Magneto ignition

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In internal combustion engines, magneto ignition is a method of ignition that 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 internal combustion engines at the end of the 19th century there were no powerful electrical energy storage devices available in the form of batteries. Up until the 1960s, it was increasingly replaced, with the exception of special areas of application, by the more cost-effective battery ignitions that are common today and exist in various versions .

Function and history

Historic low-voltage snap-off ignition.

The electrical generator used for magneto ignition is usually a permanently magnetically excited generator, consisting of a permanent magnet . In historical designs, the magnets are designed in the form of horseshoe magnets firmly mounted in the stator ; the rotating double-T armature , an early armature design in electrical machines, is located between the pole pieces of the magnets . On this rotating armature there are turns of a coil that is short-circuited until the ignition point . Up to this point in time, an electric current was induced in the short-circuited winding by the self-induction . The highest efficiency is achieved when the contact closes at the angular position of the greatest magnetic flux and the ignition point (opening of the contact) is at the angular position where no external magnetic field acts on the coil (so-called efficiency gap ). During the short circuit, the ignition energy is stored in the coil in the form of the magnetic field.

An interrupter mechanism, which is mounted on the axis of the generator and driven by the internal combustion engine, causes the circuit to be interrupted periodically. Since the coil is an inductance which has the property of allowing the current to continue flowing after the circuit has been interrupted, a high voltage pulse is generated at the open contacts of the interrupter mechanism, which can be fed to the spark plugs and lead to an ignition spark. In the Wright brothers' aircraft engine, the interrupter contact was directly in the cylinder and its breakaway sparks were used. The ignition point is determined by the angular position of the interrupter, the voltage for the spark plug is taken from slip rings in the first models . In order to avoid the failure-prone slip rings, the coil is stationary in later and current designs and the permanent magnets rotate around the coil.

Since the voltage generated by the ignition pulse is not particularly high in this design due to the limited dielectric strength of the contacts (they have to open very quickly and widely), this form is also referred to as low-voltage magneto ignition or tear-off ignition or tear-off ignition. It was patented by Siegfried Marcus in 1882/1883 , followed by structural improvements in 1887 by Robert Bosch , the founder of Robert Bosch GmbH . It is no longer in use today.

The high-voltage magneto ignition developed by Gottlob Honold , both of Robert Bosch's employees , based on preparatory work by Arnold Zähringer in 1902, represents a significant improvement in the low-voltage magneto ignition. The ignition coil has an additional second winding with a significantly higher number of turns than the first winding attached. The same magnetic field flows through the high-voltage winding as the first winding. The first winding only consists of a few turns in this design and is short-circuited until the ignition point. When the circuit breaker ignites (removal of the short circuit), the self-induction causes a voltage pulse in both coils, which can reach voltages of up to 25 kV due to the higher number of turns in the high-voltage winding  . The spark plugs are  supplied with high voltage - in multi-piston machines via an ignition distributor .

The spark performance is significantly improved by connecting a capacitor in parallel to the interrupter contact (quenching capacitor). On the one hand, this reduces spark formation and thus the contact erosion on the interrupter (it briefly takes over the flow of current at the time of opening and keeps the voltage across the contact low) and, if dimensioned appropriately, forms a so-called resonance transformer together with the coil , which leads to an additional voltage increase for the ignition comes.

Magnetic ignitions usually have a speed-dependent adjustment device for advancing the ignition point with increasing speed via a centrifugal governor . A hand-influenced adjustment value is often added to this angle adjustment by means of a cable, for manual adjustment of the ignition point or to facilitate the starting procedure. The last motorcycle from German production with this complex to operate adjustment was the sports machine BMW R 68 built up to 1955. In addition to the normal right throttle twist grip, the left grip was also designed as a twist grip for ignition adjustment.

Applications

Today magneto ignition systems are used in small motors, e.g. B. used on lawn mowers or power saws; in these, the mechanical breaker contact is replaced by an electronic circuit with power transistors and without mechanical components.

Magnetic ignition was standard on motorcycles from the beginning of the 20th century until the 1960s. Today it has become uncommon, but until the 1990s, small motorcycles and mopeds with magneto ignition systems were manufactured in significant numbers - either to be able to do without the heavy accumulator, or to ensure that the engine would run if the accumulator failed, such as for example in models from Simson . In the classic Vespa brand scooter models (with manual gearbox, 50 cm³ to 200 cm³), since 1946 until today, millions of flywheel magnetos have been installed, as these are fitted into the engine concept (cf. Vespa PX series with 2T and 4T engines). In these models, the pole wheel simultaneously functions as a flywheel and a fan wheel.

Magnetic ignitions on motorcycles can usually be recognized by their cylindrical housings (underneath or open the flywheel with magnets) in the immediate vicinity of the cylinder (s). In the early years they were often arranged in front of the cylinders, which made them prone to failure due to stones and dirt. In later years the arrangement behind the cylinders was common. Some large motorcycles were also operated with magneto ignition systems, most recently in Germany until 1969 the BMW motorcycles .

They are also used in aircraft engines . Almost all gasoline aircraft engines have magneto ignition in duplicate: In aircraft engines with spark ignition of resiliency for reasons the dual ignition provision. One of the two magneto ignitions is usually provided with a so-called snap coupling, which at very slow speeds, for example during the starting process by hand or with a slow-running starter, accelerates the opening of the breaker contacts and thus creates sufficiently strong sparking on the spark plugs.

literature

  • Konrad Reif: Otto engine management: control, regulation and monitoring . 4th edition. Springer-Verlag, 2015, ISBN 978-3-8348-2102-7 , pp. 318-323 .
  • Olaf von Fersen (ed.): A century of automobile technology . VDI Verlag, Düsseldorf 1986, ISBN 978-3-18-400620-4 .

Sources and web links

Commons : Magneto  - collection of pictures, videos and audio files

Individual evidence

  1. a b c d https://www.aerokurier.de/praxis/know-how/magnetzuendsysteme/695580 Mark Juhrig: Magnetzündsysteme, in the magazine aerokurier, issue 8/2016, accessed on August 18, 2018