The "old octane question"(s)...sorry guys 4

[RobHughes]

Does the octane rating effect the temperature within the the combustion chamber (given the same c/r) and how much?

Does higher octane fuel burn "slower" than low octane fuel?

All things being equal as far as tune goes are there ever any measurable power gains to be had by different octane fuels?

I'm really sorry for these redundant and stupid questions and I think I know the answers, but would like people more knowledgeable than I to confirm or prove my thoughts wrong.

Thanks!

 

[GregLocock]

If you look at the SAE paper I summarised it shows in some detail exactly why there is a timelag between adding lead (or other substance) to the fuel, and the change in wear rate of the seat. Off the top of my head it takes about 10 hours for the wear rate to stabilise.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[icrman]

Valve seat wear (high temp corrosion) recession or what ever you want to call it, happens in non gasoline engines as well like you acknowlegded. The mechanism causing it is the same.
Now have you ever seen valve and seat temperature charts?
I think the term "micro welding" is kind of a misnomer when talking about the wear that occurs at the valve face-seat interface. The temperatures to cause that aren't present. And if they are then the so called lead film would be boiled off and scavenged. The cure for valve seat recession is to lower the temperature. Which as proven in a 1600 cc VW aircooled engine, TEL does effectively do. And which manufacuters of various large engines accomplish with extensive seat and valve cooling. In those SAE papers do they have reference the various observed temperatures?

 

[Lou Scannon]

Is not galling a form of welding? - I've seen screws gall at essentially room temperature.
But yes, temperature is definitely a factor in valve/seat wear, as GregLockock pointed out.
Now you've got me interested... what is the mechanism for TEL to lower valve seat temperatures?

 

[GregLocock]

Microwelding is how most wear happens, and also a lot of friction. It doesn't need high temperature, just (crudely) atoms in proximity to each other. It /is/ the correct expression and is the reason why rubber tires wear - the rubber welds to the road surface, which makes it grip, and then breaks off as the contact is broken.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[swall]

This "micro welding" is known as adhesive wear and can occur at any temperature. Metals in contact under pressure can "weld" at microscopic asperities.When the components subsequently move, this micro weld is broken, tearing out a small area of metal. The torn out chunk sticks to the other component and is termed adhesive metal transfer. Adhesive wear is also known as galling. The other form of wear is abrasive wear, which is due to the presence of harder particles which wear away or abrade the softer metal. In adhesive wear, some of the torn out particles become free and may also oxidize and these can cause abrasive wear.