understanding injectors

[drewbe121212]

Hello everyone, I have been doing some looking at fuel systems and the like, and am having a bit trouble understanding the injector "terms" if you will, I think I have got them, but I am not positive as I seem to keep wondering upon descriptions with nothing but numbers and big words to which make it really hard to understand.

Please feel free to correct me or if my thoughts are in the right area...

BSFC - Brake specific fuel consumption - This is a determination of how much fuel is being consumed per horsepower every hour. So if I had a 75% BSFC, what does that mean? I fail to see the relation of the percent to the hour.

Duty Cycle - a % of the time an injector is supplied with power (makes it squirt). This percentage is calculated every 2 revolutions (otto cycle). So lets say a fuel injector is activated for 15 milliseconds at 3000 rpms, it would have a 37.5% duty cycle, or it is activated for 37.5% of the 2 revolutions, or is it just one?

Heh, it is very confusing but it makes sense at the same time. Could anyone clarify this a little bit more for me? I think it would be a lot easier to understand if I had the objects and data loggers right in front of me, but right now I am just trying to understand why.

Thank you for your feedback,
Drew

 

[drewbe121212]

Anyone? Or is this a little to techy for the newb forum? I know it is confusing to me so I am definitely looking for some answers from the experts here.

 

[luv2rallye]

Injector duty cycle is the % of the time the injector is open (sometimes called drive time) compared to the total cycle time (total cycle time = time open + time closed). Example: Lets say at some rpm the injector is open for 15ms and then closed for 60ms. So 15/(15 + 60) = 20% duty cycle. Duty cycle is computed and controlled by the ECU.

 

[drewbe121212]

Injector duty cycle is the % of the time the injector is open (sometimes called drive time) compared to the total cycle time (total cycle time = time open + time closed). Example: Lets say at some rpm the injector is open for 15ms and then closed for 60ms. So 15/(15 + 60) = 20% duty cycle. Duty cycle is computed and controlled by the ECU.

And Programs such as a S-AFC or DSM Link have the ability to control the duty cycle? Is this why you can get over sized injectors and have them still work for little horsepower or small injectors and have them putout for large power (up to 85% duty cycle).

 

[romeen]

With the SAFC the IDC's are still under the control of the ECU. The SAFC intercepts the signal from the MAS and alters it thus "tricking" the ECU in order to compensate for the larger injectors that the ECU is not programmed to operate.

 

[kenamond]

To elaborate on what romeen said, if you have larger injectors, they will flow more fuel for the same IDC. The ECU bases IDC on air mass flow rate and RPM from a fuel map. SAFC intercepts the airflow reading from your airflow sensor (MAS) and alters it so that the ECU thinks it's getting less air. This causes it to look at a different part of the fuel map which dictates a lower IDC. The SAFC lets you adjust this dishonesty for different rpm ranges and interpolates in between. You turn the dishonesty knob until you get the desired air:fuel ratio for that rpm range.

One unfortunate side effect of this relates to the fact that the ECU also has a timing map that is also based on airflow and rpm. At one fixed rpm, higher airflow demands less timing advance to prevent knock than lower airflow (more air in the cylinder means higher pressure and temperature when it's compressed). When SAFC lies about airflow to get the correct IDC, you also end up with more timing advance. This can lead to knock issues. If you are using SAFC for even larger injectors, it is even more dishonest, the timing advance is even greater, and the risk of knock goes up with it.

As far as BSFC, try wikipedia.com or howstuffworks.com. I think I read about it at one of those two places recently, but I didn't remember what I read.

 

[drewbe121212]

Cool,
Thank you for the information. I am understanding it much better now. I do want to run DSMLink when I get my car up and running, though that is still some time away. I thought the S-AFC was a trickery when it came to something (just wasn't sure what it was).

Thanks again,
Drew