What type of fuels do engine lubricants belong to in vehicles?

Engine lubricants in vehicles are classified as liquid fuels primarily because they are in a liquid state at ambient temperatures and are designed to reduce friction between moving parts, assisting in the overall functioning of the engine. These lubricants typically consist of a base oil, derived from crude oil or synthetic sources, combined with various additives that enhance performance characteristics such as viscosity, oxidation resistance, and thermal stability.

In contrast to solid fuels, which are typically in a solid state and require a different combustion process, or gaseous fuels, which exist in a vapor state and are primarily used in gas-powered engines for combustion processes, lubricants do not function as combustion fuels. They are not intended to burn as a source of energy; rather, they support the operation of the engine by minimizing wear and tear.

Combustible solids, while capable of burning to produce energy, do not apply to engine lubricants either, as lubricants do not serve the purpose of providing energy through combustion. Hence, identifying engine lubricants as liquid fuels is consistent with their physical state and their role within automotive engines.