What is the ignition system of a car?

Section 1 Overview

When the compression of gasoline engine approaches the top dead center, the combustible mixture is ignited by the spark plug, which makes the combustion do work to the outside. Therefore, the spark plug is installed in the combustion chamber of the gasoline engine. The spark plug has a center electrode and a side electrode, which are insulated. When DC voltage is applied between the two electrodes of the spark plug and the voltage rises to a certain value, the gap between the two electrodes of the spark plug will be broken down, thus generating sparks. The minimum voltage required to generate a spark between two electrodes of a spark plug is called breakdown voltage. All devices that can generate electric spark between two electrodes of spark plug are called engine ignition system. (Insert the following figure in the page)

The ignition system of an automobile engine, like other electrical equipment on the automobile, is connected by a single wire, that is, one end is grounded.

Single-wire positive grounding → old car

Negative electrode grounding → new car

Whether the positive electrode is grounded or the negative electrode is grounded, the center electrode of the spark plug should be the negative electrode at the moment of ignition, because the hot metal surface is easier to emit electrons than the cold metal surface, and the temperature of the center electrode of the spark plug is higher than that of the side electrode when the engine is working.

According to the composition and the method of generating high voltage electricity, the ignition system can be divided into two types.

Classification and composition of power supply generating high voltage

1. battery ignition system battery or generator ignition coil and circuit breaker

2. Semiconductor ignition system battery or generator ignition coil and semiconductor components

3. Non-magnetic motor ignition system

Section II Composition and Working Principle of Battery Ignition System

I. Composition

The battery ignition system is mainly composed of power supply, ignition switch, ignition coil, circuit breaker, distributor, capacitor, spark plug, high voltage conductor and additional resistor. (Insert figure below)

Second, the working principle

The power supply is a storage battery, and the voltage is 12 V or 24 v. The ignition coil and the circuit breaker * * * jointly generate a high voltage above 10000 V, which is divided into a primary loop and a secondary loop. The ignition coil is actually a transformer, which is mainly composed of a primary winding, a secondary winding and an iron core. The circuit breaker is a cam operated switch. The brake cam is driven by the engine valve cam and rotates at the same speed, that is, the camshaft rotates once for every revolution of the crankshaft gear. In order to ensure that the crankshaft rotates twice and each cylinder ignites in turn, the number of ribs of the cam of the circuit breaker is generally equal to the number of cylinders of the engine, and the contact of the circuit breaker is 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 electrified, and the current flows back to the battery cathode from the battery anode through the ignition switch, the primary winding of the ignition coil, the contact of the circuit breaker 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, which is strengthened by the action of the iron core. When the contact is pushed away by the cam of the circuit breaker, the primary circuit is cut off, and the primary circuit quickly drops to zero, and the magnetic flux in the iron core rapidly decays or even disappears, thus inducing high voltage in the secondary winding of multi-turn thin wire, causing the gap between the two poles of the spark plug to break down and generate sparks.

The faster the current in the primary winding drops, 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.

At the moment when the contacts of the circuit breaker are separated, the distributor in the secondary loop is just aligned with the side electrode, and the secondary loop flows back from the secondary winding of the ignition coil to the secondary winding through the high-voltage conductor, distributor, spark plug side electrode and storage battery. (Insert figure below)

Two flash animations are inserted here: the animation of the working diagram of the ignition system. Swf and animation of ignition circuit diagram. Sovereign wealth fund.

Third, the role of several components.

1, capacitor

The capacitor is connected in parallel with the breaker contact. When the contact is disconnected, it has two functions.

(1) Protect the contact and charge the capacitor with self-induced current to prevent the contact from burning out.

(2) Accelerate the power outage and improve the secondary voltage.

