Calculating duty cycle based on RPM and PW
 

I am looking for a formula that will calculate the % duty cycle based on the RPM and Pulse Width of the injector.

I think I have derived a formula, and I want to run it by you guys to "pick it apart".

The formula I have came up with is PW * RPM /60,000. This will give you a decimal with .5 = 50% and 1.0 = 100%.

How I came up with the constant was to figure out total Pulse width in a minute @ a given RPM. I worked on the assumption that the PW is based on the duration of firing for each revolution. Also there is 1,000 ms every second, so there is 60,000 ms in a minute.

At 6,000 rpm, and engine requires 10 ms to complete a revolution (60000/6000. Thus if my max PW @ 6,000 rpm is 10 ms.

I have done various "cross calculations" and I have related it to the standard (HP*.5BSFC)/(#Inj*%DutyCycle).

Using the above formula, and the following is the max PW @ 100% Duty Cycle for various RPMS.

RPM.....PW ms
1,000....60 ms
2,000....30 ms
3,000....20 ms
4,000....15 ms
5,000....12 ms
6,000....10 ms

I should point out, that this is if we measure PW being measured over a single revolution at a given RPM. If we measure PW every power cycle, then the constant would be 120000 (60000 x 2) and all the values about would have to be doubled.

[This message has been edited by Glenn91L98GTA (edited December 29, 2000).]

Yep,
forra single batch fire.
SEFI just double the numbers

Anyone know the formula for a TBI unit?

Used pulsewidth/available pulsewidth x 100

How do I know what the available PW is?

Grim Reaper,
Your math is correct. With that you can see why the SEFI can run a larger injector and not complain about idle BPW problems. One thing to note is that code like the $8D does not actually report the BPW via the ALDL that is sent to the injector. It also brings attention to what a BPW actually is........including the turn off/on time or not, voltage correction, etc.

EDIT: Wow, old post. Info above is for batch fire port injection.

Calculate it based on RPM

Let me take 6,000 rpm, it is a nice round number

With TBI you have 4 injector firings per crankshaft rotation. That is 2 per injector driver.

At 6,000 rpm the engine is turning 100 times per second

That means that you have 10 msecs per rotation.

The injector has 5 msec's per firing @ 100 % DC.

1 millisecond is equal to 1/1000th of 1 second.