For more than 30 years, Nexum staff have carried out both theoretical and experimental studies on fuel volatility and flammability. This has resulted in a unique capability available to clients for the analysis of new fuel blends, new applications of existing fuels or safety studies arising from the use of ordinary commercial fuels by consumers.
Nexum has extensive experimental experience and modelling capability for studying the volatility and resulting flammability of gasolines, alcohol/gasoline blends, diesel fuel and aviation kerosene. This has included studies in automobile fuel tank explosion hazards and explosions in portable consumer gas containers used for lawn and garden equipment.
Fuel Volatility and Flammability
Fuels used in transportation, such as gasoline, diesel or aviation fuel are mixtures of hundreds of individual hydrocarbon compounds. The particular components present in any given fuel sample vary due to inherent differences in chemical composition arising from different crude oil sources and refinery processes. For many purposes these differences in actual chemical composition do not matter, so physical property data such as distillation curve, heating value, density and vapour pressure allow adequate prediction of their combustion characteristics in practical situations. Many of these field tests were, in fact, specifically devised in order to characterize fuel performance in particular applications.
However, under some conditions, more detailed information on fuel properties is needed. Some examples include cold starting of gasoline engines, high altitude relight of turbojets and flammability and explosion hazards in fuel tanks. The multi-component nature of most commercial fuels can make it difficult or even impossible to analyse and predict flammability using only the usual physical property tests. This is even more difficult when fuel blends such as gasoline/alcohol mixtures are concerned. These are non-ideal blends that behave differently than pure hydrocarbon mixtures despite having similar physical properties.
Although there are some specific cases for diesel and gas turbine fuels in which the detailed volatility and flammability characteristics may become important, these characteristics are absoutely critical to performance and safety in the case of spark ignition engines. The volatility of gasoline affects the performance of spark ignition engines of all sizes. The vapour pressure characteristics required for best performance as a motor fuel depend upon the ambient temperature, expected range of ambient pressure and storage conditions to which it will be subjected. As a result, the volatility of gasolines sold for use in automobiles varies throughout the year, depending upon location and season. Having established the volatility properties necessary for acceptable performance, these same properties then produce some safety issues in storage and handling.
One case of great practical concern is that of the flammability of the so-called “headspace vapours”, that is, the mixture of air and fuel vapour lying above the liquid in a fuel tank. This mixture can present an explosion hazard depending upon the ambient temperature and fuel properties.
The gasoline produced and sold by manufacturers produces conditions in the fuel tank headspace that are normally too rich to sustain combustion at most ambient temperatures. A rich mixture in the headspace means that a flame outside the fuel tank normally cannot propagate down the filler neck into the tank, thereby avoiding an explosion that would rupture the tank and result in a catastrophic fire.
Because the vapour pressure of all components in gasoline decreases as temperature drops, the mixture in a tank headspace does eventually fall into the flammable range, from the rich side, but only at very low temperatures. This feature has become more important in recent years as gasoline vapour pressure has been reduced for environmental reasons, and as alcohol has been added. Both these effects raise the ambient temperature at which the headspace vapour becomes flammable, Headspace vapours may then become flammable at typical winter ambient temperatures.
Nexum has carried out both government and industry studies aimed at determining the changes in explosion hazard that have arisen as new fuel blends have come onto the market. Most recently, Nexum has carried out both analytical and experimental projects on the flammability of “weathered” gasoline.