Octane: what it actually means and does
#128
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I liked how he explained the genus of a flame front being increased by turbulence, which increases the exposed area of the flame - I guess kind of like an "inverse" heatsink.
#130
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I'm asking the question above because I see notes about cars that say, "to get the best performance out of this car, use 93 octane" or something like that.
Does this detection-adjustment happen only in closed loop? If it happens in WOT, it implies that it's using different tables based on the fuel's resistance to knock, no?
Does this detection-adjustment happen only in closed loop? If it happens in WOT, it implies that it's using different tables based on the fuel's resistance to knock, no?
#132
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I think our owners manual says use 93 to get best performance, but I don't know how the ECU determines octane level in the fuel tank. Is there some fancy chemical sensor? I doubt it.
Is it simply inferring from standard OBDII engine-related sensor information? If it is, I'm curious about how this is inferred and how quickly it's inferred (ie, a drive cycle?).
#133
Banned
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Well, in a contemporary internal combustion engine (the Renesis included) the ignition timing advance tables are "trimmed" in the same manner that the fuel trims affect injection timing through feedback from the knock sensor.
Many engines - especially "FlexFuel" motors and the like - are very good at this and are always playing with the ignition advance to maximize torque by listening for incipient knock.
Unfortunately, not only does the Renesis not do this (it only seeks to retard timing should knock occur and trim out advance, rather than looking for power through adding advance), it has such a poorly tuned knock feedback system that the knock sensor will never detect knock, even if the motor is knocked to death.
I've seen several datalogs of the Renesis pinging to death (breaking an apex seal in the process) and there was absolutely no indication coming from the knock sensor.
Many engines - especially "FlexFuel" motors and the like - are very good at this and are always playing with the ignition advance to maximize torque by listening for incipient knock.
Unfortunately, not only does the Renesis not do this (it only seeks to retard timing should knock occur and trim out advance, rather than looking for power through adding advance), it has such a poorly tuned knock feedback system that the knock sensor will never detect knock, even if the motor is knocked to death.
I've seen several datalogs of the Renesis pinging to death (breaking an apex seal in the process) and there was absolutely no indication coming from the knock sensor.
#134
Boosted Kiwi
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It's almost like the sensor will pick up on pinging of a very minor type yet when a full on detonation occurs it is oblivious to it .......
Last edited by Brettus; 08-04-2011 at 06:09 PM.
#135
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and thats an important distinction. other current systems actually hunt for advance looking for best torque were-as the rx8 , as jeff said, will not go looking for more power.
so the systems do not detect the octane of the fuel. they hear the knock event and try to tune around it. the manuals suggest a fuel they engineers believe will not allow knock to occur and tell you that will give the best performance.
so the systems do not detect the octane of the fuel. they hear the knock event and try to tune around it. the manuals suggest a fuel they engineers believe will not allow knock to occur and tell you that will give the best performance.
#139
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Thanks guys.
Let me see if I got that right.
In summary, there is no real octane detection happening. In recip. systems, the knock sensor is used to "hunt" for more power, whereas in the Renesis (rotaries in general?), the knock sensor is used only to decrease power, and from what I read previously, pulling ignition timing on knock event isn't that effective in rotary engines.
In piston engines, this "hunt" for power doesn't occur @ WOT, no? Does it always start out using a more aggressive WOT table then switch over to a milder table when there's a knock retard triggered?
Let me see if I got that right.
In summary, there is no real octane detection happening. In recip. systems, the knock sensor is used to "hunt" for more power, whereas in the Renesis (rotaries in general?), the knock sensor is used only to decrease power, and from what I read previously, pulling ignition timing on knock event isn't that effective in rotary engines.
In piston engines, this "hunt" for power doesn't occur @ WOT, no? Does it always start out using a more aggressive WOT table then switch over to a milder table when there's a knock retard triggered?
#140
Banned
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Thanks guys.
Let me see if I got that right.
