Bosch electronic ignition
The Bosch electronic ignition was one of the first hybride electronically controlled ignition system. In fact, it is only an amplifier of the pulses coming from the standard breaker contact. Later ignitions feature breakerless distributor pulses, followed by very sophisticated microprocessor controlled ignitions.
The first ignition is hybride because it combines electronics (amplifier) with mechanics (breaker points). This system works very well.
The main working elements are:
The breaker points are fitted inside the distributor. The points are opended and closed by a lob on the shaft of the distributor. In a normal old fashioned ignition, the current through the coil is directly controlled by the breaker points. This limits the current through the coil because the points will burn when the current is too high.
The hybride electronic ignition was designed to overcome this problem. This ignition still features points, but the points only switch a very low current. This current is amplified by an amplifier. In this way, the coil current could be increased resulting in a stronger spark, especially during high revs.
The contact angle or dwell time (the amount of time the contacts are closed) remains very important in this system, because the amplifier does not change the timing, it only amplifies breaker point pulses.
Normal contact angle or dwell time in an 8 cylinders engine amounts 30 degrees. The total time per cylinder in a fourstroke 8 cylinder engine with only one coil is 360/8 = 45 degrees. This means that the contacts are closed for 30/45 = 2/3 of the time and opened for 15/45 = 1/3 of the time.
Taking into account that the maximum revs of the 450SE is 6000 revs/min = 100 rev/sec. On a four stroke engine with one ignition coil, the distributor shaft rotates with half the engine speed = 50 rev/sec. This means that one complete rev will consume 1/50 sec = 20ms. In that case, every cylinder gets 20/8 = 2.5 ms at maximum revs of 6000 rev/sec.
Combining the two calculations gives the amount of time the coil has to charge and discharge. Charge time (when the points are closed) amount 2/3 * 2.5 = 1.6 ms. The discharge time amounts 1/3 * 2.5 = 0.8 ms. As can be seen, charge and discharge times are very short at high revs.
The ignition coil transforms the low battery voltage into a high voltage pulse capable of bridging the spark plug gap and thus producing a spark in this gap.
The ignition coil is made up of two windings, a primary and a secondary winding, with a ratio of 1:100. A low voltage on the primary coil can be transformed into a 100 times higher voltage on the secondary coil. However, this transformation will only take place when an alternating power source is used. Therefor the application of the previously described contact breaker points to make and break the current flow of the primary winding of the ignition coil.
A coil can be charged and discharged. However, it resists itself against charging and discharging. Due to this principle, a coil will try to sustain its charge (magnetic field enegy) when it is disconnected from its power source. When it is disconnected from its power source, it reacts by generating its own current to sustain the magnetic field. It utilizes the energy stored in the magnetic field that was generated when the coil was connected with the power source.
Generating a spark goes as follows:
There is only limited time to charge a coil. The lower its internal resistance, the higher the charge current, the shorter the charging time. Charge current was limited by the point which could only switch currents with a maximum of 3-5 amperes. The hybride combination of points and amplifier can raise this current to 5-7 amperes.
Balast resistors are used to limit the charge time in combinating with a coil with low internal resistance. The balast resistors limit the maximum current through a fully charged coil, thus protecting coil and points.
The amplifier is build up of a few transistors, some diodes and zeners, some resistors and capacitors. It functions as a current amplifier that amplifies the small current over the contact breaker points to a high current through the primary ignition coil.
The current through the contact breaker points is kept to a certain level (not close to zero). This current ensures that the contacts remain clean (burned clean).
Balast resistors are used to control the current through the contact breaker point and the primary ignition coil. One resistor is a permanent resistor to limit the current through the primary coil. The other resistor is used to protect the contact breaker point. This resistor is passed (not used) during start to ensure a strong spark while starting, when the battery voltage is low due to the high current demand of the starter.
The distributor contains contact breaker point, a capacitor, a vacuum advance, a centrifugal advance, contacts for the electronic ignition and a shaft with lobs that is connected (through a gear) with the crankshaft.
The distributor shaft rotates with half the speed of the cankshaft, because a four stroke engine needs only one spark per cylinder every two revs.
It has two advance mechanism: a vacuum advance and a centrifugal advance.
The centrifugal advance changes ignition timing dependent of the speed of the distributor shaft. As can be imagined, it takes some time (few miliseconds) for a fuel/air mixture to burn completely after it is ignited. To reach maximum efficiency (the highest cylinder pressure at the right moment) the fuel/air mixture must be ignited a few miliseconds before the top dead position of the cylinder is reached. Just at the moment the mixture is burned completely (the highest cylinder pressure), the cylinder will pass its top dead position ensuring the best possible efficiency.
The centrifugal advance ensures that the fuel/air mixture is ignited at the right moment, a few miliseconds before TDC. Even without centrifugal advance (at low revs) their is some advance in ignition timing. At 600 revs/min, the advance is 10 degrees which is 10/2=5 degrees distributor advance. At 600 rev/min of the engine, the distributor shaft makes a single rotation in 200ms. This means that 5 degrees distributor advance = (5/360)*200 = 2.8 ms advance in ignition timing. When the engine is reving at higher speeds, the time for a single rotation decreases. At 6000 revs/min a single rotation is performed 10 times as fast (20ms) and the fixed advance is also 10 times shorter (0.28ms). To ensure proper timing and high engine efficiency, the centrifugal advance makes sure that the correct timing is maintained even at high engine revs.
The vacuum advance changes the ignition timing dependent of the vacuum in the intake manifold. This vacuum is high at closed or part throttle when the fuel/air mixture is lean. A lean mixture needs more time to ignite and must be ignited earlier to ensure a high efficiency of the engine process (the highest cylinder pressure at the right moment). The vacuum advance advances the timing when manifold vacuum is high. At WOT (Wide Open Throttle) the fuel/air mixture is rich, ignites easily and advance is not needed. Also at WOT, vacuum is low and their is no vacuum advance.