----- Original Message -----

From: Gary Casey

To: Rotary motors in aircraft

Sent: Monday, June 22, 2009 9:44 PM

Subject: [FlyRotary] Re: Duty Cycle for Injectors

Just a little historical anecdote:  When we at Bendix were developing the very first US production fuel injection system (remember the Cosworth Vega?) we sized the injectors for about 80% on-time at maximum manifold pressure, minimum air temperature and full power.  This is a compromise, of course - a low-flow injector is desireable to have the most accuracy at idle when the pulse width is very short.  But if the injector approaches 100%, there will be a pulse width less than 100% when the injector will stay fully open, creating a step in fuel flow.  This is a normal occurence during cold engine operation when the enrichment algorithm commands more flow, but it turns out that a max engine speed there is little or no need for cold enrichment regardless of temperature, so the effect is pretty much ignored.  The idea of sizing the injectors for 80% duty cycle had nothing to do with racing applications, but it applies there, too.  The obvious problem encountered when undersizing the injectors is that once 100 duty cycle is reached the flow is limited - the engine will lean out if the rpm or air flow goes higher.  Not a good thing.

 

Inceidentally, there is no concern for injector relaibility or durability.  It is a simple solenoid valve and running at 100% duty cycle would theoretically make it last longer - it is the total number of open-close cycles that wears the injector.  As injectors wear the flow generally increases and most modern injectors are rated for a billion cycles with a flow increase of less than 3%.  The coil is cooled by the fuel itself, so overheating is not a concern.

 

Gary Casey

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Ed,

I remember reading that the 80% duty cycle rule (of thumb)was originally developed for the racing industry.

 

Given that probably 100% will never be reached in the day to day vehicle use - the 80 % duty cycle most probably doesn't apply for sizing. However bigger injectors are standard in vehicles where the RPM limit is much higher than what we may experience in our Aviation application, even if for only short periods of time.

 

Given that our RPM is on the high end for climb and take-off, but not as high as it could be (not max RPM), and cruise is for the most part only 6,000 rpm, do we really have to held to the 80% DC rule, where perhaps 90% DC (for short periods of time) may well give sufficient safety margin to maintain longevity of the injectors.

 

Not that there is a major benefit in having borderline injector duty cycle, as you so rightly pointed out -  it's just that it would be interesting to know!

George (down under)