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Why Apex Seals Fail
Most people think that that their apex seal failures are caused by detonation. More specifically, they think the detonation was caused by some combination of poor fuel, advanced ignition, and high boost. This can and does happen but is not the root cause of the seal breakage.
One of my engines, which I thought would run a very long time because of reliability mods and low boost (14psi), lost an apex seal at about 30K miles. The only good news was that upon close examination I discovered a second apex seal in the set that was cracked but had not yet broken. (Sorry the magnification isn't better.) http://i287.photobucket.com/albums/l...5-2513_IMG.jpg I was surprised to find crack was emanating from the bottom side not at the exposed working face! It makes perfect sense. The main part of the seal is supported unevenly at three points. The forth spring contact point is on the wedge portion of the seal. When the seal rocks over the high point in the housing, specifically, the spark plug openings, the tip is under pressure from the wedge. http://i287.photobucket.com/albums/l...2523_IMG-2.jpg |
So you can try a little experiment to test this hypothesis. Install a .0015” feeler gauge at the center of the apex seal simulating the raised spark plug area. The seal will be rocked-over .003“ and the wedge will put pressure on the pointed end of the apex seal. Note the exaggerated condition depicted at the top of the picture.
http://i287.photobucket.com/albums/l.../mazda2010.jpg But we still haven’t gotten to the root cause. The real problem is the raised spark plug area on the housing. There are at least three approaches to this problem: (1)- Use cold heat range plugs like NGK R7420-10 or 10.5. (This is easiest part of the solution.) (2)-The harder second part is to cool the housing better. A better pump is a good start. Second gen. cast impeller type pumps are a lot better or the Remedy pump for 3rd gens. (3)-The most difficult part would be to port the housing in the spark plug area to minimize distortion. But what is the best way? The factory racing peripheral housing have a trough cast into the inner water jacket in this area but the are for NA applications. http://i287.photobucket.com/albums/l.../mazda2014.jpg Notice that the thru-bolt bosses have been severed. |
http://i287.photobucket.com/albums/l.../mazda2005.jpg
This is what a stock housing flows like. Notice we have too much flow on #6 passage below the spark plug and not enough on #5 passage. Any thoughts? Barry |
Wow! This is awesome information. I have seen the same marking on several sets of housings and I never made the hot plug area connection.
It makes sense that the housings would grow the most next to the spark plug, both because there is a more extended high temperature condition in that region than elsewhere and because the housing is less restrained (threads tend to grow more than smooth holes). I have known spark plugs to gall in place on rotaries and that the plugs show signs of high combustion temperatures. I have also seen two housings where the apex seal left a large mark in the housing 1/2" to 3/4" after the leading plug hole (looked as if that was where the seal actually failed). I wonder if a very small bevel around the spark plugs would cause a significant loss of cold compression. This may save apex seals from the spark plug speed bump when under boost. Fooling with the coolant passages would be difficult, as they go through the intermediate iron and both end plates. Maybe a little porting on #5 (without getting too close to the plug). Maybe some experienced builders can give a better number on how many housings have this damage. I can add this as a good reason in the causes of rotary engine failure section once we have more information. Thanks for the input. |
that's some good info. thank you.
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The main problem is that the temperatures are so out of control in that spot that making headway to reducing them still hasn't brought them down to a level on par with the rest of the housing surface. I've tried all kinds of mods to minimize the growth there but nothing can completely manage it, yet. It only serves to reduce the severity. I've done a couple different styles of jacket mods, run a high flow (REmedey) waterpump, and use 10 and 10.5 plugs (NGK 7420). Still haven't needed to build my next engine to try the additional web or bridge or heat sink (chunk of AL inserted diagonally in the leading plug cooling passage on both sides of it) out. But I'm not exactly disappointed that I haven't needed another yet :) Currently floating around my mind are the idea previously mentioned, as well as running higher system pressure. Much higher. Also, Barry pointed out some NGK surface gap style plugs which appear to perhaps retain less heat given their tip design vs. the 7420's. |
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This may be a problem, they may be a little large for boosted engines. Also it would be nice if they were made of some rare earth metals like platinum or iridium. Barry |
Ok, so I have a few pictures of my own to back up Barry's Theory:
Naturally Aspirated application - 13.2:1 AFR 28 degrees BTDC Timing failed apex seal in front housing at 5400 rpm in 4th gear - after 37 seconds at WOT in road load simulation mode on Mustang dyno. This happened a while back, but my camera was acting up. Here are the Apex seals: http://i594.photobucket.com/albums/t...C/P1010581.jpg The housing in which the seals failed: http://i594.photobucket.com/albums/t...C/P1010579.jpg Another shot of the housing in which the seals failed: http://i594.photobucket.com/albums/t...C/P1010586.jpg Here is the other housing: http://i594.photobucket.com/albums/t...C/P1010585.jpg You can see that the seals failed exactly as Barry predicted. |
NoDOHC, check the remaining good seal under a microscope for a crack where the others failed.
