Cetane Number and Cetane Index

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Courtesy of Energy Institute (EI)

This article summarises the findings of an Energy Institute (EI) sponsored Technical Development Project carried out by Cliff Lilley Consultancy, and outlines a review of the suitability of the current cetane index equation used in IP 380 Calculation of Cetane Index of middle distillates by the fourvariable equation (EN ISO 4264) (ASTM D 4737 procedure A**) for calculating Cetane Index (CI), to predict the Cetane Number (CN) as measured by IP 41 Determination of the ignition quality of diesel fuels – Cetane engine method (ASTM D 613).

The EI runs the IP Gasoline and Diesel Fuel Engine Correlation Scheme*** within which up to 22 laboratories worldwide determine CN by the IP 41 (ASTM D 613)method. Up to 10 laboratories also determine density by IP 365 and up to eight laboratories determine distillation recovery temperatures by IP 123 thereby enabling CI to be calculated according to IP 380. Since 2011, six UKPIA labs have provided the density and distillation data rather than the labs in the scheme. The monthly fuels used in the correlation scheme are selected to represent European production fuels with a wide range of CNs, thus providing data on fuels that meet the BS EN 590 specification minimum of 51 CN and the BS 2869 A2 specification minimum of 45 CN. These fuels are mainly low sulphur EN 590 road fuels (50 mg/kg or less), but some of them are high sulphur non-road dyed gas oil fuels (~1,000 mg/kg). Occasionally, a marine distillate or a high cetane component for blending in conventional diesel is included.

The UK commenced biodiesel blending up to a maximum content of 5% volume from April 2008. In 2009 the limit was raised to 7% volume. Since then levels have varied depending on the economics of biodiesel blending versus ethanol in mogas to comply with the Renewable Transport Fuels Obligation (RTFO). There has been a trend for increased fatty acid methyl ester (FAME) blending. Strictly speaking, biodiesel blends are outside the scope of IP 380. Analyses in recent years have suggested a possible shift in the correlation between CN and IP 380 CI. The CI results reflect the natural cetane quality of the base diesel fuel in the important 45 to 51 CN range with respect to EN 590 compliance. All of the fuels meet the scope of IP 380 in terms of not containing cetane improver or gas-to-liquids (GTL) components.

Data sets
As the occasional fuel can have an undue effect on the trend between CN and CI, it is better to base any comparisons on larger data sets. This review looks at two ranges of data – the year 2012 and the five years covering 2008–2012. The former study period provides information on more recent fuels, whilst the latter period provides smoother overall trends, as it is less sensitive to individual fuels. The five-year study also provides year-on-year information. For the sake of brevity, the full monthly data for the five-year period are not given, but these are available from the EI.

Outliers and unusual fuels
For each fuel property, the data was first checked for unusual individual laboratory results. Any outliers detected by the Hawkins’ test, in accordance with IP 367/07 Petroleum products – Determination and application of precision data in relation to methods of test (EN ISO 4259:2006), were excluded from further analysis. Such outliers may be the result of laboratory bias or transcription errors. The means of the data, excluding outliers, was then used to provide estimates of the ‘true’ values of CN, density and distillation recovery temperatures. The latter means were then used in IP 380 to derive the CI values.

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