ENGINE Chapter 10 Ignition System

ENGINE 

Chapter 10.

Ignition System

1. Point type Ignition

The ignition system is to fire the mixture compressed in the combustion chamber making a flame with a spark plug.

When the plus and minus poles of the 12V battery is shorted, a spark can be made. However, it is too weak to ignite the mixture. Therefore, this voltage should be boosted up to 10,000∼30,000V. The ignition system consists of a device for boosting the voltage, a device for distributing the ignition timing and a series of spark plug.

To boosting the voltage, an ignition coil, an electric inducer, is used. The ignition coil comprises of a iron core shaped of rod, the secondary coil wound with about 20,000~ 30,000 turns using a hairy thin copper wire around the core and the primary coil wound with 150~300 turns using copper wire of 0.5∼1mm diameter over the secondary coil. When a current is flown at the primary coil, the iron core will be an electromagnet. And at time when the current at the primary coil is broken, a boosted high voltage is induced at the secondary coil.

The distributor has a device for controling  the current flowing at the primary coil and a cam having the same number of extruded portions in the middle of it. When this cam presses the arm by rotating, the point attached in front of the arm is opened to cut the current of the primary coil (contact breaker). And the distributor has also a device, overlapping with the contact breaker, for distributing the boosted voltage of the secondary coil to each plugs.

At the cam of the contact breaker, an advancing device for controlling the ignition timing at the variations of engine speed to send the boosted high voltage to the spark plug in time. As the cam should rotate with 1/2 of the engine rpm to match the ignition timing of the plug, it should be attached at the end of camshaft having the same rotation speed. The advancing device shall be explained at the section of 『Ignition Timing』 in detail.

After that, the high voltage generated by the ignition coil is sent to each spark plug through a series of Hihg-Tension Cord.

This ignition system is called contacting or point type. If the driving for the contact breaker is performed by a transistor, then it is called the full transistor type. Furthermore, developing the system, the distributor is driven by a computer in the distributor-less ignition device.

2. Full Transistor type Ignition

In the point type ignition device, the point is opened by the cam, and high voltage is induced at the secondary coil when the current at the primary coil is cut. Similarly, when the switch is turn off, a small unwanted spark will be made at the contact point. As the same of mechanical inertia, the electrical current has the inertia. This unwanted spark will be prevented by inserting a condenser within the ignition circuit.

Furthermore, the point would be burned or not work normally at the high speed. Therefore, for intermitting the current of the primary coil, a transistor is developed instead of the mechanical point.

As being many kinds in the transistor, the application method is various also. When it is used for a switch, NPN type transistor is used. This consists of a base made of P type semiconductor and two N type semiconductors at the both side, one is a collector, and the other is emitter.

In normal state, the current is not transmitted from the emitter to the collector of NPN type transistor. However, if a little current (base current) is applied between the emitter and the base, then an amplified current is induced from the emitter to the collector. Using this characteristic, it can be utilized as a switch.

Instead of the cam and point in the distributor, the signal generator is equipped for detecting the ignition timing, the electrical signal is sent to the igniter including a transistor. Then the base current made by the signal generator is applied to between the emitter and the base, and then an amplified current is generated from the emitter to the collector. Using this current amplifying, the intermittent current is applied to the primary coil, so a boosted voltage can be generated at the secondary coil.

The signal generator consists of a rotor having the same number of extruded portions with that of the cylinder, a permanent magnet, and a pick-up coil detecting the change of magnetic flux. As the signal rotor rotates with the 1/2 speed of the engine speed, the extruded portions passes through the gap between the pick-up coil and the permanent magnet corresponding with the ignition timing. The magnetic flux generated by the permanent magnet will be changed, so an induced current will be flown at the coil. This current is used for the base current of the transistor. Doing so, the trouble from the contact breaker can be resolved.

3. Distributor-less Ignition

In the full transistor ignition system, the primary coil current switching is performed by the signal generator and the transistor, while the advancing and distributing of secondary current is performed by the same device used in point ignition system. The distributor-less ignition system is that the advancing of ignition timing is performed by a computer using an electrical signal from the sensor for ignition timing, and the ignition is performed using generated secondary current from the ignition coil installed near the ignition coil.

The main feature of this ignition device is the high-tension cable connecting the ignition coil – the distributor – the spark plug. As this cable will be short only for connecting the spark plug and the coil just nearby or this cord is not needed in some cases, the troubles from the electromagnetic wave interference from the high current of the high-tension cable or the ignition faults from the electrical resistance of the cable can be prevented. Additionally, the electrical advance device is more compact than the mechanical one.

Inductive type crankshaft position sensor

Optical type crankshaft position sensor

In the sensor for deciding the ignition timing, the crankshaft position sensor or the camshaft position sensor, one type comprises of a timing rotor driven by the camshaft and a pick-up detecting the position of the rotor electrically. Anther type comprises of a LED attached at the rotor blade driven by the camshaft and a photo diode to decide the ignition timing.

In the ignition method, there are the individual ignition method and the bank ignition method.

Individual ignition system

Bank ignition system

In the Individual ignition method, the ignition coil is installed at each cylinder to ignite sequentially according to the order decided by the ECM. In the bank ignition method,  the ignition spark is occurred at two cylinders at the same time by one ignition coil. In this time, one cylinder is in the compression stroke and the other cylinder is in the exhaust stroke. The ignition spark at the compression stroke is effectively used, but the spark at the exhaust stroke is meaningless. In this method, the used amount of the transistors and coils is half of individual ignition method, so the cost will be down.

4. Spark Plug

The spark plug ignites at the compressed mixture to fire it according to the spark arc by the high voltage generated from the ignition coil. The spark plug contacts, at first, with the mixture having the similar temperature of outer atmosphere in the intake stroke, After that, it contacts with the exhaust gas over 2000℃ and generates high voltage of 20,000V. Therefore, it is the device working in most severe condition.

The spark plug is used for many kinds of engine in common, and manufactured with a standardizing specification internationally. There are many kinds according to the dimension, structure, performance and characteristic (especially, heat characteristic). This is distinguished by the alpha numerical indicator. This indicating character differs from the manufacturer, so be careful when replacing them. Generally, spark plug is classified as 14mm, 12mm, and 10mm according to the size of attaching screw. To make the combustion chamber compact, the smaller plug is the better. However, the smaller can be the easier to be affected by heat. It is very important to inspect the temperature situation of the engine in order to use the spark plug.

The spark plug, during driving, is affected by the various temperatures. The most affecting temperature is the combusted amount of fuel mixture per time. As the engine is high speed, the temperature of plug will be high. Even in the same driving condition, the temperature differs according to the heat range of the plug.

The heat range is the degrees of which the spark plug radiates the heat acquired from the chamber. As the plug easily radiates the heat, the heat range is high. In racing engine, the cold type having high heat range should be used to prevent the temperature from increasing easily. Contrarily, if the engine is usually used in low speed, the hot type having low heat range should be used to prevent from being cold easily.

The heat range is represented with numeric character on the plug. It differs with the manufacturer. It should be selected according to the used standard plug. If the plug not corresponding with the engine features and driving conditions, for example, if it is used in the low temperature, then carbon slug will be deposited on the end portion of the plug, so it doesn’t work. If the temperature is excessively high, then the mixture is exploded before the ignition is worked, that is pre-ignition will be occurred.