Also what water pump are you running? Did you bevel the spark plug holes? It appears that way from the pictures. Barry |
mine had the same marks:
http://i34.photobucket.com/albums/d1...1/DSC00615.jpg this was with RA classic seals, but they didnt break, it just had leaking coolant seals. it had 60k miles on stock turbo 15 psi, 7's and 9's plugs |
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Let me see if I can help in adding my schooling into the mix (Hurrah for Fluid Dynamics being applicable in all situations dealing with flow). I think the ROOT cause of the flow issue is going to take some exploring. First and foremost what do the individual passages look like? I notice from the picture you appear to have a lot of, what appears to be, leakage between the sideplate and the rotor housing. If that's just an illusion then we can forego that issue. However if it's not I think a pressure differential is developing prior to the passage. What I mean by that is that if you have a single source flowing unevenly into the various passages the incompressible nature of the fluid is going to dictate that the area with the higher pressure is going to output the more mass of the fluid (all other things being equal). Consequently ensuring you're getting an equal flow rate prior to the passage entrance would be paramount. If however there is no leakage and the input is simply just feeding it through the water ports where the waterpump seals, then the issue is indeed hardware related. Consequently since we can assume a uniform pressure distribution (not only for this experiment but for the real world application) we can see that the differences is going to be directly related to the hydraulic diameter and boundary layer conditions of the passages. This provides us with an easy enough solution as well. First and foremost; enlarging the hydraulic diameter of the passages will allow for a more unified flow condition between all ports in question. There remains however a finite amount of space to achieve that. The other option that can be excersized as well would be to instill a turbulent boundary layer on only some of the passages. Why only some? Think of it this way: we have a large passage that is putting out a lot of fluid. The issue is attempting to balance the passages to output the same amount of fluid at the end of the day. To do this we can lower the friction a majority of the fluid is exposed to in the other passages, in essence allowing for a faster flow to be achieved on smaller passages, while the undisturbed boundary layer on the larger passage causes the flow to slow down when compared to the tripped boundary layer of the smaller passages. The solution will consist of finding the appropriate balance of the two. |
Vex it sounds like your analysis is right on. Check out what RB has been offering
for years now. Looks almost like what your describing. http://www.racingbeat.com/images/ite...x280/11488.jpg They describe it here: http://www.racingbeat.com/RX7-1975-1...tem/11488.html The description talks about heat transfer being improved due to the grooves but I wonder if thats some purposeful FUD and the real solution is along the lines that your describing. It would all depend on exactly what they groove and where I guess. This is a great thread by the way. |
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I know that's confusing, but it's the best I got without drawing a picture of what I mean. Infernosig (sp?) may be able to explain what I mean better. |
Yep, thats why I said "almost" and "it would all depend". Obviously you wouldn't
groove every opening as RB has done and I'm sure it would take some intensive R&D to optimize it. Whats interesting is there are still so many improvements that can and will be made to the rotary to make it better. Think of how many refinements and designs have been wrought on the piston engine in the last 100 years to get it where it is today. The rotary has barely been in production for 50 years so far. |
Did you guys read the cooling mod thread?
http://www.rotarycarclub.com/rotary_...t=barry+bordes This thread, (Why Apex Seals Fail) is a peek into one of the conclusions that we came too. We are already running engines with this cooling mod plus some others surmised from this and other tests. Some think that this may be giving away too much valuable information. My thought is that it helps the whole rotary community. What I have noticed is that individuals start using the info and forget to give credit to the originators. But that is life... you do what is right anyway. Along that theme I would like to give credit to Carlos Lopez and Rick Engman for their thoughts on the subject. They are the community's Senior Rotary Advisors. Barry |
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Barry |
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I re-used that seal. I have over 2500 miles on the engine since the failure and have had no issues. Stock water pump, stock radiator (a lot of fins are rotted out, I am replacing it). I read this thread earlier and I found the tell-tale pattern on my housings, so I figured that beveling the edges was a good idea that may keep me from losing an engine. It survived 3000+ miles on the street just fine, but didn't survive the dyno run. I should have backed out of the throttle and had the dyno turn the loading down, My coolant temp was about 125 C when the seal went. I went to the dyno since then and had no issues (with the temp under 115 C) I think that the spark plug boss grows too much relative to the unsupported housing around it. I have considered welding a brace in (parallel with the coolant flow) to keep the surrounding housing growing at a similar rate to the spark plug hole (although the spark plug hole has threads in it). I like racing beat's idea - if I had this information before I rebuilt the engine, I probably would have done that. |
I think you're missing what I'm saying.
