Ignition

 

“The problem of problems.”  Karl Benz 1880

 

At the dawn of internal combustion motors many strange and unusual devices were invented to provide the ignition needed to make the motors work.

 

Lenoir, the original inventor of stationary gas motors way back in 1860 used the first spark plug fired by a battery buzzer coil device.   Otto used a tiny trap door with a chamber for a bit of burning coal gas to slide over a hole in the top of the combustion chamber but found that it was a poor idea..   Karl Benz used a variation of the battery vibrator ignition on the first auto and was so frustrated with the problems of reliability that he provided the quote about ignition in the subtitle above..

 

Then a magnetic oscillating magneto device using an upgraded primitive spark plug was used..  (the great scientist Siemens of Germany invented the swinging twin T magneto)  Daimler’s group came up with the strange idea of a heated ceramic tube opening into the combustion chamber. This worked well at the time and started easily. But the external fires needed to keep the tubes hot were a dangerous design.   

 

Both Count Zeppelin (who wanted to use Daimler motors in his hydrogen airships) and Emil Jellinek (Daimlers first and best salesman, racing enthusiast, and father of Mercedes Jellinek whom the Mercedes auto is named after) insisted that magneto ignition be used.   

 

Zeppelin because of the fire hazard with the hydrogen in the airships and Jellinek because he had experienced so many serious fires in his Daimlers when he was racing..

 

Robert Bosch enters at this point as the maker of most of the oscillating magneto low voltage ignition systems used..   Then one of Bosch’s employees came up with the high voltage rotary magneto and that became the standard.

 

Daimler always resented Bosch and was angry that his beloved hot tube ignition was discarded so quickly.

 

But..   There is always a but.   The magneto ignition worked well at high speeds when the motor was running but it was very difficult to start.   Since the magnets of the day were weak and lost their magnetism slowly and the amount of spark was directly dependent on crankshaft speed hand crank starting a magneto motor was difficult at best and almost impossible in the cold or wet..

 

The early racing cars carried a mechanic to adjust timing and mixture and operate the fuel pump while the race was going on.  But a big reason for the extra man was the “Run and Bump” starting used.   The driver and mechanic would run along beside the car pushing it as fast as possible.  Then they would jump in and the driver would put the car in gear and let out the clutch spinning the motor fast enough to get the magneto working.

 

The vibrator shower of sparks battery ignition (which had been around for many years) on the other hand was easy to start but very poor at higher rpm’s. (in those days 1800 rpm was considered high)   As a result many manufacturers began to use both battery igniton for starting and then switch to magneto ignition for running..   Complicated, expensive, and the dry cell batteries had to be replaced often.   The magnetos needed recharging of their magnets periodically as well.

 

 

 

 

 

 

 

Then in about 1906 one of the greatest inventors in history began working on an improved battery ignition.  Charles Franklin Kettering working with his small group in a converted barn in Ohio invented the standard ignition system used from 1909 to the present..   The Kettering ignition.

 

 

 

 

 

 

 

 

This ignition provided a single spark at the right moment and had the precision and reliability needed to make automobiles more than a toy or curiosity.  

 

This is how the Kettering system works.

 

The electrical current from the battery first goes through a resistor to limit current when the points are closed and the motor is not running and to provide a better shape to the pulse of electricity generated by the coil.  The resistor is bypassed when using the electric starter because the voltage drop from the high current used by the starter reduces the spark.  Then the current goes through the primary winding of the coil.  This is the low voltage high current side of the coil windings.

 

Then it reaches the points.  They are tungsten switch contacts inside the distributor opened by a cam which rotates at half the speed of the crankshaft.   The points are normally closed allowing current to flow to the coil primary winding.   This period when the points are closed is called the dwell time.  

 

When the points open from the passing bump on the cam the magnetic field within the coil collapses very quickly generating a high voltage pulse in the secondary winding of the coil.   There is a serious problem with the points arcing when they open and a capacitor (used to be called condensor) is in parallel with them to absorb the voltage spike that would cause them to arc and burn out quickly.

