|
Lean burn works a little bit better than cooled EGR, but there's ways around it. The Skyactiv-X I think can run at a lambda of 2.2 with significant EGR dilution which means around 70% of the gas in the cylinder is not being burned. To get there, it needs a supercharger or the mix wouldn't burn. For Euro 6 and whatever the next CARB standard is, it probably will need urea injection to pass.
IIRC with VW's SFI, they were able to run at lambda >3 by using the central direct injector to inject fuel into the piston bowl at the last moment. Much simpler than what Mazda came up with, but has the NOx problem AND a particulate matter problem (which I guess can be solved with a particulate filter), and I'm guessing it leaves more unburned fuel making it less efficient.
Another way to get very lean burn without HCCI is Mahle "jet ignition" as they use on the Mercedes F1 cars, or injecting diesel to ignite a homogeneous gasoline mix, both proven to produce very high thermal efficiency.
Slightly worse would be multi-point laser ignition, or using multiple spark plugs. Actually, the Chevy small block has only 2 valves per head, so theoretically there's plenty of space for more spark plugs.
|