I have been curious for some time regarding the rather low compression pressure that rotaries generate as compared to reciprocating piston engines. From what I understand, a new, properly broken-in 13B has a nominal compression pressure of 130 psi. Contrast this to a used 2.0 litre Opel 4-cylinder engine I installed into my 1972 GT which had a range of readings from 180-184 psi...And with a slight ring ridge felt in every cylinder before I removed the head to have a fresh valve job done. Is the relatively low pressure found in 13B's due to the complexity of the apex-corner-side seal system, or is it due to the gap present between the apex seal and the spark plug hole? How much compression reduction does the spark plug hole gap contribute toward? I have never heard or read any study dealing with this design limitation. The only thing that came close was when I read where Mazda's proposed "16X" rotary was to incorporate laser ignition. This design would eliminate the large spark plug hole in exchange for a much smaller pinhole-sized opening that would allow the apex seal to completely cover and therefore isolate the adjacent chambers,(That is, the side of the rotor undergoing compression would be sealed off from the side undergoing combustion). Theoretically, if this ignition system could be implemented, what kind of compression pressures could be expected, and what would be the gains in power?

I wanted to bounce this topic off of some of you out there who are well-versed in rotary technology, since I have not found this subject addressed elsewhere.

 

you should be able to find a download somewhere online …

 

Thank you for suggesting this book. I have downloaded it, and look forward to reading.

 

There’s more to it than this once you get into the details, but basically as I recall it, due to the general Wankel design configuration and dimensions requiring the elongated bathtub compression chamber depression on a moving rotor (x3) results in an approx. 10:1 effectiveness limit for compression on these engines.

I suppose some of the more recent RenesisX technology might be pushing it forward a small amount. Which the Rx8 Renesis was 10:1 static, compared to 9.7 on the earlier NA engines. Dorito-shaped rotors with their multi-sealing surface arrangement don’t seal as well as round piston rings in general and thus cranking pressure at low starter speed rpms tend to result in a slightly lower reading as well.

So even a decent 120 psig reading in say, an REW engine with 9:1 rotors, only works out to 8.16 assuming std sea level etc. 14.7 psig atmospheric pressure. In theory a 10:1 rotor would generate ~133 psig all else being equal. Fresh hand-blueprinted Renesis engines have exceeded that, but are rare in general practice. Other factors, like OEM apex seals sealing better at low rpm while an Iannetti ceramic sealing better at higher rpm etc. add unique influences as well.
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In my original question, I wasn't really concerned about compression ratio, only compression pressure. I did, however, have the FC S5 engine with it's 9.7:1 ratio in mind when inquiring about the rather low 130 psi reading on a fresh, well broken-in engine. According to Yamamoto in his book, any compression loss caused by the gap which exists from the leading spark plug, when the apex seal crosses over, is offset by the pressure of the combusting gasses in the adjacent chamber of a running engine. I inferred that the complex seal system of the rotor was then much less to blame. When a cranking compression test is done though, there is no combustion going on to stop this leakage past the recess caused by the leading spark plug. Hence a low but perhaps normal reading. The only way I can think of to obtain a higher reading in order to test the effectiveness of a new, broken-in apex/corner/side seal arrangement, would be to use rotor housings that have the trailing spark plug hole only, and no leading spark plug hole.I theorize that if a laser ignition was ever to be implemented, then the cranking compression would be much greater, possibly resulting in easier and faster starts.

 

Here's a clue.

Measure compression at 0rpm.

Measure it at 100rpm.

Measure it at 250rpm like you're supposed to.

Now, what would it measure at 1000rpm, or 2000rpm.

Another piece of the puzzle:

You can feel a worn out engine because it makes poor low end power, and even poor midrange power, but top end power is usually still fairly good. The engine has less time for the compression to leak past the seals, you see.


Besides all of the long seals with fairly large gaps between them, the other factor as compared to piston engines, besides that valves and round bores are easily sealed compared to chambers with corners in them, is that engine rotors have something like .2mm of side clearance, and a LOT of tip to rotor housing clearance. Pistons in a bore may have .05mm of clearance piston to bore, and in some cases the piston is actually slightly larger than the bore when at room temperature!

 

i don't have it on this computer, or i would post the pic, but the NSU spider SAE paper has a graph of piston ring length for the rotary vs the equivalent piston engine.
the rotary has longer piston rings (Apex, Side and Corner seals) than the piston engine does.

the Rotary also has 270 degree strokes, vs the piston engine which only has 180 degree strokes.

so the rotary has more places to leak compression and more time to do it.

 

don’t make me repeat the “there’s more to it” qualifier again, lol.
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pics! i should point out that this compares an NSU rotary with an NSU piston engine.
if you used a pair of Mazda engines or a 13B vs Ls3 you'd have different actual numbers