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Charles Lamont:
I have been messing about since last year with some experimental water pumps. Work on these experiments is sporadic, and I am still thinking of improvements to the test setup, but I thought a quick look at some of the work might be of interest.
 
I need a water pump for my Westbury Seagull, and I want one which provide sufficient flow at tickover to allow the engine to run indefinitely. Some initial sums indicated that a flow of about 0.3L/min would be sufficient for the engine at full power. The pipework is 5/32" (4.2mm) bore and more sums indicated that an impeller diameter of 1/2" and blade height of 5/64" would be suitable.

The first test pump, (photos 1&2) has an annular water passage cut into the body. This pump was also intended to experiment with the shaft seal. I tried a stainless to PTFE face seal, but it did not work well and the pump was quite wet at higher heads. Grease lubrication of the shaft was more than the test rig motor could manage, so the pump was modified to take a ptfe bearing sleeve.

I wanted to see if a proper spiral volute would make a difference and built pump no 2 (photos 3-6). The spiral passage was milled in steps on the rotatry table. The slight scollops can be seen in the last photo. This pump uses an o-ring seal at the drive end, so that the ptfe sleeve would run wet.

Next: impeller variants   

Charles Lamont:
In classical centrifugal pump design theory, vector diagrams are used in working out the best shape for the impeller blades. However, in very small pumps like these, friction and viscosity effects are large, and the textbooks just don't go this small. One of them does say that for small pumps, the calculations will result in blades with a steep backward rake. In general, a backward rake should produce higher pressure and lower flow rate, while forward swept blades should give relatively higher flow rate and lower pressure, with radial blades inbetween.

So far I have tried impellers with: four straight backswept blades,  six radial ones, four forward swept, and three backswept curved ones that are typical of larger pumps.

I have also spent a little while trying CFD (computational fluid dynamics) simulations with SimScale, but have not managed to get decent results, and don't think I can justify investing much more time in following that rabbit hole.

I have so far carried out over 200 runs on the test rig, and the main conclusion so far is that at this scale the blade rake makes much less difference to the performance curve than I expected, typically something like 10% on flow rate at the same speed and head. Much more significant, I have lately found by error, is the axial clearance between the blades and the cover. The spiral volute with a small clearance at the tongue or cutwater performs better than the annulus as I would expect, but the two pump bodies are not exactly comparable, so that may be unfair.

Next: a different type of pump.     

Vixen:
Hello Charles,

Thanks for posting the results of your water pump experiments. They have appeared at just the right time for me, as I am in the middle of making the water pumps for the two Mercedes engines.

Thanks again

Mike

Charles Lamont:
Unsure if the little centrifugal pump would have sufficient output at low speed, and with the investigation taking on a life of its own, I decided to see If I could make a regenerative pump with a 1/2" rotor. Initially I did not relish having to make a tiny woodruff-like cutter to mill the vanes. Instead, I was able to cut radial slits and Loctite little pieces of 0.015" brass shim in place, finish turning afterwards. The water passage arcs in body and cover were milled with a 1/8" ball-nose cutter.

On test, I found it does produce more pressure at low speed than the centrifugal pumps, but I think the curve may be too steep for practical use at higher speed. At 2000 rpm the head at zero flow was 45cm, while the centrifugal pumps need 4000 rpm to reach the same pressure.

This pump has been an interesting exercise to make and test, but I probably won't use this type with the Seagull.

Next: the test rig



 

Roger B:
Interesting work  :ThumbsUp:  :ThumbsUp:  :)

I have used gear pumps as cooling water pumps on a couple of engines as I think in small sizes they are less effected by tolerances/clearances. I may be completely wrong   :headscratch:

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