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OMP Oil Metering pump output and modification

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OMP Oil Metering pump output and modification

Old 01-02-2019, 10:05 PM
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So I want to chime my 2 cents in here as a NASA Engineer (although I don't claim to be a fluids expert). For background, my Renesis is dying from a coolant seal leak and I am trying to put together a list of the best upgrades I can do to make the replacement motor last as long as possible. Also its late and I am tires so forgive me if I make some incomplete sentences (Yay Government shutdown and furlough. Without it I would not have the time to ramble on about this).
  1. So lets go back to pump 101 and remember why fluids move - pressure gradients. I'm going to postulate that a good amount of the forcing function that drives oil injection in our engines is in fact suction in the combustion chamber. I say this because the oil injectors appear to be on the top where the intake "stroke" occurs and also from reading an article on RX-7 club which discusses the purpose of applying vacuum to the oil injectors. For reference, this is the link: https://www.rx7club.com/3rd-generati...d-open-846007/ (Note I am assuming our system works on a similar principal. I've never taken a Renesis apart so I don't know.) This makes sense for several reasons to be as I will discribe next.
  2. If this is true, I also postulate that the function of the OMP is less to "pump" but more to prime the lines and also to supply the right amount of oil needed to be "sucked" into the combustion chamber. If we following the KISS formula, why not just provide high pressure engine oil to the injectors to be ingested that way? Well, that would provide a positive pressure gradient from the oil pumper (via a negative suction controlled by combustion) which would introduce too much oil - by using the metering pump, we not only reduce the injector line pressures to prevent excess ingestion, which would also prevent injection of oil when the engine is OFF but there is residual pressure in the oil system, we also smooth out the effects of oil pressure transients, can add extra oil during starting, and also reduce temperature of the injected oil (Not sure how oil atomization is affected by temperature. Would be interesting to look that up.) In short, by using the metering pump to feed the injectors only the amount of oil the engine should try to "suck in," you solve several engineering problems - it really is an ingenious solution if I am on the right track with my idea.
    1. Why then, you might ask, are there videos of folks pumping oil by spinning the OMP with a drill and filling up cups! Refer to 1 please - pressure is higher than atmosphere, so the oil moves! As I will cover below, flow at that point is dependent entirely on the length and composition of the lines from the pump to the injectors assuming constant dP from the pump outlet.
  3. With this in mind, I am proposing the following - If you can use Series 2 housings in a Series 1 stack-up to get the third oil injector, complete the mods described above to provide additional OMP oil flow then tee the third injector into the nominal lines for each housing - since the driving function for the oil injection is primarily suction, we should get very uniform oil application through all three. Note this assumes similar tube lengths, low viscosity (we can argue why that doesn't really apply later if needed), no/equal friction between all branches if the tee, and similar pressure gradients between the OMP and combustion chamber. It also assumes EVEN PRESSURE across the apex seal, and not some sort of bell curve shaped gradient (Which very well could be the cause, but actually will probably be OKAY! Why? And why do this? See below...
  4. Using my wife's research library access, I have requested SAE papers 2014-01-1664 and 2014-01-1665, the latter dealing with "Rotary Engine Oil Transport Mechanisms." From the abstract: "Oil transport from both metered oil and internal oil is observed. Starting from inside, oil accumulates in the rotor land during inward motion of the rotor created by its eccentric motion. Oil seals are then scraping the oil outward due to seal-housing clearance asymmetry between inward and outward motion. Cut-off seal does not provide an additional barrier to internal oil consumption. Internal oil then mixes with metered oil brought to the side of the rotor by gas leakage. Oil is finally pushed outward by centrifugal force, passes the side seals, and is thrown off in the combustion chamber."
    1. I think adding the third injector, even if the exact amount of oil injected can't be perfectly metered, will only serve to cool the center of the Apex seal and then aid in side seal cooling/lubrication. And if pressure is LOWER in the middle, it will only help to draw MORE oil into the middle which should, hypothetically, be evenly distributed to each side of the rotor.
  5. On a side note, from SAW 1664 abstract, "The dominant cause of internal oil consumption is the non-conformability of the oil seals to the housing distortion generating net outward scraping, particularly next to the intake and exhaust port where the housing distortion valleys are deep and narrow. Simulation with housing transverse waviness shows that increasing spring force can lead to an unexpected increase in internal oil consumption." This applies primarily to Brettus as I know he does FI applications, but this makes me believe that increased pressures inside the engine during combustion in FI applications, and aftermarket apex seals with stiffer springs, might negatively contribute to what I am imagining as "oil blow-by" - I need to do more research, but can oil that moves too fast fail to provide necessary lubrication? I'd say yes based on my understanding of Navier-Stokes equations - in a nutshell, if you move oil too quick, your shear gradient is HUGE and the thickness of your boundry layer is small. I'd again postulate that, as somebody who is NOT a lubrication engineer, that there is an ideal layer thickness to provide thermal and friction protection. Perhaps boosting the RX-8 kills engines early via older 13Bs because of the change to the location of the intake / exhaust ports and increased oil blow-by and reduced effectiveness as a lubrication agent? I won't know until I get the full paper, but I'd hope a 2014 paper would use a Renesis and not an older 13B or 12A design....
Anyways, I am done rambling for the night. On a side note, does anybody have a good VIDEO of an OMP tear-down or more detailed photos of how the cam interacts with all the parts? I have a descent idea of how it all works, but until my Model 3 shows up from Tesla I can't tear the thing apart as my 8 is my Daily Driver.
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Old 01-02-2019, 10:38 PM
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Last edited by G2barbour; 01-02-2019 at 10:46 PM.
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Old 01-02-2019, 10:44 PM
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Originally Posted by TheAMAZINGNorad View Post
  1. So lets go back to pump 101 and remember why fluids move - pressure gradients. I'm going to postulate that a good amount of the forcing function that drives oil injection in our engines is in fact suction in the combustion chamber. I say this because the oil injectors appear to be on the top where the intake "stroke" occurs and also from reading an article on RX-7 club which discusses the purpose of applying vacuum to the oil injectors. For reference, this is the link: https://www.rx7club.com/3rd-generati...d-open-846007/ (Note I am assuming our system works on a similar principal. I've never taken a Renesis apart so I don't know.) This makes sense for several reasons to be as I will discribe next
  2. If this is true, I also postulate that the function of the OMP is less to "pump" but more to prime the lines and also to supply the right amount of oil needed to be "sucked" into the combustion chamber. If we following the KISS formula, why not just provide high pressure engine oil to the injectors to be ingested that way? Well, that would provide a positive pressure gradient from the oil pumper (via a negative suction controlled by combustion) which would introduce too much oil - by using the metering pump, we not only reduce the injector line pressures to prevent excess ingestion, which would also prevent injection of oil when the engine is OFF but there is residual pressure in the oil system, we also smooth out the effects of oil pressure transients, can add extra oil during starting, and also reduce temperature of the injected oil (Not sure how oil atomization is affected by temperature. Would be interesting to look that up.) In short, by using the metering pump to feed the injectors only the amount of oil the engine should try to "suck in,"
  3. With this in mind, I am proposing the following - If you can use Series 2 housings in a Series 1 stack-up to get the third oil injector, complete the mods described above to provide additional OMP oil flow then tee the third injector into the nominal lines for each housing - since the driving function for the oil injection is primarily suction, we should get very uniform oil application through all three. Note this assumes similar tube lengths, low viscosity (we can argue why that doesn't really apply later if needed), no/equal friction between all branches if the tee, and similar pressure gradients between the OMP and combustion chamber. It also assumes EVEN PRESSURE across the apex seal, and not some sort of bell curve shaped gradient (Which very well could be the cause, but actually will probably be OKAY! Why? And why do this? See below...
from what I can tell, the output of the OMP is not synchronized in any way with the intake cycle of the engine. Oil is simply pumped to the injectors at a certain rate and made available for the next intake stroke where the engine then draws the oil in. To be clear, the OMP most certainly is a positive displacement pump. The engine has the capacity to draw oil from the injectors but does not draw it through the lines. Also, a common misconception is that we hook up a vacuum line to the oil injector manifold, and this is simply not true. It is the engine intake stroke providing vacuum on one side and atmospheric pressure on the other, as the line to the oil injectors is before the throttle body. The only reason for this line to exist is so the air drawn past the oil injectors has passed over the MAF first, or it would be unmetered air. The function of the intake vacuum on the oil is not as much to pump it in as it is to atomize it so that it isn't just dribbling out of the holes. *this is probably one of the major reasons the renesis does not have a good reputation for reliability under boost. The oil injectors were never designed to be used in that way. I believe that you were saying the same thing I just clarified in paragraph 2.

