Volkswagen has denied claims it built defeat devices into a second engine model, hours before chancellor Angela Merkel arrives in Frankfurt to open the city's international motor show.
Documents leaked to German public television suggest the EA288 motor - the heart of hundreds of thousands of VW group cars since 2012 - uses so-called "cycle-recognition" software to detect if exhaust tests are being carried out.
The EA288 motor, classified under the Euro-6 norm, is built into the Volkswagen Golf, Tiguan and Passat vehicles as well as models of the Audi, Skoda and Seat brands.
German broadcaster SWR said the documents show EA288 engine software adjusts the amount of Adblue ammonia solution released - to neutralise harmful exhaust gases - if the car's wheels - but not the steering wheel - are moving.
The documents about the current European vehicle test cycle - acronym NEFZ - refer to "use and recognition of the NEFZ ... to trigger switching" of the exhaust process depending on the stretch the vehicles is driving.
The documents also reportedly contain a "description of the dosing strategy" of ammonia solution (AdBlue) "during and outside the cycle".
Volkswagen declined to comment on what it said were confidential documents but insisted: "Vehicles with the diesel motor EA288 ... contain no cycle recognition."
Car experts consulted by the broadcaster disagree, saying the processes described in the EA288 documents bear similarities to processes discovered by US investigators, triggering the 2015 VW diesel scandal that has cost the Wolfsburg group more than €30 billion to date.
More than 11 million cars with the EA189 motor only met emissions standards on the test ramp and emitted 40 times more harmful nitrogen dioxide than legally permitted during real-world driving.
Axel Friedrich, an exhaust expert involved in uncovering the first diesel scandal, said the documents suggest vehicles with the EA288 motor also "recognise whether it is on the test ramp, and then sufficient AdBlue is injected, but in normal use on the street much less is used".
German legal expert Andreas Baier told the SWR broadcaster the documents shifted the diesel scandal to a "new level (and) means for us that Volkswagen tricked customers, too, with the successor model" to the EA 189 motor examined by US environmental authorities.
The revelations come at an awkward time for VW, as it presents a new range of electric cars at the Frankfurt IAA trade show including its highly-anticipated ID3 model.
VW chief executive Herbert Diess - under investigation by German prosecutors as part of the diesel scandal - said at the presentation of the ID3: "This is the car that is expected of us."
Environmental groups are picketing the show, demanding cleaner vehicles and an inner-city ban on SUVs. Last Friday the driver of an SUV mounted a pavement in Berlin's city centre and killed four people, including a three year-old boy.
Around 20,000 protestors are expected on Saturday, the trade fair's first public day. In addition, trade fair organisers have organised citizen discussions between car executives, politicians, citizens and environmental NGOs.
Two Fridays for Future members have been invited to a podium discussion with the head of Germany's car lobby organisation (VDA).
Leading Volkswagen officials have accused environmental protestors of spreading panic about the industry.
"In the public discussion at present one gets the impression that that car is nothing more than a singular risk," said Bernd Osterloh, VW works council chief, to the Handelsblatt business daily. "On the danger scale it lies somewhere between Ebola and North Korean missiles."