Assessment of Car and Fuel Factors Affecting Vapour Locking Tendency

Abstract

‘Vapour lock’ is a term used to describe an adverse change in the performance of the fuel system resulting from the evolution of vapour from the fuel. It may occur anywhere in the fuel system although petrol pump vapour lock is generally the most serious. The most common form of this trouble is that which arises when attempts are made to re-start a car which has been stationary for a few minutes, after having been previously driven at sustained high speed. The mechanism of vapour lock is shown to depend both upon the amount of vapour formed and the ability of the fuel system to handle vapour, but the greatest improvements are likely to be made in reducing the amount of fuel vapour formed. The effects of temperature, pressure, and fuel volatility on vapour formation are discussed, including the conflicting volatility requirements for freedom from vapour lock on the one hand, and good starting and warm-up characteristics on the other. It is shown that distillation and vapour pressure characteristics are not suitable criteria for quality control purposes, and that only by the laboratory determination of vapour/liquid ratio (V/L) versus temperature relations can a proper estimate of fuel vapour locking tendency be obtained. Laboratory apparatus for determining of V/L ratios is described, and results for some fuel combinations are presented. The various driving conditions encountered in service are discussed in relation to their effects upon vapour locking tendency. Open-road driving at full throttle followed by a stationary ‘soaking period’, during which the under-bonnet temperatures rise to maximum values, is a very severe condition, but hill climbing or city traffic conditions may be just as severe. A full throttle soak-test method for the determination of the volatility tolerance of cars on the road, making use of a double series of reference fuels, is described; it is shown how the results may be interpreted in terms of the ambient temperature for freedom from vapour lock using any given fuel. A description is also given of a simplified road technique using a single series of reference fuels. This test technique requires no vehicle instrumentation, and a predetermined V/L ratio framework may be used to interpret the results.