Limiting pressure to a gauge

I am rebuilding a 1910 5 litre petrol engine. On start up the oil pressure peaks at 70 psi but the running pressure is 15 psi. I need to limit the pressure reaching the gauge on start up as the gauge only reads to 30 psi?

Edited By vintage engineer on 16/10/2019 21:40:16

Put a 30psi pressure relief valve before the gauge.

If originality is not important an easy solution it to fit a higher pressure gauge. True, it will not allow accurate interpretation of low pressure values but mostly with engines its not the actual pressure of the oil that is important, its the presence of pressure that matters.

There are probably pressure limiting valves (some vehicle rear brakes were so equipped to stop wheels locking up) but whether anything ready made exists for your application I have no idea.

If the high pressure is only present for a very short period then a restrictor close to the source of the pressure would act as a snubber and the pressure 'event' would be over before the gauge had time to respond and suffer damage.

Ian P

Where does the 'relieved' oil or pressure go?

Ian P

back into the sump, but the PRV would have to pass quite a bit of the pump flow to be effective, and that oil should be going round the engine, not back into the sump. You could put a restrictor in front of the PRV, but that would be prone to getting blocked and stopping the gauge working

If the oil pressure gauge isn't original could you put an end stop on the bourdon tube (so that it doesn't bend the needle) and simply let it hit this end stop at about 30psi. I doubt it would harm the gauge.

Why?

Is this because the oil is viscous when it is cold, or perhaps the engine is a bit tight yet and will slacken off when the running in process is complete.

Cure the problem, not the symptoms.

I'm uncomfortable with the idea of dumping oil flow back to sump during start-up, that's when you most need the oil in the bearings where it belongs. High pressure on start up suggests the oil isn't getting where it needs to be - or at least not in the quantities it should. I'm assuming the oil pump is a positive displacement device so the rise in pressure indicates a restriction in the flow around the engine. If it is actually caused by the oil being a bit thick and treacly when it is cold then there are other oils to choose, at least for the running in phase.

I'd fit a higher scale pressure gauge as a temporary measure and see how this problem develops as the engine hours accumulate.

HTH Simon

Quite, back to the sump. The restrictor should work fine. When the oil is cold and viscous it will reduce flow to the PRV (which will protect the gauge) and when the oil is hot the PRV won't open anyway so the restrictor is not utilised.

Do remember that a positive displacement pump must have an outlet - or it will destroy itself. Unless it is built to resist extremely high pressure. The drive is often the limiting area, but I have seen hydraulic pump housings cracked in half (normally operating at close to 300 Bar, mind).

There's an idea for a very simple restrictor here;

https://www.stangnet.com/mustang-forums/threads/autometer-oil-pressure-gauge-hose.853817/

..restricting flow is an accepted method of reducing "spikes" in gauges. You could fit it in a union at some accessible point so that If it does block, it is easy to service.

Personally, I would most emphatically NOT put any sort of pressure relief valve in the gauge line.. the consequences of a mishap are out of all proportion to the benefits..

First, if you have data for the pressure gauge, check what it can withstand. It may be OK with 70 PSI. A restrictor will only help if there is sufficient air or other compressible fluid or resilient expansion between the gauge and restriction or in the gauge itself. If the gauge is mounted above the oil level then air in the pipework and gauge my provide this if the restrictor is at the engine end of the connection The ratio between the restrictor flow and compressible volume must be high enough that the oil pressure has reduced to below the maximum non-damage pressure rating of the gauge before enough has flowed through the restrictor to compress the volume of air to the limit. If the gauge and pipework can fill with oil and not drain you could use a small spring loaded accumulator (cylinder with a spring loaded piston designed for full compression of the spring at a bit more than the gauge max pressure rating) and restrictor.

Robert G8RPI.

I presume that your gauge has to look the part, ie vintage. If it is solely for safety reasons, a 100 psi digital one would be accurate at 15 psi and not need any special connections. For an actual 30 psi gauge, you would need an adjustable relief valve (spring loaded ball), with a sintered inlet to stop muck,and a return pipe back to the sump. A restrictor between the sintered filter and the relief valve would ensure that very little oil was used therefore not starving the bearings.

