Octane, Compression Ratio, and Detonation. (Full Version)

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Octane, Compression Ratio, and Detonation.

  

slipper -> Octane, Compression Ratio, and Detonation. (9/13/2006 6:43:41 AM)

I've decided to create this thread due to an overwhelming amount of "what octane" threads and misinformation.

For those who insist on using premium, if it makes you feel better or if its a sense of assurance go for it.

First of all Honda would not risk a potential hundreds of thousands of dollars if not more in waranty work and publish 86 octane for no reason. They are the largest engine manufacturer in the world and the largest motorcycle manufacturer in the world. Honda has been known for extensive R&D before releasing something into the market. This is the exact reason i trust Honda automobiles and motorcycles.

Second. For those who use the compression ratio arguement, this is only a fraction of the story. Its kinda like saying a 8 ounce salmon filet has just as much fat as a 8 ounch steak. One is an abundant source of omega 3 fatty acids which is good, the other is full of saturated fats. Sure the compression ratio is 12.0:1 but thats a very very generic figure. How is this generic you may ask. 12.0:1 is a static figure and only factors the volume with the piston at bottom dead center divided by the piston at full compression aka top dead center. A very simple explaination to this would be this. Say we have a 4 cylinder engine with a total displacement of 2000cc's, each cylinder displaces 500cc's. the combustion chamber in the cylinder head displaces 50cc's. 500 divided by 50 equals 10, in other words a 10.0:1 compression ratio. so just like my analogy to fat content in salmon and steak, static compression ratio is insignificant.

In the bigger picture we would see the pistons moving up and down, rod ratios, intake and exhaust valves opening and closing, valve overlap, and of course the static compression ratio. Dynamic compression ratio factors in all of the above, a much more complex calculation. a typical dynamic compression ratio for a pump gas motor would most likely be in the range of 7.5 to 8.5:1. with stock or street cams this would typically translate to a 10.0 to 12.0:1 static compression ratio. Of course more info would be needed for an exact figure.

Even more complex is detonation (aka pre-ignition,ping, or knock). Detonation is the burning of the air fuel mixture in a volitle and uncontrollable manner creating a violent spike in cylinder pressure. In ideal conditions the air fuel mixture will burn in an even and controlled fashion. The higher the octane the lower the energy content (british thermal units) which results in a better the resistance to detonation. The higher the compression ratio the less resistant the engine will be towards detonation. Again this is only half the story. Detonation is also determined by the temperature of the ambient air entering the engine, temperature of the engine components, air density, engine load, combustion chamber design, engine speed as well as gasoline octane and compression ratio.

Yes i am a noob... to the forum. However i have much experience in the motorsports field. This thread was not meant to offend anybody.

Sticky material?

  
woot -> RE: Octane, Compression Ratio, and Detonation. (9/13/2006 8:33:02 AM)

I've been argueing that side till I was blue in the face... it's a good post.

The thing that most people don't get is the relationship between preasure and tempature... you didn't explicitly state it, however, I knew from your post you understood it.

Amonton's law.
Preasure (low compression)/ Temp (low compression = Preasure (high compression) / Temp (high compression)

You quirt in cold gas, into a low preasure system (old carb bikes really were a mini-vacuum, that's how they worked) - and then you preasurize it incredibly. In so preasurizing it the tempature goes up proportionally. In english - if it was cold and low preasure, it's now hot and high preasure.

Gas has a flash point. If you reach that tempature it goes boom. Well that's a bit too much of a simplification but it'll do. So in old cars, carboned cylanders would have little embers that stayed alight - causing pre-ignition... fuel addtives were to wash those away and get fix the pre-ignition problem.

In a high compression engine you can heat the gas so much as you preasurize it - as the original poster stated - it reaches the flash point too early. Pre-ignition.

-

Lets pause for a moment and thing about how a 4-stroke works. As simply worded as possible.

1- Intake
The piston is travelling down, the intake valve opens and fuel/air enter the chamber. The exhaust valve is closed, so as the piston travels down the preasure drops. It is also being cooled by the cooler air coming in. Low peasure and low tempature. (See why an cold air intake matters? The lower the temp the lower the preasure at this stage... more fuel/air mix put in. Also note that it will raise the temp of the compressed gas later so might require higher octanes)

2- Compression
The intake valve has now closed. The piston travels up compressing the fuel/air mixture. Preasure goes up, so tempature has to.

3- Power stroke.
Some point before the piston reached the top of the compression stroke the spark plug fired. It takes a split second for that flame reaction to really kick in, but on the other hand, a high revving engine would also take a split second to start the downward stroke... timing is where we set the time the spark occures. We want it to spark just in time to get a full power stroke, but not too early as to fight against the piston travelling up in the compression stroke.