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It didn’t seem to help much to alleviate the growth problem as I had hoped. The next set of mods included severed fins and larger balanced passages. |
Barry,
You said you were using reliability mods.. What mods exactly were you using? Ported? etc, etc, Just curious :) |
It seems to me that the stain is in line perfectly with the fin. It appears to me that the fin has kept that area from growing. Effectively retracting it as the rest of the housing grows. Looking at the 16x and the renni, they seem to aleviate some of this material. I'm wondering if this is in an attempt to not contain it, but to let it grow a the same rate. Thoughts?
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Increasing the engergy transfer around the spark plug hole I believe will yeild more results than not. Something else to consider is that the cooling of the combustion walls is not necessarily the root cause of the problem. I'll have to do some research into the material properties when I'm back home, but could it be the atomic structure of the materials developing fissures over time due to heat cycling? For instance would voids and imperfections in the casting develop the same event? Quote:
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The cracks in the OEM Apex Seals are due to their hardness. I noticed 20-30 compression gains on rotor housings when re-freshed using our Apex Seals and these are housings that would otherwise go to the trash. A few years ago we posted a 135PSI 12A engine that we built but no one believed now Judge ito has been building 130 to 140PSI Engines daily with used (refreshed) rotor housings but one thing is for sure the raised spark plug area is a problem if not addressed. http://rotarycarclub.com/rotary_foru...t=12967&page=5 |
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I believe the factory started grinding either the apex seals or the housing surface that way for better fit while running. Barry |
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another problem on used housings, this is why they shrink, and why we measure for width, is that the steel liner actually collapses in the compression area, so the center of the housing bows in. its very bad to have leakage between chambers... mike |
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Notice the temps where we want to provide extra cooling are doubled that of atmospheric boiling water. Barry http://i287.photobucket.com/albums/l...usingtemps.jpg |
that very well could be. next time you have some used housings handy, take a straight edge and lay it across the surface, its above the spark plugs that seems to do the worst.
i did build a couple of motors with a stock diameter stud kit, and i was told to tighten it to 50lbs, but i think this is actually bad. it also certainly didn't solve the actual problem which was stock ecu/injectors with a street port and big turbo! or more correctly the owner. after thinking about it, i think the small diameter studs actually hurt more than they help. the studs DO NOT locate any of the housings any better than the stock bolts, plus #50lbs of torque the rotor housings probably ARE bowing, which is bad! mike |
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So I did a little more research and I think this modified picture is quite enlightening. Sorry it's hard to make out but it's the best I can do with GIMP at the moment.
http://www.rotarycarclub.com/rotary_...1&d=1306892507 We now have flame fronts with angles of eccentric shaft and the temperatures at those flame fronts that match up with the chart. I have a few more pieces of information that I will be posting throughout the week coming from SAE paper 860560: Material Technology Development Applied to Rotary Engine at Mazda by Takumi Muroki and Jun Miyata |
This is a very interesting post but the title "why apex seals break" may be misleading.
