broken turbo

[West, David]

Greg,

Please see comments below and correct at will if you feel like it.

Does bsfc  = basic fuel consumption?

Reg

Dw

-----Original Message-----
From:	bearbvd at sni.net [SMTP:bearbvd at sni.net]
Sent:	Monday, 21 December 1998 14:02
To:	diy_efi at efi332.eng.ohio-state.edu
Subject:	RE: broken turbo

>Greg,
>
>Just thinking,
>
>Maybe I am confused about the definition of duty cycle (%)
>
>My duty cycle meter is a simple multimeter jobby that I think 
relates
>on time to off time as a %.
>
>I thought that duty cycle is the % of time the injector is open
>compare to the time is can be open until it has to open again.

It is.
>
>So as rpm increases the available open time decreases.

It does.
>
>Therefore at 6000 rpm,
>
>Assuming the injectors fire once every cycle (these actually fire
>twice on this motor) that gives 3000 pulses every minute.
>
>Which equals 50 pulses per second
>Which gives a maximum open time of 20 msec
>Therefore 80% = 16 msec

Correct.
>
>Now at 1000 rpm - 8.33 pulses per second
>Which gives a maximum open time of 120 msec
>Therefore 2% = 2.4 msec

Correct.
>
>So from this I get 15% total power which equates to 30 hp (different
>from my other figure)

I don't see how you got to this.

For simplicity with the numbers, assume that bsfc at idle is same as 
at
WOT, 6000 rpm, as it is at idle. I know--not true, but really only 10% 
or
so different, so no biggy in terms of order of magnitude calcs.

OK--with the 80% duty cycle @ WOT 6000 rpm, the injectors are open 80% 
of
the time (16 ms/20ms) and are therefore flowing 80% of their rated 
flow to
make 200 HP. With the 2% duty cycle @ idle, the injectors are open 2% 
of
the time (2.4 ms./120ms.) and are therefore flowing 2% of their rated 
flow,
(Please bare with me)
This is where I think there could be a problem - 2% duty cycle doesn't 
mean 2% of rated flow I don't think, as we both agree that duty cycle 
is relative to available open time which varies with rpm.
I think that pulse width is what determines flow.
2% at 1000 rpm is a lot greater width than 2% at 6000 rpm (when I say 
greater width I of course mean time length)

Ie: in my engine max horsepower and rev limit at approximately 6000 
rpm (to make figures nice) so maximum pulse width is 20 ms and 80% is 
16 ms so therefore 200HP = 16 ms. One can then assume (I think) that 8 
ms = 100 hp and so on (of couse we aren't taking into account ramp up 
and ramp down of the injector which would have much more of an effect 
on flow at low pulse width)

This brings about another scenario - My injectors are shorted to earth 
to energise the solenoid. Therefore when they are earthed the duty 
meter starts measuring open time and when the 12 volts is returned the 
duty meter starts measuring closed time. Therefore the flow that 
exists when the injector is closing is actually occurring when the 
duty meter is measuring closed time.  I think that is takes about 0.5 
msec to open and close. So a 2.4 ms pulse has 1.4ms full flow and 1 ms 
reduced flow of which only 0.5 ms is included in the pulse width time. 
- just some more food for thought but probably totally useless.

So my value of 30hp was arrived at in the following way:

2.4 ms as a % of 16 ms is 15% and we know that 16 ms equals 200 hp 
therefore 15% of 200 hp equals 30hp

Now based on my assumptions of 0.5 ms for opening and closing each 
(which could be totally wrong) and we would be getting a small amount 
of flow during this phase,  I estimate an equivalent full flow width 
of 1.8 ms (another big big assumption and I have probably confused all 
hell out of you now)

So 1.8 ms is 11.25 percent of 16ms  which equals 22.5 hp - still a 
lot

Not withstanding all this rubbish your value of 5-6 hp seems rather 
neat and accurate for idle hp.



or 1/40 (2/80)  as much as they are at WOT 6000 rpm, and the engine is 
now
burning 1/40 as much fuel, and therefore (remember we assumed equal 
bsfc
for the two conditions) making 1/40 as much horsepower, 200/40, or 5 
HP at
idle.  Even if you correct for real world bsfc numbers at WOT and 
idle,
this means only 5.5 or 6.0 HP at idle. Which is about right. 
(Friction, oil
pump, water pump, alternator, fan, pumping losses, etc.)

I think what is tripping you up is that duty cycle is based on real 
time
available between squirts, not the (far shorter than the time between
squirts at idle) max PW as programmed into the ecu.

Merry Christmas!!
Ho Ho Ho
Regards, Greg
>
>Now there could be any number of reasons why all this is total 
garbage
>and I sure, and hope, that someone will set me straight.
>
>Kind Regards,
>
>David West