"Powell, Ken" wrote:
snip everthing .....
Paul and Vance,
After following this discussion I decided to make a couple of observations (thank goodness Paul keeps things civil) and to
make a few comments. As you know I'm not an engineer but at one time I could make a 2-stroke go pretty well (National #8 in
the 1988 AMA Hare Scrambles Championship/Class B - wasn't good enough to be an 'A' rider though). Bill Jepson is our real
2-stroke expert so maybe he will correct me if I make any mistakes.
1. A wankel cannot be compared to either a piston 4-stroke OR a piston 2-stroke; a wankel is - well - a wankel.
2. a wankel does operate on a 4-stroke cycle but ... The 270 degree rotational cycle rules out any direct comparision to
conventional piston engines. This makes displacement comparisons invalid.
3. 2-strokes - these are not suitable for comparison because:
a.) the lower compression ratios obtainable due to the exhaust port
b.) the lack of control over overlap while maintaining sufficient port size (I'm really thinking of piston ported engines
here - some of the other methods allow some flexibility but have other problems). It is interesting to note that a 2-stroke
optimised for max power at 7000 RPM will suffer from dismal BSFC at low RPM due to the mixture being pushed out the open
exhaust port before the rising piston enters the compression phase.
One last comment. A 2-stroke is a wonderful, durable engine with great performance if you can live with the poor BSFC. You
must keep the egt temps under control; too rich you will foul plugs and stick the rings; too lean you will seize or hole the
piston.
Ken Powell
Comparing the engines in the sense you are comparing them is valid.
All I am doing is comparing the potential quantity of fuel and air processed in unit
time. In other words HP potential or displacement. Now these engines all have their
advantages and disadvantages. I won't go into them in detail
but IMHO the advantages of the Wankel on balance make it the winner for light
aircraft engines.
Paul Lamar
Rolf Pfeiffer wrote:
The comments from Ken, I think, are right on. Based on this I would like to add a few more points. The Wankel sure is very
robust and a good aircraft engine. It might suffer a small fuel penalty, but sure is durable and relative small in size.
A good reference for comparing engines is the Mazda RX-7 or now RX-8, where the displacement of one chamber is roughly 40 in3
(654 cc). So far so good, no one disputes that.
Summary of points:
1. Two-cycle or four-cycle?
The chambers travel through two TDC?s for every single power stroke, and no scavenging of gases is taking place as is necessary
for two-cycle operation. After passing through TDC (top dead centre) and the then following power stoke, the chamber passes
through a second TDC with volume reduction followed by volume increase that is solely used to expel the spend gases through the
exhaust port and introduce fresh gases via the inlet port into the chamber. This equals the classic four-cycle engine operation.
2. Length of power stroke.
Power stroke in RPE = 180 deg. Same in Wankel between shaft and rotor, however, between shaft and stationary housing it is 270
deg.
Noticing that, is not just being picky, in fact, this is the sole reason why the two engine types are different and why the WE
cannot be compared to an RPE in the sense that all things are being equal simultaneously.
Since one stroke takes 270 degrees, it takes 4 x 270 =1080 degrees of shaft rotation to complete the four strokes of power,
exhaust, intake and compression of a chamber, which corresponds to the three shaft rotations per one rotor rotation.
Point 1. The output shaft of the Wankel engine will turn 1.5 times faster than the crankshaft of the RPE when the duration of
power strokes are the same.
3. Number of chambers.
A) The triangular piston forms three chambers within the trochoidal shaped housing, that is equal to three cylinders in an RPE.
The fact that the three chambers are formed and rotate within the same housing is irrelevant.
B) When one face has turned 90 deg. during one power stroke, it takes another 30 degrees of rotor rotation to complete 120
degrees, before the next face or chamber reaches TDC.
In eccentric shaft degrees, there is power (at least in theory) over 270 degrees and a further 90 degrees are required before
the next chamber is at TDC to begin it's power stroke. There is a 1/3 gap between the end of one power stroke and the beginning
of the next. An RPE of equal gap in power strokes is a three cylinder 4-cycle engine. The single rotor Wankel is equal to a
three-cylinder four-cycle RPE with regard to equal overlap (or lack thereof) of power strokes.
Point 2. A single rotor Wankel is equivalent in power stroke sequence (or overlap) to a three-cylinder four-cycle RPE.
A 4-cylinder reciprocating engine has no gap, the Wankel does. That's why the 4-cylinder, four-cycle RPE is also not a direct
equivalent to the single rotor WE which has only three "pistons", meaning three cavities that follow in a four-cycle event. One
cavity is missing. Is it
not obvious?, it has three chambers, A, B, and C. D is not there as it would be in a four-cylinder four-cycle RPE.
