I fitted the new smaller helix pump onto my two cylinder engine, the details are here:
https://www.modelenginemaker.com/index.php/topic,7883.msg248312/topicseen.htmlThere were various problems with this design:
1) The O ring seal between the pump element and pump body requires close control of tolerances and careful smoothing of almost hidden edges to avoid damaging the O rings.
I conceived this design to allow for hardened pump elements and a modular system to make a multi cylinder pump. I’m now not so sure that this is an optimal solution, maybe a simple hardened sleeve shrunk/Loctited into an aluminium body might be better
Care would be needed to ensure that the inlet port in the sleeve lined up with the port in the body, easy with Loctite, harder with a shrink fit.
2) Fuel leaks past the plunger.
I accept that there will always be leakage past a metal pump plunger in a metal barrel. This could possibly be reduced by using some form of seal around the end of the plunger but I think that would be another level of technology with a 2mm diameter seal
The other option is to manage the leakage. With the current vertical layout the fuel leakage flows down onto the operating mechanism and washes the lubricant away resulting in wear and seizing. This would be less critical with diesel which has some lubricating properties. My first two injection systems have/had horizontal pumps so that leakage past the plunger does not get into the mechanism.
Maybe a new design with the pump horizontal would be better as it would allow for the fuel leakage to by led away from the mechanism.
3) Stroke/volume control and the inlet port.
My first pump designs were based on full sized diesel pumps where the inlet valve is a port in the side of the pump barrel. This removes the need for an actual inlet valve and any sealing problems. It also allows for a precise control of start of injection which is necessary for true diesel operation. For operation on petrol (gas, benzine) it creates a problem as the cylinder is under vacuum when the port opens causing the petrol to vaporise. This can be solved, as in full size practise, by pressurising the fuel above it’s vapor pressure ~0.8 bar/10psi.
Controlling the volume by controlling the pump stroke works and is quite simple but does not give constant timing, which is not a problem for manifold petrol injection. It is however very sensitive to wear in the operating mechanism. Even if the actual working stoke remains the same increased clearances in pivots etc. reduce the delivered volume. When the working stroke is around 0.2mm this is a problem.
Controlling the volume with a helix ground in the plunger is more complicated. It gives a constant start of injection point and seems fairly linear. This seemed to work ok with diesel, but with petrol the leakage past the plunger washed the lubricant away from the rack and gear make the movement stiff and jerky.
Next steps.
I think I need to split the development into two streams:
a) The helix controlled pump for diesel, continuing on the current path but using a simple hardened sleeve.
b) A horizontal pump with separate inlet and delivery valves for petrol. This will reduce the sensitivity to wear in the operating mechanism, hopefully remove the need for a separate pressure feed pump and allow separation of the leakage past the plunger.
The picture are taken from the two cylinder engine thread so the numbers are out of sequence.