peak compression at tdc?
 

If you rotate the engine by hand towards TDC or the leading timing mark should you hit a compression peak at TDC as you would with a piston engine or will compression and exhaust happen just before. Looks to me that compression is peaking around 20 degrees before which doesn't sound right to me. Aligns with the timing I am seeing too. But then all my pulleys and marks seem to tie up with CAS position?! Confused as hell!

Cheers
Lee

Peak compression will occur at TDC just cranking the engine, however with the engine running you will find that your peak combustion pressure will occur a bit after TDC.

If you look at the shape of the housing and plates at TDC compression you wont have any ports open to the rotor face under compression.

That is what I assumed. My engine appears to have a peak compression point around 20degrees before TDC. I assume that is not a different rotors peak point? This kind of lines up with the timing error my light is showing. But all the marks seem to tie up and correspond with the marks for the original 20b pulley? I have not changed the pulley hub so assume it is original 20b. So I am very confused as to what is going on!?

Can you describe the process that you are using to determine your peak compression pressure?

and just on a side note: Assumption = The mother of all fuck ups

Not a dig or anything just a life lesson I've learnt too many times.

Hah haa. No offence taken. I completely agree. All I am doing is turning the engine by hand and when you hear it compress and then the point it pops past and exhausts if you know what I mean. Not taking thus as an exact point but a guide give or take a few degrees. Trying to determine why my timing is showing up as 20 degrees advanced. The compression point I can feel and hear is roughly 20degrees ahead of my timing marks. Yet if I swap my 20b pulley back on align leading mark and check CAS position it all seems to tally up. I have never touched the original hub unless it somehow had 20b pullies on a different hub before. And I never touched the CAS. Just wanted to check it to be sure. I thought the compression-exhaust point could be a good gauge of what is going on?!

1st things first we are dealing with two things, engine timing and ignition timing. If I was you I would physically confirm your pulley timing marks line up with where the engine is actually at.

Then work on your ignition timing...What is the firing order have you got it setup for?

Sorry I have not been following your thread very closely. I do have my engine down with the oil pan off with FC cas. If you want, you can ask whatever picture you want off my engine as I rotate it. Engine dynod over 800 and timing is spot-on. Let me know.

By moving the e-shaft by hand to feel the compression you are feeling more than the compression in the rotary.

The compression increases as you turn the e-shaft, but also the air charge is pushed through the center of the rotor to get to the other side of the pinch in the rotor housing.

You might be feeling the resistance on the e-shaft decrease as the compression charge blows down through the rotor slot to the lower side of the pinch.

I wish there was a video that really highlighted the gas transport inside the rotary engine and was in slow motion.

At 2:41-2:42 the compression charge is still only above the pinch in the rotor housing. The rotor slot is not yet open to the pinch in the housing.

By 2:43 the compression charge has blown down through the rotor slot and pressure has equalized on top and bottom of rotor housing pinch.

Blue TII, not sure I follow entirely? There is a distinct change in sound from compression to exhaust as well as the feel around 20 degrees before tdc. Presume that cannot be confused with what you are talking about?

Stickman.. Thanks for the offer. I will pm you if I need anything else. But first a good check would be cas pointer position with engine lined up on leading pulley mark.

HSmidy, I presume it can only have one firing order as the e-shaft would have to change. If that wad off would it not be out by like 120degrees and too much to run any sense? For sure I need to verify pulley marks ate correct to engine position first which is why I am trying to check tdc with the compression. FC 13b and 20b pulley hubs are the same if I recall? 13b FD is the only different one right and that has little dowels in it to help fit the pullies which makes it stand out. Also would a 13b FD hub give a 20dgree ish timing error in advance or much more? I remember there is a thread somewhere showing all three. Maybe I can use the position of the bolt heads at TDC to help somehow? Really want to avoid removing hub having heard horror stories and the fact it will be bloody tight!
Thank you
Lee

When you rotate the eccentric shaft on a rotary you are doing more work than just decreasing the volume of one rotor face.

If I understand you correctly, you are noticing you are putting in more work at 20 before TDC than at TDC.

I couldn't insert the pic, but this link should take you to the chart showing the change in volume of the rotary engine.

