Diesel fuel is generally a liquid fuel used in diesel engines, which ignites the fuel without sparking by compressing the inlet air mixture and then injecting the fuel. (Glow plugs, grid heaters, and block heaters help achieve high combustion temperatures during cold weather starting.) The most common type of diesel fuel is specific fractional distillation of fuel oil, but alternatives that are not derived from oil, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted. To distinguish these types, in some academic circles, petroleum-derived diesel fuel is increasingly referred to as oil-oil. Ultra Low Sulfur Diesel (ULSD) is the standard for determining substantially reduced sulfur diesel.