Environment and Climate

Global WLTP roll-out for more realistic results in fuel consumption

Questions and answers regarding the new international test procedure: Lawmakers require standardized test procedures to measure how much fuel a car consumes and whether it complies with the emissions limits. The new “Worldwide harmonized Light-duty vehicles Test Procedure” (WLTP) will apply to the type approval of new passenger cars across the EU since September 1, 2017. It will succeed the NEDC (New European Driving Cycle), which has been in force since 1992. It comprises both a new driving profile on test benches as well as more precise and up-to-date conditions for the entire test and is therefore intended to result in more realistic consumption data. What are the implications of this change? Seven questions, seven answers.

How are plug-in hybrids and electric cars measured?

In order to obtain type approval, electrified vehicles will also have to be tested according to the new WLTP rules in Europe. Just as the fuel consumption figures for vehicles with internal combustion engines will change with the WLTP’s roll-out, the same will apply to the range specifications of pure electric cars and plug-in hybrid vehicles. The range specifications are more representative with the WLTP. The electrical range that is actually usable for customers will not change.

For purely battery operated electric vehicles, this means that the higher average speed of the new test cycle leads to a higher energy consumption. This energy is stated, however, not in liters, but in kilowatt-hours (kWh) per 100 kilometers. The measurement is carried out as prescribed in the previous fuel consumption measurement specification: the battery must be fully charged at the start of test bench test. Immediately after testing, test engineers reconnect the vehicle to a charger. The cable is equipped with an electricity meter. This meter measures the total amount of current, which has the advantage that the battery’s energy losses during charging are detected as well. The resulting value is divided by the range determined in test bench testing.

Roll-out of the WLTP signifies a major change for plug-in hybrid vehicles, which have both an electric drive and a combustion engine and can be externally recharged. These vehicles complete the test several times. They start up with a full battery. The cycle is repeated until the battery is empty. The combustion engine operates for a longer time each cycle. Emissions are measured with each cycle. This is followed by a measurement with an empty battery in which the drive energy originates solely from the combustion engine and regenerative braking. This multi-stage measurement can not only be used to determine fuel consumption and CO2 emissions more precisely, but the electrical range and total range as well. The CO2 value to be determined is then calculated as the ratio of the electrical range to the total range. At the same time, a so-called “utility factor” (UF) is introduced.

The UF represents the proportion of vehicle distance traveled electrically. In the case of pure electric vehicles, a UF of 100% applies. In the case of traditional internal combustion engines, the UF is 0%. In the case of plug-in hybrid vehicles, the UF increases with their electrical range. Lawmakers can use the UF to evaluate a vehicle’s ability to drive without emissions. The higher the electrical range, the lower the CO2 emissions. This is quite close to real-life conditions since the driver of a plug-in hybrid will have to refuel less often when he or she has sufficient current available, e.g. to drive typical commutes purely electrically. In practice, the actual consumption behavior of a car with plug-in hybrid drive will vary widely from one user to another. In the case of long-distance trips, the electrical distance traveled is negligible and the consumption will be on a par with the traditional combustion engine. On the other hand, many short-distance trips and commutes can be covered almost entirely electrically, with actual fuel consumption close to 0 l/100 km.

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