Actual compression chamber volume is indeed lowest at TDC like a piston engine, though the rate of decrease slows very much near TDC.

Because the rate of volume decrease is slower, you are able to feel the work of the other things you are doing by rotating the e-shaft.


http://cp_www.tripod.com/rotary/images/pg29_30b.gif

No 20degrees before TDC it passes compression completely and goes slack. i.e to exhaust if I didn't know any better. As I said you can hear the squeezing compression sound follow by a Ttttsss.. and it starts to exhaust.
I have picked up and old thread just now with some good pulley pictures to discuss around so guess topic can be continued over there rather than copy all the pictures and chat here!?

Thanks
Lee

Are you turning the motor slowly then?

If that is the case, the compression stoke will bleed out across the apex seal as it crosses the large hole of the Leading spark plug that is now also open to the exhaust stroke.

In actual operation the Leading spark plug is placed so that the compression stroke above and the exhaust stroke below are at the same pressure when the apex seal crosses the hole so there is minimal gas movement between chambers.

Note- NOT balanced pressure when there is no combustion/exhaust ^^

Turning the engine over by hand doesn't mean anything. If you have a scope with a pressure transducer, you will find that 100% of the time, peak pressure is at TDC while cranking with no combustion. This is a mechanical axiom. The reason it gets "easier" is because the seals are bleeding off pressure and equalize.

Does anyone think there might be some lag due to clearances in the bearings and the stationary gears that is showing the retarded timing? When turning by hand one is turning the assembly backwards. Under power, the rotor forces the crank. In a piston engine there is only the two bearing clearances that would add up to very little by the way of timing..

blue TII, thanks for the in depth reply. I see what you mean. So I could be feeling and bearing blow by as the seal crosses. Maybe need a backup method. To verify tdc. I read somewhere about put wire into the plug hole to see when the apex seal passes. Not super kean but as I have half an idea where it is I should be able to rotate it carefully and check. So for a 20b which rotor will pass which plug for L1 TDC. I assume I want it passing a trailing hole which is smaller and I am looking for it to be centred in the plug hole? I may be able to setup a mirror and light or borrow a bore scope to check.
Thanks for all the help and explanations. Always learning!

A dial indicator micrometer in the Leading spark plug hole will show you true TDC.

Get as close to TDC as possible before setting up the micrometer though as the surface of the rotor varies the further from TDC you get (the rotor recess shape).

And as TonyD89 indicates you should find TDC by rotating the rotor slowly backward from ATDC to take up the gear lash in the correct direction.

Find TDC with the engine apart, mark the flywheel, calibrate the flywheel versus whatever you are using as a crank sensor, crank the engine with the starter while watching the crank sensor and a pressure transducer on a 2 channel scope.

Anything else you're doing is trying to measure something finely marked with a worn out crayon.

Peejay..that would be great if the engine was in bits out of the car but I isn't!
Blue TII this sounds like good info and should be accurate and hopefully no danger to the rotor tip. I assume This is looking at L1 and based on the tip of the rotor being centre of the L1 hole and that will give TDC? I don't imagine there to be a lot of gear slack but will certainly see if there is any back rock after I find TDC rotating it al clockwise.
Cheers
Lee

I have been thinking about this some more and there is a lot or mis-information in other topics on this. But reading around I think I might have figured out how to find TDC.
So for the 13B you can use the plugs holes (assuming they are equal distance from BDC) to position the rear rotor to BDC and hence find TDC for rotor #1. by marking the pulley as the rotor tip passes centre of each plug hole and then taking the midpoint with a protractor.
On the 20B that won't work because the rotors are split 120°. However you could measure and find BDC with this method on rotor 1 and then take the point 180° around the pulley to find TDC. Seems accurate enough but does rely on being sure the plug holes are equally split around BDC. Can anyone confirm.

I think Blue TII you are saying to swing the rotor around to near where you think TDC is and use a dial gauge down the plug hole. I imagine more likely trailing as it probably won't fit down the leading one? and then see when the chamber in the rotor is perpendicular to the plug hole. i.e the gauge stops moving and than starts dropping back. That will show you TDC on that rotor. This assumes the chamber on the rotor to be a flat surface across the plug holes? Can anyone confirm this please? Crap on the rotor could give you a slight misreading but it should be pretty damn accurate if the rotor face is flat surface adjacent to the plug holes.

