Fuel-economy testing — What is WLTP?

WLTP stands for Worldwide harmonized Light vehicles Test Procedure. There we’ve told you!

But to fully understand this new global fuel consumption standard and how it fits into the auto space, please indulge us while we tell the back story. You see, fuel economy and exhaust emissions have been in the news lately... for all the wrong reasons.

When car companies haven't been telling outright porkies about nitrogen oxide emissions – and using engine management firmware to conceal the facts — they've been using every known loophole in the book to lead consumers to believe the cars they're buying are more fuel efficient than is actually the case.

Pivotal to this misunderstanding was NEDC (the New European Driving Cycle) a fuel economy/emissions test standard (that also informed our local ADR fuel tests) that left enough wriggle room in its testing procedure for car companies to present their test results in a highly favourable way.

NEDC testing was a contrived scenario allowing the use of overinflated tyres and the disconnection of engine ancillaries.

Within the regulatory framework for NEDC, the car companies were legally entitled to exploit the loopholes. But more and more, the NEDC results were being questioned by legislators and consumers, based on real-world experience.

In response to pressure from the public, and in the face of resistance from car manufacturers in Europe, the United Nations and regulators from Europe, India and Japan have introduced a new standard for fuel efficiency and emissions. This new standard, implemented in September 2017, is WLTP.

For cars sourced out of Europe, the previous NEDC data supplied by the manufacturers from their own laboratory testing was used unquestioningly to comply with our own ADR 81/02 standard.

Now, however, cars are trickling through to the Australian market with fuel consumption figures based on WLTP data.

Indeed, the same day this article was published French brands Peugeot and Citroen announced that their entire Australian passenger car line-ups are now WLTP protocol-compliant.

One example of the tougher regime imposed by WLTP is the Range Rover SDV6, announced for the Australian market in July 2018.

With a bi-turbo V6 diesel engine the Range Rover's combined-cycle fuel consumption figure is 7.7L/100km, according to WLTP. That's a significantly worse number than the NEDC figure of 6.9L/100km for the single-turbo V6 diesel in the Range Rover TDV6 that the new model replaced.

Yet the newer engine is cleaner running, JLR Australia advises, and complies with the latest European emissions legislation – Euro 6C. That means it emits fewer nitrogen oxides than the older TDV6 engine, which doesn't comply with Euro 6c and is being dropped from the line-up.

So the bi-turbo SDV6 can produce fewer than 80mg/km of nitrogen oxides, but the older TDV6 engine can't. Despite that, SDV6 is rated worse for fuel consumption.

This highlights the difference between the original NEDC standard (applied to TDV6) and the newer WLTP standard for the SDV6, based on RDE – 'Real Driving Emissions'.

But that's extrapolating real-world fuel efficiency based on compliance with emissions standards; is that relevant?

When an engine draws in 'X' amount of fuel and burns it in the combustion process, it creates a proportional quantity of emissions – whether those emissions are nitrogen oxides or carbon dioxide, to name two types.

Diesel fuel, being richer in carbon content than petrol, produces a higher quantity of CO2 (carbon dioxide) at the exhaust, for every drop burned, relative to volume.

To illustrate, the Audi A6 Bi-Turbo – a 3.0-litre diesel V6 – is rated at 6.1L/100km for fuel economy. BMW's 520i – a 2.0-litre petrol four-cylinder – achieves 6.2L/100km. Very little separates the two for fuel economy, but CO2 emissions, according to Australia's Green Vehicle Guide are 161g/km for the diesel Audi, versus 141g/km for the petrol BMW.

Fuel economy favours the Audi, but the BMW produces fewer CO2 particles at the exhaust. This ecoscore website provides more detail.

Setting aside the difference in fuel economy/emissions for diesels and petrol vehicles, there is a direct correlation between the exhaust pipe emissions of CO2 and fuel consumption. If you know the CO2 emissions of a vehicle, it's possible to reverse-calculate the vehicle's fuel economy.

Euro 6c, the new emissions standard which came into force from September 1, 2017, restricts vehicles to 60mg/km of nitrogen oxide for petrol engines or 80mg/km for diesels. The standard is based around WLTP, which is essentially a fuel economy standard, but has implications for exhaust emissions standards as well.

Vehicles are allocated to one of three classes, based on power to weight ratio (PWR). This number is calculated by dividing the car's engine output in kilowatts by its kerb mass in tonnes. A larger SUV weighing two tonnes with a diesel engine developing 140kW would rate a PWR of 70 (140/2).

Class 1 is for low-powered vehicles with a PWR below or equal to 22. Class 2 is for vehicles with a PWR between 22 and 34. Class 3 is for a PWR above 34. Most modern cars are Class 3 ('high-power') vehicles.

Each Class 3 vehicle is subjected to four different tests, mimicking real-world driving in Low, Medium, High, and Extra High speed conditions.

A Class 3 test cycle lasts 30 minutes (1800 seconds). During that time, the vehicle will accelerate at no more than 1.6m/s² (metres per second squared – 0-100km/h at this rate takes 15.5sec) and decelerate at no more than -1.5m/s². For the extra-high-speed phase, maximum permissible acceleration is as low as 1.055m/s².

For the low-speed phase, which is up to 56.5km/h, the duration is 589 seconds, including a total time of 150 seconds at standstill. From the start the vehicle repeatedly accelerates and decelerates to different speeds. During the course of this phase, the vehicle stops no less than five times, each occasion of differing duration.

For the medium-speed phase of the test cycle, the total duration is 433 seconds and total time spent at standstill is just 49 seconds. During this phase of the test, the vehicle only stops the once – right at the end of the low-speed phase. The maximum speed permitted is 76.6km/h.

The third phase of the test cycle is the high-speed component – 455 seconds duration, including 31 seconds stopped. Maximum speed for this phase is 97.4km/h. As for the medium-speed phase, the vehicle is only stopped at the start and the end of the high-speed phase.

That's true of the extra-high-speed testing too. The duration of this test phase is 323 seconds, including eight seconds stopped. Maximum speed is 131.3km/h.

In total, the vehicle is supposed to cover 23.266km during the 30-minute test, comprising roughly three kilometres at low speed, 4.8km at medium speed, 7.2km at high speed and 8.3km at extra-high speed.

WLTP is an improvement on NEDC, and the figures produced from the testing process are certainly more realistic, but like NEDC the newer testing procedure makes no allowance for hills. And the acceleration limits are considered to be too low, when drivers typically accelerate up to the low-speed maximum in under 10 seconds – versus the 15 seconds allowed by the test.

There's one specific way in which WLTP is distinctly better than NEDC – the newer standard factors in vehicle internal parasitic losses and rotational inertia of and undriven axle. On a single-axle dynamometer, the front wheels of a rear-wheel drive car or the rear wheels of a front-wheel drive car remain stationary. On a road those wheels would be turning in unison with the drive wheels. WLTP bumps up the fuel usage to three per cent during 'coast-down tests' to compensate for this, whereas NEDC didn't.

Ultimately though, WLTP remains fundamentally what the NEDC was – a way of consistently comparing vehicles for fuel consumption and emissions in a repeatable process.

At least WLTP can't be manipulated quite as easily...