Choosing afr

I didn't understand this module at all and especially the graph.

I might need a little more information on exactly where you need help in order to answer you properly. If you can be more specific I can give you better answers.

If you're meaning the graph of AFR vs power and emissions then this is simply showing how the engine power and fuel consumption are affected as we swing the AFR from very rich to very lean - We see that we make the most power somewhere around 12.5:1 and get the best fuel economy at around 15.4:1.

The next graph breaks up the operating range of the engine since we aren't going to target the same AFR everywhere in the load and rpm range. We're using the fuel as a way of controlling the combustion temperature, so when we are at full throttle and the engine is making maximum power, it's also producing a lot of heat and we use a richer AFR to combat this heat and keep the engine safe. Under cruise conditions we're more interested in economy and we aren't producing so much heat in the engine so we can use a leaner AFR.