Ignition (combustion engine)
When the ignition is called the internal combustion engine , the ignition of the compressed fuel-air mixture in the combustion chamber of the cylinder . In gasoline engines, a high-voltage spark ignites the compressed fuel-air mixture on the spark plug ; in diesel engines , the fuel ignites by itself when it is finely dispersed into highly compressed, hot air through a nozzle ( self-ignition ).
The first form of ignition was glow tube ignition , in which a small platinum tube was made to glow in the combustion chamber of the cylinder. This led to ignition when the gas mixture was compressed. However, this device was prone to failure and could not be regulated.
Types of ignition
Magneto ignition
The magneto ignition does not require any additional electrical energy storage such as an accumulator or battery for the ignition process . Instead, the electrical energy for the ignition spark is obtained from an electrical generator driven by the internal combustion engine . It is one of the oldest ignition methods for internal combustion engines, since in the early days of the first internal combustion engines at the end of the 19th century, no powerful electrical energy storage devices in the form of batteries were available.
From the turn of the century in 1900 to around 1960, magneto ignition was the standard variant in motor vehicles. After that, the on-board electrical system was powerful enough to supply the ignition. From the 1970s / 80s, power electronics became available and inexpensive, and enabled wear-free electronic ignition systems. Magnetic ignition is still used in applications that do not have their own power supply, such as lawn mowers, mopeds and aircraft engines .
Battery ignition
The ignition voltage is generated in the ignition coil by self-induction , as is the case with magneto ignition . In contrast to magneto flywheel ignition, the power for the ignition coil comes from the on-board battery. For this purpose, a current flows through the primary winding of the ignition coil, which is briefly interrupted by a mechanical breaker contact. In the brief moment in which the electrical voltage breaks down in the primary winding of the ignition coil, a change in the magnetic field occurs, which induces a high voltage in the secondary winding of the ignition coil . This is transmitted via a cable to a spark plug, where a spark is created that ignites the gas mixture (gasoline and air).
An ignition capacitor (usually 0.22 µF) is connected in parallel to the contact , which on the one hand reduces the burn-off caused by the arc on the breaker contacts and on the other hand forms an oscillating circuit with the primary coil that has the same resonance frequency as the secondary coil. In this way, the energy transfer from the primary to the secondary circuit is improved.
In engines with several cylinders, there are different ways to control the individual spark plugs: In the simplest case, with two-cylinder engines, the spark plugs are connected in series and ignite at the same time. One of the two sparks hits the end of the exhaust stroke and has no effect ( wasted spark ). In the case of multi-cylinder engines, otherwise, ignition distributors were installed, which conducted the high voltage of the secondary winding of the ignition coil either to one of the spark plugs. For this purpose, the distributor has a rotating contact, the distributor finger, which moves one after the other close to the contacts that are connected to the spark plugs via the ignition cables. With each ignition, the ignition interrupter must interrupt the flow of current to the primary winding of the ignition coil once so that the ignition current from the rotating finger can generate a spark at the contact in the distributor cap and at the spark plug. The most complex is the use of a separate ignition system for each cylinder, i.e. consisting of an interrupter contact, ignition coil and spark plug. In modern engines, static ignitions are widespread, in which the interrupter is designed as an electronic circuit and each one ignition coil feeds two spark plugs. A sensor on the flywheel measures the crank angle and its signal is processed by a control computer.
In older vehicles, the ignition must be adjusted regularly. This requires a bit of skill: the spark on the spark plug occurs when the ignition contact is opened ( self-induction ). When setting the ignition point, a small indicator lamp connected via the breaker contact can help. First the ignition contact spacing is set and then the ignition timing, since, conversely, the ignition timing initially set would be readjusted by a subsequent change in the contact spacing.
The correct ignition contact spacing is important for the functioning of the ignition system, which has a direct effect on the dwell angle : If the contact spacing is too large, the magnetic field is too weak (too short a time to build up current) and, as a result, an ignition spark that is too weak, especially at higher speeds. If the contact distance is too small, the breaker contact will wear out more, because the collapsing magnetic field when the contact opens means that the current can continue to flow there (the contact opens too slowly). The ignition capacitor is only of limited help here for spark suppression - the contacts burn off faster than usual.
