Posted by Tom Fleming on July 25, 2010 at 04:28:14 from (206.193.253.243):
In Reply to: gasoline octane.. posted by mike paulson on July 24, 2010 at 12:15:57:
Owen, I agree that Pete23 gave a really good example. However, the tetraehtyl lead (TEL) was not originally added as lubricant for the valve seats, the higher octane it produced in gasoline did 2 things: by raising the octane it reduced pre-ignition (knocking) and it reduced the heat of the combustion. Both of which would batter and erode valves. That is the reason for the hardened valve seats in unleaded heads, so they could run on lower octane fuels (stellite valve seats have been around even longer than TEL, as it came into use around 1916). Stellite valve seats were also added to "high end" engines. Increased compression and higher RPM engines require higher octane fuel. TEL was at the time, the most cost effective way to boost octane rating.
In fact, the lead "cleaners" 1,2-dibromoethane and 1,2-dichloroethane are added with TEL to clean the lead oxide out of the combustion chambers and valves. So I agree, the residual lead oxide would have lubricated the valve seats, but once again, that was not the original intent of the TEL. Early on, before the "cleaners" were added, the lead would build up on the valves and other parts of the engine, and were actually worse than carbon.
I am not trying to bust anyone's chops here, but the chemistry behind this is very specific and simple. Lot's of urban legend on why TEL was added, and what it did. most of those explanations are not completely correct, or flat out wrong. They have been passed from person to person over the years.
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