How to create a new fluid properties when simulating a dual-fuel engine using FIRE
 

Dear experts,
I am trying to simulate the combustion process inside the cylinder of a 4 stroke dual-fuel engine using ESE Diesel provided by AVL FIRE, in which, methane will be injected into the intake port to premixed with fresh air prior to supplying to the cylinder in the intake stroke. My simulation will be conducted from IVC to EVO. This means that, at the start of the compression process, the fluid inside the cylinder must be a mixture of methane and fresh air. Therefore, the Fluid properties in the simulation must also be a mixture of methane and air, not only AIR (as default setup).
My problem is I don't know how to create a new (or modify) Fluid properties to change the default setup (AIR) to my desired Fluid properties (methane-air mixture). So if you have any experience in this issue, please give me some advice and guidance.
Any help will be very helpful to me.
Thanks in advance!
Chien Pham Van