The dynamic setting of the ignition point (function of the centrifugal force adjustment of the ignition point) is carried out with a stroboscope, which is triggered inductively via the ignition cable of the first cylinder. This allows you to observe the markings made on the motor shaft.
Transistor ignition systems (TSZ-h / TSZ-i / TSZ-k)
The T ransistor- S pulen for ündanlage (TSZ) works like the breaker ignition with centrifugal and vacuum advance. The interrupter contact is provided as a switch-wearing by a power transistor is replaced with a high current for more ignition, the capacitor loaded against the contact wear is eliminated. The ignition signal usually comes from a donor , either the Hall sensor (TSZ-h) or an inductive sensor (TSZ-i). Older transistor ignition systems with a series resistor always switched precisely thanks to an unloaded mechanical interrupter contact (TSZ-k).
Electronic ignition system (EZ)
It differs from transistor ignition in that a microcomputer calculates the ignition point based on the permanently stored values of an ignition map . The ignition is triggered electronically in the control unit.
Fully electronic ignition system (VEZ)
The VEZ is an electronic ignition system in which the rotating distributor has also been replaced by electronics (referred to as stationary ignition distribution )
Advantages:
- Higher operational safety due to few high voltage connections
- Wear-free by dispensing with moving (rotating) parts
- Less radio interference, as no sparks occur outside the combustion chamber
Disadvantage:
- Higher integration effort, fewer standard components, often engine-specific production of the components
The VEZ processes the signals from four sensors:
- load
- Engine speed
- Motor temperature (optional)
- Knock sensor (optional)
There are two types of ignition coils that a VEZ can be equipped with:
Single spark coil
Each cylinder has its own ignition coil, which is activated and regulated by the control unit or the ECM (Electronic Control Module).
Double spark coil
Two cylinders with an ignition interval of 360 ° are simultaneously supplied with ignition sparks by a so-called double spark coil. One spark ignites the fuel-air mixture in a cylinder at the end of the compression stroke, the other in the parallel cylinder at the end of the exhaust stroke is called a "support spark", this ignites the unburned hydrocarbons in this cylinder and thus contributes to clean combustion. Double ignition coils are not like conventional ignition coils as autotransformer executed, but have separate primary and secondary windings, the spark plugs are connected in series .
control
The electronic control, English Electronic Control Module , ECM , works with a read-only memory such as an EEPROM (Electrically Erasable Programmable Read Only Memory) or flash memory . Until the 1990s, EPROMs (Erasable Programmable Read Only Memory) were in use, which could only be reprogrammed with great difficulty. When using flash memory and EEPROM, the ECM can be reprogrammed when it is closed. The analog signals, for example from the coolant temperature sensor, are converted into digital signals in the ECM with analog-digital converters so that the microprocessor can process them. The ignition map in the microprocessor is usually matched according to the following criteria:
- Consumption reduction
- Pollution reduction
- Torque increase at low speed
- Increase in performance
- Improvement of the running smoothness of the engine
- Installed parts, depending on the supplier
In all operating states, such as start, full load, partial load, overrun, ignition angle corrections can be made if external influencing variables (e.g. engine temperature, intake air temperature, battery voltage) require it.
Other additional functions integrated in the ECM are, for example:
- Idle speed control
- Speed limitation (variably adjustable)
- Knock control
- Emergency program
- Sensor monitoring
- Self-diagnosis
The previously independent control unit is now mostly integrated in a combined ignition and injection control unit, which means it is connected to all other electronic components in the car.
High-voltage capacitor ignition (HKZ), thyristor ignition
The high voltage capacitor ignition (MSI), also called thyristor or English Capacitor Discharging Ignition (CDI) , employs a capacitor , which on an equation voltage of 500 V is charged and discharged abruptly via the ignition transformer to the spark plugs at ignition. As an essential criterion, the energy storage for the ignition does not take place in an ignition coil , but in the eponymous capacitor.
