In various technical texts, the term “switch” can be found. What it is? In the most general sense, it is a device for switching electrical circuits (signals), which can be electronic, electron beam or electromechanical.
In the narrow sense, this is usually called the ignition switch, which is equipped with any vehicles with gasoline engines. This type of switch, mainly automotive, is the subject of this article.
Background to ignition systems
As you know, in each cycle of the gasoline internal combustion engine, there is a stage of preparation of a fuel-air combustible mixture and a stage of its combustion. But for the mixture to burn, you need to set it on fire with something.
The first solution used in the earliest automobile ICEs was to ignite the mixture from a glow tube inserted into the cylinder and preheated before starting the engine. During its operation, the temperature of this tube was constantly maintained due to the mixture burning in each cycle of operation.
Interestingly, the magneto spark ignition system was used in parallel with the ignition ignition of auto engines, but at first only for industrial gas ICEs. This principle was quickly adopted by car manufacturers, and after the invention of R. Bosch in 1902, the usual spark plug spark system became generally accepted.
Spark ignition principle
Currently, the most common battery ignition system, containing a current source in the form of a car battery during start-up and a car generator when the engine is running, an ignition coil, which is a transformer with a high-voltage secondary winding, to which a spark-forming spark plug is connected, as well as an ignition distributor (switch) . The operation of the switch consists in periodically interrupting the current circuit of the primary winding of the ignition coil. With each such interruption of the current, its magnetic field existing at the points of space occupied by the wires of the secondary winding of the ignition coil decreases very quickly. Moreover, in accordance with the law of electromagnetic induction, a very large vortex electric field arises at the same points in space , the intensity of which creates a high (up to 25 kV) EMF in the secondary winding of the ignition coil, broken by the spark plug electrodes. The voltage between them quickly reaches a value sufficient to breakdown the air gap, and then an electric spark jumps, igniting the fuel-air mixture.
What is switched in the ignition system?
So, car switch. What is it and why is it needed? In short, this is a device whose task is to break the current circuit in the primary winding of the ignition coil at the most favorable moment for this.
In a four-stroke internal combustion engine, this moment occurs at the end of the compression stroke (2nd engine operation cycle), shortly before the piston reaches the so-called top dead center (TDC), in which the distance from any piston point to the axis of rotation of the engine's crankshaft is maximum. Since the crankshaft performs a circular rotational motion, the moment of interruption of the current is tied to a certain position before it reaches the TDC and the piston. The angle between this crankshaft position and the vertical plane is called the ignition timing. It ranges from 1 to 30 degrees.
Given the story, the question: “Car switch: what is it?” - it should be answered that this is first mechanical, and later, as technology develops, an electronic current chopper in the ignition coil.
Mechanical predecessor of the ignition switch
Actually, this device was called a switch only in recent years, after it became fully electronic. And before, starting in 1910, when an automatic ignition system first appeared on Cadillac cars, its function, along with other tasks, was performed by a breaker-distributor (distributor). Such a duality of the name arose due to its dual function in the ignition system. On the one hand, the current in the primary winding of the ignition coil must be interrupted - hence the “interrupter”. On the other hand, the voltage of the high-voltage winding of the ignition coil must be distributed alternately among the candles of all cylinders, and with the desired lead angle. Hence the second half of the name - "distributor".
How did the distributors work?
The breaker-distributor has an internal shaft driven in rotation from the crankshaft, on which a dielectric rotor-slider is mounted with a rotating current-carrying plate at its end. A spring-loaded carbon brush slides across the plate, connected to a high-voltage central contact in the distributor cover, which, in turn, is connected to the secondary winding of the ignition coil. The current-carrying plate periodically approaches the contacts of the high-voltage wires located in the distributor cover that go to the spark plugs of the cylinders. At this moment, a high voltage appears in the secondary winding of the coil , which breaks through two air gaps: between the current-difference plate and the wire contact to this candle and between the electrode electrodes.
Cams are installed on the same shaft, the number of which is equal to the number of cylinders, and the protrusions of each cam open simultaneously with the connection of a specific candle the contacts of the current chopper included in the primary circuit of the ignition coil.
In order to prevent sparks between the breaker contacts when opening, a large capacitor is connected in parallel with it. When the contacts of the chopper of the induction emf are opened in the primary winding, it causes a charge current of the capacitor, but due to its large capacity, the voltage on it, and therefore between the open contacts, does not reach the value of air breakdown.
But what about the lead angle?
As you know, with a decrease in the crankshaft speed, the mixture in the cylinders must be ignited in the compression stroke later, right in front of the TDC itself, i.e. the ignition timing should be reduced. On the contrary, with an increase in the rotational speed, the mixture in the compression stroke must be ignited earlier, i.e. lead angle increase. In distributors, this function was performed by a centrifugal regulator, mechanically connected to the cams of the current chopper. He turned them on the distributor shaft in such a way that they, earlier or later in the compression stroke of the mixture, opened the breaker contacts.
Changing the lead angle is also necessary at a constant frequency, when the load on the engine changes. This work was performed by a special device - a vacuum ignition regulator.