4- The piston reaches the bottom of the power stroke filled with waste gas and some unburned fuel. Can't be helped. The exhaust valve opens and the piston travels up pushing the waste out the exhaust.

--

Why all that ramble - as the previous poster pointed out - it's all about the timing between compression and power stroke - the instant where the piston stops travelling up (creating preasure) and starts travelling down (creating power). If it ignites too soon some of the power is wasted fighting against the compression upstroke. Using too low an octane, carbon build-up, the wrong timing etc can lead to pre-ignition. Pre-ignition is the engine fighting with itself and not working efficiently.

However, the counter balance as the original poster pointed out is that - high octane fuel has less energy. Using too high an octane fuel won't hurt the bike, like preignition would, but it will rob you of money, power, and fuel milage. Not much, but it does.

The trick is to use lowest fuel octane you can use in the vehicle and NOT have preignition. Honda as stated, knows what they are talking about so use what ever it says in the manual.

added background, but the O.P. had it down.
woot.

Will this topic ever die? Nope... even if stickied people won't find it :(






doggy69 -> RE: Octane, Compression Ratio, and Detonation. (9/13/2006 10:19:26 AM)

So what are you guys trying to say....lol....j/k

hondadriver4life -> RE: Octane, Compression Ratio, and Detonation. (9/13/2006 10:31:13 AM)

OW MY BRAIN HURTS NOW LOL

woot -> RE: Octane, Compression Ratio, and Detonation. (9/13/2006 10:54:53 AM)


Short answer?

It's cheapest to run the lowest octane that doesn't pre-ignite.

Honda knew what they were doing, just do what they said.

If you need higher octane than specified by Honda, due to preignition, then you should find out why. Maybe it is simply the modifications to the bike that affect the tempature of the cylanders, and/or carb deposists.

IF that doesn't work for you then use higher octane - but please don't tell me anymore that I need to run higher octane. :p


slipper -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 3:44:57 AM)

Im pretty surprised, no one here wants to object or put up an educated arguement?

chainstretcher -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 7:41:34 AM)

There is no logical argument -- peeps have been trained by the oil companies for years that higher octane = better performance. And if super costs about 5 cents a gallon more to make and they charge 20 cents at the pump you can see why.

Generally the super contains extra detergents -- so if you get carbon buildup on your piston crown from incomplete burn then hopefully the detergents will clean it up[:)]


DrDRFT -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 8:01:05 AM)

just a quick question regarding octane for 06' 1krr.

Here in Australia it is stated in the honda manual and on the sticker on the tank to run a minimuim of 95 octane!
I think in the US it is 91 octane?

My question is,

IF the only reason for higher octane is, so you can run more advanced timing without detination(thus making more power)? Then would this mean that the Australian Model is running more advanced comparied to US? If this is true? Would this mean that the Australian model is actually more powerful? If it is not? than why would Australians have to run 95 as to 91 in US?

I dont fully understand thats why I am hoping others could explain... Thanks


chainstretcher -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 5:15:42 PM)

Sorry dude but I don't think that's the case. There are at least three major ways to measure octane for the pump and depending on where you live in the world determines what formula they post on the pumps. In the U.S. we use the R+M/2. You guys may use the RON only -- you'd have to check the sticker on the pump to see. There is no way I know of to compare them unless they are using the same formula for the rating.

Here's an article that I've posted here before that explains it in non-engineering terms. I thinks it's an excellent read if you like that sort of stuff. Of course my 14 year old man-child tells me I'm a geek[8|]

quote:

The octane number assigned to a motor fuel has very little to do with the actual chemical "octanes" in the fuel and everything to do with how well the fuel resists detonation (which is directly related to the amount of energy (heat) required to get the fuel burning in the first place).

Therefore, it is possible to assign octane VALUES to fuel which contain no octanes whatsoever.


WHY DO WE CARE ABOUT OCTANE? WHAT IS PREIGNITION? WHAT IS DETONATION?
The octane value of a fuel is an empirical measure of its ability to resist detonation and, to a limited extent, preignition. Technically, octane ratings measure a fuel's ability to resist the spontaneous ignition of unburnt end-gases under controlled test conditions.

What is preignition?

Preignition occurs when the fuel/air mixture in a cylinder ignites before the spark plug fires.
It can be caused by burning contaminates (such as carbon, or a spark plug of the wrong heat range) in the cylinder or by extreme overheating.

What is detonation?

