To counter that, the complementary RDE test is being introduced at the same time. As the name suggests, this test is not laboratory-based. Although the procedure sets the types of roads and topography, it can be performed on any roads anywhere in the world. It is this aspect of the new test that poses the toughest challenge for manufacturers developing cars today.
How does WLTP work and how does it differ from NEDC?
Both take place in controlled conditions to provide repeatable results. The key differences are that WLTP will test cars with and without optional equipment offered to customers to give the best and worst case scenarios, will include higher speeds than NEDC, will look at harder acceleration and deceleration, and has a tougher set-up process for vehicles ahead of the test.
NEDC is based on a theoretical driving profile and consists of two drive cycles, urban and extra urban. The combined figures normally quoted are calculated from those two. WLTP will comprise several test cycles (Worldwide Harmonised Light Vehicles Test Cycles - WLTC), each one designed to suit a particular category of cars based on factors such as power-to-weight ratio and maximum speed. There’s also a series of test cycles for hybrid and electric vehicles and, overall, the regime is far more detailed and sophisticated than before.
WLTP test cycles are based on real-world driving data collected worldwide. The new cycles are split into four parts, based on low, medium, high and extra high average speeds. Each speed range will comprise a variety of phases, including stops, acceleration and braking. The test cycles are longer at 30 minutes instead of 20 minutes and the highest speed is now 81.4mph rather than 75mph. Where NEDC relied on fixed gearshift points, WLTP specifies different shift points for each vehicle.
NEDC fails to take into account options such as larger wheels or any items that increase the standard kerb weight. Neither does it account for what is now common standard equipment such as heated seats, air conditioning, four-wheel drive and heated windows. WLTP takes all of those factors into consideration.
Where does the Real Driving Emissions test figure in all this?
However improved WLTP is, it is still a laboratory test and cannot take into account whether an individual driver’s style is aggressive or sedate, the traffic conditions, weather conditions, gradients or load on the car. All of these things have an effect on fuel consumption and NOx emissions. The RDE test is carried out by fitting Portable Emissions Measuring (PEMS) equipment to the car to record exhaust emissions at the tailpipe.