HIGHER volatility is required in an aircraft-engine fuel than in the ordinary internal-combustion-engine fuel, and provision must be made for easy starting, good distribution, freedom from vapor lock and decreased fire hazard. Each of these requirements requires the evaporation or non-evaporation of a certain percentage of the fuel under given temperature and pressure conditions that vary from one engine to another and with atmospheric-temperature changes. Changes in fuel volatility to suit the engine have no appreciable effect on performance, but small differences in the engine, particularly those that are found in the induction system, overshadow to a far greater degree any change that might occur in the volatility of the fuel.
The ideal fuel is one whose boiling point is sufficiently high to avoid vapor lock and minimize the fire hazard but which will evaporate approximately 20 per cent at the initial boiling-point. A more complete removal of the hump in the distillation curve without lowering the initial boiling-point will give better starting and warming-up characteristics.