18 Oct 2019

High-octane petrol (HOP) study: making gasoline relevant for the future of transport

Gasoline is a complex mixture of hydrocarbons and other chemical compounds used as fuel for spark-ignition internal combustion engines (ICEs), primarily in passenger cars and other light-duty transportation vehicles (LDVs). Gasoline in the European Union (EU) has to meet more than a dozen individual specifications, for which the technical standards and analytical methods are specified under EN 228, the European Standard for unleaded petrol. The European Commission (EC) also sets limits on other components of finished gasoline.

Mandatory environmental regulations for several fuel properties were first introduced in 1998 (Directive 98/70/EC), and were revised in 2003 (Directive 2003/17/EC) and 2009 (Directive 2009/30/EC1). Other industry specifications limit the tendency and ability of the gasoline blend to foul, damage or corrode gasoline storage facilities as well as the components of vehicle combustion and exhaust systems. These specifications include gum deposition, oxidation stability, colour, NACE corrosion and phosphorous levels. Petroleum refineries are the main source of finished gasoline and blendstocks for oxygenate blending (BOBs).

Ethanol represents approximately 5 volume percent (vol%) of the finished gasoline consumed in the EU. To burn, liquid gasoline must be vaporised and mixed with oxygen (air). Since gasoline is a blend of hundreds of molecules with different characteristics, gasoline boils (vaporises) over a range of temperatures and must be blended in a way that vaporisation will occur over the entire range of engine operating temperatures. Specifications which measure and control the vaporisation performance of gasoline include:

  • Reid vapour pressure (RVP);
  • distillation;
  • drivability index; and
  • vapour-liquid ratio.

Apart from the volatility characteristics mentioned above, the other important fuel quality to be considered is ignition quality. The octane number of a fuel is a measure of its resistance to auto-ignition. Gasoline spark-ignited engines need a high-octane fuel to avoid knocking; this contrasts with diesel engines which rely on auto-ignition and therefore require a low-octane (or high cetane number) fuel. The octane number of a fuel is measured in a special test engine known as a CFR engine, which is a single-cylinder test engine with variable compression ratio dating from 1928.

Although the test has been progressively improved over the years, the basic engine configuration and test conditions remain the same. Tests in the early 1930s demonstrated that the knocking behaviour of fuels in vehicles of that era did not correlate with the measured Research Octane Number (RON), therefore a new, more severe Motor Octane Number (MON) was developed. Both methods are still in use today, although in modern passenger cars the relevance of MON is more questionable. A fuel’s octane number is determined by comparing and extrapolating its performance in the engine with blends of pure compounds: iso-octane, defined as 100 octane; and n-heptane, defined as having a zero octane number.

Gasoline octane, particularly the RON, is a critical consideration in the design of today’s engines which are optimised for particular octane numbers. A number of studies carried out over the years by the engine manufacturers as well as Concawe members have suggested that engines with higher compression ratios realise improvements in engine performance and efficiency, but they require gasoline with higher octane ratings. A more recent modelling and vehicle study carried out by Concawe has confirmed the results of previous studies and will be the subject of a separate article in the next Concawe Review.

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