Detonation occurs when the flame-front in a cylinder does not proceed smoothly from the point of ignition (the spark plug) to the opposite side of the cylinder.
It refers to the spontaneous ignition of the entire charge in the cylinder. This ignition is often caused by the extreme pressure rise in the cylinder that occurs when the charge is first ignited (by the spark plug).



WHAT ABOUT FUELS?
There are six things to consider when comparing hydrocarbon fuels:

1. Volatility. In short, what's the fuel's propensity to vaporize. This effects the ability to easily mix the fuel with air and the fuel's tendency to vapor-lock. It also determines the pollution characteristics of the fuel where evaporative pollution is a concern.
2. Pre-ignition & knock resistance. Referred to as "Octane value." How much energy does it take to get the fuel burning - how much does it resist auto-ignition from compressive heat? Also, what is the rate of burn of the fuel (which affects the rate of pressure rise)?
3. Energy content. How much energy can be extracted from the fuel as a percentage of its volume or mass.
4. Heat of evaporation.
5. Chemical stability, neutrality, and cleanliness. What additives does the fuel contain to retard gum formation? Prevent icing? Prevent corrosion? Reduce deposits?
6. Safety
The first three factors are often confused and interrelated when, in fact, they measure three completely separate things. There is no natural collelation between them.

General rules:

Heavy fuels (diesel, jet): Low volatility, low knock resistance, high energy per volume
Light fuels (gasoline): High volatility, high knock resistance, low energy per volume

Note that gasoline, partially, makes up for its (relatively) low energy-per gallon by the fact that a gallon of gasoline weighs less (by about 15%) than a gallon of jet fuel.

Octane rating is in no way correlated with engine power or efficiency. There is more potential energy in a gallon of diesel fuel than a gallon of gasoline, yet the diesel fuel has a much lower octane value (more on that below).


HOW DO YOU DETERMINE OCTANE?
Ok, then, how is octane rating determined? First, you go out and get a suitable supply of the fuel which you wish to test. Then,

rrasco -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 5:24:58 PM)

damn stretch, i got to "The octane number..." then i scrolled down, picking up my jaw off the floor, i am trying to type this post. but thanks for the post slipper...undoubtably, you will get someone who thinks you are wrong, and thinks we all NEED to run atleast 91. my bike actually runs better on 87 than higher octanes. so no argument here.

DThompson -> RE: Octane, Compression Ratio, and Detonation. (9/18/2006 10:38:09 PM)

There will be someone in here that wants to argue. i have argued in 2 different threads about how you should run what they recomend. Everyone else seems to think its a matter of being cheap or something. I think its funny really. There are alot of people on this forum that "think they know, but they have no idea."

HAHA[;)]

rrasco -> RE: Octane, Compression Ratio, and Detonation. (9/19/2006 9:49:48 AM)

quote:

ORIGINAL: DThompson

There will be someone in here that wants to argue. i have argued in 2 different threads about how you should run what they recomend. Everyone else seems to think its a matter of being cheap or something. I think its funny really. There are alot of people on this forum that "think they know, but they have no idea."

HAHA[;)]


come on dthompson, you know everyone here is an expert, and there isnt possibly anything you could tell them that isnt either wrong or they already know. sometimes i wonder why people even ask questions if they wont accept an answer. like the n00bs who want to get a 1000rr as a first bike.

Thor -> RE: Octane, Compression Ratio, and Detonation. (9/20/2006 6:31:21 AM)

quote:

ORIGINAL: DrDRFT

just a quick question regarding octane for 06' 1krr.

Here in Australia it is stated in the honda manual and on the sticker on the tank to run a minimuim of 95 octane!
I think in the US it is 91 octane?

My question is,

IF the only reason for higher octane is, so you can run more advanced timing without detination(thus making more power)? Then would this mean that the Australian Model is running more advanced comparied to US? If this is true? Would this mean that the Australian model is actually more powerful? If it is not? than why would Australians have to run 95 as to 91 in US?

I dont fully understand thats why I am hoping others could explain... Thanks




Your bike is more powerful. U.S. spec bikes are retarded (timing)

x

DrDRFT -> RE: Octane, Compression Ratio, and Detonation. (9/20/2006 7:53:51 AM)

Where did you get the info that the US bikes are Retarded in timing as compared to the Australian version? I really want to find out bout this as I have done a few dyno runs and people are all saying that it is not possible for me to get those numbers??? But they are comparing to US model bikes. This is why I think that the two versions are running different maps and are producing different horsepower???

chainstretcher -> RE: Octane, Compression Ratio, and Detonation. (9/20/2006 8:06:09 AM)

Ya, you better have reputable sources for that bombshell. I've found absolutely zero evidence that U.S. 1kRRs are tuned differently from other countries.

  

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