When the magnetic flux in the ignition coil core changes, not only high voltage (mutual inductance voltage) is generated in the secondary winding, but also self-induced voltage and current are generated in the primary winding. At the moment when the contacts are separated and the primary current drops, the self-induced current is in the same direction as the original primary current, and its induced voltage is as high as about 300V, which produces a strong spark between the contacts and makes the contacts burn quickly. Affect the normal operation of the circuit breaker. At the same time, the change rate of primary current decreases and the induced voltage in secondary winding decreases. The spark in the spark plug gap becomes weak, making it difficult to ignite the mixture.

When the contact is closed and the primary current increases, there is also self-induced current in the primary winding, which is opposite to the primary current, slowing down the growth rate of the primary current and reducing the voltage generated by the secondary winding.

2. Additional resistance

The additional resistor is connected in series with the primary winding of the ignition coil.

The additional resistor is connected in series with the primary winding of the ignition coil to adjust the primary current and keep it basically stable.

The characteristic of additional resistance is that the higher the temperature, the greater the resistance, so it is also called thermistor.

The secondary voltage is related to the primary current. The greater the primary current, the stronger the magnetic field in the iron core, the greater the change rate of magnetic flux when the contacts are separated, and the higher the induced secondary voltage. Therefore, the current in the primary winding should be increased as much as possible. However, after the circuit breaker contacts are closed, the primary current gradually increases from zero according to the exponential law, and it takes some time to reach the stable value obtained by ohm's law.

When the engine speed is high, the contact closing time is short, the current is small when the primary circuit is disconnected, and the induced secondary voltage is low; On the contrary, when the engine speed is low, the contact closure time is long, the current is large when the primary is disconnected, and the induced secondary voltage is high. If the ignition coil is designed according to the high speed of the engine, the primary current is too large at the low speed, which is easy to overheat the ignition coil; If the ignition coil is designed according to the low engine speed, the primary current is too small and the secondary voltage is too low at high engine speed to ensure reliable ignition.

Additional resistance is to solve this contradiction. When the engine speed decreases, the primary current increases, and the resistance value of the additional resistor increases with the increase of its temperature, so that the primary current decreases and the ignition coil will not overheat. When the engine speed increases, the primary current decreases, and the resistance value of the additional resistor decreases with the decrease of its temperature.

During start-up, short-circuit the additional resistor to ensure the necessary intensity of the primary current.

The third section, before ignition.

First, why do you want to ignite in advance?

Ignition timing has a great influence on engine performance. It takes a certain time from the ignition of the spark plug to the combustion of most of the mixture in the cylinder, which produces a high explosive force. Although this time is very short, but because of the high speed of crankshaft, during this time, the angle of crankshaft rotation is still very large. If ignition is carried out at compression top dead center, the mixture will burn, and at the same time, the piston will move down, increasing the cylinder volume, resulting in low combustion pressure and reducing engine power. Therefore, it is necessary to ignite when the compression is close to the top dead center, that is, the ignition is advanced. When the spark plug is ignited, the included angle between the crankshaft position and the crankshaft position when the piston is at the compression top dead center is called the ignition advance angle.

Second, the influencing factors of ignition advance

The best ignition advance angle varies with many factors, the most important ones are engine speed and combustion speed of mixture, and the combustion speed of mixture is related to the composition of mixture, the shape of combustion chamber, compression ratio and other factors.

When the engine speed is constant, with the increase of load, the throttle valve opens, the combustible mixture entering the cylinder increases, and the pressure and temperature at the end of compression increase. At the same time, the proportion of residual exhaust gas in the cylinder is reduced and the combustion speed of mixed gas is accelerated. At this time, the ignition advance angle should be reduced appropriately. On the contrary, when the engine load decreases, the ignition advance angle should be increased appropriately.

When the throttle opening of the engine is constant, the crankshaft angle occupied by the combustion process increases with the increase of the speed, so the ignition advance angle should be appropriately increased. The ignition advance angle should be appropriately increased with the increase of rotating speed.

In addition, the ignition advance angle is also related to the antiknock performance of gasoline. When using gasoline with high octane number and good antiknock performance, the ignition advance angle should be larger.

