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# Configuring Trigger Angle

### Tech Articles

Discussion and questions related to the course Practical Standalone Tuning

Hi,

I have a question regarding Trigger Angle configuration. What I am trying to understand is how exactly the crank angle BTDC is calculated for the remaining cylinders after the first tooth or first trigger. An example would be a 4 cylinder engine with a 24 tooth wheel and 90 degrees for each cylinder (90*x4 cyl = 360*) which a 90* trigger angle would make perfect sense in which case there would be 6 teeth for every cylinder until the cycle ends and it starts over. My question is what if you choose a trigger angle that is not perfectly divisible?

For instance take the same 24 tooth wheel on a 4 cylinder engine but with a trigger angle of 60*. In this case case it seems like there are teeth that are not being "used" (60*x4 cyl = 240*). does the ecu just ignore the last 120* of teeth until the cycle is over and then it starts again after the synch event? In other words, if the trigger angle is not set to the perfect amount of degrees between each cylinder are the extra teeth then simply wasted?

It seems that I am missing something and I apologize if it is obvious but for some reason I can't wrap my head around this?

Thank You!

What you are missing is a 4-stroke engine turns 720 degrees for each cycle (one firing of each cylinder), not 4 times per 360 degree rotation.

The ECU has high-resolution timers internally. So at a given crank speed ( the time between triggers) and angle (number of teeth since the trigger, plus the current time), it can figure out when is the correct time to open injectors, or start coil charging, or fire the ignition.

For normal 4-stroke (even fire) engines, the TDC of each cylinder happens every (720 deg / number of cylinders) degrees of crank rotation. For odd-fire engine, you have to specify to the ECU what the crank angles are for each cylinder TDC.

The Trigger Angle is the difference between some reference point (cam signal, missing tooth on the crank triggers, etc) and the TDC of cylinder 1. It is not related to the number of teeth between cylinders.