Wouldn't the ratio of oxygen to nitrogen remain the same regardless of the volume?
The "Lean" aspect applies to the volume of air relative to the volume of fuel so although the volume of air is decreased the actual mixture of the air does not alter.
Cheers , Pete.
That's true before the mixture burns. Once ignited only the oxygen combines with the fuel to produce combustion. If there's left over O2 in during combustion (eg lean mixture) it will try to latch onto other molecules, such as nitrogen.
I don't know about separating it, but that's more or less what EGR systems do. They mix some exhaust into the intake stream, which replaces some oxygen-rich air in the cylinder. Less oxygen in the mix means reduced NOx emissions.
Actually, that isn't how EGR works. EGR works because the extra inert gasses in the chamber absorb energy from combustion and the combustion is cooler because of that(also gives more to expand from heat of combustion). Additionally, the extra gasses take up space in the manifold and reduce vacuum a lot like our WAI are doing.
The actual oxygen/fuel ratio never changes with EGR. All stock EGR cars still run 14.7:1 on freeway cruise which is where EGR is active.
[...] Additionally, the extra gasses take up space in the manifold and reduce vacuum a lot like our WAI are doing.
Uh... What? Warm air intakes work because they reduce the density of the air being pulled into the engine. Lower density means there's less oxygen (and nitrogen for that matter) per volume of air, so less fuel needs to be injected to reach a stoichiometric ratio. Manifold vacuum is unaffected by the temperature of the air.
Originally Posted by dkjones96
The actual oxygen/fuel ratio never changes with EGR.
No, the volume of air/fuel mixture being pulled into the cylinder during each intake stroke is reduced. The mixture is diluted by the exhaust gasses. Less air and fuel being burned means less heat released in the cylinder, lower combustion temperature and therefor reduced NOx emissions.