Capacitor discharge ignition
Encyclopedia
Capacitor discharge ignition (CDI) or thyristor
ignition is a type of automotive electronic ignition system which is widely used in outboard motor
s, motorcycle
s, lawn mower
s, chainsaw
s, small engines, turbine
-powered aircraft
, and some car
s. It was originally developed to overcome the long charging times associated with high inductance
coils used in inductive discharge ignition
(IDI) systems, making the ignition system
more suitable for high engine speeds (for small engines, racing engines and rotary piston engines). The capacitive-discharge ignition uses capacitor
discharge current output to fire the spark plug
s.
was the first to propose such an ignition system. In U.S.
patent
#609250 first filed February 17, 1897, Tesla writes 'Any suitable moving portion of the apparatus is caused to mechanically control the charging of a condenser
and its discharge through a circuit
in inductive relation to a secondary circuit leading to the terminals between which the discharge is to occur, so that at the desired intervals the condenser may be discharged through its circuit and induce in the other circuit a current of high potential
which produces the desired discharge.'
The patent also describes very generally with a drawing, a mechanical means to accomplish this. In the late 1940s an attempt to make one using mechanical means to trigger the capacitor's discharge was tried. It used extra contact switches in addition to the usual points (contact breaker
), but suffered from timing problems and was unreliable. The quest for an electronic means of producing a CD ignition began in earnest during the 1950s. In the mid 1950s, the Engineering Research Institute of the University of Michigan
in cooperation with Chrysler Corporation in the United States worked to find a method to produce a viable unit.
They were unsuccessful, but did provide much data on the advantages of such a system, should one be built. Namely; a fast voltage
rise time to fire-fouled spark plug
s, high energy throughout the RPM range resulting in better starting, more power
and economy, and lower emissions
. A few engineers, scientists, and hobbyists had built CD ignitions throughout the 1950s, using thyratron
s which required a warm-up period, and thyratrons were vulnerable to the effects of vibration
as well. In an automotive application, the Thyratron triggered CD ignition would fail in either weeks or months. The unreliability of those early Thyratron CD ignitions, made them unsuitable for mass production. One company at least, Tung-Sol (a manufacturer of vacuum tubes) marketed the Thyratron triggered CD ignition, model Tung-Sol EI-4 in 1962, but it was expensive. Despite the failings of Thyratron (vacuum tube) type CD ignitions, the improved ignition that they gave made them a worthwhile addition for some drivers. Silicon-controlled rectifier
s (SCR) or thyristor
s came later thanks to Bill Gutzwiller and his team at General Electric
. The SCR was rugged with an indefinite lifetime, but very prone to unwanted trigger impulses which would turn the SCR 'on'. Unwanted trigger impulses in early attempts at using SCRs for CD ignitions was caused by electrical effects, but mainly 'points bounce'. Points bounce is a feature of a points-triggered system. In the standard system with points
, distributor
, coil
, ignition
(Kettering system) points bounce prevents the coil from saturating fully as RPM increases resulting in a weak spark, thus limiting high speed potential. In a CD ignition, at least those early attempts, the points bounce created unwanted trigger pulses to the SCR (thyristor) that resulted in a series of weak, untimed sparks that caused extreme misfiring. There were two possible solutions to the problem.
The first would be to develop another means of triggering the discharge of the capacitor
to one discharge per power stroke by replacing the points with something else. This could be done magnetically or optically, but that would necessitate more electronics and an expensive distributor. The other option was to keep the points, as they were already in use and reliable, and find a way to overcome the 'points bounce' problem. This was accomplished in April 1962 by a Canadian, RCAF officer F.L. Winterburn working in his basement in Ottawa
, Ontario
.
The design used an inexpensive method that would only recognize the first opening of the points and ignore subsequent openings when the points bounced.
