Nitrogen Oxides (NOx) Emissions
![(EGR) Exhaust Gas Recirculating Valve](graphics/egr-valve-2.jpg)
The ECM controls the EGR or exhaust gas recirculation valve according to operating conditions. The valve prevents high combustion chamber temperatures from forming NOx oxides of nitrogen, a smog-forming emission. This harmful gas forms if the EGR valve is not opening correctly.
![EGR Exhaust Gas Recirculating Valve and NOX](graphics/performance-egr-nox.png)
The valve provides a regulated flow of inert exhaust gases into the intake manifold, lowering combustion chamber temperatures. Blending exhaust gas into the chamber keeps it at an acceptable range (below 2500° F), preventing nitrogen and oxygen from bonding. Excessive NOx results in pollution and smog.
(VVT) Variable Valve Timing
![](graphics/variable-valve-timing-1.jpg)
Most modern engines have variable valve timing. VVT systems time the exhaust and intake valves, improving low-end and high-end performance. For example, holding the exhaust valve open a little longer allows some exhaust into the combustion chamber during the intake stroke, preventing excessive NOx formation.
Three-way Catalytic Converter
![A catalytic converter cutaway.](graphics/catalytic-converter-cutaway.jpg)
A catalytic converter substrate consists of an aluminum oxide honeycomb coated with materials like platinum and palladium. They react and remove (CO) carbon monoxide and (HC) hydrocarbon from the exhaust stream.
![Three way catalytic converter](graphics/three-way-catalytic-converter.png)
A two-way catalytic converter works by oxidizing CO and HC to (CO2) carbon dioxide and water. A three-way catalytic converter also removes (NOx) oxides of nitrogen from the exhaust.