Volkswagen rigged their emissions tests using a device familiar to car engineer and University of Manitoba associate professor Ed Hohenberg. 

The automaker cheated using something known as a defeat device, which makes a car's engine run cleaner when undergoing emissions testing, he said.

"The engine software recognizes when the engine is being run through the EPA emission testing procedure."

It has a special calibration that meets the emissions regulations, said Hohenberg.

But when the engine does not recognize an emissions test is running, "it operates under a separate calibration and generates emissions that are in excess of what the EPA allows," he said.

Volkswagen could face fines as high as $18 billion for breaking emissions rules put in place by the U.S. Environmental Protection Agency and its counterpart, Environment Canada, and potentially billions more in class action lawsuits.

Volkswagen CEO Martin Winterkorn stepped down on Wednesday in response to the controversy. More resignations are expected.

Customer satisfaction might explain why the German automaker would put so much at risk to trick EPA tests, Hohenberg said. The EPA-compliant calibration "may result in poor throttle response, less torque, less horsepower, worse fuel economy," he said.

VW not alone in using a 'defeat device'

Hohenberg is familiar with defeat devices because they are not new. Several heavy-duty diesel engine manufacturers, including Caterpillar, Detroit Diesel and Volvo, were found guilty of using the devices in 1998 and settled with the EPA.

In the last 10 or 15 years, emissions regulators have become much more strict in allowable diesel particulate matter and nitrogen oxide emissions, he said.

Hohenberg said it's "certainly possible" Volkswagen is not the only diesel carmaker guilty of rigging engines to pass the stricter tests.

On Tuesday, Volkswagen's former CEO pledged $10 billion to update cars that did not meet EPA emission standards.