A company was formed in Ottawa in early 1963 called Hyland Electronics building CD ignitions using the Winterburn design. It provided a 75 milijoule spark at all engine speeds up to 5,000 rpm on an eight cylinder (10,000 rpm on a four-cylinder) and consumed only four ampere
s at that speed. Dynamometer
testing during 1963 and 1964 showed a minimum of 5% increase in horsepower
with the system, with 10% the norm. One example, a Ford Falcon, had an increase in horsepower of 17%. Spark plug lifespan was increased to at least 50,000 miles and points lifespan was greatly extended from 8,000 miles to at least 60,000 miles. Points lifespan became a factor of rubbing block (cam follower) wear and the life cycle of the spring
with some lasting almost 100,000 miles.
The Hyland unit was tolerant of varied points gaps. The system could be switched back to standard inductive discharge ignition
by the swapping of two wires. The Hyland CD ignition was the first commercially produced solid-state CD ignition and retailed for $39.95 Canadian. The patents were applied for by Winterburn on September 23, 1963 (United States patent# 3,564,581). The design was leaked to the United States in the summer of 1963 when Hyland exposed the design to a US company in an effort to expand sales. Afterward, numerous companies started building their own throughout the 1960s and 1970s without licence. Some were direct copies of the Winterburn circuit. In 1971 Bosch bought the European patent rights (German, French, British) from Winterburn as their own CD ignition was based upon the Winterburn design.
(IDI) systems, which are solely relying on the electric inductance
at the coil to produce high-voltage
electricity
to the spark plug
s as the magnetic field
breaks down when the current
to the primary coil winding is disconnected (disruptive discharge
). In a CDI system, a charging circuit charges a high voltage capacitor
, and at the instant of ignition the system stops charging the capacitor, allowing the capacitor to discharge its output to the ignition coil before reaching the spark plug.
A typical CDI module consists of a small transformer
, a charging circuit, a triggering circuit and a main capacitor. First, the system voltage is raised up to 400-600 volt
s by a transformer inside the CDI module. Then, the electric current flows to the charging circuit and charges the capacitor. The rectifier
inside the charging circuit prevents capacitor discharge before the moment of ignition. When the triggering circuit receives triggering signals, the triggering circuit stops the operation of the charging circuit, allowing the capacitor to discharge its output rapidly to the low inductance ignition coil, which increase the 400-600 V capacitor discharge to up to 40 kV at the secondary winding at the spark plug. When there's no triggering signal, the charging circuit is re-connected to charge back the capacitor.
The amount of energy the CDI system can store for the generation of a spark is dependent on the voltage and capacitance
of the capacitors used, but usually it's around 50 mJ
, or more. The standard points/coil/distributor ignition, more properly called the inductive discharge ignition
system or Kettering ignition system, produces 25mJ at low speed and drops off quickly as speed increases.
Most CDI modules are generally of two types:
Note that not all small engine ignition systems are CDI. Some older engines, and engines like older Briggs and Stratton use magneto ignition. The entire ignition system, coil and points, are under the magnetized flywheel.
Another sort of ignition system commonly used on small off-road motorcycles in the 1960s and 1970s was called Energy Transfer. A coil under the flywheel generated a strong DC current pulse as the flywheel magnet moved over it. This DC current flowed through a wire to an ignition coil mounted outside of the engine. The points sometimes were under the flywheel for two-stroke engines, and commonly on the camshaft for four-stroke engines. This system worked like all Kettering (points/coil) ignition systems... the opening points trigger the collapse of the magnetic field in the ignition coil, producing a high voltage pulse which flows through the spark plug wire to the spark plug.
If the engine was rotated while examining the wave-form output of the coil with an oscilloscope, it would appear to be AC. But you must consider that since the charge-time of the coil corresponds to much less than a full revolution of the crank, the coil really 'sees' only DC current for charging the external ignition coil.
Some electronic ignition systems exist that are not CDI. These systems use a transistor to switch the charging current to the coil off and on at the appropriate times. This eliminated the problem of burned and worn points, and provided a hotter spark because of the faster voltage rise and collapse time in the ignition coil.
