Achieving near-zero exhaust emission targets requires a systems approach. Engine manufacturers are focusing on ways to control engine operation to reduce engine out emissions as low as possible and reduce the burden on the catalysts.
Approaches aimed at reducing cold start emissions involve retarding the ignition timing so as to allow some hydrocarbons to pass through in the exhaust and light off the catalyst sooner. Variable valve timing (VVT) is being used to introduce some fraction of exhaust gas into the combustion process and reduce HC and NOx emissions. On clean diesel engines, Exhaust Gas Recirculation (EGR) is used to dilute intake air with some fraction of exhaust gas to lower the combustion temperatures resulting in lower engine out NOx emissions.
Direct injection of fuel into the cylinders rather than port injection has allowed better control of the air fuel ratio during combustion and resulted in better fuel utilization. Improved turbulence and mixing in the intake port of some low emission engines have resulted in a 24% fuel savings. Clean diesel engines have benefited significantly from common rail fuel injection which allows for electronically controlled injection at very high pressures. Through the use of pilot and retarded injection strategies or in combination with injection rate shaping clean diesels have achieved significant reduction in NOx over conventional diesel injection such as pipe-line or unit injection. Common rail and electronic injection control is very effective in carefully controlling post injection of fuel making it suitable for use with emission control devices such as particulate filters, NOx adsorbers and lean NOx catalysts requiring brief periods of fuel rich exhaust to facilitate regeneration of the catalyst or filter.