Small-displacement engines pollute more than larger engines in real-world testing, automakers have found, pushing them to shift towards larger displacement engines, reports Reuters.

After years of down-sizing, a lower limit on engine capacity appears to have been reached, roughly 1.5 litres in displacement.

“We’re reaching the limits of downsizing,” said Alain Raposo, head of powertrain for Renault-Nissan.

“The techniques we’ve used to reduce engine capacities will no longer allow us to meet emissions standards.”

Under specific testing conditions, very small-displacement turbocharged engines could be tuned to produce minimal emissions, but in real-world driving these diminutive engines are often over-worked, causing them to spew excessive emissions due to thermal-regulation challenges.

Sub-1.5-litre 3- and 4-cylinder engines often have to inject excess fuel into their combustion chambers to prevent overheating, but this results in higher unburned hydrocarbons, carbon monoxide, and smog-forming fine-particle emissions.

General Motors, VW, and several other car companies are expected to ditch their smallest displacement engines in favour of larger mills in the coming months and years as a result of the counter-intuitive emissions quagmire.