snip again

The forum seems to be open again?
Anyway, I know, a lot has been said about the displacement of the rotary.
My comments refer to engine calculations, or engine math, if you will.
Mep in particular.

As I see it, EVERYTHING IS WRONG with calling the Mazda Rotary a 1.3
litre engine. Here is why:

First, it is a four cycle engine and after all, in comparison to an RPE
two shaft rotations must be counted to be equivalent.
I think it is not correct to say any four-cycle engine performs like a
two-cycle engine of half its displacement, regardless of Rotary or RPE.

Secondly, one measure of engine performance and of its internal loading
is the 'mean effective pressure' (mep). This is the reference piston
pressure, the average between maximum and minimum gas pressure during
the power stroke. (Work is calculated by using the stroke as the
distance travelled.)

The hp of an engine is influenced only by three things: displacement,
mep and engine rpm, while torque is the product of displacement and mep
only. Even the ratio of stroke-to-piston diameter has no impact on
torque. Most people would dispute that, however.

In four-stroke engines the hp is calculated thus:
Hp = displacement * rpm / 120 * mep. (for 2-strokes the divisor is 60).
In metric power is in kW, displacement is in litre, rpm in 1/min, mep in
MPa. (No other factor is required.)

Using the imperial system the displacement is in cu-inch and the mep in
psi. A factor of 6600 is needed, since 6600 inlb/s equals one hp. (550
ftlbs/s = 1 hp)

In numbers: hp = displacement (cuin) * mep (psi) * n (rpm) / 792000.
(6600 * 120 = 792000)
mep in psi = hp * 792000 / ( displacement (cuin) * n (rpm) )

Example, 2.6 litre, 4 cycle RPE, generating about 180 hp at 6000 rpm.
(The number of cylinders is irrelevant). 2.6 litre equals ~ 160 cuin.

Mep = 180 hp * 792000 / (160 * 6000) = ~ 150 psi. If 200 hp are
generated, the mep is 165 psi.

Today’s car engine average around 150 to 170 psi. 20 years ago it was
about 10 to 30 psi lower. With charging, mep can be pushed up to as much
as 200 psi. F1 engine are in the 230 psi range. Industrial engine are
near 90 to 100 psi, even Diesels. However, they last “forever” having
10,000 hrs in life or more is not at all unusual. On the other hand F1
engine with ~ 235 psi last for about 5 hours, that’ all. However, their
rpm is continuously very high.

The mep is a measure of power output per displacement and also a good
indicator of the life of the engine.

When using 80 cuin or 1.3 litre displacement for the Mazda Rotary would
result in near 300 psi mep. That number is unheard of for an ICE. That
just can’t be right. The mep of the rotary is also near 150 psi in
naturally aspirated use.

Calculating with 2.6 litre (160 cuin) displacement, its mep is near 150
psi, as it should be. Because of that, the Rotary can not be a 1.3 litre
engine, but it must have a displacement of 2.6 litres.

Of the three variables for power, mep is influenced mainly by the
compression ratio, the porting and the resulting VE, as well as by the
pressure of the intake charge, NA or charged.

Regarding its displacement to overall size, the Rotary has an advantage
there. An RPE of 2.6 litre displacement takes more space and is heavier.

Rpm is limited in all engines by the inertia forces that are linear with
the respective masses and proportional to the square of rpm. This is
true not only for the RPE but also for the Rotary where there is no
linear reciprocation, but a constant centrifugal force of rotor and
eccentric. The direction of acceleration is pointing from the centre of
the eccentric to centre of the eccentric shaft. A limit to the rpm
applies to the Rotary just the same.

Coming back to the displacement of the Rotary, calculating all three
chambers would also be wrong as it takes three shaft revolutions for all
three chambers to fire and that would also be incorrect as the speed is
1.5 times higher compared to an RPE of 2.6 litres of same output. So the
rotary is not a 3.9 litre engine either, it simply is an equivalent 2.6
litre four-cycle engine. Two shaft rotations need be calculated. Period.

On using 3.9 litre displacement its mep would also be incorrect at near
100 psi which is lower than is being measured.

What really counts is the displacement in volume per time at a certain
engine rpm that must be compared. The Mazda Rotary compares to 2.6
litres (160 cuin) displacement with regard to equivalent rpm to a four
cycle RPE. The power output of the rotary also jives with that. Only 2.6
litre can be right.

Regards
Rolf




The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. http://www.rotaryeng.net
Copyright 1998-2006 All world wide rights reserved.