big snip
Dave Arbogast wrote:
I think there is a bias in the debate... Try this paradigm - Look at the
engine as a black box... ignore what is inside and how it works. Measure the
volume the box takes in for one revolution of the output shaft. In the end,
nothing else matters. Right? End of discussion...
-dave>
--
-dave
mailto:dave@arb.net
http://arb.net
I totally and unequivocally agree.
Paul Lamar
That is a good statement.
Say the RX-7 (or RX-8) turns at 9000 rpm shaft rotation, then it will displace: 2 Rotors * 40 in3 *9000 rpm = 720,000 in3 per
minute throughput.
Say a six cylinder four-cycle reciprocating engine (in-line or radial, it does not matter) having a cylinder displacement of
also 40 in3 and turns at only 6000 rpm will have a throughput of 6 cylinders * 40 in3 * 6000 /2 = 720,000 in3 per minute. The
throughput is the same, with equal efficiency both engine should make the same hp.
Note, that the displacement of the six cylinder four-cycle is 6 *40 = 240 in3. Both engine have six chambers/cylinders each of
40 in3. Both have the same throughput, the same duration of power strokes and the same overlap (see Vance), the same everything,
except for the output speed. The RX-7 has to turn 1.5 times faster than the Recip to obtain the same throughput.
If the RX-7 also turns at only 6000 rpm shaft speed, then it will only have a throughput of 2/3 or 480,000 in3/min. To be equal,
the displacement of the REP has to be reduced to 2/3 * 240 = 160 in3. At 6000 rpm it too will have 480,000 in3/min throughput.
Regarding total throughput it does not matter if the RPE is a six cylinder engine of each 26.7 in3 per cylinder displacement, or
if it is a 4 cylinder engine of 40 in3 per cylinder, the throughput is the same and the output shaft speed is the same.
A true comparison to the RX-7 is a six cylinder RPE of 240 in3 displacement turning at only 2/3 shaft speed. If we deviate from
that, we are into a compromise situation. If we use equal displacement at equal speed, then the RPE has 160 in3 displacement. If
we use a six cylinder version then the sequence of power strokes are the same but not the chamber size and number of power
stokes per minute. If we use the four-cylinder version than the chamber size and beginning sequence is the same but not the
overlap of power strokes.
Any deviation from the 6 cylinder 240 in3 RPE is an adaptation of a compromise. In one case the sequence is the same in the
latter chamber size and beginning sequence of power strokes are the same. At same shaft speed the duration of power strokes is
1.5 times longer in the Wankel, is this a true comparison? Likely not. With an early outlet port opening the duration is
somewhat reduced, making the four-cylinder of 160 in3 displacement perhaps the most "equal" candidate or the closest one to
compare to an RX-7.
Since each chamber "runs" through two TDC positions during one rotation and during one power stroke, it clearly operates on the
four-cycle principle. We also do not say that a four cylinder four-cycle engine runs like a two cylinder two-stroke.
RP
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