The ignition point characterizes the crankshaft position of an internal combustion engine with external ignition ( Otto engine ) at which the spark is triggered on the spark plug . It is specified either in "degrees of crank angle before TDC" ( top dead center = highest position of the piston in the cylinder ) or, more rarely, in "mm before TDC" in relation to the respective piston. As a rule, the setting for all cylinders of an engine is made using the orientation of the first cylinder.
Optimal ignition timing
The optimal ignition point depends on the speed , the piston diameter and the load (amount of the fuel-air mixture in the combustion chamber , which determines the final compression pressure and thus the speed of propagation of the flame front). On the one hand, the presence of an ignitable mixture on the spark plug must be guaranteed and, on the other hand, the flame spread must be observed. The maximum cylinder pressure must be after top dead center, otherwise serious engine damage can occur. This would result in uncontrolled combustion processes with high pressure and temperature peaks. Also because the piston would have to work against the pressure wave. With this knocking combustion , the engine components that form the combustion chamber - i.e. pistons, cylinders and cylinder head - are subjected to very high mechanical loads, which can lead to the destruction of the engine.
At low speeds, the ignition must therefore take place late, ie the piston is close to TDC.
Since the speed of flame propagation is independent of the speed, the ignition point is shifted to advance with increasing speed, ie further before TDC. There is still an ignition delay of around one millisecond from the point of ignition to the ignition of the mixture.
These parameters require a complex ignition angle map , which could only be put into practice in modern, electronically controlled engines. The ignition point is depending on the speed, the load and the working principle ( two-stroke or four-stroke ) between 6 ° and 40 ° crank angle before TDC.
Static ignition engines have only one ignition timing. A setting of the ignition timing must be selected here that produces enough power at low speeds and avoids uncontrolled combustion processes with high pressure and temperature peaks at high speeds.
In modern engines, what is known as variable ignition is used. The ignition timing is adjusted automatically at low to high engine speeds.
Measurement of the ignition timing
The ignition point can be made visible with the aid of a stroboscopic lamp (flash lamp ). For this purpose, the ignition voltage is tapped capacitively , inductively or galvanically by the ignition system and a lightning bolt is generated precisely at the ignition point . The crankshaft appears to be standing still; the ignition angle can be read from the offset of the TDC marking.
Correction of the ignition timing
In modern engines in particular, changing the ignition timing is only possible with the help of a program change in the engine management. This program change can be done by chip tuning (replacing the microchip that contains the ignition angle map), intervening in the existing engine control unit using suitable software or an additional control unit (piggyback).
For engines that still have an ignition distributor, the ignition point can be corrected by turning the ignition distributor slightly. This is mostly driven by the camshaft. The ignition point is determined by the ignition interrupter (break contact ) or the Hall sensor. Since these components are located in the distributor, the ignition point is set by turning the entire distributor.
Especially in motorcycle engines there is often no ignition distributor. The ignition timing is controlled here either via a single break contact (for one- and two-cylinder engines) or via a pulse generator. If the pulse generator or the interrupter contact is mounted on a plate together with the alternator, the ignition point can usually be adjusted by turning the entire arrangement. Motorcycles with carburettor engines lack the ability to determine the engine load. The ignition point is only adjusted depending on the speed, but not depending on the load. This means that fuel consumption is significantly higher than actually necessary.
In simple engines, such as those used in chainsaws , the ignition point can only be changed by replacing the ignition module, which in newer devices also works electronically and has a speed-dependent ignition angle map.
- ^ RT Peters: time of burning. In: Waspenblech Archive. RT Peters, May 21, 2016, accessed on December 12, 2018 (German).
- ^ Richard van Basshuysen , Fred Schäfer (ed.): Handbook internal combustion engine. Basics, components, systems, perspectives. 2nd improved edition. Vieweg, Braunschweig et al. 2002, ISBN 3-528-13933-1 , pp. 301ff.