Re: Unleaded gas or use a lead additive for new en

Posted by Skyhawk Greg on August 30, 2006 at 13:00:05 from (70.228.121.60):

In Reply to: Unleaded gas or use a lead additive for new engine posted by scott#2 on August 30, 2006 at 05:23:13:

Scott,

Short answer - Run straight unleaded.

Long answer - Detonation has been a problem since gasoline engines first appeared. A spark would start the combustion process in a controlled fashion, but some of the fuel would start to decompose before the flamefront within the cylinder would reach it. A byproduct of the decomposition would be compounds that could burn without a spark. These are called auto-ignition compounds, and their presence would eventually result in an uncontrolled, and over-rapid burning of the remaining air and fuel mixture which we recognize as detonation. Working backwards, we can see that the decomposition is caused by the heat of compression before the spark, the residual heat in the cylinder and heads, and the heat produced by the ignition of the fuel itself. Several chemicals were tried in an effort to eliminate detonation. Aniline, iodine, selenium oxychloride, methylclopentadienyl manganese tricarbonly, and a list that goes on and on. Finally, in the 1920's, it was noticed that certain lead products had a very high natural knock resistance. Soon, tetra-ethyl lead was used as an easy way to increase the octane numbers, which allowed higher compressions, and as a result, more power output. A side benefit of using lead was that, after the lead was involved in the burning process, a lead oxide then coated the valve seating area. This helped to stop the valve seat recession which was another problem of the time when using the cast iron heads. The early engines also had no valve seals, and the oil would drip down the valve stem. This caused a little bit of smoke, but it lubricated the valve stem where it contacted the cast iron head. When valve seals came into use, the lead became more important for a small amount of lubrication between the valves and the head. Lead was here to stay, at least for a while. One of the drawbacks of using the lead though, was that it would tend to build up on the insides of the engines. To reduce this, ethylene dibromide and ethylene dichloride were added to act as scavengers and keep the engines cleaner. Anyway, this explains why the lead used to be there. Today, newer engines, and rebuilt engines, use hardened valve seats and alloy valve guides that do not require leaded fuels for valve guide lubrication. How the octane numbers have been increased without the lead is a whole different thread.

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