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.