why is the rotary so inefficient??
12-15-02, 08:29 PM
#76
Banned. I got OWNED!!!
Originally posted by Nathan Kwok
The shape of the combustion chamber makes it really difficult to get a complete burn. The flame front has to travel a long ways. If you read the SAE paper on the renesis, the side ports allow the chamber to act as a centrafuge, the heavier unburnt fuel is pushed to the very periphery of the housing. By placing the exhaust port slighly toward the center, the unburnt fuel is actually swept past the exhaust port and stays in the chamber to be burned in the next cycle, while still allowing the exhaust gases to exit efficiently. Its genius imho, such an awesome idea and only possible with a rotary.
The shape of the combustion chamber makes it really difficult to get a complete burn. The flame front has to travel a long ways. If you read the SAE paper on the renesis, the side ports allow the chamber to act as a centrafuge, the heavier unburnt fuel is pushed to the very periphery of the housing. By placing the exhaust port slighly toward the center, the unburnt fuel is actually swept past the exhaust port and stays in the chamber to be burned in the next cycle, while still allowing the exhaust gases to exit efficiently. Its genius imho, such an awesome idea and only possible with a rotary.
Any engine that recirculates burnt or partially burt fuel/air form a previous cycle is not "efficient" in producing maximum power, you need overlap to produce VE% that are high for maximum specific power.
Emissions and low speed fuel efficiency due to being able to run a much leaner a/f mixture stabily are what the RENISIS are good at from preliminary SAE reports looking at the benifit of side exhaust ports, but It is beaten in specific output by the Honda S2000 engine which has been in production now for quite a while.
BSFC will be allot better for low load as stated however BSFC at maximum power is not any better if compression ratio difference are accounted for in the comparison, take this into account with the increased engine speed and the specific output looks quite ordinary compared to what is available from Honda.
Some early reports of the in vehicle economy of the RX8 are pretty ordinary, MPG's in the range of 22 to 23mpg when used in city tests combined with highway, but I am unsure of the amount of "spirited driving" ???
Either way, Emissions is the winner, power ? well if you applied the increased peak speed and compression to a Peripheral exhaust port engine, then you would most likley have more power.
12-16-02, 09:47 AM
#77
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How and why does a renesis make more power than previous rotaries?
1. Larger displacement - No. There are rumors of a future increase in rotor width from 80mm to 90mm.
2. Better volumetric efficiency (How much air can we get in the engine?)
a. The side exhaust allows an intake port approx 100% bigger than previous production rotaries.
b. The side exhaust allows the intake port to be opened earlier without causing too much overlap (horrible low end).
c. The side exhaust allows dual exhaust ports approx 30% bigger than previous production rotaries. Unfortuately, the port is not open as long as previous production rotaries and the exit path for exhaust is not as straight.
d. The intake manifold has been greatly improved to provide laminar airflow and uses 3 different length paths to tune the dynamic effect of the intake air pulses. This improves the supply of air to the engine for the entire powerband.
3. Better thermal efficiency (Better use of the air/fuel mixture for power)
a. Better atomization of fuel due to ultra fine fuel injectors
b. More powerful spark due to higher power coils
c. Higher compression (unknown how high?) - [limited by sealing improvements and reliability]
d. Improved combustion stability/efficiency at low speed & light load due to no overlap.
e. Better thermal efficiency due to longer power stroke (exhaust port opens later)
4. Better mechanical efficiency (Less friction / resistance to movement)
a. 14% Lighter rotors. Rotors will be approx 8.2 lb (9.54 * .86) leading safely to higher revs.
b. Chrome-molybdenum alloy on housing = lower coefficient of friction?
c. Carbon-fiber should lead to reduced drivetrain losses?
Peripheral exhaust is definitely out of the question for new production vehicles because of tightening emissions standards worldwide. There are new tier 2 standards for 2004-2005 in the US and new stage IV standards in Europe for 2005-2006. Preliminary EPA fuel economy estimates run from 19-20 city to 24-30 highway.
Mazda has studied peripheral induction though. Per Mazda: "The advantage of peripheral induction, high power output, compensates for larger overlapping and deceleration unstable combustion".
The current design has 6 intake ports matched with 3 different length intake runners to maximize the dynamic charging effect. It may be practical for Mazda to create a peripheral port for the high rpm path. This could be closed at low rpms to eliminate overlap and unstable combustion and opened by shutter valves at high speeds so that the overlap will provide greater torque and power from scavenging.
Brian
1. Larger displacement - No. There are rumors of a future increase in rotor width from 80mm to 90mm.
2. Better volumetric efficiency (How much air can we get in the engine?)
a. The side exhaust allows an intake port approx 100% bigger than previous production rotaries.
b. The side exhaust allows the intake port to be opened earlier without causing too much overlap (horrible low end).
c. The side exhaust allows dual exhaust ports approx 30% bigger than previous production rotaries. Unfortuately, the port is not open as long as previous production rotaries and the exit path for exhaust is not as straight.
d. The intake manifold has been greatly improved to provide laminar airflow and uses 3 different length paths to tune the dynamic effect of the intake air pulses. This improves the supply of air to the engine for the entire powerband.
