Diesel cleans up

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The anti-diesel headlines of recent years often overlook the progress that has been made in cleaning up oil-burners’ acts. Craig Thomas looks at the realistic prospects for the fuel in our Road to Zero future.

Engineers have been busy in recent years meeting the challenges of the current Euro 6 standard

Engineers have been busy in recent years meeting the challenges of the current Euro 6 standard

The market for diesel cars – and diesel as a fuel – all changed in September 2015. Prior to that, diesel-fuelled vehicles held sway in monthly sales figures, with around 50% of the market: today, they’re hovering just under 30%. Supposedly, that’s largely thanks to Volkswagen.

Except, of course, there was something rotten in the state of diesel long before the United States Environmental Protection Agency (EPA) slapped a violation notice of on the Volkswagen Group, after the discovery of ‘defeat devices’ that enabled diesel-engined cars to cheat on emissions tests. There were already warnings about diesel NOx and particulate emissions, and their effect on air quality (especially in cities), but they had been largely overlooked or ignored. Dieselgate opened the floodgates, though, and the public and environmental campaigners demanded change.

Cleaner diesels

Plans were already in place for improvements to emissions testing before the Volkswagen revelations, but these became newsworthy and the introduction of Worldwide Harmonised Light Vehicle Test Procedure (WLTP) and Real Driving Emissions (RDE) has focused the minds of everyone on the automotive industry – and its customers in the fleet sector.

So does diesel have a future, or are the improvements in smaller, turbocharged petrol engines going to be sufficient to tide us over until hybrids have usable electric-only ranges or – in the next decade or so – we can switch to fully electric or hydrogen fuel cell vehicles?

Reports of diesel’s demise are certainly premature, if you speak to engineers involved in developing new powertrains. They’ve been busy in recent years meeting the challenges of the current Euro 6 standard – which will continue as additional WLTP and RDE regulations come into force. But the feeling in the engineering community seems to be that the new diesel standards are already a major improvement, so the next stage is targeting other pollutants.

Jon Andersson, global technical expert for emissions measurements and standards at Ricardo – a global strategic engineering and environmental consultancy – told us: “WLTP has created a load of extra challenges. Some of those are around things like evaporative emissions, but there are also new pollutants coming. In addition to the historical pollutants – hydrocarbons, CO, particulate matter, particulate number, nitrogen oxide, NOx – in the future we will see a whole series of new species that have to be controlled.

“It’s not entirely certain what those will be, but they will include, for example, ammonia and nitrogen dioxide, and partly oxidised species such as formaldehyde and others. We know that there’s going to be an extension to the sweep of emissions that we have to control and that, in itself, is a whole series of new challenges that we’re getting to grips with at the moment.

“On the diesel side, there’s much less focus on the engine. There are still some tweaks being made to engine efficiency, but it’s much more focused on after-treatment. I think what we’re seeing from the OEMs is a move towards a really robust lean NOx trap and a selective catalytic reduction system (SCR) filter solution, with sufficient SCR volumes to allow good NOx control at high speed, low speed and even the most progressive low-temperature urban conditions.

“My guess is that with the after-treatment solutions on diesel and gasoline that we will have at the end of the Euro 6d, before we transition to the full WLTP, we probably won’t have to do too much with diesel after-treatment, because we will already have virtually all the capabilities in place.”

The hurry for hybrids?

The progress of diesel technology is clearly good news, but the concern is that it will do little to persuade those who have jettisoned diesel in favour of petrol – which could prove problematic for the carmakers who have based their long-term emissions reduction plans on the basis of diesel playing a more significant role.

Ken Pendlebury, Ricardo’s director for technical consulting, outlined this secondary effect, saying: “To meet with the next generation of CO2 legislation, a lot of the OEMs in Europe were relying on 50% penetration of diesel engines and even a small shift back to gasoline means a fairly radical rethink of methods to improve the CO2 of gasoline engines. A consequence is that companies such as Volvo are saying that all their powertrains will be electrified moving forward and this is, to a certain extent, a dash to hybridisation.”

It’s clear that what the Society of Motor Manufacturers and Traders refers to as alternatively fuelled vehicles (AFVs) – which incorporates hybrids, plug-in hybrids (PHEVs), battery electric vehicles (EVs) and hydrogen fuel cell vehicles (FCEVs) – are increasing in popularity, with fleets buying in (literally) to the new technologies. They still only account for 8% of the overall car market, so there’s a long way to go before the government’s 2040 target for the ending of petrol and diesel vehicle sales.

The increase in AFVs will undoubtedly be helped by the numerous new PHEVs that carmakers have launched and will continue to launch in the next few years. There could well be a hiccup in sales as the switchover from old NEDC emissions testing figures to WLTP figures makes them less attractive than they previously were – official economy numbers in excess of 130mpg will be a thing of the past – but we could see developments in battery capacity to give them longer electric-only ranges that will add to their appeal.

Major manufacturers such as Volkswagen, Mercedes-Benz and BMW are also planning to introduce new EV models and sub-brands to their line-ups in the next two or three years that will help popularise battery cars, especially when they have real-world ranges of 250-300 miles.

In the meantime, however, there’s still a place for diesel, as Andy Eastlake, managing director of the Low Carbon Vehicle Partnership explained: “If you are doing 40,000 miles a year in a car, a diesel is still probably the right thing to choose, from a low carbon perspective and efficiency.

“Some of the background discussions in getting the government’s Road to Zero strategy out were around how we acknowledge the role of diesel and the role that it has to play in the near and medium term, while not undermining the importance of pushing the electrification or zero-emissions agenda.

“So, it’s absolutely part of the agenda and the fact that we are progressing on looking at renewable and sustainable diesel solutions, and developing the after-treatment for diesel, is an acknowledgment that there’s still a role to play there. Nobody’s suggesting we stop development of diesel, because we’re going to need combustion fuels in a number of applications for a long time to come.

“In the car sector, I think there’s certainly a focus on electrification and zero emissions in city centres, short journeys, and trying to get people to adopt that in far greater numbers where it’s appropriate for them.

“But we do have to acknowledge and accept that there are a number of applications where diesel is still arguably the right choice to make right now, particularly with the latest generation as of Euro 6d, where those diesels are, from an air-quality perspective, down at the levels that petrol vehicles are delivering.”

Automotive technology – irrespective of the type powertrain in the vehicle – will continue to develop cleaner, more efficient engines to help reduce emissions and improve air quality. Diesel will play a part in that progress for some time to come, so fleet owners should ignore the more lurid headlines and remain confident that it can still be the right choice for them.

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Craig Thomas

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