Laser ignition
The laser ignition is an ignition system in which the combustion by a focused laser beam is triggered. A plasma with a core temperature of over 10,000 Kelvin is generated by ionization at the focal point of the laser beam . The high temperature and a pressure wave emanating from the plasma core at supersonic speed ignites the mixture.
The advantage of laser ignition is, among other things, the free choice of ignition location; an ignition remote from the wall with its advantages in terms of wear and combustion efficiency can thus be easily produced. Due to the high ignition energy, laser ignition, in contrast to spark ignition, can also ignite very lean mixtures.
The laser ignition was implemented, for example, in a single-cylinder test engine from the Vienna University of Technology . Some of the major hurdles for use in vehicles are the size, price and energy requirements of laser ignition. In a cooperation between CTR and AVL List , a laser spark plug is being developed that is suitable for mobile applications.
Overall, it must be stated that laser ignition is still at the research stage.
Interference suppression
The ignition sparks generate high-frequency interference pulses that must be suppressed. There are the following measures:
- The spark plug connector or the spark plugs contain a built-in interference suppression resistor of approx. 5 kΩ. It limits the maximum current, the rate of current rise and thus also the generated interference radiation.
- The ignition cables are laid as close as possible to the engine block.
- The distributor has a shielding metal cap.
- Complete shielding of the ignition (spark plug connector, cable, ignition distributor)
- Ignition capacitors over the breaker contacts (are required for the function, but also reduce interference)
- Back-up capacitors to ground in the ignition supply circuit; they prevent the spread of disturbances in the vehicle electrical system.
A distinction is made between two classes of interference suppression: The remote interference suppression required by law for all vehicles and the near-end interference suppression not required by law.
The aim of remote interference suppression is to reduce the interference field strength to protect radio and television reception in the vicinity of the vehicle (at least 5 kΩ per ignition circuit are required by law). 15 kΩ per ignition circuit should not be exceeded, otherwise the ignition spark is weakened too much.
The local interference suppression in vehicles with built-in radio receivers not only includes a possibly higher interference suppression resistor, but in particular blocking capacitors in the primary ignition circuit. The audio technology in the car often has to be protected by additional filters in order to suppress interference from the alternator.
Ignition timing
For an efficient power delivery with the lowest possible fuel using the ignition timing is set so that the highest combustion pressure is about at all speeds and load cases 10 ° to 20 ° crank angle after the upper dead center occurs (OT). The focus of combustion , that is the point in time at which 50% of the fuel mass used is burned, is then around 5 ° to 8 ° CA after TDC. The fuel-air mixture must therefore be ignited before TDC.
However, since the combustion time of the fuel-air mixture is approx. 2 ms regardless of the speed, the time of ignition must always be before TDC as the engine speed increases.
If the ignition point is set too early, uncontrolled combustion processes with high pressure and temperature peaks can occur. With this knocking combustion, the engine components that form the combustion chamber - piston and cylinder head - are subjected to very high mechanical loads, which can destroy the engine . In addition, the composition of the exhaust gas deteriorates and there are power losses. However, these relationships cannot be generalized as they also depend on other parameters. The mixture composition (too rich or lean mixture), the shape of the combustion chamber and the position of the spark plugs in the combustion chamber are also decisive for the engine's tendency to knock.
If the ignition point is selected too late, the piston has already moved far in the direction of bottom dead center before the fuel-air mixture is completely burned. More energy from the fuel used is lost with the exhaust gases. The temperature of the gas in the cylinder is still very high when the exhaust valve opens. The consequences are: poor efficiency, higher fuel consumption, overheating problems, possible destruction of the engine.
With a microprocessor computer, the ignition point can be better adapted to the operating state of the engine using an ignition map . This ignition map is often changed by so-called chip tuning in favor of more power, which is usually at the expense of service life, fuel consumption and environmental compatibility.
