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Hello, first time caller, long time listener... (I searched but did not find anything dedicated to this discussion)
I have a question about Effective compression ratio. I know there are a couple of different formulas that are used. But the one I am looking is this: ECR = ((14.7+Boost)x CR)/14.7
My engine was assembled with 9.7 compression rotors. I know that the power gained from compression is far less than the peak power gained with more boost. This is pretty much proven numerous times with numerous examples.
My reason for the higher compression is the use that my car will see. Fun weekend driver with the goal of fun low end, with a broad usable powerband.
So, I have been really looking at this whole "Effective Compression Ratio" and I see a lot of knee-jerk reactions as to what boost levels to run with what compression ratios.
(Fuel choice obviously changes this as well)
Here is a table I put together:
So the reason for the rectangles is this: If you look at the point the horizontal lines touch each ECR curve (color coded for each compression ratio), then follow the the vertical lines down to the x axis... you will find which two boost numbers will yield the same Effective Compression Ratio respective to the rotors being used in the engine.
So the golden question is: Is this just theory, or should I be able to run the same efective compression ratio as the 9.0 guys do with similar setups?
Example hypothesis: (assuming sea level)
If someone runs 20psi boost on 9.0 rotors with water injection, then 9.7 compression engines should be able to run 17.5 psi boost with water injection.
Results would theoretically be about a 3% bump down low and for much of the first half of the curve, with the sacrifice of 6-9% (comparitively) for the second half of the curve.
Is my brain fried from my other studies, or am I on the right track?
It's very difficult to quantify x amount of boost to go with y compression ratio. It's more a case of how much pressure you can make in the engine at the time of compression without detonation.
The more efficient a turbo you have, the better an IC you have, the more boost you can run. It's more about efficiency and how far you can go before detonation than a fixed number - this compression ratio is limited to this much boost.
It's very difficult to quantify x amount of boost to go with y compression ratio. It's more a case of how much pressure you can make in the engine at the time of compression without detonation.
The more efficient a turbo you have, the better an IC you have, the more boost you can run. It's more about efficiency and how far you can go before detonation than a fixed number - this compression ratio is limited to this much boost.
Dale
And all of this makes sense to me.
Except that pressure and compression and detonation are nearly directly correlated due to adiabatic heating.
What I am looking for: is all things equal, except rotors.
So if all things are equal, the internal pressures should follow the pattern (not necessarily the actual numbers due to overlap and other factors as theory numbers work in perfect scenarios)
My goal is to dispel the myths of not running more than 10psi on high comp rotors by showing a correlation between two different compression ratios at similar boost pressures.
Except that pressure and compression and detonation are nearly directly correlated due to adiabatic heating.
What I am looking for: is all things equal, except rotors.
So if all things are equal, the internal pressures should follow the pattern (not necessarily the actual numbers due to overlap and other factors as theory numbers work in perfect scenarios)
My goal is to dispel the myths of not running more than 10psi on high comp rotors by showing a correlation between two different compression ratios at similar boost pressures.
Dang I had a long reply typed but my phone flipped.
I've read before and at peak tq are dangerous times for detonation. You can increase timing after. I'd say race gas or ethanol are more reliable and safer than relying on water injection. I think that's suggested to run on top of a tune not to be tuned with. Why not try tuning with stock knock sensors and turning boost up slowly until you see readings increase. But sure I'd like to see if you can predict it though your research and then see how right you are on your car and a stock 9:1 comp.
I have a similar issue to work out with my High comp 10.5:1 piston TT engine. Interesting test and setup with those rotors as the FD is gutless without the turbos. I bet it would be more fun to drive down low but I also like a high powered fd and I'd hate to limit boost. I'd also hate to do something that would increase detonation as that's the #1 hands down thing the engine can't tolerate, like a kid can't share candy fairly. Good luck
Dang I had a long reply typed but my phone flipped.
