The use of lead-free gasoline in conventional passenger car engines poses some problems that are discussed in this paper. Under heavy-duty operation, severe exhaust valve seat wear may occur. This will eventually result in one or more valves remaining open with extremely high exhaust emissions.
The combustion chamber deposits formed in the absence of lead are typically more carbonaceous. These deposits have a higher heat capacity than lead deposits and the result, after extended mileage, is higher octane number requirements for the engines operated on nonleaded gasoline.
The use of aromatic blending stocks to increase the octane number of nonleaded fuels to approach the octane quality of today's leaded gasolines increases undesirable exhaust emissions. The amounts of phenol, benzaldehyde, and total aromatic aldehydes in the exhaust gas are directly proportional to the aromaticity of the fuel. Polynuclear aromatics in the exhaust are also proportionally increased with an increase in aromaticity. In addition, the photochemical reactivity of automobile emissions may be increased by as much as 38%.