Since the CDI system only provides a short spark, it's also possible to combine this ignition system with ionization measurement. This is done by connecting a low voltage (about 80 V) to the spark plug, except when fired. The current flow over the spark plug can then be used to calculate the temperature and pressure inside the cylinder.
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
ignition is a type of automotive electronic ignition system which is widely used in outboard motor
Outboard motor
An outboard motor is a propulsion system for boats, consisting of a self-contained unit that includes engine, gearbox and propeller or jet drive, designed to be affixed to the outside of the transom and are the most common motorized method of propelling small watercraft...
s, motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...
s, lawn mower
Lawn mower
A lawn mower is a machine that uses a revolving blade or blades to cut a lawn at an even length.Lawn mowers employing a blade that rotates about a vertical axis are known as rotary mowers, while those employing a blade assembly that rotates about a horizontal axis are known as cylinder or reel...
s, chainsaw
Chainsaw
A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...
s, small engines, turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
-powered aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
, and some car
Čar
Čar is a village in the municipality of Bujanovac, Serbia. According to the 2002 census, the town has a population of 296 people.-References:...
s. It was originally developed to overcome the long charging times associated with high inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
coils used in inductive discharge ignition
Inductive discharge ignition
Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air-fuel mixture in the combustion chamber of internal combustion engines. The first versions were magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors...
(IDI) systems, making the ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
more suitable for high engine speeds (for small engines, racing engines and rotary piston engines). The capacitive-discharge ignition uses capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
discharge current output to fire the spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s.
History
The history of the capacitor discharge ignition system can be traced back to the 1890s when it is believed that Nikola TeslaNikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
was the first to propose such an ignition system. In U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
#609250 first filed February 17, 1897, Tesla writes 'Any suitable moving portion of the apparatus is caused to mechanically control the charging of a condenser
Condenser
Condenser may refer to:*Condenser , a device or unit used to condense vapor into liquid. More specific articles on some types include:*Air coil used in HVAC refrigeration systems...
and its discharge through a circuit
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
in inductive relation to a secondary circuit leading to the terminals between which the discharge is to occur, so that at the desired intervals the condenser may be discharged through its circuit and induce in the other circuit a current of high potential
Potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
which produces the desired discharge.'
The patent also describes very generally with a drawing, a mechanical means to accomplish this. In the late 1940s an attempt to make one using mechanical means to trigger the capacitor's discharge was tried. It used extra contact switches in addition to the usual points (contact breaker
Contact breaker
A contact breaker is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of spark-ignition internal combustion engines.-Purpose:...
), but suffered from timing problems and was unreliable. The quest for an electronic means of producing a CD ignition began in earnest during the 1950s. In the mid 1950s, the Engineering Research Institute of the University of Michigan
University of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...
in cooperation with Chrysler Corporation in the United States worked to find a method to produce a viable unit.
They were unsuccessful, but did provide much data on the advantages of such a system, should one be built. Namely; a fast voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
rise time to fire-fouled spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s, high energy throughout the RPM range resulting in better starting, more power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
and economy, and lower emissions
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...
. A few engineers, scientists, and hobbyists had built CD ignitions throughout the 1950s, using thyratron
Thyratron
A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
s which required a warm-up period, and thyratrons were vulnerable to the effects of vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
as well. In an automotive application, the Thyratron triggered CD ignition would fail in either weeks or months. The unreliability of those early Thyratron CD ignitions, made them unsuitable for mass production. One company at least, Tung-Sol (a manufacturer of vacuum tubes) marketed the Thyratron triggered CD ignition, model Tung-Sol EI-4 in 1962, but it was expensive. Despite the failings of Thyratron (vacuum tube) type CD ignitions, the improved ignition that they gave made them a worthwhile addition for some drivers. Silicon-controlled rectifier
Silicon-controlled rectifier
A silicon-controlled rectifier is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W...
s (SCR) or thyristor
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
s came later thanks to Bill Gutzwiller and his team at General Electric
General Electric
General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...
