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Stoichiometric AFR lean definition / safe margin

Understanding AFR

Discussion and questions related to the course Understand AFR


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About the Stoichiometric AFR, we already heard from Andre in this module that 14.7 is the TEORICALLY perfect mixture with the fuel pump petrol.

Is correct to say that TEORICALLY a lean mixture (so leaner than 14,7) could be dangerous anyway we drive?

Or could be dangerous only to wide open throttle and could be safe in cruise operation?

Thanks

First just to be clear, the stoichiometric AFR is the AFR at which we should be achieving complete combustion. That's not to say it's the perfect mixture or one we will always want to run. In general for maximum power we will need to tune richer than stoichiometric and under these conditions a leaner than stoich AFR may be dangerous. At cruise however we can often find a small improvement in fuel economy by tuning leaner than stoich.

How about turbo engine car? I am using 3SGTE on MR2. Is it still safe to set target AFR of 1.05 during cruising on a close loop?

What's the manifold pressure at cruise?

Cruising manifold pressure is 40-140kPa within 2500-3500rpm

Lambda 1.05 is certainly OK for 40-60 kPa, but I would target richer to avoid knock above that. Probably taper to .0.88-0.90 at 100 kPA, and 0.78-.82 at 140