Engine Displacement Theory - Rotary vs Normal

Big Rat

Big Rat

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I'm gonna dumb this down a bit so that we all can be on the same page.
SO i take the RX8 and the Astra 200IE as my example.
2 rotor vs 2Ltr 4 cylinder.
So why does Rotaries have more power? I mean 1.3ltr makes 170 kw vs the 115kw for the 2.0.
My theory...
The normal roraty motor we know is the 13b. Twin roter, 1.3 liter.
How do we get 1.3 Liter?
Well, simply each rotor has three combustion chambers as it spins. each chamber is 650cc. Two rotors makes 1.3.
But recall that each rotor has 3 such chambers. so technically, for each rotation of a rotor it fires 3 times, making 1950cc used for the bang. times 2 rotors and we actually have 3.9ltr.
But then the design comes back into play, the crack spins three times for each revolution of a rotor, meaning for each bang the crank makes one revolution. 3.9/3, back to 1.3. Ok, so Rotary is truly a 1.3ltr per each revolution of the crank.

How does a 2.0 inline 4 compare?
Well, piston 1 fires and pushes the piston down, 180deg turn of the crank. Piston 2 fires, pushing piston one back up, and the crank does another 180, so one full revolution. Piston 3 fires and other 180 deg, Piston one comes down, sucking in airfuel mixture, Piston 2 goes up and pushed exhaust gas out. Piston 4 fires, another 180, making two full revolutions, and all 4 pistons fired.
So for all 4 pistons to play, you need two revolutions. For me that means a 2.0ltr inline4 is only a 1ltr motor for each revolution.
That makes sense that the rotary would be more powerful as it has a bigger capacity for each turn of the crank.

Off course design comes in, 3 moving parts for Rotary ( 2 rotors and one crank) vs 27 for inline 4 (4 pistons, 4 condords, 16 valves, 2 cams, one crank). Revving ability obviously, but the initial starting point has the rotary actually having a bigger capacity per revolution of the crank.

How much out of whack am i here?
 
Way out.
The rotary motor has 3 chambers, yes.
1. For letting in air and fuel,
2. For the combustion process
3. For outlet/exhaust.


Because it can "spin" much faster, thus go through the 3 strokes faster than a 4 stroke ICE.
Think of it as bandwidth vs. latency.
 
Yes, three chambers, so for one complete revolution of that rotor all th
swakop_toe said:
Way out.
The rotary motor has 3 chambers, yes.
1. For letting in air and fuel,
2. For the combustion process
3. For outlet/exhaust.


Because it can "spin" much faster, thus go through the 3 strokes faster than a 4 stroke ICE.
Think of it as bandwidth vs. latency
Click to expand...

I get that.
But when the bang occurs the next chamber is sucking in air. and the chamber infront of the air exploding is pushing the air out. Make one full rotation and all three chambers did a suck bang blow.
One rotor revolution = 3 chambers having a pop.
All three chambers are doing their job in one sequence and one revolution.
First One sucks, two bang, three blows.
Then One bangs, two blows, three sucks,
Lastly One Blows, two sucks, Three Bangs.
One revolution of the rotor, all three chambers had a bang. but again, one revolution of teherotor is three turns of the crank, so it comes back to one bang per crank revolution.

And it is actually not faster, at least not the rotor. So as i said one full turn of a rotor mans 3 full revolutions of the crank. So if i rev to 12000RPM, that is crank speed, the rotor is actually going 4000RPM.
Still 12000 bang per rotor, so 24000 Bangs with two rotos makes that pretty sound for us.
 
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In my view, in simplistic terms, rotaries are more 'power' efficient since they have 'separate' chambers for different strokes of the cycle while conventional IC engines use the same chamber, meaning the rotary can effectively have more power strokes per revolution than the conventional IC engine. They are also more efficient in terms of 'energy' since there is little energy lost to accelerate any part of the moving assembly such as pistons, conrods, etc. that come to a momentary stop before changing direction at TDC or BDC, along with them not wasting 'energy' on valvetrain since they use ports.
 