Lets be careful and not add to much theory and 1000 dollar words and confuse some of the laymen who are just coming to the rotary scene. We all know that the web is full of data but gotten to the point that true information is lost or siphoning out the good stuff becomes almost imposable, with that in mind lets keep this forum nonsense free. Now the specifics on why this post can be misleading is because the breakage on the end of the long part (hypotenuse) only happens on OEM 93-94 2 peace seals. Also the end tip on the OEM seals gets within a few thou of the irons of either side, where the sharp tip may catch the iron when the engine detonates and rotor tip hits the irons. We see this often with drag cars pushing 40+ lbs of boost. Cutting the rotor faces alleviates some of these issues. (Aftermarket seals including ours have a larger Side peace than OEM.) This also happened on engines with fresh new housings and new OEM Apex seals. Therefore if the apex seal cracked (hypotenuse) it happened when the engine leaned out or detonated, these are drag cars which never get any road millage where the (hypotenuse) area can slowly crack then one day let go completely. Even if the hypotenuse area of the apex seal was further inward or even if you used a 1 piece apex seal, leaning fuel, TOO much advanced ignition or detonation are still why apex seals break. We have been working on Apex Seal technology for almost 10 years with collaboration from engine builders both stateside and Puerto Rico and agree this breakage (hypotenuse) end tip break off is primarily a oem seal issue. Glory to GOD! |
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You are working with racers that are pushing the envelope. Although I test often for detonation with my TFX in-chamber sensor rig I have not experienced any detonation yet. Thank Goodness! When your seals do fail where is the damage normally located? Good luck with your Goopy seals and housings. Barry |
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the drag vs street vs road race thing actually is very important too. in road racing our warmup is 15 minutes, which is actually LONGER than the entire drag racing SEASON. and street driving is again different, the WOT periods are short, but it needs to sit at idle and low rpm for a long time (traffic), plus mileage is important there is nothing at all wrong with drag racing, 1000hp 13B is an achievement! it just requires a different engine/tune up etc etc |
As promised:
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I'll work on getting some of the figures posted up for everyone's benefit. But just looking at them I see something rather interesting; As load is increased both BMEP and temperature peak at 4000 RPM and both begin to decrease proportionately to HP rating from 4k RPM to 5k RPM. If I'm understanding this correctly it correlates perfectly with what has been stated by others here. Burning super lean even in low load conditions will increase the instantaneous heat in the chamber as well as the heat transfer into the combustion face material (as can be seen in another figure showcasing why iron wasn't used for the rotor housing). Given the nature of the defects and what the SAE paper has stated concerning a stock configuration it can be intimated that failure of the Apex Seals is going to be derived from lean conditions increasing the heat of the combustion chamber beyond design constraints. The result; heavy thermal loading of metalic parts. To tackle thermal expansion and any subsequent interference from the Apex Seal and side housings will need to be evaluated on merit (I will actually be attempting a simulation here shortly to test just that). |
Good post Vex!
I especially like the way you tied Barry's theory to other's experience. I can tell from my last dyno session that the more the timing was advanced, the more quickly the engine overheated. The closer I ran to max power AFR, the more quickly the engine overheated. In my previous dyno session, I ran all the way to 9,000 RPM without severe overheating at 11.2:1 AFR and 22 degrees of ignition timing (I also only made 175 WHp). I was making 175 WHp by 5,000 RPM on my next run, but the significantly more optimum AFR and 28 degree ignition timing proved too much for the engine. Last time I ran for hours at optimum AFR with no issues, but I couldn't advance the timing enough to make much power as the engine would overheat immediately. Basically, there was no difference in the engine internals between the dyno pulls, but keeping the engine from overheating was key in keeping the engine intact. There are those who would claim that the advanced ignition killed the apex seals, but I think it was more a temperature thing, as I ran 38 degrees of total advance at my last session, but I couldn't get past 4,000 RPM without overheating. As long as I stopped when the coolant temp hit about 115C I saw no problems with seal failure. Interestingly enough, the only time I ever got to 9,000 RPM in 3rd gear on the dyno without overheating was when I was running 17:1 AFR. |
A couple of observations.
Detonation can be a cause of breaking apex seals but it is not a root cause. Leaning out at high boost and too much advance can be a root causes, but how many of us run conservative maps and still have problems? There is something else going on. Also, no one has brought up the warping of apex seals. Using OEM I haven't seen this. Can anyone share their experiences with this problem? Barry |
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Also the leaning out of the AFR does not necessarily have to be in positive pressure to cause the issues of thermal induced mechanical interference. Note that the SAE paper only took the motors up to 5000 RPM for 45 seconds. They were not in positive pressure, but were loaded. The result can be the same and is dependent upon the AFR and the resultant flame front temperature. If you look at the picture I posted previously, I would assume that the peak temperature would appear at the peak temperature at normal operation (assuming timing remains constant and the fuel itself is adjusted). This means if we're running lean AFR at normal timing the result is going to be increased temperature at the Leading spark plug boss, where we may, or may not have appropriate energy transfer. |
Vex from what I have read they seem to be talking about softer seals that warp or bow in the apex groove causing the seal to jam and thus lose sealing ability... but it can be straightened to be possibly used again.
Barry |
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the SAE papers are on NA engines, and they point to the root cause being temperature related. when you add the turbo this doesn't change, except that you can have "conservative" maps and still be too hot. so the question is what is too hot? and when is it too hot? note this is going to vary with the apex seal. we know the stock seals want about 900-950c EGT at peak power with a max of 1100c. warping is a symptom i think. i don't think i've seen it, but i really don't have a good way to tell. mike |
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