which is dependent on 
the values of capacitor C5 and resistor R5. The configuration and 
functionality of diode D7, capacitor C5, and resistor R5 is to be called a 
Signal Level Extender Circuit. The purpose of OGOS is to turn off the 
drive to transistor T2. The drive to transistor T2 is to be turned off 
long enough for all of the energy contained in the capacitive Discharge 
Capacitor CDC to be dissipated when the SCR S1 is turned on. 
A Free Running Oscillator is constructed with the logic gates A6 and A7. 
The frequency of oscillation is controlled by the values of resistor R6 
and capacitor C6. The frequency of the oscillator is chosen such that when 
transistor T2 is on that a magnetic field is built up in the transformer 
L1 to the point where saturation of the magnetic field is about to occur. 
The oscillator drive to the base of transistor T2 is turned off when the 
OGOS 7 is in the logical 1 state. Also drive to the base of transistor T2 
is turned off when the Voltage Achieved Signal (VAS) 6 is at a logical 0 
state indicating that the minimum voltage has been achieved across the CDC 
which is called the Capacitive Discharge Voltage (CDV). The VAS signal has 
no effect if the OGOS is in the logical 1 state. 
Transformer L1 is configured as a voltage step up transformer. Energy is 
stored in the transformer L1 when the transistor T2 is on. When transistor 
T2 is turned off the collapse of the magnetic field in the transformer L1 
causes a much greater voltage to be built up in the primary coil winding 
of the transformer L1 and this voltage is passed to the secondary winding 
output and it is amplified by a factor which is equal to the turns ratio 
of the primary to secondary windings. The voltage at the secondary winding 
of a transformer L1 is rectified by the Full Wave Bridge Rectifier B. The 
output of the Full Wave Bridge Rectifier B is stored on the CDC. The value 
of the voltage stored across the CDC for any given time period is 
dependent on the frequency of signal 8 which is the output of the Free 
Running Oscillator and on the turns ratio of the transformer L1 along with 
the size of the magnetic field that is stored in the transformer L1. This 
circuit configuration which places and controls the CDV on the CDC is 
called the Regulated Supply Voltage Booster. The circuit elements and 
configuration that causes transistor T2 to turn on and off and also 
containing transistor T2 is called the Transformer Drive Circuitry. 
The signal SCR-F-S 5 turns on the SCR S1. Diode D5 shorts out negative 
going voltage spikes that result as a turning on of the SCR S1. When SCR 
S1 turns on it causes the voltage that was stored on the CDC to be applied 
across the primary winding of the transformer L2. This causes a voltage 
oscillation to occur and a voltage to appear across the secondary winding 
of the transformer L2 such that it causes an electrical spark to jump 
across the electrodes of the spark plug SP. The energy that is dissipated 
in the firing of the spark plug is dependent on the value of voltage that 
was present across the CDC and on the capacitance value of the CDC. The 
firing voltage of the spark plug is dependent on the voltage across the 
CDC and on the turns ratio of the transformer L2. The firing of the spark 
plug by means of applying a charged capacitor to the primary of 
transformer to create a high enough voltage at the secondary of the 
transformer so that an electrical spark will jump an air gap is known as 
capacitive discharge firing of a spark plug.