Specifications
The Regulations specify minimum limits of 45 for CN and 47 for CI. Diesel is required to meet at least one of these limits. These limits are unchanged since fuel quality was regulated in 1988.
Because of the limitations described below, CI should not be used as an optional method for expressing CN. However for the reasons discussed in the previous section this is a common practice in the industry.
The relationship between CN and CI depends on how the diesel is made. Diesel can be made using a number of processes and from a variety of feedstock. Processes include:
- straight-run fuel distillation,
- catalytical cracking,
- thermal cracking.
In addition CN can be significantly improved by adding a cetane improver. However the methods used to calculate CI do not take the effect of adding improvers into account.
Diesel made at Marsden Point and by most Australian refineries comes from a mixture of straight-run and hydrocracked cracked stocks.2 CN for diesel made from thermally cracked stocks can be lower than CI by up to 3.5.
When the diesel specification was initially drawn up in 1988, diesel was supplied mainly from Marsden Point. Cetane improver was not used to raise cetane number. At that time the difference between CI and CN for domestic diesel was about two. On the basis of the relationship between CN and CI for New Zealand diesel, the value of CI was set in regulation at 2 higher than the value of CN.
Diesel for the New Zealand domestic market is now obtained from a variety of sources and a variety of technologies are used to manufacture the product. For this reason this study was undertaken to investigate the relationship between CN and CI for diesel currently in the New Zealand market. In addition to reinforcing the existing FQM programme, these results can be used to indicate whether the limits for CN and CI and the test method chosen for CI determination should be reviewed.
During routine FQM sampling in the period 1 May 1997 to 30 May 1998 double samples of diesel were collected. The first can of each sample was tested in the normal way as part of the FQM programme. The second can of each sample was kept in storage until the end of the period, when all cans were sent to Singapore in one shipment for testing.
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