. The SCR was rugged with an indefinite lifetime, but very prone to unwanted trigger impulses which would turn the SCR 'on'. Unwanted trigger impulses in early attempts at using SCRs for CD ignitions was caused by electrical effects, but mainly 'points bounce'. Points bounce is a feature of a points-triggered system. In the standard system with points
Contact breaker
A contact breaker is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of spark-ignition internal combustion engines.-Purpose:...
, distributor
Distributor
A distributor is a device in the ignition system of an internal combustion engine that routes high voltage from the ignition coil to the spark plugs in the correct firing order. The first reliable battery operated ignition was developed by Dayton Engineering Laboratories Co. and introduced in the...
, coil
Coil
A coil is a series of loops. A coiled coil is a structure in which the coil itself is in turn also looping.-Electromagnetic coils:An electromagnetic coil is formed when a conductor is wound around a core or form to create an inductor or electromagnet...
, ignition
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
(Kettering system) points bounce prevents the coil from saturating fully as RPM increases resulting in a weak spark, thus limiting high speed potential. In a CD ignition, at least those early attempts, the points bounce created unwanted trigger pulses to the SCR (thyristor) that resulted in a series of weak, untimed sparks that caused extreme misfiring. There were two possible solutions to the problem.
The first would be to develop another means of triggering the discharge of the capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
to one discharge per power stroke by replacing the points with something else. This could be done magnetically or optically, but that would necessitate more electronics and an expensive distributor. The other option was to keep the points, as they were already in use and reliable, and find a way to overcome the 'points bounce' problem. This was accomplished in April 1962 by a Canadian, RCAF officer F.L. Winterburn working in his basement in Ottawa
Ottawa
Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...
, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
.
The design used an inexpensive method that would only recognize the first opening of the points and ignore subsequent openings when the points bounced.
A company was formed in Ottawa in early 1963 called Hyland Electronics building CD ignitions using the Winterburn design. It provided a 75 milijoule spark at all engine speeds up to 5,000 rpm on an eight cylinder (10,000 rpm on a four-cylinder) and consumed only four ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s at that speed. Dynamometer
Dynamometer
A dynamometer or "dyno" for short, is a device for measuring force, moment of force , or power. For example, the power produced by an engine, motor or other rotating prime mover can be calculated by simultaneously measuring torque and rotational speed .A dynamometer can also be used to determine...
testing during 1963 and 1964 showed a minimum of 5% increase in horsepower
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
with the system, with 10% the norm. One example, a Ford Falcon, had an increase in horsepower of 17%. Spark plug lifespan was increased to at least 50,000 miles and points lifespan was greatly extended from 8,000 miles to at least 60,000 miles. Points lifespan became a factor of rubbing block (cam follower) wear and the life cycle of the spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
with some lasting almost 100,000 miles.
The Hyland unit was tolerant of varied points gaps. The system could be switched back to standard inductive discharge ignition
Inductive discharge ignition
Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air-fuel mixture in the combustion chamber of internal combustion engines. The first versions were magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors...
by the swapping of two wires. The Hyland CD ignition was the first commercially produced solid-state CD ignition and retailed for $39.95 Canadian. The patents were applied for by Winterburn on September 23, 1963 (United States patent# 3,564,581). The design was leaked to the United States in the summer of 1963 when Hyland exposed the design to a US company in an effort to expand sales. Afterward, numerous companies started building their own throughout the 1960s and 1970s without licence. Some were direct copies of the Winterburn circuit. In 1971 Bosch bought the European patent rights (German, French, British) from Winterburn as their own CD ignition was based upon the Winterburn design.
The basic principle
Most ignition systems used in cars are inductive discharge ignitionInductive discharge ignition
Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air-fuel mixture in the combustion chamber of internal combustion engines. The first versions were magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors...
