To know the combustion properties, you may calculate the value of heat content ( calorific value).It will vary from bio-diesel to bio-diesel i,e from the plant oil to plant oils.You may use pour point improver for their suitability in the colder resion,also use some oxidation stability to mainten the quality upto certain time period.You may also study the properties of Viscosity as well as the spray pattern, so that low temperature combustion properties of bio-diesel can be well explained.