After years of moving away from larger engines in favor of smaller displacements, fewer cylinders, and in cases turbocharging, on-the-road emissions tests to begin in Europe next year are causing automakers to reverse direction.

Why? On-the-road emission tests expose real-world emissions better than do easygoing lab tests on dynamometers which have fluffed up CO2 scores and exaggerated mpg figures. With higher scrutiny now calling for more effective tests, the gig is reportedly now up for a direction to which automakers and regulators until now turned a blind eye.

According to an investigative report from Reuters, small engines are not going to meet these effectively stricter emissions regulations in Europe and automakers are bringing back larger engines.

“These mini-motors sailed through official lab tests conducted – until now – on rollers at unrealistically moderate temperatures and speeds,” wrote Reuters in its exclusive report. “Carmakers, regulators and green groups knew that real-world CO2 and nitrogen oxide (NOx) emissions were much higher, but the discrepancy remained unresolved. … “Carmakers have kept understandably quiet about the scale of the problem or how they plan to address it. But industry sources shared details of a retreat already underway.”

Dieselgate A Tipping Point

The Volkswagen diesel emissions cheating scandal begun last year has thrown regulators and automakers sideways on the emissions testing front, along with beginning what could be the end of diesel engines, and less reliance on smaller gasoline engines, as powertrain options in cars. Tougher emissions tests may kill diesel engines smaller than 1.5 liters and gasoline engines below about 1.2 liters, executives and analysts interviewed by Reuters predict. The scandal may also point to the end of simulated lab tests for emissions and the necessity of on the road testing.

Hitting European Union CO2 reduction targets in recent years has led to adopting smaller and lighter engines integrated with new technologies, especially turbochargers, to make up for lost power from smaller engines. Three-cylinder motors below one liter have become more popular, with a few, such as Fiat models, running on twin-cylinders.

Renault, General Motors’s Opel, and VW are preparing to enlarge or scrap some of their best-selling small car engines over the next three years, industry sources told Reuters. Other manufacturers are expected to follow, with both diesels and gasoline engines affected, according to Reuters.

Announcements this year by Volkswagen and Daimler on ambitious plug-in electrified vehicle campaigns, and their unveilings at the Paris Motor Show, may be driven by the pressure to meet stricter emissions rules. Volkswagen had invested heavily in engine downsizing and alternative powertrains such as turbocharging.

“The techniques we’ve used to reduce engine capacities will no longer allow us to meet emissions standards,” Alain Raposo, head of powertrain at the Renault-Nissan alliance, told Reuters at the Paris auto show. “We’re reaching the limits of downsizing.”

Renault, VW and GM’s Opel all declined to comment on specific engine plans with Reuters.

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Cars with small but powerful engines had sailed through lab tests in Europe in recent years. Since the U.S. Environmental Protection Agency’s announcement last year on Volkswagen diesel car emissions, testing standards have fallen under heavy scrutiny. Automakers knew they only had to pass simulated drive cycle tests on dynamometers. These tests were typically conducted at unrealistically moderate temperatures and speeds.

Carmakers, regulators, and environmental groups knew that real-world CO2 and nitrogen oxide (NOx) emissions were much higher, but the discrepancy remained unresolved, Reuters said.

Starting next year, new models in Europe will be subjected to on-the-road testing for NOx, with all cars required to comply by 2019. Independent testing in the wake of VW’s exposure last year as a U.S. diesel emissions cheat shed light on the limitations of simulated testing.

“They might be doing OK in the current European test cycle, but in the real world they are not performing,” said Pavan Potluri, an analyst with IHS Automotive.

“So there’s actually a bit of ‘upsizing’ going on, particularly in diesel.”

Global automakers have been hawking their new fuel efficient technologies for the past 10 years as emissions standards toughened, fuel prices temporarily spiked, and consumer interest grew in fuel efficient vehicles. New technologies brought forced air/fuel mixture into turbo engines, helping small engines perform as if they were larger and had more cylinders.

The Ford EcoBoost has been an example of this trend, along with Mazda’s SkyActiv engine. The SkyActiv system compresses the air-fuel mixture in the cylinders and harnesses more energy from every drop of fuel, Mazda said.

Fiat, Renault and Opel have the worst real NOx emissions among the newest “Euro 6” diesels, according to test data. They now “face the biggest burden” of compliance costs, brokerage Evercore ISI recently warned.

These findings are the inevitable result of on-the-road testing, said Thomas Weber, head of research and development at Mercedes, which has nothing below four cylinders in its vehicles.

“It becomes apparent that a small engine is not an advantage,” Weber told Reuters. “That’s why we didn’t jump on the three-cylinder engine trend.”

GM will not replace its current 1.2-litre diesel when the engines are updated on a new architecture arriving in 2019, sources told Reuters. The smallest engine in the range will be 25 to 30 percent larger.

Renault is planning enlargement of its 1.6 liter R9M diesel that will be nearly 10 percent large in size, which had replaced a 1.9-litre model in 2011, industry sources said. VW is replacing its 1.4 liter three-cylinder diesel with a four-cylinder 1.6, they said.

VW is counting on PEVs to emerge from the diesel cheating scandal and it strict global targets for emissions reductions. The German automaker announced in June that it plans to sell two-to-three million electric cars annually by 2025, which would be about a quarter of its current vehicle production.

“You can’t downsize beyond a certain point, so the focus is shifting to a combination of solutions,” said Sudeep Kaippalli, a Frost & Sullivan analyst who predicts a hybrids surge.

In the future, he said, “downsizing will mean you take a smaller engine and add an electric motor to it.”