Micro Fuel Injector Trade Study Complete!

[RodRico]

Nozzle trade study completed using ethanol (which is fine for comparison of configurations). It turns out the simplest solution wins... single small orifice with an offset feed into the needle chamber inducing swirl. The plot below summarizes the characteristics and performance of the final configuration. At 275 psi, the injector mass flow is 7.09E-5 kg/s which results in delivery of the target charge (at 0.386 equivalency) over 8.7 degrees of rotor rotation (the injector is sized so it can support equivalency of 0.801 before the intake port closes).

Now I just have to find detailed characteristics of diesel so I can adopt the dimensions for it's increased viscosity, finalize the design, and move on to soliciting quotes from micro-machine shops. As it stands, I think plunge EDM is the best choice, so I'll be focusing on those suppliers first.

 

[enginesrus]

Next will be a good filtration system.

 

[RodRico]

Yes, a very fine filter will be required. My orifice is 51 microns in diameter, so these 37 micron stainless steel filter sheets should do the trick. I have plenty of surface area available for filtering fuel into the injector using these screens. Since these filters will be somewhat difficult to replace (just like the filters in any fuel injector), there will also be a bulk fuel filter (there are many with sub-50 micron capability).

 

[BrianPetersen]

If there is a needle closing off the injector opening from the inside, you need to filter much finer than that, because something that gets into that gap between the needle and seat will hang it open.

Diesel fuel systems have similar issues, so diesel fuel filters to 5 microns, or finer, are readily available. A screen can serve as last-ditch fail-protection, but do include a good, real fuel filter. The smallest one you can find will probably be bigger than your entire engine, but so what. You've got a ton of other headaches to solve. Fuel filtration will be among the least of them.

 

[RodRico]

Brian,

The needle tip is flat and simply covers the orifice opening (I adjust the tip diameter to control the ratio of initial opening pressure versus hold open pressure). Nonetheless, I imagine something very small could hold it open a bit. That may not be an issue, however, since there's no pressure in the injector after injection completes... it's a unit injector with a cam driven pump piston to generate injection pressure starting at the right time and another cam driven relief valve dumps the pressure when the injection event is done. Note the phase of both cams is adjustable, but I don't expect to be moving the start of injection, so only the cam controlling pressure relief is dynamically adjusted during operation. Do you think I could get by with the just the 25 micron filter given the operation of the injector?

Rod

 

[BrianPetersen]

Why risk it? Just throw a 5 micron diesel fuel filter in the flow stream and be done with it. You've got enough other issues to concern yourself with to not have to worry about this one.

 

[RodRico]

Brian, Good point. Rod

 

[moon161]

You can get yourself a ready made 5 micron filter for less than $10 and a filter head for less than $20, and your head scratching will only be how to plumb it and where to put it.

 

[RodRico]

Yep. I've found a good number of 5 micron diesel fuel filters/water separators for a very low price on-line. They're far too large for my need, but that's not a problem for the prototype. I've also found a number of suppliers for 5 micron screen that should allow construction of a miniature 5 micron filter for production deliveries should I make it that far down the road.