Quote:
Originally Posted by christofoo
By minimizing pumping loss by maximizing manifold pressure.
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Correct. One way to do this is to deliberately introduce more oxygen than is strictly needed into the cylinder, while keeping constant the amount of fuel being consumed per cycle. The resulting mixture is lean.
Why do we do this? Remember that varying the amount of oxygen required that we vary the intake manifold absolute pressure. Increase the absolute intake manifold pressure, and we increase the amount of oxygen available for combustion. This has the effect of lowering intake manifold vacuum, since we are lowering the difference in pressure between the intake manifold and the exhaust manifold (which we had previously agreed was equivalent to atmospheric pressure).
Lower intake manifold vacuum, and we lower the required amount of pumping work. Since we kept the amount of fuel being consumed a constant, per cycle, we also kept constant the amount of produced work. However, since we agreed that available work is the produced work minus the pumping work, we increased the amount of available work. Right?