Spark Ignition Timing

To remember the important points of the ignition system, you can watch this video:

on my site: www.urcar-engine.blogspot.com

The spark timing is usually produced before the piston reaches the TDC by the end of compression stroke of about (5-40) degrees. This is required to utilize the maximum pressure of cylinder. The fuel also needs a period to start the ignition. This period is known as delay period. Delay period is divided into two periods: chemical period and physical period. The period that is required for fuel molecules to be the chemical bonds are broken is called chemical delay period. The period that is required for fuel to be as vapor form is called physical delay period. The delay period is determined by the chemical construction of fuel and fuel volatility.

The optimum spark timing is usually selected experimentally to produce the maximum cylinder pressure, maximum torque and maximum brake power and minimum brake specific fuel consumption. The optimum spark timing may vary with engine speed and different loads. The ignition distributor contains mechanisms to adjust the spark timing according to engine speed and different loads of engine.

-Centrifugal (governor) advance mechanism. - Vacuum advance mechanism.

The ignition timing is controlled by centrifugal advance according to engine speed and by vacuum advance according to engine load.

Setting the spark timing according to engine speed:

When your vehicle is running at high speed, this means the crank shaft speed increases and consequently the cam shaft speed increases also (don't forget that the crank shaft speed is double the cam shaft speed in 4-stroke engine). At the time that the electric circuit opens, the rotor in the distributor is in the contact with the high voltage leads and the spark is generated. The ignition distributor is driven by a gear on the cam shaft; so increasing the engine speed retards the spark timing and leads to power loss.

The centrifugal advance mechanism is responsible for advancing the spark timing with increasing of engine speed. The centrifugal advance mechanism is located inside the ignition distributor. This mechanism consists of fly weights, cam plate and governor springs. As increasing engine speed, the movement of fly weight under centrifugal force advances the angular position of cam relative to rotor of distributor. The cam plate moves the distributor rotor in the direction of rotation until the centrifugal force balances with spring forces. In this new cam position, the breaker points opens early and the spark is advanced.

Setting the spark timing according to different loads:

we know well the different loads of engine are:

At starting: rich mixture (F/A=1/9). At part load: lean mixture ( F/A=1/18). At full load: rich mixture (F/A=1/12). At no load or idling: (F/A=1/9).

when the mixture is rich, the flame speed is higher than in the case of lean mixture. When your vehicle is running at part load, the flame speed is the slowest and the time of combustion process in the combustion chamber is the longest; so it must advance the spark timing to ensure complete combustion and maximum brake power.

The vacuum advance mechanism is responsible for advancing the spark timing during engine part load. This mechanism is connected to a small port above throttle valve. vacuum advance mechanism consists of diaphragm and spring. The vacuum pressure pulls the diaphragm and the breaker plate moves in the opposite direction of rotation. In this new position, the breaker points open early and the spark is advanced.

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