Worldwide Harmonized Light Vehicles Test Procedure. It’s a Mouthful, but How Does It Work?

It’s the reason that Volkswagen Group might have to stockpile vehicles, and it’s causing headaches for automakers across Europe. But what is WLTP, should you care, and how does it work?

For decades, the European Union has used its own test cycle to measure vehicle emissions and set fuel economy ratings for passenger cars. Called the New European Driving Cycle, the NEDC is anything but new. And as it turned out, it didn’t quite represent the real European driving cycle either.

Last updated in 1997, the NEDC has two driving cycles, one urban and one extra-urban. the urban cycle is designed to represent driving in a European city. And no, that doesn’t mean sitting in traffic for hours while stuck in a roundabout.

Strap a cold car onto a chassis dyno, strap it down, and start the engine. A computer, or a really precise driver, follows the test procedure using the gas and brakes. The dyno is loaded to simulate the weight and aero of the car, making it real-world applicable. More or less.

The engine starts, the car waits 11 seconds, then slowly accelerates to 9.3 mph (if these speed numbers seem odd, it’s because the test is metric). Then the car cruises for eight seconds, brakes to a stop in five seconds, and stops for 21 seconds. It goes on in intervals like that of slow acceleration, slow braking, and a bit of idling for 13 minutes. It covers about 2.5 miles of distance and has a top speed of 31 mph. Except that it’s all virtual distance since the test is done on a test stand. That is, the car doesn’t go anywhere.

The Extra-urban cycle is a similar set of slow acceleration and slow braking events. Top speed is 74.4 mph, and the average speed is just 38.8 mph. Oh yeah, and automakers can do the tests 1.2 mph below the described speed, using special eco-mode settings, with the climate control off, and with the mirrors and roof rack removed. Even with higher tire pressures.

Compare that with the US EPA’s test, which was last updated in 2008. In the city, the test has a top speed of 56.7 mph, an average of 21.2 mph, and a theoretical distance of 11 miles. There are also two start-up cycles, with one cold and one hot. The highway test averages 48.3 mph and has a 60 mph top speed. To make them even more real-world, whatever economy results the city test returns are decreased by 10% and the highway figures are decreased by 22%. There is also a supplemental test to simulate aggressive driving with a top speed of 80 mph, and another that uses the air conditioning system.

Those differences are why you’ll see electric car ranges that have massive differences between initial reporting and real results. Like the last-gen Nissan Leaf that saw a 155-mile range in Europe and 107 in the US. It’s not the automaker’s fault, they’re just following the test.

With a test that doesn’t reflect the real world and the knowledge that multiple automakers have figured out how to manipulate the test in their favor, it was time for a new way of doing things.

Enter the Worldwide Harmonized Light Vehicles Test Procedure, or WLTP. Just ignore that it isn’t quite used worldwide, at least for now. It was designed with guidance from the EU, Japan, India, and the UN.

The WLTP has three big vehicle classes, based on power to weight in kW/tonne. And yes, that’s tonne. 1,000 kg, or about 2,200 lbs. Most cars are Class 3, > 34 kW/tonne. Some larger vehicles like vans can fall into Class 2, which is 22-34 kW/tonne.

Instead of fixed-rpm shift points for every manual-equipped vehicle, the test has an algorithm that calculates optimal shift points for each one. It also filters out frequent gear changes, which can skew results.

The new Class 3 test runs for a total of 14.4 miles. Nearly six times the old test. The maximum speed on the test is 81.4 mph, and the average speed without stops is 33 mph. It puts low, middle, and high speeds all into one test.

Plug-in hybrids get an even more interesting version of the cycle. They start with a full battery, and then do the test over until the battery is flat. Then they do it with a flat battery too. It’s because automakers can’t predict how often you’ll actually charge your PHEV. Some drivers never do, treating it as a slightly less efficient conventional hybrid.

There’s a second component, too. This one’s the real kicker. The Real Driving Emissions (RDE) test. The WTLP testing is still done in a lab. Basically because for all practical purposes it has to be. Have fun controlling the temperature outside if you happen to be trying to certify your new model in February, for example.

RDE means that automakers have to strap a real test rig onto the back of a real car and drive it on real roads. For real. It effectively eliminates the possibility of TDI-style emissions defeat devices or even just vehicles that happen to do better on the test than outside.

At this point, I should probably mention that the tests don’t actually measure fuel economy. They measure CO2, nitrogen oxides, carbon monoxide, and particulates. Then fuel economy is mathed back out the other end.

The kicker for automakers is that all WLTP lab testing has to be done by September 1, 2018. Though they get a break on the RDE part, which doesn’t have to be done for an extra year.

This is where it gets complicated, though. The NEDC test doesn’t care if you fit optional bits like big wheels or a roof rack. The WLTP does. So automakers have to test every car and every engine. Plus they have to test each optional tire size. And anything else that they think might make a difference, and you had best believe they’re going to err on the side of caution this time. Nobody wants another emissions issue in the paper.

So do you need to care? Maybe. It depends on where you live. In the EU? This makes a difference to the economy numbers you see. Reading the latest news from automakers and seeing NEDC figures? Then it matters to you because the new numbers will be lower, but more realistic. That’s how it works and how they reached those figures.