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.

<a href="https://www.youtube.com/watch?v=GHcJAl6gp80" target="_blank">http://www.youtube.com/watch?v=GHcJAl6gp80</a>

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.

<a href="https://www.youtube.com/watch?v=yVcLcY32dkY" target="_blank">http://www.youtube.com/watch?v=yVcLcY32dkY</a>

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.   

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

[Roger B]

Thank you Mike   

The spring is a commercial item, full details below.  I decided at the beginning of my fuel injection experiments to use commercial springs for consistency and to allow others to copy what I have done.

The nuts are made from 2mm brass hex.

For actual use the injectors could be made smaller by soldering the fuel pipe directly to the body without using a union. The nozzle hexagon could also be reduced in size.

The injection pump will follow the two cylinder design below using a 2mm diameter shuttle with a maximum stroke of 2mm. Previous trials have shown that this is more that enough to petrol inject a 25cc cylinder.

To allow the pump sleeve to be fitted to the body without damaging the O rings one of the O ring grooves is cut in the body. For this I need a narrow internal grooving tool that will enter a 12 mm bore. They are available, at a price, so I decided to try and modify a small Proxxon boring bar.

I very rarely try and grind lathe tools, preferring to use inserts for consistency, usually DCGT 070204. I ground something that I thought looked right, checking with an offcut with a 12mm hole that there was some clearance.
After a little experimentation with tool height and angle I got a good cut 

[Vixen]

Hello Roger,

Thank you for the spring information, I hope I have not distracted you too much from your excellent miniature injector thread.

I looked back through your previous posts but could not find much spring info. I see you tried a larger diameter spring which needed changes to the lock nuts to keep it centralised, not sure if the performance was better or worse than the 'standard' spring.
 
Am I correct in thinking you give the spring a 2.0mm preload? which would appear to give about 1.0 N  closing force on the needle.
 
I can find similar (but not identical) springs from the Lee Springs Ltd catalogue. Which way should I bias my choice? Is a higher spring rate preferable to a lower rate?

The more I look at the Lucas shuttle valve metering unit, the more I think it may be an excellent choice to meet our model engine requirements. The shuttle will ensure equal fuel delivery to each cylinder and the movable shuttle end stop looks like it is begging to have it's position set by a simple wedge. The wedge position being set either by manifold vacuum or by the throttle position.

The proposed size of you two cylinder Lucas injection pump is also interesting. You calculate a 2mm diameter shuttle with a 2mm stroke will be more that enough to petrol inject a 25cc cylinder. If we consider methanol requires approx. three time the quantity compared with petrol; then a similar 2mm diameter shuttle with a 2mm stroke will be more that enough to methanol inject an 8.0cc cylinder (917?). I calculated a 2mm diameter shuttle with a 1.0 mm stroke for methanol, so we are well within the same ballpark. Hopefully there should be good design read across.

Thanks again. 

Mike 

[dieselpilot]

The more I look at the Lucas shuttle valve metering unit, the more I think it may be an excellent choice to meet our model engine requirements. The shuttle will ensure equal fuel delivery to each cylinder and the movable shuttle end stop looks like it is begging to have it's position set by a simple wedge. The wedge position being set either by manifold vacuum or by the throttle position.

That's exactly how the system worked. Race versions used a throttle linked fuel cam. Road car systems used a manifold reference. The challenge will be feeding 6 or 12 cylinders.

The previous thread had me thinking I should try to build one for an RC aircraft engine.

[dieselpilot]

Roger, will you be using a diaphragm where the original does?

[Roger B]

I haven’t got into the details of pressures with this system yet. I am going to try the original Lucas pressures so that the injector should open at around 3 bar. That means a lower spring rate may be better. My running trials so far have been made with an injection pump that will reach at least 100 bar.

My 25cc horizontal engine has a 2mm bore pump and the working stroke is around 1mm so there is plenty of capacity to spare on petrol.

My initial trials will use a mechanical connection between the throttle and the shuttle position. Moving on to a diaphragm will require a good vacuum signal which requires a multi cylinder engine with a not too wild cam.

This first version of the shuttle pump is very much a proof of concept so I won’t be hardening and lapping, just using silver steel in reamed bores. The silver steel is typically 0.01mm undersize (in spec.) and the reamed hole are typically 0.01mm oversize (in spec. for H7) which give a running clearance of 0.02mm. This is too much for a 100bar diesel pump but may be ok for a 7 bar petrol pump 

The pump sleeve is made from 12 mm silver steel drilled 5.8 mm and reamed 6mm.

The pump rotor is 6 mm silver steel drilled 1.9mm and reamed 2mm. The ends are threaded m4 x 0.5.

[Roger B]

The sleeves (I made an additional as a spare) were then set up inn the mill using an edge finder and drilled through 1.5 mm.

The outlet ports were opened out with a 4.5 mm end mill for the sealing O ring.

The sleeves were then turned 180° and positioned with a 1.5mm drill to mill the fuel inlet passage on the other side.

[Vixen]

 Following every move 

Mike

[jcge]

Nice work Roger. I'm fascinated to see how this goes.
Regards
John

[steamer]

Watching VERY  closely!

Dave

[Roger B]

Thank you all for your support and interest 

Next up was to drill the ports in the rotor. The two holes have to be at 180° and offset by 10mm. I decided to use my small Proxxon RT as I could get one hole between a pair of jaws and the other outside.

With hindsight (so accurate) It would probably be better to drill the rotor and sleeve to the required depth, drill the required ports in a RT or dividing head whilst still attached to the stock, part off and then ream the bore to size. This would probably be essential for a 4 or 6 cylinder sleeve.

This operation went ok and I could assemble the rotor in the sleeve and put a 1.4mm drill into both ports at the same time 

I am still working on the design of the body. One end will have the cam/wedge to control the shuttle position, the other some form of drive coupling (very small Oldham style?). Both ends will have to have a return line for fuel that leaks past the shuttle or rotor. I will also need to select a suitable way to retain the fixed shuttle stop. For the finished item some form of Loctite would work, but in the experimental phase it needs to be a removable mechanical fixing. I am looking at either a very small grubscrew in the rotor, maybe M1.6 x 2, or a M1.4 screw from the coupling end.

Lots to think about