== EGR vacuum switching valve == Despite the rather lengthy name the EGR-VSV (exhaust gas recirculation vacuum switching valve) does little more than allow vacuum to flow through the system depending on engine coolant temperature.
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that is between two carbon atoms, can be found in alkenes. Many types of double bonds exist between two different elements. For example, in a carbonyl group with a carbon atom and an oxygen atom. Other common double bonds are found in azo compounds (N=N), imines (C=N) and sulfoxides (S=O).
A covalent bond is a strong bond between two non-metal atoms. It consists of a shared pair of electrons. A covalent bond can be represented by a straight line or dot-and-cross diagram. Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.