The optimal effect of the air-fuel mixture is only achieved if the mixture is ignited at the right time. In addition, reliable ignition under all conditions is a prerequisite for the perfect operation of the catalytic converter. If misfiring occurs, the catalyst can be damaged or even completely destroyed due to overheating during the afterburning of the unburned mixture.
The ignition process is a complex process. For example, if the engine of your A4 rotates at a frequency of 3000 revolutions per minute, then about 50 ignition sparks are distributed to individual cylinders with great accuracy per second. Not only time, but also the ignition energy of the combustible mixture must correspond: at least 0.2 mJ per ignition with a favorable ratio between air and fuel and more than 0.3 mJ in the case of a rich or lean mixture. If there is not enough energy, then the combustible mixture does not ignite, and fatal misfires occur.
Ignition control
From ignition to complete combustion of the mixture takes two milliseconds. With the same composition of the mixture, this time remains constant. For this reason, the ignition spark must occur in time so that the optimum combustion pressure is ensured in any engine operating mode.
The task of determining the exact moment of ignition lies with the control device, which also controls the operation of the ignition system. The processor is programmed for different ignition timing for different engine operating modes. In order for the appropriate ignition timing to be selected from the memory, the control device receives data on the engine operating mode. The control unit evaluates the data received from the individual sensors, such as data on the temperature and engine speed, as well as the position of the throttle valve and camshaft.
Optimum ignition timing
The ignition timing must be chosen in such a way that the following four requirements are met:
- maximum engine power,
- economical fuel consumption,
- engine knock prevention
- as clean exhaust gases as possible.
For example, if you lightly press the accelerator pedal (partial load) the mixture in the combustion chambers burns more slowly. In this case, in order to ensure that the energy of the fuel is fully used despite this, the control device starts the ignition earlier. The best values are set when the air-fuel mixture is ignited at the moment of maximum compression. In a four-stroke engine, this is when the piston starts to transition from moving up (compression stroke) to move down (working cycle).
Ignition and combustion
However, the ignition timing does not exactly coincide with top dead center, since the fuel particles need three thousandths of a second to ignite. Therefore, the ignition spark begins its work already during the upward movement of the piston (earlier ignition). On the contrary, the combustion pressure is set immediately after the piston passes top dead center. Since it always takes the same time to ignite the air-fuel mixture, ignition occurs earlier as the engine speed increases.
Position of the crankshaft and piston at the time of ignition with early ignition Z
Pressure change in the combustion chamber:
1. Zt: ignition at the appropriate time,
2. ZB: ignition too early (detonation combustion),
3. Zc: too late ignition.
Detonation combustion
In positive ignition engines, abnormal combustion processes may occur under certain conditions. These processes limit the growth of power and efficiency. This unwanted combustion process is called detonation combustion. It occurs in the form of instantaneous combustion of mixture particles that are not yet covered by the flame front. In this case, the ignition timing is too far in the early ignition direction.
In this case, the flame propagation speed can reach 2000 m/s, while during normal combustion the flame speed is only 30 m/s. If flash combustion with excessive pressure increase continues for a long time, the cylinder head gasket, pistons, bearings and spark plugs may be damaged.
The knock sensors register fluctuations in uneven combustion, and the control device shifts the ignition timing towards late ignition. Thanks to anti-knock regulation, the engine, which was designed for gasoline class «Super», can be operated with normal gasoline for some time.
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