ignition system Can generate enough voltage to break through spark plug gap
The voltage required for spark plug electrode breakdown to generate spark is called breakdown voltage. The secondary voltage generated by the ignition system must be higher than the breakdown voltage in order to make the spark plug jump. The breakdown voltage is affected by many factors, including:
1) Spark plug electrode gap and shape: the larger the spark plug electrode gap, the higher the breakdown voltage; The tip of the electrode is sharp and the required breakdown voltage is low.
2) Pressure and temperature of the mixed gas in the cylinder: the higher the pressure of the mixed gas, the lower the temperature, and the higher the breakdown voltage.
3) Electrode temperature: the higher the temperature of the spark plug electrode, the smaller the gas density around the electrode, and the lower the breakdown voltage. [2]
The spark shall have sufficient energy
When the engine works normally, only 1~5mJ spark energy is needed because the temperature at the end of the mixed gas compression is close to its self ignition temperature. However, when the mixture is too rich or too lean, the engine needs higher spark energy when starting, idling, or when the throttle is opened sharply. With the improvement of economy and exhaust purification requirements of modern engines, it is urgent to improve spark energy. Therefore, in order to ensure reliable ignition, the high-energy electronic ignition system should generally have a spark energy of 80~100mJ, which should be higher than 100mJ when starting.
The ignition time shall adapt to the working conditions of the engine
First, the ignition system shall ignite according to the working order of the engine. Secondly, ignition must be carried out at the most favorable time.
Because the mixture burns in the cylinder for a certain time, the mixture should not be ignited at the top dead center of the compression stroke, but should be advanced appropriately so that when the piston reaches the top dead center, the mixture has been fully burned, so that the engine can obtain greater power. The ignition time is generally expressed by the ignition advance angle, that is, the angle at which the crankshaft rotates during the period from the time when the spark is sent to the time when the piston reaches TDC.
If the ignition is too late, and the ignition is only started when the piston reaches TDC, the combustion of the mixture is mainly completed during the downward process of the piston, that is, the combustion process is carried out when the volume increases, so that the area of contact between the hot gas and the cylinder wall increases, thus the heat amount converted into effective work is relatively reduced, and the maximum combustion pressure in the cylinder decreases, leading to engine overheating and power reduction. If the ignition is too early, because the combustion of the mixture is completely in the compression process, the combustion pressure in the cylinder rises sharply and reaches the maximum before the piston reaches TDC, which makes the piston recoil and the engine perform negative work, which not only reduces the power of the engine, but also may lead to deflagration and unstable operation, accelerating the damage of moving parts and bearings. [3]
Traditional ignition system
The power supply is the battery, whose voltage is 12V or 24V, and the ignition coil and the breaker together produce a high voltage of more than 10000V. It is divided into primary circuit and secondary circuit. The ignition coil is actually a transformer, which is mainly composed of primary winding, secondary winding and iron core. The breaker is a cam operated switch. The cam of the breaker is driven by the engine gas distribution cam and rotates at the same speed, that is, the crankshaft gear rotates twice and the camshaft rotates once. In order to ensure that the crankshaft rotates twice and each cylinder ignites in turn, the number of ridges of the breaker cam is generally equal to the number of cylinders of the engine. The contacts of the breaker are connected in series with the primary winding of the ignition coil to cut off or connect the circuit of the primary winding.
When the contact is closed, the primary circuit is energized, and the primary current flows from the positive pole of the battery to the negative pole of the battery through the ignition switch, the primary winding of the ignition coil, the contact arm of the breaker, the contact, and the ground, which is a low-voltage circuit. When the contact is disconnected, when the primary winding is energized, a magnetic field is generated around it and strengthened by the role of the iron core. When the breaker cam opens the contact, the primary circuit is cut off, the primary circuit rapidly drops to zero, and the magnetic flux in the iron core then rapidly decays and disappears. Therefore, a very high voltage is induced in the secondary pole winding with many turns and thin wires, so that the gap between the two poles of the spark plug is broken down and sparks are generated.
The greater the speed of current decrease in the primary winding, the greater the change of magnetic flux in the iron core, and the higher the induced voltage in the secondary winding. The primary circuit is a low-voltage circuit, and the secondary circuit is a high-voltage circuit.
Traditional ignition system Electronic ignition system
When the traditional ignition system works, the contact of the breaker will produce sparks at the contact and burn the contact at the moment when the contact is separated. When the spark plug deposits carbon, it is easy to leak electricity, the secondary pole voltage cannot go up, and it cannot ignite reliably, resulting in high-speed misfire. The semiconductor ignition system overcomes these shortcomings and has a strong ability to jump fire, making ignition reliable.
Electronic ignition systems are generally divided into the following three categories:
(1) The ignition system composed of non-contact circuit breakers composed of electromagnetic, infrared or Hall components is called contactless igniter. Its amplification circuit is divided into transistor circuit and capacitor discharge circuit.
(2) The ignition system controlled by the ECU (Electronic Control Unit) uses the microprocessor in the ECU to determine the ignition time according to the signal of the crankshaft angle sensor, so it has no breaker, only the distributor, and directly controls the connection and disconnection of the primary circuit of the ignition coil according to the signal sent by the ECU.
(3) Distributor less ignition system is the most advanced ignition system at present. The crankshaft sensor sends not only the ignition time signal, but also the cylinder identification signal, so that the ignition system can send the ignition signal to the designated cylinder at the designated time. This requires that each cylinder is equipped with an independent ignition coil, but if it is a six cylinder engine, cylinders 1, 6 Cylinders 2, 5 and 3, 4 share one ignition coil respectively, that is, there are three ignition coils in total. Obviously, when each ignition coil is ignited, there is always one cylinder that is ignited empty, which should be noted during detection.
The contactless ignition system can use a low impedance inductance coil, which can greatly increase the primary current, make the secondary voltage up to more than 30kV, enhance the ignition energy to improve the ability to ignite the lean mixture, improve fuel economy and reduce exhaust pollution at the same time. The distributor free ignition system is completely an electronic device without mechanical moving parts, which completely solves a series of failures caused by cam and bearing wear and contact ablation gap misalignment.