The advent of the first switches
By the end of the 70s of the last century, it became clear that the weakest node of the distributor are the breaker contacts, through which the full current of the primary winding flowed. They constantly burned and failed. Therefore, the first solution was a special electronic circuit of the switch to interrupt the current in the coil. In its input low-current circuit, wires from the terminals of the traditional contact switch of the distributor were connected. However, now his contacts were interrupted not by the full current of the ignition coil, but by a small current in the input circuit of the switch.
Actually, the electronic switch was structurally made in a separate unit and connected (at the request of the driver) to the classic distributor. Such an ignition system is called contact electronic. She was very popular in the 80s of the last century. And in our time, you can still find cars equipped with it.
The switch circuit of the contact electronic system was assembled on transistors.
The next step is to reject the contact breaker.
The contact current chopper, even in the low-current version used in the contact electronic ignition system, remained a very unreliable unit. Therefore, automakers made considerable efforts to exclude it. These efforts have been successful since the creation of the proximity sensor distributor based on the Hall sensor.
Now, instead of several cams, a cylindrical hollow screen with slots and blinds between them began to be installed on the distributor shaft, the number of blinds and slots being equal to the number of engine cylinders. The curtains and slots of the screen move in a magnetic field created by a permanent magnet, past the miniature Hall sensor. While the screen curtain moves past it, the output voltage of the Hall sensor is missing. When the curtain is replaced by a slot, the front of the voltage pulse is removed from the Hall sensor by an electronic circuit, indicating the need to interrupt the current in the primary winding of the ignition coil. This voltage pulse is transmitted through wires to the current switch unit in the ignition coil, where it is pre-amplified and then used to control the main power switching cascade.
Another option for a non-contact distribution sensor is an assembly with an optical sensor, which uses a phototransistor instead of a Hall sensor, and an LED instead of a permanent magnet. The optical sensor has the same rotating screen with slots and shutters.
The appearance of the switch as such
So, in a non-contact ignition system, instead of one contact distributor, two separate nodes appeared: a non-contact (but only at a low voltage) distribution sensor and an electronic switch. The function of distributing the high voltage voltage to the spark plugs in the distribution sensor is still performed by a mechanical rotor runner with a current-carrying plate.
But what about the regulation of the ignition angle? These tasks are still performed by centrifugal and vacuum controllers as part of the distribution valve. The first of them now rotates not the cams on the shaft, but shifts the curtains of the screen, thereby changing the ignition angle. The vacuum regulator has the ability to move the Hall sensor with its base plate, also adjusting this angle.
Given the above, to the question: "Modern car switch: what is it?" - It should be answered that this is a structurally isolated electronic unit of a contactless ignition system.
Failure to distribute high voltage
The longest time in the switch was the mechanical distributor of high voltage across the spark plugs of the cylinders. The most interesting thing is that this unit was quite reliable and did not cause big complaints. However, time does not stand still, and at the beginning of our century, the switch connection diagram underwent another major change.
In modern cars, there is no distribution of high voltage from one coil to different candles. On the contrary, the coils themselves “multiplied” in them and became the accessories of the candles of each cylinder. Now, instead of the contact switching of candles for high voltage, non-contact switching of their coils at low voltage is performed. Of course, this complicates the switch circuit, but the capabilities of modern circuitry are much wider.
In modern cars with injection engines, the switch is controlled by either an autonomous engine control unit or the vehicle's on-board computer. These control devices analyze not only the speed of rotation of the crankshaft, but many other parameters characterizing the fuel and coolant, the temperature of various components and the environment. Based on their analysis in real time, the ignition timing settings also change.
Switch Failures
The most common malfunction of a mechanical distributor is the burning of its contacts: both moving and high-voltage contacts of candles. To prevent this from happening (at least not too fast), you need to regularly inspect them, and if soot has formed on them, then it should be removed with a file or small skin.
If a capacitor connected in parallel with the contacts of the chopper or a resistor in the circuit of the central high-voltage electrode fails, they can be replaced.
Malfunctions of the electronic switch caused by the failure of the Hall sensor pulse amplifier or coil current switch are usually not resolved, since such a switch is not separable. In this case, as a rule, the faulty block is simply replaced by a new one.
How to check the switch?
If the engine idle “floats”, or it stalls on the go, or does not start at all, then check for the presence of sparks on the candles connected to the ignition distributor with the Hall sensor. To do this, you need to unscrew them, put on the tips of the armored wires, put the candles on the “mass” and “crank” the crankshaft with the starter. If there is no spark or if it is weak, go to the switch.
But how to check the switch? Ignition should be switched on and assess how the arrow of the voltmeter deviates. If the switch is operational, then it should deviate in two stages. First, the arrow occupies a certain intermediate position, in which it remains for 2-3 seconds, and then goes into the final (regular) position. If the arrow immediately takes its final position, then you can try replacing the switch.
Switch Connection
How to connect the switch to a contactless ignition system? It should be remembered that its terminal block is connected by two wires to the terminals “B” and “K” of the ignition coil, a three-wire harness with a connector - to the Hall sensor on the distribution sensor and one wire to the “ground”. With the output “+” of the battery, the switch circuit is connected at the terminal “B” of the coil.