4. Equal chamber displacement. (40 cu-in)
To be equal to the Wankel, the three cylinder RPE will have 40 cu-in displacement per cylinder for a total of 120 cu-in. This is
equivalent to counting all three chambers in the WE that are there!
5. Equal throughput.
A further point of equality is equal throughput per unit time. If our 120 in3 RPE turns, say 6000 rpm, then the WE will turn 1.5
times faster, or at 9000 rpm to have exactly the same throughput. Also the same are total displacement, chamber displacement,
the same starting sequence of power strokes as well as their duration and overlap.
This three cylinder 4-cycle RPE is an exact duplicate to the single rotor Wankel. The only difference is the faster speed of the
output shaft. Well, so what, all other things are equal. After all, the rotor turns at only at 3000 rpm.
(RPE= 120 in3 *6000/2= 360,000 in3/min. WE= 40 in3*3*3000 rpm(rotor)= 360,000 in3/min)
Should shaft speed matter? 9000 rpm vs 6000 rpm in the RPE. Well, the German Authorities gave the Wankel a brake and allowed the
shafts speeds be equal for determining the displacement of the Wankel. This simply means that instead of three shaft rotation
they
allowed two rotation and only two chambers to be counted. This comparison does make sense as we shall see.
The technical justification was to set the displacement of both engines at two shaft rotations which is needed to complete one
four-cycle operation in the RPE, and since the Wankel also operates in the four cycle principle, this seems fair. This then
results in equal total engine displacement at equal shaft speed when counting two chambers rather than three.
Doing this gives the Wankel an advantage in 50 % more power stroke overlap and a 50 % longer power stroke in time.
6. Total engine displacement.
A) Counting only two rotations, our equivalent three cylinder RPE will have only 80 in3 or 26.7 in3 per cylinder and will have
equal sequence of power strokes and equal throughput at equal shaft speed of say 6000 rpm.
Different however, are the following: During two shaft rotations the WE fires twice and the RPE three times, the WE has 50%
fewer chambers, has 50% larger chambers (40 versus 26.7 cu-in) and it has 50% longer duration of the power strokes. Is this a
fair comparison? Not squarely.
B) Comparing the single Wankel to a two cylinder RPE of 40 in3 per cylinder. First we note that it too fires once per revolution
or twice at two revolutions like the Wankel does. Further, we have equal volumes of cylinders and chambers, equal throughput at
same shaft
speed and the same beginning sequence of power strokes.
The WE still has an advantage by having a 50% longer power stroke duration and therefore more overlap. However, with an early
outlet port opening, this advantage is somewhat reduced.
In summary, the Wankel engine can best be compared with, or it comes closest to, or the ?most appropriate? RPE is a two cylinder
four-cycle engine, it has the least differences.
The conflict that exists originates solely from the unequal shaft speed per power stroke duration.
7. The RX-7 or RX-8
Doubling it all up, a two-rotor Wankel such as the RX-7 is comparable to a four-cylinder RPE of 160 cu-in displacement (40 cu-in
per cylinder) with the advantage of a somewhat longer duration power stroke in the WE. Only with regard to power stroke overlap
it compares to a six cylinder RPE of 160 cu-in. The earlier outlet port opening and difficulties in porting and removal of spent
gases makes the WE closer in performance to a four-cylinder RPE, or perhaps, it acts more like a five cylinder RPE in this
regard. Yet, it still equals a 4-cylinder RPE in all other aspects.
(RPE= 160 in3 *6000/2= 480,000 in3/min. WE= 160 in3*6000/2 rpm(shaft)= 480,000 in3/min, or 40 in3/chamber*6 *2000 rpm(rotor)=
480,000 in3/min, the RX-7 is equal to an 160 in3 RPE). (A single rotor RX-7 at 6000 rpm displaces 240,000 in3/m, equal to
360,000 in3/m at 9000rpm).
8. BMEP
With equal throughput at equal shaft speed, the calculations for torque, bmep and displacement are also equal in both engines.
The calculation of bmep (brake mean effective pressure) is a constant times torque divided by displacement. Bmep in psi = 150.8
* torque in ft·lbs / displacement in in3.
An RX-8 may have a peak torque near 160 ft·lbs. Bmep =150.8*160 ft·lb/160 in3 = ~ 150 psi. This is on the low side with regard
to NA RPE?s, but it is comparable. The bmep varies only within a narrow range in all engine types. It is dependent on
compression ratio and volumetric efficiency, and these do not vary that much, except with charging.
If it were only 80 in3 displacement the bmep would be near 300 psi, that would be incorrect and would not represent reality, so
a two rotor RX-7 has to be 160 in3 in displacement.
It can be said that displacement equals torque and piston area equals power. This applies to the Wankel as well. The piston area
can not be varied within a specific size Wankel.
This meets with Ken, just elaborated and being put in different words.
Rolf Pfeiffer