A PCV valve, controlled by manifold pressure, regulates gas flow between the crankcase and the inlet manifold. With the engine off, the valve is closed. Air cannot enter the inlet manifold. This allows the engine to start. . To reduce exhaust emissions, some devices or systems control the combustion process itself, while others treat the resulting exhaust gases.
Most modern vehicles use a combination.
Combustion chamber design can affect the combustion process, and the level of emissions. Designs that increase gas flow rate and promote vaporisation, distribute fuel more evenly in the chamber. . The EGR valve lets some exhaust gases pass into the intake system. During combustion, these exhaust gases absorb heat. This lowers peak combustion temperatures, and reduces formation of oxides of nitrogen. .