The manifold is a straight, metal tube that directs the gas from the main gas valve to the burners. The burners are usually open tubes and each has a small nozzle inside known as “spuds” through which the manifold disperses the gas or propane; there can anywhere from 2-10 burners with correlating spuds. Intake manifold heaters, both electric and of the burner type, have also been used for cold-start assist of diesel engines with the goal of globally raising the intake air temperature. Electric manifold heaters consist of coil elements which through Joule heating increase the temperature of the intake air. In contrast, an exhaust manifold collects the exhaust gases from multiple cylinders into a smaller number of pipes – often down to one pipe. The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s).