Diesel engines: your questions answered
The lower compression ratios of petrol engines have traditionally resulted in their production of considerably less NOx than diesels from combustion. Neither in the past have they had the diesel’s exhaust ash problem, requiring a particulate filter. But the situation could be about to change. The rise of more frugal, harder-working, smaller-capacity turbocharged petrol engines – with higher resultant compression ratios – is likely to mean they will also make more NOx. There are also strong indications that they may produce finer particulates that may be harmful and may need filters of their own. Research continues…
The most urgent problem, identified by London mayor Khan, appears to be the profusion of old-school diesels – notably, well-worn and decades-old taxis, trucks and delivery vans as well as passenger cars – on our roads. The mayor has already hit the headlines, and been rebuffed by the government, for proposing a £500m scrappage scheme that would pay diesel owners up to £3500 for ditching old diesel cars.
Before he left office, London’s previous mayor, Boris Johnson, also proposed a 2020 clean air directive that would penalise pre-EU6 (2014) diesels and pre-EU4 (2005) petrol cars by charging their drivers £12 on top of the regular daily congestion charge to enter London’s congestion zone. Latest indications are that Khan’s administration will keep the idea but bring it forward by a year. The writing seems to be on the wall for old diesels. Those who need older cars for inner London will do better owning petrol models.
What should the concerned private car owner do about diesel ownership? First, say the experts, consider the kind of driving you do. If you have a healthy early diesel and never drive in urban areas, it’s no crime to keep it. You won’t be contributing to urban pollution, the urgent problem.
Steve Gill, director of powertrain engineering at Ford of Europe, says: “Our concern is that customers will move away from diesels as a result of unbalanced coverage of the emissions situation. We’re not trying to force diesels on customers – they should buy what they want – but we’ve had to reposition some of our products and we’re having discussions now about where to put our money in the future.”
In Autocar’s opinion, diesel car ownership must not be off the agenda, because a bedrock of diesel sales is necessary to keep CO2 levels falling, the latest cars are clean and the industry needs the sales to fund the electrification that is coming. There is no doubt that the cleanest cars, with both their particulate and NOx outputs ‘trapped’, are EU6 models, produced from 2014. We’d choose one of those.