Specific question about octane rating with respect to compression ratio

In an alt.home.repair thread, some were proposing 92 AKI gasoline for lawn equipment, where I questioned the logic, based on compression ratio alone.

AFAIK, the octane rating just means the gas reacts differently to a given compression ratio (and other detonation factors such as heat & timing).

For example, given whatever the compression ratio in my lawn equipment is, I get along just fine with 87 AKI California gasoline; yet some propose 92 AKI fuels instead.

My question: Of the engines that *require* high-octane fuels, is the (main) *reason* for that requirement the compression ratio? [We can ignore detergents for this discussion.]

Or are there other factors (ignoring heat & timing, which are a given) that increase the need for fuels rated at a higher anti-knock index?

Restated: Q: Is the *need* for premium directly in proportion to the compression ratio of the engine?

Reply to
Danny D
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Just to answer one part, I would expect premium fuel to make most lawn equipment harder to start. If the manufacturer doesn't specifically say you NEED to use premium there is rarely any benefit to using it. There are some cars that can use either because they have a knock sensor that will adjust the timing to compensate. When its possible to do that there is rarely much gained using premium... It's mainly a marking gimmick so they can test it with premium and get 305 hp instead of the 295 they get with regular gas. Might make it possible to best the 300 hp competitions engine so they have bragging rights.

Reply to
Ashton Crusher

it used to be thought that octane requirement was a function of compression ratio, but that thinking has changed. now, it more a function of cylinder head design. correspondingly, motors that require high octane do so because of antiquated head design.

Reply to
jim beam

To simplify your question:

Is the engine detonating using fuel with octane X? If the answer is no, there is NO point in using higher octane fuel.

Reply to
Alan Baker

Not exactly correct.

Compression ratio is the largest factor in when an engine will experience detonation, but not the only one.

But increasing compression ratio remains a valid method for extracting the most work from an engine, so the need for higher octane gas will remain wherever a manufacturer has chosen to pursue higher specific output.

Reply to
Alan Baker

respectfully, it's not that simple. biggest factor in detonation is cylinder head design. modern heads go out of their way to make sure all internal features are smoothed out, and that there are no combustion dead spots. this ensures a smooth flame front progression, and that two fronts aren't meeting somewhere they shouldn't. that's what causes detonation.

see above. there are standard production engines today running 15:1 compression with regular gasoline. according to the old school, that's impossible. good head design is the key.

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Reply to
jim beam

No. By far the biggest factor is compression ratio and the heating of the charge that it causes. Cylinder head design can certainly be used to even out any hot spots, but without compression, there would BE no hot spots.

Which "standard production engines" are running 15:1 compression on regular gasoline? And moreover, how does that negate what I've said.

Yes: by careful design of the cylinder head (and other things) you can raise the compression ratio, but only by a smallish fraction.

Reply to
Alan Baker

you're about 30 years out of date.

Reply to
jim beam

jim beam wrote in news:ko4vbt$5nn$ snipped-for-privacy@dont-email.me:

so you admit you pulled the 15 to 1 figure out of your butt, I thought so. KB

Reply to
Kevin Bottorff

I'm not aware of any, but he does have a valid point; I've seen production engines with 10+ to one compression and forced induction running on premium pump gas. Clearly this would have been impossible

30-40 years ago... head design, cam profiles, and sophisticated engine management systems all contribute to make it possible.

Back in the day you could have 10:1 compression OR supercharging and either one by itself would require premium; both together would require high-octane race fuel.

nate

Reply to
N8N

N8N wrote in news: snipped-for-privacy@z8g2000yqd.googlegroups.com:

there is a world of difference between 10 to 1 and 15 to 1. 15 to 1 is low compression diesel range. your not running that in a gas eng with out some real high octane or exotic fuel. nothing you can get at the pump, except E85. KB

Reply to
Kevin Bottorff

One way would be to have an open valve during the compression stroke such that the effective compression was significantly less than would be expected with "normal" valve timing and a high CR.

Reply to
.

Kevin Bottorff wrote in news:ko56ji$s87$ snipped-for-privacy@news.albasani.net:

There are some "hybrid car" engines that are running 13:1. I think the Prius is one of them. But those engines are meant to run in a very narrow load-band, so the engineers can afford to run that kind of compression.

Compression ratios are meant to be suitable for the engine's entire range of usage. Limit that range, and ultra-high compression ratios become more practical without exotic solutions or fuels.

Reply to
Tegger

So if compression ratio is such a minor factor, why not raise compression ratios to 20:1, 50:1, 100:1?

Reply to
Alan Baker

because you can't compress past liquefaction. up above the 20's, that's what you're getting.

btw, reading back, i mis-typed earlier - i meant 12:1, not 15:1. honda have a number of production engines with this ratio. race engines can go much higher.

Reply to
jim beam

WHAT????

Tell us all:

What component of the air/fuel mixture liquifies at a compression ratio of 20:1? Please provide the appropriate phase diagram for confirmation.

And thus you make my point.

  1. The growth in compression ratios has been relatively small for all that work on the head, etc.

  1. Race engines run higher compression ratios because they use much higher octane fuel.

Reply to
Alan Baker

i'm repeating this:

but i think you want to make a point here, so i'd like to see /your/ phase diagram and where the boundary actually lies.

you said: "the need for higher octane gas will remain wherever a manufacturer has chosen to pursue higher specific output" which is not true. f1 engines use 17:1 and gasoline that includes the octane of ordinary pump gas. they're using better physical design and understanding of both gas flows and flame front propagation to raise compression ratios [and thus outputs] /without/ having to resort to high octanes.

"all that work" simply comprises removal of dead spots and angular features that were previously left from machining and considered irrelevant. but maybe you meant "all that research".

no, the bottom end of the f1 fuel is the same as we buy on the road.

even if it were 98, it's still below aviation gas.

Reply to
jim beam

What part? Neither "liqui" nor "ratio" appear in the entire piece

Nope. This is your claim to support, so go for it.

Got proof of F1 compression ratio? And that they [sic] "use gasoline that includes the octane of ordinary pump gas"?

Research is work.

  1. "the bottom end" implies there is a "top end", right?
  2. 2009.

  1. The other large factor in detonation is TIME and at F1 RPMs, there's a lot less time for the conditions of detonation to develop.

Reply to
Alan Baker

you're trying to misinterpret. it says: "the fuel mixture is compressed into nearly-solid form before ignition. Cylinders run on the verge of hydraulic lock."

"hydraulics" means liquids, by definition. as does "near solid" for most substances, but that wouldn't suit your argument either.

you're the one saying it's wrong, so show me what i apparently don't know. i can't even find a phase diagram for air [since you mentioned it].

i've got the same resources as you. if i can find these numbers, so can you.

so say "research" then! unless you're deliberately being imprecise to give yourself more "opportunities" to pick nits. there's no additional "work" required to the head during manufacture.

didn't you read the article? or are you trying to avoid some of the other material it contained regarding properties of lower octane use?

so find a more recent one!

from what i can gather, knock is typically a sub-millisecond event. even at 18krpm, that's still an order of magnitude less time than the engine speed window.

Reply to
jim beam

And a dragster running nitro-methane which has its own oxygen for combustion bound in it is relevant to our discussion how? They could be running "on the verge", because the cylinder is nearly filled with liquid to begin with.

I don't believe you have "found these numbers". That's why I'm asking.

No. Because there's more to it that simply research.

Nope. YOU find one.

The knock itself? Yes.

The time it takes for the air/fuel mixture to develop the conditions which will lead to knock? Not so much.

Reply to
Alan Baker

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