In modern diesel engines, the EGR gas is usually cooled with a heat exchanger to allow the introduction of a greater mass of recirculated gas. Uncooled EGR designs do however exist, often referred to as hot gas recirculation (HGR). Unlike spark-ignition engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, when there is otherwise a large excess of air) in controlling emissions. Exhaust recirculated back into the cylinder can increase engine wear as carbon particulates blow by the piston rings and into the oil.