selecting a solvent and a range of lubricant solvent ratios l s the low end will have a minimal amount of lubricant such as the 5 ratio in the example of fig2 the high end will be an optimized ratio depending on the viscosity of the lubricant and the amount of solvent needed to prevent adverse effects on the atomization of the test fluid into the device 10 in the example of fig2 the high end is 30 but less or more lubricant could be mixed it is expected that the high end of the ratio will result in a lubricant solvent mixture having a viscosity comparable to that of diesel fuel as long as test results are not affected by the mixture viscosity lubricant percentages greater than 30 may be possible or even desirable step 32 is mixing the lubricant composition with the solvent at a first ratio of the selected range of ratios step 33 is using the constant volume combustion device to measure ignition delay id at that ratio the sample size is small typically less than 100 ml if there remain ratios within the range to be tested step 34 is incrementing the ratio so that a different ratio is next tested an example of a suitable increment is an additional 5 of the lubricant composition after various lubricant solvent ratios have been tested step 35 is relating the ignition delay values associated with those ratios to a reactivity index alternatively as stated above steps 31 34 can be repeated for a second solvent so that.