And as for paragraph 3, I've been thinking about the exact same thing recently!

Some great info thank you. FYI the bullet points play havoc on quoting you in replies.

Also, I have many pictures of the OMP torn down. If I can get Admin to approve my posts, I will upload them. This morning I I actually uploaded a reply to an earlier question about where I tapped the OMP for the internal pressures and it hasn't been approved yet
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Old 01-03-2019, 08:05 AM
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IIRC, the air line we connect to the oil injectors should be just a hair under atmospheric pressure. It connects to the accordion tube between the MAF and TB so any pressure drop will be most due to the minimal restriction of the air filter and air moving across the hose port.
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Old 01-03-2019, 09:06 AM
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Originally Posted by NotAPreppie View Post
IIRC, the air line we connect to the oil injectors should be just a hair under atmospheric pressure. It connects to the accordion tube between the MAF and TB so any pressure drop will be most due to the minimal restriction of the air filter and air moving across the hose port.
That sounds right but that slight vacuum would be incidental to ensuring that the air is metered through the MAF. The important thing is that, relative to engine intake vacuum, it is a positive pressure, not a vacuum.

For all practical purposes, atmospheric pressure is being used to atomize the injection oil.

I guess my point is more semantics than anything, but it is important to keep things clarified
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Old 01-03-2019, 05:01 PM
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My understanding of the OMP injectors is that the oil is injected into a stream of air provided by the vacuum of the intake stroke, through the injector and one-way valve at the upper portion of the injector, with back up air provided by the small plastic manifold connected to the intake tube.
So the oil travels into a stream of air, probably of decent velocity, but not really under vacuum, because of the check valve in the upper banjo requires some pop-off oil pressure greater than the vacuum it is blocking from draining the oil lines at low manifold/rotor chamber pressure.

Mazda saw fit to put a check valve in the upper banjo, because vacuum was not only not required for oil delivery to the engine from the pump, it also was unwanted because it could cause siphoning of oil out of the lines themselves during high vacuum conditions. However vacuum is used to provide a high velocity stream of air to mix with the oil and provide some energy to hopefully disperse it within the chamber.

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Old 01-03-2019, 05:09 PM
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Hopefully you are able to post those SAE papers, I would like to read them. Thanks.
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Old 01-03-2019, 05:14 PM
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I have some S2 housings and extra injectors that I am going to use for an s1 build soon, in exactly the way you describe.

I am very glad to see so much hands-on info on here about this topic now. Thank much to all involved for taking the time to run these experiments!
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