The relationship between the period, pulse width, pulse repetition frequency and duty cycle is illustrated in the example below. The units of measure in this example are in time. The period (T) is the amount of time between the start of each pulse. The pulse width (t) is the length of time that the pulse is on. The pulse repetition frequency (PRF) is the number of pulses per second. The duty cycle is the percentage of time that the pulse is on with respect to the time that it is off, or in other words, the percentage of time on during each period.

T = Period = .50 sec.

t - Pulse Width = .25 sec.

PRF = Pulse Repetition Frequency = 2 pulses/sec.

 

Duty Cycle = PRF * t = 2 * .25 = .50 = 50%

Also,

Duty Cycle = t / T = .25 / .50 = .50 = 50%

DUTY CYCLE = 1.1 x DWELL

This diagram illustrates the reasoning behind the concept that a dwell meter can be used to measure duty cycle, and that the duty cycle is equal to 1.1 times the dwell angle.

When on the 4-cylinder scale, a reading of 20 degrees dwell indicates that the points are closed (conducting) a total of 80 degrees (4 * 20) for each 360 degrees of distributor cam rotation, or they are closed 22% (80 / 360) of the time. In other words, the duty cycle for the point closure is 22%. This is the same as the dwell * 1.1 (i.e. 20 * 1.1 = 22% duty cycle.)

In this example, 360 degrees is the unit of measure that is used instead of one second. We have a PRF of 4 pulses per 360 degrees, or 4 pulses per unit of measure. So, if we want to use the equation Duty Cycle = PRF * t, then we will have to divide the value of t by 360.

Duty Cycle = PRF * t = 4 * 20/360 = .22 = 22%

Also,

Duty Cycle = t / T = 20/90 = .22 = 22%

FREQUENCY = RPM / 30

What follows is the "proof" that a tachometer can be used as a frequency counter, and that frequency will be equal to the RPM reading divided by 30.

Assume a 4-cylinder engine is running at 500 RPM. Then the distributor cam is turning at 250 RPM and the four lobes on the cam will yield 1000 pulses per minute. In this example the tachometer would read 500 RPM but is actually pulsed 1000 times per minute or 16.67 pulses per second. This is the same as RPM / 30 = 500 / 30 = 16.67 pulses per second, or in other words, 16.67 Hz.

Determining Pulse Width (t)

So, with our analyzer we have the capability to measure duty cycle and frequency. Substituting these readings into the last equation below allows us to determine the pulse width. It is pulse width of the fuel injectors that we are actually interested in.

Duty Cycle = PRF * t

Then:

t = Duty Cycle / PRF