MazdaManiac

iTrader: (3)

The focus of the thread that hadn't produced any useful information in over a year.
Do you just forget stuff every 6 months or so and start over?
We know you post with your fingers in your ears. Maybe you poked something along the way.

olddragger

iTrader: (3)

Damn--if i can post with my fingers in my ears then i have a talent with my other appendages!

That is my last post.
OD

dynamho

That's a really nicely written piece - concise, unassuming, and to the point.

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.

dynamho

Burning question: How does engine management detect octane level?
I read in Corvette forums that it uses the knock sensor to detect octane. Is that right?

dynamho

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?

Brettus

iTrader: (2)

The knock sensor cannot detect octane , it is more of a saftey device . In the case of the 8 it is about as effective as a ledge half way down the cliff you just fell off .

dynamho

I thought so. It can't be the knock sensor and that's what makes me more curious.

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?).

MazdaManiac

iTrader: (3)

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.

Brettus

iTrader: (2)

Conversely - I've seen data logs of Renesis engines running perfectly with (seemingly) no hint of knock yet timing is being pulled regardless .

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 .......

zoom44

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.

MazdaManiac

iTrader: (3)

Not the same thing.
Did you actually see knock retard (which is a separate PID) or did you just see timing being pulled?

The latter is a different interaction.

Brettus

iTrader: (2)

Don't know if 'knock retard' was activated - just saw timing being pulled . What else would make the ECU pull timing other than the knock sensor ?

MazdaManiac

iTrader: (3)

IAT relative to load, RPM delta relative to load and ECT relative to load.

The main interaction is the RPM delta tables, but IAT plays into final timing and there is even an IAT delta table.

dynamho

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?

MazdaManiac

iTrader: (3)

Precisely.

It depends on the engine management scheme.
Mazda's contemporary piston motor engine management schemes have two separate knock feedback loops for WOT and non-WOT.

Thoroughscrub

My 8

MazdaManiac

iTrader: (3)

If, in fact, this ^^ is what you did and this vv
is your conclusion, than there couldn't possibly be a simple answer to your questions.

alnielsen

iTrader: (4)

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.
You can download a PDF version of your owners manual at www.mazdausa.com.

olddragger

iTrader: (3)

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.

Thoroughscrub

I was expecting MM to say something pointless. Thanks to those that gave a strait forward answer.

wcs

iTrader: (2)

No, but instead you do by suggesting he would.
Really smooth.

GTDave

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

ganseg

iTrader: (5)

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.

GTDave

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.

ganseg

iTrader: (5)

For a stock tune, how would I know if the octane was sufficient?