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Old 01-20-2012, 01:25 AM   #68 (permalink)
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Actually I believe the advent of continuously variable valve timing is also taking care of NOx. From Fine Tuning Engine Performance with Variable Valve Timing - TechConnect Online News Blog

"The use of an EGR valve was an early method for reducing the formation of NOx. Exhaust gas is reintroduced into the intake manifold through a valve, diluting the intake charge and effectively reducing combustion chamber temperatures and the formation of NOx. A side effect of introducing external EGR to reduce NOx is that it causes the hydrocarbon (HC) levels to increase.

Internal EGR

A more effective method of controlling emissions is to increase intake and exhaust valve overlap, a version of internal EGR. Valve overlap refers to the amount of time in the four-cycle engine event when both the intake and exhaust valves are open. A reversion occurs in the cylinder as the piston is moving down while both valves are open. Exhaust gas is drawn back into the cylinder, simulating an EGR function. Being able to control the length of this event can substantially lower NOx. HC levels are also reduced by re-burning the tail of the exhaust event that is rich in hydrocarbons. However, placing the camshafts in a permanently increased overlap position would affect idle and low rpm performance. The greater the overlap, the lower the intake manifold vacuum levels.

Fixed camshafts compromise between smooth idle, good low-rpm torque and high-rpm power. But variable camshaft timing accommodates the sometimes divergent needs for power, drivability, economy and emission control.

Variable valve timing uses a cam phaser to dynamically change valve timing events relative to piston timing by controlling the camshaft. This allows the position of the camshaft to be changed, dependent on need. At idle and low engine load, overlap is minimum, improving idle quality. At higher engine speed and load, overlap is increased, allowing emissions to decrease.

The cam phaser allows the PCM to change the relationship of the camshaft relative to the crankshaft, permitting better control over emissions and performance. "
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