In summary, there is no real octane detection happening. In recip. systems, the knock sensor is used to "hunt" for more power, whereas in the Renesis (rotaries in general?), the knock sensor is used only to decrease power, and from what I read previously, pulling ignition timing on knock event isn't that effective in rotary engines.
Let me see if I got that right.
In summary, there is no real octane detection happening. In recip. systems, the knock sensor is used to "hunt" for more power, whereas in the Renesis (rotaries in general?), the knock sensor is used only to decrease power, and from what I read previously, pulling ignition timing on knock event isn't that effective in rotary engines.
Mazda's contemporary piston motor engine management schemes have two separate knock feedback loops for WOT and non-WOT.
#143
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Call me retarded... I read every post. Every, post.
A lot of the chemistry terms used in the original post were over my head, but I tried my best to understand and look up terms that I didn't.
The one person that I saw ask if there was a serious need to use (for the sake of simplicity) Premium gas, was given no answer, only a face being smashed in to a wall.
MM, you go on to say that you put a gallon of somethingorother in your tank that is 117 octane, every so often. I am assuming you're stating, via this action, that using higher octane gas is 'better' for rotary engines. Am I right?
I comprehend the difference between low and high octane gas, and why high performance engines utilize high octane gas. Ultimately my question is, does my 8s motor fall in to THAT spectrum of high performance engines. My 8 came with no owners manual, I have only assumed premium was the right choice for my engine.
A lot of the chemistry terms used in the original post were over my head, but I tried my best to understand and look up terms that I didn't.
The one person that I saw ask if there was a serious need to use (for the sake of simplicity) Premium gas, was given no answer, only a face being smashed in to a wall.
MM, you go on to say that you put a gallon of somethingorother in your tank that is 117 octane, every so often. I am assuming you're stating, via this action, that using higher octane gas is 'better' for rotary engines. Am I right?
I comprehend the difference between low and high octane gas, and why high performance engines utilize high octane gas. Ultimately my question is, does my 8s motor fall in to THAT spectrum of high performance engines. My 8 came with no owners manual, I have only assumed premium was the right choice for my engine.
You can download a PDF version of your owners manual at www.mazdausa.com.
#144
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very true. Actually in it's NA form this engine is pretty stout for street use and the occassional track day. Cant you make a little more power by tuning it to a little lower octane? IDK for sure but I have heard of other engines that could.
If you FI--then its a totally different world. Tuning becomes even more important and the engines fuel (octane and quality) is a big part of the tune.
If you FI--then its a totally different world. Tuning becomes even more important and the engines fuel (octane and quality) is a big part of the tune.
#147
Many people have the wrong idea about Pre-ignition and detonation. Maybe this will clear it up for you.
KEY DEFINITIONS
Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.
Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.
The entire article is here http://www.contactmagazine.com/Issue...ineBasics.html
KEY DEFINITIONS
Detonation: Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.
Pre-ignition: Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.
The entire article is here http://www.contactmagazine.com/Issue...ineBasics.html
Last edited by GTDave; 02-22-2012 at 02:53 PM.
#148
Nature vs. Nurture
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What are the unique issues in a rotary? In past piston engines, I would reduce octane based on circumstances. With my 1.8T, during the winter, i would go down to midgrade because temps were lower and the knock sensor waqsn't retarding the ignition. On the rotary, i was always afraid to do that because I read the knock sensor wasn't effective. Too much octane may be contributing to carbon build up, but I decided i wuld rather have extra carbon buildup vs some possible worse consequence.
#149
Throttle position sensor, Mass air flow sensor, Manifold absolute pressure sensor, Engine temp sensor, Air intake sensor, Engine speed. and probably a few more I can't think of right now.
#150
Nature vs. Nurture
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A rotary engine isn't octane sensitive. It is tuning sensitive. If MM is using gasoline with that high octane (I didn't know that grade was possible), it's because his car is turbocharged. A regular rotary car can run fine on lower than premium grades of gas. When I was racing, I ran and set the timing for regular grade gas.