 

The high voltage pulse in the secondary winding of the coil is led through the rotor to the proper contact on the distributor cap for the cylinder which is firing.  Then through specially insulated high voltage wire to the sparkplug.  At the sparkplug a spark jumps the gap between the outer ground electrode and the center positive electrode lighting the fuel/air mixture at the proper instant.

 

The timing of the spark is critical for proper operation of the motor and at first it was the responsibility of the driver.  The ignition advance/retard was manually done with a lever on the steering column..    Retard for starting and advance for more power when running but listen for pinging and retard as necessary..   Quite an interactive situation..   Drivers were much busier in the early days..

 

Then several inventions took this responsibility away from the driver and made it part of the mechanism.   Advancing the ignition as rpm increased compensated for the  amount of time it took for the mixture to burn.   For starting the spark would be retarded automatically and using springs and centrifugal weights the advance would increase with rpm.  See picture below for the weights and springs of the advance mechanism.

 

 

 

 

 

Then it was found that when driving at low speeds with the throttle almost shut the reduced filling of the cylinder caused the mixture to burn more slowly than at full throttle.  So an additional advance mechanism was invented that had a vacuum diaphram and spring to advance the timing at high vacuum conditions when the throttle was almost closed and the motor was running at much reduced power.

 

This combination served very well to make the ignition function so smoothly and unobtrusively as to be ignored until a failure from lack of maintenance occurred.

 

The weakest link in the chain of reliability was the points.  After that the spark plugs..    The points were only capable of about 5-10 thousand miles of operation before they needed adjustment or replacement.   The tiny spark every time they opened eventually destroyed them..   Consider that in 5000 miles on a v8 motor that is about fifty million nasty little sparks eating away at the tungsten contacts..

 

As the points were disintegrating it would cause the gap to widen and the dwell time to increase.  This would increase the current beyond what was needed to generate the maximum magnetic field in the coil causing it to heat up and reduce it’s output.    Also with the larger gap the timing would be retarded, sometimes enough to cause overheating in the summer..   Too little or too much gap both could cause problems.

 

Meanwhile in the motor the spark plugs were losing metal from the sparking and the gap would widen and the electrodes would be rounded both causing more voltage to be needed to fire the mixture properly..

 

Then about 45 or 50 years after Kettering’s invention (a long time for technology to stay the same) a major improvement to his ignition system was made.

 

Others had similar devices but I will discuss the GM HEI Ignition system.   This concept brought Kettering’s 19^th century design into the 20^th century.

 

The points were discarded and replaced by a magnetic reluctor.   This is a star shaped piece of metal rotating within a circular magnetic pole with matching points directed inward toward the center.   See picture below for a view of the reluctor and sensor pole in the HEI.

 

 

 

 

The change in magnetic flux in the reluctor/sensor assembly was detected by a winding at the base of the sensor and provided an input to the transistorized switching circuit in the ignition module.   The output of the ignition module went to a coil similar to the Kettering design but GM chose to package the coil in the top of the distributor cap to make the ignition a one piece unit.  

 

  There was no need for the capacitor to protect points but one was installed to prevent interference to the car radio..

  

The reluctor/sensor/switch module arrangement took the place of the unreliable points and could go for hundreds of thousands of miles without problems..  A most impressive advance.  

 

 

See picture above for a look at the complete integrated system.   And picture below shows a schematic of the HEI system.

 

 

 

 

The voltage available from the HEI is much greater than the old points style system.  About 35-40 thousand volts compared to 20-25 thousand  from the points stystem.   This allowed the sparkplugs to run a larger gap and be less sensitive to wear and fouling.  A superior system in every way except one.  If you are ever shocked by the high voltage from a HEI system you will never forget it.  

 

The GM HEI is a fine reliable system for stock autos but..  (another one of them)  

 

It does require some changes to make it suitable for high performance use.   Changes to make the timing more precise, to make the advance curve provide maximum horsepower, and to allow higher rpm operation.

 

These issues will be addressed in my next article..

 

Rick Draganowski

Gold Beach, Oregon