Lean burn for cruise condition

[DerekAE86]

I've been toying with the idea to setting up a Lambda offset for cruise condition to lean out the engine.

Something like if the RPM is within +/-200 rpm of the cruise zone with TPS delta is less than a certain amount for around 30s for example it turn on the Lambda offset table.

I know there are some manufactures that do this already. Apparently Mazda's Skyactiv leans out to 65:1 which is like 4.400 Lambda - But I assume the engine is specifically designed with this in mind.

Has anyone played with lean burn for low load conditions before? Is there any tips for determining what your engine will safely achieve?

I assume it'll be influnced by many factors; na/turbo, compression ratio, port/direct injection, how efficient your cooling is?

[dx4picco]

from my experience you struggle having a stable combustion at light loads with lambda much higher than 1.1-1.15.

Keep an eye on your EGTs aswell.

[DerekAE86]

9 minutes ago, dx4picco said:

you struggle having a stable combustion at light loads

I wonder if engines with higher compression can overcome this a little more? Or perhaps adjusting the timing along with the lambda target could help?

[dx4picco]

Most likely yes.

the only reason you would want to lean burn is for efficiency, so just changing lambda isn't the best way of doing.

burning lean will change burn rate and MBT timing aswell. you will need to tune on steady state dyno to find that

[Adamw]

On 10/13/2023 at 8:46 PM, DerekAE86 said:

I know there are some manufactures that do this already. Apparently Mazda's Skyactiv leans out to 65:1 which is like 4.400 Lambda - But I assume the engine is specifically designed with this in mind.

DI engines yes, but it is rare to see anything but 1.0 lambda on any modern port injected engine as NOX goes through the roof.  With DI they generally use a stratified charge type strategy - basically they have a non-homogenous mixture inside the combustion chamber, so the overall average air to fuel ratio is lean, but the mixture in a localised area around the spark plug is richer to support good combustion initialisation.

You can often go a little leaner than 1.0 with port injection, however for that to translate into better fuel economy you need to reduce any possibility of lean misfire to the bare minimum.  The ignition system needs to be powerful enough to support a large spark gap (higher voltage), injector atomisation, spray pattern and alignment etc needs to be near ideal, cylinder to cylinder air and fuel flow balance needs to be very close, and the ignition curve needs to be well optimised.