For much of the past decade, European car makers have been rolling out smaller engines, often with fewer cylinders, to meet tightening emissions regulations. These engines are fitted with turbochargers to boost power to levels at or beyond the engines they replace.
In the wake of the Volkswagen's Dieselgate affair, the EU will begin subjecting new vehicles to more realistic real-world NOx (oxides of nitrogen) tests from next year. All vehicles sold in Europe will need to be compliant by 2019.
A new global standard for real-world testing of fuel economy and CO2 output will reportedly come into force from 2021.
Above: Real-world emissions testing rig.
Volkswagen was able to cheat its way past regulators by using a defeat device to detect conditions typical of a laboratory emissions test, and then enabling more stringent emissions controls for the duration of the test.
Current small capacity turbo engines from other automakers are able to legally pass lab emissions tests because the tests require the cars to be driven at abnormally low speeds and at relatively moderate temperatures.
In the real world, though, these small engines often have hot and overworked turbos, and when in this state they produce far less favourable results.
According to Reuters, recent real-world testing has shown that these small diesel engines can produce up to 15 times the permitted levels of NOx when their turbochargers start running hot.
In similar situations, downsized turbocharged petrol engines guzzle more gas, and emit more fine particulate matter and carbon monoxide than they are meant to.
Above: Renault Clio.
At this year's Paris motor show, Alain Raposo, head of powertrain development at Renault-Nissan, confirmed the gist of the report. He told the news wire: "The techniques we've used to reduce engine capacities will no longer allow us to meet emissions standards. We're reaching the limits of downsizing."
Although no automakers have gone on record about their future engine plans, it's understood that some European car makers, including Renault, GM and Volkswagen, are planning to scrap or upsize their smallest petrol and diesel engines.
While European car makers begin upsizing their engines, it's said that vehicles sold primarily in North America and China still have room to benefit from downsizing, as engine capacities in those markets are generally larger.