I've read before and at peak tq are dangerous times for detonation. You can increase timing after. I'd say race gas or ethanol are more reliable and safer than relying on water injection. I think that's suggested to run on top of a tune not to be tuned with. Why not try tuning with stock knock sensors and turning boost up slowly until you see readings increase. But sure I'd like to see if you can predict it though your research and then see how right you are on your car and a stock 9:1 comp.
I have a similar issue to work out with my High comp 10.5:1 piston TT engine. Interesting test and setup with those rotors as the FD is gutless without the turbos. I bet it would be more fun to drive down low but I also like a high powered fd and I'd hate to limit boost. I'd also hate to do something that would increase detonation as that's the #1 hands down thing the engine can't tolerate, like a kid can't share candy fairly. Good luck
I definitely agree that taking a very cautious approach is going to be of the utmost importance. I don't have easy access to e85 where I am in the Northwest, so I will be doing what I can on premium.
I am curious if the higher compression ratio changes the dynamic of how gas adiabatic heating occurs? So even though the pressures end up the same, is there a difference in how quickly one gets there? It doesn't add up in my mind, but maybe there is something I don't know and that is why I posted this.
In theory, it should only be a 2-3psi difference in peak pressure capabilities with all other factors being equal... But theory doesn't pay for a new engine. Haha!
As for limiting boost, I am not seeing the loss of 3psi (since I was limited by oem apex seals to begin with) as drastic if I have a broader flatter powerband. I am ultimately curious as to how the dyno is going to look with the efr8374
Might want to get Howard Coleman involved, it's a rare topic. But very interesting. I'm being told I won't be able to run much boost at all unless it's with e85 or race gas. They suggested around 10 on pump where I think it could be 15+ on lower comp. The normal comp is 9.6 and I'm at 10.5. I think the rotary will be even more affected by higher comp because of its high hp to cubic inch ratio, high combustion/ exhaust gas Temps and it's sensitivity to detonation.
Might want to get Howard Coleman involved, it's a rare topic. But very interesting. I'm being told I won't be able to run much boost at all unless it's with e85 or race gas. They suggested around 10 on pump where I think it could be 15+ on lower comp. The normal comp is 9.6 and I'm at 10.5. I think the rotary will be even more affected by higher comp because of its high hp to cubic inch ratio, high combustion/ exhaust gas Temps and it's sensitivity to detonation.
But if the combustion chamber pressures are the same between 9.7@17.5psi and 9.0@20psi, then those factors listed should be the same.
It's the arbitrary numbers game that I don't like.
The only way to figure this out "effective compression ratio" for the rotary is to put a pressure transducer in the compression area.
I wouldn't bother trying to use piston engine rules of thumb such as you posted.
My experience is limited to 8.5:1 and 9:1 rotors and I favored the 8.5:1 as the 9:1 couldn't do my usual 14psi on the gas available in my area without detonation (probably running 87 octane often).
I couldn't imagine trying 9.7:1 and boost on pump gas.
Renesis has a very advanced ECU and needs 91 octane to run 10:1 CR with no boost. My friends with stock RX-8 here often get detonation at high rpm. I haven't had that problem since my RX-8 auto only revs to 7,500rpm or so. I did try 87 octane and immediately got a check engine light and knock sensor codes.
Doesn't the ecu pull timing when it sees knock with the stock ecu? I know it does on the FD.
If you have a stock engine and ecu that should work no?
In my opinion the fact that the FD doesnt have a mass air flow meter as well as the map sensor like all other piston engine turbo cars its retarded. That's just one reason why they get such a bad name, not only a sensitive engine but components around it were designed carelessly.
Snook Doesn't the ecu pull timing when it sees knock with the stock ecu? I know it does on the FD.
If you have a stock engine and ecu that should work no?Yes
Yes.
IDK if the knock sensor activates timing retard at all rpms on RX-8.
Earlier rotaries ignored knock readings in higher rpms to avoid false readings.
RX-8 seems to throw and log a code for ANYTHING out of the ordinary it sees. I have gotten to where I just ignore the check engine lights and let them clear in normal drive cycles.
