ADR fuel consumption figures: A guide at best

The carefully mapped out test procedure for official fuel consumption and emission figures is essential as a yardstick for inter-vehicle comparison. But it doesn't reflect the real world, and it's likely in for a major update with the increase in plug-in hybrids.

Where do those 'official' fuel consumption and emissions figures come from? You know - those  numbers you find on every road test here and on the pages of any motoring publication or spec sheet you'd care to pick up.

The figures also appear on the official fuel consumption label federal law requires all new passenger cars, 4WDs and light commercial vehicles to display on their windscreens. The label denotes fuel consumption in three modes: 'urban' (aka city), 'extra-urban' (aka highway) and 'combined' cycle. It also includes a CO2 emissions figure in grams per kilometre.

But where do these numbers come from? What lucky people spend their entire working lives going for drives around town and out into the country and calculating the fuel consumption off the petrol receipts? Can these people be trusted?

Actually, there's more to it than that. And less. Let us explain...

The exact nature of testing is dictated by Australian Design Rules - ADR 81/02 Fuel Consumption Labelling for Light Vehicles, to be specific. It draws its procedures from United Nations Economic Commission for Europe (UN ECE) Regulations.

The vehicle travels nowhere. It goes for a 20 minute spin on a chassis dynamometer - the same kind of rolling test bench used to measure power (kW) and torque (Nm) figures.

Fuel consumption testing is divided into two sequential phases. Phase one, the 'urban' cycle, emulates stop-start traffic, while phase two, the 'extra-urban' cycle, emulates acceleration to a higher peak speed. The commonly quoted combined-cycle figure takes in both phases.

Much effort is put into making the test as realistic as possible. The dyno is calibrated to simulate aerodynamic drag and inertia - resistance to movement caused by the vehicle's mass. A fan set up ahead of it imitates the flow of air into its front-end intakes at varying speeds.

But there's a natural trade-off between standardisation and real-worldness, and for these purposes it's standardisation that's more important. After all, this is first and foremost about providing a specific yardstick against which vehicles can be tested. That means removing all the variables - road quality, weather conditions, driver behaviours, topography etc - that influence fuel consumption and emissions in the course of individual journeys by individual vehicles.

It's this standardisation that allows us to use the test and its results as a reliable ready-reckoner in comparing the environmental performance of different models, a spokesman from the Department of Infrastructure and Transport told motoring.com.au.

But bear in mind that standardising it reduces the results to the level of the theoretical, he adds. "It allows objective comparison between vehicles, but obviously no one procedure can simulate all real-world driving conditions. Actual on-road fuel consumption and emissions will vary depending on traffic conditions, vehicle condition and load and how individuals drive."

The official test takes 1180 seconds - just under 20 minutes. From the viewpoint of a manufacturer, it's not a big thing, says Ford spokesman Peter Fadeyev. "It's a small formality for our people with the arrival of each new model. The whole thing - setting up, running the test for 20 minutes and dismantling it afterwards - takes about an hour. Our people do it on site - the big car companies do. But of course we have to get our labs accredited independently."

The urban component of the test cycle, taking up about 800 seconds (a little over 13 minutes), assumes an average speed of 19 km/h. Some 30 per cent of this is spent idling, but that's divided up into 13 intervals, interspersed with a series of stop-starts designed to mimic heavy urban traffic.

Our departmental spokesman says this clear division between urban and extra-urban cycles is important, with the majority of drivers spending much of their time at the wheel in city traffic. "So it gives a more accurate pointer to the fuel consumption they'll actually experience than the combined figure."

The extra-urban cycle takes up the last 380 seconds - a little over six minutes. It assumes a relatively high average speed of 63 km/h. It peaks at 120 km/h, but doesn't sustain such speeds for any length of time because when the test was conceived it was decided a more elastic acceleration and deceleration envelope would better approximate real-world driving conditions and therefore fuel consumption. (If you have a trip computer, it's easy to confirm the principle behind this. Next time you take a long trip, reset it and check your average speed when you've finished - chances are you'll find it's much lower than it seemed.)

Tests are typically conducted on a sample vehicle from a vehicle platform using each available powertrain rather than individual models. Take, for example, Volkswagen's Golf. It comes in hatch, wagon and cabrio variants (sedan, too, until mid-2011 when the Jetta was announced as an independent platform) with an array of petrol and diesel engines of varying power output married to manual and DSG transmissions. Rather than test every variation, testers put each powertrain on the dyno and use a look-up table to account for weight differences between the different body types.

Because some European models are made to run on PULP, Australian testing uses 95RON fuel for all petrol cars, to level the playing field.

How does the test affect hybrid vehicles? Does it put systems like Toyota's Hybrid Synergy Drive or Honda's Integrated Motor Assist at any advantage or disadvantage?

The testing process is the same for hybrid vehicles as it is for IC engines. Asked if the current regimen functions as well for hybrids, Toyota spokesman Mike Breen told motoring.com.au the company's Regulations and Certification division had found nothing indicating ADR81/02 puts petrol-electric hybrids like the Prius and the Lexus lineup at any disadvantage.

It will, however, likely need re-evaluation when PHEVs like Toyota's Prius plug-in and Holden's Volt reach the mainstream market.

"Whatever test procedures are in place, they have to evaluate all kinds of PHEV systems - petrol-electric and electric-only drive - and properly evaluate the total energy consumption," Breen said. What he's alluding to here is the considerable technological variation between different interpretations of the 'plug-in hybrid' concept.

Both the Prius Plug-In and the Volt combine electric motors with petrol engines, but the Toyota is essentially a standard Prius with the battery pack expanded to extend its all-electric driving range, whereas the Volt has been developed from scratch - there is no 'Volt lite' model.

The Volt has drawn criticism from some quarters because it's not a series-hybrid (range-extended EV), as it was originally understood to be. The petrol engine in the Volt also provides some motive drive as well as recharging batteries. In this respect it's the same as the plug-in Prius. For both cars the power pack is externally chargeable; when the battery pack runs down the system reverts to normal series/parallel hybrid drive that employs both petrol and electric power to the drive wheels.

Breen said by Toyota's reckoning, those vested with the job of developing and evaluating future testing methods will need to find ways of accommodating these different approaches and take into account actual PHEV usage in the market. "It will be important, too, to establish globally harmonised fuel consumption test procedures across Europe, the US and Japan to help advance the technology as a whole, with the same PHEVs offered in every market," he added.

This is hardly an issue in Australia for the time being, with virtually no PHEV or EV presence on the road. But within two years, the picture will be different.