Fuel Additives

- Iain Wakefield

Modern unleaded fuel is made up from a complex blend of refined hydrocarbons mixed with a small percentage (usually five percent in the UK) of ethanol. Fuel is sold by its octane rating and the Research Octane Numbers (RON) for commercially available petrol in this country is a minimum of 95 RON for ordinary unleaded and 97 RON for premium fuels such as BP Ultimate or Shell V-Power.

Unleaded fuel, or 98 RON four star, was withdrawn from UK forecourts on 1 January 2000. Owners of older cars were then left with a serious dilemma of either having to have harder valve seats fitted to their car’s cylinder head or dosing every tank of fuel with a lead-replacement additive such as Castrol Valvemaster.

Although refineries add MMT (methylcyclopentadienyl manganese tricarbonyl) to unleaded fuel to boost its octane level, lead replacement additives act as a substitute for the small amount of tetraethyl lead that was first added to motor spirit in the Twenties to stop engines suffering from pre-ignition or knocking.

Pre-ignition is when the compressed fuel/air mixture in the combustion chamber ignites before the ignition sparks and over time, severe knocking or pinking can seriously erode the valve seats and the engine components.