Edgar Westbury Wallaby ignition

Hi all,

I am purchasing a Wallaby engine which has no ignition components other than the on engine points etc. There looks like a condenser or something similar on the engine between the points Low Tension post and engine body but not sure until i receive it what this actually is. Moving on i am wondering what i can use as a coil setup or even CDI setup (maybe dual) i assume for this type i would need to replace the points with pickup sensor. I would like to maintain the points, i have the instructions which contains ignition diagrams but seems to indicate a distributor with HT outputs whereas setups i have seen look to have a coil with 2 HT outputs one to each plug and LT sides connected together and to the points LT post, does this suggest that both plugs spark always even if not on the compression stroke. Any advice on setup and equipment that maintains the points would be appreciated.

This is the circuit diagram from the original article but it is not unusual to have spark plugs firing each revolution instead of every other.

Edited By Nick Clarke 3 on 15/09/2022 11:38:34

Thanks Nick,

Thats the diagram i have also, the engine and most i have seen dont seem to have a distributor that has HT lead outputs. All i have seen suggest that the coil in the diagram is a twin coil with x2 HT outputs direct to the plugs. i got this from the below video which part way through does a slow walk around showing coil etc. one thing it doesnt show is the battery connection.

https://www.youtube.com/watch?v=BpmBdPD8euY

As a follow up after doing some investigation i think the engine was setup for a wasted spark ignition system using a twin coil. I have noted that the engine has a capacitor between the points and block so basically points LT side to ground. If i used a 6V twin coil would i still need a condenser with the capacitor. So i think it would be 6V battery to coil + terminal and engine block------coil negative ( secondary ) to LT side of points also to condenser ( other side of condenser to block ground) , plus capacitor, points i assume are already connected to ground via the block. HT leads connected to plugs. Meaning i guess that as one cylinder is on compression and the other on exhaust that both spark and so on. Not sure if i got this right so any comments good or bad appreciated especially about the capacitor and condenser setup.

The drawing posted above shows the distributor being driven at half engine revs so you will only get the spark fire once during the 4-stroke cycle for each cylinder

Lost or waste spark is more common when the contacts are on the crank shaft

Andy. A condenser is the old name for a capacitor.

Roy

Thanks all,

I think i am getting there, it looks like the points opening/closing is crank driven. The capacitor is a ceramic type not electrolytic but a guess they perform the same function. I would upload a diag of my understanding but havnt figured out how to do that yet.

Batt + -------> coil +

BATT neg------->----engine block----points non LT input side

Coil LT ---------> points plus capacitor/condenser

Capacitor -------->coil LT side and engine block (GND)

Coil HT1-----plug 1

Coil HT2 -----plug 2

Forgot Points non LT input -------GND i.e. connected when points closed

Yes points or contact is off the crank so as it is shown the spark will go to No 1 cylinder, next full rotation of the crank the "D" shaped rotor in the distributor will have turned 180deg so the spark goes to No 2 cylinder

So one spark per engine rotation going alternately to No1 then No2 then No1 etc

The primary purpose of the capacitor is to reduce arcing across the points as they open. This reduces erosion of the points, and also helps reduce radiated emissions. Adding the capactor, reduces dv/dt and hence peak voltage, as the points open. This might seem to be a bad thing as the voltage across the spark plug will be proportional to the voltage across the points as they open.

However the capacitor and coil primary inductance form a series resonant circuit which will oscillate when the points open. The voltage at the internal node of a series resonant curcuit can be much higher than the driving voltage, the value being dependent upon the Q, and this will be reflected in the voltage at the spark plug. In this way the loss of dv/dt is compensated for, at least to some extent.

The capacitor will carry the full coil primary current and the voltage across the capacitor will be plus and minus wrt 0V. So a ceramic or film capacitor will be needed. An electrolytic capacitor is unsuitable.

