vksgeneric (Regular) View all posts in this topic  
Sat Jan 20, 2007 1:41 am
Re: Long Mileage Test        

What you say is quite interesting. I can't see a reasonable explanation to your observations, so I am obviously missing something fundamental, something that is quite different in bike versus auto engines. Not something I've heard covered in school either. The only somewhat relevant thing that I can think of is that some (all?) hydraulically actuated valves never need adjustment by design, but it does not sound like your Protege had one of those. I am really no expert when it comes to these questions.

As for the valve stress, I imagine the valve mass does have some effect, I do not know how much, so I did not really take that into account. What I was thinking of is that each time a valve opens and closes, it comes in contact with the seat, and there is some friction involved. Not much, but over millions or billions of iterations that causes the bulk of the wear. During the compression stroke there is further stress on the valve that promotes wear. However that pressure is probably roughly the same for any gasoline engine because the compression ratios for all gasoline engines are similar, the autoignition temperature and the pressure at which gasoline self-ignites (I do not know the proper word for that concept) do not change, and the ratio of the valve area to the valve seat area is probably similar for most valves (not quite 100% sure about the last statement, but that's my understanding). A third factor is that the valves will spend less time in a _completely_ closed position, thus having less of a chance to dissipate the heat (thinking about it, now I can see what you said about the valve mass. The lighter the valve, the higher its acceleration can be, and the quicker it can get from fully open to fully closed, thus, allowing for more aggressive cam design, and being able to spend more time in full contact with the seat).

Ok, so I just completely demolished my own argument about why the valves on a bike may need to be adjusted significantly more often than those on most cars. The only other thing that I can think of is that if you have more ignitions per unit of time in a higher RPM engine, the temperature will be higher, thus further promoting wear. The aluminum block may be able to dissipate the extra heat just fine, but the valves are not quite as fortunate. Of course, that doesn't explain your statement about your bike having a really low red line, and still exhibiting the need for frequent adjustments.

Interesting stuff to think about. Of course, I am way over my head here. I don't know anything about engine design, just like figuring out how things work:)

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