Lean burn engine development is still going on.

Here’s is an article from today on plasma spark plugs to ignite lean air to fuel ratios

https://www.wired.com/story/is-lightning-fast-plasma-the-key-to-a-cleaner-car-engine/

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The efficiency of a combustion engine is deeply dependent on how it's being used. For example, engines tend to be least efficient at low speeds and low power, a scenario that might be encountered when driving through a city. So Sjöberg and the Transient Plasma team tested the ignition system in the engine across a range of operating modes that loosely correlated with different scenarios, like driving on the highway. During the tests at Sandia, Transient Plasma System’s plug demonstrated that it could offer up to a 20 percent improvement in fuel efficiency in these driving modes, relative to the performance of a commercial combustion engine with a conventional spark plug.

Singleton says the results from the Sandia tests soon attracted the attention of the auto industry. For the past few years, Transient Plasma Systems has been working with several undisclosed car manufacturers to test the system with the companies’ engines. He says he is optimistic that the first cars to use the system could be on the road within the next five years.

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