Question re: injectors

[robert dingli]

Frank Deutchmann writes,

> 
> I have a semi-theoretical question on fuel injectors:
> 
> Given an injector which is sized such that it can operate synchronus with
> the intake valve at a given RPM (i.e.: the injector is large enough that it
> can deliver the full quantity of fuel required within the opening window of
> the intake valve, but not so large that the injector open time window error
> component becomes significant), is there any benefit to placing the injector
> farther from the intake valve?
> 

In steady state conditions, yes, but in transient conditions, no.

> In particular, I am wondering if there is some benefit of increased fuel
> evaporation if the injector is moved farther away from the intake valve -- I
> am assuming a straight intake runner to the valve, however.  Also note that
> I am not concerned about fuel pooling in the intake tract at lower or higher
> RPMs (i.e.: assuming airflow is great enough that the fuel injection cone
> does not widen enough to contact the intake runner walls).
> 

The main benefit is in the mixture preparation.  The fuel and air will get 
more time to mix homogeneously.  Regardless of fuel injector spray pattern,
you should count on a significant portion of wall wetting (sometimes up to a
millimetre or more of fuel film thickness).  In a typical system where the 
injector sprays directly onto the inlet valve, 10 to 40% of the injected
fuel will join the fuel puddle on the walls to be inducted in later cycles.
There is also the transition delay from injector to port, which may limit 
your controller response.

> As a final question, how would such a scenario compare to a smaller injector
> placed closer to the intake valve, and thus deriving evaporation benefit from
> firing fuel onto the back of the hot valve?

In their Le Mans winning quad rotor race engine, the engineers at Mazda
utilized two injectors per rotor.  One close to the peripheral port for
fast response during transients, and one further upstream for steady state 
operation for better mixture preparation.  Fuel efficiency was a major
consideration in their class.

> 
> I wouwd appreciate pointers to any research in this area!
> 

>From what I have sitting on my desk in front of me:
SAE 930766 which explains modelling and control of fuel wall wetting
There are a quadzillion SAE papers on injector design, port design for
fuel injection and injector placement.  I you could be more specific,
I will sift through the papers that I have collected.

Robert Dingli

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

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 9344 7966                          (+613) 9344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
----------------------------------------------------------------------