Measuring oil pressure

[Swede]

I have a setup where I'd like to measure engine oil pressure, but I've got a problem - the pressure pulsates violently. Rather than a nice, steady reading, like 30 PSI, my pressure gauge needle flails through +/- 10 PSI. Besides being distracting and not particularly elegant, it's destined to wreck the pressure gauge.

While I have things torn apart, I'm wondering if there is anything I can do to eliminate this. I have a glycerin-filled pressure gauge which should help. I'm thinking a tiny, tiny orifice somewhere in the line should dampen or eliminate pulsations. Since the oil has viscosity, the pressure pulses shouldn't be transmitted through the orifice.

I'm thinking a #80 drill through a brass round section, maybe 1/4" long, inserted in the line, should do the trick.

Am I on the right track?

 

[Noitoen]

On electronic transducers, the thin orifice is used. This eliminates the pulsing. You could also use such a transducer and a electronic circuit to average the signal.

 

[doubleboost]

Small hole in the feed line will sort the job
As you say act as a damper
John

 

[Catminer]

Swede;

In the real size world the orifice is in the inlet of the gauge, called a snubber.

Peter

 

[Maryak]

I'm thinking a #80 drill through a brass round section, maybe 1/4" long, inserted in the line, should do the trick.

Am I on the right track?

Swede,

Yep that's the way to do it. One word of caution, if you make the hole too small the gauge will show pressure when there isn't any
Maybe it's not a big deal for your application........................ but ???

Best Regards
Bob

 

[lordedmond]

as a fix why not try a filter from a oil fired central heating nozzle

the snubbers may be to big for your case


Stuart

 

[steamer]

they also make pressure gauge snubbers which are threaded on in fron of the gage....They may be too big for you application...but ...google is your friend ;D

Dave

 

[boatmadman]

If you have an isolation cock on the pressure gauge, just slowly shut that in to dampen the fluctuations. Just dont shut off completely!

Ian

 

[MachineTom]

For oil I doubt a short lenght of #80 hole will have any effect, a Snubber/Choke is a simple answer. Make your own, a snubber is like a very fine threaded screw,like 80tpi pressed inside a unthreaded body, this makes for a very long path for the oil to travel, which is what is need to reduce the pulsations. You can figure the rest.

 

[milotrain]

One thing of note is that Wika will build you damn near anything you want and it doesn't cost you any more (just time). Also they build all there gauges in the USA. I've ordered a few custom gauges through these guys and they were fantastic.

http://www.dfs-gauges.com/


(Warning, total assumption, no data, no experience)
One thing in regards to snubber length and pulsations is that the length of the snubber must should have to be a function of the frequency of the pulsations. The faster the pulses the shorter the snubber should have to be, the slower the pulse the longer the length should have to be. However I'd bet that the snubber length should be at least as long as the wavelenght of the frequency of the pulse to really work well.

You could use a double slit interference filter and see if that cancels out the pulsing but you might need to have a cat hidden in a shoebox somewhere in your house to make sure the waveform collapses into a particle. ;D

 

[Lakc]

The quick and dirty way just to take measurements is install a longer hose to the gauge. Keeping a section of the more pliable flexible hydraulic hose inline instead of all copper tubing would be recommended if you want a more permanent mounting. Its good design practice to avoid all metal lines and too short of rubber hoses just because of the fatigue factor alone. The coiled loop often seen in steam pressure gauges will also dampen vibrations and act as an anti-hammer device as well.

 

[mu38&Bg#]

What I'm wondering is why there it so much fluctuation to begin with? Is it a gear pump?

 

[Lakc]

What I'm wondering is why there it so much fluctuation to begin with? Is it a gear pump?

Im guessing its his Hodgson radial, and if it oils like the Edwards, its a cam actuated piston pump. I was going to buy a set of plans from Lee at NAMES this year but walked home with a dividing head instead.

 

[Swede]

Great stuff, guys, thank you. Yes my Hodgson is being completely rebuilt, painted, jazzed up. I've been asking unrelated questions on paint, propellers, decals, etc, and been having a lot of fun revisiting this project. The stand and firewall components have been painted with a dark red and green, the new 3-bladed adjustable-pitch propeller looks nice. I'm upgrading the ignition while it's apart.

The oil pump is a gear pump, and in theory should be very smooth. It's possible that the location of the pressure gauge on the firewall may simply be harmonizing with normal engine vibrations, and if that's the case, snubbers or orifices won't help, but it's worth a try. One thing I've noticed with this design, at least as it was when I built it, was that the pressure pump overwhelms the scavenging system, and if the oil metering valve (a simple needle valve) is opened more than 2 turns, the crankcase gets flooded with oil, and it eventually gets pumped out of the front main bearing.

So it is a matter of tweaking the oil valve to try and find a spot somewhere between NO oil (very bad) and too much oil. I had hoped that the pressure gauge would help... open the oil metering needle valve, and as soon as I get pressure, leave it where it's at. I did add a pressure relief valve on the oil pump itself. Being a gear pump, the possibility existed of producing crazy-high pressures internally, and the valve was set to dump excess oil above 60 PSI or so. Unfortunately, this pressure relief valve is internal and not accessible from the outside.

 

[Lakc]

I did add a pressure relief valve on the oil pump itself. Being a gear pump, the possibility existed of producing crazy-high pressures internally, and the valve was set to dump excess oil above 60 PSI or so. Unfortunately, this pressure relief valve is internal and not accessible from the outside.

Do you think its possible your getting an oscillation from this valve, and that's causing the pressure fluctuations? An oil hole uncovering somewhere?

I went back and reviewed your pump pics but couldn't tell what was going on without a drawing.

 

[Swede]

I don't think so... normal operating pressure for this pump is well below the 60 PSI I set the relief valve at, so unless the relief screw moved, it shouldn't be oscillating. But it's a thought.

The word "oscillating" got me thinking... it might be this - the oil source needle valve does not permit a strong continuous flow of oil into the pump inlet. Gear pumps must be primed,, and they don't pump air well. Perhaps what I am seeing is the pump loafs along until adequate oil is present; the efficiency immediately soars, and the available oil is pumped out. There's a moment of cavitation, the source oil then catches up, and the cycle repeats at 5 to 10 hertz. Maybe...

One other thing I'm trying - the original plumbing from engine to gauge was solid copper tube. I'm trying a new pipe that will be split and reconnected using viton tube, so there will be a flexible joint in the line that may help with mechanical vibrations. But if I can't fix this, it's no big deal.

One other thought for anyone doing a scavenging system like this - make the pathway that the used oil takes to the sump or collection area WIDE OPEN. It'll definitely help. I have a couple of choke points internally that prevents oil from flowing downhill, and I think that's part of my problem.