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Keeping the factory knock control is the reason I did a ROM tuned ECU on my FD instead of plug and play standalone.
But I raised the redline on that ECU to 9,000rpm and killed the motor that way... (a single stuck side seal from rotor touching side housing).
It's the arbitrary numbers game that I don't like.
Originally Posted by BLUE TII
The only way to figure this out "effective compression ratio" for the rotary is to put a pressure transducer in the compression area.
I wouldn't bother trying to use piston engine rules of thumb such as you posted.
its not arbitrary, we just don't know what they are.
for a start though, we know from reading Kenichi Yamamoto's book, that the power vs compression ratio is in a different number range than, say a small block Chevy. in a small block chevy 12:1 would be considered high compression, but as we see from the graph in the rotary book, that 9.2:1 in a rotary actually has better BSFC at 1500rpm, and only looses at 5,000rpm by a small percentage.
there are a couple of reasons why this might be so, the first is the K number. it dictates in part the ratio of BDC to TDC, which is the compression ratio range available.
the second is something shared with a piston engine, and that is combustion chamber shape, in a piston engine if you put too large a dome on the piston, power goes down, because the chamber shape gets too weird. the rotary seems to do something similar. also with the rotary the combustion chamber moves. the air(+fuel) has to travel all the way around the engine. when you raise the compression to the maximum that the K number lets you, the combustion recess gets small, and this effects the airflow around the engine internally.
its not arbitrary, we just don't know what they are.
for a start though, we know from reading Kenichi Yamamoto's book, that the power vs compression ratio is in a different number range than, say a small block Chevy. in a small block chevy 12:1 would be considered high compression, but as we see from the graph in the rotary book, that 9.2:1 in a rotary actually has better BSFC at 1500rpm, and only looses at 5,000rpm by a small percentage.
there are a couple of reasons why this might be so, the first is the K number. it dictates in part the ratio of BDC to TDC, which is the compression ratio range available.
the second is something shared with a piston engine, and that is combustion chamber shape, in a piston engine if you put too large a dome on the piston, power goes down, because the chamber shape gets too weird. the rotary seems to do something similar. also with the rotary the combustion chamber moves. the air(+fuel) has to travel all the way around the engine. when you raise the compression to the maximum that the K number lets you, the combustion recess gets small, and this effects the airflow around the engine internally.
So there is no real validity to the method I used while keeping constant the combustion chamber shape, engine type and all other variables?
its not arbitrary, we just don't know what they are.
for a start though, we know from reading Kenichi Yamamoto's book, that the power vs compression ratio is in a different number range than, say a small block Chevy. in a small block chevy 12:1 would be considered high compression, but as we see from the graph in the rotary book, that 9.2:1 in a rotary actually has better BSFC at 1500rpm, and only looses at 5,000rpm by a small percentage.
there are a couple of reasons why this might be so, the first is the K number. it dictates in part the ratio of BDC to TDC, which is the compression ratio range available.
the second is something shared with a piston engine, and that is combustion chamber shape, in a piston engine if you put too large a dome on the piston, power goes down, because the chamber shape gets too weird. the rotary seems to do something similar. also with the rotary the combustion chamber moves. the air(+fuel) has to travel all the way around the engine. when you raise the compression to the maximum that the K number lets you, the combustion recess gets small, and this effects the airflow around the engine internally.
To elaborate on this as it wasn't spelled out.
The rotary is completely different animal than the piston engine in this regard. The recess in the rotor is not just some combustion chamber, it is also a transfer port.
The rotary moves its gas charge all through the motor from intake port to exhaust port. When the intake charge is in the compression stroke and the rotor moves toward TDC the pinch in the peanut shaped rotor housing separates the charge above and the charge below the pinch.
The entire intake charge must go through the recess in the rotor to get to the side of the rotor housing that can push on the side of the rotor to create work during the combustion strokes expansion.
