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The problem with lean burn is NOX emissions. Direct injection allows higher compression on lower quality fuel. It may also allow higher air-fuel ratios without NOX issues. Not sure about that but the higher compression accomplishes the same result without NOX issues.
Many manufacturers tried lean burn a couple of decades ago. The best comparison of the mileage difference would be the Federal and California versions of the Honda Civic VX.
Direct injection cools the in combustion temperature by transferring the heat to the incoming fuel charge for better atomization and higher compression. It may eventually evolve into a real HCCI design which will allow true lean burning with virtually no regulated emissions.
If (or when) they perfect a HCCI IC gasoline engine, the potential elimination of any after combustion treatment of exhaust emissions may actually be a consequence of that evolution of the IC engine. 60% conversion of fuel energy, combined with integrated pulse and glide of the power planet while the vehicle operates independently of the power plants function will produce the proverbial 100 MPG car.
regards
Mech
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