Here is an old message from the CD that might answer some port timing
questions.
Paul Yaw wrote:
Rogers, Bob J. wrote:
I have a couple of questions about port timing on the Second and Third
generation turbo rotaries that I am hoping someone can answer.
First - I was looking at the specifications section of the Mazda Service
Manuals for the Second Generation Turbo Rotary engine (1991 13B Turbo) and
for the Third Generation Turbo Rotary (1994 13B Turbo) and noticed that the
Primary intake port timing for the 94 Turbo is open for 90 degrees longer
than the 91 Turbo. I was astounded by this and wondered if it was a
misprint. Here are the port timing figures that were in the manuals.
It certainly was a misprint!
91 Mazda Turbo:
Intake Primary Open 45 degrees ATDC, Close 50 degrees ABDC -
Duration 185 degrees
Intake Secondary Open 32 degrees ATDC, Close 50 degrees ABDC -
Duration 198 degrees
Exhaust Open 75 degrees BBDC, Close 48 degrees ATDC -
Duration 303 degrees
94 Mazda Turbo
Intake Primary Open 45 degrees BTDC, Close 50 degrees ABDC -
Duration 275 degrees
Intake Secondary Open 32 degrees BTDC, Close 50 degrees ABDC -
Duration 262 degrees
Exhaust Open 75 degrees BBDC, Close 48 degrees ATDC -
Duration 303 degrees
The port timing for both engines is the same, but the ports themselves
are shaped a bit differently. The timing stated for the early model is
correct. The intake ports open AFTER TDC. The duration figures are
also incorrect. Were they printed as such, or did you determine that?
It takes 270, not 180 degrees to go from TDC to BDC in a rotary. That
makes the duration 275 for the primary, 288 for the secondary, and 393
for the exhaust. Looking carefully at the exhaust duration it is no
wonder they don't make any low speed torque.
The exhaust timing is the same for both engines, and the point at which the
primary and secondary intake ports close is the same for both engines, but
the point at which the intake ports OPEN is dramatically different between
the Second and Third generation engines. Interestingly, the secondary port
in the 91 Turbo engine opens 13 degrees earlier than the primary, whereas in
the 94 Turbo, just the opposite is true. Because the respective numerical
degree marks for the intake port opening points are the same in both engines
(45 degrees and 32 degrees) and only the designation of AFTER or BEFORE top
dead center changes, I was suspicious that there might be a typographical
error inone of the manuals. Changing the letter "A" to the letter "B" in
the designation ATDC (After [Before] Top Dead Center makes a big difference.
Can you confirm that the port timing specifications listed above are
correct? If so, the extra 90 degrees that the primary intake port (and the
extra 64 degrees that the secondary intake port) is open in the Third
generation turbo could make a big difference in potential power output.
What do you think?
This leads to my second question. When I drew a chart of when the ports
opened and closed on a piece of paper, I noticed that there was no time in
a 360 degree circle when both the intake and exhaust port were closed. I am
sure that there is an explanation for this, but it does not come to me. I
understand that there can and should be some overlap between the opening and
closing of the intake and exhaust ports to take advantage of the inertia of
the gas moving into and out of the combustion chamber, but it seems that
both the intake and the exhaust ports should be closed at some point in the
cycle, so that there can be compression, ignition, and power. What am I
forgetting/overlooking? Since the engine obviously runs, I am sure that
there is a perfectly good explanation for this.
Thanks in advance for your explanation and clarification.
The geometry of the rotary engine is pretty neat, but it makes for a lot
of confusion. I wrote an article several months ago describing the port
timing which includes illustrations that I stole from "The Rotary
Engine" by Kenichi Yamamoto You can find the article here:
http://personal.riverusers.com/~yawpower/techindx.html
This was the first of a series of technical articles written for the Bay
Area RX-7 Club newsletter. You might find the subsequent articles
interesting as well. At least I hope you do!
Paul Yaw
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