Make that smaller and you are increasing pumping losses for the engine and adding heat from friction to the compression charge.
Adding compression to the rotary really aids the throttle tip in feel and response and... not as much else as we would hope.
You want torque on a turbo rotary, you put boost to it. You choose at what rpm you want the boost.
Low comp rotors and low rpm boost.
It actually drives like a cammed V8 which isn't much fun after driving rotaries. No increase in torque with rpms. You can shift at 8,000rpm if you want, but it is exactly as fast to shift at 6,500rpm.
You start to feel like you are wasting time waiting around for 8,000rpm to come...
Make that smaller and you are increasing pumping losses for the engine and adding heat from friction to the compression charge.
This is the only part I disagree with on a science level. Adiabatic heating would be the same since the resultant pressure is the same. (When using two sister points in my graph above, and leaving all other variables the same)
"Make that smaller and you are increasing pumping losses for the engine and adding heat from friction to the compression charge."
This is the only part I disagree with on a science level. Adiabatic heating would be the same since the resultant pressure is the same. (When using two sister points in my graph above, and leaving all other variables the same)
Well, in my example low compression rotor versus high compression rotor were both NA.
There is more adiabatic heating from the high compression rotor not just from the compression, but from the transportation of the compression charge through the transfer port of the rotor recess.
That is all I meant- was not comparing forced induction to NA in that example.
Just showing why raising the compression ratio of a rotary does not increase power in the same way in a rotary as in a piston engine. You have an additional factor (gas transport) that is saps efficiency.
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In general, you should look at raising compression ratio as increasing Otto Cycle efficiency (longer expansion stroke) and forced induction as adding displacement.
You *can* use forced induction to also increase efficiency by longer expansion stroke by changing to Miller Cycle.
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If you were to compare NA to FI with the same compression pressure what would be each engines respective compression temperature?
We could build an "improved" Wankel rotary that had high compression with no rotor recess and transported the intake charge through an external passage with intercooling much like a piston air compressor.
If you were to compare NA to FI with the same compression pressure what would be each engines respective compression temperature?
The adiabatic heating would be the same. Just has been divided at different stages. One would have a chance to be Intercooled prior to the engine stroke compression.
But if the starting temperatures are the same, and the ending pressure is the same, then the adiabatic heating will be the same. The only difference will be if more heat is added from the turbo charger or from the engine pumping, on top of the adiabatic process.
But when we are talking about a difference of 0.7:1 in compression and a difference of 2.5psi boost... I highly, and I mean very highly doubt that 10psi is the safe limit for a 9.7 engine. But all of the reading I can find tend to freak out at double digit boost levels at that ratio.
When the combustion chamber is shaped the same, engine design the same, displacement the same, etc.... It just doesn't make sense.
As for power difference, compression cannot make up for mass volume. So even with the same net pressures in the combustion chamber, the lower compression/higher boost engine will make more peak power, consistently from what I have read and from my math on compressed volumes of air/fuel.
My goal is to point out that people run 18 psi on pump gas with 9.0 compression, so 15psi on 9.7 should not be a problem since the pressures will be the same. Peak torque will shift left, along with peak hp, peak horsepower will drop.
When the combustion chamber is shaped the same, engine design the same, displacement the same, etc.... It just doesn't make sense.
this is because the shape of the chamber DID change in a meaningful way. the 9.7 rotors do not flow air as well inside the engine as the 9.0 engines do, at a big air flow number, this is important.
secondly, you're assuming that 0.7 of a change isn't very big, but imagine a piston engine going from 12.5:1 and then raising that 0.7. the actual number derived from a rotary LOOKS comparable to a piston engine, but it is not.
Mazda's own engines show this. the Rx8 and the sky active have the highest compression ratios that they can practically have, and the piston is running 14:1 and the rotary is 10:1. the K number on the rotary changes the compression numbers that are available in a rotary engine.
Fair enough. I didn't realize the combustion chamber was shaped so much differently between the two. I'll have to research that more.
