Back before Saturn was given the axe by GM, they were experimenting with something called HCCI* fuel technology. Essentially, the ignition was via compression like a diesel, but the engine ran on a super lean mixture of gasoline or E85. Details after the jump.

*Homogenous Charge Compression Ignition

Instead of glow plugs, the engine relied on spark plugs when cold or when under load, but the rest of the time would switch to compression ignition, leading to a cleaner burn and something like 15% better fuel economy. Because it burns gasoline rather than diesel it requires conventional exhaust treatments, rather than the expensive and extensive catalyst systems that diesel engines need.

In 2008 Saturn applied this technology to an Aura, converting a 2.2L Ecotec engine to the new system. Thanks to direct injection and VVT/VVL on both cams, it provided a very respectable 180hp and 170ft-lbs. The crossover from spark to compression ignition was managed by a sophisticated engine controller that relied on input from cylinder pressure sensors.

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This technology is highly impressive, and Saturn nearly had it refined to the point of being production ready but unfortunately the brand was killed off before that could happen. You can read more about it here and here.