Changing the ignition timing
Ignition ahead - ignition of the working mixture in the engine cylinder until the piston reaches the top dead center .
The ignition timing has a big impact on engine performance. During the operation of a four-stroke ICE during the compression stroke, before reaching the TDC , the ignition of the working mixture in the combustion chamber by means of a spark plug occurs. Ignition of the working mixture, expansion of the working gases, and the next cycle is performed - the working stroke. In fact, the combustion of the working mixture does not occur instantly. From the moment a spark appears to the moment when the entire mixture ignites and the gas pressure reaches its maximum value, several milliseconds of time passes. This period of time is very small, but since the speed of rotation of the crankshaft is very large, even during this time the piston manages to go some way from the position at which the ignition of the mixture began. Therefore, if the mixture is ignited at TDC , combustion occurs with an increasing volume (the beginning of the working stroke) and ends when the piston goes some distance and the maximum gas pressure is less than if the entire mixture were burned before TDC was reached. If the mixture ignites too early, then the gas pressure reaches a significant value before the piston approaches TDC and counteracts the movement of the piston. All this leads to a decrease in engine power, its overheating. Therefore, with the right choice of the ignition moment, the gas pressure reaches its maximum value after about 10-12 degrees of crankshaft rotation after the piston passes the top dead center. Ignition timing is characterized by the ignition timing .
Ignition timing - the angle of rotation of the crank from the moment at which voltage begins to be applied to the spark plug to break through the spark gap before the piston occupies the top dead center .
The most favorable ignition timing mainly depends on the ratio between the burning rate of the mixture and the engine speed. The higher the engine speed, the greater the ignition timing should be, and the greater the burning rate of the mixture, the lower. The burning rate depends on the design of the engine , on the composition of the working mixture and some other factors. The greatest influence on the combustion rate is exerted by the content of residual gases in the working mixture. With a small throttle opening, the percentage of residual exhaust gas is large, the mixture burns slowly, so the ignition timing should be large. As the throttle valve opens, more and more fresh combustible mixture enters the cylinder , and the amount of exhaust gases remains approximately unchanged, as a result, their percentage decreases and the mixture burns faster - the ignition timing should decrease. With a simultaneous change in the throttle position (load change) and the number of revolutions, the most advantageous ignition timing depends on both factors at the same time and depending on the engine operating conditions, both factors can affect the most advantageous timing in one or in different directions.
To change the ignition timing, depending on the speed of the crankshaft, centrifugal regulators , usually located in breakers , are used. When the engine load changes and its revolutions are kept constant, the centrifugal regulator does not change the ignition timing, while under these conditions (constant revolutions and variable load) the ignition timing should change. For this, the centrifugal regulator is supplemented with a vacuum regulator .
All this is true provided that the fuel allows detonation-free operation of the engine. However, in reality, the ignition timing limit is limited by the knock phenomenon in the engine. Therefore, when switching from fuel of one quality to another, which differs from the first by antiknock properties, the ignition setting must be changed. This is carried out using a special device - an octane corrector, which allows you to adjust the ignition setting depending on the quality of the fuel used.
In modern injection systems, the onboard computer (ECM) is engaged in the installation of a UOZ based on the program and the readings of the sensors, including the knock sensor , so the installation of centrifugal regulators, octane correctors and other elements of carburetor systems is not required. Since, often, each spark plug has its own ignition coil, the ECM can control the SPD of each cylinder individually. The same can be achieved on the so-called. Tramble ignition systems, since the ECM also controls the timing of the spark.