Method and apparatus for calculating derived cetane numbers

A method of calculating a derived cetane number of a liquid hydrocarbon sample, the method comprising:

• (a) injecting a sample into a combustion chamber, the combustion chamber being held at a constant volume;
• (b) combusting the sample in the combustion chamber;
• (c) measuring a pressure in the constant volume combustion chamber as a function of time after the injection until combustion is completed;
• (d) obtaining a pressure versus time combustion profile of the sample injected into said constant volume combustion chamber, wherein the profile comprises a first region and a second region, the first region including the start of combustion, and the second region relating to a later time than the first region;
• (e) selecting a single data point from the second region of the combustion profile, said data point representing a combustion delay of the combustion profile;
• and
• (f) calculating the derived cetane number for the sample using the time value associated with said single data point;
• and wherein
• the method further comprises the step of performing a blockage check, the blockage check comprising:
• (i) opening the combustion chamber;
• (ii) measuring the pressure in the chamber over time until a pre-determined pressure is reached;
• (iii) determining the time, t1, between opening the chamber and the chamber reaching the predetermined pressure;
• (iv) comparing t1 to a pre-specified threshold time, tt;
• and
• (v) providing an indication if t1>tt.