Cummins Inc. has agreed to pay an over $1.67 billion penalty to settle claims by regulators that the engine manufacturer unlawfully altered hundreds of thousands of pickup truck engines to bypass emissions tests. According to the U.S. Justice Department, which announced the agreement in principle Thursday, Cummins' alleged actions violated the Clear Air Act — a federal law that requires car and engine manufacturers to comply with emission limits. The $1.675 billion fine would be the largest civil penalty the Justice Department has secured under the Clear Air Act to date and second largest environmental penalty ever secured.