FRANKFURT — In 2014, as evidence mounted about the harmful effects of diesel exhaust on human health, scientists in an Albuquerque laboratory conducted an unusual experiment: Ten monkeys squatted in airtight chambers, watching cartoons for entertainment as they inhaled fumes from a diesel Volkswagen Beetle.

German automakers had financed the experiment in an attempt to prove that diesel vehicles with the latest technology were cleaner than the smoky models of old. But the American scientists conducting the test were unaware of one critical fact: The Beetle provided by Volkswagen had been rigged to produce pollution levels that were far less harmful in the lab than they were on the road.

The results were being deliberately manipulated.

The Albuquerque monkey research, which has not been previously reported, is a new dimension in a global emissions scandal that has already forced Volkswagen to plead guilty to federal fraud and conspiracy charges in the United States and to pay more than $26 billion in fines.

The company admitted to installing software in vehicles that enabled them to cheat on emissions tests. But legal proceedings and government records show that Volkswagen and other European automakers were also engaged in a prolonged, well-financed effort to produce academic research that they hoped would influence political debate and preserve tax privileges for diesel fuel.