Any further thoughts or corrections to what I have written?

Thank you
Lee

That is what I was thinking, but like peejay says it is going to get messy/complicated quickly.

Leading (Lower) is the big hole. Trailing (Top) is the little hole.
The spark plug locations are not split evenly on either side of TDC.

Now that I think about how I would try to go about it; I can't say with confidence you could find TDC through the plug holes with a dial indicator micrometer.

It also depends on the layer of carbon and where the micrometer actually hits the rotor face. It may even hit the corner of the bowl, or somewhere else.

The only real 100% way to verify TDC 100% of the time, is to have the engine apart and mark it on the flywheel. Calibrate to a CAS pointer. And then use a pressure transducer to verify the marks line up, but as long as the marks on the pulleys are correct from the get go, you should be fine. You end up splitting hairs on a half a degree or so by playing around like that.

Back to the drawing board then.. Surely someone has measured the angle between the rotor tip crossing the Trailing plug hole and TDC. If someone had that info it would be a easy accurate solution everytime within a fraction of a degree? Anyone have that?

Thanks
Lee

It would really depend on which series of engine. The location of the plug holes changed between S4 and S5 and up engines.

I went back to your original post just to recenter what you are trying to ask and accomplish.

It sounds like you wanted to know why you had more "effort" at 20 BTDC, vs @TDC.

I work on cars all day. I am very familiar with the feeling of rotating an engine over. It does physically get more difficult as you approach TDC. Once you are close, I have never measured honestly but let's use 20 BTDC, the engine gets a bit easier. You are basically bleeding the pressure out of the combustion chamber as you go up to TDC and then @ TDC, the pressure is equalized and it feels "easy" to rotate.

On a piston engine, depending on rod ratio, the crank pin can move back and forth a few degrees without moving the piston a whole lot. It's really easy to turn the engine at this point. This doesn't mean that peak pressure happens before or after TDC. Peak pressure, when cranking and even running is ALWAYS at TDC.

This is a running compression test on a vehicle at idle. The tallest peaks are at peak compression at TDC.


https://cimg0.ibsrv.net/gimg/www.rx7...eb2e0b8cb9.jpg



With a labscope, similar to this one, i can measure the peak pressure and determine cam timing, ignition timing, valve overlap, VVT position, etc. It all depends on how I hook up my scope. I usually hook up 4 leads total. Pressure in cylinder, cam ref, crank ref, and ignition trigger #1, or on the cylinder that I'm testing.

If I know the commanded ignition advance, I can use the time divisions to find out where the degrees advance is in relation to time. Sometimes I can set ignition timing to 0 degrees and see the spark line line up EXACTLY with the peak pressure on the screen.


Here is the reality, I feel like you are trying to put too much thought into how the engine felt as you rotated it in relation to TDC and timing. As long as you use matched pulleys on an engine and set your ignition timing accordingly, you will be just fine. As soon as you start swapping pulleys around, it becomes a different story. As far as measuring through the plug holes and such, you may be able to come up with an answer, but it will likely not apply to every engine out there. You may still end up with a degree or two of variance.

The only true way to line up TDC with physical marks to 100% perfection is to have the engine torn down and mark the flywheel and trigger it off that way.

The other way, that may of may not work, is fill the chamber on the compression stroke with oil and have a tube coming off the trailing plug hole. Fill it all the way up until you have a line that you can mark on the tube. Rotate the engine back and forth and line up your pulley to TDC. The highest point of fluid level should be TDC.

Jon Hujbin (sp?, home built four rotor in the 2nd gen build thread) used oil in the chamber and a clear hose/hoses in the plug holes. TDC is when the oil previously dumped in the chamber before turning to TDC is the highest in the hose. Kinda like a water level.

That's the technique I'm referring to. I saw the way he did it quite a ways back.

John also marked the locations as he built his engine on the flywheel, so I think he used the oil in the chambers as a proof of concept.

