Comparing Piston and Rotary Displacement

"LandSpeed-DSM" (LandSpeed-DSM) 08/14/2013 at 16:06 • Filed to: None

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(Image courtesy of RotorMaster.com.au)

Since we have been on the topic of rotaries lately, I shared my perspective on how to attempt an apples-apples comparison in terms of displacement and performance.

Fellow Jalopnik member OneRotor shared a contribution to an RX7 board made by Howard Coleman:
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At one point he mentions "airflow" in which I have more than a passing interest, and would like to believe a decent grasp of as it relates to Compressors, Cams, Engine Demand Flow (VE), Displacement and so forth. But ultimately the ability to extract energy from fuel via combustion to produce heat and make power.

In his words: "A turbo is exactly what the rotary needs to take advantage of the motor’s immense breathing ability. While the rotary doesn’t make as much hp as a piston engine per given unit of airflow it can flow much more air and that’s where the turbo enters the picture."

Now I certainly am not disagreeing with the notion of putting a turbo on a rotary. Quite the opposite. Instead it was the phrasing of " While the rotary doesn’t make as much hp as a piston engine per given unit of airflow it can flow much more air "

This is where I take pause. In post intended to create some dialogue on another forum long ago, I shared my view of the matter, and figured I would do so again here to try and garner the input of others. My understanding of their operation places the idea that a "13B" is not 1.3L

Alright, flame-suit is on. Have at:

"The only thing that would detract from what was shared is in the way displacement is usually measured for rotaries, and this is highlighted in his comment about airflow with respect to rotaries vs. piston engines.

In addition to the fact that they have fewer rotating parts which is a good thing, they are seemingly less stressed at a given power level because for whatever reason Mazda was able to get sanctioning bodies to only take the displacement of one face per rotor.

Big thing to remember is these are not directly comparable to piston engines for reasons that are both obvious and not.

The rotor its' self only spins once for every 3 rotations of the crankshaft. For a piston engine to send one piston through all 4 cycles takes 720* of crank rotation

Each rotor has 3 "faces", so for a full 4 cycles in a 2-rotor setup one face firing every 180* means it takes 1080* of rotation for all 6 faces to run through the full 4 cycles.

So to compare the 2-rotor to a 4-cyl lets use the standard 720* cycle and compare. If it takes 6 faces 1080* to complete their cycles, 4 faces will have gone through combustion in 720*

With that said, here's how I compare displacement of a 2-rotor like the Mazda "13B" to a 4 cylinder..

If each rotor face will run through a 40 cubic inch (~654cc) swept volume, and 4 faces will go through the a combustion cycle, with only two completing all four phases in 720* (same as the piston engine needs for two combustion events, but only one cylinder finishing all four phases in that period) that would give us the equivalent of about a 2.6L 4 cylinder because 4 rotor faces x 654cc = 2.616L

As should be obvious now, each rotor case you add is a significant increase in potential.

In my mind I usually think of it as the total volume, where if we are counting all 6 faces you would have 3.9L for 2-Rotor 13B, but for this exercise if we are looking at crank speed related to cylinder events to make the comparison it helps some people wrap their minds around it. Helped me at least when it was explained that way.

But at the same time my mind looks at it as the total displacement is 3.9L because you measure all the pistons in a reciprocating motor whether or not they are working through a given period lol

These cases are basically stackable too, each case/rotor assembly being about 1.9L for all 3 faces.. 3 rotors then would be ~5.88L and 4 rotors coming to a whopping ~7.85L

Which is about how they perform vs their piston counterparts. But for their size and weight, I do concede the power potential is baffling.

Mazda calls the 2-rotor which depending on interpretation is either 2.6L or 3.9L the "13B" because they are referencing displacement to rotor rotation. Two rotors, one 654cc face completing 4 cycles in one rotation would then be 1308cc"

Note: This is my first proper, published Oppo post. So there may be some issues with my formatting initially as I test things out!

The WB > LandSpeed-DSM 08/14/2013 at 16:13

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That's a good comparison. I would consider all swept volume of each firing event during a complete cycle of the engine to compare to a piston engine. Another thing to consider is some sort of equivalent to bore/stroke ratio.

I don't know how that would work, but I'm curious what the effective stroke length is of a rotary.

Furthermore, I'd be curious to see a comparison of BMEP between piston and rotary engines using your proposed displacement and the "book" considered displacement. That would give you a relative measure of effectiveness of the rating systems.

LandSpeed-DSM > The WB 08/14/2013 at 16:56

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I've pondered an R/S equivalency.. something based on the crank-lobe is what I keep coming back to. Or the change in distance from the rotor face to the case in the combustion chamber. But would that account for changes in "aggressiveness" of the rotor, as in how flat vs. convex the face is as a whole.

As far as the BMEP is concerned, it quickly drops from something like 19Bar @ "1.3L" (NA 13B, 238HP @ 8500rpm SAE Net) down to ~9.5 Bar at 2.6L and a dismal ~6.3 Bar when measuring all 6 faces for 3.9L.

I don't think those figures are too far off though when you account for the additional 200-400*F EGTs coming out of the ports on a rotary vs. its gasoline piston counterpart.. that is all energy not being put to use on the rotors.

The WB > LandSpeed-DSM 08/14/2013 at 17:16

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Hmm. What is a typical compression ratio for a rotary? Or equivalent?

LandSpeed-DSM > The WB 08/14/2013 at 17:19

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Static compression ratio for the RX8 (2004) is listed as 10.0:1

Effective/Dynamic compression is a different story though.

MyT13B > LandSpeed-DSM 02/19/2016 at 17:05

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I have been pondering what the equivalent bore and stroke for a 13B is. based on the stroke of the eccentric shaft the stroke is about 1.25". The face of the rotor has the same area as a round bore of about 6.4". The torque output of these engines also support these figures.....