You hear it all the time… but what exactly IS the Injector Duty Cycle and where do these numbers come from? It is really quite simple. As with most other terms you may have learned browsing this site, it is a calculation. Simply put, it is the amount of time a fuel injector is energized or on during an engine cycle (intake, compression, combustion and exhaust).

Duty Cycle Formula

A four stroke engine takes 2 complete rotations of the crankshaft to complete a cycle. The RPM of the motor is what is going to determine the “time”. Let us use the example of an engine running at 600 RPM’s. This would give us 300 cycles per minute (600 / 2 = 300). Next we need to convert those converted to cycles per second. Because RPM is a “per minute”, we need to divide by 60 (60 seconds in a minute). This would be 300 / 60 = 5. We want this number to represent seconds per cycle (the “time) so we would represent this number as 1/5 (or 200 milliseconds, ms) and equated to the engine cycle taking 200 ms to complete.

Now we need to determine the time the injector is energized at this point in the fuel map. Lets just say for ease of understanding that the injector is pulsed at 20ms. This would be (20 / 200 = 1/10 ) 20 (ms injector is directed to pulse) / 200 (time it took for the engine to complete a cycle) would give you = 1/10 OR a 10% Duty cycle. Simple, right?

The Misconception

I have seen it more than once that people report they ran a duty higher than 100%… it is not possible. Your injector has completely maxed out and only ran 100%, you just ran it past its operating range and the calculation shows that. It is impossible to physically run an injector HIGHER than 100%.

Calculator for Duty Cycle

You can use the calculator below to determine injector duty cycle when engine speed and injector pulse width are known. You can enter numbers in the white boxes. The grey box will show you the injector duty cycle %. As you can see, the example outlined above is the default. The “Reset fields” button writes the default values into the white boxes. Press “Calculate” to determine injector duty cycle (IDC). As you can see, it will show the same results we came up with above in the example. There is not much to this topic so this simple calculator is not very interesting but should help you.

   RPM       IPW (ms)    IDC