LPG needs higher compression than gasoline for maximum efficiency. It can run perfectly fine as is, but we leave a whole lot of economy and power on the table by using a dual-fuel LPG system.
Though with regular gasoline engines now hitting 13:1 to 14:1 compression ratios, the point is nearly moot. An ideal situation would be using LPG with liquid injection (the more common ones still spray gaseous propane) via direct injection... but that would leave more injectors in the chamber than we probably have space for.
What's funny is Mazda is running their SkyActiv Diesel at an ultra-low 14:1. I wonder how hard it would be to merge the SkyActiv Diesel and Gas motors...
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I just drove the 2011 Cruze diesel last month for a review article. Good engine. Decent torque (though nowhere near what the Ford PSA TDCi and VW TDi units have), decent top end. Our test unit smoked a bit... it's likely the previous tester put in a contaminated batch of fuel, because it cleared up after two or three days.
Fuel economy averaged between 30-38 mpg on the highway. This unit used a six-speed automatic with a locking converter. The big issue is that sixth gear is so long, I had a hard time getting it to lock up on the highway below 70 mph... Most of the time I was slurring around at around 1500 rpm with the torque converter happily bleeding away power. It should do near-50 mpg easily, but getting higher would require a very long trip and more aggressive TC locking in 5th gear.
Ford and VW use a superior dual-clutch unit, but a traditional torque converter is smoother in stop-and-go, which is why Chevrolet went with that.
As for the "sporty" part, the Cruze has a great chassis, and the extra diesel power does give it some oomph over the naturally aspirated Cruze, but the long-geared, slow-shifting and indecisive automatic, plus that heavy iron-block up front make it less thrilling to drive than a small Ford or VW diesel sedan.