Starting aids, ignition aids
Petrol engines
Starting aid for a gasoline engine when the outside temperature is low and the engine is cold can consist of spraying special, highly volatile hydrocarbons into the air filter so that carburetor engines in particular have sufficient evaporated fuel in the cylinder when the spark plug is sparking.
Model construction engines can be equipped with a glow plug, which is electrically heated before starting and then remains glowing during operation with fluctuating temperature.
Not to be confused with the type of jump starter in which a vehicle with a weak starter battery is supplied with power via a jump lead, from another vehicle or another power source during the starting process.
Diesel engines
At least on the Jenbacher (single-cylinder) engines JW 8 and JW 15 with 8 or 15 HP and about 1.5 liters displacement, there was a pin protruding from the side into the combustion chamber, in the tubular end of which a fuse was inserted that either Even when screwing in the finger glowed or an ignition aid made of white fibers in the shape of a roll, a piece of gray or red soaked. In the compression stroke, the fuel-air mixture heats up and the ignition is promoted by the fuse.
"Zündfix-Diesel-Self-igniter" is a starting aid / fuse / ignition fuse for starting diesel or crude oil engines without an electrical preheating system. Zündfix is available with 4, 5, 6, 7 and 8 mm nominal diameters; the 7 mm version is about 30 mm long, fibrous and white, and about 7 mm wide impregnated in red at one end. There are 100 rolls in a tin can to protect against getting wet. The fibrous roller is inserted with the light end clamped into the sleeve of the "ignition key", this is screwed into the engine and must be tightened to seal. When the engine is started, this insert begins to glow or burn due to the heating before the fuel-air mixture ignites by itself. The self-ignition of the jump start reliably ignites the fuel-air mixture even when the engine is still cold.
Alternatively, there is the (emergency) method of sticking a piece of wood into the "ignition key", letting the flame of the piece of wood burn down and, if it is only glowing at the charred end, inserting the ignition key into the cylinder. The brief continued glowing of the charcoal in the cylinder temporarily forms a good ignition source for the fuel-air mixture.
Specialist literature
Reference books
- Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition. Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3
- Jürgen Kasedorf, Richard Koch: Service primer for vehicle electrics. An introduction to automotive electronics. 14th revised edition. Vogel Buchverlag, Würzburg 2001, ISBN 3-8023-1881-1 , ( Vogel-Fachbuch: Service-Fibel )
- Rudolf Hüppen, Dieter Korp: Car electrics. Ignition, battery, alternator, starter, instruments, devices, lighting. 7th edition. Motorbuchverlag, Stuttgart 1972, ISBN 3-87943-059-4 , ( Now I'm helping myself 20)
Technical brochures
- Bosch: Technical instruction for radio interference suppression. 1st edition, Robert Bosch GmbH, Stuttgart, 1978, VDT-U 1/2 DE
- Bosch: Technical instruction on circuit symbols and circuit diagrams for vehicle electrics. 2nd edition, Robert Bosch GmbH, Stuttgart, 1974, VDT-UBE 001/10
Individual evidence
- ↑ Deutz standing motor - start with air pressure georgenrone, youtube.com, published August 22, 2013, accessed August 27, 2018, video (1: 33/5: 21). - about 7 mm in diameter and 5 times as long, the opening in the ignition key is slightly narrowed at the open end.
- ↑ HERTH + BUSS diesel self-igniting ignition fix ensures easy and safe starting for diesel or crude oil engines. ø 7 mm
- ↑ Jenbacher JW8 Kaltstart dreschkirtag, youtube.com, published September 16, 2012, accessed August 27, 2018, video (4:47). - 2 flywheels, ignition fix.
- ↑ Starting the Jenbach JW15 county1454, youtube.com, March 4, 2009, accessed August 27, 2018, video (1:31). - @ 0:11: "Zündfix" from the yellow-red can.
- ↑ Majstor Ljubina pilana Žabalj - 2 od 3 davors85, youtube.com, July 28, 2011, accessed August 27, 2018. Video (7:00) - Wood chips are put into the ignition key, burns off in 1:40 when the chip is it is only used and started glowing.