Discussion and questions related to the course Understand AFR
Can anyone shed some light on DI engine target AFR, I am just about to tune my first DI car, I have noticed the stock maps are well leaner than i would run normally run.
Any rough figures people using for boosted application?! Or should I just target a normal AFR i would on a non DI engine?!
Some DI engines use what's referred to as a stratified charge where the fuel distribution in the cylinder is focussed primarily around the spark plug to ensure good ignition. This allows a stratified charge DI engine to run with a leaner AFR than a conventional PI engine which provides a homogenous charge where the fuel is relatively equally dispersed through the combustion chamber. In this sort of engine I tend to use the factory targets AFR targets as a guide and test from there to see what the engine wants.
I was going to ask the same question when I found this topic. I just started working on a place where a lot of turbo BMW cars with DI come for a tune, and I noticed the same as Dmac, stock maps with a 0.90 lambda at full load.
I know we should run a richer mixture to prevent detonation or get a higher limit for knocking. So according to your experience on these DI engines, would a 0.85 lambda be okay? or for a safer tune I should just go with 0.80 at full load/high rpm as I do for PI?
Also, there are some others guys running higher boost level than stock on these BMW (stock engine) with Nitrous, so what AFR should I tune at to give them a safer tune and help their engines to not blowing up.
I have been tinkering with the med17 ecu on a vw.
It seems they have a internally modeled egt map. Which calculates what the egt should be. And if exceeds a certain "calculated" temp. It uses a different afr target table.
This table is around normal efi numbers. 0.8ish lambda.
I tend to be a little conservative with my DI tuning and unless I have a specific goal such as fuel economy in mind, I'll tend to tune with similar AFR targets to a PI engine. Regardless of DI or PI though, you can still test the effect of different AFR targets on the dyno. Often with DI you can find that leaner AFRs don't result in the engine being more prone to knock so it's not necessary to run richer in order to achieve MBT timing.
Keep in mind, the I6 BMW engines (N54/N55 and newer S55 or B58) aren't actually running making a very high specific output (torque per liter or BMEP). So they don't knock as hard, and don't need the combustion to be so retarded, and therefore don't need to run so rich to keep EGT's down. Look at the 2.0 BMW engines (N20 etc) and you'll see they can run pretty rich. It's not as rich as an Evo (4G63 for example), but it's still plenty rich.