Fuel economy and emissions: how WLTP differs from NEDC...

Europe is employing a new drive cycle for emissions testing and claimed fuel-consumption figures. Will it be realistic this time around?

You're in the market for a new car and the most important consideration in a time of skyrocketing fuel prices is efficiency. Can you trust the manufacturer’s claimed consumption figures? Up to now, the answer was a resounding “no”, as explained in the article Claimed vs. real fuel consumption which appeared in CAR back in August 2013. The feature focused on the shortcomings of the New European Drive Cycle (NEDC) used for emissions testing and claimed fuel-consumption figures. The article concluded with a prediction of a possible change of drive cycle in the future to yield more realistic results. That watershed moment arrived and, as of 1 September 2018, the new Worldwide Harmonised Light Vehicle Test Procedure (WLTP) was introduced.

Background of WLTP

The need for a more realistic test cycle was recognised as far back as 2007 by the United Nations. Contrary to popular belief, the development of the WLTP preceded the Dieselgate emissions scandal which shocked the automotive world in 2015. The aim of the new cycle is to provide a test that could be run under controlled conditions in a laboratory and would produce repeatable results (as is the case with NEDC) but be more realistic of actual driving on the road. Countries such as the United States, Europe, India, Korea and Japan participated in providing real-world driving data for the development process.

NEDC vs. WLTP

To understand why WLTP is more representative, we investigate the differences between it and NEDC.

Test conditions

Both tests are run in a laboratory on rollers under controlled conditions after the vehicle has been soaked at room temperature (for at least six hours in the case of NEDC) to ensure a cold-start condition. The NEDC test could take place with ambient temperatures between 20 and 30 degrees Celsius while WLTP requires an exact 23. For the first time, electric energy consumption is also measured on hybrid (and electric vehicles) during the WLTP test; this should put an end to the unrealistic NEDC fuel-economy figures published by manufacturers.

It is clear from the speed profiles WLTP is a more transient cycle featuring higher accelerations as well as a higher maximum speed. The tolerance band when following the specified speed profile is reduced and this can lead to more scrapped emissions tests than before. It is more than double the distance and the vehicle spends less time stationary than on the NEDC cycle (one of the reasons stop/start technology was introduced). Lastly, gearshift points are not fixed as before and calculated for each vehicle prior to testing.

The test vehicle

The NEDC test was performed only on the base model of each vehicle range, with the fewest options fitted (best-case scenario). The only weight added to the vehicle was the driver and a small reference mass; for example, 25 kg for light commercial vehicles.

The WLTP demands the base vehicle and top-spec example of a model range (worst-case) are tested. This time, the effect of the mass of possible options of the vehicle and the aerodynamic influence (for example, roof racks) and 15 percent of the possible payload are included. In the end, the emissions result of all the vehicles in the model range within the two extremes must be calculated (a time-consuming exercise), considering all the permutations which can result in a vast matrix of data.

The results

Current independent studies will ultimately determine the difference between the results of the two cycles. Early indications are that CO₂ emissions – directly related to fuel consumption – do increase in the order of 10 to 30 percent depending on the model and powertrain combination (petrol or diesel). What skews the results slightly is that some markets, like India, exclude the “extra high” speed section of the WLTP speed profile because it is irrelevant to their congested roadways.

CAR’s policy of adding 20 percent to the conservative NEDC fuel economy results – called the CAR fuel index – is therefore warranted in most cases. Criticism levelled against WLTP is that it is still too conservative, especially the slow rate of acceleration at low speeds. On the road, this increase in speed always varies. A complex equation is used to back-calculate the NEDC results from the WLTP data as required by some markets (including ours) not introducing WLTP results at the moment.

Real driving emissions (RDE) 

The next step to genuine results is to measure it on the road. Although not strictly scientific without repeatable results because of variables such as climatic conditions, traffic patterns and driving style, it should still result in emissions and fuel-consumption measurements in the ballpark of what was achieved in the laboratory. Dieselgate came about when certain emissions measured on the road were 40 times higher than the NEDC cycle, pointing to dubious software used when the vehicle was in the laboratory; different calibration maps were employed for when the same car was driven on the road.

Authorities have caught on to this and certification for type approval of vehicles (to be able to sell them in the relevant market) now also involves an on-road testing component for verification purposes. The emissions limits allowed on the road are higher but it still prevents any beating-the-curve techniques when it comes to engine emission calibration.

Impact on manufacturers

Manufacturers have been hit hard by the introduction of WLTP testing. The longer emissions cycle and at least twice the number of vehicles needing testing for type approval plus the upgrading of emissions laboratories (four-bag, tail-pipe sampling for the four stages of WLTP compared to two-bag sampling for the two-stage NEDC) require massive financial investment.

German manufacturers with many models and vast options lists were severely affected as a backlog of testing (and certification) prevented the sale of many models after the introduction of WLTP. Many OEMs reported a global drop in vehicle sales of 18%, directly attributed to the change-over of emission cycles. It is conceivable model line-ups and options lists on vehicles will be trimmed in the future to limit emissions-certification costs and time to market.

South African context

The bad news right now is, locally, there is no set date for the introduction of the WLTP figures for new vehicles. This is a particularly sensitive subject because CO2 figures are linked to tax paid to the government, as well as the prices of new vehicles. As the WLTP yields higher CO2 and claimed fuel-economy figures, it will lead to higher vehicle prices for the consumer although the actual vehicle (and technology) is exactly the same.

The ideal scenario would be to introduce the new WLTP figures to give more realistic claimed fuel-consumption figures to the consumer but alter the CO2 tax equation so that a similar amount of tax is paid to keep vehicle pricing constant.

Author: Nicol Louw

Original article from Car