In recent years， engine manufacturers have been required to reduce the levels of nitrogen oxides (NOx) in diesel engine exhaust to meet Tier 3 emission standards required by the Environmental Protection Agency (EPA) One of the reasons for this mandate by the EPA is because NOx has been associated with respiratory disease and This requirement is accomplished by changes in engine designs that include retarded timing， raised piston rings， selective catalytic reduction and the use of exhaust gas recirculating (EGR)New engine designs use EGR to control NOx emissions by recirculating exhaust gases back into the combustion chamber to be burned a second time， thereby reducing emissions associated with health The amount of exhaust gas introduced into the combustion chamber will displace oxygen， creating cooler In doing this， many of the exhaust contaminants end up in the engine lubricating Diesel engine oils are now exposed to a higher level of contamination that can degrade the oil and damage engine There is concern that exhaust gas recirculation can have a detrimental effect on engine durability and its effects on the Oils exposed to the EGR environment show an increase in soot content， acid number (AN) and viscosity， while the engine and oil are both exposed to corrosive/acidic gases and particle