Third, the ignition advance angle adjusting device

Automatic adjusting device: centrifugal ignition advance adjusting device.

Vacuum ignition advance adjusting device

Manual adjustment device: octane number corrector

Section 4 Main components of battery ignition system

I. Distributors

Function: Connect or disconnect the primary circuit.

The high voltage generated by the ignition coil is distributed to the spark plugs of each cylinder according to the engine.

Automatically adjust the ignition time according to the engine speed and load.

Composition: The distributor consists of circuit breaker, distributor, capacitor and ignition advance adjusting device.

Insert the following figure

The function of the circuit breaker is to turn on and off the primary circuit periodically, so that the primary current changes, thus inducing the secondary voltage in the ignition coil. The contact clearance of circuit breaker is generally 0.35~0.45 mm, which can be changed by adjusting the position of static contact.

The function of the distributor is to distribute the high-voltage electricity generated in the ignition coil to the spark plugs of each cylinder in turn according to the working order of the engine.

The capacitor is connected in parallel with the contacts of the circuit breaker, and its function is to reduce the electric spark generated between the contacts when the primary circuit of the ignition coil is disconnected, prevent the contacts from burning, speed up the change speed of magnetic flux in the ignition coil and improve the ignition voltage.

The ignition advance adjusting device is located at the lower part of the distributor and consists of a centrifugal ignition advance adjusting device (Figure 8-8) and a vacuum ignition advance adjusting device.

Two flashlights are inserted here: centrifugal ignition advance regulator. Vacuum ignition advance regulator. Sovereign wealth fund.

Second, the ignition coil

The ignition coil converts the low-voltage electricity of the power supply into the high-voltage electricity required for ignition of the spark plug. According to its core structure, there are two types:

Open magnetic circuit ignition coil: the open magnetic circuit ignition coil adopts a cylindrical iron core, the upper and lower ends are not connected together, and the magnetic field lines form a magnetic circuit through air.

Ignition coil with closed magnetic circuit: The iron core of the ignition coil with closed magnetic circuit is formed by stacking iron sheets in the shape of "mouth" or "sun". Magnetic circuit closed. (Insert figure below)

Third, the spark plug

Function: Introduce high-voltage electricity into the combustion chamber to generate sparks and ignite the mixture.

If the self-purification temperature is higher than 500 ~ 600℃ and the skirt temperature is lower than this temperature, the oil particles falling on the skirt of the insulator cannot burn immediately, forming carbon deposit and causing leakage.

hot spot

Spark plugs used in different engines are heated differently, which requires insulating skirts of different lengths. According to the different skirts, spark plugs are divided into cold types (the length of skirts is equal to 8 mm). Medium (skirt length is equal to 1 1mm and14 mm); Hot type (skirt length is equal to 16mm and 20mm).

Insert the following figure

Section 5 Semiconductor Ignition System

When the battery ignition system works, when the contacts of the circuit breaker are separated, sparks will be generated at the contacts, which will burn the contacts. Spark plug carbon deposits, easy to leak electricity, secondary voltage can not go up, can not be reliably ignited, leading to high-speed flameout. The semiconductor ignition system overcomes these shortcomings, with strong ignition jumping ability and reliable ignition. Semiconductor ignition systems can be divided into three categories: semiconductor auxiliary ignition systems, contactless semiconductor ignition systems and computer-controlled semiconductor ignition systems. (Insert figure below)

The working principle of the semiconductor ignition system is basically the same as that of the battery ignition system, except that the semiconductor ignition system and the battery ignition system generate high voltage in different ways. It uses some semiconductor components instead of circuit breakers in the battery ignition system to generate ignition pulse signals. For example, in the contactless semiconductor ignition system, an ignition generator (sensor) is used instead of a circuit breaker. The commonly used sensors are Hall type, magnetoelectric type and photoelectric type.

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