Combustion engines are heat engines driven by the heat of a combustion process. Gas turbines are rotary engines that extract energy from a flow of combustion gas. Methanol is occasionally used to fuel internal combustion engines. Syngas is combustible and can be used as a fuel of internal combustion engines. Sulfates occur as microscopic particles (aerosols) resulting from fossil fuel and biomass combustion. Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. Motor oil is used for lubrication of internal combustion engines. For HICEV see List of hydrogen internal combustion engine vehicles. Trolleybuses are quieter than internal combustion engine vehicles. The spark plugs ignite a small pulse of lean mixture injected into the combustion chamber. Engine displacement is an important factor in the power of an internal combustion engine. Highly concentrated sources of oxygen promote rapid combustion. This action is similar to that of the cylinder in an internal combustion engine. The reaction is analogous to the combustion of methane. The engine is the four combustion chamber RD-170.