In an interview with the Telegraph, he admitted that the trend of downsizing engine has come to an end as emissions tend to go up as engines get smaller.
He went on to explain, “Small diesels are just not economic. The percentage of diesel-powered Polos sold stands at 30 percent now, but as diesel gets more expensive (to meet real-world emissions tests), it will not be as popular." It was also stated in the article that Diess will not expect to see diesel capacity go below 1.6-litre in terms of engine capacity.
Engineers have been known to be battling overheating turbochargers in small capacity engines. To counter this problem, automakers tend to inject excess fuel to prevent these overheating issues which in turn will result in poorer emissions.