3. Better thermal efficiency (Better use of the air/fuel mixture for power)
a. Better atomization of fuel due to ultra fine fuel injectors
b. More powerful spark due to higher power coils
c. Higher compression (unknown how high?) - [limited by sealing improvements and reliability]
d. Improved combustion stability/efficiency at low speed & light load due to no overlap.
e. Better thermal efficiency due to longer power stroke (exhaust port opens later)
4. Better mechanical efficiency (Less friction / resistance to movement)
a. 14% Lighter rotors. Rotors will be approx 8.2 lb (9.54 * .86) leading safely to higher revs.
b. Chrome-molybdenum alloy on housing = lower coefficient of friction?
c. Carbon-fiber should lead to reduced drivetrain losses?
Peripheral exhaust is definitely out of the question for new production vehicles because of tightening emissions standards worldwide. There are new tier 2 standards for 2004-2005 in the US and new stage IV standards in Europe for 2005-2006. Preliminary EPA fuel economy estimates run from 19-20 city to 24-30 highway.
Mazda has studied peripheral induction though. Per Mazda: "The advantage of peripheral induction, high power output, compensates for larger overlapping and deceleration unstable combustion".
The current design has 6 intake ports matched with 3 different length intake runners to maximize the dynamic charging effect. It may be practical for Mazda to create a peripheral port for the high rpm path. This could be closed at low rpms to eliminate overlap and unstable combustion and opened by shutter valves at high speeds so that the overlap will provide greater torque and power from scavenging.
Brian
12-18-02, 06:54 PM
#78
Got Boost?
high exhaust temps arent good for a turbo, its the airflow over the exhaust turbine. If you could have freezing exhaust gas (i dont know how you would do that...) it would work the same, its all a matter of airflow
This, by the way , is how rotaries can at least turn their thermal ineffeincies into power on demand. (still does jack for low load fuel consumption)
Besides the renesis, what other production piston engine has no overlap? It may be misleading to point out overlap as a cause of previous rotaries fuel economy woes if all piston engines have overlap also?
Last edited by fatboy7; 12-18-02 at 07:00 PM.
12-19-02, 04:33 PM
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Ijneb wrote that high exhaust heat was bad for turbos and that it was the exhaust airflow that drives turbos.
In responding to Ijnebs post from last month, fatboy wrote:
Hi Fatboy,
Are you saying that higher exhaust temps will make a turbo last longer? I believe that is what Ijneb was talking about. If you are just talking about how higher temps lead to more airflow because of the higher pressure, you have just repeated what Ijneb was saying... Of course you are aware that one of the unique properties of rotaries is that they generate strong pulses of air from both the intake and exhaust ports? The strong pulses are one of the reasons why a rotary will more easily accomodate a T-78 than a piston.
I didn't have an idea of what the average overlap for was for most production engines. That is why I asked about overlap and piston engines and I was hoping that someone would post something useful if they had that kind of information.
The 13b has about 16 degrees of overlap since the exhaust port closes at 48 degrees ATDC and the primary and secondary intake ports open at 32 degrees ATDC. Agreed?
Are you contending that "most" production piston engines have about 4-5 degrees of overlap? I did a quick search on "stock overlap cams" and the first website that came up is below:
http://members.aol.com/solomiata/cams.html
The website has the below #s:
USA spec 1990-93 Miata 1.6 DOHC 5-speed
Overlap 20 degrees (-5-15)
USA spec 1.8 BP8 90-93 HLA (Protégé, Escort GT)
Overlap 19 degrees (-5-14)
Unless these are completely not representative of piston engine overlaps, it would not seem that previous rotaries had much more overlap than piston engines. In these examples, the rotary had less overlap. While I'm sure there are probably many examples of stock cars that have 5 degrees of overlap or less, there are also tons of examples of production piston cars with 16 degrees or more of overlap. If you posted info of cars from the rx-7 time period with 5 degrees overlap or less, it may add to this discussion.
Brian
In responding to Ijnebs post from last month, fatboy wrote:
Not true... the same mount of gas (mass) with higher temps has more potential energy stored in it, not to mention that it will decidedly take up more volume and/or be at a higher pressure than it cooler counterpart. it's this principle that makes engines work (heat cycle), the turbo is just another engine really that has a unique job, filling the bigger engine next to it with more air. The hotter higher pressure air will kick those turbos silly with power. How many 2.6L piston engines can easily accomidate a T-78 without serious lag issues?