They wouldn't have had relief valves and restrictors back in 1910. Just do what the original manufacturer did. Must be information available somewhere. Or others on the net with same kind of engine. Probably just let the gauge peg when cold and carried on regardless. Lot of that old stuff was pretty crude yet still functioned.

Most vintage Harley engines are much the same. If you fit an oil pressure gauge it pegs it on cold start up then settles down to 10psi running pressure. The gauges seem to survive.

Edited By Hopper on 18/10/2019 09:17:22

Thank you all for you advise. I will do some experiments with restrictors or pegging the gauge. The engine is a Dennis Brothers engine and is one of four experimental engines they made for testing a new design, so information is non existing!

I spoke to John Dennis and he checked his records and confirmed that all four engines were scrapped! We think someone sold them out the back door instead of scrapping them!

Ah well, that is something rare! I suppose you could put a petcock in line with the gauge and shut if off on start up then open it up once pressure has settled down. Danger is, like petcocks in motorcycle oil supply lines to stop wet-sumping, it's only a matter of time before someone forgets to open the petcock. Which in youir case might give a false reading when in fact there was no oil pressure. Maybe add a common oil pressure switch and light as a back up?

I fitted a 100psi gauge to my Royal Enfield Constellation. To make a flexible pipe to get round the steering head, I pulled the cable out of a length of HT lead, it being large diameter with a small hole and it worked quite well. The pressure when cold went up to at least 90psi, but only reached 15psi when hot. There was nothing wrong, but that 15psi seemed awfully low at the time. The pressure and scavenge pumps on these bikes are oscillating type like the Mamod toy steam engines.

Every four-stroke I have ever had in bits has an oil pressure relief valve somewhere to solve exactly this problem. Engines are designed to run with hot oil - ie thin. Many designs use the 'blown-off' oil rather than just letting it flow to the sump, eg by feeding it between skew gears which always benefit from the lube.

It may be that the oil-way from the original blow-off valve is blocked, squashed etc, and this stops the blow-off from working with cold oil.

And at that age the big-ends are likey to be lubricated by dippers, tubes on the bog ends which dip[ in an oiled trough as the crank rotates. This system survived until fairly recent times on Honda 50s and lawn-mower engines.

Tim

Tim has hit the nail on the headf!, there must be a pressure relief valve, usually in the pump, or in the filter housing if it has onebut maybe in an oil gallery, and it will vent back to the sump, sounds like it is sticking when cold!

This is a 1910 engine with open oil galleries drip feeding the main bearings and dippers feeding the big ends. The oil is pumped down the exhaust cam, which has oil ports drilled to line up with the galleries as the came rotates. There is no pressure relief valve and it is quite common on early engines not to have one. These engines have large oil pumps and work by supplying a large volumes of oil, rather than low volumes and high pressure.

If you must install a pressure limiting device to protect your gauge, then this is the type of device you need.....

maybe you could make something that is functionally the same without costing the earth...

https://www.wika.co.uk/910_13_en_co.WIKA?ProductGroup=72400

But as suggested, if the pressure is rising to the levels you state, then perhaps there is an issue elsewhere?

Edited By John Rudd on 20/10/2019 11:40:45

Having chewed this over in my mind for several days, now, I wonder if it is worth trying to take advantage of one of the properties of fluid dynamics, and simply try a long length (coiled against the bulkhead?) of as small-a-bore pipe as you can find - cold oil will find this a singularly difficult passage to negotiate for it's entire length when thick, and with an open-gallery system, find some other route. All you need is a temporary damping of the extreme pressure of start-up.

It needn't cost any more than the price of some pipe to experiment, and I can't see how any damage can result.

I'm half-sure that in the past I have seen this rigged on a low-pressure oil gauge.. - does it ring any bells with anyone else?