Andrew

Thanks again, this engine doesn't currently have a distributor hence the wasted spark setup i beleive. In the future i might look at a modern CDI system but for now i will maintain the points as they seem to have plenty of meat on the contacts.

Dont admint to fully understanding the electronic principles but very good explanation. Not being electronic minded what was confusing me and still is, is that in diagrams i see that whether the points are open or closed there would appear to be a path to GND i.e. always via the condenser but i guess that is not a real path as its a capacitor ? i.e. why does opening the points affect anything causing the primary coil to collapse ?. This is just for my curiosity.

Capacitors, Resistors, and Inductors have properties that can be built into useful circuits.

Resistors limit the passage of AC and DC current equally well. They're good for dropping voltages and preventing over-current. Physically resistance wire or carbon.

Capacitors block DC, but allows AC to pass. They do this by storing energy, charging and discharging. The frequency that can pass depends on the capacitance, so 50Hz mains requires hundreds of microfarads, whilst 5MHz will flow through 1000 picofarads. No heat is lost. Capacitors can be used to short unwanted AC to ground, or to pass AC from one circuit to another, without effecting DC functions. Physically, two plates separated by an insulator. It behaves like a secondary battery, except massively faster.

Inductors block AC while allowing DC to pass. They do this by storing energy, charging and discharging like a capacitor but in the opposite sense. The frequency that's blocked depends on the inductance, so 50Hz mains requires several henrys, whilst 5MHz requires about 100 micro-henries. Not much heat is lost. Inductors can be used to short DC to ground whilst keeping AC, or to pass DC from one circuit to another whilst blocking unwanted AC - a choke. Inductors generate a magnetic field, which can be picked up by an adjacent coil - a transformer. The ratio of turns converts volt-amps on one side to different volt-amps on the other, converting say, 10V @ 1A AC to 1000V @ 0.01A . Physically a coil of insulated wire,

Gets interesting when these characteristics are combined. For example, a single DC pulse fed into a capacitor and inductor connected in parallel will charge the inductor up by creating a magnetic field. When the inductor is fully charged, the field collapses causing current to flow into the capacitor until it is fully charged and the inductor is empty. At which point the capacitor discharges into the inductor, and the circuit oscillates at a frequency dependent on the combined capacitance and inductance. It would oscillate forever where it not for losses in the components.

In the ignition circuit:

• The contact breaker rotates switching DC power from the battery ON/OFF to the LT windings of the coil (a transformer) The OFF's are set to occur near TDC by moving the cam.
• When the contract breaker is closed, power from the battery flows into a low number of turns at the base of the coil and creates a magnetic field.
• When the contact breaker opens, the collapsing magnetic field induces a voltage into a large number of turns in the upper coil. As the magnetic field is changing, the induced voltage looks like AC to the coil, and so is magnified up to several thousand volts at the HT terminal. The energy is dissipated as a spark inside the cylinder where it ignites the fuel/air mix.

The capacitor has more than one function:

• The spark generates a large amount of Radio Frequency energy that can interfere with radios and TV's. The capacitor value is chosen to suppress these frequencies by shorting them to ground. Something between 0.2 and 0.01microfarads AC is usual. Note it's earthing high frequency AC whilst the DC battery is unaffected.
• The capacitor absorbs energy that would otherwise cause the contacts to arc as they open, so the contacts last much longer when one is fitted.
• The capacitor absorbs energy extra quickly when the magnetic field collapses after the contact breaker opens and the faster changing collapse puts more energy and voltage into the spark.

The circuit is an efficient and reliable way of creating timed high-voltage sparks from a low DC source. It exploits both AC and DC characteristics. No resistor in this particular circuit, but they were used in the past to drive a 6V coil with a 12V battery. The extra 6V causes the magnetic field to change faster and boosts the spark, but it's essential not to overload the coil. The resistor, or 'ballast', prevents the coil from getting more than 6V continuous. No doubt the extra-fat spark improves combustion, but I believe the main reason was better cold-starting. Cranking a heavy engine full of gummy oil on a cold morning would cause an ineffective spark due to the voltage of a chilled 6V battery being pulled down low.