Thanks guys good info. ACR-RX7. You clearly know your stuff and have all the gear around you to do what I need to. I am stuck with more basic tools and good old fashioned measurements! I see on a piston engine there is a very small window where the pulley might loosen at TDC due to the rod going over centre. Not sure how that works out on rotary though? I don't get where the bleed is happening. Bleed across the plugs would surely be much more than 20degrees BTDC? Anyway. That aside this was something I noticed when trying to stab in my CAS as my timing light shows up I am 15-20degrees advanced on L1. Checked with two lights twice both give the same result. Yet, lining up my pulley marks and CAS all looks good. I then noticed it felt like TDC on the engine when rotating it by hand was around 20degrees advanced from my TDC marking. So I am here trying to get to the bottom of it. There looks to be some error in the pulley marks referring to some old images Banzai racing posted. So trying to figure out why mine are in a different place to start with. But they would in fact put the timing out even further!
So basically I need to confine where TDC is first off and I can go from there. Need to figure the best way to get within a degree with the engine in the car.
I was wondering if I could pull a plug and connect a hose and syringe to the end and rotate the engine and see when the syringe stops moving to get TDC. Maybe a drop of oil in the rotor first to seal the tips better. So what you have described is pretty much that. Do I need to entirely fill the rotor? Presumably not but a good drop? Then rotate forwards and back around TDC a few times and split the difference on the markings if any? Or take the marks when rotating the engine backwards only to cater for potential gear play as mentioned earlier?
Cheers
Lee

Two things:
1: Are you 100% sure you stabbed the CAS correctly? I mean without any doubt. It's been awhile, but I think 1 tooth is about 20 degrees. Did you pull the cover on the CAS to make sure it was lined up? Have the pulleys been swapped or changed at any point in time? This is critical. I do feel like you are falling down the rabbit hole on this one, instead of simply having the CAS installed incorrectly.

2: The seals are not perfectly airtight. When you roll the engine over, the seals bleed the pressure into the next chamber, or crankcase if it's a piston engine. At cranking speed, and when running, they seal just fine, but not when slowly rotating over by hand.

Think about it this way, during a cylinder leakdown test on a piston engine, upwards of 20% leakage is considered acceptable, in some cases. You will never have a 100% perfect seal. In fact, valves in a piston engine do not perfectly seal every single time during a combustion event. There is always a tiny bit of leakage. The place where the air goes is right past the gaps in the side seals and apex seals. In a piston engine, the piston rings have end gaps. This is to allow gasses to get behind the rings and push them into the cylinder walls. It also allows for heat expansion.

If these gaps were not present, you frankly would have a hell of a time turning the engine over by hand at all. You would compress the air and the engine would keep springing back at you. The effort you felt subsided because you reached a point where all of the compressed air had bled out and you were very likely at TDC, where the effort for rotation is the least.

Nope. The apex seals and side seals are not tight enough in any engine to test this way.




Yes.


Gear play really doesn't play in at this point. If you do have substantial gear play, the engine is trash. However, after trolling for literally 20 minutes, I found the post where John and his 4-Rotor FC find TDC.

https://www.rx7club.com/build-thread.../#post11767773




I'll close with this. Make sure your CAS is in correctly. If it is, and your marks are still off, then try John's method. This is as good as you can get without pulling the engine apart, or using a scope.





Tell ya what. I have a factory sealed engine in my garage that has never had the CAS out of it. When I bought it, it still had a nylon cap on the mounting bolt, so I will pull the CAS cover off and line it up with the marks on the pulley. I can take pictures and give you an idea of where it all should line up, but I'm willing to bet that someone already has this information out there.

