A car’s fuel consumption is essentially determined by its driving resistance values, i.e. by its mass, aerodynamic drag and rolling resistance. The new WLTP procedure, which will be in force since September 2017 for type approval in Europe, integrates these physically dictated driving resistance values more fully than the NEDC which has been used so far, and is thus considerably more representative. Like the NEDC, the WLTP is also carried out in certified test labs under precisely defined conditions. As a result, the measurement results are both stable and reproducible while enabling a direct comparison of different vehicles regardless of the test bench or test lab used.
A driving cycle defines the speed at which a vehicle is driven on the chassis dynamometer for every second of the test. This results in a so-called velocity profile. The WLTP differs from the NEDC, which it will replace, in a number of aspects: The new test does not only take longer, namely 30 instead of 20 minutes, it also accelerates up to Germany’s recommended highway speed of 130 km/h much more often. The gear shift points during acceleration have also been readjusted. And the amount of vehicle idle time has also been reduced. This results in a significantly more dynamic cycle and a higher average speed of 46.6 km/h compared to the one in the old cycle which was only 33.6 km/h. Physically, this means an increase in the energy expended for mass acceleration and therefore higher fuel consumption.
Another significant change is that, according to the WLTP, measurements must be carried out on a fully equipped vehicle. Until now, this was done only with a vehicle’s standard equipment. When purchasing a car, every additional equipment option can increase the vehicle’s weight and thus the energy expended to accelerate the car and keep it in motion. Due to the large number of special equipment packages and their different energy consumptions, car manufacturers will perform additional measurements with other equipment levels in addition to measuring a fully equipped vehicle. Car body versions that influence aerodynamics or different tires with individual rolling resistance values will also be taken into account. This will result in a range of fuel consumption values that is more specific and therefore better reflects real-world driving.
An important detail for technophiles: test bench engineers do not measure the actual volumetric fuel consumption in liters, but the weight of the carbon-containing components, including carbon dioxide (CO2), in the exhaust gas. Since standardized fuels are used for the bench test, the consumption value can be calculated directly in liters per 100 kilometers based on the CO2 weight and the distance traveled on the test bench. This form of measurement, which leads to a more exact determination of the greenhouse gas, was already mandatory in the 1990s.
The test specifications for temperature and the tire composition will also become more specific with the roll-out of the WLTP since some external factors that can influence fuel consumption may differ from one of the participating countries to another. For example, the average temperature in Europe is 14 degrees Celsius, and the average idle time of a vehicle is nine hours. This is why an additional test is carried out at 14 degrees Celsius to supplement the general emissions measurement at 23 degrees Celsius. Until now, only a range of 20 to 30 degrees Celsius was used in the NEDC. The vehicle to be tested is therefore parked in a climatic chamber over a defined period of 12 to 36 hours at 23 degrees Celsius in a first step and then at 14 degrees during the subsequent test without being cooled (natural soak). Lowering the starting temperature increases the consumption determined by the test, because the oil in the engine, transmission and axle components become more viscous with decreasing temperature. Moreover, the requirements on tire pressure and tread depth have been made more specific.
These factors raise the fuel consumption values determined in the WLTP compared to the former NEDC values considerably. Nevertheless, the amount of energy that is expended for a given driving profile remains the only decisive factor in determining actual fuel consumption. The nominal consumption value will increase by an average of around 20 percent if the same vehicle is first tested in the NEDC according to the old procedure and then in the WLTP with the new procedure. However, this does not affect fuel consumption at all when the vehicle is operated by the customer. This can be compared to measuring the outside temperature in Celsius and Fahrenheit – regardless of the unit of measurement you will stay just as cold or warm when you go outside.