98 RON fuels: are they worth the extra coin?

Are 98 RON 'ultra premium' fuels like Shell's V-Power, BP's Ultimate and Caltex's Vortex 98 worth the 10-20 cent a litre premium they command over standard 91 RON ULP (fast on the way out) and E10?

For the limited number of cars explicitly demanding it, obviously. For less high-strung machinery, it's a matter of doing the maths and balancing out your spending tolerances against the love you feel for your machine.

In launching its V-Power product in Western Australia recently, Shell went to considerable lengths to demonstrate its efficacy – not just in the high-performance engines with which 98 is normally associated, but in any.

Shell made much of its long-standing technical partnership with Ferrari. V-Power, it says, is the culmination of six decades of combined effort, and it's near enough to the formula they put into Fernando Alonso and Felipe Massa's Scuderia cars on GP days to be interchangeable with the track fuel.

NRMA senior policy advisor Jack Haley is ambivalent about ultra premium fuels. He says there are so many variables from car to car it's not possible to come up with a blanket yes or no.

"I'm sure it does have benefits, but it's a matter of magnitude against cost," said Haley. "Really, the only way to tell is to put it to the test in your own car."

He recommends testing using ten tankfuls either side of an upgrade from what you're using to 98.

That is, ten tankfuls of the old, carefully logging fuel consumption as you go, then ten tankfuls of 98 logging in the same way.

"It'll probably take several months, and it's good to do it over a period where there's no aberration to your normal driving patterns – no long trips, for example, if you're an urban commuter," Haley said.

To put its case for its premium product, Shell brought fuel chemist John Lambert, a 35-year veteran of its UK research labs, to the Perth launch to explain its benefits in detail. Lambert put a compelling case for spending the extra $10 or so extra you pay to fill a medium fuel tank with V-Power over 91 or E10.

Firstly, it burns differently. Perhaps counterintuitively given its association with high-performance engines, a higher octane rating makes fuel less volatile. This makes it less prone to premature (or pre-) ignition, the cause of a phenomenon known as knock, or pinging. That's the rattling sound an engine makes when fuel in the combustion chamber detonates spontaneously, before the piston has assumed the optimal position to transfer the energy burst from combustion down to the crankshaft and out through the transmission.

To demonstrate this, Lambert used the analogy of the crank on a bicycle, rotating under the power of the rider's legs.

"The most efficient transfer of energy from leg to pedal and eventually to road comes by pressing on the pedal when the crank's rotated just a few degrees forward of 12 o'clock," he explains.

"In the combustion chamber, the best moment to spark is the same – not when the piston's at the point of maximum compression but just beyond, when the crank's already rotated that couple of degrees beyond full reach and the piston's just begun heading downwards again."

Knock can cause all sorts of damage to pistons, rods, valves and beyond. Modern cars use knock sensors to retard the timing a fraction to work in with it. But that means ignition happens when the crank is further around its 360-degree cycle than that optimal couple of degrees, which effectively cruels the engine's efficiency. High octane fuel makes it easier for the engine to bring it all into alignment to get the most out of the bang.

NRMA's Haley adds an important caveat to this.

"As a rule of thumb, each octane increase delivers a fuel consumption improvement of about one per cent," he says.

"But that's only in cars built and tuned to take advantage of it."

This means in most cars, the benefits on this front go wasted.

But that's not Shell's primary selling point for V-Power. That's in strongest evidence in a proprietary detergent additive that dislodges and disposes of grimy deposits on valves and other vulnerable components. This is the key point of attraction for owners of older cars and models that don't need PULP.

By Lambert's explanation, it acts in two ways: it loosens up existing build-ups and gets rid of them, then it stops any new deposits accumulating.

Lambert suggests that while it varies with external conditions such as climate, and from car to car, drivers of older cars will likely start feeling its benefits after two or three consecutive tankfuls.

Again, NRMA's Jack Haley thinks this needs qualification. Once again, he says, it's a question of balancing out the magnitude of benefits against cost.

"All petrols now have detergent in them," he says.

"And they need it much less than they used to anyway. Now they have no lead, they've got rid of the chief culprit in leaving damaging deposits. Any build-ups you find in engines nowadays are soft carbons, which engines find easier to get rid of in any event."

Shell's other selling point for its super premium lies in another proprietary additive designed to reduce friction between piston rings and cylinder wall. Lambert says the benefits here are twofold: firstly, it helps strengthen the seal of the combustion chamber.

But there's a fundamental conflict in the way it works, because while the seal needs to be as close as possible to absolute, the piston needs to be able to run its course as freely as possible – in a four-stroke engine running at 3000rpm, each piston is executing 100 stroke movements a second.

Squirted into the chamber on the induction stroke, the 'friction modification technology' in the fuel lubricates the cylinder walls for the rings to move across in the lead-up to combustion, during which it's harmlessly burned away and replaced next time round – 25 times a second at 3000rpm.

Haley has no doubt about the benefits of reducing friction as much as possible.

"But you have to look at the context it's operating in. The engine comprises only one source of friction among many. A normal car engine is a very inefficient device – it only delivers about 12 per cent of its fuel's energy value to the road," explained Haley.

"The rest is lost in heat, powertrain resistance, aerodynamic drag, rolling resistance and the like. So by the time you factor all that in, reducing engine friction will undoubtedly have some benefit, but the net effect is going to be small."

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