Diesel… who'd call it a fuel of the future now? The VW emissions scandal of 2015 spurred a purge that's seen executives jailed and billion dollar fines levelled.
But perception of it leaving the public filthy? Diesel is our highest-volume fuel with an annual burn of 135 gross petajoules (22 million barrels, each barrel equating to 159 litres) versus 115 gross petajoules of petrol. Obviously, it's the lifeblood of commercial land transport that keeps the economic wheels turning, yet we're still pumping plenty into private vehicles.
Diesel dedication from SUVs, the top-selling new vehicle type, now commanding more than 60 per cent of new car sales in any given month, is still strong from big models, if less so from small to mediums. Any loss there is surely offset by our big infatuation with one-tonne utes, whose best years have been since the VW saga exploded. All drink the stuff. This isn't a fuel we can be proud of, right?
Only if you're a car buyer. Until VW's admission to covering up, diesel passenger vehicles were cementing as better driving choices. This on strength of presenting 20 per cent lower CO2 emissions and 20 per cent better official fuel economy than their petrol counterparts.
READ MORE: * What not to put in your fuel tank this summer * VW crisis widens to petrol vehicles after carbon-dioxide probe * How Volkswagen's diesel duping was caught
Of course, now we know some makers were tailoring their cars to perform better in tests for nitrogen oxides (NOx) output than they could in real life. Most culprits were in the VW family, yet not all. Latest news is that Mercedes-Benz parent Daimler seems set to be fined up to $1.12 billion for violations.
How polluting are diesel vehicles now?
All still produce particulates, the tiny flakes of soot linked to cancer, and all still create NOx, a generic for nitric oxide (NO) and nitrogen dioxide (NO2).
In fairness, all vehicle exhaust emissions matter from a human health point of view. Petrol engines produce more carbon monoxide, another nasty, and they also pump NOx.
The World Harmonised Light Vehicle Test (WLTP) used in Europe, where most diesel cars are produced and the private diesel driving habit is strongest, and introduction of the next step in a phased emissions reduction strategy have been powerful changes for good.
Cars meeting the latest, Euro 6, standard are the cleanest in history. NOx emissions cannot exceed 0.08g/km; six times lower than the 2000 level. Getting there requires a full force of tech, well beyond using diesel particulate filters (DPFs). Most of the NOx converts to largely harmless nitrogen and water.
What about my new ute, where does that sit?
It's just as well the light commercial vehicle category, into which one-tonne utes fall, only has to comply to EU5, the standard that EU6 supersedes, because even though the EU6 requirement for LCVs isn't as tough as it is for cars, its 125g/km target is well out of reach. Distributor data suggests none of the big selling utes achieve less than 200g/km.
We should also be glad diesel utes aren't common in Japan. Used import vehicles only have to meet EU4, so are smuttier.
We're hearing DPFs can be troublesome. What's the issue?
Acknowledgement by Toyota here and in Australia that its Hilux 2.8-litre turbo diesel engine has a DPF issue is worrying. Whereas the NZ problems are claimed to be contained to a small ownership count, in the lower North Island, the Aussie woes seem to be national and big enough to spur a legal action.
A DPF is a steel chamber packed with cordierite and silicon carbide filter walls to trap nanoparticles before remaining gases pass out of the exhaust pipe. In plain English, it's designed to capture and store the soot.
When electronics detect the capsule is full, a burn cycle begins, whereby an injector blasts the DPF with fuel and heat and oxidises the particles into tinier matter.
That's how it's supposed to work. However, there's growing evidence DPFs are prone to clogging. When that happens, a vehicle might exhibit cooling problems, increased fuel use, increased idle speed, a change in engine note, maybe a nasty exhaust aroma. A clogged DPF means that your vehicle won't work the way it is supposed to.
Prolonged low-speed or short duration urban driving is a risk. It often doesn't induce sufficient engine temperatures required for regeneration. DPFs need to heat up to 600 deg Celsius to properly burn off.
So, generally, regular open road runs usually do the trick, though I've had a Subaru diesel (that, agreed, spent too long in town) that remained troublesome even after that regime, only coming right after the gunge was blown out then the controlling software recalibrated. A new DPF would have cost around $7000.
The risk also markedly diminishes when an engine has an additional feature of Selective Catalytic Reduction, which uses a reductant solution (one third urea, two thirds ionised water) called AdBlue, held in a separate tank and injected into a catalytic converter, to neutralise the higher levels of NOx. Some brands are now backing up SCRs with an ammonia filter. Ironically, this tech was available to VW years ago, but was ignored on cost. In hindsight…
When I have a choice of fuels, what makes diesel better?
Efficiency. In the right circumstances. Diesel contains marginally more energy than petrol so less fuel can be used to achieve the same outcome. Yes, diesel engines cost more, and the Road User Charge is a pain, but if you have a high annual mileage, do a lot of open road driving or tow hefty loads, then diesel shines.
Breakthrough in heavy-duty electrics might change everything. Yet regardless of advances with buses and rubbish trucks, the ideal of a big rig, tractor or dozer working all day on battery oomph alone is years from realisation.
Powerful advocates for keeping diesel include Bosch which, though a leading EV powertrain developer, argues diesel's outstanding efficiency is still needed to limit global warming and suggests emissions "can be cut even further, further than any current legislation requires."