Syracuse315

Sounds good Cliffy, thanks for the help.

By the way, you said:

This is because the injectors can't modulate the pulse width past 90%, correct? Sorry for all the questions, just the mechanical engineer in me.

cliffyk

Duty cycle for a fuel injector refers to the percentage of the time it is open vs. the time it could possibly be open (and do anything purposeful). In a 4-stroke/cycle engine each cylinder has an intake stroke every other crank revolution, so the longest an injector could be open--and be useful--is 2 engine revolutions.

So, injector duty cycle is the injector pulse width / ((rpm /2 /60)^-1, with the fractional result expressed as a percentage. This is rpm / 2 because the cylinder only fires once for each 2 crank revolutions, then divided by 60 to get firings per second, and the the reciprocal of that to move the value from the frequency domain to the time domain (same as the injector pulse width).

PWinj / (rpm / 2 / 60)^-1, or PWinj / (120 / rpm)

To put numbers to that, assume a 15ms pulse width and engine speed of 3000 rpm; 0.015 / (120 / 3000) = 0.375 = 37.5%

As the duty cycle get much over 90% valve overlap and other timing consideration wreak havoc with the initial and final moments of the injector's opening and closing. At 100% duty cycle, I.e. always open (commonly called "going static") it may still be not enough fuel to meet the intake air requirements the mix goes lean, with f/i this can be very lean very fast and "it" hits the fan.

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You said you are an ME, me too--MSME MIT '71, which seems like a powerfully long time ago...

Syracuse315

Makes perfect sense, nice to see the technical explanation behind how the injection system works.

As you may have guessed from the name, I graduated from Syracuse University (BSME '09). Surprisingly, the closest thing I got to a hands-on automotive class there was thermodynamics (carnot, stirling, rankine cycle, etc...). Glad I'm learning this at some point though!

cliffyk

Have you ever read John Heywood's Internal Combustion Engine Fundamentals or Charles Taylor's Internal Combustion Engine 2 volume set?

Both very heavy reading, however if you want to really "get inside" the ICE process one or both are essential (nearly Biblical). I like Heywood best, though I am partial as I studied with him in the late 60s.

A good, more contemporary and less complex tome is Greg Banish's Engine Management: Advanced Tuning

Syracuse315

I haven't read any of them, but they will be going on my reading list. I see Taylor was a professor at MIT, did you ever study with/under him as well?

cliffyk

I did not, he retired in 1960--before my time...