Theoretical max pulse lenght limited by RPM?

tekniq · Post by **tekniq** » Sat Mar 17, 2007 8:51 am

Hi,

I'm doing some calculations on the injector pulse length and max duty cycle at max RPM, and find some things i'm a bit troubled about, maybe someone can help me understand.

Suppose:
We have a 1.4 liter 4cyl turbo engine, with 4 port injectors rated at 19 ppm. which should be good for about 120 HP according to megamanual.
Req_fuel per cilinder for 1 injector is 12ms as calculated in megamanual

for ease of calculation lets assume:
Max RPM = 6000 = 100 revs\sec = 200 ignitions per sec.
Max boost = 1 BAR = 14.7 PSI = 200 kPa MAP
VE at this range is 100%
Max duty cycle is 85 %

Situation 1:
Sequential injection: 1 injector per cilinder every 2 revs.
Time between injections = 1000 ms / 100 revs * 2 = 20 ms
Max pulse lenght = 20 * 0,85 = 17 ms
Required pulse lenght = 12 * 200/100 (MAP)* 100/100 (VE) = 24 ms

Ahhrgg. this is way to much.. and not even counting enrichments.
I know this is a lot of boost and VE of 100 looks resonable to me as i look in the documentation. Do we really need huge injectors for this small engine and HP. (not according to injector_size calculator)

changing number of squirts doesnt help.
situation 2:
2 Squirts alternate:
Time between injections = 1000 ms / 100 revs = 10 ms. (every bank of 2 injectors fires once every rev.)
Max pulse lenght = 10 * 0.85 = 8.5 ms
Req_fuel = 12 / 2 = 6 ms (for 2 injectors)
req Pulse length = 6 * 200/100 * 100/100 = 12 ms

same problem for simulanious:
1 Big Squirt 2 times a rev.
Time between inj. = 1000ms / 100 revs = 5 ms.
Max pulse lenght = 5 * 0.85 = 4.25 ms.
Req_fuel = 12/4 = 3 ms
req Pulse lenght = 3 * 2 = 6 ms
-------------
At 6000 rpm the time to inject is just too short, how do we put in enought fuel???

I know that with NA MAP and VE are much smaller but with enrichments, inj_opening time and acceleration even here this seems a big problem too me.

Even if we were to put in gigantic injectors it wouldn't idle because idle pulse length with these injectors is:
MAP = 20 kPa. Ve = 50, inj open 1.0 ms
pulse lengh = 12 * 20/100 * 50/100 + 1.0 = 2.2 ms (stil just ok)

What then??

tekniq · Post by **tekniq** » Sun Mar 18, 2007 4:24 am

I still don't understand,

At 6000 rpm, 85% duty cycle, 4 injectors, 4 squirts per cycle, alternating:
Time between 2 squirts on the same injector-bank is just 10*0.85= 8.5 ms

if Req_fuel should be between 6 and 12 ms. how can it add fuel under boost or enrichment?

Mike_Robert · Post by **Mike_Robert** » Sun Mar 18, 2007 5:37 am

You can't "make" more time to fit the required fuel. You can either fit larger injectors or more injectors firing in a staged scheme. Your calcs show injectors that are too small for the application. BTW, I idle at 1.1 -1.2 ms under an MS2 processor. Also, your table VE/needed fuel is going to be less when past the torque peak so fuel per cycle goes down at rpm past peak volumetric efficiency (not referring to VE in the MS tables). AE is generally not needed so much at high rpm/high load.