For the rotary inclined engineers among us,
what are the performance/fabrication advantages of
the rotary engince vs. the piston engine.

I've looked at a lot of posts, I just want to see if we
can get them in one list, instead of scattered all over
the forum.

Rotation Vs. Reciprocation

A lot less inertia in the operation of the engine, which
is why we got away with iron rotors in '79 and I don't
think there have been iron pistons since '12 because
of the high weight. Advantage to the rotary, less losses
due to changing direction.

Ports Vs. Valvetrain

The rotary can get "straight" airflow into the chambers,
no valvestem, head passages, etc in the way.
no moving parts, no frictional or operation of the
camshaft/valves losses.

Chamber shape:
rotary chamber has a high surface-to-volume, so
the fuel efficiency is less. (0.75 BSFC vs. 0.50 BSFC on
a good piston engine.) I hope I have that abbreviation
right. Higher turbulence in the chamber means the
rotary is less prone to pre-ignition/detonation
(heard this, don't know if it's true.)

The rotary design is very lightweight, and I think the
castings are easier to fabricate, due to the engine
being somewhat of a "stack" of plates.

Everyone feel free to correct me where I'm wrong.
(Not on "A V8 is lighter than a rotary... I think we've heard that debate enough.)
There should be a lot more to add to this list...
intake port/exhaust port placement and overlap, etc...
that I'm not clear enough on to put in myself

Thanks in Advance,
BlackRex

 

I wonder why they don't use iron pistons in boingers. A piston could be designed that is about the same weight as an aluminum piston, but the iron piston would withstand MUCH more abuse...

 

Read this:

http://home.earthlink.net/~rotaryeng...le-rotary2.txt

(I'm not going to copy and paste the text here because of copyright crap).

 

Nope. They must be machined and plated to exacting tolerences. They also require rotary-specific factory infrastructure.

 

Yeah, construction and tolerancing is definitely the biggest disadvantage. Just look at the shape, its common sense that it would be much harder to mfg. The other disadvantage is no possibility for variable valve timing, or in our case, variable port timing. We can compensate somewhat by using variable intake runners on the intake side, but there's no way around exhaust port sizing. We have the same sized ports for exhaust at low and high rpm, and you can only optimize exhaust scavenging for certain rpms. With valves, although the size of the valve is constant, you can alter lift and timing to optimize filling on the intake side and scavenging on the exhaust side at different rpms.

Biggest advantages are: few moving parts to fail, much much higher rpm limits (essentially more than we'll ever have a use for), higher exhaust temps result in faster catalytic converter lightoff (important nowadays, not many people know, but the new RX-8 is reputed to achieve LEV status using ONE UNDER FLOOR catalytic converter, an amazing feat), and the rest you mentioned .

 

Nathan, no variable port timing? Have you ever heard of the 6-port engines? Mazda had the first variable intake timing systems in the '81 6-port 12A (Japan only) and was first seen in the US in the '84 GSL-SE. Afterwards all N/A 13Bs hwere 6-port.

 

the jeeps in ww2 used iron pistons. rotary in 2 rotor from also has a rev limit because the eccentric shaft is not supported in the middle, and it will flex. the rotary also has less friction because there is no cam/valve train to spin. therefore the weight distribution if the engine is a little better; cams are heavy and 2 of them on top of your motor....

mike

 

Thanks for all the replies.

BlackRex

 

The rotaries used to have a weight advantage back when piston engines were mostly made of cheap low-nickel cast iron and RPMs were kept low for emissions purposes. But now it's no matter to make a 4-cylinder engine that weighs half as much and makes more power, as well as revving higher.

People who swap Ford 302s in their 1st-gens find that the front ride height stays the same. Rotaries are a lot heavier than one may think.