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Originally Posted by octinum
I remember reading that small VW engines with both turbochargers and direct injection consume more than larger NA engines with similar output. However, this comment might be for an older generation TFSI.
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They're supposed to be more fuel-efficient than a larger naturally-aspirated engine with port injection, but they're not as easy to service. No wonder in some countries Volkswagen only offers the 4-cyl naturally-aspirated 1.6L for models available elsewhere with the 1.0 and 1.4 TSI engines.
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Alfa Romeo JTS engines suffered from some soot buildup; long time users say equally maintained Twin Sparks (port injected) "keep" their performance longer than JTS because of this.
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Oily vapors from the crankcase vent would stick to the manifold and then trap some soot from the EGR in an engine with direct injection. With port-injection on the other hand, not only the fuel vaporizes and burns cleaner, but the oily vapors dillute more effectively with the air/fuel mixture than it would happen with charge air only.
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fuel vaporized in the intake manifold would also help a denser charge, lowering charge temperature
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It will depend on the expansion ratio of LPG while it vaporizes. So, even with a colder charge, it might not get so denser at all.
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The injectors could provide more fuel per cycle as well; as the vaporization would happen _after_ injection. I believe it would be beneficial in any case; better than vapor injection at least.
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Could absorb the same amount of heat from the ambient air with a fewer amount of fuel, which would be good for fuel-economy without too much harm to performance.
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I haven't found any references to supplemental injectors too. I have a feeling that the manufacturers never bothered with port injected ones when the trend is toward direct injection, and the extra cost for liquid injection tech would be harder to justify for an already mature market of port injected vapor phase kits.
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When I refer to supplemental, it's what your car has. The injectors dedicated to operate with LPG are supplemental to the original fuel system fitted to the engine. And sure the trend toward direct injection may seem to not justify a switch from vapour-phase to liquid-phase on port-injection setups, but it's also worth to notice how pointless are some port-injection CNG adaptations on vehicles originally fitted with direct injection, which still require an amount of the original liquid fuel to be used in order to prevent damages to the stock injectors which are directly exposed to the flame spread.