Diesel Engines

Part of an old diesel engine.

EES | Cases by Practice Area

Clean Air Act Diesel Engine Cases

United States v. Caterpillar, Inc.
United States v. Cummins Engine Company
United States v. Detroit Diesel Corporation
United States v. Mack Trucks, Incorporated
United States v. Navistar International Transportation Corporation
United States v. Renault Vehicules Industriels
United States. v. Volvo Truck Corporation

The emission of pollution – including carbon monoxide, oxides of nitrogen, and hydrocarbons – from cars and trucks is regulated by the Clean Air Act. The Environmental Protection Agency (EPA) promulgates rules implementing the requirements, including test procedures used to show compliance with emission limits before engines or vehicles can be sold.

With the advent of the use of on-board computers to control engine operations in the 1980s, it became possible for engine and vehicle manufacturers to circumvent EPA’s test procedures by programming the computer to operate an engine or vehicle one way on the EPA test, reducing certain pollutants, but another way in actual use. Manufacturers might do this to obtain better real-world fuel economy or for other reasons, but the changed operation can lead to increased pollution. These practices are illegal. The Clean Air Act and EPA’s regulations prohibit the use of “defeat devices” which reduce the effectiveness of the emission control system, except in certain narrow circumstances that did not apply here.

In the 1990s, EPA testing showed that the manufacturers of heavy-duty diesel engines were using computer programs to show compliance with emission limits on the EPA test but to change the fueling of the engines in actual use to reduce fuel consumption, but in way that increased emissions of oxides of nitrogen or “NOx.”

NOx contributes to the formation of ground level ozone (smog), soot, and dust. These pollutants can cause premature death, asthma attacks, bronchitis, and reduced lung functions and other breathing problems, especially in the elderly and children. NOx also causes acid rain, which causes damage to agricultural crops, pollutes drinking water, and causes acid deposition in water bodies.

EPA referred the matter to the Environmental Enforcement Section early in 1998. The companies involved were Caterpillar Inc., Cummins Engine Company, Detroit Diesel Corporation, Mack Trucks, Inc., Navistar International Transportation Corporation, Renault Vehicules Industriels, s.a. and Volvo Truck Corporation, representing 95 percent of the U.S. heavy duty diesel engine market.

Intensive and highly technical negotiations ensued, leading to the lodging of proposed Consent Decrees with the U.S. District Court for the District of Columbia in October 1998. In July 1999, the Court entered the Consent Decrees.


In addition to the payment of $83.4 million civil penalties – the largest in environmental enforcement history at the time – and performance of projects to offset the substantial excess emissions from the companies’ violations, the Consent Decrees require the companies to modify their engines to limit and eliminate the use of the defeat devices and thereby lower the emissions from new engines.


Updated May 14, 2015

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