Looks like I just saved myself some time.

https://www.rx7club.com/2nd-generati.../#post10883854

Thanks again ACR_RX7. Good info. I will check out Johns thread again. I followed it most of the way through! I think I will have to try this though. I have been trying to get an image of an installed CAS with an engine aligned on the L1 pulley mark but not found one confirming this was the case. The pointers hardly move when you install it then? My pulley leading mark and pointers line up just like this. The issue is this puts my timing way off when I put a light on it. So something is not right. Not really enough CAS adjustment to get the timing back in on the light and if you try the engine doesn't like it. I would guess the timing is in the right ball park and somehow my pulley marks are off. You may not have read the full story over the various posts. But, long story short. I got a junk FC with a 20b in it in stock form so I am led to believe. Swapped it into my FD. Changed the front trigger wheel but left the hub on which I assume is a 20b one but now I am starting to wonder if somewhere down the line it got an FC one fitted? I transferred the 20b marks across to the FD trigger wheel by mating them together and assuming the marks on the original pulley are correct mine are in 100% the same place. Plus I added TDC using a protractor for good measure. So, car idles pretty well if a little rich and possibly runs a touch warm for my liking. Had a break in dyno and made 420bhp ish flywheel at something like 5000rpm and 8psi boost. Which seems reasonable. Nevertheless I decided I wanted to check my timing before messing with any timing maps and it came up 15-20degress off. So I have been trying to figure out why since! Electrics are all wired correctly as I double checked last night. So there is either a global correction I have missed in the ECU or the timing marks are wrong and the CAS is all off. Hence I need to verify if my marks are I the ball park or 20degrees off! Johns idea seems sound to me certainly will tell me within a small margin.
When you say about marling tdc on flywheel is better how come? Is it because this is usually located on a dowel so more accurate? I presume you make a pointer marker on the rear iron some where to align too?
To be fair I would be surprised if you got half a degree of play in the front pulley bolt holes?
Thanks again
Lee

Just another thought that might help me. O have seen images. Of the different pullies and hubs. But can someone confirm roughly where the bolts are pointing on a 20b hub when the leadin marking is aligned? Around 15degrees off horizontal? Angled down or up? Conversely if you took the 20b hub and off a swapped it for FC or FD which I think have the same bolt pattern and then fitted the 20b pulley back and aligned the leading mark again where do the bolts point now. This might give me a quick answer as to whether the motor has an FC hub on instead of the 20b one. Then I have another gauge I can work with. As I recall doing that puts the marks out around 15degrees. Just not sure if that shows as advanced or not.

Thank you
Lee

I'm willing to bet that the pulley marks are 20 degrees off, instead of your CAS at this point. The 20B CAS and 13B ones are the same, as far as I have been led to believe. It would not surprise me if the hub is not matched, or if the pulleys were swapped around at some point by the previous owner.

In any event, the CAS installation is the same as a 13B so it should be fine. You have not mentioned what engine controller you are using, but what you can do is set the timing lock to a know value of, say 0 or stock -5 degrees. Mark the pulley there and you might have to make a new pointer, or just set your trigger offset differently in your engine controller.

If your global trigger offset is incorrect, this is why your timing light is not lining up correctly.


As far as marking the flywheel, the engine is stacked back to front. As you set the first rotor down, you just mark the flywheel in relation to any point of your choosing. Usually, the flywheel has no pulley bolt play in it, like the front does. If it does, the keyway is worn out. As you stack the engine, you rotate to TDC for that rotor. mark again and continue for however many rotors you have.

180 degrees out for a 2 rotor, 120 for 3 rotor, 90 for 4 rotor. This is just for physical marking of the flywheel and then if there are any issues on the front, you can refer to the back. I know that hubs and pulleys should always be a matched set, so if they get swapped all hell can break loose.

I have been thinking about this tonight. I could be being dumb but. If the timing marks are off as suspected. I have aligned my engine to TDC with these marks and then set the CAS to these marks as well. So would that not mean that if I timed the engine or should show correct even if it was wrong unless the CAS jumped a tooth which it hasn't as my pointers line up perfect on the corners of the CAS sensors. So essentially the engine should think that is correct TDC as the sensors is what triggers the coil to fire via the ECU. Correct. Even more reason to confirm my TDC is right anyway as I need to know my baseline is right. Does this point to ECU global correction or CAS at wrong polarity even though wired up as per the manual maybe? I am on a microtech by the way ltx-12.

