Researchers at MIT have demonstrated how ordinary spark-ignition car engines can, under certain driving conditions, move into a spark-free operating mode that is more fuel-efficient and cleaner. The new capability could be available in production models within a few years, improving fuel economy by several miles per gallon. Over time, it could cut oil demand in the U.S. by a million barrels a day.

Switching a spark-ignition (SI) engine to homogeneous charge compression ignition (HCCI) mode pushes up its fuel efficiency. In an HCCI engine, fuel and air are mixed together and injected into the cylinder. The piston compresses the mixture until spontaneous combustion occurs. The engine combines fuel-and-air premixing with spontaneous ignition. The result is that combustion occurs simultaneously at many locations throughout the combustion chamber.

Using the results of their engine tests as a guide, the researchers developed an inexpensive technique that should enable a single engine to run in SI mode but switch to HCCI mode whenever possible. A simple temperature sensor determines whether the upcoming cycle should be in SI or HCCI mode.

The researchers estimate that the increase in fuel efficiency would be a few miles per gallon, which may not seem like an impressive improvement, but, according to the team, if all cars in the US improved that much, it could be worth a million barrels of oil per day, which is impressive.

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