Supporting > Engine Ancillaries
Lucas Mechanical PI System Model
Roger B:
At first I was not sure whether to write a thread on this as it is quite specialised. It started as a discussion on the Porsche fuel injection thread and then I realised that if it worked it would be a good solution for one of my future engine designs.
The injectors are based on my previous poppet/mushroom design but reduced in size as they will not have to fit my existing diesel engines and don’t need the compression seal.
As this system has a quantity of petrol pressurised to around 7 bar (if I have to use the same pressure as the full size system) some though will need to be given to safety.
The injector nozzles are turned from 6 mm hex brass with the bore drilled 0.8mm and reamed 1mm. there is then a clearance bore for the needle and a larger clearance for the spring.
The needle is made in the same way as the larger ones using 1.5mm silver steel:
Centre drill with a 0.5mm bit.
Turn the portion that will be threaded down to 1mm.
Thread M1.
Roger B:
Continuing the needle sequence:
Support the needle with a 1mm bore brass bush held in the tailstock chuck and turn the next section down to 1mm.
Turn the section before the cone down to 0.8mm to allow the fuel to pass.
Set the cross slide to 5° and turn the cone.
Part off using a 1.2mm bore brass bush to set the parting tool in the right place on the cone.
Roger B:
Next the union nipples were turned from 3.5mm brass rod.
All the turning was done using an Applitec 1mm parting and grooving tool.
The injector body and M4 x 0.5 union were turned from 5mm brass hex also using the Applitec parting tool to avoid changing tools and having to reset the cross slide zero position.
The cones on the needles were carefully polished with a 400 grade diamond lap. I decided to try not lapping the cone onto the nozzle as I have done with previous poppet injectors.
Finally I made up an M4 x 0.5 to M5 x 0.5 adaptor pipe using 2 mm od 1 mm bore copper pipe so I could make some trials.
Roger B:
The injector was fitted to my test pump and tested with lamp oil (kerosene that doesn’t smell). The spray pattern was quite good but not sufficiently atomised to be ignited with a flame.
http://www.youtube.com/watch?v=GHcJAl6gp80
I made an adaptor from a short length of plastic pipe to fit the inlet manifold on my 12 cc twin. After blowing out the lamp oil and priming with petrol. It ran quite well, but obviously much too rich due to the reduced pressure on the helix pump reducing the leakage.
http://www.youtube.com/watch?v=yVcLcY32dkY
Manually adjusting the fuel rack improved the running, however a different control cam will be required.
Vixen:
Hello Roger,
What an excellent tutorial of how to make the parts for one of your miniature injectors. :ThumbsUp: :ThumbsUp: :ThumbsUp:
What can you tell us about the tiny spring, which is obviously key to getting the tiny injectors to perform so well? Are they commercially available, or you need to 'roll your own'? Perhaps the more important parameters are spring rate and free length. What closing force on the needle are you currently trying?
Thanks for posting this detail.
Mike
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