The truth about premium diesel fuel | Auto Expert John Cadogan

What about premium diesel fuels, how do they work?

We should go the full Sir David Attenborough and look at the ecosystem, in which an unholy trinity rules the diesel roost. You’ve got the vehicle manufacturers, the fuel companies and the government. A triangle of weasels, packed tight with mutual distrust and tri-lateral contempt.

Yet the vehicles are useless without the fuels, and the fuels have no purpose without the vehicles. And of course without the government there would be many more unemployed lawyer arseholes on the street, begging in bad Armani.

Vehicles are designed to run on a particular kind of fuel - and those particulars are defined by a technical standard. The fuel is manufactured to meet that standard. And, hypothetically, the government cracks the whip.

Here in ‘Straya the applicable standard for diesel is driven by a piece of legislation with the catchy name: Fuel Standard (Automotive Diesel) Determination 2001.

In total there are 19 different physical properties in the standard for diesel - things like cetane number and cetane index (which is - kinda - like octane rating for gasoline).

There’s density, viscosity, lubricity, conductivity, water content and flashpoint (attention fez-wearing terrorists everywhere: diesel is a crap fuel for a Molotov cocktail - flashpoint 61.5 degrees C - which is also why it’s so much safer for remote area refuelling from a jerry can - just saying).

There’s also limits for impurities in that legislation - five per cent maximum biodiesel content (which is undeclared if it’s in there). Ash, carbon residue, water and sediment, and of course: sulphur content (10 parts per million - currently). That’s almost a homeopathy treatment dose for sulphur, right there.

The point is: Your diesel engine is designed to run on diesel fuel that meets this standard. And it cannot be sold here unless it meets the standard. And this means the truth about premium diesel is 95 per cent marketing bullshit.

Your diesel will run happily on any automotive diesel sold at a bowser. Truck diesel relates to the pump - which offers a fatter nozzle and a higher volume flow rate.

Anyway, a contemporary modern long-haul truck is a very hi-tech healthy six-figure asset in a cutthroat business where every cent of operating cost really matters. Any suggestion that truck diesel might be in some way inferior is categorically nuts, when you think about it.
Unlike premium gasoline, which offers a higher octane rating than regular, and therefore a slight performance or economy benefit, there is no cetane rating jump with premium diesel. Engine performance will remain the same on premium diesel.

Premium diesel’s alleged benefits are … fluffy … at best. It has an allegedly superior additive package that is allegedly designed to make your engine allegedly cleaner. Whatever that means.
It’s BP’s piranhas all over again. And it foams less on re-filling.

Shell says its Diesel Extra premium offering will help you (quote) “use less fuel” but there’s a disclaimer there should you read the fine print:

Actual fuel savings and other product benefits will vary depending on age and condition of engine. Shell Diesel Extra is designed to avoid rising fuel consumption over the lifetime of your vehicle by helping to keep your engine running in accordance with manufacturer's specifications.

Finally, there are a few different flavours of diesel across regions and seasons in Australia and they all have to do with a thing called the ‘cloud point’. If you want to understand cloud point, get a bottle of olive oil and put it in the refrigerator. Come back in an hour or so.
Olive oil starts to turn to wax at about four or five degrees C. Put it back in the pantry - the clouds of wax in the oil disappear. That’s the cloud point. Diesel does the same thing. And wax in diesel is of course undesirable because it doesn’t flow down the fuel lines. That’s kinda bad, if you want your engine to start.

So they tweak the chemistry of diesel to reduce the cloud point in colder regions according to a standard called AS 3570 - 1998. The maximum permitted cloud point is typically about 15 degrees C in warmer regions in summer. But it drops to minus three degrees in colder places in winter.

What often happens is that the fuel in the capital cities at this time of year has a cloud point of zero or minus one degrees. And that’s what’s in the tank, doing a good impersonation of being quasi solid, because the ambient temperature is lower than the cloud point.

If you had arranged instead to arrive at the ski fields in a low-fuel state and filled up - most likely with a fuel like Caltex Alpine Diesel - the cloud point of which is well below zero - your vehicle's reliability would not be compromised by the fuel effectively turning to wax overnight.