Are you saying that higher exhaust temps will make a turbo last longer? I believe that is what Ijneb was talking about. If you are just talking about how higher temps lead to more airflow because of the higher pressure, you have just repeated what Ijneb was saying... Of course you are aware that one of the unique properties of rotaries is that they generate strong pulses of air from both the intake and exhaust ports? The strong pulses are one of the reasons why a rotary will more easily accomodate a T-78 than a piston.
Originally posted by fatboy7
Do you have any idea what kind of overlap a 13B has vs. most production piston engines? like 3 - 4 times as much.... this is bad for fuel economy, under low load situations.
Do you have any idea what kind of overlap a 13B has vs. most production piston engines? like 3 - 4 times as much.... this is bad for fuel economy, under low load situations.
The 13b has about 16 degrees of overlap since the exhaust port closes at 48 degrees ATDC and the primary and secondary intake ports open at 32 degrees ATDC. Agreed?
Are you contending that "most" production piston engines have about 4-5 degrees of overlap? I did a quick search on "stock overlap cams" and the first website that came up is below:
http://members.aol.com/solomiata/cams.html
The website has the below #s:
USA spec 1990-93 Miata 1.6 DOHC 5-speed
Overlap 20 degrees (-5-15)
USA spec 1.8 BP8 90-93 HLA (Protégé, Escort GT)
Overlap 19 degrees (-5-14)
Unless these are completely not representative of piston engine overlaps, it would not seem that previous rotaries had much more overlap than piston engines. In these examples, the rotary had less overlap. While I'm sure there are probably many examples of stock cars that have 5 degrees of overlap or less, there are also tons of examples of production piston cars with 16 degrees or more of overlap. If you posted info of cars from the rx-7 time period with 5 degrees overlap or less, it may add to this discussion.
Brian
Last edited by Buger; 12-19-02 at 04:40 PM.
12-28-04, 12:54 AM
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Well, I am a Ford/Mazda lover. I have owned an RX3, RX4, XR4TI, and now the RX8. Some people love to calculate their mileage, I look forward to that calculation like Catholics look forward to confessional. My lowest mileage was in the RX4, consistently in the low teens (why does such a dinky motor have a four barrel carb?) I get good mileage in nothing. None of these got better than 22MPG with me. The RX8 just got 15 MPG in a city/freeway mix. All of them felt pretty powerful. The RX4 got up to about 135 MPH (buzzer went off in 4th), the XR4TI 128 MPH, and I will not tempt the RX8, because its too late to die young.
The rotary motor is not that hot, it just revvs high. The Cinderella story is told after 5,500 RPM. If the RPMs are restricted to 5,500 like most other motors, their advantage is lost.
Now another point:
The rotory power output is peaky. To access the peak power, the engine is kept in a narrow part of the powerband, say 6,000 to 9,000 RPM. The equivalent for a 6 cylinder would be 4400 to 6500 RPM. The rotary will offer peak power right at the 8900 RPM, while the 6 will peak a little before redline, say 6200 RPM. This gives the 6 motor more time at near peak power than the rotary.
Of all these cars, the RX4 was the definite killer. The contemporary was the 260Z, which had less power and was more fragile. I once raced a 260 for the merging lane, easily got in front. I stayed well into the "buzzer" and looked back at a cloud of smoke behind the 260 as he pulled over.
The RX8 is fantastic, but the competetion is stupendous. Who would have thought that the Chevy 350 would make 400 street HP with decent mileage, or the 3500 Nissan could do 280 HP, and get 22 MPG?
If you really want mileage get the 3. At 30 MPG, and decent get up and go its a winner (consumer reports loves the car).
Otherwise its just a question of how big a grin you get out per $ put in.
The rotary motor is not that hot, it just revvs high. The Cinderella story is told after 5,500 RPM. If the RPMs are restricted to 5,500 like most other motors, their advantage is lost.
Now another point:
The rotory power output is peaky. To access the peak power, the engine is kept in a narrow part of the powerband, say 6,000 to 9,000 RPM. The equivalent for a 6 cylinder would be 4400 to 6500 RPM. The rotary will offer peak power right at the 8900 RPM, while the 6 will peak a little before redline, say 6200 RPM. This gives the 6 motor more time at near peak power than the rotary.
Of all these cars, the RX4 was the definite killer. The contemporary was the 260Z, which had less power and was more fragile. I once raced a 260 for the merging lane, easily got in front. I stayed well into the "buzzer" and looked back at a cloud of smoke behind the 260 as he pulled over.
The RX8 is fantastic, but the competetion is stupendous. Who would have thought that the Chevy 350 would make 400 street HP with decent mileage, or the 3500 Nissan could do 280 HP, and get 22 MPG?
If you really want mileage get the 3. At 30 MPG, and decent get up and go its a winner (consumer reports loves the car).
Otherwise its just a question of how big a grin you get out per $ put in.
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