Duty Cycle of Fuel Injectors

[zachattack45]

Ok im trying to use this fuel injector sizing calculator at this website, http://www.smokemup.com/auto_math/fuel_injector.php
but i dont know how to figure out my Duty Cycle, anyone?

[Injuneer]

Duty cycle is defined as the length of the injector pulse width in milliSeconds, divided by the time it takes the engine to complete twp crankshaft revolutions, in milliSeconds. So duty cycle is a function of the PCM's calculated pulse width, and the RPM of the engine at that specific time. A "milleSecond" (mS) is 1/1000th of a second.

As an example, it takes the engine 0.020 seconds, or 20 mS to complete two crankshaft revolutions at 6,000 RPM. If your PCM was calling for an injector pulse width of 18 mS, your duty cycle would be 18 / 20 = 0.90 = 90%

When you use an injector sizing program, you have to input the:

Flywheel HP (example: 500HP)
Number of Cylinders = 8
Brake Specific Fuel Consumption (BSFC) - see below
Injector Duty Cycle you WANT to achieve - see below


BSFC:

The typical online injector sizing program assumes a BSFC of 0.500, and a duty cycle of 80%. It does this for the following reasons:

BSFC can only be measured by running the engine on an engine dyno. It is the rate at which the engine consumes fuel, measured in pounds (#) per hour (HR) per HP.

A very conservative assumption of BSFC is 0.500 #/HR/HP. That means for each HP the engine is producing, it needs .5 #/HR of fuel. The programs use that because there is no "one size fits all" value for BSFC. A very well tuned, normally aspirated engine may approach 0.40 #/HR/HP. I have the data for my engine on the engine dyno, and it ran at 0.44 #/HR/HP at peak torque point. When the engine was run with nitrous, which requires a much richer A/F ratio, the BSFC was 0.53 #/HR/HP. A supercharged engine might see BSFC's exceeding 0.55 #/HR/HP, because of the fact the flywheel HP does not reflect the HP taken off the front of the crankshaft to drive the blower.

DUTY CYCLE:

An injector is constantly opening and closing. Try to open and close it too fast, and it doesn't perform correctly. For many types of injectors, if you try to cycle them fast enough to reach their rated flow capacity at high RPM, they lose the spray pattern, and never fully open or close, reducing the actual fuel deliverd to as little as 1/2 of rated flow. If this happens.... the injector can't deliver the fuel, the A/F mixture runs lean, and you get into detonation and destroy piston, heads, rings, etc. So you want to limit the injector open time to about 80% of the time available in two revs of the crank, or 80% DUTY CYCLE.

80% tends to be a little on the conservative time also. Some injectors are comfortable at 85-90% duty cycle. Many people are able to drive the injectors close to 100% DC for bried periods, like the mear fraction of a second the engine is at peak power on a dyno. But pushing the injectors to this level repeatedly wears out the injector, loses spray pattern, and runs the risk of running your engine lean.

So.......

The typical injector sizing program looks like this:

500HP X 0.500 #/HR/HP / 8 cylinders / 0.80 = 39 #/HR injector

But if you take a less convervative approach.

500HP x 0.450 #/HR/HP / 8 cylinders / 0.85 = 33 #/HR injector

So.... you need to "guess at" BSFC and DC to use the equations. I would personally recommend 0.48 #/HR/HP on an N/A engine, with no more than 85% (0.85) DC. That gives you these limits for HP vs injector size:

24# - 340 HP
28# - 400 HP
30# - 450 HP
36# - 510 HP
42# - 600 HP

And that is still reasonably conservative. There is probably going to be a few people telling me.... "I make 400rwHP on stock 24's".... and they may and they may not. As noted above, that's based on a dyno pull, and a dyno pull 1) does not put the max load on the injectors for more than a second, and 2) just because the "SAE corrected" HP is 400rwHP, it does not mean the enigne actually ran at that power level. If it was a hot, humid low barometer day, the engine actually made a bit less than that HP.... it is only the SAE corrections that "said" it WOULD make that HP if it was run at the low temp, low humidity, 29.96" barometer that SAE corrects to.

And.... wow this is getting verbose..... I've seen other posts where you ask about using nitrous with you motor. If you run a "wet" nitrous system, you size the injectors for the N/A HP. If you run a "dry" nitrous system, you size the injectors for the N/A + N2O power..... but you also have to adjust for the fuel pressure, because some dry kits elevate the injector fuel pressure to get increased flow.

There is really no "down side" to a larger injector.... as long as the PCM is properly setup with the correct flow rate and "offsets", and as long as it isn't so large that the PCM can't "turn it down" to the less-than-1 mS pulse widths required to idle.

I run 64 #/HR Bosch injectors, at 58psi, making them in effect 78# injectors. At 800HP on dry nitrous, the duty cycle is only 70%.