broken turbo

[Greg Hermann]

>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,
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!!

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