A test bench test under defined boundary conditions is the only way to produce objective and reproducible conditions so that emission and fuel consumption values of different vehicles from different manufacturers can be compared with one another. One important objective is for the test to be representative of average driving behavior, which is why real-world driving data from people around the globe was examined before defining the WLTP that has been applicable since September 2017. However, even a representative bench test like the WLTP cannot take into account all the factors affecting fuel consumption in the real world.
The main consumption-increasing factors that can be considered only to a limited extent in a standardized lab test include the individual driving style but also the geographical conditions of the route, the individual load, the climatic conditions and the energy used for accessories such as seat heating and air conditioning. Depending on the outside air’s humidity and temperature and the driving profile, the air conditioning system can generate an additional fuel consumption of over one liters of fuel per 100 kilometers.
In recent years, suppliers specializing in the development of air conditioning systems have developed solutions to lower this additional consumption, for example by automatically adding a certain amount of pre-cooled recirculated air. Nevertheless, fuel consumption measurements according to the WLTP are always carried out with the air conditioning system switched off. This makes it possible to compare vehicles independently of their air-conditioning solutions. Furthermore, the frequency of use of air conditioning systems is just as variable as the air temperature in temperate zones.
The geographical characteristics of individual routes cannot be influenced by humans either. Not only does every vehicle that drives uphill have to fight against gravity – traversing narrow mountain passes also increases fuel consumption due to the required steering effort. This is because tires build up cornering forces, without which the vehicle would drift out of the curve, and thereby increase the rolling resistance. Additionally, almost all modern cars have power steering assistance. The development of electromechanical steering aids, which are increasingly replacing traditional power steering systems, has allowed the automotive industry to reduce fuel consumption significantly. However, both the steering system and the topography are omitted from the WLTP, since it is impossible to create a bench test simulation which is representative worldwide. People plan their own routes and have their own driving behavior.
Despite these limitations, the roll-out of the WLTP will reduce the gap between the average fuel consumption values determined and standardized by lawmakers and the individual fuel consumption of many motorists. Experts estimate that the values measured and standardized in the WLTP are on average 20 percent higher than the values determined in the NEDC. The fuel consumption values determined in the WLTP are therefore more realistic because they are more up to date, although they are not “real” in the sense of being applicable to every driver.