If someone can answer with where the pulley bolts should be pointing at TDC that will give me something to go off.
Thanks
Lee

I think you may have answered your question. In all reality, the ECU doesn't care where it's lined up as long as the trigger and offset are programmed correctly. You can have a car run off of any point. I have worked on V8 engines where the dizzy was stabbed in incorrectly. This was a buddy of mine and it was night time. He replaced his intake that night and just needed it to run so that he could get to his second job. So, I pulled the wires off the cap. Hopped them around until it ran. Then we timed it with the dizzy several teeth off. Sure the #1 plug wire was not on the #1 post, but it ran good enough for him to get to work. The next day I pulled the dizzy out and stabbed it in correctly. The engine doesn't care as long as it gets spark to the correct plug at the right time. Where it comes from is a different story.

The ECU is only firing the spark where it's told. It has no idea where the pulley marks are located. The CAS may be wired backwards, but you would have to check the MicroTech subforum for diagrams for that.

I can't help you here, but hopefully someone else can.

With the keyway at 9 o'clock, there are a set of bolts on the pulley that are very close to vertical.

Hi HSMIDY, that is with FC and FD hub right? The the 20b hub is around counterclockwise about 15degs from vertical. Will check mine later. I remember seeing some images that confused me that are on the other thread started by F1pilot. There is a picture showing a 20b pulley fixed on an eshaft. The keyway is rotated to 9 oclock. If you look at the timing marks these are counterclockwise from vertical 10degrees or so. Look at the front cover of a 20b. The timing pointer is around 10degrees clockwise of vertical. So if TDC is 9 o'clock for the keyway. How on earth van the timing marks line up to the pointer? I will try and repost the pic.

Picture of pulley fitted to shaft. Will get one later of the front cover pointer to show what I mean about the relative position.

https://cimg1.ibsrv.net/gimg/www.rx7...49865cebab.jpg

Here is a couple of pics from mine. 20b D series if it makes any difference and what I believe to be original.

http://i64.tinypic.com/9tekn6.jpg

http://i68.tinypic.com/rh5oab.jpg

Well that is interesting.. Look where your keyway is compared to the picture above. Spot the difference? So that would imply you had and FC or FD 13b hub if all the other info I have flubd is correct. Which is what I suspect I might have. So if you align you leading timing marking with the pointer on the cover your bolts are clockwise of vertical? I will swap back my 20b pulley later and post a pic buy best I can see looking now is bolts maybe slightly counterclockwise of vertical a few degrees, if at all, with leading 1 lined up. I would guess the pointer is set around 5 degrees clockwise from vertical for reference. So yes looking again at the marks in relation to the pulley bolts it could set your keyway at 90 degrees.

This is the post I am referring to. Note the bolt pattern in relation to the keyway. Ignore the FD trigger wheel. Not the same as what you have shown. FD and FC look the same. Although it looms like maybe FC holes are off a degree or two. Maybe camera angle.

In addition Banzai posted this showing the 20b marks put on a FD trigger wheel mounted on a 20b hub. First thing I spotted is mine are not quite in the same place. Yet have been copied exactly from my 20b pulley.

Some food for thought.

Same pulley setup as shown above on the engine.

http://i65.tinypic.com/2a7yc9j.jpg

http://i67.tinypic.com/vno2l2.jpg

Okay.. Thanks for the picture.. Interesting. Looks like it would line up pretty well on TDC. However, unless there are two versions of 20b hub-pulley setup that should not work?!

20B Leading timing mark in relation to bolt on 20B hub

https://cimg9.ibsrv.net/gimg/www.rx7...e009e80503.jpg

20B Hub keyway, leading mark at 12

https://cimg0.ibsrv.net/gimg/www.rx7...7cb454cfdd.jpg

FC Hub with 20B pulley, leading mark at 12

https://cimg6.ibsrv.net/gimg/www.rx7...730680719c.jpg

I am looking again.. All three hubs have a slightly different keyway position don't they. FD is in line with bolts holes. FC is around about 5 degrees or so and 20B is around about 10-15 degrees by the look of it. These hubs and pulley configurations are a mine field when you consider most people probably don't even realise they are different!

So HSMIDY you must have an FD hub your engine then?

Cheers
Lee

FD hub has pins and I've had the pulley off the hub and cant see any signs of there ever being pins on it.

Could be an FC one maybe? Doesn't look like a 20b one though. Might be worth checking your TDC out like me.