Beginning to consider taking my engine apart to put 9.0's back in. Just hate to have spent so much money on the lightening and balancing of the 9.7's which are supposedly induction hardened like FD rotors from lpg test engines. Adam at rotaryengine.com would know more on that.
My goal is to point out that people run 18 psi on pump gas with 9.0 compression, so 15psi on 9.7 should not be a problem since the pressures will be the same. Peak torque will shift left, along with peak hp, peak horsepower will drop.
Unless I am totally missing something.
The only thing you are missing is that you will never really see a difference between 8.5:1 rotors and 10:1 rotors on the dyno sheet in the low end or anywhere else because everything else is more of a factor.
You will however see the difference of running 20psi boost at 3,000rpm on 8.5:1 rotors and 15psi boost at 3,000rpm on 9:1 rotors because it is a 50ft/lb difference in torque.
High comp rotors will give you better tip in throttle feel, low comp rotors with more boost will actually give you more torque... everywhere.
I went from the 7670 to the 8374 so I didn't have to run as high of PR with the higher comp rotors. Thinking of dropping compression back down. But also thinking of just trying out since it's already built.
Hello, first time caller, long time listener... (I searched but did not find anything dedicated to this discussion)
I have a question about Effective compression ratio. I know there are a couple of different formulas that are used. But the one I am looking is this: ECR = ((14.7+Boost)x CR)/14.7
My engine was assembled with 9.7 compression rotors. I know that the power gained from compression is far less than the peak power gained with more boost. This is pretty much proven numerous times with numerous examples.
My reason for the higher compression is the use that my car will see. Fun weekend driver with the goal of fun low end, with a broad usable powerband.
So, I have been really looking at this whole "Effective Compression Ratio" and I see a lot of knee-jerk reactions as to what boost levels to run with what compression ratios.
(Fuel choice obviously changes this as well)
Here is a table I put together:
So the reason for the rectangles is this: If you look at the point the horizontal lines touch each ECR curve (color coded for each compression ratio), then follow the the vertical lines down to the x axis... you will find which two boost numbers will yield the same Effective Compression Ratio respective to the rotors being used in the engine.
So the golden question is: Is this just theory, or should I be able to run the same efective compression ratio as the 9.0 guys do with similar setups?
Example hypothesis: (assuming sea level)
If someone runs 20psi boost on 9.0 rotors with water injection, then 9.7 compression engines should be able to run 17.5 psi boost with water injection.
Results would theoretically be about a 3% bump down low and for much of the first half of the curve, with the sacrifice of 6-9% (comparitively) for the second half of the curve.
Is my brain fried from my other studies, or am I on the right track?
All the "effective compression ratio" mumbo jumbo you read on the internet for pistons and rotaries is way too oversimplified and is basically useless. It's a geometric measurement that's only used for gas exchange events. Ie, on a piston engine when considering how late the intake valve closes.
x axis is piston engine crank angle degrees ATDC firing, where 540 is BDC compression, lower number is earlier, later number is later (720 = TDC firing). So yeah, on a piston engine if your intake valve closes before or after BDC, your effective compression ratio goes down.
This effective compression ratio calculation is used in stock ECU's to calculate variable valve timing targets (obviously not used in a rotary because there are none in production right now that would need such a feature).
What OP is talking about is really a gas flow model. So the amount of air actually trapped in the engine (piston or rotary really) has to do with the pressure and temperature of the air entering, yes, but the amount of residual gas in the combustion chamber and gases trapped in crevices makes all the difference.
What that means is, OP's ((14.7+Boost)x CR)/14.7 formula doesn't mean anything because it doesn't take into account gases that are ingested into the engine during overlap due to backpressure. That's a function of turbo design, boost, rpm, etc.
The way to calculate engine load (indicated mean effective pressure) on a rotary is in the 1989 Watanabe SAE paper. It's a curiosity only, it has no bearing on any of our purposes here.
01-31-16 | 08:17 PM