Neuk_ said:
In my view, in simplistic terms, rotaries are more 'power' efficient since they have 'separate' chambers for different strokes of the cycle while conventional IC engines use the same chamber, meaning the rotary can effectively have more power strokes per revolution than the conventional IC engine. They are also more efficient in terms of 'energy' since there is little energy lost to accelerate any part of the moving assembly such as pistons, conrods, etc. that come to a momentary stop before changing direction at TDC or BDC, along with them not wasting 'energy' on valvetrain since they use ports.
Click to expand...
Yup. Three moving parts, in the same direction vs 27 (simplified) all in opposite directions the whole time. I fully agree. But then they lose some of their efficiency due to low compression ratio. (9:1 IIRC)Lots of good things for rotary, but lack of compression is a buzzkill. Enter the turbo and yeah baby...
 
1.3L Rotary uses the same amount of fuel as a 2.4L conventional engine.
 
Big Rat said:
I'm gonna dumb this down a bit so that we all can be on the same page.
SO i take the RX8 and the Astra 200IE as my example.
2 rotor vs 2Ltr 4 cylinder.
So why does Rotaries have more power? I mean 1.3ltr makes 170 kw vs the 115kw for the 2.0.
My theory...
The normal roraty motor we know is the 13b. Twin roter, 1.3 liter.
How do we get 1.3 Liter?
Well, simply each rotor has three combustion chambers as it spins. each chamber is 650cc. Two rotors makes 1.3.
But recall that each rotor has 3 such chambers. so technically, for each rotation of a rotor it fires 3 times, making 1950cc used for the bang. times 2 rotors and we actually have 3.9ltr.
But then the design comes back into play, the crack spins three times for each revolution of a rotor, meaning for each bang the crank makes one revolution. 3.9/3, back to 1.3. Ok, so Rotary is truly a 1.3ltr per each revolution of the crank.

How does a 2.0 inline 4 compare?
Well, piston 1 fires and pushes the piston down, 180deg turn of the crank. Piston 2 fires, pushing piston one back up, and the crank does another 180, so one full revolution. Piston 3 fires and other 180 deg, Piston one comes down, sucking in airfuel mixture, Piston 2 goes up and pushed exhaust gas out. Piston 4 fires, another 180, making two full revolutions, and all 4 pistons fired.
So for all 4 pistons to play, you need two revolutions. For me that means a 2.0ltr inline4 is only a 1ltr motor for each revolution.
That makes sense that the rotary would be more powerful as it has a bigger capacity for each turn of the crank.

Off course design comes in, 3 moving parts for Rotary ( 2 rotors and one crank) vs 27 for inline 4 (4 pistons, 4 condords, 16 valves, 2 cams, one crank). Revving ability obviously, but the initial starting point has the rotary actually having a bigger capacity per revolution of the crank.

How much out of whack am i here?
Click to expand...
Short version... because brap, brap, brap, brap, braaaaaaaaaaaaaaaap!
 
LMACGPTFY (Let Me Ask Chat GPT For YOU) :
Your explanation has some valid points, but there are a few factors to consider in understanding why rotary engines like the one in the RX8 can produce more power compared to a traditional inline 4 engine like the one in the Astra 200IE.

One crucial factor is the higher RPM capability of rotary engines. Rotary engines can rev much higher compared to piston engines, which allows them to produce more power. The design of rotary engines, with their fewer moving parts, contributes to their ability to spin at high RPMs more smoothly.

Additionally, the rotary engine's unique combustion process contributes to its power output. The rotary engine's triangular-shaped rotor creates three combustion chambers during each rotation, leading to a higher power density per displacement compared to a piston engine. This, combined with the higher RPM capability, allows rotary engines to generate more power.

However, it's worth noting that power output is not the only factor that determines overall engine performance. Factors such as torque delivery, efficiency, and emissions also play a role in evaluating engine performance.

In conclusion, while your explanation touches on some aspects of why rotary engines can produce more power, other factors like RPM capability and combustion process also contribute to their higher power output.
Click to expand...
 
ChrisThomas said:
I remember Tiff reviewing the Rx8 1.3 and it had won awards in the 2.5 to 3.0 category
Click to expand...
Jip, and then promptly got the worst engine ever by all enthusiasts who wanted to turbo the thing or juice it up. :ROFL: Apparantly, the way it is ported, it does not work so lekker for turbo. If you wanted that shape with more oomph you needed to get a 13b, port it and plonk your turbo on.
 

Insanely expensive to rebuilt, check.
Overly high tolerances required, check.
Unreliable, check.
Uses insane amounts of fuel and oil, check.

Compared to "braaap 1.3" it is just easier to get a v8 and call it a day IMO
 
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