# 4.9.1. Equations To Solve

    # Sierra > Fuego Procedure > Fuego Region
    Begin Solution Options

      # DEFINE WHAT EQUATIONS TO SOLVE
      Activate Equation Continuity
      Activate Equation X_Momentum
      Activate Equation Y_Momentum
      Activate Equation Turbulent_Kinetic_Energy

Equations are activated with the `Activate Equation X` command. Every Fuego simulation must have the continuity and momentum equations active, so these are optional to include in `Solution Options`.

The various turbulence models require different equations to be activated. These are noted in the list below, but Fuego will activate them automatically for you (and will warn and ignore any you specify that are not valid for your selected turbulence model).

Possible equations that can be activated are:

> - `Continuity` - Solves mass conservation for `pressure` - always active by default.

> - `X_Momentum` - Solves x-momentum conservation for `x_velocity` - always active by default.

> - `Y_Momentum` - Solves y-momentum conservation for `y_velocity` - always active by default.

> - `Z_Momentum` - Solves z-momentum conservation for `z_velocity` - always active by default in 3D, inactive in 2D.

> - `Enthalpy` - Solves enthalpy conservation for `enthalpy`, activates a post-processor utility to compute `temperature` after each solve.

> - `Species` - Solves species conservation equations for `mass_fraction`, the last species mass fraction is evaluated analytically to ensure the mass fractions sum to 1.

> - `Soot` - Solves transport of soot mass fraction for the EDC soot model.

> - `Nuclei` - Solves transport of soot nuclei number density for the EDC soot model. Will be activated automatically if `soot` is activated.

> - `EDC_Product` - Solves transport of the EDC product species, used with the EDC combustion model. Not commonly used.

> - `Mixture_Fraction` - Solves transport of the first fuel mixture fraction when using a flamelet combustion model

> - `Second_Mixture_Fraction` - Solves transport of the second fuel mixture fraction when using a flamelet combustion model

> - `Scalar_Variance` - Solves transport of scalar variance when using a flamelet combustion model

> - `Conserved_Enthalpy` - Solves transport of conserved enthalpy when using a flamelet combustion model

> - `Progress_Variable` - Solves transport of an arbitrary scalar quantity. This is sometimes used with a flamelet combustion model, but can also be used independently to transport an arbitrary scalar quantity.

> - `Turbulent_Kinetic_Energy` - Solves transport of turbulent kinetic energy for `turbulent_kinetic_energy`. Required for all turbulence models except `SMAG` and `DSMAG`. Will be activated automatically based on the selected turbulence model.

> - `Turbulent_Dissipation` - Solves transport of turbulent dissipation for `turbulent_dissipation`. Required for `KE`, `LRKE`, and `V2F`. Will be activated automatically based on the selected turbulence model.

> - `Turbulent_Frequency` - Solves transport of turbulent frequency for `turbulent_frequency`. Required for `KW`, `LRKW`, `SST`, `LRSST`, and `SSTDES`. Will be activated automatically based on the selected turbulence model.

> - `Turbulent_V2` - Solves transport of turbulent frequency for `turbulent_v2`. Required for `V2F`. Will be activated automatically based on the selected turbulence model.

> - `Turbulent_Helmholtz_Function` - Solves transport of turbulent frequency for `turbulent_f`. Required for `V2F`. Will be activated automatically based on the selected turbulence model.

> - `Volume_Of_Fluid` - Solves for a volume fraction of liquid, `volume_of_fluid`, used for multi-phase simulations.

> - `X_Solid_Momentum` - Solves for conservation of solid momentum in the x-direction for the granular phase model.

> - `Y_Solid_Momentum` - Solves for conservation of solid momentum in the y-direction for the granular phase model.

> - `Z_Solid_Momentum` - Solves for conservation of solid momentum in the z-direction for the granular phase model.

> - `Solid_Volume_Fraction` - Solves for conservation of solid volume fraction for the granular phase model.