Injector selection

[robert dingli]

Hi all,

Matt writes ..
> 
> 
> Hi.  I am in the process of injector selection for our FSAE car
> 
> Basically we have a honda CBR 600 engine with an air restrictor
> It is 4 cylinders, and will never make more than 90 HP
> It will run at up to 12,500 RPM
> 
> For my calculations, I guessed at 25% efficiency (better to guess low).
> Using those figures, I come up with 13.6 lb/hr  (gasoline)
> 
> Since we dont want to run at 100% duty cycle, we would need the next 
> larger size, right?  That would be 16 lb/hr.
> Basically our choices are 13, 16, 19, or 21 lb/hr.  The system is 
> non-sequential.
> 

Matt, the flow rate of an injector is for a given fuel pressure.  It is
straight forward to buy regulators which operate at different pressures
and adjustable units are readily available.

I originally used a 220 kPa Bosch regulator but later found a 250 kPa
Bosch unit to replace it.  I also have a 300 kPa regulator for when I
modify the engine further and require more fuel flow.  One of our customers 
uses a 500 kPa regulator in their racing application.  Without changing
my injectors (which normally cost around Aus$200 each here new) I can
increase the maximum fuel flow rate very easily when required.

To maintain resolution it's important to not oversize the injector/pressure
regulator combination.  For applications where there is a very large dynamic
fuelling range (eg turbo and rotary applications) we use extra injectors
which only get turned on when the load is reasonably high.  When injectors
are operated close to the point of injector cutoff during idle, the idle 
stability can suffer due to very slight injector mismatch and variability.

The turnon/off time of an injector is approximately 0.8 to 1.0mS.  
13500 rpm -> 4.44 mS per revolution or 8.88 mS per cycle
Injecting every cycle is recommended to avoid a fuel flow nonlinearity
between injecting at just below 100% and injecting at 100% duty cycle.

One of our customers is fitting an injection system to a Honda 6 cyl bike
stroked to 1200cc and fitted with a turbo charger.  I'll let you all know
how it runs later.    :-)

Robert

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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 344 7966                           (+613) 344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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