If engines ran leaner — burnt more air and less fuel — they would produce significantly smaller NOx emissions.
This doesn't sound right to me. I'm pretty sure engines can run lambda>1 right now, and this gives better fuel economy, but it is not allowed due to emissions requirements. lambda>1 might give less inherent NOx, but lambda=1 is required for catalytic converters to work and the decrease in NOx from higher lambda is not enough to outweigh the loss of the catalytic converter.
This doesn't sound right to me. I'm pretty sure engines can run lambda>1 right now, and this gives better fuel economy, but it is not allowed due to emissions requirements. lambda>1 might give less inherent NOx, but lambda=1 is required for catalytic converters to work and the decrease in NOx from higher lambda is not enough to outweigh the loss of the catalytic converter.