Background
Prior to the removal of price control on the petroleum fuel distribution industry, fuel quality was controlled through the specifications being set and modified as necessary by agreements between Government, the oil industry and motor vehicle manufacturers.
In 1988 the Petroleum Sector Reform Act deregulated the petroleum distribution industry. Deregulation may have reduced incentives for suppliers to produce and market fuels of acceptable or consistent quality. To address this the Government introduced minimum quality specifications through regulations in order to provide for consumer, health and safety, and environmental protection, referred to in this report as the Regulations.1
The petroleum fuel quality monitoring (FQM) programme was set up in 1989 to monitor and enforce compliance with these specifications.
The Regulations specify a number of properties and limits for both petrol and diesel. For diesel, two of these properties are cetane number (CN) and cetane index (CI).
Diesel engines dispense with an externally generated ignition spark. The fuel must ignite spontaneously and with minimal delay (ignition lag) upon being injected into the hot, compressed air in the combustion chamber. Cetane number expresses the fuel's ignition quality, i.e. suitability for spontaneous self-ignition in a diesel engine. The higher the cetane number, the shorter the ignition delay period and the smaller the quantity of fuel in the combustion chamber when the fuel ignites. Consequently, higher cetane number fuels generally cause lower rates of pressure rise and permit improved control of combustion. This in turn improves engine efficiency and power output, and reduces hydrocarbon and NOX emissions. N-hexadecane (cetane) CH3(CH2)14CH3 which ignites very easily is assigned CN 100. Heptamethylnonane, with a low ignition tendency, carries the CN 15.
Cetane number is determined using a test engine. However because cetane engines are more expensive and require more highly skilled operators than octane engines, they are uncommon. For instance the only cetane engine in Australasia is in New South Wales.
For this reason cetane index, which is an approximation to CN based on an empirical relationship between density and volatility parameters, is widely used for routine monitoring of diesel ignition quality. It should be noted that, technically speaking, CI should not be relied upon exclusively as a measure of CN even though it is a common practice in the industry worldwide to rely upon CI as an indicator of diesel quality.
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