(IDI) systems, which are solely relying on the electric inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
at the coil to produce high-voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
to the spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s as the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
breaks down when the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
to the primary coil winding is disconnected (disruptive discharge
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....
). In a CDI system, a charging circuit charges a high voltage capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, and at the instant of ignition the system stops charging the capacitor, allowing the capacitor to discharge its output to the ignition coil before reaching the spark plug.
A typical CDI module consists of a small transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, a charging circuit, a triggering circuit and a main capacitor. First, the system voltage is raised up to 400-600 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s by a transformer inside the CDI module. Then, the electric current flows to the charging circuit and charges the capacitor. The rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
inside the charging circuit prevents capacitor discharge before the moment of ignition. When the triggering circuit receives triggering signals, the triggering circuit stops the operation of the charging circuit, allowing the capacitor to discharge its output rapidly to the low inductance ignition coil, which increase the 400-600 V capacitor discharge to up to 40 kV at the secondary winding at the spark plug. When there's no triggering signal, the charging circuit is re-connected to charge back the capacitor.
The amount of energy the CDI system can store for the generation of a spark is dependent on the voltage and capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of the capacitors used, but usually it's around 50 mJ
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
, or more. The standard points/coil/distributor ignition, more properly called the inductive discharge ignition
Inductive discharge ignition
Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air-fuel mixture in the combustion chamber of internal combustion engines. The first versions were magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors...
system or Kettering ignition system, produces 25mJ at low speed and drops off quickly as speed increases.
Most CDI modules are generally of two types:
- AC-CDI - The AC-CDI module obtains its electricity source solely from the alternating currentAlternating currentIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
produced by the alternator. The AC-CDI system is the most basic CDI system which is widely used in small engines.
Note that not all small engine ignition systems are CDI. Some older engines, and engines like older Briggs and Stratton use magneto ignition. The entire ignition system, coil and points, are under the magnetized flywheel.
Another sort of ignition system commonly used on small off-road motorcycles in the 1960s and 1970s was called Energy Transfer. A coil under the flywheel generated a strong DC current pulse as the flywheel magnet moved over it. This DC current flowed through a wire to an ignition coil mounted outside of the engine. The points sometimes were under the flywheel for two-stroke engines, and commonly on the camshaft for four-stroke engines. This system worked like all Kettering (points/coil) ignition systems... the opening points trigger the collapse of the magnetic field in the ignition coil, producing a high voltage pulse which flows through the spark plug wire to the spark plug.
If the engine was rotated while examining the wave-form output of the coil with an oscilloscope, it would appear to be AC. But you must consider that since the charge-time of the coil corresponds to much less than a full revolution of the crank, the coil really 'sees' only DC current for charging the external ignition coil.
Some electronic ignition systems exist that are not CDI. These systems use a transistor to switch the charging current to the coil off and on at the appropriate times. This eliminated the problem of burned and worn points, and provided a hotter spark because of the faster voltage rise and collapse time in the ignition coil.
- DC-CDI - The DC-CDI module is powered by the battery, and therefore an additional DC/AC inverter circuit is included in the CDI module to raise the 12 V DC to 400-600 V DC, making the CDI module slightly larger. However, vehicles that use DC-CDI systems have more precise ignition timing and the engine can be started more easily when cold.
Advantages and Disadvantages of CDI
A CDI system has a short charging time, a fast voltage rise (between 3 ~ 10 kV/μs) compared to typical inductive systems (300 ~ 500 V/μs) and a short spark duration limited to about 50-80 µs. The fast voltage rise makes CDI systems insensitive to shunt resistance, but the limited spark duration can for some applications be too short to provide reliable ignition. The insensitivity to shunt resistance and the ability to fire multiple sparks can provide improved cold starting ability.Since the CDI system only provides a short spark, it's also possible to combine this ignition system with ionization measurement. This is done by connecting a low voltage (about 80 V) to the spark plug, except when fired. The current flow over the spark plug can then be used to calculate the temperature and pressure inside the cylinder.