Opinions Please - True Or False
Discussion
Is the below correct or incorrect - I will not reply to any posts but will read through the replies with great interest -
1) The capacitor plays a very important part in the points and coil ignition system
2) When the contact points open the capacitor allows the several amps of current flowing in the coil to drop to zero in a rapid but controlled manner
3) This change in current induces a peak of 100 to 200 volts on the coil primary which is stepped up to 7 to 14 kilovolts at the secondary – enough to cause a spark at the plugs under operating conditions from idle to full power
4) The capacitor value is a compromise. If it’s too small more arcing takes place at the points reducing their life. If it’s too large the current changes too slowly and the peak voltage is reduced
5)The ideal value of the capacitor is in the range 0.2 to 0.4 microfarads but is not unduly critical
6) Lucas originally chose 0.4 microfarads for the Austin 7, but later replacements may be 0.2 microfarads
7) With no capacitor fitted not only does severe arcing take place at the points but more importantly even with new points the primary and secondary voltages are severely reduced
8) With no capacitor fitted tests showed no more than 7 kilovolts even with the coil output open circuit
SUMMARY
With no capacitor fitted the result is that the engine may start with difficulty and idle but when the plugs demand greater firing voltage as the throttle is opened and cylinder pressure increases a bad misfire will ensue. Continued operation allows the points to burn up and go high resistance, causing a further reduction in output voltage and a worsening misfire
1) The capacitor plays a very important part in the points and coil ignition system
2) When the contact points open the capacitor allows the several amps of current flowing in the coil to drop to zero in a rapid but controlled manner
3) This change in current induces a peak of 100 to 200 volts on the coil primary which is stepped up to 7 to 14 kilovolts at the secondary – enough to cause a spark at the plugs under operating conditions from idle to full power
4) The capacitor value is a compromise. If it’s too small more arcing takes place at the points reducing their life. If it’s too large the current changes too slowly and the peak voltage is reduced
5)The ideal value of the capacitor is in the range 0.2 to 0.4 microfarads but is not unduly critical
6) Lucas originally chose 0.4 microfarads for the Austin 7, but later replacements may be 0.2 microfarads
7) With no capacitor fitted not only does severe arcing take place at the points but more importantly even with new points the primary and secondary voltages are severely reduced
8) With no capacitor fitted tests showed no more than 7 kilovolts even with the coil output open circuit
SUMMARY
With no capacitor fitted the result is that the engine may start with difficulty and idle but when the plugs demand greater firing voltage as the throttle is opened and cylinder pressure increases a bad misfire will ensue. Continued operation allows the points to burn up and go high resistance, causing a further reduction in output voltage and a worsening misfire
mauld said:
The capacitor is fitted to suppress electromagnetic interference (caused by arcing) with old AM radio and TV signals.
You are thinking of a different capacitor. The capacitor fitted across the points (often refereed to as a 'condensor' by older enthusiasts) is there to increase the voltage output of the coil and reduce burning of the points. More modern cars sometimes have a capacitor on the 12v side of the coil which is there to reduce RF emissions from the cars wiring.Mr2Mike said:
You are thinking of a different capacitor. The capacitor fitted across the points (often refereed to as a 'condensor' by older enthusiasts) is there to increase the voltage output of the coil and reduce burning of the points. More modern cars sometimes have a capacitor on the 12v side of the coil which is there to reduce RF emissions from the cars wiring.
Firstly the OP is talking about contact braker ignition systems not modern systems.secondly the capacitor, condensor, suppressor, is fitted across the points which is the output of the coil to ground. It suppresses arcing caused by the high back EMF from the coil which is the the main cause of Electromagnetic radiation. There are other sources of EMI such as rotor arm, spark plug leads etc. The capacitor does all the other things but the OP didn't mention EMI. Very early radio transmitters were basicaly contact braker ignition systems producing a high voltage spark of very high power, the braker being the morse key.
Here is a link to book explaning interferance supression.
https://books.google.co.uk/books?id=lSVuDKZ_-PoC&a...
mauld said:
Firstly the OP is talking about contact braker ignition systems not modern systems.
secondly the capacitor, condensor, suppressor, is fitted across the points which is the output of the coil to ground. It suppresses arcing caused by the high back EMF from the coil which is the the main cause of Electromagnetic radiation. There are other sources of EMI such as rotor arm, spark plug leads etc. The capacitor does all the other things but the OP didn't mention EMI. Very early radio transmitters were basicaly contact braker ignition systems producing a high voltage spark of very high power, the braker being the morse key.
Here is a link to book explaning interferance supression.
https://books.google.co.uk/books?id=lSVuDKZ_-PoC&a...
I suggest you read the book again, more carefully secondly the capacitor, condensor, suppressor, is fitted across the points which is the output of the coil to ground. It suppresses arcing caused by the high back EMF from the coil which is the the main cause of Electromagnetic radiation. There are other sources of EMI such as rotor arm, spark plug leads etc. The capacitor does all the other things but the OP didn't mention EMI. Very early radio transmitters were basicaly contact braker ignition systems producing a high voltage spark of very high power, the braker being the morse key.
Here is a link to book explaning interferance supression.
https://books.google.co.uk/books?id=lSVuDKZ_-PoC&a...
The capacitor across the points is there for the correct operation of the ignition system, is NOT present just for EMI suppression. Removing the capacitor may increase EMI, but only because it will promote excessive arcing at the points and ignition system will not perform correctly (greatly reduced HT output).A capacitor connected from the coil supply to ground is used to suppress EMI, and as I stated is fitted to more modern cars. This isn't because the ignition systems are fundamentally different, they are not. In modern cars, an electronic switch (e.g. MOSFET, IGTB) simply replaces the points but in all other aspects it works on exactly the same principals as a classic Kettering ignition. EMI suppression is added simply because the law demands it, and cars are now fitted with far more electronic systems that could be affected were the suppression not fitted.
Adding a capacitor from ignition coil supply to ground on a classic car with points ignition will also assist in reducing radiated emissions from the wiring.
Edited by Mr2Mike on Wednesday 4th March 10:13
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