The WLTP cycle for electric cars in depth, how much is real about it?

On January 1, 2021, the new European WLTP emissions regulations came into force (Worldwide harmonized Light vehicles Test Procedureand), which modifies the gas measurement parameters of vehicles, making them stricter. When applied to electric vehicles, the same model now homologates lower autonomy figures than in the previous NEDC cycle (New European Driving Cycle), adjusting somewhat more to reality. Thus, a Renault Zoe homologates 395 kilometers of autonomy, a Volkswagen ID.4, 517 kilometers and a Tesla Model 3 Long Range, 602 kilometers. Figures that, although possible, need very favorable conditions to be real. Why is this happening?

To understand the procedure used to calculate the autonomies that the manufacturers then indicate in their technical sheets, it is necessary to know what is behind the WLTP cycle (Globally harmonized light vehicle test procedure). To illustrate its details, we take a specific model as an example: the Volkswagen ID.4 with a battery of 77 kWh of usable capacity (82 kWh gross).

Volkswagen indicates an average consumption for this electric car of 16.9kWh/100km according to the new WLTP cycle. With this data, and knowing the useful capacity of the battery, the autonomy that would be obtained can be estimated with a simple mathematical operation: 456 kilometers. In the data sheet, however, Volkswagen offers two different figures for the ID.4: a combined autonomy of 517 kilometers and a city autonomy of 701 kilometers.

The WLTP cycle inside

To understand why this happens, it is necessary to take a close look at the internals that the WLTP procedure (P, for procedure) uses to calculate autonomy. First, the test conditions are specified. 23 degrees centigradean optimal temperature for electric cars and that is easy to ensure in a laboratory, but that is highly variable in reality.

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Time-speed graph of the WLTP cycle showing the speed variation in the four cycles that are carried out. Source: Wikipedia.

The basis of the WLTP is the driving cycles, WLTC (C for Cycle), that is, the speed curve that must be accurately followed on a dynamometer under those laboratory conditions. The WLTC consists of four subcycles: Low (Low), Middle (Medium), High (High) and Extra-high (Extra High). The names refer to speed levels or speed phases:

  • Low, low speed: up to almost 60 km/h (589 seconds). The maximum speed is 56.5 km/h, but in 56% of the 3,095 meters of this section, the electric car is stopped on the test bench to simulate a traffic light phase or intermittent traffic.
  • Middlemedium speed: up to almost 80 km/h (433 seconds).
  • Highhigh speed: up to almost 100 km/h (455 seconds).
  • extra-high, very high speed: up to 131.3 km/h (323 seconds). The highest speed at which you drive during homologation is reached for just a few seconds.

If the complete WLTC driving cycle is considered, combining the four subcycles, the average speed, including the stopping phases, is only 46.5km/h. Anyone who has driven an electric car knows that speed is one of the most critical parameters in energy consumption and, consequently, in range. There is therefore a clear discrepancy between the combined value of the test bench that gives an average of 46.5 km/h and reality, especially for those who regularly drive on the motorway.

Together, the Low and Middle subcycles form the so-called city ​​cycle (City Cycle). Here it should be noted that the autonomy specification for plug-in hybrid vehicles (PHEV) refers exclusively to this cycle. Therefore, the result is that it is easier for them to achieve a minimum autonomy which, in many states, allows them to take advantage of tax relief, purchase aid and ecological labels. If the full WLTC were applied to them, all of them would be left out of these advantages.

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For 100% electric vehicles, WLTP uses a “short” test procedure and divides it into two dynamic and two constant segments.

In the case of 100% electric vehiclesWLTP uses a “shorthand” test procedure and divides it into two dynamic and two constant segments. In the dynamic segment, the first and third subcycles, acceleration and deceleration phases take place, while the constant, the second and fourth subcycles, involve driving at a continuous speed of 100 km/h.

Although only subcycles 1 and 3 are needed to evaluate the dynamic part, the full WLTC cycle is performed, followed by the City cycle. This results in a distance of 31,113km. In this case, the dynamic segments are used to determine the energy consumption during driving and the constants (2 and 4) are used to perform a accelerated battery emptying, with the aim of reducing the measurement time on the test bench. Therefore, the duration of the 100 km/h sequences depends on the battery capacity. Throughout the abbreviated test procedure, battery current and voltage are continuously measured.

As a result, several different values ​​are collected during a single long measurement: firstly, the actual useful capacity of the traction battery and secondly, the current energy consumption in the aforementioned dynamic cycles.

The measurement on the test bench is considered finished when the electric vehicle can no longer maintain a constant speed of 100 km/h on the last stretch. This is when the battery is considered completely discharged. Because losses occur during the conversion of alternating current from the power grid to direct current during charging, there are different values ​​in the energy used and the electricity recharged.

To obtain the combined autonomy according to the WLTP cycle the two complete cycles, low, medium, high and extra high, in segments 1 and 3 are relevant. In them, the energy consumption is identical although the driving curves are the same. This is because there is a cold start phase at the beginning, which is naturally less important for electric cars than for cars with combustion engines. In these the oil has to be heated first. In electric vehicles, the traction battery is initially so full that recovery is not possible immediately. As a result, consumption in segment 1 increases slightly.

The combined autonomy according to WLTP, i.e. 517 km in the case of the Volkswagen ID.4, is obtained by dividing the energy content measured when charging the battery by the weighted average electricity consumption of the two full WLT cycles. The autonomy in the city It is obtained by dividing the energy by the values ​​of the City cycle (taking into account that these are the Low and Medium speed subcycles), which results in 701 km for the ID.4 case.

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Calculations made by the Volkswagen ID.4’s on-board computer: an electric car can achieve approved ranges on the road when the weather conditions are favorable and with a low speed profile far from that recommended on motorways. In the city, the more favorable traffic conditions allow the homologation to be more in line with reality.

In the case of combined energy consumption, the 16.9 kWh/100 km includes load losses, since these must be paid by the owner. For now, an exact specification for charging (power and type of current) has not been defined, but it is expected to be included soon in the revision of the legislation.

The real life

In real life, users of an electric car experience that they can achieve approved autonomies on the road when the weather conditions are favorable and with a low speed profile, far from those recommended on motorways.

However, at high speeds, and in cold weather conditions, power consumption increases significantly due to air resistance and less efficient chemical reactions with the cooler battery. If these two circumstances are combined, the forecasts of autonomy are very far from reality.

Jan Dornoff, Emissions Specialist at ICCT (International Council on Clean Transportation), an independent non-profit organization that provides technical and scientific analysis to environmental regulators, go here a potential for improvement for the cycle. The test procedure at 23 degrees and without auxiliary consumers such as the air conditioning system leads to idealized measured values. In addition, energy consumption and autonomy must be measured in a low-temperature test for all vehicles: “There is already a low-temperature test for electric vehicles at the UN ECE level. This is done at minus seven degrees and with auxiliary units activated,” explains Dornoff. There is currently no known timetable for the introduction of this procedure in the European Union, but “we at the ICCT recommend that it be as soon as possible to create transparency for clients and legislators.”

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