In a way, the Art of Electronics is manipulating mixtures of AC and DC to create wanted effects. As the number of effects is almost unlimited, electronics is an entire field of engineering - bigger and more complicated than anything done by rude mechanicals!

Dave

wow thanks for that it has explained the dark magic surrounding the condenser part in the ignition CCT. The main point being it stops DC so i get now how it doesn't provide an alternate path to GND which is good for me. I now know i can get a double HT 6V coil wire it up and hey presto all that mysterious elastictrickery will apply its own physics and fingers crossed work. Just need to get hold of the engine now.

Thanks to all again

In the 70's Ford (Cortina and Escort era) used a 6V or Ballast coil but instead of a ballast resistor used a resistive LT wire (pink from memory) from the running position on the ignition switch and bypassing this with a direct wire when starting.

Several time we had cars coming in which would not start where this resisitive wire had been replaced by a normal one and totalled the coil or it had been removed as superfluous as there was 'already a wire there'.

Fortunately, there are fewer home fiddlers without the required knowledge today!

The joys of buying second hand who knows what improvements ! previous owners have done. I think for my engine the below ebay reference might do it ? i assume that the small red wire is BATT positive and the white lead the coil LT side. The plugs in the engine are champion plugs with nut attachment so would just need to put fork connectors or round on the HT lead ends with some insulation.

115488968805

If staying with conventional points/condenser ignition but using a twin outlet lost spark coil make sure you DONT get a coil for for electronic ignition, the resistance of the low tension side is much lower & you will almost certainly find if the engine stops with the points closed then the points will pass so much current they will glow red, if you cant find anything suitable then you could try fitting a ballast resistor in series with the coil but this will give you a weaker spark.

Thanks i checked the listing for the above coil from Ebay and it does state Points but doesnt give the resistance value.

I found another one which states secondary output resistance 10.8---11.2kohms

I would describe the action of the circuiit slightly differently to SOD.

Let's imagine that at time zero the points are closed, the full voltage is across the primary coil, but no current is flowing. The capacitor is shorted by the points, so holds no charge and hence the voltage across it is zero. At time zero plus a small amount a current will start to flow through the primary and points. The rate at which this current increases will be determined largely by the inductance of the coil. As we wait the current will build as 1 minus an exponential to an asymtote. The value of the asymtote will be V/R, ie, the applied voltage divided by the resistance of the circuit, due to the inductor and the points. So in the final steady state the value of the inductance plays no part in the final current value. In practice an ignition system will not allow the current to flow for anywhere long enough to allow the current to build to the maximum. The points normally open long before that. Usefully the first part of the current curve is almost a straight line, which makes the maths easier.

At a time after zero we have a current flowing in the primary and the points. If the points open without the capacitor the current will fall to zero and the voltage between the coil and points will rise in an attempt to maintain the current. This voltage is then amplified by the ratio of primary to secondary turns in the ignition coil to provide a voltage high enough to jump the spark plug gap. From a practical point the increase in voltage in the primary circuit may be enough to arc across the points, eroding same and radiating energy.

However, if we have the capacitor the current will continue to flow but into the capacitor, charging it up and creating a voltage across the capacitor. At some point the voltage across the capacitor will equal the applied voltage so no current will flow. But the inductance and capacitance form a series resonant curcuit. So on the way to the final stable DC state the energy will oscillate between inductor and capacitor. The voltage where the inductor and capacitor are joined can be many times higher than the applied voltage during this oscillation and is dependent upon the Q of the resonant circuit. As SOD says these oscillations die down due to losses in the components, mostly series resistance in the inductor. As the oscillations die down they decrease in amplitude until reaching a steady state DC value equal to the applied voltage.

Andrew