Lucas Mechanical PI System Model

[Vixen]

Hello Roger,

That's better news today, sounds like you are back on track. Still following along

Mike

[steamer]

yup  the little gear pumps are persnickety!..   In the lathe Ive had good luck with toolmakers bushings to line up on each bore,

Watching along as well

Dave

[Roger B]

Thank you both 

Next the Oilite bushes were pressed in place, one top hat bush and one I turned the flange off earlier. I did not fit the lip seal at this point as if I have to remove it it will probably be damaged. Finally the M3 fixings for the cover were drilled and tapped.

When I fitted the cover the gears became tight, they were rubbing on the cover and needed to be slightly thinned. To do this I put the idler gear on the end of the drive gear shaft and clamped them in a 13mm ER collet. This would hopefully keep everything balanced and square.

[Roger B]

I then needed to make a couple of hose fittings for the first test. The pump was fixed in a 4mm collet in my Proxxon FD150 lathe with a toolmakers clamp to stop it rotating. The tank was filled with lamp oil (non-smelly kerosene) and off we go.
The first test produced quite a good flow:

<a href="https://www.youtube.com/watch?v=wixuh8Tt0Xc" target="_blank">http://www.youtube.com/watch?v=wixuh8Tt0Xc</a>

The pressure developed was not so good, barely reaching 1 bar at 3000 rpm.

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

I measured the flow rate as 10ml in 4 seconds at around 1000rpm (the big ring spanner is to stop the measuring cylinder falling over).

It pumped, but as I somewhat expected it did not achieve the pressure I need which is why I didn’t initially fit the lip seal. The maximum pressure is a balance between the amount of liquid carried round in the gear teeth compared to the amount of leakage. I could probably reduce some of the clearances to reduce the leakage however the Ali Express gears were not very precise and not quite circular. The other option is to go to bigger teeth so that more liquid is pumped to compensate for the leakage.

I do have some 12T Mod 1 Delrin gears that I have used in my water pumps. To use these I would have to open up the cavities from 13 to 14mm. As I have a 14mm plug gauge this will help with improving the precision.

Watch this space   

[Admiral_dk]

A nice steady flow and that is not a bad thing   

Something tells me that I need to go back and read up on this system again ....
as I could have sworn that the amount of Fuel is Metered by the Shuttle travel and not by the pump 

Per       

[Roger B]

That is correct , however the pump needs to deliver the injection pressure ~7 bar.

[Roger B]

Some analysis:

The pump delivered 10 cm3 in 4 seconds at 1000rpm so in 66.6 revolutions. There are two 24 tooth gears so 66.6 revolutions is 3200 tooth volumes for 10 cm3. The pumped volume from 1 tooth is then 0.003 cm3 = 3 mm3. The teeth are 5mm wide so the required tooth cross-sectional area is 0.6 mm2.
A 0.8mm drill shank has a cross-sectional area of about 0.5mm2 and fits into the tooth space so the tooth area is a bit more than this. This suggests that without back pressure the pump is fairly efficient.

Next I inspected the gears again. There is once again too much clearance 

https://youtube.com/shorts/XExcCt8jywU?feature=share

<a href="https://www.youtube.com/watch?v=XExcCt8jywU" target="_blank">http://www.youtube.com/watch?v=XExcCt8jywU</a>

The gear centres seem to have reduced to 11.8mm and I could slide a 0.2mm drill shank between the teeth at the meshing point. A 0.3mm drill wouldn’t quite go in. A 0.2mm wide 5mm long leakage path is an area of 1mm2, not surprising it wouldn’t generate any pressure 

What is the problem? Is the brass used in these gears too soft? Was the surface finish so bad that more than 0.1mm has been polished off? They were from Ali Express.

I have a couple of options:

I can bore the cavities out for the MOD 1 Delrin gears, but I think this would give too much flow or I can use the steel MOD 0.5 gears as in the Junkers oil pump. They are slightly thinner so I would have to skim about 0.5mm from the body and the bores are 5mm instead of 4mm so new shafts would be required.

Once again lots to think about 

[Admiral_dk]

To my eyes picture 157 in reply 43 looks to have the same clearance as what you show now - I know it can be my eyes deceiving me ....

Is it possible to make a pump where the 'freewheeling gear' sits in an offset bush - so turning the bush adjust the clearance between the gears 

Per   

[Roger B]

Thank you Per. The clearance is hard to measure and even harder to photograph. Unfortunately, an excentric bush would not work as the gear axis must be concentric with the cavity. As Dave (Steamer) said, these are tricky little things.

I decided to try the steel MOD 0.5 gears. Firstly the bosses were parted off and then a new idler shaft was turned from a piece of 5mm silver steel. Another piece was centre drilled and then knurled for a length of 3mm. This was then pressed into one of the gears.

[Roger B]

The gear was then held in a 13mm ER collet and supported with the tailstock centre to turn the shaft down to 4mm. The gears seem to mesh well with no obvious clearance. The body was then skimmed to give minimal clearance for the gears and the pump was assembled for testing.

This gave around three times the pressure from the previous version. I had to change to a different pressure gauge.

<a href="https://www.youtube.com/watch?v=OOZ8jw60siI" target="_blank">http://www.youtube.com/watch?v=OOZ8jw60siI</a>

For this test I had an injector set for low pressure to allow some flow through the pump. There was some fuel leakage from the back of the pump which was sprayed around by the lathe chuck. It could be from the drive shaft as I haven’t fitted the lip seal yet or from the idler shaft that is just floating and is not yet Loctited in the body.

Checking the pump after the test showed some marks on the cover plate and the gear mesh still looks good.

I think that the next step is to try with MOD 1 gears, maybe Delrin, maybe steel 

[Roger B]

As the minimum order value on my source of MOD 1 steel gears was rather high I decided to try the Delrin gears, I can always change them for steel later.

I followed a similar process to before so I won’t put in much detail. The body was squared up and the two shaft holes were drilled and reamed at a centre distance of 11.9mm. The bosses were parted off from the gears and they were faced to 8.90 – 8.95mm width.

The inlet and outlet ports were drilled and tapped as before and the body was mounted on the 4mm spigot for boring the cavities. This time I used a carriage stop to set the basic depth and the compound slide to home in on the 9mm depth.

[Roger B]

Both the cavities were opened out using the graduations on the cross slide handwheel combined with frequent measurements. This allowed me to get within a couple of hundredths of 14mm.

Now I need to bore one shaft out to 8mm for the bearings and oil seal and then drill and tap the cover fixing holes.

[Admiral_dk]

Fingers crossed - as we hope for success 

Per         

[Chipmaster]

Stay with it Roger 
Andy

[Roger B]

Thank you both 

My fuel injection work is a fairly lonely path however there are some who are interested so it is worth posting. My diesel work has come to a (hopefully) temporary stop as I no longer have access to a suitable test area either outside with a power supply or inside with good extraction.

On with the gear pump: The drive shaft hole was opened out to 8 mm for the Oilite bushes and lip seal as before. I knurled the end of the drive shaft and pressed the gear in place. As knurling just displaces material I hope that the plastic of the gears flows a little to fit with the knurl without causing too much force and splitting the gear. It seems to work. The fixings for the cover were spotted through then drilled and tapped as before.