Both my Range Rovers are converted to run on propane, and unlike on petrol the LPG system runs the engine very lean. If you use a special glass sparkplug (those old "Colortune ones!) you can see the flame is a lovely bright blue on propane and quite orange on petrol, with the mixtures set correctly.

Measured at the downpipe with the sniffer in the lambda sensor hole it shows no (or almost no) CO and a bit more NOx than you'd really want, but after the cat there's no measurable NOx either. The tailpipe emits basically just warm carbon dioxide and water vapour.