Diesel is dead because diesels are dirty.

Ever since 2015’s dieselgate emissions scandal, when Volkswagen manipulated its vehicles to produce fewer oxides of nitrogen (NOx) only while the vehicle detected it was being tested, this has been the hyped accusation.

Europeans have long loved diesel engines, but North America has rejected them, while Australia (as with its geography) sits mid-field. Last year diesel made up 35 per cent of petroleum sales, with diesel fuel sales increasing by around 105 per cent since 2000.

SUVs and 4×4 utes powering Aussie diesel growth

Thank (or blame) the mining boom and our love of SUVs and dual-cab 4×4 utes, which has seen the segments move from 71,268 and 35,718 sales respectively in 1997, to a whopping 456,646 and 165,797 in 2017. Of last year’s numbers, about half of all SUVs and a huge 147,190 dual-cabs were diesel, while for context, petrol-powered passenger cars fell from 540,353 in 1997 to 450,012 in 2017. So buyers still want diesel in big cars, then.

Diesel contains marginally more energy than petrol (35.8 versus 34.2 million joules per litre) so less fuel can be used to achieve the same outcome. But diesel can also ignite spontaneously without the spark plugs of a petrol engine – called compression ignition – thanks to its tolerance for an extremely high compression ratio. High ratios work each piston stroke harder to deliver more torque, reduced fuel usage and a combination thereof.

The negatives of diesel

The downside is the engine and driveline components have to be built stronger to withstand the pressure, and these days they need turbocharging to perform. Hence Australia’s cheapest diesel, the Hyundai i30 Go, asks $2500 more than its petrol equivalent. In the small car class that percentage increase becomes hefty, and the economy difference usually isn’t as broad as it is in heavier vehicles – a double whammy for diesel at that end.

With Australia’s favourite vehicle, the HiLux ute, Toyota last year ditched the 4.0-litre non-turbo petrol V6 from the range due to lack of demand. It had a 10:1 compression, 175kW of power plus 376Nm of torque, and glugged 12.0 litres per 100 kilometres. Everyone for the same price picked the 2.8-litre turbo-diesel with 15.6:1 compression, 130kW, 430Nm, and 8.1L/100km – or about $1000 saved on fuel over 15,000km.

Oh, and high-versus-low compression is the reason you don’t want to fill a diesel with petrol, as it will detonate the engine causing severe and permanent damage.

Environmental and climate change benefits of diesel

There are few climate change-related diesel downsides. Burning a litre of diesel creates 2.7kg of CO2 versus 2.3kg for petrol, which is typically offset by the efficiency of an oiler. Local diesel is also closer to European standards than petrol, with the former achieving a sulphur mandate of 10 parts per million (ppm) since last decade, whereas the latter is stuck at 50-150ppm, making it one of the dirtiest forms of unleaded anywhere in the world.

Service stations will attempt to separate truck diesel from passenger (or ‘premium’) diesel as more families turn up on bowser forecourts previously dominated by greasy trucks, but it’s mostly in the pump itself – high-flow being for trucks and smaller nozzles for cars/SUVs. The premium stuff does contain an increased number of cleaning and anti-foaming agents, with the latter helping to fill a tank faster and more completely, but they’re minor points.

Unlike with premium unleaded, which boosts a fuel’s octane rating, diesel’s cetene rating equivalent is unchanged. The only other difference emerges in cold areas where ‘alpine’ diesel helps resist liquid turning lumpy in fuel lines. That isn’t a problem for petrol.

Diesel and its vehicle emission problems

Vehicle emissions is the real issue, though. Diesel doesn’t burn as cleanly as petrol and it contributes significantly more to air pollution via carcinogens such as oxides of nitrogen (NOx) and other particulates.

Current Euro V emissions standards allow diesel passenger vehicles to emit 180 milligrams of NOx per kilometre driven, three times higher than petrol’s 80ml/km limit. Meanwhile diesel light-commercial vehicles (LCVs) can emit 280mg/km versus a petrol LCV’s 82mg/km. Many vehicles already meet stricter Euro VI emissions standards, but most diesel utes don’t and there is no timeline for the stricter mandates to be introduced locally.

It’s only a matter of time before the NOx noose attempts to swing diesel’s way, because eventually that Euro VI standard will more than halve passenger diesel NOx limits to 80g/km, while the HiLux class will be in for more than half a cut to 125g/km.

Indeed, a HiLux diesel goes right up to Euro V limits with 162.6mg/km (2.8L auto) to 261.5mg/km (2.4L auto), where a Euro VI-rated Volkswagen Amarok V6 manages 109.1mg/km.

Tips to reduce diesels emission issues

Diesel will live or die on its post-combustion exhaust treatment of such emissions. Most have diesel particulate filters (DPFs), which is a steel chamber packed with cordierite and silicon carbide filter walls to trap nanoparticles before remaining gases pass out of the exhaust pipe. When electronics detect they’re full, an injector blasts the DPF with fuel and heat and oxidises the particles into tinier matter that matters less.

Low-speed urban driving with insufficient engine temperatures causes the DPF to fill without time to automatically burn off, however. With HiLux, Toyota recommends revving the engine for up to 30 or 40 minutes at idle. Most say to drive at 20km/h or higher for 20 minutes to allow ‘regeneration’ to occur. In any case the best way to avoid clogging is to take diesels for regular freeway runs.

DPFs also rely on the engine to be well-maintained with low-ash oil and tight intake hoses to obtain the correct feedback from all sensors – which could degrade in the long term.

Meanwhile, exhaust gas recirculation (EGR) swirls burnt gases back into the cylinders to help reduce NOx emissions, and some vehicles are equipped with AdBlue; which is a one-third urea to two-thirds ionised water fluid that further curbs NOx through selective catalytic reduction (SCR). AdBlue can be bought from a service station for as little as $1 per litre, a separate tank of which should last 5000km to 20,000km.

Brands such as Volvo have called off diesel development, but Mercedes-Benz has targeted 30g/km of NOx for its diesels by 2020, while Bosch believes that refinement of current technologies could see the elimination of particulates from diesel engines by the same year.

Either way, a new Benz A-Class diesel already achieves NOx of less than 60mg/km owing to, “exhaust-gas aftertreatment close to the engine as well as multiple exhaust gas recirculation (EGR) with high- and low-pressure EGR [and] an SCR catalyst with AdBlue exhaust fluid.”

It all highlights how post-combustion treatments will need to hold up diesel’s entire future.