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The end of the diesel engine? Why oil-burners are on the way out in passenger cars but not in utes - yet

Most examples of the best-selling Toyota HiLux ute are sold with a diesel engine.

Cast your mind back a decade-and-a-half or so, and a turbo-diesel car was the new black.

The must-have in new cars, turbo-diesel technology had finally hit Australia as a response to a new Australian standard for diesel fuel (with greatly reduced sulphur levels) that suddenly had importers falling over themselves to grab a piece of the action.

We’d had diesel cars before, but these were mainly sold to farmers who filled them from the tractor fuel tank, and they were anything but mainstream.

But in the first few years of the new century, we were lapping diesel up.

That was mainly at the expense of some petrol-engined options as well as the LPG-fuelled cars which took a dive about the same time.

But these days? Diesel is not such a sexy commodity, and while we’re behind Europe to an extent, we’re still seeing diesel’s fortunes ebb away as a range of factors take hold.

So, what’s killing diesel cars? For a start, it’s a global thing, not just an Aussie trend.

Germany, once the poster child for diesel passenger cars, saw just on half its new passenger cars sold in 2015 powered by the stuff. But by 2017, that had dropped to just 41 per cent.

In Europe as a whole, diesel-engined passenger cars peaked in 2012 and have tapered from that point. Banning diesel cars from some major European city centres played a part, to be sure.

Volkswagen’s infamous Dieselgate furore didn’t help either, of course, but neither did VW’s decision to drop diesel-powered variants of its Golf and Polo models.

The fact is that ever-tougher emissions regulations in Europe are killing off the models we once chose from, and that’s impossible to get around.

Even the stunning rise of the SUV hasn’t enabled diesel to retain its fashionista status.

Although generally bigger, heavier vehicles and, therefore more suited to diesel power, SUVs in this country have also moved away from diesel to petrol and petrol-hybrid power.

The exception is the flourishing dual-cab ute market, which is all diesel, all the way, and with the two best-selling models in the country falling into this category, that’s giving our numbers a skew that Europe isn’t experiencing.

But beyond utes, the affordability of a hybrid driveline has also given diesel a fright, largely because a hybrid driveline can often beat a diesel for fuel economy in stop-start and urban running where the majority of Australian cars do the majority of their work.

Considering that buyers of some new Toyota models are ticking the hybrid driveline option-box in more than 60 per cent of cases, you can see the trend.

Up until its recent facelift, the Hyundai i30 small hatch was available with a diesel engine option. Up until its recent facelift, the Hyundai i30 small hatch was available with a diesel engine option.

And there are other factors.

Part of the problem is that attempting to run a clean, low-emissions engine with modern performance while still fuelling it with diesel is a bit like trying to make clean, renewable electricity while still burning coal.

The inputs and outcomes are just chemically at odds with each other.

Diesel engines have lower carbon-dioxide outputs compared with a petrol engine doing the same job, but it’s oxides of nitrogen that are the diesel’s bogey-man.

Oxides of nitrogen have been shown to be carcinogenic, generally harmful and an unavoidable by-product of burning diesel in an internal-combustion engine.

Higher states of tune making engines more thermally efficient have helped, so has computer-control of the injection process.

Ultimately, though, the diesel-engine industry has decided that the best way to clean up tailpipe emissions is to fit a Diesel Particulate Filter (DPF).

The filters, however, have proven to be troublesome, leading to critical engine problems as well as leading to much greater tailpipe emissions if they’re (the DPFs) not working properly (as they seem often not to be).

Probably the kindest thing you can say about DPF technology is that it’s a stop-gap measure, but it will always be compared with spraying perfume on a dog; it will only work until the hound finds something new to roll in.

AdBlue (a solution of about one-third urea and two-thirds water) can help by chemically transforming some of the oxides of nitrogen to water (vapour) and nitrogen.

But it requires pumps, injectors, tanks and controllers (and top-ups) and works in conjunction with a DPF, not in place of it.

For all that a diesel engine is typically more thermally efficient than a petrol engine and will, therefore, go further on every litre, the emissions side of things increasingly outweighs that consumption benefit.

Early generations of the Toyota Camry mid-size sedan were offered with diesel, but now hybrid is all the rage. Early generations of the Toyota Camry mid-size sedan were offered with diesel, but now hybrid is all the rage.

The counter-view holds for heavy machinery and the transport industry where fuel economy and range are major financial considerations, but for passenger cars, the pendulum is swinging away from lower fuel consumption as enough of a reason.

From a manufacturing point of view, diesel engines are also more expensive to make than a petrol engine of equivalent performance.

Even though modern petrol powerplants also use a turbocharger, they are lighter than a diesel and are simpler to package from a centre of gravity point of view.

Petrol engines have also closed the gap on the diesel’s one big advantage: fuel consumption.

While the gap was once a big one, technical advances in fuel-injection, turbocharging (variable-vane geometry) and electronic engine management (allowing higher compression ratios) have brought with them big leaps in petrol efficiency, to the point where a good petrol engine will use very little more fuel than a diesel of the same output.

Another of the diesel engine’s great benefits was its relaxed nature and good low-down torque that worked especially well with the automatic transmissions that the world now favours.

But improvements in modern transmissions including dual-clutch and CVT technology as well as big reductions in drivetrain losses in even conventional torque-converter automatics, have closed that gap, allowing petrol engines to work extremely well in those applications.

The democratisation of hybrid drivelines (and stop-go technology) has also pushed the petrol’s cause as the constant stopping and restarting of petrol engine is far less intrusive in a noise, vibration and harshness sense compared with a diesel.

And while those factors have helped the petrol close the gap on the diesel’s advantages, they’ve also allowed the petrol to gain an even bigger advantage in terms of refinement and driveability, not to mention anything like a fun driving experience.

And finally, there’s the day-to-day experience of living with a diesel-powered car.

Because it doesn’t evaporate, diesel clings to the bowser nozzle, and any spilled on the forecourt remains for you to stand in and eventually turn your car’s carpets into diesel-sponges.