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

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.