View Full Version : Engine fueling calculations

10-22-2008, 09:47 PM
Ok all, I need your input here.

I am trying to make some calculations on injector duty cycle here.

For starters, I made an excel sheet. one row i have rpm. then i take that rpm, divide it by 2, then divide it by 60 seconds. I take that result x, and calculate its inverse (1/x) to obtain how many seconds per cycle(intake+compression+combust+exhaust).

So for example, at 800 rpms's, 1 complete cycle takes .15 seconds..( 1/((800/2)/60)).

Then, I divide that cycle time by 4 to get the time of the intake stroke.
.15/4 == .0375 seconds.

So if I fire my injector the entire intake stroke, it will be on for .0375 seconds.

I did a test/cleaning of my injectors. I took the rail out, attached the injectors, an inline fuel pump and filter, and ran each for 15 seconds. Each output ~70 cc's.

So 70cc's for 15 seconds, that is 4.6 cc's per second. 4.6 per second, by .0375 seconds, gives me .175 cc's of fuel in that stroke. So let's say I have the perfect mix, and have 14.7 cc's of air for every cc of fuel. I have

.175 cc of fuel + (.175 *14.7)air == 2.74 cc's of combustible mixture.

I have a 1998 cc engine( lets use 2000 for simplicity). So at each cylinder i have 500 cc's. Now I dont know the CR of my own engine(surprising as ive been playing with it for two years :smokin: ). But let's say it is 10:1. At full compression I have 50 cc's of space, but only 2.74 cc's of material. this doesnt make sense to me.

What am I missing?

10-23-2008, 01:55 AM
I dont know the answers, but soppose you try working backwards. Start with the 50cc and figure out what should be air and what should be fuel, and maybe you can find your error, it could be a digit or decimal mistake.

10-23-2008, 03:11 AM
I think that you error is the time that the injector inject fuel. The injector fire one quantity of fuel by complete combustion cycle, wich mean 4 cycle wich mean 2 revolution. So, at 800 RPM, one injector will inject 400 x "fuel quantity" in on minute.

So, if wee are saying that you are using some 280cc/min injectors and that your engine is running at 800 RPM and that your injector duty cycle is 30%:

30% of 280 = 84cc/min. 84/400 = 0.21cc/complete combustion cylce (2 revolution)

0.21 x 14.7 because you want a 14.7/1 air/fuel ratio = 3.087 + 0.21 = 3.291 cc of air/fuel mixture.

This is my answer. I cannot prove it, but I don't think that a 500cc cylinder is completely filled on an intake stroke. Maybe I'm wrong on this... but this is my though.

I'm sure that there is some thermodynamic calculation under this... like air take more volume when heated etc...

10-23-2008, 04:07 PM
Ok, so am I wrong in assuming the injectors only fire during intake?

Thanks all :)