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Which Octane petrol?

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Re: Which Octane petrol?

Postby Ducked » 26 May 2012, 14:29

sulavaca wrote:
Ducked wrote: concentrating on pre/auto-ignition is still misleading, since octane number is defined by detonation.


It shouldn't be misleading, as issues with a lower octane fuel are generally to do with auto-ignition, not necessarily pre-detonation as that can include reasons such as hot spots. I am referring to the pressure inside the cylinder of the OP's motorcycle and the potential for auto-ignition, which can create knock.

In your sentence you say that Octane is defined by detonation, and auto-ignition is a means of detonation. Detonation is how diesel engines work too, without a spark. These things are all related.
I think you are making things more complicated and confusing than they are.


I didn't make these things "complicated and confusing". They are complicated and confusing.

What makes them often impossible to discuss is that hardly anyone can be bothered to apply the terminology correctly, probably because its too complicated and confusing.

There is no such thing as "pre-detonation" since detonation never occurs at any time in a correctly operating petrol engine, and the term pre-detonation implies that its normal, but happening at the wrong time.

Detonation (AKA knock AKA pinking AKA pinging) and pre-ignition are different things, and the term "pre-detonation" rather obviously combines and confuses them.

You seem to be defining auto-ignition as compression ignition, caused by the compression in the cylinder, taking the diesel engine as a special case "pure" example.

Thats a restricted definition of auto-ignition, which is more generally defined as any ignition not initiated by the timed spark, and could include ignition by a hot-spot.

You also seem to be equating detonation with auto-ignition (or at least saying its caused by auto-ignition). That is misleading, since detonation usually occurs after the timed spark, due, among other things, to the compression and radiant heat from the normally propagating flame front.

Its not independent of the timed spark, so its not autonomous, so it isn't auto-ignition.

Of course the details probably don't matter to the OP, but words have meaning, technical terms especially so, and if they're going to be used they should be used correctly.
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Re: Which Octane petrol?

Postby sulavaca » 26 May 2012, 18:44

Ducked wrote:Of course the details probably don't matter to the OP, but words have meaning, technical terms especially so, and if they're going to be used they should be used correctly.


Indeed.
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Re: Which Octane petrol?

Postby Durbanite » 27 May 2012, 15:42

It's all good, I am learning some new terms ... hehehe
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